Multi-layer synchronous microplastic observation system and method based on ocean front tracking

Abstract

The application discloses a multilayer synchronous micro-plastic observation system and method based on ocean front tracking, which comprises a front tracking ship and a cross-front observation ship; the front tracking ship is provided with a Manta trawl net and a sampling pump; and the cross-front observation ship is provided with an ADCP and a CTD. The method comprises the following steps: selecting one or more voyage sections on the front, performing micro-plastic tracking voyage along the front direction by the front tracking ship, collecting micro-plastic particles in the surface layer and vertical section, performing cross-front voyage perpendicular to the front direction by the cross-front observation ship, measuring key physical parameters of the front intensity, counting the micro-plastic abundance of each voyage section, calculating the salt density horizontal gradient based on interpolation, calculating the horizontal flow velocity divergence, and then performing observation data fitting to obtain the corresponding relationship between the front intensity and the micro-plastic abundance. The application realizes the collection of micro-plastics at the ocean front and the observation of physical elements by synchronously collecting key data through the double-ship system, and establishes the influence and action relationship of the ocean front physical elements on the micro-plastics.

Description

TECHNICAL FIELD

[0001] The present application relates to a sampling method for marine microplastic pollution, in particular to a multi-layer synchronous microplastic observation system and method based on marine front tracking, belonging to the technical field of marine observation. BACKGROUND

[0002] Marine microplastics (plastic particles with a particle size of less than 5 mm) have become a global environmental pollutant, and its sources include plastic waste degradation, washing wastewater discharge, and industrial raw material leakage. According to research statistics, the global stock of microplastics in the ocean has exceeded one million tons, and the annual increase is hundreds of thousands of tons. These particles can enter the food chain through biological feeding, adsorb and transfer toxic substances (such as heavy metals and persistent organic pollutants), and ultimately threaten marine ecological safety and human health. Therefore, the great severity of marine microplastic pollution and the high monitoring demand, accurate quantification of the spatiotemporal distribution of microplastics, especially its transport and aggregation mechanism in key dynamic regions of the ocean, are the scientific basis for assessing pollution risks and developing governance strategies.

[0003] Although microplastic monitoring technology is constantly developing, the existing microplastic sampling technology still has limitations, especially the traditional sampling method has significant defects in revealing the vertical distribution of water bodies and responding to ocean dynamic processes:

[0004] 1. The singleness of surface sampling: Although the traditional trawl sampling is a commonly used method in the industry, it can only collect water samples from the surface of about 0.3 meters deep, and cannot reflect the vertical profile of microplastic distribution in the water body. In fact, a large amount of microplastics will gather in the thermocline and other subsurface regions, and microplastics can be distributed to hundreds of meters deep under the action of turbulence and sedimentation. Relying solely on surface data will seriously underestimate the pollution load.

[0005] 2. Asynchronous and inefficiency of depth sampling: Although ship-mounted pumps or layered nylon nets can collect samples at specific depths, a single operation can only target a single or a small number of pre-set depths. If a complete profile is to be obtained, the equipment needs to be repeatedly lowered for several hours. In dynamic areas such as ocean fronts, the water mass structure may change significantly under the action of tides and runoff, resulting in a mismatch in time and space of samples at different depths, and failing to capture the instantaneous aggregation effect of microplastics in the vertical water layer. Although the layered sampling based on CTD water samplers can simultaneously obtain water samples and temperature and salinity data, the core of its design is water chemical parameter measurement: low microplastic sampling efficiency: a single trigger only obtains a limited volume (usually 2-10 L), which is much lower than pump sampling (hundreds of liters to cubic meters), making it difficult to capture representative samples in low-concentration areas; lack of anti-pollution design: the opening and closing mechanism easily introduces plastic component wear debris, and lacks a dedicated filtration unit, which requires additional water transfer, increasing the risk of microplastic loss or pollution; limited coverage: high-frequency operation is required for high-resolution sampling in a large range of front areas, which has low practical feasibility.

[0006] In addition, the prior art also has a blind spot in the study of the microplastic aggregation effect of the ocean front. The ocean front is the junction of different water masses, and its characteristics are a narrow area with a sharp increase in temperature, salinity or density gradient in the horizontal direction. The front significantly drives the horizontal transport and vertical redistribution of nutrients, plankton and pollutants by generating vertical circulation and turbulent mixing. Existing studies have shown that the front can induce the enrichment of microplastics at the thermocline, and the concentration can be several times that of the background area. However, traditional microplastic sampling methods rely on fixed sections or grid sampling, which cannot accurately track the migration path of the front; and are insufficient to analyze the aggregation structure of microplastics driven by the front.

[0007] In summary, the prior art cannot solve the core problem of "how to simultaneously and efficiently obtain multi-depth microplastic samples and corresponding physical environmental parameters in the ocean front area" through "along the front" and "across the front" sampling, which seriously restricts the understanding of the microplastic transport-aggregation mechanism regulated by the front. Therefore, it is urgent to develop an integrated microplastic monitoring method for the front environment that supports multi-layer simultaneous sampling. SUMMARY

[0008] The technical problem to be solved by the present application is to overcome the deficiencies of existing ocean microplastic sampling technology in sampling depth, synchronicity and front specificity, to provide a multi-layer simultaneous microplastic observation system and method based on ocean front tracking, which can accurately target the ocean front structure, achieve multi-layer simultaneous sampling, and effectively distinguish the influence of the presence and intensity of the front on the distribution of microplastics.

[0009] Specifically, the following problems need to be solved: First, break through the limitations of traditional sampling equipment that cannot obtain vertical profile microplastic distribution data, and achieve multi-layer simultaneous sampling from the surface to the near bottom; Second, solve the problem that existing technology cannot complete multi-depth sampling at the same spatiotemporal point, and cannot capture the rapid changes of microplastics in the front area; Third, change the current situation that existing methods lack systematic sampling design in the front area, and reveal the influence mechanism of the front on the transport, aggregation and distribution of microplastics through "along the front" and "across the front" comparative sampling, to provide more comprehensive and accurate support for the monitoring, evaluation and management of ocean microplastic pollution.

[0010] The multi-layer simultaneous microplastic observation system based on ocean front tracking is characterized by including an along-front tracking ship and a cross-front observation ship.

[0011] The along-front observation ship sails along the front direction for microplastic tracking, and is used for collecting microplastic particles in the surface layer and vertical section. The ship tail is provided with a Manta trawl net, the net opening direction is consistent with the sailing direction, and is used for collecting marine surface microplastics. The ship is provided with a mechanical flow meter to calculate the water volume passing through the trawl net. The ship side is provided with a telescopic sampling rod covering the vertical section from the subsurface layer to the near bottom layer, and sampling pumps are installed on the rod at equal intervals. The outlet of each sampling pump is connected with a stainless steel filtering unit, and a 5-micron filter film is arranged in the filtering unit. The water sample is directly filtered through the filter film to intercept microplastics after being pumped.

[0012] The cross-front observation ship sails across the front direction perpendicular to the front midpoint, and is used for measuring the front strength key physical parameters. The ship side is provided with an ADCP and a CTD rod. The ADCP detection direction is vertically downward, and is used for measuring the vertical velocity profile of the whole water column, and reflecting the front dynamic characteristics by calculating the vertical velocity divergence. The CTD rod is provided with CTDs installed at equal intervals, so as to measure the depth and salinity data of the water body in real time, and reflect the physical characteristics of the front by calculating the salinity gradient.

[0013] The Manta trawl net has a net opening size of 1m*0.5m and a mesh size of 333 microns.

[0014] The 5-micron polycarbonate filter film is ultrasonically cleaned with ultrapure water for 3 times, 10 minutes each time, and is sterilized and dried for standby use.

[0015] The multilayer synchronous microplastic observation method based on ocean front tracking is characterized in that the method uses the above observation system, and comprises the following steps.

[0016] (1) Obtain the front position;

[0017] (2) Design a double-ship cooperative route:

[0018] Select one or more sailing sections on the front, and select the sailing order of each section;

[0019] (3) Start the operation of the two ships synchronously

[0020] For each section, the along-front tracking ship sails along the front at a speed of 2 knots after reaching one end of the section on the front, simultaneously starts the Manta trawl net to continuously sample, synchronously starts all the layered sampling pumps on the sampling rod to collect water samples, intercepts microplastics through the filter film in the filtering unit, and stabilizes the water inlet depth of the trawl net at 0.2-0.3m;

[0021] The cross-front observation ship crosses the front along the direction perpendicular to the front midpoint at a speed of 2 knots, synchronously starts the ADCP to measure the vertical velocity profile, and starts the CTD to record the salinity and depth distribution;

[0022] (4) Sampling recovery and observation data storage

[0023] When the along-front ship sails away from the sampling area boundary, the Manta trawl is hauled up, the Manta trawl sampling volume V of this leg is calculated; the sampling pump is stopped to avoid invalid collection in the non-sampling area; the microplastic samples trapped by the Manta trawl net are collected and placed in a 4°C refrigerator to prevent sample degradation or contamination; the ADCP, CTD and turbidity meter on the across-front observation ship are simultaneously stopped;

[0024] All filter membrane samples on the along-front observation ship are transferred to a -20°C freezer (to inhibit microbial degradation); the trawl samples are frozen together with the flushing water to avoid direct sunlight;

[0025] The salinity and depth data of the across-front observation ship are combined with the ADCP data, and the sampling time, location and route path data are labeled;

[0026] (5) Both ships cooperatively complete all leg observations

[0027] Both ships are simultaneously adjusted to the next observation starting point, and steps (3) and (4) are repeated until all leg sampling is completed;

[0028] (6) Process observation data

[0029] (6.1) Calculate the microplastic abundance of each leg;

[0030] (6.2) Use the salinity S, horizontal flow rate u, v, and corresponding time t, depth z data collected by each across-front observation ship to construct a time-depth two-dimensional original data matrix, and interpolate in the depth direction;

[0031] (6.3) Calculate the interpolated salinity horizontal gradient |▽S| based on the data collected by each across-front observation ship;

[0032] (6.4) Calculate the flow rate divergence based on the interpolated ADCP flow rate data and latitude and longitude coordinates collected by each across-front observation ship;

[0033] (7) Observation data fitting

[0034] Use the microplastic abundance data obtained in step (6) and each item of marine physical element data to perform fitting, thereby obtaining the corresponding relationship between the front intensity and the microplastic abundance.

[0035] Before the work ship starts the operation, the mechanical flow meter on the along-front observation ship is calibrated with a known volume of water sample before sampling to obtain a correction coefficient R, so as to ensure that the calculation error of the sampling volume calculation formula V=R×Q×A is ≤5%, V: sampling volume, Q: flow meter reading difference, A: net opening area.

[0036] The microplastic abundance of each flight segment is counted, as follows:

[0037] (1) All samples brought back to the laboratory are first placed in a 30 °C oven for drying for 24 h;

[0038] (2) After cooling to room temperature, plastic particle selection is performed according to industry standards, and weighing is performed;

[0039] (3) The volume and number of microplastic particles of each flight segment are recorded;

[0040] (4) The microplastic abundance of the flight segment is calculated using the flow meter readings of each flight segment to obtain the sampling volume.

[0041] The two-dimensional raw observation data matrix of time-depth is constructed, as follows:

[0042] The time sequence of observations is taken as the first dimension (t1, t2,..., t n ), and the measured depth corresponding to each time point is taken as the second dimension (z1, z2,..., z m ). In combination with the data of salinity S, horizontal flow rate u, v collected by the cross-front observation ship on the profile, the raw data matrix S (t, z), u (t, z), v (t, z) is formed, wherein each matrix element represents the salinity and horizontal flow observation value at a time and a depth.

[0043] The interpolation in the depth direction is an interpolation in the depth direction of the vertical profile at each time point for the discrete depth observation data, to generate regular depth data, with the following steps:

[0044] (1) Depth grid setting: according to the measured maximum depth (such as 0-10 m), the regular depth interval after interpolation (such as 0.5 m) is set;

[0045] (2) Interpolation calculation: for each time point t i , the interp1 function of MATLAB is called to perform vertical interpolation on the original depth salinity S and flow rate u, v data. Taking the salinity data as an example: S_interp(t i , Z) = interp1(z_obs(t i ), S_obs(t i , z_obs), Z, 'linear'); wherein z_obs(t i ) is the measured depth sequence at time t i , S_obs(t iS_obs(t,z) is the corresponding measured salinity data, 'linear' means linear interpolation to ensure the continuity of vertical salinity data; after interpolation, regular depth grid data S interp(t,z), u interp(t,z), v interp(t,z) are obtained, that is, each time point corresponds to complete vertical depth profile data.

[0046] The calculation is based on the interpolated salinity level profile gradient |▽S|, and the steps are as follows:

[0047] (1) Distance calculation: for adjacent time points t k and t k+1 , the horizontal distance formula between the two points is calculated according to the latitude and longitude coordinates: ΔD k = 111319.5 × cos(lat k ) × |lon k+1 - lon k | × (π / 180) + 111133 × |lat k+1 - lat k | × (π / 180), where 111319.5m is the distance corresponding to 1° longitude (at the equator), 111133m is the distance corresponding to 1° latitude, (π / 180) is the angle to radian coefficient, and the unit of ΔD k is meters;

[0048] (2) Salinity difference calculation: for each depth z, the salinity difference between adjacent time points is calculated: ΔS k (z) = S interp(t k+1 ,z) - S interp(t k ,z);

[0049] (3) Salinity gradient calculation: the salinity gradient is the ratio of the salinity difference to the horizontal distance, and the absolute value is taken as the quantitative index: |▽S|(t k ,z) = |ΔS k (z)| / ΔD k ;

[0050] Where the unit of |▽S| is psu / m, and the larger the value, the more intense the spatial variation of salinity at that depth between t k and t k+1 . The core feature of the estuary front is that the salinity of different water masses changes dramatically in space, forming a clear transition zone. The rate of spatial variation of salinity directly reflects the intensity of water mass convergence. Therefore, when the salinity gradient is larger, it means that the front is stronger.

[0051] The horizontal flow velocity divergence |div| is calculated based on the interpolated ADCP flow velocity data and latitude and longitude coordinates, and the steps are as follows:

[0052] (1) Flow velocity spatial variation rate calculation: for adjacent time points t k-1 , t k , t k+1 , the spatial variation rates of the horizontal flow velocity u and v components are calculated:

[0053] du_dx(t k ,Z) = [u(t k+1 ,Z) - u(t k-1 ,Z)] / (ΔD k-1 + ΔD k );

[0054] dv_dy(t k ,Z) = [v(t k+1 ,Z) - v(t k-1 ,Z)] / (ΔD k-1 + ΔD k );

[0055] wherein, ΔD k-1 is the horizontal distance between t k-1 and t k , and ΔD k is the horizontal distance between t k and t k+1 , and the calculation method is the same as above;

[0056] (2) Flow velocity divergence calculation: the horizontal flow velocity divergence is the algebraic sum of the spatial variation rates of the u and v components, and after taking the absolute value, it is taken as a quantitative index:

[0057] div(t k ,Z) = du_dx(t k ,Z) + dv_dy(t k ,Z);

[0058] |div|(t k ,Z) = |div(t k ,Z)|;

[0059] wherein, the unit of |div| is 1 / s, if div>0, it indicates that the water flow is divergent, if div<0, it indicates that the water flow is convergent, and the absolute value size reflects the convergence and divergence intensity. In the frontal area, due to the difference in the movement speed of different water masses, the convergence or divergence of the water flow is easy to form, and when the convergence occurs, the microplastics will be gathered in the frontal area, therefore, the flow velocity divergence can be used as a core index for representing the dynamic strength of the frontal surface, and reflects the strength of the frontal surface in the hydrodynamic action level.

[0060] Compared with the prior art, the present application not only realizes the collection of microplastics at the ocean front and the observation of physical elements, but also establishes the influence of the ocean front physical elements on the microplastics and the relationship between the ocean front physical elements and the microplastics. Through the double-ship system, three types of key data are synchronously collected: microplastic concentration, physical environmental parameters (temperature, salinity, turbidity, depth), and dynamic parameters (front strength calculated by the vertical gradient of CTD, and flow velocity data of ADCP carried by the unmanned ship). Through the matching data integration module, the "front strength-microplastic concentration" correlation curve can be directly generated, thereby providing direct data support for revealing the mechanism of "front driving microplastic aggregation".

[0061] The collection and observation process of the present application has multiple synchronizations, thereby improving the timeliness and relevance of the data. The traditional method needs to lower the equipment multiple times to collect samples at different depths (e.g., 5-10 minutes are consumed for sampling at each layer), while the present application can complete the synchronous sampling of the surface layer, the middle layer and the near-bottom layer at the same time through the integrated design of the multi-layer CTD rod on the working ship. This feature solves the "time difference error" caused by the rapid change of the water structure in the front area (e.g., the thermocline depth changes by 1-2 m within 10 minutes), and ensures that the multi-layer microplastic concentration data are strictly matched with the corresponding temperature, salinity and turbidity parameters at the depth, thereby providing a reliable data basis for analyzing the vertical distribution rule.

[0062] The samples obtained by the present application are more representative, and the pollution control is more optimal. The sampling pump is directly connected to the stainless steel filtration unit (to avoid the contact of the plastic parts with the water sample), and a 5-micron aperture polycarbonate filter membrane (resistant to seawater corrosion and without plastic dissolution) is used. The water sample is filtered immediately after being pumped out, thereby reducing the pollution risk in the multiple links of "water sample storage - transfer - filtration" in the traditional method.

[0063] The operation of the present application is more feasible, and the scene adaptability is strong.

[0064] The device design takes into account the actual conditions of marine operations: the multi-layer CTD rod is made of carbon fiber material (weight ≤10 kg), and can be manually adjusted (inclination angle 0°-60° to adapt to different ship heights); the verticality can be maintained through the anchoring device (the weight of the counterweight is calculated according to the water depth, e.g., 5 kg counterweight for 10 m water depth), and the wind and wave resistance level reaches Beaufort 4 (wind speed 6-8 m / s), which can be used in various front environments (temperature front, salinity front, density front) such as nearshore, estuary and open sea. The sampling (cooperation of the working ship and the unmanned ship) can cover an area of 5-10 km of sea, and the time consumption is about 1-2 hours, which is 80% higher than the traditional method (6-8 hours are needed for the same range). BRIEF DESCRIPTION OF DRAWINGS

[0065] Figure 1 Figure 2 is a schematic diagram of two ship track trajectories in a river estuary frontal region.

[0066] Figure 2 Figure 3 is a three-dimensional schematic diagram of a multi-layer synchronous microplastic observation system based on ocean frontal tracking.

[0067] Figure 3 Figure 4 is a schematic diagram of a Manta trawl net.

[0068] Figure 4 Figure 5 is a schematic diagram of a cross-frontal tracking ship CTD rod.

[0069] Figure 5 Figure 6 is a schematic diagram of an along-frontal observation ship equipment configuration.

[0070] Figure 6 Figure 7 is a correlation curve diagram of microplastic abundance and frontal strength parameters (salinity gradient and divergence).

[0071] wherein, Figures 1 to 6 The correspondence between the reference signs and the components is as follows:

[0072] 1 along-frontal tracking ship; 2 cross-frontal observation ship; 3 Manta trawl net, 301 is a light hard aluminum floating plate, 302 is a mechanical flow meter, and 303 is a net; 4 CTD rod assembly, 401 is a CTD rod, 402 is an extension rope, and 403 is a CTD; 5 sampling rod assembly, 501 is a sampling pump. DETAILED DESCRIPTION

[0073] The present application provides an observation system and method for marine microplastics, particularly an observation method near a frontal surface. A double working ship cooperative observation platform is adopted to realize the correlation acquisition of the microplastic distribution characteristics in the frontal region and the frontal strength parameters through synchronous operation. This method can not only accurately collect microplastic samples at the frontal surface, but also can simultaneously measure key indicators such as frontal salinity gradient and vertical flow velocity divergence, thereby revealing the driving mechanism of frontal strength on microplastic aggregation. The observation system includes two cooperative professional observation ships and their special equipment, which are as follows.

[0074] I. System composition (as shown in Figure 2 )

[0075] 1. Along-frontal tracking ship

[0076] The along-frontal tracking ship is used for tracking navigation along the frontal surface, and is mainly used for collecting microplastic particles in the surface layer and vertical profile to provide sample support for microplastic distribution characteristic analysis. The core configuration includes:

[0077] Manta trawl net: as Figure 3As shown, mounted on the stern, the net opening direction is consistent with the sailing direction, used for collecting microplastics in the ocean surface layer (about 30 cm deep), including a light hard aluminum floating plate 301, a net 303, equipped with a mechanical flow meter 302, and the sampling water volume is accurately calculated through a calibration formula (V = R x Q x A, where V is the sampling volume, R is the calibration coefficient, Q is the flow meter reading, and A is the net opening area).

[0078] Sampling rod: as shown in Figure 5 As shown, a telescopic carbon fiber rod is fixed on the side of the ship, and layered sampling pumps 501 are installed on the rod at uniform intervals, covering the vertical profile from the subsurface layer to the near bottom layer. The sampling rod, sampling pump 501 and related accessories form a sampling rod assembly 5. The outlet of each sampling pump 501 is connected to a stainless steel filter unit (with a 5 μm filter membrane inside), and the water sample is directly filtered through the filter membrane to trap microplastics after being pumped. After sampling is completed, the filter membrane is taken out through sterile operation and labeled with information such as depth, position, volume, etc., and is stored in a cold storage for laboratory analysis (extraction, identification and counting).

[0079] 2. Cross-front observation ship

[0080] Cross-front sailing is perpendicular to the front direction, mainly used for measuring key physical parameters of front strength, providing data support for quantifying front strength. Its core configuration includes:

[0081] ADCP (Acoustic Doppler Current Profiler): installed on the side of the ship, with the detection direction vertically downward, used for measuring the vertical flow velocity profile of the whole water column, and reflecting the dynamic characteristics of the front by calculating the vertical flow velocity divergence.

[0082] CTD rod 401: as shown in Figure 4 Consistent with the sampling rod 5, it is fixed on the side of the ship. CTD 403 (temperature, salinity, depth sensor) is bound on the rod at uniform intervals, and a turbidity meter can also be added to measure the salinity, temperature and turbidity distribution of the water body in real time, and the physical properties of the front are reflected by calculating the salinity gradient. The end of the rod is provided with a telescopic rope 402 for adjusting the angle. The CTD rod 401, telescopic rope 402 and CTD 403 constitute a CTD rod assembly.

[0083] Front strength quantification: through the horizontal salinity gradient (|▽S|) and the flow velocity divergence (|div|), as the core index representing the front strength.

[0084] II. Multi-layer synchronous microplastic observation system and method based on ocean front tracking, specifically as follows:

[0085] S1, preliminary preparation stage

[0086] 1.1 Front position acquisition method

[0087] The existing technology can be used to access and download satellite remote sensing data and images (such as Sentinel-3 ocean color satellite, HY-1C China ocean color satellite) to obtain target sea area water color, sea surface temperature (SST) data, combined with historical front data of the target sea area (including frequency, location, migration range, type), and obtain high-precision weather forecast (covering wind speed, wind direction, wave, precipitation), to realize accurate acquisition of front position. The focus is to ensure that the wind speed in the sampling window period is less than 8 m / s, and there is no severe weather, to determine: the specific date suitable for double-ship cooperative sampling and the specific location of the front core area as the core coverage range of double-ship observation.

[0088] 1.2 Double-ship cooperative route design of double working ships

[0089] Select one or more segments on the front, and the length of the segment is selected according to the actual situation, such as 1 km; select the sailing order of each segment.

[0090] Front observation ship route: design a continuous tracking path parallel to the front direction, such as Figure 1 indicated by the red line, the route length is about 1 km, and the speed is 2 knots;

[0091] Cross-front observation ship route: design a cross-section path perpendicular to the front direction, such as Figure 1 indicated by the blue line, the width is 1 km (enough to cover the front width), and the speed is 2 knots;

[0092] Two ships set the same start time (error ≤1 minute).

[0093] 1.3 Specific equipment as follows:

[0094] 1.3.1 Front observation ship (sailing along the front, collecting microplastics)

[0095] Manta plankton trawl: net opening size 1m×0.5m, mesh size 333μm, fixed at the stern. Equipped with a mechanical flowmeter, which needs to be calibrated with a known volume of water sample before sampling to obtain the correction coefficient R (such as R=0.16), to ensure that the calculation error of the sampling volume calculation formula V=R×Q×A (V: sampling volume, Q: flowmeter reading, A: net opening area) is ≤5%.

[0096] Sampling pump array: Select a carbon fiber rod with a length that can cover the subsurface to near bottom depth, such as a 10m long and 5cm diameter carbon fiber rod in estuarine areas, and fix it to the ship side. Install 10 layer sampling pumps evenly on the rod at 1m intervals (pump head made of stainless steel, pump pipe made of silicone). Connect each pump outlet to a 316 stainless steel filter cup (with a 5μm polycarbonate filter membrane inside, the filter membrane needs to be ultrasonically cleaned with ultrapure water for 10 minutes three times in the laboratory, and then dried aseptically for standby).

[0097] 1.3.2 Cross-front observation ship (cross-front sailing, measure front intensity)

[0098] ADCP (Acoustic Doppler Current Profiler, Model: Signature500): Installed on the ship side (≥1m from the ship body to avoid turbulence interference), the detection direction is vertically downward, used to measure the vertical flow velocity profile of the whole water column.

[0099] Sensor array: Use the same type of 10m carbon fiber rod, fixed on the ship side. CTD (Conductivity-Temperature-Depth, Model: RBR concerto) and turbidity meter (MRS Scientific) are evenly bound on the rod at 1m intervals to measure the temperature, salinity, depth, and turbidity of the vertical profile of the water body.

[0100] S2, two ships arrive and equipment deployment calibration

[0101] Two working ships arrive at the reference point at the same time, and complete equipment installation and calibration respectively:

[0102] Equipment preparation for front tracking ship

[0103] Manta trawl deployment: Fix the trawl net at the stern with a special fixing device, adjust the length of the hawser to 15m, control the net opening water depth to 0.2-0.3m; calibrate the flowmeter with standard volume water sample on site, record the correction coefficient R.

[0104] Sampling pump deployment: Fix the carbon fiber telescopic rod on the ship side to ensure firm installation and smooth extension. After extending the rod to a horizontal state, slowly lower it into the water with an electric winch; after entering the water, stand still for 5 minutes, monitor the corresponding CTD salinity of each depth sampling pump through the data acquisition system to ensure that there is no obvious fluctuation (fluctuation ≤0.01psu), and ensure that the sensor is stable. Start the sampling pump, test the pumping-filtering process (no blockage, no water leakage), and confirm that the filter membrane and the water bottle are connected normally.

[0105] Equipment preparation for cross-front observation ship

[0106] ADCP installation: Fix the ADCP on the ship side, ensure that the detection direction is vertically downward, and avoid interference from the ship body; turn on and preheat for 10 minutes, test the stability of the flow velocity data acquisition.

[0107] Sensor deployment: CTD and nephelometer (bound on CTD) are fixed on the carbon fiber telescopic pole on the ship side at 1m intervals; the pole is slowly put into the water (the process is the same as the frontal observation ship), and after entering the water, it is left for 5 minutes. If there is no obvious fluctuation in the CTD salinity and nephelometer turbidity readings (CTD salinity fluctuation ≤0.01psu), it is ensured that the sensor is working properly.

[0108] S3, start observation simultaneously by two ships

[0109] Two working ships start operation simultaneously at the preset time, with the frontal observation ship as the core operation area. The gradient observation ship simultaneously carries out cross-frontal observation, and the specific process is as follows:

[0110] 3.1 Start-up phase

[0111] 3.1.1 Along the frontal tracking ship:

[0112] When reaching the boundary of the sampling area at the frontal position, sail according to the predetermined trajectory at a speed of 2 knots, immediately start the Manta trawl net for continuous sampling, and record the initial reading Q1 of the mechanical flowmeter simultaneously as the starting data for calculating the sampling volume of this section.

[0113] Start all stratified sampling pumps on the sampling pole to collect water samples, and filter and trap microplastics through 316 stainless steel filter cups (with 5μm polycarbonate filter membrane inside).

[0114] After starting, monitor the equipment operation status in real time to ensure that the trawl net mouth enters the water at a depth of 0.2-0.3m, the sampling pump water pumping-filtering process is smooth without blockage, and the CTD salinity fluctuation is ≤0.01psu.

[0115] 3.1.2 Cross-frontal observation ship:

[0116] Start ADCP to measure the vertical flow velocity profile, start CTD and nephelometer on the telescopic pole to record salinity and turbidity distribution, and sail along the line perpendicular to the frontal direction at a speed of 2 knots.

[0117] 3.2 Sampling stage

[0118] 3.2.1 Along the frontal tracking ship

[0119] Sail along the frontal direction at a speed of 2 knots in the frontal area, use real-time water depth data obtained by the shipboard depth sounder to monitor the angle and depth changes of the telescopic pole in real time: if it touches the bottom quickly, adjust the angle to shorten the temperature telescopic pole to prevent damage to the equipment or cause data distortion. And monitor the position of the trawl net and the state of the sampling pump in real time.

[0120] 3.2.2 Cross-frontal observation ship

[0121] The ADCP outputs the vertical current data in real time, and the CTD and the turbidity meter record the changes of salinity and turbidity continuously while the ship keeps sailing along the vertical front.

[0122] 3.3 Leaving the sampling area stage

[0123] 3.3.1 Along-frontal observation ship

[0124] When the ship is about to leave the boundary of the sampling area at the front, the Manta trawl is hauled up, the flowmeter end reading Q2 is recorded, the cumulative reading is calculated by the formula Q = Q2-Q1, and the trawl sampling volume V of this section is accurately calculated according to V = R x Q x A (R is the correction factor, such as 0.16; A is the net opening area).

[0125] Stop the sampling pump on the telescopic rod to avoid invalid collection in the non-sampling area.

[0126] 3.3.2 Cross-frontal observation ship

[0127] Stop the ADCP, CTD and turbidity meter at the same time.

[0128] 3.4 After completing the sampling at the front

[0129] 3.4.1 Along-frontal observation ship equipment recovery and cleaning

[0130] Slowly recover the sampling rod and Manta trawl to the deck, avoid equipment collision during the recovery process, check whether the trawl net is damaged and whether the sampling rod is connected firmly, and record the equipment status. Carefully collect the microplastic samples trapped by the Manta trawl net, put them into the pre-numbered 140-mesh 106-um nylon net, and ensure that the samples correspond to the sampling area, time, etc. Put the sample bottle containing the microplastic sample into the 4°C refrigerator to prevent the sample from degrading or being contaminated. Thoroughly rinse the Manta trawl net body and collection cup with sufficient pure water, and the number of rinsing times is not less than 3 times, and the water volume of each rinsing is not less than 500 mL. Collect the rinsed water sample and also put it into the refrigerator for return. Through rinsing, the microplastics remaining on the trawl net are collected to ensure the cleanliness and sampling accuracy of the trawl net in the next sampling area.

[0131] Immediately take out the filter membrane in each filter cup with a strictly sterilized sterile forceps, and strictly prohibit touching the surface of the filter membrane during the operation, and put it into a pre-numbered sterile sample bag. The sample bag is numbered according to the sampling station + sampling time + sampling depth. And quickly transfer to the 4°C refrigerator for storage.

[0132] 3.4.2 Cross-frontal observation ship data and equipment recovery

[0133] Export ADCP flow velocity data and salinity, turbidity data of CTD and nephelometer on the telescopic pole. Recover ADCP and telescopic pole, rinse the surface of the equipment with fresh water to remove salt on the surface of the instrument, check the sensor probe to ensure that there is no biological attachment or physical damage.

[0134] 3.4.3.2 Repeat observation of the same location by two ships

[0135] When multiple segments are selected, multiple repeat observations are made on different locations of the front. The two ships are adjusted to the next observation starting point (still within the range of the front) at the same time, and the sampling rod and Manta trawl net are redeployed along the front tracking ship. After deployment, necessary post-dive checks and tests are performed again, including sampling pump function tests.

[0136] The cross-front observation ship adjusts the route at the same time, and necessary post-dive checks and tests are also performed, including CTD and nephelometer sensor stability tests. Keep the same synchronization period as the front observation ship, and repeat the above observation process until the preset number of observations is completed.

[0137] S4: Sample and data on-site processing (end of daily operation)

[0138] 4.1 Sample processing

[0139] Transfer all filter samples from the front observation ship from the 4°C refrigerator to the -20°C freezer (to inhibit microbial degradation); freeze the trawl samples together with the rinse water to avoid direct sunlight.

[0140] Merge and store the salinity, turbidity data and ADCP data of the cross-front observation ship, and label the sampling time, location, and route data.

[0141] 4.2 Data processing

[0142] Import data from both ships into a dedicated computer and use professional software (such as MATLAB) for processing: front observation ship data includes surface microplastic sampling volume, vertical sampling volume, etc.; gradient observation ship data includes salinity, flow rate, temperature, turbidity, etc.

[0143] 4.2.1 Microplastic abundance data

[0144] 1) Drying and weighing: all samples returned to the laboratory are first placed in a 30°C oven for 24 hours; after cooling to room temperature, record the dry weight (accurate to 0.1 mg) using a precision 0.0001 g balance.

[0145] 2) Selection of plastic granules: Adopting industry standards, use metal tweezers to pick up granules that meet all of the following criteria: ① No visible cells or organic structures; ② No friction noise when picked up; ③ Uniform coloring; ④ Moderate hardness / toughness, not easy to break, and smooth surface; ⑤ Neat edges; ⑥ Fine filaments with smooth surface, uniform thickness, three-dimensional curvature, and no branching.

[0146] 3) Record the volume and quantity of microplastic particles for each flight segment;

[0147] 4) Calculate the microplastic abundance at a station by obtaining the sampling volume from the flow meter readings of each section.

[0148] 4.2.2 Preprocessing of data across fronts

[0149] The salinity (S), horizontal velocity components (u, v), and corresponding time (t), depth (z) data collected by the cross-frontal observation vessel were processed to construct a two-dimensional time-depth raw data matrix. Specifically, the first dimension (t1, t2, ..., t...) is used as the observation time series. n The second dimension is the measured depth at each time point (z1, z2, ..., z). m This forms the original data matrices S(t,z), u(t,z), and v(t,z), where each matrix element represents an observation at a certain time and depth.

[0150] 4.2.3 Using MATLAB for depth interpolation

[0151] For discrete depth observation data, MATLAB is used to interpolate the depth direction of the vertical profile at each time point to generate regular depth data. The steps are as follows:

[0152] 1) Depth grid setting: Based on the measured maximum depth (e.g., 0-10m), set the interpolated regular depth interval (e.g., 0.5m).

[0153] 2) Interpolation calculation: For each time point t i The `interp1` function in MATLAB is used to perform vertical interpolation on the original depth salinity (S) and flow velocities (u, v). Taking salinity data as an example:

[0154] S_interp(t i ,Z) = interp1(z_obs(t i ), S_obs(t i ,z_obs), Z, 'linear'); where, z_obs (t i () represents time t i The measured depth sequence, S_obs (t iS_obs(t,z) is the corresponding observed salinity data, 'linear' means linear interpolation to ensure the continuity of vertical salinity data; after interpolation, regular depth grid data S interp(t,z), u interp(t,z), v interp(t,z) are obtained, that is, each time point corresponds to complete vertical depth profile data.

[0155] 4.2.3 Salinity horizontal gradient (|▽S|) calculation

[0156] Based on the interpolated salinity vertical profile data salinity gradient, the steps are as follows:

[0157] 1) Distance calculation: for adjacent time points t k and t k+1 , the horizontal distance formula between two points is calculated according to the latitude and longitude coordinates: ΔD k = 111319.5 × cos(lat k ) × |lon k+1 - lon k | × (π / 180) + 111133 × |lat k+1 - lat k | × (π / 180), where 111319.5m is the distance corresponding to 1° longitude (at the equator), 111133m is the distance corresponding to 1° latitude, (π / 180) is the angle to radian coefficient, and the unit of ΔD k is meter;

[0158] 2) Salinity difference calculation: for each depth z, the salinity difference of adjacent time points is calculated: ΔS k (z) = S interp(t k+1 ,z) - S interp(t k ,z);

[0159] 3) Salinity gradient calculation: the salinity gradient is the ratio of the salinity difference to the horizontal distance, and the absolute value is taken as the quantitative index: |▽S|(t k ,z) = |ΔS k (z)| / ΔD k ;

[0160] Wherein, the unit of |▽S| is psu / m, and the larger the value, the more intense the spatial change of salinity at that depth in that time interval. The core feature of the estuarine front is that the salinity of different water masses changes sharply in space, forming a clear transition zone. The rate of spatial change of salinity directly reflects the intensity of water mass convergence. Therefore, when the salinity gradient is larger, it means that the front is stronger;

[0161] 4.2.4 Calculation of flow velocity divergence (|div|)

[0162] Based on the interpolated ADCP flow velocity data and latitude and longitude coordinates, the horizontal flow velocity divergence is calculated, with the following steps:

[0163] 1) Calculation of spatial variation rate of flow velocity: for adjacent time points t k-1 , t k , t k+1 , the spatial variation rates of the horizontal flow velocity u and v components are calculated:

[0164] du_dx(t k , Z) = [u(t k+1 , Z) - u(t k-1 , Z)] / (ΔD k-1 + ΔD k );

[0165] dv_dy(t k , Z) = [v(t k+1 , Z) - v(t k-1 , Z)] / (ΔD k-1 + ΔD k );

[0166] where ΔD k-1 is the horizontal distance between t k-1 and t k , and ΔD k is the horizontal distance between t k and t k+1 , and the calculation method is the same as step 1) in 4.2.3;

[0167] 2) Calculation of flow velocity divergence: the horizontal flow velocity divergence is the algebraic sum of the spatial variation rates of the u and v components, and after taking the absolute value, it is taken as the quantitative index:

[0168] div(t k , Z) = du_dx(t k , Z) + dv_dy(t k , Z);

[0169] |div|(t k , Z) = |div(t k , Z)|;

[0170] Here, |div| is measured in units of 1 / s. If div > 0, it indicates water flow divergence; if div < 0, it indicates convergence. The absolute value reflects the intensity of convergence and divergence. In frontal regions, due to differences in the movement speed of different water masses, convergence or divergence of water flow is easily formed. Convergence can cause microplastics to accumulate in the frontal region. Therefore, velocity divergence can serve as a core indicator characterizing the dynamic intensity of a front, reflecting the strength of the front's hydrodynamic impact.

[0171] 4.3) Data Fitting:

[0172] This invention utilizes a frontal observation vessel to collect microplastics at the frontal zone, and then obtains microplastic abundance data through statistical analysis. After processing and calculating the data collected by the cross-frontal observation vessel using the above steps, the marine physical elements of the observation route can be obtained. After integrating these data, the correspondence between frontal intensity and microplastic abundance can be obtained. Furthermore, by using MATLAB for calculation, the fitting relationship between the variables can be obtained.

[0173] like Figure 6 As shown, in one observation, the fitted formula is: Microplastic abundance = 108×|▽S| +1351×div - 2.86.

[0174] S5: Equipment Cleaning and Maintenance

[0175] All parts that come into contact with the water sample (sampling pump tubing, filter cup, tweezers, and trawl) should be rinsed three times with ultrapure water (≥500mL each time) to remove residual microplastics and salts; the filter membrane support should be wiped with 75% alcohol for disinfection and then air-dried for later use.

[0176] Check equipment status: flow meter calibration accuracy, ADCP flow rate measurement deviation, CTD salinity error (recalibrate to ≤±0.002psu) to ensure that equipment performance meets standards for the next day's operation.

Claims

1. A multi-layer synchronous microplastic observation method based on ocean front tracking, characterized in that The method utilizes a multi-layer synchronous microplastic observation system based on ocean front tracking, The system comprises a front tracking ship and a cross-front observation ship; The front tracking ship sails along the front to track microplastics, and is used to collect microplastic particles in the surface layer and vertical profile. A Manta trawl net is installed at the stern of the ship, the net opening direction is consistent with the sailing direction, and is used to collect marine surface microplastics. A mechanical flowmeter is provided to calculate the water volume passing through the trawl net. A retractable sampling rod is provided on the side of the ship, covering the vertical profile from the subsurface layer to the near bottom layer. Sampling pumps are installed at equal intervals on the rod. Each sampling pump outlet is connected to a stainless steel filter unit, which has a built-in 5μm filter membrane. After the water sample is extracted, it is directly passed through the filter membrane to intercept microplastics. The cross-front observation ship sails across the front to measure the strength of the front. An ADCP and a CTD rod are installed on the side of the ship. The ADCP probe detects vertically downward to measure the vertical flow velocity profile of the whole water column, and reflects the dynamic characteristics of the front by calculating the vertical flow velocity divergence. The CTD rod is installed at equal intervals to measure the depth and salinity data of the water body in real time, and reflects the physical characteristics of the front by calculating the salinity gradient. The observation method comprises the following steps: (1) Obtain the front position; (2) Design a cooperative route for the two working ships: Select one or more sailing sections on the front, and select the sailing order of each section; (3) Start the operation of the two working ships synchronously For each section, the front tracking ship sails along the front at a speed of 2 knots after reaching one end of the section on the front, and simultaneously starts the Manta trawl net for continuous sampling. All the layered sampling pumps on the sampling rod are started synchronously to collect water samples, and the filter membrane in the filter unit is used to intercept microplastics. The depth of the trawl net opening into the water is stabilized at 0.2-0.3m; The cross-front observation ship crosses the front along the direction perpendicular to the midpoint of the front at a speed of 2 knots, and simultaneously starts the ADCP to measure the vertical flow velocity profile and starts the CTD to record the salinity and depth distribution; (4) Sampling recovery and observation data storage When the front tracking ship leaves the sampling area boundary at the front, the Manta trawl net is salvaged, and the Manta trawl net sampling volume V of the section is calculated. The sampling pump is stopped to avoid invalid collection in the non-sampling area. The microplastic samples intercepted by the Manta trawl net are collected and stored in a 4°C refrigerator to prevent sample degradation or contamination. The ADCP and CTD of the cross-front observation ship are stopped synchronously; All the filter membrane samples of the front tracking ship are transferred to a -20°C freezer; the trawl net samples and the flushing water are frozen together to avoid direct sunlight; The salinity and depth data of the cross-front observation ship and the ADCP data are stored together, and the sampling time, position, route data are labeled; (5) The two ships cooperatively complete the observation of all sections The two ships are adjusted to the next observation starting point synchronously, and steps (3) and (4) are repeated until the sampling of all sections is completed; (6) Process the observation data (6.1) Calculate the microplastic abundance of each section; (6.2) Constructing the time-depth two-dimensional original data matrix with the data of salinity S, horizontal flow velocity u, v, corresponding time t, and depth z collected by each cross-front observation ship, and interpolating in the depth direction; (6.3) Calculating the horizontal gradient |▽S| of the interpolated salinity based on the data collected by each cross-front observation ship; (6.4) Calculating the flow velocity divergence based on the interpolated ADCP flow velocity data and the latitude and longitude coordinates collected by each cross-front observation ship; (7) Observation data fitting Fitting the microplastic abundance data obtained in step (6) and each marine physical element data to obtain the corresponding relationship between the front strength and the microplastic abundance.

2. The multi-layer synchronous microplastic observation method based on ocean front tracking according to claim 1, characterized in that The Manta trawl has a mesh size of 333 μm.

3. The multi-layer synchronous microplastic observation method based on ocean front tracking according to claim 1, characterized in that The built-in 5 μm filter is ultrasonically cleaned with ultrapure water for 3 times, 10 minutes each time, sterilized and dried for standby use in the laboratory.

4. The multi-layer synchronous microplastic observation method based on ocean front tracking according to claim 1, characterized in that before the work ship starts the operation, the mechanical flow meter along the front tracking ship is calibrated with a known volume of water sample before sampling to obtain a correction coefficient R, so as to ensure that the calculation error of the sampling volume calculation formula V=R×Q×A is ≤5%, V: sampling volume, Q: flow meter reading difference, A: mesh opening area.

5. The multi-layer synchronous microplastic observation method based on ocean front tracking according to claim 1, characterized in that The microplastic abundance of each voyage is calculated as follows: (1) All samples returned to the laboratory are first placed in a 30 °C oven and dried for 24 h; (2) After cooling to room temperature, the plastic particles are selected according to the industry standard, and weighed; (3) Record the volume and number of microplastic particles in each voyage; (4) Calculate the microplastic abundance of each voyage by using the flow meter reading to obtain the sampling volume.

6. The multi-layer synchronous microplastic observation method based on ocean front tracking according to claim 1, characterized in that The time-depth two-dimensional original observation data matrix is constructed as follows: With the observation time series as the first dimension t1, t2,..., t n , with the measured depth corresponding to each time point as the second dimension z1, z2,..., z m , combined with the data of salinity S, horizontal flow velocity u, v collected by the cross-front observation ship on the profile, the original data matrix S (t, z), u (t, z), v (t, z) is formed, wherein each matrix element represents the salinity and horizontal flow observation value at a certain time and a certain depth.

7. The multi-layer synchronous microplastic observation method based on ocean front tracking according to claim 6, characterized in that The interpolation in the depth direction is to interpolate the depth direction of the vertical profile at each time point for the discrete depth observation data to generate regular depth data, and the steps are as follows: (1) Depth grid setting: according to the maximum depth measured, set the regular depth interval after interpolation; (2) Interpolation calculation: for each time point t i , call MATLAB's interp1 function to vertically interpolate the original depth salinity S and flow rate u, v data, taking the salinity data as an example: S_interp(t i ,Z) = interp1(z_obs(t i ), S_obs(t i ,z_obs), Z, 'linear'); where z_obs(t i ) is the measured depth sequence at time tᵢ, S_obs(t i ,z_obs) is the corresponding measured salinity data, and 'linear' indicates linear interpolation to ensure the continuity of the vertical salinity data; after interpolation, regular depth grid data S_interp(t,Z), u_interp(t,Z), v_interp(t,Z) are obtained, i.e. complete vertical depth profile data corresponding to each time point.

8. The multi-layer synchronous microplastic observation method based on ocean front tracking according to claim 1, characterized in that The calculation of the horizontal profile gradient |▽S| based on the interpolated salinity is as follows: (1) Distance calculation: for adjacent time points t k and t k+1 , the horizontal distance formula between two points is calculated according to the latitude and longitude coordinates: ΔD k = 111319.5 × cos(lat k ) × |lon k+1 - lon k | × (π / 180) + 111133 × |lat k+1 - lat k | × (π / 180), wherein 111319.5m is the distance corresponding to 1° longitude, 111133m is the distance corresponding to 1° latitude, (π / 180) is the angle to radian coefficient, and the unit of ΔD k is meters; (2) Salinity difference calculation: For each depth z, calculate the salinity difference between adjacent time points: AS k (z) = S interp(t k+1 ,z) - S interp(t k ,z); (3) Salinity gradient calculation: The salinity gradient is the ratio of the salinity difference to the horizontal distance, and the absolute value is taken as the quantitative index: |▽S|(t k ,z) = |ΔS k (z)| / ΔD k ; wherein |▽S| has units of psu / m, and a larger value indicates a more rapid spatial variation of salinity at the depth between t k and t k+1 .

9. The multi-layer synchronous microplastic observation method based on ocean front tracking according to claim 8, characterized in that The horizontal flow velocity divergence |div| is calculated based on the interpolated ADCP flow velocity data and the latitude and longitude coordinates, and the steps are as follows: (1) Flow rate spatial variation rate calculation: For adjacent time points t k-1 , t k , t k+1 , the spatial variation rate of the horizontal flow rate u, v components is calculated: du dx (t k ,Z) = [u(t k+1 ,Z) - u(t k-1 ,Z)] / (ΔD k-1 + ΔD k ). dv dy(t k ,Z) = [v(t k+1 ,Z) - v(t k-1 ,Z)] / (ΔD k-1 + ΔD k ). where ΔD k-1 is the horizontal distance between t k-1 and t k , ΔD k is the horizontal distance between t k and t k+1 , and the calculation method is the same as claim 8. (2) Flow velocity divergence calculation: the horizontal flow velocity divergence is the algebraic sum of the spatial variation rates of u, v components, and the absolute value is taken as the quantitative index: div(t k ,Z) = du_dx(t k ,Z) + dv_dy(t k ,Z) |div(t k ,Z) = |div(t k ,Z)|; Wherein, the unit of |div| is 1 / s, if div>0, it indicates that the water flow is divergent, if div<0, it indicates that the water flow is convergent, and the absolute value size reflects the convergence and divergence intensity.

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