How do you sample water for microplastics?
Sampling water for microplastics primarily involves capturing particles floating on or suspended in the water using specialized nets or collecting water volumes for filtration.
Collecting water samples is a critical initial step in studying microplastic pollution in various aquatic environments, including oceans, rivers, and lakes. The specific method used often depends on the research goals, the water body being sampled, and the size range of microplastics being targeted.
Common Methods for Surface Waters
For surface waters, where many buoyant microplastics accumulate, specific types of trawls are widely used. As noted in many studies, most water monitoring studies use Neuston or Manta trawl with mesh sizes of 20–1000 μm to collect microplastics in the surface waters. These methods are designed to efficiently capture particles floating on or near the water surface as a boat tows the net.
What are Neuston and Manta Trawls?
- Manta Trawl: Named for its shape resembling a manta ray, this trawl is towed behind a vessel. It features a wide, rigid mouth that skims the water surface, directing the top layer of water and any floating material into a collecting net bag. It is particularly effective for capturing microplastics and other debris right on the water's surface.
- Neuston Trawl: Similar in principle to the Manta trawl, the Neuston trawl is also designed to collect organisms and particles from the water's surface layer (the neuston layer). It can often sample a slightly thicker slice of the surface water than a Manta trawl, though both target the uppermost few centimeters.
The Importance of Mesh Size
The size of the mesh in the collecting net is a crucial parameter. Using mesh sizes in the range of 20–1000 μm allows researchers to capture microplastic particles within the defined microplastic size range (typically <5 mm down to microscopic levels).
- A smaller mesh size (e.g., 20 μm) can capture smaller microplastic fragments and fibers. However, these nets can clog more easily with plankton and other organic matter, reducing sampling efficiency over time.
- A larger mesh size (e.g., 1000 μm or 1 mm, which is the upper limit for microplastics) is useful for capturing larger microplastic fragments but will miss smaller particles.
The choice of mesh size depends on the study's objectives and the expected size distribution of microplastics in the sampling area.
Other Sampling Considerations
Effective water sampling for microplastics requires careful planning to ensure representative and uncontaminated samples.
- Location and Transect: Sampling should occur in relevant areas (e.g., pollution hotspots, open water zones) following defined transects to cover a representative area.
- Towing Speed and Duration: Consistent speed and duration of trawls help standardize the volume of water sampled.
- Depth: While trawls target the surface, other methods like pumps or Niskin bottles are used to collect water from specific depths in the water column.
- Contamination Control: Avoiding contamination from synthetic clothing, ropes, paints, or other plastic items on the boat is paramount. Using pre-cleaned equipment and minimizing exposure to potential sources is essential.
Beyond Trawls: Alternative Methods
While trawling is standard for surface samples, other methods are employed for different sample types:
- Bulk Water Sampling: Collecting discrete volumes of water using containers like stainless steel buckets or Niskin bottles. This water is then typically filtered through mesh or filters with specific pore sizes (often much smaller than trawl mesh) to capture smaller microplastics and those suspended in the water column.
- In-situ Pumping/Filtration: Using submersible pumps and filter units to filter large volumes of water directly at different depths in the water column.
Summary of Common Methods
| Method | Water Layer Sampled | Typical Application | Mesh/Filter Size Range | Notes |
|---|---|---|---|---|
| Manta/Neuston Trawl | Surface (top few cm) | Floating microplastics & debris | 20–1000 μm (Net) | Towed behind boat, collects surface layer |
| Bulk Water Sample | Surface or Depth | Suspended microplastics, smaller fractions | Varies (Filter pore size) | Collected in containers, filtered later |
| In-situ Filtration | Specific Depth | Microplastics at depth | Varies (Filter pore size) | Filters water directly in place |
After sampling, the collected material is typically processed in a laboratory to separate microplastics from organic matter, identify them, and quantify their abundance.
