As reports over the last 3-4 decades demonstrate increasing plastic concentration in the oceans, public and regulatory concerns have grown in parallel. Sequoia Scientific Inc. and other instrument manufactures are thus frequently contacted by scientists and regulators seeking tools that measure and/or detect plastic particles in situ in the ocean. However, there is a large roadblock to this endeavor: at present time, no instrument exists that can accomplish such a feat.
First, it is important to realize that the number concentration of plastic particles in the ocean is generally between zero and a few 100 particles/m3. For example, Kooi et al. (2016) reports mean values of 0.68 particles/m3 in the surface layer of the North Atlantic accumulation zone. Reisser et al. (2016) reports median values of 1.69 particles/m3 in the surface layer of the North Atlantic Gyre. Goldstein et al. (2012) reports 0.116 particles/m3 in the North Pacific Subtropical Gyre. Choy et al. (2019) reports 16 particles/m3 at a depth of a few 100 m in Monterey Bay, CA.
These low number concentrations have unfortunate implications for in situ measurement and detection of plastic particles. At a number concentration of a particles, the volume on average containing exactly one particle is 1/a. This must be compared to the sensing volume, SV, of any sensor designed to detect plastic particles. If each measurement is completely independent of the next, i.e. all water in the sensing volume has been replaced between two consecutive measurements, the measurements required to detect one plastic particle is (1/a)/SV. It is immediately seen that only when (1/a) >= SV can we expect to detect a particle.
To exemplify this, consider Sequoia’s LISST-Holo2, with a sampling volume SV of 1.5 cm3 = 1.5*10-6 m3. At a range of plastic number concentrations typical for the world’s oceans, the table shows the number of holograms and time required (at a 20 Hz sampling frequency) to observe just one plastic particle.
| Number concentration (m-3) | Independent holograms required to see just one plastic particle in a 1.5 cm3 sensing volume | Time (s) (hh:mm:ss) required to collect holograms @ 20 Hz |
|---|---|---|
| 0.7 | 952,000 | 47,600 (13:13:39) |
| 1 | 666,667 | 33,300 (09:15:33) |
| 10 | 66,667 | 3,300 (00:55:33) |
| 16 | 41,667 | 2,100 (00:34:43) |
| 100 | 6,667 | 333 (00:05:33) |
| 1,000 | 667 | 33 |
| 5,000 | 133 | 7 |
| 10,000 | 67 | 3 |
| 333,000 | 2 | 0.1 |
| 666,000 | 1 | 0.05 |
It is evident from that with a sampling volume the size of the LISST-Holo2, it is not feasible to attempt to see plastic particles unless their concentration is of the order of at least 10,000’s particles/m3. At the concentration typically encountered in the ocean of 1 to 100 particles/m3, a sensor with a sensing volume of the order of 0.01 to 1 m3 is required.
At present, no commercially available sensor with a sensing volume of this size exists. Honjo et al. (1984) describes a camera system with a sensing volume of 1.06 x 1.56 x 0.4 m = 0.66 m3 and a resolution of 200 μm. However, their system simply took pictures of the particles. For automated and efficient analysis, it is necessary for the camera to automatically discriminate plastic particles from non-plastic particles from the images. It is unknown if this is feasible. Finally, a camera with a sensing volume of this size would not work in coastal zones where the particulate concentration from sediment and plankton particles is already of the order of millions or billions of particles/m3.
References:
Choy CA, Robison BH, Gagne TO, Erwin B, Firl E, Halden RU, Hamilton JA, Katija K, Lisin S, Rolsky C, Van Houtan KS (2019: The vertical distribution and biological transport of marine microplastics across the epipelagic and mesopelagic water column. Scientific Reports 9, 7843. https://doi.org/10.1038/s41598-019-44117-2
Goldstein MC, Rosenberg M, Cheng L (2012): Increased oceanic microplastic debris enhances oviposition in an endemic pelagic insect. Biology Letters 8: 817-820. https://doi.org/10.1098/rsbl.2012.0298
Honjo S, Doherty KW, Agrawal YC, Asper VL (1984): Direct optical assessment of large amorphous aggregates (marine snow) in the deep ocean. Deep Sea Research Part A. Oceanographic Research Papers 31(1): 67-76. https://doi.org/10.1016/0198-0149(84)90073-6
Kooi M, Reisser J, Slat B, Ferrari FF, Schmid MS, Cunsolo S, Brambini R, Noble K, Sirks L-A, Linders TEW, Schoeneich-Argent RI, Koelmans AA (2016). The effect of particle properties on the depth profile of buoyant plastics in the ocean. Scientific Reports 6, 33882. https://doi.org/10.1038/srep33882
Reisser J, Slat B, Noble K, du Pleiss K, Epp M, Proietti M, de Sonneville J, Becker T, Pattiaratchi C (2016): The vertical distribution of buoyant plastics at sea: an observational study in the North Atlantic Gyre. Biogeosciences 12: 1249–1256. https://doi.org/10.5194/bg-12-1249-2015
©Sequoia Scientific, Inc., 2019