Zooplankton is ubiquitous in marine environments and the zooplankton species composition is often used to determine the ecological quality of marine water bodies. Furthermore, being at the base of the food chain, thus serving as food for higher trophic levels, zooplankton is a crucial component of the marine ecosystem. Therefore zooplankton sampling is a crucial part of the LifeWatch campaigns. During monthly campaigns, zooplankton is sampled at 9 onshore stations; during seasonal campaigns, zooplankton is sampled at 17 on and offshore stations. To counter the diurnal migration of zooplankton, a vertical WP2 plankton net is used at all stations.
To process large samples of zooplankton semi-automatically, the Flanders Marine Institute (VLIZ) installed a ZooSCAN (HYDROPTIC) system as part of the Belgian LifeWatch observatory. The device is used to (1) generate a digital copy of zooplankton samples, (2) process and analyze collected zooplankton samples in a fast and semi-automatic way, and (3) capture data on rapidly degenerating gelatinous plankton. The device is located at the LifeWatch laboratories at the Marine Station Ostend (MSO).
The ZooSCAN system uses scanner technology with custom lightning and a watertight scanning chamber into which liquid zooplankton samples can be placed. During scanning, one digital image per sample is generated and the sample can be recovered without damage afterwards. The ZooSCAN generates high resolution pictures or vignettes (14,150 x 22,640 pixels, up to 4800 dpi). With a resolution of approximately 10µm, the ZooSCAN is optimal for objects larger than 200µm and thus optimal for zooplankton. The digital images can then be analyzed by semi-automatic computer processing: all organisms/particles from the samples are annotated to several taxonomic groups (e.g. Appendicularia, Gammaridae, Calanoida, etc.), but also debris, sand, plastics, etc. Identification of the organisms/particles is done by semi-automatic comparison of each organism/particle in the scanned image with a library set. Such library can be built for each individual survey or can be imported from a previous survey. The latest machine learning algorithm allows high recognition levels; complementary manual sorting is recommended to achieve a high number of taxonomic groups. Although the resolution of the digitized zooplankton images is lower than that of the images obtained using a binocular microscope, the ZooSCAN technique has proven to be more than adequate for large sample sets.
Top: the ZooSCAN device with connected computer (©VLIZ) - Bottom: some examples of scanned zooplankton images (©VLIZ)