Conditions for assessing zooplankton abundance with LOPC in coastal waters

Recent technical advances in laser-based systems to measure zooplankton distribution have opened new perspectives in ecological and behavioral studies by significantly improving the horizontal and vertical sampling resolution, providing information on zooplankton patchiness and on the influence of small scale physical processes. The application of laser-based systems also led to new challenges on the identification of organisms vs. particulate matter. In areas with high detritus abundances, zooplankton abundances might be overestimated by counting plankton and detritus together. We investigated the contribution of detritus in Laser Optical Plankton Counter (LOPC) data collected during two cruises on the continental shelf of the Gulf of Lion (NW Mediterranean Sea). The study area was characterized by several types of ecoregions owing to the influence of winds, freshwater runoff and intrusion of oligotrophic waters from offshore. We identified the main mechanisms leading to the formation of detritus as a function of environmental conditions and developed a method to assess the contribution of detritus in LOPC counts based on the proportion of large particles (multi-element plankton, MEPs). Highest percentages of detritus (up to 90% of the counts, mainly particulate organic matter from various sources) were found in stratified conditions associated with relatively high chlorophyll a concentration (chl-a; ca 2 mg m−3). Discontinuities in density profiles alone also resulted in peaks of particles concentrations. We suggested a threshold of 2% of MEPs in LOPC counts above which the LOPC is most likely counting more detritus than organisms. This easy check of the detritus contribution to total LOPC counts was applied to datasets from different marine ecological situations (glacial input, clear water, productive shelf) and gave successful results in different biogeographical regions (e.g. high latitude and tropical habitats).