Measuring selectivity of feeding by estuarine copepods using image analysis combined with microscopic and Coulter counting

Although estuarine zooplankters are generally believed to be detritivorous, high clearance rates by the estuarine copepods Eurytemora affinis and Acartia tonsa on natural estuarine microplankton have been reported in the literature. In order to enable detection of possible selectivity for these microplankton organisms over detritus, a method that measures clearance rates on total particulate matter is proposed. Image analysis is used to measure copepod gut contents, and combined with Coulter counter measurements of total particulate matter concentration, allows one to calculate clearance rates on the latter. The first results indicate that, in the Westerschelde estuary (SW Netherlands), both E. affinis and A. tonsa are capable of selecting dominant phytoplankton cells over the total particulate matter. The possibility of using image as a sole technique in these studies is discussed