MEASURING SELECTIVITY OF FEEDING BY ESTUARINE COPEPODS USING IMAGE-ANALYSIS COMBINED WITH MICROSCOPIC AND COULTER COUNTING

Although estuarine zooplankters are generally believed to be detritivo rous, high clearance rates by the estuarine copepods Eurytemora affini s and Acartia tonsa on natural estuarine microplankton have been repor ted in the literature. In order to enable detection of possible select ivity for these microplankton organisms over detritus, a method that m easures clearance rates on total particulate matter is proposed. Image analysis is used to measure copepod gut contents, and combined with C oulter counter measurements of total particulate matter concentration, allows one to calculate clearance rates;,on the latter. The first res ults indicate that, in the Westerschelde estuary (SW Netherlands), bot h E. affinis and A. tonsa are capable of selecting dominant phytoplank ton cells over the total particulate matter. The possibility of using image analysis as a sole technique in these studies is discussed. (C) 1995 International Council for the Exploration of the Sea.