Selective feeding of Eurytemora affinis (Copepoda, Calanoida) in temperate estuaries: model and field observations

The copepod Eurytemora affinis generally lives under estuarine conditions, where the suspended particulate matter (SPM) is strongly dominated by non-living particles. This article investigates as to how far E. affinis is capable of feeding selectively on phytoplankton under these extreme circumstances. Selectivity for phytoplankton by E. affinis was studied from samples taken from the Westerschelde (Belgium/The Netherlands). Copepod clearance rates exerted on natural phytoplankton quantified from the gut pigment content were significantly higher than those exerted on total particulate matter, calculated from microscopic image analysis of total gut content and total particulate matter concentration in the water. In addition, gut pigment content data on adult E. affinis measured during spring in three European estuaries (the Gironde (France), the Westerschelde and the Elbe (Germany)) were used to study the quantitative response of gut pigment content to varying ratios of phytoplankton to total SPM. A model, expressing maximum gut fluorescence as a function of body weight and gut fluorescence as a product of maximum gut fluorescence with the proportion of phytoplankton to total SPM in the feeding medium, satisfactory fitted the gut fluorescence observations made in the three estuaries. Over the range of phytoplankton-carbon/particulate organic carbon ratios observed in the three estuaries, gut fluorescence decreased with SPM concentration, but maximum gut fluorescence was reached when phytoplankton-carbon was >5% of particulate organic carbon. Limitation of selective feeding apparently only occurred under extremely high SPM loads, such as observed in the Gironde. Maximum gut fluorescence values corresponded exactly to those obtained previously for planktonic copepods in general, affirming the relationship with copepod size.