Investigating the spatial and seasonal distribution of the copepods Eurytemora affinis and Eurytemora velox: a case study of the Scheldt estuary

Distribution and abundance of the two calanoid copepod species Eurytemora affinis (Poppe, 1880) and Eurytemora velox (Lilljeborg, 1853) were studied in the Scheldt estuary (Belgium, The Netherlands) in order to look for general patterns of both species’ distribution in relation to environmental factors and compared with their distribution in the tributaries of the Scheldt estuary. Species data, collected at five stations between 2018 and 2022 were analyzed by Generalized Linear Models (GLM) and the results interpreted in the context of ecosystem functioning. E. velox develops best in the fresh water reach of the estuary, peaking at the most upstream stations Uitbergen and Dendermonde, receiving water from the non-tidal tributaries that harbor the most E. velox and high phytoplankton biomass. E. affinis also develops best in the fresh water reach, peaking at Dendermonde but also at the oligohaline station Kruibeke. The results agree with both species’ preferences observed in the tributaries and suggest that E. affinis might be suffering from hyperoxia-induced oxidative stress under high oxygen conditions Such limitation could explain its prevalence in estuarine waters under tidal influence in which O₂ concentration is tempered by decomposition of organic matter along the estuary.