Reproductive response of Euterpina acutifrons in two estuaries of the Basque Country (Bay of Biscay) with contrasting nutritional environment

The reproductive response involving egg number and size was studied for the egg carrying copepod Euterpina acutifrons in two estuaries with different nutritional environment (Bilbao and Mundaka). To characterise the available food for copepods, the concentration of particulate organic matter (POM), the combined concentration (PGL) and proportion of particulate protein, carbohydrate and lipids and the concentration of chlorophyll a were measured in the <20 μm seston fraction. Data on female size (prosome length) and abiotic factors, such as temperature, salinity and oxygen saturation in water, were also obtained to relate them to reproductive and nutritional variables. Correlation and stepwise regression analyses indicated that temperature via female size was the main factor controlling brood volume in a temporal context, whilst the nutritional environment had a secondary role. Nevertheless, the nutritional environment caused noticeable between-estuary differences in the reproductive response of this species. ANOVA tests showed that E. acutifrons clutch size and brood volume were significantly bigger in the estuary of Bilbao, which was richer in both total (POM) and labile (PGL) organic particulate matter than the estuary of Mundaka. In contrast, egg size, female size and phytoplanktonic food quantity did not differ significantly between estuaries. These results suggest that the organic enrichment with particles of detritical-heterotrophic origin enhanced E. acutifrons clutch size and brood volume in Bilbao. Egg size was closely related to phytoplankton in both estuaries, but a clear trade-off between egg size and number per clutch was observed only in Mundaka. Thus, we suggest that the reduction of egg number in order to increase egg size might happen only under food-limiting conditions, but not in organically rich and quite constant nutritional environments such as found those in Bilbao.