Seasonal and fine-scale spatial variations in egg production and triacylglycerol content of the copepod Acartia tonsa in a river-dominated estuary and its coastal plume

Abstract

We measured egg production rates of the estuarine calanoid copepod Acartia tonsa in Mobile Bay, an estuary in the northern Gulf of Mexico. Two stations were sampled approximately monthly, one at the mouth of the bay and the other just beyond the mouth in the salinity front between bay and coastal waters. Over the whole year, temperature was the most important environmental variable controlling egg production. Rates increased with temperature up to 30°C and 140 eggs female-1 day-1. We found no evidence of food limitation. There was no correlation between egg production and phytoplankton abundance, nor increased egg production in response to supplements of phytoplankton added to natural food, suggesting that non-phytoplankton food was important in the diet. At the highest egg production rates, the amount of the storage lipid triacylglycerol (TAG) in adult females was greatly diminished, to <50 ng female-1. This suggests that lipids in the diet can be very tightly coupled to egg production. Both egg production and TAG content of females showed significant variability on spatial scales of 3-15 km, especially in relation to the salinity front separating water outwelling from the bay from open coastal water. For organisms that are using copepods and their eggs as food, this variability would result in a heterogeneous food environment, both in terms of the abundance of food and its nutritional content.