Importance and nutritional value of large ciliates for the reproduction of Acartia clausi during the post spring-bloom period in the North Sea

Abstract

Shipboard experiments were performed to examine the qualitative importance of large marine microzooplankton for the reproduction of Acartia clausi in the North Sea. Feeding and egg production were compared in 2 treatments in which females were fed natural seston or natural seston selectively enriched with large prey (>20 μm). The mineral (C, N) and lipid contents of the food suspensions were determined for size-fractionated samples to characterize the nutritional composition of prey. Large oligotrich ciliates and Strobiliidae dominated the seston biomass. Ciliates, particularly the oligotrich Laboea strobila, were the preferred food and made up more than 86% of the diet (2.9 μg C female-1 d-1) in the natural treatment. Small thecate dinoflagellates composed the remaining part. Egg production was high (>25 eggs female-1 d-1), achieving maximum rates recorded for this species in the North Sea. Ingestion and egg production of females increased equally in the enriched treatment, largely due to ingestion of additional ciliates. This allowed calculation of the gross growth efficiency for egg production on a diet of mixed ciliates, which was high at ∼30%. Measurements of the mineral and lipid content confirmed a high seston nutritional quality. The 20 to 48 μm fraction reflected the composition of female diets and indicated an enrichment of N and polyunsaturated fatty acids (PUFA) in the diet compared to total seston. The fraction >48 μm consisted mostly of ciliates (>97%) and was particularly important in providing N to copepods. Ciliates were also rich in PUFA and eicosapentaenoic acid (EPA), which is normally characteristic of diatoms. We conclude that large ciliates constitute an excellent food source for the reproduction of A. clausi. The occurrence of biomass peaks of large ciliates following the spring diatom bloom probably contributes substantially to the spring recruitment of the species in the North Sea. © Inter-Research 2008.