Trophic interactions between benthic copepods and algal assemblages: a laboratory study

Abstract

Meiofauna are potentially capable of controlling microphytobenthic biomass. Grazing by copepods in high densities has been hypothesized to lead to successional changes in algal assemblages toward less-edible algal species with consequent effects on copepod survival and growth. In our study, adult Attheyella trispinosa were offered monospecific algal films of 3 benthic diatom species and 1 benthic cyanobacterial species and algal assemblages consisting of different combinations of these species. Changes in abundances of algal species were observed over 17 d. Nauplii of A. trispinosa and Bryocamptus minutus were grown with a similar range of algal assemblages supplied as food, and their survival and development rates were measured. Feeding by A. trispinosa decreased the density of the diatom Nitzschia perminuta, whereas biomass of all other diatom species generally was unaffected by copepod feeding. The biomass of the cyanobacterium Leptolyngbya foveolarum was either unaffected or slightly enhanced by copepod feeding. Nevertheless, the architecture of the algal assemblages changed. Leptolyngbya foveolarum was ingested and partly digested by the copepods, and copepods were able to develop on this diet. However, L. foveolarum alone or combined with diatoms reduced the survival and development rate of nauplii and diminished copepodid sizes of A. trispinosa and B. minutus. Our results suggest that these algal species can exert a strong effect on harpacticoid copepods. Specific effects of copepod grazing on bispecific algal assemblages were masked to a large extent by the interactions among autotrophic species. In contradiction to our hypothesis, copepods grazing on L. foveolarum altered the physiognomy of algal assemblages and promoted the persistence of (edible) diatom species otherwise overgrown by L. foveolarum of lower food quality.