DOES THE INFLUENCE OF THE EPIBENTHIC PREDATOR CRANGON-CRANGON L (BROWN SHRIMP) EXTEND TO SEDIMENT MICROALGAE AND BACTERIA

The effect of juvenile Crangon crangon L. (brown shrimp) on the microb ial part of the food web of marine shallow-water sandy sediment was in vestigated in two experiments using an outdoor flow-through system. Bi omass, composition and productivity of microalgae and bacteria, POC an d PON content in the sediment, as well as nutrient and oxygen fluxes, were measured in the absence and presence of two natural densities (50 and 100 ind.m-2) of juvenile Crangon. Stimulating effects of the pres ence of juvenile Crangon on both microalgae and bacteria were observed after three weeks. However, the number of statistically significant e ffects was low, despite the conspicuous change in sediment characteris tics caused by Crangon activity. To explain the possible trophic effec t of Crangon via meiofaunal grazing, the results were compared with me iofaunal grazing rates (dual labelling by C-14-bicarbonate and H-3-thy midine) and changes in the meiofaunal and macrofaunal community. The r easons for the low number of significant trophic effects of Crangon on algae and bacteria were that the overall meiofaunal grazing pressure on microbiota was low, and that only part of the meiofauna (mainly har pacticoid copepods) was significantly affected by the predator. Meiofa una thus appeared to be a weak mediating link in the sandy sediment sy stem investigated. No effect of Crangon on oxygen fluxes, or POC and P ON content in the sediment was observed. Neither were the nutrient con centrations in the overlying water affected, but increased nutrient le vels in the pore water were observed. According to theoretical calcula tions, excretion from the shrimp may imply a considerable nutrient add ition which can be rapidly re-utilized by the microbes directly at the sediment/water interface. Bioturbation by Crangon appeared to counter act flaking of the microalgal mat.