Habitat modification mediated by motile surface stirrers versus semi-motile burrowers: potential for a positive feedback mechanism in a eutrophied ecosystem

We performed a 4 wk laboratory experiment with the semi-motile burrowers Macoma balthica and Marenzelleria spp. and the motile surface sediment stirrers Monoporeia affinis and Mysis mixta to study their effects on the transport of tracer particles (mean diameter [circle divide] 1 mu m) in the sediment, sediment parameters (depth of the oxidized layer, water content, organic matter content), water turbidity, and nutrient fluxes across the sediment-water interface (PO43-, NO2- + NO3-, NH4+; measured weekly). Two densities were included for each taxon, representing a low and a high field density. M balthica significantly increased particle mixing, the rates being 59.6 and 61.9 x 10(-3) cm(2) d(-1). While all taxa increased turbidity in the overlying water, the strongest effects were caused by M. affinis and M mixta, resulting in 112- and 45-fold increases, respectively. In addition, these 2 motile species increased oxidation of the sediment surface layers. A distinctive difference in the nutrient fluxes was observed between the semi-motile (M. balthica, Marenzelleria spp.) and the motile taxa (M affinis, M mixta). The former increased the efflux of both PO43- and NH4+, while the latter suppressed the efflux of NH4+ and decreased the sediment uptake of NOx indicating enhanced N removal. Higher nutrient exchange rates were observed at the higher animal densities. We conclude that a shift in the benthic communities from the 2 motile to the 2 semi-motile taxa, observed throughout the northern Baltic Sea, may notably alter the regime of benthic nutrient cycling and thereby the performance of the entire ecosystem.