ALTERED BENTHIC PREY-AVAILABILITY DUE TO EPISODIC OXYGEN DEFICIENCY CAUSED BY DRIFTING ALGAL MATS

Large quantities of drifting algal mats have become a common phenomeno n on shallow sandy bottoms in the northern Baltic Sea. A decomposing a lgal mat rapidly induces hypoxic or anoxic conditions in the underlyin g sediment and interferes with the normal living-mode of the benthos, i.e., mobility, feeding, and predator-prey relationships. Field survey s have shown that bivalves, such as Macoma balthica, avoid the unfavou rable anoxic conditions by emerging at the sediment surface. Due to lo w reburrowing rates these bivalves are exposed to epibenthic predators once the drift algae disperse. A series of aquarium experiments were conducted to test for differences in the survival of M. balthica when exposed to continuous predation without prior stress and short-term pr edation after hypoxic stress induced by algae (approximate to 20% O-2- saturation). Sublethally stressed adult M. balthica, that normally are outside the size-range of the isopod predator Saduria entomon, became significantly more susceptible to predation when lying exposed at the sediment surface. The same effects were found with two other predator s, the brown-shrimp Crangon crangon and the flounder Platichthys flesu s. Drift algal mars induced an escape-reaction in the infauna more rap idly than mere hypoxia. This response is affected by temperature, whic h further underlines the importance of drift algae influencing prey av ailability. This paper discusses the role of the drift algae as a regu lating factor for the infauna and demonstrates the importance of exter nal structuring factors, such as hypoxic stress and algal mars, on pre dator-prey interactions.