The impact of loose-lying algal mats and predation by the brown shrimp Crangon crangon (L.) on infaunal prey dispersal and survival

Abstract

When detached, macroalgal debris accumulates on shallow soft bottoms in the northern Baltic Sea and loose-lying algal mats are formed on normally uncovered areas. The role of patchy occurrences of algal mats and predation by Crangon crangon in determining survival and distribution of infaunal species was tested in an aquarium experiment over 4 days. The test-organisms, the amphipod Corophium volutator, the polychaete Nereis diversicolor, and juveniles of the bivalve Macoma balthica, all exhibit different levels of mobility and tolerance to stress induced by drift algal mats. The responses of the test-organisms were studied in relation to mere predator presence, the presence of an algal mat and combinations of algal mats and predation. To study possible horizontal migrations between algal mats and bare sand, the experimental aquaria were divided into two halves (not separated during the experiment). The influence of algal mats was studied by covering one half (50% coverage) of the aquaria and leaving the other halfs uncovered. The behaviour of C. crangon and C. volutator was followed. Both species exhibited a diurnal activity pattern. C. volutator and N. diversicolor were preferred by C. crangon but exhibited active migrations to the refuge provided by the algal mat. Juvenile M. balthica were most susceptible to stress induced by algal mats and not preferred as a prey species. Drifting algal mats cause local hypoxia but also offer refuge from predation by epibenthic predators. The results emphasize the importance of drifting algal mats in stabilizing the interaction between mobile prey, such as C. volutator and N. diversicolor and epibenthic predators. The patchy occurrences of drift algae mimicked here, underline the importance of the processes occurring along the interface between algal mats and bare sand areas.