Trophic role of the omnivorous grass shrimp Palaemon elegans in a Swedish eelgrass system

Algal grazers are thought to play a critical role in preventing overgrowth by ephemeral algae in seagrass beds. We studied the interaction between the omnivorous grass shrimp Palaemon elegans, the algal grazer Cammarus locusta and the filamentous green alga Ulva sp. (syn. Enteromorpha sp.) to assess whether the shrimp has a positive or negative effect on the growth of ephemeral algae in eelgrass beds on the Swedish west coast. Laboratory experiments demonstrated that P. elegans were effective predators on gammarids <8 mm total length (TL), as well as effective grazers of filamentous Ulva sp. The shrimp appeared to prefer gammarid prey over algae, and only consumed significant amounts of Ulva sp. when the gammarid prey was too large to be consumed efficiently. The total consumption of Ulva sp. was positively affected by shrimp when gammarid prey were >5 mm TL, but negatively affected when G. locusta was smaller. In a field cage experiment, enclosed P. elegans decreased the abundance of gammarids by on average 68%, causing a trophic cascade that increased the biomass of filamentous Ulva sp. 7-fold. These results suggest that P. elegans on the Swedish northwest coast feed mainly on amphipods, including the algal grazer G. locusta, resulting in a positive net trophic effect. on the growth of filamentous Ulva sp. This study indicates that P. elegans may be an import ant link between overexploitation of cod in northwest Sweden and the recent increase of algal mats and loss of eelgrass in the area.