Variable and complementary effects of herbivores on different life stages of bloom-forming macroalgae

Increased nutrient loading favors macroalgal blooms in eutrophied coastal e cosystems. The main counteracting factor on this bottom-up support is top-d own control by consumers. We asked (1) whether herbivore control on 2 bloom -forming macroalgae in the Baltic Sea varies between different algal life s tages, (2) whether herbivores selectively feed on Enteromorpha spp. (Chloro phyceae) thereby supporting dominance of Pilayella littoralis (Phaeophyceae ), and (3) whether various herbivore species differ in their effects. In co mparative field and laboratory experiments, we analyzed herbivore pressure and selectivity on germling density and adult thalli of Enteromorpha spp. a nd P. littoralis. In the field, herbivores reduced macroalgal recruitment b y 80% within 14 d indicating strong herbivore control at early life stages. Recruits of Enteromorpha spp, were significantly preferred over P. littora lis Adult thalli of both algae showed similar growth rates, but grazing rat es were significantly higher on Enteromorpha spp. In laboratory experiments , Idotea chelipes (Isopoda), Littorina saxatilis (Gastropoda) and Gammarus locusta (Amphipoda) strongly reduced germling density, whereas effects of L . littorea were weak. I. chelipes and L. saxatilis significantly preferred germlings of Enteromorpha spp. over P. littoralis. I. chelipes had strong e ffects on adult Enteromorpha spp. but not on P. littoralis. The effects of G. locusta and L. littorea on both adult algae were smaller and not selecti ve, and L. saxatilis did not feed on adults at all. Different herbivore fee ding modes are discussed. We conclude that strong and selective herbivory o n juvenile and adult Enteromorpha spp. favors dominance of P. littoralis in the Baltic. An effective control of macroalgal blooms in eutrophied areas can be supported by sustaining high herbivore density and species richness considering the variable and complementary effects of different herbivore s pecies on different algal life stages.