In the Baltic Sea, increased dominance of ephemeral and bloom-forming algae is presently attributed to increased nutrient loads. Simultaneously, coastal predatory fish are in strong decline. Using field data from nine areas covering a 700-km coastline, we examined whether formation of macroalgal blooms could be linked to the composition of the fish community. We then tested whether predator or nutrient availability could explain the field patterns in two small-scale field experiments, by comparing joint effects on algal net production from nutrient enrichment with agricultural fertilizer and exclusion of larger predatory fish with cages. We also manipulated the presence of invertebrate grazers. The abundance of piscivorous fish had a strong negative correlation with the large-scale distribution of bloom-forming macroalgae. Areas with depleted top-predator communities displayed massive increases in their prey, small-bodied fish, and high covers of ephemeral algae. Combining the results from the two experiments showed that excluding larger piscivorous fish: (1) increased the abundance of small-bodied predatory fish; (2) changed the size distribution of the dominating grazers, decreasing the smaller gastropod scrapers; and (3) increased the net production of ephemeral macroalgae. Effects of removing top predators and nutrient enrichment were similar and additive, together increasing the abundance of ephemeral algae many times. Predator effects depended on invertebrate grazers; in the absence of invertebrates there were no significant effects of predator exclusion on algal production. Our results provide strong support for regional declines of larger predatory fish in the Baltic Sea promoting algal production by decreasing invertebrate grazer control. This highlights the importance of trophic interactions for ecosystem responses to eutrophication. The view emerges that to achieve management goals for water quality we need to consider the interplay between top-down and bottom-up processes in future ecosystem management of marine resources.

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Consumer presence and nutrient availability can have contrasting and interactive effects on plant diversity. In a factorial experiment, we manipulated two levels of nutrient supply and the presence of two moderately specialized grazers in different combinations (no grazers, two species in monoculture, and both in combination). We tested how nutrients and grazers regulated the biomass of marine coastal epiphytes and the diversity of algal assemblages, based on the prediction that the effect of consumers on prey diversity depends on productivity and consumer specialization. Nutrient enrichment increased the epiphytic load, while monocultures of single grazer species partly prevented epiphyte growth. However, only the presence of two species with complementary feeding preferences effectively prevented epiphyte overgrowth. The epiphytes comprised micro- and macroalgal species, and the diversity of these algal assemblages differed, depending on grazer identity. For the microalgae, diversity was reduced by nutrient addition when grazer control was inefficient, but not when specialist microalgal grazers were present. Macroalgal diversity was reduced in ambient water with specialist macroalgal grazers compared to the treatment with inefficient ones. These results indicate that grazer composition and productivity are crucial in determining whether consumer pressure will have a positive or negative effect on algal diversity.

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With increasing eutrophication in coastal areas, mass developments of annual filamentous algae, such as Pilayella littoralis and Enteromorpha spp., have increased. Simultaneously, the perennial macroalga Fucus vesiculosus has declined in many areas in the Baltic Sea. To test the effects of P. littoralis on F. vesiculosus we performed a number of laboratory and field studies. The most severe negative effect observed was the reduced number of settled germlings (95%) when P. littoralis was present on the substrate prior to seeding of F. vesiculosus eggs. A low concentration (0.1%) of exudates from P. littoralis reduced the attachment rate of F. vesiculosus during the initial 12 h, while higher concentrations of exudates (5 to 10%) negatively affected germination and rhizoid development. These results could explain for the negative correlations that have been observed between F. vesiculosus and annual filamentous algae in the Baltic Sea. Our field data showed that the biomass of filamentous algae was higher in the summer than in the autumn, suggesting that recruits from the summer-reproducing F. vesiculosus may encounter more competition from P. littoralis and other filamentous algae than do recruits from the autumn-reproducing populations. The summerand autumn-reproducing F. vesiculosus host similar abundance and biomass of their associated flora and fauna; hence, a potential shift from summer-to autumn-reproducing F. vesiculosus would not change the overall community composition in this ecosystem.