Contribution of scale-dependent environmental variability on the biomass patterns of drift algae and associated invertebrates in the Gulf of Riga, northern Baltic Sea

Increased eutrophication has caused a bloom of opportunistic macroalgae and development of drift algal mats in coastal seas worldwide including the Baltic Sea. To date the patterns of the drift algae have been described at one scale (usually at a sampling unit scale i.e. < 100 m) ignoring an infinite variety of other possibilities. Therefore, these analyses do not capture a substantial fraction of overall variance and thus, have limited ecological significance and descriptive power. The objective of this study was to investigate in which of studied spatial scales (sampling unit, 100, 1000 and 10,000 m scales) abiotic environmental variability explained best the biomass patterns of drifting algae and associated invertebrates in the Gulf of Riga, northern Baltic Sea. In addition we assessed whether the species composition and biomass structure of drift algae were related to the biomass structure of three invertebrate functional groups. The biomass distribution of drift algae was better explained by large-scale environmental variability than small-scale environmental variability. The occurrence of topographic depressions at 10,000 m scale seems very important for drifting algal mats to remain in the study area. Non-migrating herbivore community was more strongly related to the biomass patterns of drift algae than mobile herbivore community. The biomass pattern of non-migrating suspension feeders was mainly explained by coastal morphology at different spatial scales indicating that the drift algae had minor or no effects on this functional group.