Effect of short-term elevated nutrients and mesoherbivore grazing on photosynthesis of macroalgal communities

Abstract

Marine macroalgal communities are among the most productive habitats worldwide. They provide energy and matter to higher trophic levels and support other important functions for ecosystems and services for human society. To date it is not clear to what extent irradiance, nutrient loading, and mesoherbivores regulate the primary productivity of a community. In a factorial field experiment we evaluated the interactive effect of short-term pulses of elevated nutrients and of the activity of grazers on the photosynthesis (in terms of the rate of oxygen production per unit mass) of communities dominated by the perennial Fucus vesiculosus and the ephemeral Cladophora glomerata in the northern Baltic Sea. This experimental manipulation had no effect on the community dominated by F. vesiculosus. A 12-hour addition of herbivores decreased the photosynthetic production of the macroalgal community dominated by C. glomerata in spring but increased its production in summer. The simultaneous addition of nutrients and herbivores in summer reversed the effect. A 4-times longer manipulation had no effect on the C. glomerata production. Differences in the responses between separate and interactive effects imply that the photosynthetic production of a community cannot be predicted by separate effects of the same variables. Our experiment also indicated that macroalgal communities dominated by F. vesiculosus covered by epiphytic macrophytes performed stably under different stress regimes and could buffer moderate short-term disturbances due to elevated nutrient loads and/or herbivory of either natural or human origin.