RELATIVE IMPORTANCE OF GRAZING AND NUTRIENT CONTROLS OF MACROALGAL BIOMASS IN 3 TEMPERATE SHALLOW ESTUARIES

Macroalgal biomass and competitive interactions among primary producer s in coastal ecosystems may be controlled by bottom-up processes such as nutrient supply and top-down processes such as grazing, as well as other environmental factors. To determine the relative importance of b ottom-up and top-down processes under different nutrient loading condi tions, we estimated potential amphipod and isopod grazer impact on a d ominant macroalgal species in three estuaries in Waquoit Bay, Cape God , Massachusetts, that are subject to different nitrogen loading rates. We calculated growth increases and grazing losses in each estuary bas ed on monthly benthic survey data of macrophyte biomass and herbivore abundance, field grazing rates of amphipods (Microdeutopus gryllotalpa and Cymadusa compta) and an isopod (Idotea baltica) on the preferred and most abundant macroalga (Cladophora vagabunda) and laboratory graz ing rates for the remaining species, and in situ macroalgal growth rat es. As nitrogen Loading rates increased, macroalgal biomass increased (3x), eelgrass (Zostera marina) was lost, and herbivore abundance decr eased (1/4x). Grazing rates increased with relative size of grazer (I. baltica > C. compta > M. gryllotalpa) and, for two of the three speci es investigated, were faster on algae from the high-nitrogen estuary i n comparison to the low-nitrogen estuary, paralleling the increased ma croalgal tissue percent nitrogen with nitrogen load. Macroalgal growth rates increased (2x) with increasing nitrogen loading rate. The compa rison between estimated growth increases versus losses of C. vagabunda biomass to grazing suggested first, that grazers could lower macroalg al biomass in midsummer, but only in estuaries subject to lower nitrog en loads. Second, the impact of grazing decreased as nitrogen loading rate increased as a result of the increased macroalgal growth rates an d biomass, plus the diminished abundance of grazers. This study sugges ts the relative impact of top-down and bottom-up controls on primary p roducers varies depending on rate of nitrogen loading, and specificall y, that the impact of herbivory on macroalgal biomass decreases with i ncreasing nitrogen load to estuaries.