| Biomass and competitive interactions among primary producers 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. Eelgrass (Zostera marina L.), a light-limited species, typically dominates benthic production in shallow temperate estuaries. Over the past few decades, however, there has been extensive loss of eelgrass habitat worldwide, coincident with both increased delivery of anthropogenic nitrogen from watersheds to estuaries and increased standing stocks of nitrogen-limited algal producers that may shade and eventually replace eelgrass.; I used the estuaries of Waquoit Bay, MA, USA, in a space-for-time substitution to infer the time course of increased eutrophication created by increasing urbanization of watersheds. Over the range of nitrogen loads delivered to Waquoit estuaries (∼5–410 kg N ha−1 y −1), eelgrass areal cover, shoot density, and rates of production decreased exponentially as nitrogen loading rates increased. Maximum macroalgal canopy height increased linearly as nitrogen loading rate increased.; To evaluate how increased macroalgal biomass affects eelgrass productivity, I conducted macroalgal enclosure/exclosure experiments within two eelgrass populations subject to different rates of nitrogen loading. Rates of eelgrass loss increased (due to decreased recruitment), growth rates decreased, and aboveground summer production decreased exponentially with increasing macroalgal canopy height. Macroalgal cover is a proximate cause for overall loss of eelgrass in the higher N estuary (30 kg N ha−1 y−1 ), since removal of macroalgae resulted in a 500% increase in summer aboveground net production.; To determine the relative importance of bottom-up and top-down processes in controlling macroalgal biomass, I estimated potential amphipod and isopod grazer impact on macroalgae in three estuaries subject to different nitrogen loading rates (based on benthic survey data of macrophyte biomass and herbivore abundance, grazing rates, and in situ macroalgal growth rates). Comparison between estimated macroalgai growth increases versus grazing losses suggested that (1) grazers could lower macroalgal biomass in midsummer, but only in estuaries subject to lower nitrogen loads, and (2) the relative importance of grazing decreased as nitrogen loads increased. In summary, increased nitrogen loads uncouple top-down controls of macroalgal biomass, leading to competitive exclusion of eelgrass. |
