Macroalgal canopies contribute to eelgrass (Zostera marina) decline in temperate estuarine ecosystems

Loss of eelgrass (Zostera marina) habitat from temperate estuaries worldwid e often coincides with increased macroalgal accumulations resulting from in creased delivery of anthropogenic nitrogen. We conducted macroalgal enclosu re/exclosure experiments during summer 1998 within eelgrass populations in two estuaries of Waquoit Bay, Massachusetts, USA, to evaluate how increased macroalgal biomass affects density, recruitment, growth rate, and producti on of eelgrass. One estuary featured a low nitrogen loading rate and sustai ned a relatively pristine eelgrass population with a 2 cm high macroalgal c anopy. The other estuary had a sixfold higher nitrogen loading rate and a d eclining eelgrass population with a 9 cm high macroalgal canopy. Experiment al units were 1 X 1 m plots of eelgrass fenced within 50 cm high plastic me sh that excluded or included macroalgae at canopy heights ranging from 0 to 25 cm. In both estuaries, rates of eelgrass loss increased, largely a resu lt of decreased recruitment, and growth rates decreased (due to decreased r ates of leaf appearance) with increasing macroalgal canopy height. Abovegro und summer production in both estuaries decreased exponentially as macroalg al canopy heights increased. We conclude that macroalgal cover is a proxima te cause for loss of eelgrass in the higher N estuary since, upon removal o f macroalgae, we observed an increase in shoot density, a 55% increase in s ummer growth, and a 500% increase in summer aboveground net production. Bas ed on summer growth data and density of shoots in our experimental plots th e following spring, we suggest that the negative impacts of macroalgal cano pies persist, but also that eelgrass recovery upon removal of macroalgae ma y be possible. To identify the mechanisms by which macroalgae potentially inhibit eelgrass production, we measured changes in nutrient and oxygen concentrations resu lting from macroalgal canopies and estimated the relative importance of sum mer standing stocks of phytoplankton, epiphytes, and macroalgae to potentia l shading of eelgrass in both estuaries. We document both (1) unfavorable b iogeochemical conditions (lowered redox conditions and potentially toxic co ncentrations of NH4+) imposed by the presence of macroalgal canopies and (2 ) potential light limitation of eelgrass by standing stocks of producers in the higher N estuary, with estimates of light reduction via macroalgae num erically more important than light sequestration by phytoplankton and epiph ytes for newly recruiting shoots. Increased macroalgal biomass associated w ith increased nitrogen loading to estuaries can lead to eelgrass disappeara nce, and we identify an approximate 9-12-cm critical macroalgal canopy heig ht at which eelgrass declines.