MACROALGAL BLOOMS IN SHALLOW ESTUARIES - CONTROLS AND ECOPHYSIOLOGICAL AND ECOSYSTEM CONSEQUENCES

Macroalgal blooms are produced by nutrient enrichment of estuaries in which the sea floor lies within the photic zone. We review features of macroalgal blooms pointed out in recent literature and summarize work done in the Waquoit Bay Land Margin Ecosystems Research project which suggests that nutrient loads, water residence times, presence of frin ging salt marshes, and grazing affect macroalgal blooms. Increases in nitrogen supply raise macroalgal N uptake rates, N contents of tissues , photosynthesis-irradiance curves and P-max, and accelerate growth of fronds. The resulting increase in macroalgal biomass is the macroalga l bloom, which can displace other estuarine producers. Fringing marshe s and brief water residence impair the intensity of macroalgal blooms. Grazing pressure may control blooms of palatable macroalgae, but only at lower N loading rates. Macroalgal blooms end when growth of the ph ytoplankton attenuates irradiation reaching the bottom. In estuaries w ith brief water residence times; phytoplankton may not have enough tim e to grow and shade macrophytes. High phytoplankton division rates ach ieved at high nutrient concentrations may compensate for the brief tim e to divide before cells are transported out of the estuary. Increased N loads and associated macroalgal blooms pervasively and fundamentall y alter estuarine ecosystems. Macroalgae intercept nutrients regenerat ed from sediments and thus uncouple biogeochemical sedimentary cycles from those in the water column. Macroalgae take up so much N that wate r quality seen; high even where N loads are high. Macroalgal C moves m ore readily through microbial and consumer food webs than C derived fr om seagrasses that were replaced by macroalgae. Macroalgae dominate O- 2 profiles of the water columns of shallow estuaries and thus alter th e biogeochemistry of the sediments. More frequent hypoxia and habitat changes associated with macroalgal blooms also changes the abundance o f benthic fauna in affected estuaries. Approaches to remediation of th e many pervasive effects of macroalgal blooms need to include intercep tion of nutrients at their watershed sources and perhaps removal by ha rvest of macroalgae or by increased flushing. Although we have much kn owledge of macroalgal dynamics, all such management initiatives will r equire additional information.