Macroalgal blooms in shallow estuaries: Controls and ecophysiological and ecosystem consequences

Abstract

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 fringing 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 macroalgal bloom, which can displace other estuarine producers. Fringing marshes 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 phytoplankton attenuates irradiation reaching the bottom. In estuaries with brief water residence times, phytoplankton may not have enough time to grow and shade macrophytes. High phytoplankton division rates achieved at high nutrient concentrations may compensate for the brief time to divide before cells are transported out of the estuary. Increased N loads and associated macroalgal blooms pervasively and fundamentally alter estuarine ecosystems. Macroalgae intercept nutrients regenerated from sediments and thus uncouple biogeochemical sedimentary cycles from those in the water column. Macroalgae take up so much N that water quality seems high even where N loads are high. Macroalgal C moves more readily through microbial and consumer food webs than C derived from seagrasses that were replaced by macroalgae. Macroalgae dominate O 2 profiles of the water columns of shallow estuaries and thus alter the biogeochemistry of the sediments. More frequent hypoxia and habitat changes associated with macroalgal blooms also changes the abundance of benthic fauna in affected estuaries. Approaches to remediation of the many pervasive effects of macroalgal blooms need to include interception of nutrients at their watershed sources and perhaps removal by harvest of macroalgae or by increased flushing. Although we have much knowledge of macroalgal dynamics, all such management initiatives will require additional information.