No abstract

Macroalgal blooms arc produced by nutrient enrichment of estuaries in which the sea floor lies within the photic zone. We review fcaturcs 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 residcncc 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,,,.,, 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 macroalgac, but only at lower N loading rates. Macroalgal blooms end when growth of the phytoplankton attenuates irradiation reaching the bottom. In cstuarics with brief water rcsidencc times, phytoplankton may not have enough time to grow and shade macrophytcs. High phytoplankton division rates achieved at high nutrient concentrations may compensate for the brief time to divide before cells arc transported out of the estuary.Increased N loads and associated macroalgal blooms pervasively and fundamentally alter estuarinc ecosystems. Macroalgae intercept nutrients regenerated from sediments and thus uncoupIe biogeochemical sedimentary cycles from those in the water column. Macroalgae take up so much N that water quality seen:? high even where N loads are high. Macroalgal C moves more readily through microbial and consumer food webs than C derived from seagrasscs that were replaced by macroalgae. Macroalgae dominate 0, profiles of the water columns of shallow estuaries and thus alter the biogeochemistry of the sediments. Marc frequent hypoxia and habitat changes associated with macroalgal blooms also changes the abundance of bcnthic fauna in affected estuaries.Approaches to rcmediation of the many pervasive cffccts of macroalgal blooms riced to include interception of nutrients at their watcrshcd 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.

Nutrient enrichment as a result of anthropogenic activity concentrated along the land-sea margin is increasing eutrophication of near-shore waters across the globe. Management of eutrophication in the coastal zone has been ' hampered by the lack of a direct method to trace nitrogen sources from land into coastal food webs. Stable isotope data from a series of estuaries receiving nitrogen loads from 2 to 467 kg N ha-' yr-' from the Waquoit Bay watershed, Cape Cod, Massachusetts, indicate that producer and consumer 15N-to-14N ratios record increases in wastewater nitrogen inputs. Nitrate from groundwater-borne wastewater introduces a '"N-enriched tracer to estuaries. This study explicitly links anthropogenically derived nitrogen from watersheds to nitrogen in estuarine plants and animals, and suggests that wastewater nitrogen may be detectable in estuarine biota at relatively low loading rates, before eutrophication leads to major changes in species composition and abundance within estuarine food webs.

No abstract

It is clear that anthropogenic nitrogen inputs from watersheds to estuaries stimulate eutrophication. It has been difficult, however, to explicitly link anthropogenic N entering estuaries to N found in estuarine producers. To explore this link, we compared stable isotope ratios of N in groundwater and producers from the Waquoit Bay watershedestuary system, Cape Cod, Massachusetts. The δ15N values of groundwater nitrate within the Waquoit Bay watershed increase from −0.9‰ to + 14.9‰ as wastewater contributions increase from 4 to 86% of the total N pool. As a result, the average δ15N of dissolved inorganic nitrogen (DIN, nitrate + ammonium) received by different estuaries around Waquoit Bay increases from +0.5‰ to +9.5‰. This increase is strongly correlated to increases in δ15N of eelgrass, macroalgae, cordgrass, and suspended particulate organic matter. The increase of all producers examined in Waquoit Bay with increasing δ15N of DIN in groundwater demonstrates a tight coupling between N contributed to coastal watersheds and N used by primary producers in estuaries. The ability to identify effects of increasing wastewater N loads on δ15N of estuarine producers may provide a means to reliably identify incipient eutrophication in coastal waters.