Stable isotopic tracking of land-derived nitrogen into estuarine food webs.

摘要

Anthropogenic nitrogen loads stimulate eutrophication of coastal waters. As eutrophication intensifies, increases in primary production are accompanied by major shifts in the dominant flora. In some shallow temperate estuaries, eelgrass is replaced by phytoplankton and macroalgae. This shift, in turn, has direct and indirect affects on consumers. I used stable isotopes to trace land-derived nitrogen into food webs of shallow estuaries receiving different N loads from the Waquoit Bay watershed, Cape Cod, MA. I explored the potential of nitrogen stable isotopes as a tool for detecting incipient eutrophication, and examined how pathways of nitrogen flow through estuarine food webs change as eelgrass is replaced by algae.; Stable isotope data from Waquoit Bay indicate that producer {dollar}sp{lcub}15{rcub}{dollar}N-to-{dollar}sp{lcub}14{rcub}{dollar}N ratios (expressed as {dollar}deltasp{lcub}15{rcub}{dollar}N in per mil units) record increases in wastewater nitrogen inputs to estuaries. Nitrate from groundwater-borne wastewater acts as a {dollar}sp{lcub}15{rcub}{dollar}N-enriched tracer. The {dollar}deltasp{lcub}15{rcub}{dollar}N of groundwater nitrate within the Waquoit Bay watershed increases from {dollar}{lcub}-{rcub}0.9perthous{dollar} to +14.9{dollar}perthous{dollar} as wastewater contributions increase from 4% to 86%. As a result, the average {dollar}deltasp{lcub}15{rcub}{dollar}N of DIN (nitrate + ammonium) received by different estuaries around Waquoit Bay ranges from +0.5{dollar}perthous{dollar} to +9.5{dollar}perthous.{dollar} The {dollar}deltasp{lcub}15{rcub}{dollar}N of DIN entering the estuaries is strongly correlated to the {dollar}deltasp{lcub}15{rcub}{dollar}N of primary producers growing within them. Producer {dollar}deltasp{lcub}15{rcub}{dollar}N reflects wastewater N loads to the Waquoit Bay estuaries at relatively low loading rates, suggesting that producer {dollar}deltasp{lcub}15{rcub}{dollar}N may be a useful indicator of incipient eutrophication.; The wastewater signal recorded by producers is passed on to consumers in Waquoit Bay. Dietary changes as a secondary effect of increased N loads, however, additionally influence the {dollar}deltasp{lcub}15{rcub}{dollar}N of estuarine consumers. Carbon and nitrogen stable isotope data indicate that phytoplankton and macroalgae are major food sources of primary consumers under both low and high nitrogen conditions, but that eelgrass is also an important food source where nitrogen loading is low. Eelgrass C and N are passed on to benthic as well as pelagic secondary consumers. With loss of eelgrass as a consequence of nitrogen loading, an important pathway through which land-derived nitrogen enters food webs of Waquoit Bay is eliminated. This fundamental change probably affects the rate that land-derived nitrogen is cycled within estuaries, as well as its ultimate fate.