Effect of nitrogen loading on enzyme activity of macroalgae in estuaries in Waquoit Bay

Assays of nitrate reductase activity (NRA), glutamine synthetase activity ( GSA), and alkaline phosphatase activity (APA) were used to measure nitrate, ammonium, and phosphate assimilation in five macroalgal species in three e stuaries of Waquoit Bay, Massachusetts, U.S.A., subject to different N load ing rates. The NRA significantly increased as loading rates of nitrate incr eased in Cladophora vagabunda and Gracilaria tikvahiae, with C. vagabunda e xhibiting the greatest response. The GSA was always higher than the recorde d NRA activity, but did not respond to the nitrogen loading gradient in any of the macroalgal species. The APA depended on nitrogen load in Cladophora vagabunda only. The NRA increased as internal nitrogen pools increased in Cladophora vagabunda, Gracilaria tikvahiae, and Fucus vesiculosus. Higher p ercent tissue N and lower C: N were found in Cladophora vagabunda, Gracilar ia tikvahiae, and Fucus vesiculosus in higher N estuaries. Increases in per cent tissue N, and increases in NRA, were due to higher loads of nitrate en tering the estuary. Increased nitrogen loads correlated to increased C:N in macroalgal tissue, which correlated to increased NRA in Cladophora vagabun da and Gracilaria tikvahiae. This link illustrates that these two opportuni stic macroalgal species best access the available inorganic nitrate:pool in estuaries subject to eutrophication in Waquoit Bay.