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 estuaries of Waquoit Bay, Massachusetts, U. S. A., subject to different N loading rates. The NRA significantly increased as loading rates of nitrate increased in Cladophora vagabunda and Gracilaria tikvahiae , with C. vagabunda exhibiting the greatest response. The GSA was always higher than the recorded 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 percent tissue N and lower C : N were found in Cladophora vagabunda , Gracilaria tikvahiae , and Fucus vesiculosus in higher N estuaries. Increases in percent tissue N, and increases in NRA, were due to higher loads of nitrate entering the estuary. Increased nitrogen loads correlated to increased C : N in macroalgal tissue, which correlated to increased NRA in Cladophora vagabunda and Gracilaria tikvahiae . This link illustrates that these two opportunistic macroalgal species best access the available inorganic nitrate pool in estuaries subject to eutrophication in Waquoit Bay.