Shallow coastal systems worldwide are exhibiting increased algal growth in response to nutrient enrichment. This study evaluates primary production patterns in an estuarine system (Bass Harbor Marsh, ME, USA) receiving low levels of anthropogenic nitrogen. Biomass, areal coverage and in situ oxygen production of green macroalgae, Ruppia Maritima, and Phytoplankton were measured over a growing season to determine net ecosystem production. Macroalgae and R. maritima exhibited seasonal biomass curves with early summer peaks; however, peak biomass of macroalgae [150 g dry weight (wt) m-2] was substantially greater that R. maritima (33 g dry wt m-2) Phytoplankton biomass, measured as chlorophyll a, was low (<1 ??g 1-1) early in the season and peaked (11 ??g 1-1) following a mid-summer decline in macroalgal biomass, suggesting a competitive interaction with macroalgae. Instantaneous net production rates varied over the growing season for all 3 primary producers. R. maritima net production ranged from near zero to 2.7 mg C g-1 dry wt h-1, with higher rates during summer and much of the seasonal variability explained by temperature. Macroalgal (0.88 to 5.0 mg C g-1 dry wt h-1) and phytoplankton (0 to 28 mg C m-3 h-1) net production did not exhibit any clear seasonal signal. Net primary production calculated on an areal basis demonstrated macroalgae's dominance in the lower basin of Bass Harbor Marsh, with peak summer rates (400 mg C m-2 h-1) greatly exceeding maximum rates for both R maritima (70 mg C m-2h-1) and phytoplankton (12 mg C m-2 h-1). When compared to other New England estuarine sites with short residence times, nutrient loading and peak green macroalgal biomass in Bass Harbor Marsh are relatively low; however, the strong dominance of opportunistic green macroalgae is a pattern that is characteristic of shallow coastal systems undergoing eutrophication.