Dissolved inorganic carbon (DIC) is rarely considered limiting for macroalgae, but some research suggests that under conditions of N sufficiency, photosynthetic capacity is enhanced with DIC enrichment. During spring (April-May) and summer (July-August) 1993, we investigated the interactive effects of nitrogen (N) and DIC on photosynthetic capacity, growth, and nutrient uptake rates of the macroalgae, CIadophora vagabunda (L.) van den Hoek and Gracilaria tikvahiae (McLachlan), dominant species in a temperate eutrophic estuary (Cape Cod, Massachusetts, USA). Water-column CO2 concentrations showed significant diurnal fluctuations, ranging from a morning CO 2 peak (21 pM) to an afternoon low (13 ~M) during summer, probably associated with metabolic activities in a thick algal mat. Results from instantaneous photosynthesis measurements and microcosm experiments indicate that DIC limits photosynthetic capacity and growth rates of C. vagabunda during summer, perhaps related to tissue N sufficiency and low water-column CO 2 concentrations. For example, this species showed enhanced growth (F--8.69, P< 0.02) under DIC but not N enrichment. G. tikvahiae showed marginal DIC enhancement of maximum photosynthetic rate, while growth was significantly stimulated by addition of N. Reduced thallus N of this species during the summer further identifies N as the primary factor limiting growth. In addition, G. tikvahiae has the ability to use DIC in its several forms, while C. vagabunda primarily uses dissolved CO~. DIC enrichment resulted in Communicated by J. R Grassle, New Brunswick J. S. Rivers ~ 9 R Peckol ([])