Growth,nutrient uptake capacities and tissue constituents of the macroalgae Cladophora vagabunda and Gracilaria tikvahiae related to site-specific nitrogen loading rates

Cladophora vagabunda (L.) van den Hoek and Gracilaria tikvahiae (McLachlan) have become dominant components of the macroalgal assemblage in Waquoit Bay, a Massachusetts embayment, possibly due to nitrogen (N) enrichment from anthropogenic inputs transported via groundwater. During 1989–1993, we measured site-related growth, ammonium uptake rates and tissue constituents of these macroalgae from areas subject to high N loading rates (Childs River) and lower N loadings rates (Sage Lot Pond). We also conducted in situ and microcosm enrichment experiments to determine what limited algal growth throughout the year. Our results indicated that these species are strongly affected by and have a strong impact on the N environment of this embayment. For example, C. vagabunda and G. tikvahiae from Childs River had higher light-harvesting pigments and tissue-N concentrations than Sage Lot Pond populations. Additionally, both Childs River populations showed greater site-specific growth and N uptake rates, particularly during the summer period of peak growth. In fact, maximum uptake rates of 90 and over 140 mol dry wt g^-1 h^-1 for Childs River C. vagabunda and G. tikvahiae, respectively, suggest that these species can remove substanital quantities of N from overlying waters, and may be responsible for low (often (<1 M) water-column nutrient concentrations during summer. In situ and tank enrichment experiments indicated that growth rates were limited by available N during summer, while P may be limiting during a brief period toward the end of the annual growth cycle (autumn). Under experimental enrichment, growth rates of Sage Lot Pond algae were similar to values measured at the site receiving higher N inputs, and generally, G. tikvahiae showed growth enhancement (up to 0.2 doublings d^-1) under light-saturating conditions (0.5 m) while C. vagabunda showed nutrient-enhanced growth at 2.5 m. The effects of available nutrients on algal growth were strongly influenced by irradiance and temperature, resulting in a complex seasonal interaction that emphasized the dynamic nature of species response to N loading. Dominance by these two macroalgae in Waquoit Bay, as in other areas undergoing eutrophication, is likely related to physiological strategies that enable these species to tolerate large environmental variations, to take advantage of greater N availability and to survive indirect effects of N loading (e.g. reduced irradiance, anoxia).