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 (McLac hlan) have become dominant components of the macroalgal assemblage in Waquoit Bay, a Massachusetts embayment, possibly due to nitrogen (N) e nrichment from anthropogenic inputs transported via groundwater. Durin g 1989-1993, we measured site-related growth, ammonium uptake rates an d tissue constituents of these macroalgae from areas subject to high N loading rates (Childs River) and lower N loadings rates (Sage Lot Pon d). We also conducted in situ and microcosm enrichment experiments to determine what limited algal growth throughout the year. Our results i ndicated that these species are strongly affected by and have a strong impact on the N environment of this embayment. For example, C. vagabu nda and G. tikvahiae from Childs River had higher light-harvesting pig ments and tissue-N concentrations than Sage Lot Pond populations. Addi tionally, both Childs River populations showed greater site-specific g rowth and N uptake rates, particularly during the summer period of pea k growth. In fact, maximum uptake rates of 90 and over 140 mu mol dry wt g(-1) h(-1) for Childs River C. vagabunda and G. tikvahiae, respect ively, suggest that these species can remove substanital quantities of N from overlying waters, and may be responsible for low (often (<<1 m u M) water-column nutrient concentrations during summer. In situ and t ank enrichment experiments indicated that growth rates were limited by available N during summer, while P may be limiting during a brief per iod toward the end of the annual growth cycle (autumn). Under experime ntal enrichment, growth rates of Sage Lot Pond algae were similar to v alues measured at the site receiving higher N inputs, and generally, G . tikvahiae showed growth enhancement (up to 0.2 doublings d(-1)) unde r light-saturating conditions (0.5 m) while C. vagabunda showed nutrie nt-enhanced growth at 2.5 m. The effects of available nutrients on alg al growth were strongly influenced by irradiance and temperature, resu lting in a complex seasonal interaction that emphasized the dynamic na ture of species response to N loading. Dominance by these two macroalg ae in Waquoit Bay, as in other areas undergoing eutrophication, is lik ely related to physiological strategies that enable these species to t olerate large environmental variations, to take advantage of greater N availability and to survive indirect effects of N loading (e.g. reduc ed irradiance, anoxia).