PHYSIOLOGICAL-RESPONSES OF THE OPPORTUNISTIC MACROALGAE CLADOPHORA-VAGABUNDA (L) VANDENHOEK AND GRACILARIA-TIKVAHIAE (MCLACHLAN) TO ENVIRONMENTAL DISTURBANCES ASSOCIATED WITH EUTROPHICATION

Ability of macroalgae to tolerate environmental disturbances often con tributes to their success in marine communities. We investigated the e ffects of episodic and chronic disturbances (hypoxia/anoxia, elevated ammonium, reduced light) associated with eutrophication on physiologic al responses (respiration, photosynthesis, growth, nutrient uptake) of the macroalgae, Cladophora vagabunda (L.) van den Hock and Gracilaria tikvahiae (McLachlan). These opportunistic species accumulate into th ick (0.3 to > 1 m) mats during summer in a eutrophic embayment (Waquoi t Bay, Massachusetts), resulting in a highly reducing environment char acterized by elevated ammonium concentrations. Elevated (100 mu M) amm onium levels depressed growth and ammonium uptake rates of both specie s. In fact, C. vagabunda had high rates of ammonium release under cond itions similar to the in-mat environment (dark, elevated N). Ammonium uptake rate was 2-3 times higher for G. tikvahiae under lighted vs. da rk conditions. This light/dark effect was more dramatic for C. vagabun da; presence of light, characteristic of the surface of the mat, subst antially reduced the inhibitory effect of the elevated ammonium treatm ent. In the field and laboratory, C. vagabunda and G. tikvahiae showed reduced respiration rates under stagnant conditions and hypoxia, resp ectively. As hypoxic, stagnant conditions occur frequently within the thick mats, a reduction in respiration may enable these species to sur vive these disturbances. We also investigated the potential for recove ry of algal thalli buried within the anoxic region of the mat. C. vaga bunda demonstrated remarkable resilience; following a 2-day acclimatio n period under lighted, aerated conditions, blackened fronds taken fro m anoxic regions of the mat showed nearly full physiological recovery, namely, photosynthetic performance and nutrient uptake rates approach ed values measured for surface-collected fronds. Previous field measur ements indicated that while C. vagabunda is capable of surviving long- term burial (> 3 months) within the mat, turnover of the mat is a freq uent event during summer due to heavy motor-boat activity. The distinc tive species' patterns may be critical acclimation responses to large swings in the oxygen and nutrient environments of the algal mat, contr ibuting to the successful dominance of both Cladophora spp. and G. tik vahiae in this and other areas undergoing eutrophication.