Physiological responses of the opportunistic macroalgae Cladophora vagabunda (L.) van den Hoek and Gracilaria tikvahiae (McLachlan) to environmental disturbances associated with eutrophication

Abstract

Ability of macroalgae to tolerate environmental disturbances often contributes to their success in marine communities. We investigated the effects of episodic and chronic disturbances (hypoxia/anoxia, elevated ammonium, reduced light) associated with eutrophication on physiological responses (respiration, photosynthesis, growth, nutrient uptake) of the macroalgae, Cladophora vagabunda (L.) van den Hoek and Gracilaria tikvahiae (McLachlan). These opportunistic species accumulate into thick (0.3 to > 1 m) mats during summer in a eutrophic embayment (Waquoit Bay, Massachusetts), resulting in a highly reducing environment characterized by elevated ammonium concentrations. Elevated (100 μM) ammonium levels depressed growth and ammonium uptake rates of both species. In fact, C. vagabunda had high rates of ammonium release under conditions similar to the in-mat environment (dark, elevated N). Ammonium uptake rate was 2–3 times higher for G. tikvahiae under lighted vs. dark conditions. This light/dark effect was more dramatic for C. vagabunda; presence of light, characteristic of the surface of the mat, substantially reduced the inhibitory effect of the elevated ammonium treatment. In the field and laboratory, C. vagabunda and G. tikvahiae showed reduced respiration rates under stagnant conditions and hypoxia, respectively. As hypoxic, stagnant conditions occur frequently within the thick mats, a reduction in respiration may enable these species to survive these disturbances. We also investigated the potential for recovery of algal thalli buried within the anoxic region of the mat. C. vagabunda demonstrated remarkable resilience; following a 2-day acclimation period under lighted, aerated conditions, blackened fronds taken from anoxic regions of the mat showed nearly full physiological recovery, namely, photosynthetic performance and nutrient uptake rates approached values measured for surface-collected fronds. Previous field measurements indicated that while C. vagabunda is capable of surviving long-term burial (> 3 months) within the mat, turnover of the mat is a frequent event during summer due to heavy motor-boat activity. The distinctive species' patterns may be critical acclimation responses to large swings in the oxygen and nutrient environments of the algal mat, contributing to the successful dominance of both Cladophora spp. and G. tikvahiae in this and other areas undergoing eutrophication.