SUMMER DECLINE OF ULVA LACTUCA (CHLOROPHYTA) IN A EUTROPHIC EMBAYMENT: INTERACTIVE EFFECTS OF TEMPERATURE AND NITROGEN AVAILABILITY? 1

Throughout the summer, abundance of Ulva lactuca L. declined while biomass of Cladophora vagabunda ( L.) van den Hoek and Gracilaria tikvahiae McLachlan increased in a New England embayment undergoing eu‐trophication (Waquoit Bay, Massachusetts). We investigated the physiological basis for the summer dieback, focusing on temporal variations in photosynthetic performance and tissue nitrogen (N). We also compared photosynthetic and N uptake capabilities of U. lactuca with other abundant species in this eutrophic system. Photosynthetic egiciency and capacity of U. lactuca declined markedly at 25°C, compared with a spring (15°C) peak in photosynthetic performance; P max was 4.6 ± 0.3 and 1.8 ± 0.6 μmol O 2 .m −2. s −1 during spring and summer, respectively. Notably, summer p max of other abundant species of the embayment was 1.5–3 × higher than that measured for U. lactuca. Ulva lactuca showed a signifciant photosynthetic response to dissolved inorganic carbon enrichment during summer, when water‐column‐dissolved CO 2 levels were 20% of spring values. Although ammonium uptake rates of U. lactuca were extremely high at both subsaturating (15μM) and saturating (75 μM) N concentrations, as predicted by the functional‐form hypothesis, tissue N fell to 1% by late summer. We suggest that a carbon imbalance, initiated by rising water temperatures and declining water‐column N; thermal stress; and biological factors (competition, grazing) all contribute to the recurrent summer decline of U. lactuca in this shallow, eutrophic embayment. Thus, while the morphology of U . lactuca might be considered a successful strategy for disturbed, or “stressed” (sensu Littler and Littler 1980), habitats, its inability to persist and flourish in this environment emphasizes the complexity of factors at work in natural systems .