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

Abstract

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; Pmax was 4.6 ± 0.3 and 1.8 ± 0.6 μmol O2.m−2.s−1 during spring and summer, respectively. Notably, summer pmax 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 CO2 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.

The retreat of large brown seaweeds on the north coast of Spain: the case ofSaccorhiza polyschides

“…factors interact and contribute to the growth performance of a seaweed, since most studies focus on the effects of a single factor and the interactive effects of several factors are less well known. However, there is some evidence that an interaction between high temperatures and low availability of nitrate is responsible for a decline in the photosynthetic performance of Saccharina latissima (Gerard, 1997) and Ulva lactuca (Rivers & Peckol, 1995), as well as in the reproductive allocation and spore standing-stock in Macrocystis pyrifera (Reed et al, 1996) and in the distribution, abundance and size of Californian kelps (Dayton et al, 1999). Something similar could be affecting the growth of Saccorhiza polyschides but it is difficult to find a straightforward relationship between ecological responses and changes in the abiotic environment.…”

The effect of photoacclimation on the photosynthetic physiology ofUlva curvataandUlva rotundata(Ulvales, Chlorophyta)

“…Developing eutrophication modifies biotic and abiotic factors in such a way that the growth of some key species is promoted. Many of these species belong to the genus Ulva, although conflicting data concerning the success of Ulva species in eutrophicated areas have also been reported (Rivers & Peckol, 1995).…”

Persistent occurrence of floating Ulva green tide in Hiroshima Bay, Japan: seasonal succession and growth patterns of Ulva pertusa and Ulva spp. (Chlorophyta, Ulvales)

“…In our results, DIP was selected as a correlated variable on the RGR of U. pertusa cultured in the cages. In many previous reports in temperate regions, nitrogen has been indicated to be more important than phosphorus, which limits algal growth and production Ménesguen & Piriou, 1995;Rivers & Peckol, 1995;Fong et al, 1996;Viaroli et al, 1996b;Malta & Verschuure, 1997;Sfriso & Marcomini, 1997;Naldi & Viaroli, 2002). For now, we are not sure if summer growth of U. pertusa is phosphorus-limited in Hiroshima Bay, but it is possible that growth of U. pertusa is influenced more seriously by nutrient shortage in summer than Ulva spp.…”

“…Seasonal changes in dominant species or species composition have often been observed in green tides worldwide (Pregnall & Rudy, 1985;Rivers & Peckol, 1995;Fong et al, 1996;Pihl et al, 1996). In some cases, environmental shifts or catastrophic events caused absolute replacement of dominant species .…”

“…However, there are also differences in some eco-physiological traits among species, e.g., abilities in nutrient uptake and stock, tolerance to high temperature or anaerobic conditions. These differences among species can lead to seasonal or incidental successions in green tides Rivers & Peckol, 1995).…”