Physiological ecology and seasonality of Ulva pertusa on a temperate rocky shore

Green tides, i.e. blooms of Ulva species, occur on rocky intertidal shores over a wide geographic area in Korea. For the first time we report seasonal fluctuations in biomass, photosynthetic performance and chemical composition of the green-tide forming Ulva pertusa on the southern coast of Korea. Water temperature, salinity, inorganic nutrients and precipitation influencing Ulva mat dynamics were also monitored. There was a pronounced seasonality, not only in the biomass of U. pertusa, but also in photosynthetic performance and in variation in tissue pigments and nutrients. In addition to a primary seasonal response, significant variation in biomass was correlated in parts of the year with nutrient inputs from the surrounding watershed, heavy rain events and summer desiccation. There was a unimodal seasonal pattern in which biomass peaked in May (2.2 kg fw m^-2) and dropped significantly from June to September. Recovery of Ulva mats, as indicated by recruitment of new plants, began during autumn. Photosynthetic rates, maximum photosynthesis (P _max) and photosynthetic efficiency (α) were highest during the growth period and were lowest when biomass peaked or declined in May and July. Tissue pigments had a less clear seasonal pattern. Relative amounts of nitrogen and phosphorus bound in U. pertusa also displayed an obscure seasonal trend. Complex interactions among biological and environmental variables precluded strong correlations with any particular environmental factor. Stochastic events also played a major role in seasonality and can override normal physiological adaptations.