Effect of environmental parameters on the growth of Ulva rigida (Thau lagoon, France)

Fixed populations of Ulva rigida C. Agardh were studied, in the Thau lagoon, over two years (1994-1995) at 1.5 m depth (biomass, concentrations of N and P and pigment). Simultaneously, the growth of young thalli in cages was studied in relation to climatic (global radiation) and hydrological environmental parameters (temperature, salinity, dissolved oxygen, incident light, dissolved N and P, chlorophyll-a in the water, suspended particulate matter). Nutrient availability was generally good in this area and maximal growth period (RGR > 5%) was found between April and June. In this period, only the phosphate level could, in certain cases, delay the start of growth. The limiting parameters were physical i.e. growth occurs when temperatures were between 10-24 degrees C and incident light in cages between 80 and 280 mu E m(-2) s(-1). No growth occurred below 10 degrees C and only limited growth was observed between 10 and 12 degrees C, optimal growth occurring between 12 and 23 degrees C. Temperatures in excess of 24 degrees C were responsible for the halt in growth in summer. In autumn, despite favourable temperatures, the growth of Ulva rigida was low due to reduced light transmission caused by the phytoplankton bloom. Succinctly, the lack of establishment of the optimal physical conditions, necessary for the growth of this species could explain the absence of Ulva blooms in this lagoon, despite the fact that nutrient concentrations were generally favourable.