An investigation of non-steady-state algal growth. I. An experimental model ecosystem

In order to test rigorously the transient behaviour of mathematical models of algal growth, detailed laboratory data sets with good temporal resolution are required. A series of algal growth experiments was conducted in transient conditions. Monoculture growth of, and competition for nutrients between, three contrasting species of phytoplankton (the diatom Thalassiosira pseudonana, the harmful flagellate Heterosigma carterae and the toxic dinoflagellate Alexandrium minutum) were investigated in different temperature, light and nutrient regimes. Although growth dynamics were qualitatively similar in batch culture, quantitative differences were evident in the growth response of the different species when grown in single yield-limiting nutrient conditions in identical physical conditions. Quantities such as the carbon:nitrogen (C:N) ratio and C and N per cell varied between species and within species under different growth conditions. Such results have particular significance to the development of mathematical models, which commonly represent algal populations as a single homogeneous group using a single currency such as numbers, C or N. Changes in light and temperature regime influenced algal growth: Alexandrium failed to grow at low temperatures, while specific growth rates of Thalassiosira were more sensitive to changes in temperature than those of Heterosigma. Changes in the dominant organism(s) and/or its size or nutrient status may influence the transfer of nutrients within the food web. Commonly, mathematical models make cell growth a function of a single yield-limiting nutrient. Decreased growth rates and high residual nutrient concentrations in competition experiments indicate that this approach is unlikely to be successful in conditions of limited supply of more than one nutrient, where multiple nutrient stresses will be significant.