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

In order to test rigorously the transient behaviour of mathematical modelsof algal growth, detailed laboratory data sets with good temporalresolution are required. A series of algal growth experiments was conductedin transient conditions. Monoculture growth of, and competition fornutrients between, three contrasting species of phytoplankton (the diatomThalassiosira pseudonana, the harmful flagellateHeterosigma carterae and the toxic dinoflagellateAlexandrium minutum) were investigated in differenttemperature, light and nutrient regimes. Although growth dynamics werequalitatively similar in batch culture, quantitative differences wereevident in the growth response of the different species when grown insingle yield-limiting nutrient conditions in identical physical conditions.Quantities such as the carbon:nitrogen (C:N) ratio and C and N per cellvaried between species and within species under different growthconditions. Such results have particular significance to the development ofmathematical models, which commonly represent algal populations as a singlehomogeneous group using a single currency such as numbers, C or N. Changesin light and temperature regime influenced algal growth:Alexandrium failed to grow at low temperatures, whilespecific growth rates of Thalassiosira were moresensitive to changes in temperature than those ofHeterosigma. Changes in the dominant organism(s)and/or its size or nutrient status may influence the transfer of nutrientswithin the food web. Commonly, mathematical models make cell growth afunction of a single yield-limiting nutrient. Decreased growth rates andhigh residual nutrient concentrations in competition experiments indicatethat this approach is unlikely to be successful in conditions of limitedsupply of more than one nutrient, where multiple nutrient stresses will besignificant.