Time-lag algal growth dynamics: biological constraints on primary production in aquatic environments

Abstract

Changes in the nutrient regime of phytoplankton cells induce variable time lags before the onset of cell division (quasi-instantaneous response to more than 24 h lags), dependent mainly on the algal species and the magnitude of the nutrient pulse. The latter parameter appears to be a main controlling factor in algal growth dynamics under transient conditions, overriding other variables such as temperature, irradiance and nutritional state. The presence of such phenomena puts intrinsic limits on primary production in a variable nutrient environment because algal cells are not made up of synthetic components only (sensu Williams 1971), but also of structural and genetic material. Under these conditions, high nutrient uptake rates over short time periods do not necessarily lead to high growth rates (defined as increase in cell numbers) over comparable time scales, even ~f cell quota increase very rapidly follow~ng nutrient resupply. Data from different groups of investigators on uptake-growth coupling, internal nutrient pools, growth lag, and carbon-nitrogen uptake interactions show consistent patterns as follows. for phytoplankton in a vanable nutrient environment, 2 essential strategies emerge at the genus level. One is the 'growth' response, exhibited by the genera Durialiella and Chaetoceros, which do not accumulate internal pools of inorganic nutrients, whose uptake and growth are closely coupled, and which therefore process nutrient pulses very rapidly into new cells. The other is the 'storage' response, found in genera such as Thalassiosira or Amphidinium, which have the capability of accumulating large internal nutrient pools, present extensive uncoupling between uptake and growth, and exhibit lags in cell &vision of up to 24 h following a single addition of the Limiting nutrient. The latter response type presents an ecological advantage when nutrient pulsing frequency is lower than cell dlvision rate; the first response type would provide a competitive advantage at high frequency pulses.