Résumé
The effect of light and nutrients on a temperate water planktonic microbial community was investigated
via experiments conducted in Scottish coastal waters during summer. Experimental water was collected from the depth of maximum chlorophyll concentration in the Clyde Sea on the 7th and 9th of August 2001. For each experiment, the microbial community was incubated for 36 h under one of two differing natural light regimes: continuous 3% solar irradiation or alternating 1-hour periods of 80% solar irradiation and darkness. Water incubated with the alternating light regime was also subjected to three initial nutrient manipulations: added nitrate, added nitrate and phosphate, or no nutrient addition. The four treatments were designed to simulate stratified or mixed water column conditions; the latter with or without nutrient entrainment. During the first experiment, the photosynthetic capacity and efficiency of phytoplankton were significantly reduced after 36 h incubation under the alternating light regime, regardless of nutrient addition; however phytoplankton in the second experiment showed not such response. Higher incident light intensities, recorded towards the end of the first experiment and throughout the second, may have resulted in photoinhibition of phytoplankton during the last hours of the first experiment, with cells acclimatised by the end of the second experiment, despite low nitrogen availability. In general, the four treatments had relatively little effect on the taxonomic composition and abundance of the microbial community; however, significant differences were observed in the abundance and activity of some microbial groups and species. Bacterial production increased during incubation across all treatments and a positive relationship was observed between bacterial specific growth and photosynthetic capacity. This suggests strong coupling between phytoplankton and heterotrophic bacteria
via dissolved organic material allowing bacteria and phytoplankton to sustain their growth under fluctuating environmental conditions. By contrast, the dinoflagellates
Alexandrium sp. exhibited highest abundance when exposed to the alternating light regime but only during the first experiment. Reduced competition for ammonium, due to photoinhibition of low-light adapted diatoms, may explain this observation suggesting that growth rate of potentially toxic dinoflagellates, such as
Alexandrium spp., can be limited in temperate waters by ammonium availability rather than irradiance.