Mesocosm Experiments to Investigate Phytoplankton Competition

Several years of mesocosm experiments in the Norwegian fjords near Bergen (where Ehux is a common part of the phytoplankton succession) looked at the effects of nutrient and other factors on phytoplankton species composition. In particular, nutrients were added in different amounts and different ratios to different bags, and the resulting phytoplankton species numbers were counted. The mesocosm bags experienced natural temperatures and irradiances (they were suspended in the fjord water). The polyethylene mesocosm bags (90% light transmittance) were 4 metres deep, 2 metres in diameter, and constantly stirred. Each bag was initially filled by pumping in unfiltered fjord water, and so natural communities of zooplankton, bacteria and viruses were introduced at the beginning of each experiment. See (Egge & Hemidal, 1994) for a full description of the experimental set-up.

Many phytoplankton species were present in the bags, but the abundances of just two have been plotted against various parameters, in the diagram shown above. Concentrating solely on Ehux cell numbers, the diagram shows no correlation with nitrate, greater Ehux numbers at low phosphate, and a tendency for blooms to occur at higher temperatures and irradiances. Irradiances are averaged over the previous 5 days. Temperature and light will be highly correlated in the shallow mesocosm bags, and it is thought likely that the correlation with high temperature is probably because Ehux does better at high light, rather than because it competes better at high temperatures.

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