Competition between pelagic and benthic microalgae for phosphorus and light

Abstract

In freshwater ecosystems production of both pelagic and benthic microalgae tends to be limited by phosphorus and light. However, the availability of these resources to pelagic and benthic communities differs due to differences in habitat structure. In a well mixed epilimnion individual phytoplankton cells should receive similar light intensities and nutrient concentrations per unit time. Benthic microalgae colonize substrates forming three-dimensional mats. Their access to nutrients and light is partially determined by the vertical architecture of the mat and often restricted to the canopy layer algae. In addition, algal mats will always be shaded by the phytoplankton inhabiting the water column above.

We investigated competition between pelagic and benthic freshwater microalgae in a three factorial experiment, in which we manipulated phosphorus concentration, light intensity and the source of the inoculum (pelagic, benthic and benthic-pelagic). We found significant and interacting effects of phosphorus, light and inoculum on algal biovolume, chlorophyll-a concentration and cellular phosphorus content. When cultured separately, both benthic and pelagic microalgae benefited from the enhanced supply of phosphorus and light (higher biovolume, Chl-a). In combined culture, pelagic microalgae benefited from the increased light supply and became dominant irrespective of whether high or low phosphorus concentrations were added. In contrast, benthic microalgae profited from low light intensity combined with high nutrient concentrations. Cellular phosphorus contents in benthic and in mixed cultures were highest under low light supply, whereas in pelagic cultures no signifi- cant differences in cellular phosphorus content were detected.