Asymmetric competition for nutrients and light between benthic and pelagic algae: Theory, field data, and an experimental test system

Background/Question/Methods

In shallow aquatic systems benthic and pelagic algae compete asymmetrically for nutrients and light. The asymmetry arises because pelagic algae attenuate light on the way to the benthic habitat, and benthic algae intercept sediment nutrients diffusing into the pelagic habitat. We present a mathematical model of cross-habitat resource competition between benthic and pelagic algae in which the supply with nutrients depends on the nutrient concentration in the sediment and the transfer rate across the sediment-water interface, and the supply with light on surface irradiation, abiotic light attenuation, and water column depth. We also designed a laboratory test system that allows independent manipulation of all these factors. 

Results/Conclusions

The model predicts that benthic algae competitively exclude pelagic algae at high light and low nutrient supply through interception of the nutrient flux from the sediment. Increasing the nutrient supply, decreasing the light supply, or both will first enable coexistence of benthic and pelagic algae, but eventually leads to the competitive exclusion of benthic algae through shading by pelagic algae. Alternative states where either species can out-compete the other through priority effects are possible but unlikely for plausible parameter values. The model qualitatively and quantitatively reproduces empirical patterns found across shallow lakes from temperate and arctic regions varying in nutrient concentration. There, benthic primary production decreased and pelagic primary production increased with increasing phosphorus concentration in the lake water. The experimental system consists of a lower compartment representing the sediment pore water, an upper compartment representing the pelagic water column, and an interface between the two representing the sediment surface. This interface consists of glass fiber filters that serve as an easily sampled substrate for benthic algae and that allow diffusive nutrient flow between the two compartments. We describe technical aspects of the experimental system and present data from a first test of the benthic and pelagic algae asymmetric competition.