Submitted to: Aquatic Botany
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/5/2010
Publication Date: 5/1/2010
Citation: Spencer, D.F., Rejmanek, M. 2010. Competition between two submersed aquatic macrophytes, Potamogeton pectinatus and Potamogeton gramineus, across a light gradient. Aquatic Botany. 93:75-82 Interpretive Summary: The mechanisms by which species compete with one another are poorly known. We observed that two species of pondweeds did not often occur in the same patch in irrigation canals. Building on previous work, we performed experiments in which these species were grown in mixture combinations at different daily light levels. The results showed that the species which produced more of it stem material near the surface of the water was a stronger competitor than the species which had more or less uniform distribution of leaves in the water column. However, these results were most pronounced at high light levels. This finding supports a theory for plant community organization proposed by the British ecologist, Phillip Grime. It also explains the distribution of two aquatic weeds common in western irrigation canals, and is also one of few studies that demonstrates that competition for light by plants is an important factor regulating species distributions.
Technical Abstract: Submersed aquatic macrophyte communities, are often limited by the availability of light. Thus, they offer a unique opportunity to evaluate competition when light is the limiting resource. Competitive abilities of Potamogeton pectinatus (L.) Börner and Potamogeton gramineus L. were estimated using additive series experiments at each of four total daily irradiances, 1.24 , 2.01, 3.64, 7.75 (M m-2 day -1). Results for P. gramineus showed that its mean weight per plant was reduced more by the presence of other P. gramineus plants than by changes in the density of P. pectinatus. However, the difference was statistically significant only at the higher light levels examined. In the case of P. pectinatus, P. gramineus exerted a stronger impact on P. pectinatus plants than did other P. pectinatus plants. Coefficients representing interspecific competition were significantly greater than those representing intraspecific competition at higher light levels. These results indicate that P. gramineus was a stronger competitor when light levels were high. They further indicate that at low light levels, there was little evidence that competition was a strong force in determining either species performance. These results support a theory offered by Grime. However, it was possible to predict that P. gramineus would be a stronger competitor based on its light compensation point which was lower than the one for P. pectinatus which demonstrates the utility of the R* approach proposed by Tilman. Aspects of both approaches may aid ecologists in understanding community structure.