Does light heterogeneity affect structure and biomass of submerged macrophyte communities?

Environmental heterogeneity is universal and occurs at various spatial scales. Many studies have examined effects of environmental heterogeneity on growth of individual species, but few have tested the effects on species composition and biomass of plant communities, especially those consisting of submerged macrophytes. Moreover, no study has tested effects of scale of heterogeneity on plant communities. We constructed communities with four submerged macrophytes (i.e., Ceratophyllum demersum, Hydrilla verticillata, Egeria densa and Myriophyllum verticillatum) and subjected the communities to three homogeneous light treatments (100%, 65% and 30% of full light and coded as high, medium and low light treatment, respectively) and two heterogeneous light treatments differing in patch size (large vs. small patch treatment). The total amount of light received by the whole communities in the two patchy treatments was the same as that in the homogeneous medium light treatment. Under homogeneous treatments, decreasing light intensity significantly decreased total biomass, total number of nodes and total shoot length of the submerged macrophyte communities and of H. verticillata and E. densa, but did not significantly affect growth of M. verticillatum or C. demersum. Light heterogeneity, i.e., light patchiness or patch scale, did not affect these three growth variables of the community as well as those of the individual macrophytes. Therefore, light intensity can affect structure and biomass of submerged macrophyte communities, but light heterogeneity may not. [ABSTRACT FROM AUTHOR]