Nutrient enrichment, grazer identity, and their effects on epiphytic algal assemblages: field experiments in subtropical turtlegrass Thalassia testudinum meadows

Authors

Lesley P. Baggett, Dauphin Island Sea Lab, Department of Marine Science, University of South AlabamaFollow
Kenneth L. Heck Jr., Dauphin Island Sea Lab, Department of Marine Science, University of South AlabamaFollow
Thomas A. Frankovich, Marine Science Program, Department of Biological Sciences and Southeast Environmental Research Center, Florida International UniversityFollow
A R. Armitage, Marine Science Program, Department of Biological Sciences and Southeast Environmental Research Center, Florida International University, Department of Marine Biology, Texas A&MFollow
James W. Fourqurean, Marine Science Program, Department of Biological Sciences and Southeast Environmental Research Center, Florida International UniversityFollow

Abstract

We tested the relative importance of top-down and bottom-up effects by experimentally evaluating the combined and separate effects of nutrient availability and grazer species composition on epiphyte communities and seagrass condition in Florida Bay. Although we succeeded in substantially enriching our experimental cylinders, as indicated by elevated nitrogen concentrations in epiphytes and seagrass leaves, we did not observe any major increases in epiphyte biomass or major loss of Thalassia testudinum by algal overgrowth. Additionally, we did not detect any strong grazer effects and found very few significant nutrient-grazer interactions. While this might suggest that there was no important differential response to nutrients by individual grazer species or by various combinations of grazers, our results were complicated by the lack of significant differences between control and grazer treatments, and as such, these results are best explained by the presence of unwanted amphipod grazers (mean = 471 ind. m–2) in the control cylinders. Our estimates of grazing rates and epiphyte productivities indicate that amphipods in the control cylinders could have lowered epiphyte biomass to the same level that the experimental grazers did, thus effectively transforming the control treatments into grazer treatments. If so, our experiments suggest that the effects of invertebrate grazing (and those of amphipods alone) were stronger than the effects of nutrient enrichment on epiphytic algae, and that it does not require a large density

Comments

This material is based upon work supported by the National Science Foundation through the Florida Coastal Everglades Long-Term Ecological Research program under Cooperative Agreements #DBI-0620409 and #DEB-9910514. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Recommended Citation

Baggett, L.P., K.L. HeckJr., T.A. Frankovich, A.R. Armitage, J.W. Fourqurean. 2010. Nutrient enrichment, grazer identity and their effects on epiphytic algal assemblages: field experiments in sub-tropical turtlegrass (Thalassia testudinum) meadows. Marine Ecology Progress Series 406: 33-45.