MEPS 406:33-45 (2010)  -  DOI: https://doi.org/10.3354/meps08533

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

Lesley P. Baggett1,2,*, Kenneth L. Heck Jr.1,2, Thomas A. Frankovich3, Anna R. Armitage3,4, James W. Fourqurean3

1Dauphin Island Sea Lab, 101 Bienville Boulevard, Dauphin Island, Alabama 36528, USA
2Department of Marine Science, University of South Alabama, Mobile, Alabama 36688, USA
3Marine Science Program, Department of Biological Sciences and Southeast Environmental Research Center, Florida International University, 3000 NE 151st Street, North Miami, Florida 33181, USA
4Department of Marine Biology, Texas A&M University at Galveston, PO Box 1675, Galveston, Texas 77553, USA

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 of grazers to control epiphyte biomass even when nutrient loading rates are substantially elevated.


KEY WORDS: Nutrient enrichment · Grazing · Seagrass · Epiphytes · Gastropods · Caridean shrimp · Hermit crab


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Cite this article as: Baggett LP, Heck KL Jr, Frankovich TA, Armitage AR, Fourqurean JW (2010) Nutrient enrichment, grazer identity, and their effects on epiphytic algal assemblages: field experiments in subtropical turtlegrass Thalassia testudinum meadows. Mar Ecol Prog Ser 406:33-45. https://doi.org/10.3354/meps08533

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