Estuarine,Coastal and Shelf Scimu (1991)3~ 567-579 Bird Colonies Cause Seagrass Enrichment in a Subtropical Estuary: Observational and Experimental Evidence George v. N. Powella, James W. Fourqureana,b, W.Judson Kenworthy' andJoseph C. Ziemanb "NatJ'onal AudubonSociety,115Indian Mound Trail, Tavernier,FL 33070, U.S,A" 6nepartmmt of Emlironmnual Scimu, Clark Hall, Uniwrsiry of Virginia, Charlouewille, VA 22903, U.S,A, andcNational Marin4 FisMM Senliu, SoutMast FisMria Cmter, Beau/ort Laboratory, Beaufort, NC 28516-9722, U,S.A. Received 26 JtDu 1989 and in retlisedf- 2January 1991 Keywords: birds; nutrients; 5eagrasses; fertilization; Florida Keys Colonies/roosts of piscivorous birds in Florida Bay, a subtroPical estuary, con- centratenutrients by feeding awayfrom their colonies/roosts and returning with food for young and to dcfaecate. Seagrass beds surroundinl the colony islands were markedly different from those around similar islands that did not contain colonies. Seaarass standing crop was enhanced up to 200m from bird colony islands compared with islands without colonies. The species of seagrass werealso different at colonies, whereHalodulewrightii and Ruppiamaritima predominated in zones close to the colony islands. Around islands without colonies. only Thalassia ustudinumwaspresent. Experimental bird perches placed to stimulate concentrated bird presence produced changes in adjacentseagrass meadows that were similar to differences between islands with colonies and thosewithout. Over 5 years. seagrass standinl crop increased around the experimental perches. and species dominance shifted from T. testudinumto H. wrightii. No similar changesoccurred at control locations. These experimental results indicate that the bird concentrations are respooaible for the observeddifferences in seagrass communities surrounding islandsthat contain colonies. These enriched areas are significant to the seagrass ecosystem because many seagrasses in Florida Bay appear to be nutrient-limited. Demenal fish and invertebratedensity and species richness havebeen shown to be a function of the seagrass standing crop and species composition. so the changes in seagrasses stimulated by localizedbird concentrations have the capacity to alter the entire community structure. Introduction Historically, the southernmost tip of the Florida peninsulasupported largepopulations of birds that roostedand nestedcolonially in Florida Bay and the adjacentcoastal wetlands. While thosebird populations havebeendrastically reduced(Frederick & Collopy, 1988), remnant populations show similar distribution patterns. These piscivorous birds forage 0272-7714/91/060567 + 13103.00/0 @ 1991Academic PressLimited