In the fall of 1987, a widespread, rapid die-off of turtle grass, Thalassia testudinum, began in Florida Bay. This disturbance was thought to threaten the Bay's water quality, sport fishery, and nursery function. It was hypothesized that over the short-term, grass canopy loss and declining environmental conditions would lead to shifts in species composition and reduced abundance of grass canopy dependent organisms. Over a longer-term increasing seagrass habitat heterogeneity resulting from recolonization of denuded bottom by algae and Halodule wrightii, shoalgrass, would lead to enhanced nursery function and an improved sport fishery.

A detailed quantitative database, covering the period October 1984 through April 1987, is available from Johnson Key Basin in western Florida Bay prior to the onset of seagrass die-off. These data describe community dynamics and spatial relationships among seagrass associated caridean shrimp, fishes and the pink shrimp, Penaeus duorarum, in relation to an undisturbed seagrass habitat. As such, the database serves as a baseline against which to assess the response of a seagrass fauna to habitat changes occurring in Florida Bay following seagrass die-off. The experimental design and sampling protocols employed previously in Johnson Key Basin have been duplicated between October 1994 to April 1997 in order to address the following objectives: 1) to document changes in grass bed structure and habitat complexity in Johnson Key Basin since the mid-1980's; 2) to compare the seasonality of characteristic seagrass associated shrimp and fish populations in Johnson Key Basin prior to and following seagrass die-off; and 3) to compare quantitative relationships between grass bed structure and animal density and species composition observed in an undisturbed versus a disturbed seagrass system.

Quantitative animal samples were collected using a 1 m² throw trap. At each station four replicate throw trap samples were collected. All fishes, caridean shrimp and pink shrimp were removed from each throw trap and identified, counted and sized as appropriate in the laboratory. Thirty stations in Johnson Key Basin had been sampled originally and associated with quantitative estimates of grass bed micro-habitat were associated with each throw trap sample. These measures included: seagrass standing crop and blade density; algal biomass; sediment texture, depth, compaction, sediment organic content; root and rhizome biomass; and water depth.

A comparison with habitat data collected in 1985 indicated that the character of the seagrass community in Johnson Key Basin has changed significantly over the past ten years. Combined 1995 January and May estimates of the standing crop of Thalassia, indicate that turtlegrass has declined by 72.3 percent in Johnson Key Basin when compared to 1985. No significant difference in the standing crop of Halodule was observed between 1985 and 1995 at the thirty stations sampled. Syringodium, relatively widespread (present at 12 of 30 stations in May 1985, dominant at 1 station) but never abundant, was absent in 1995. Following the decline of Thalassia, living root and rhizome biomass among these thirty stations also declined, 49.8 percent when compared to 1985. These changes resulted in a marked shift in seagrass species community dominance in Johnson Key Basin. In May of 1985, Thalassia was the dominant seagrass at 24 of 30 stations in Johnson Key Basin. Halodule dominated at 5 stations, all of them near-key stations. By May of 1995, Thalassia dominated at 14 stations while Halodule had expanded its presence into deeper water and was the dominant seagrass present at 14 of 30 stations. A new habitat, bare sediment, not present in 1985, dominated at 2 stations in 1995.

Observed patterns of abundance and species composition between 1985 and 1995 are in line with hypothesized short-term expectations following loss of seagrass habitat. The abundances of caribbean shrimp and fish were lower in 1995 when compared to either January or May of 1985. These patterns of abundance in Johnson Key Basin are similar to those observed when comparing animal densities in adjacent patches of die-off and undamaged seagrass habitat.

Differences in the species composition of fish and caridean shrimp were observed. The rainwater killifish, Lucania parva, the most abundant fish in the Johnson Key Basin seagrass bed prior to seagrass die-off, representing over 62 percent of catch in 1985, dropped to less than 3 percent of catch in 1995. Since fish abundance in the Basin in 1995 was less than half that observed in 1985, L. parva experienced over a 98 percent decline in abundance over the ten year period. Similarly, Thor Floridanus, the numerically dominant caridean shrimp in Johnson Key Basin in 1985, representing over 81 percent of the caridean shrimp captured, dropped to 26.6 percent of catch in 1995, experiencing over a 93 percent decline in abundance. Other animals declined in abundance following seagrass die-off as well, notably the killifish, Floridichthys carpio, and the toadfish, Opsanus beta, and the caridean shrimp, Periclimenes longicaudis. In contrast, the code goby, Gobiosoma robustum, the bay anchovy, Anchoa mitchilli and the burrowing caridean shrimp, Alpheus herterochaelis, were found in greater numbers in 1995. The appearance of the bay anchovy and the Spanish sardine, Sardinella aurita, for the first time in throw trap collections is thought to be a response to the algal blooms which now characterize the formerly clear water Johnson Key Basin. The distinct changes in species dominance in the Johnson Key Basin seagrass bed can not be completely explained by loss of seagrass habitat since substantial seagrass is still present in the Basin and at the thirty stations sampled and both L. parva and T. floridanus were most abundant in shoalgrass habitats in 1985.

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