In the fall of 1987, a widespread, rapid die-off of turtle grass, Thalassia testudinum, began in Florida Bay. Die-off occurred in areas of dense seagrass cover and principally in and around Rankin Lake, Rabbit Key Basin and Johnson Key Basin in western Florida Bay. Increasingly extensive and persistent turbidity and algal blooms, apparently linked to the loss of seagrass cover, have been associated with active seagrass die-off sites since 1988 and have characterized western and central Florida Bay since 1991. Recolonization of impacted grass bed habitats by shoal grass, Halodule wrightii, is occurring.

Loss of seagrass habitat on the scale observed in Florida Bay is unprecedented in tropical seagrass systems and hypothesized to threaten the Bay's water quality, sportfishery, and nursery function. In the short-term, grass canopy loss and declining environmental conditions may lead to shifts in species composition and reduced abundance of grass canopy dependent organisms. Over the long-term increasing seagrass habitat heterogeneity may lead to enhanced nursery function and an improve sportfishery.

A detailed quantitative database is available from Johnson Key Basin from October 1984 to April 1987 prior to seagrass die-off. Limited additional data is available from between May 1989 and August 1991, a period following the onset of seagrass die-off in the Bay but prior to the extensive and persistent plankton blooms which have characterized it since 1991. For caridean shrimps, fishes and pink shrimp this database documents population and community dynamics and spatial relationships with seagrass habitat. Therefore, it provides an excellent baseline against which to observe the response of characteristic seagrass associated species in Florida Bay to grass canopy loss, seagrass community change, and changing environmental conditions following seagrass die-off. The purpose of this project is to duplicate over the period, October 1994 to April 1997, the experimental design and sampling protocols employed previously in Johnson Key Basin prior to seagrass die-off in order to address the following objectives: 1) to document changes in seagrass community structure and habitat complexity following seagrass die-off; 2) to document changes in species composition, abundance, and seasonality of caridean shrimps and fishes with changes in seagrass habitat; 3) to document temporal and spatial abundance and size frequency distribution of the pink shrimp, Penaeus duorarum, in relation to changes in seagrass habitat; and 4) to evaluate quantitative relationships between animal abundance and species composition and grass bed micro-structure and habitat complexity.

In 1984 thirty stations were established in Johnson Key Basin. Stations were located generally with no a priori consideration of the seagrass habitat present. The stations were evenly stratified among the principal seagrass macro-habitat types present in Florida Bay: bank, basin, and near-key. Nine of these thirty stations, 3 within each macro-habitat type, were repetitively sampled on a six-week interval between October 1984 and April 1987 in order to address questions of timing. These nine stations were also sampled between August and December on a six-week interval between 1989 and 1991. All thirty stations were sampled four times (January 1985, May 1985, May 1989, and January 1990) in order to address animal versus habitat questions.

Quantitative animal samples were collected using a throw trap. The throw trap consisted of an open-ended 1 m2 aluminum box, 45 cm deep, with panels of nylon netting (0.16 mm stretch mesh DELTA netting) attached on parallel edges at the top of the throw trap. Each panel of netting was large enough to cover the top of the throw trap when it was used in water deeper than 45 cm. At each station four replicate throw trap samples were located along a 20 m transect, one each in each 5 m segment. After the trap was dropped in place, it was cleared of animals with three separate passes of a 1 m wide frame sweep net of mesh size similar to the panels. It has been estimated that three sweeps collect at least 95% of target species present in the trap. SCUBA was used while clearing the trap in deep water. All fishes, caridean and penaeid shrimps were removed from each throw trap, identified, counted and sized as appropriate in the laboratory. When all thirty stations were sampled the connection between these seagrass associated animals and grass bed structure was made by associating each throw trap collection with estimates of grass bed micro-structure including: seagrass standing crop and blade density; algal biomass; sediment texture, depth, organic content and compaction; root and rhizome biomass; and water depth.

During the current effort six-week interval sampling has been ongoing since October 1994 (9 collections have been made) and the thirty stations have been sampled in January and May 1995. Dr. Mike Marshall of Mote Marine Laboratory is processing samples and identifying the organisms recovered. To date processing efforts have emphasized back-logged samples originally collected between 1989 and 1990 and recent samples collected during January and February 1990. Results reported here focus on a comparison among the thirty stations sampled in January/February of 1985, 1990 and 1995.

Not all of the thirty stations were sampled in each of the three years. Extreme low water in Florida Bay characteristic of January and February precluded sampling high bank stations successfully in all three years. Because of this results reported here are based on the twenty-four stations (5 bank, 9 basin and 10 near-key) sampled in each year. At these sites distinct changes in seagrass habitat have occurred since 1985. By 1995 the standing crop of Thalassia had declined by 82% as compared to 1995. Similarly, Halodule had declined by 53% and Syringodium has completely disappeared. These changes resulted in a marked shift in seagrass dominance among the 24 stations. In 1985 Thalassia was the dominant grass at 17 of 24 stations in Johnson Key Basin; Halodule dominated at 5 stations, all of them near-key habitats. By 1995 Thalassia dominated at only 9 stations while Halodule had expanded its presence into deeper water and dominate at 11 of 24 stations. Further, by 1995 Syringodium, never common, had disappeared and a new habitat, bare sediment, not present in 1985, dominated at 4 stations.

The abundance of seagrass associated caridean shrimps, fishes and pink shrimp was lower in 1995 when compared to either 1985 or 1990. Habitat change due to seagrass die-off in 1990 was localized within Johnson Key Basin and apparently affected only 6 of the 24 stations unlike 1995 when all but 6 stations evidence significant habitat change. Species composition differences were evident in 1995 when compared to 1985 and 1990. The killifishes, Lucania parva, and Floridichthys carpio, and the toadfish, Opsanus beta, were present in significantly lower numbers than in previous years. In contrast, the code goby, Gobiosoma robustum, and the bay anchovy, Anchoa mitchelli were found in greater numbers. The killifishes were community dominants within the Johnson Key Basin grass bed prior to widespread habitat change and virtually absent in 1995. Among the caridean shrimps, Alpheus sp. have increased in abundance, however, this result may be a sampling artifact due to the loss of the grass canopy. Evidence was not found supporting the hypothesis that increasing coverage by Halodule would translate to increasing recruitment of the pink shrimp, Penaeus duorarum.

At this point the project will continue with six-week interval sampling through April 1997. On completion, effects of seagrass die-off on seasonal timing can be addressed in addition to animal/habitat relationships. Additionally, data collected in this project will continue to contribute to ongoing statistical and population dynamics models of the pink shrimp in relation to the Tortugas fishery and to the expansion of the ATLSS fish and invertebrate model into Florida Bay.

(This abstract was taken from the Florida Bay Science Conference Proceedings, 1995)

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