Fish and Shrimp in Relation to Seagrass Habitat Change in Johnson Key Basin, Western Florida Bay (1985 – 1995)
Michael B. Robblee, André Daniels
USGS, Water and Restoration Studies Center, Miami, FL
In the fall of 1987, a widespread, rapid die-off of the dominant seagrass, Thalassia testudinum, began in dense seagrass habitats in western Florida Bay. Increasingly extensive and persistent turbidity and algal blooms, apparently linked to the loss of turtle grass, were associated with active seagrass die-off sites initially and have characterized western and central Florida Bay generally since 1991. In Johnson Key Basin denuded bottom was rapidly colonized by shoal grass, Halodule wrightii, following the loss of turtle grass. Syringodium filiforme, manatee grass had disappeared from the basin by 1995 possibly in response to reduced light availability.
In 1985, prior to seagrass die-off, thirty sampling stations had been established in Johnson Key Basin. Stations had been located generally with no a priori consideration of the seagrass habitat present and were evenly stratified among the principal seagrass macro-habitat types present in Florida Bay: bank, basin, and near key. These thirty stations were sampled on a five-year interval (January 1985, May 1985, May 1989, January 1990, January 1995 and May 1995) providing the opportunity to observe fish and invertebrate community responses in numbers and species composition to change in seagrass habitat within the basin over the decade, 1985-1995. Quantitative animal samples of seagrass associated fish and invertebrates, including the pink shrimp, were collected using a 1m2 throw-trap. The throw-trap operated over the full range of water depths occurring in Johnson Key Basin with a sampling efficiency estimated to exceed 95% for seagrass associated fish and invertebrates. Each animal sample was coupled with measurements estimating seagrass canopy structure and local environmental conditions: standing crops of seagrass and associated macroalgae, seagrass blade densities by species, sediment texture and organic content, water and sediment depth, compaction, salinity and temperature.
Unlike some areas in western Florida Bay, seagrass die-off in Johnson Key Basin was patchy with severe to no visible impacts among the thirty stations. Seagrass habitat change was most extensive between 1990 and 1995 when algal blooms were always present in the basin. The cumulative affect of seagrass loss and recovery and reduced water clarity between 1985 and 1995 was a shift away from a turtle grass dominated seagrass meadow to one exhibiting greater habitat heterogeneity. Over the decade the standing crop of Thalassia declined by 71% in Johnson Key Basin (Figure 1A). Standing crop of Halodule increased by 24%
Figure 1. Comparison of habitat changes and associated caridean shrimp, and fish communities in Johnson Key Basin over the decade 1985 to 1995.
in the basin. Syringodium had disappeared from the thirty stations sampled though it was reported as present in the basin. Thalassia, the dominant seagrass in 1985 at 17 stations in Johnson Key Basin was the dominant at only 9 in 1995. Halodule, present at 13 stations in 1985, was found at 18 by 1995. By 1995 bare sediment, not present as an appreciable habitat type in 1985, characterized the bottom in Johnson Key Basin at 4 of the thirty stations.
Faunal changes were observed accompanying these habitat changes. Comparing 1985 to 1995 the abundance (January and May averaged) of seagrass associated caridean shrimps had declined by about 65% while seagrass fishes had declined by 81% (Figure 1). Densities of pink shrimp, Farfantepenaeus duorarum, in 1995 were half that observed in 1985, (1.9/m2 vs 4.0/m2, respectively). Salinity and water temperature conditions, during sampling in 1985 and 1995, did not differ appreciably (31.5‰ vs 28.4‰; 25 oC vs 24.9 oC, respectively).
The fish and shrimp communities changed dramatically over the decade in Johnson Key Basin. In 1985, the killifish, Lucania parva and the caridean shrimp, Thor floridanus dominated numerically (62% and 81% of individuals, January and May averaged, respectively). By 1995 these populations had declined to 3% and 27% of fish and caridean shrimp collected, respectively. Six fishes, Anchoa mitchilli, Sardinella aurita, Achirus lineatus, Gobiosoma robustum, and Hippocampus zosterae. were numerically more abundant in 1995 and accounted for 65% of fish collected. Five caridean shrimps, Periclimenes americanus, Alpheus heterochaelis, Thor floridanus, Hippolyte plueracanthus, and Hippolyte zostericola, accounted for 81% of the caridean shrimps collected in 1995.
A preliminary factor analysis (non-rotated PCA) was used to reduce 9 habitat variables to 4 interpretable components. PC1 accounting for 25.3% of the variability among the original habitat variables was interpreted as a Thalassia gradient. Halodule is negatively correlated with Thalassia on PC1. PC2 (16%) was interpreted as a shallow water Halodule gradient. PC3 (12.4%) was interpreted as a Syringodium gradient. PC4 (10.4%) was interpreted as a macro algal gradient. Interpretation of PC’s 1-3 indicate Thalassia and Syringodium decline in abundance with time while Halodule increases over time.
Individual species and species groups relate to habitat differently in Johnson Key Basin. The pink shrimp, an abundant penaeid shrimp, is significantly correlated to PC2 and 3 indicating an affinity for shallow water shoal grass and manatee grass habitats. The killifish, Lucania parva, was significantly correlated to PC1 and 2. In Johnson Key basin this fish is associated with seagrass generally and is most abundant, as is the pink shrimp, in shallow water shoal grass habitats. The dominant caridean shrimp, Thor floridanus, is a ubiquitous species in the Johnson Key Basin grass beds, abundant in 1985 and scarce in 1995. It was positively associated with each of the 4 principal components.
More detailed analyses focusing on defining seagrass canopy and seagrass habitat change with time, the relationship of pink shrimp size to seagrass habitat and individual species relationships with seagrass habitat will be presented.
Robblee, Michael, Water and Restoration Studies Center, Miami, FL 33178, Fax: 305-242-7855, Phone: 305-242-7832, , Question 5