Topical Area: Higher Trophic Levels

William G. Lyons, Florida Department of Environmental Protection, Florida Marine Research Institute, St. Petersburg, FL

Objectives: Knowledge of the relationships between environmental change, habitat change, and the recruitment, growth, and survivorship of constituent fauna is requisite for restoration efforts to be effective in Florida Bay. The work described here is being conducted to map the composition and distribution of benthic faunal assemblages in Florida Bay and to assess changes in those assemblages related to changes in the salinity regime. Living mollusks are being used as representatives of that benthic fauna. The molluscan fauna of Florida Bay is rich and diverse, approaching 200 species which are distributed variously among the Bay's several environmental regimes (Turney and Perkins, 1972; Lyons, 1996). Environmental conditions, especially salinity, fluctuate widely in the bay, and changes in molluscan species composition in core samples from various parts of the bay have been interpreted to indicate the influence of oscillating salinity on the resident benthic fauna (Brewster-Wingard et al., 1997). By elucidating the responses of living molluscan assemblages to known changes in ambient salinity, we can improve our ability to interpret the evidence of environmental change in these cores, and we will acquire information that will enable more accurate predictions of the consequences of activities which would alter the salinity regime of the bay.

Progress: Using methods described by Lyons (1995), we sampled living mollusks quantitatively between 1994 and 1996 at 101 sites distributed evenly throughout the bay. Sampling was conducted in summer 1994 during a period of hypersalinity, when salinity during sampling exceeded 30 °/oo at 96 sites, exceeded 36 °/oo at 75 of the sites, and ranged upward to 52 °/oo; values <18 °/oo were seen at only 2 sites. Salinity declined markedly after summer 1994. In summer 1995, the highest salinity measured at the above sites was 35.5 °/oo; salinity still exceeded 30 °/oo at 58 sites, but values ranged from >18 to 30 °/oo at 32 sites and from 0 to 18 °/oo at 11 sites. Salinity increased somewhat in 1996 and, during the summer sampling period, ranged from 0.8 to 40.9 °/oo; values exceeded 30 °/oo at 62 sites, ranged from 0.8 to 18 °/oo at 10 sites, and ranged from >18 to 30 °/oo at 29 sites. These values were generally more moderate than in either 1994 or 1995 but more resembled those of 1995.

The 101 sites yielded 95 species comprising nearly 14,000 specimens during summer 1994. A mussel, Brachidontes exustus, contributed 78% of all of the specimens. Comparisons of intersite similarity and abundance were mapped and indicated three major site groups corresponding closely to the Interior, Transitional, and Gulf Subenvironments of Turney and Perkins (1972). A fourth group consisted of 2 sites in Joe Bay, where Turney and Perkins did not sample. A major difference between groups in the 1994 map and those of Turney and Perkins involved the failure in the 1994 study to distinguish the Northern and Atlantic Subenvironments identified by those earlier authors. Reasons for these differences were explained by (Lyons, in press). The Northern Subenvironment was based on a single species which, although distinctive, occurs with several other species that cause sites in that "zone" to classify with other nearby groups. The original Atlantic Subenvironment extended well west of the present study area and included several sites on the Atlantic side of the Keys; most of the species that characterized that zone are rare or absent in the small portion of the zone sampled in the present study.

In a difference of greater concern, an area of approximately 200 km² north of the Matecumbe Keys that was included in the Transitional Subenvironment by Turney and Perkins was classified in the Interior Subenvironment in the 1994 map. Most of the area in question is located in the path of flow of high-salinity water and microalgal/cyanobacterial blooms that swept the central and southwestern bay during much of the decade, suggesting that the composition of benthic assemblages might have been affected by these events.

The molluscan fauna was monitored at a subset of 15-30 sites, selected from the 1994 groupings, during summer and fall/winter 1995 and spring 1996 to track seasonal fluctuations and changes related to large shifts of salinity in the eastern and central bay. Results of those efforts provided evidence of (1) a marked reduction in species richness and abundance in the eastern bay, where salinities declined from euhaline and polyhaline to mesohaline values; (2) displacement a few kilometers southward of the mesohaline fauna formerly characteristic of the Bay's northern rim; and (3) incursion of a more robust and diverse fauna into areas of the central bay that were sparsely populated during the episode of higher salinity.

All 101 sites were again sampled in summer 1996, yielding 127 species in nearly 8,500 specimens. The lower number of specimens reflected a decrease in abundance of the mussel, B. exustus, which contributed only 29% of all specimens. Comparisons of intersite similarity and abundance were again mapped, producing the same four major site groups, but the group affiliations of many sites changed in 1996, evidently influenced by faunal shifts as responses to the changing salinity regimes. In perhaps the most significant shift, nearly all sites in the 200-km² area north of the Matecumbe Keys that clustered with the Interior Subenvironment in 1994 were classified with the Transitional group in 1996. This change in subenvironmental affiliation, brought about by the recruitment of many species northward and eastward into the area where salinities were formerly high, provides support for the zonal boundaries proposed by Turney and Perkins.

Sampling during three years in Florida Bay has produced living representatives of 155 species of mollusks. These mollusks include a small mesohaline group that occurs along the northern bay rim, another small group of northern species tolerant of widely fluctuating salinity, and a larger group of about 30-40 species whose frequencies and abundances are indicative of a widespread resident bay fauna. Another group consists of several dozen species that are allied with the Gulf Subenvironment but seldom if ever occur in the more shallow bay. Most of the remaining species are occasional visitors from Gulf and Atlantic assemblages west and south of the bay. Species richness is lowest in the northern bay, where salinity fluctuations are greatest, but then increases through several concentric zones toward the more stable environments to the south and west.

Brewster-Wingard, G. L., S. E. Ishman, D. A. Willard, L. E. Edwards, and C. W. Holmes. 1997. Preliminary paleontologic report on cores 19A and 19B, from Russell Bank, Everglades National Park, Florida Bay. U. S. Geological Survey Open-File Report 97-460. 29 pp.

Lyons, W. G. 1995. Mapping Florida Bay benthic assemblages: Using mollusks to assess faunal change. Pp. 167-169 in: Florida Bay Science Conference: A report by principal investigators, October 17 & 18, 1995. Gainesville, Florida.

Lyons, W. G. 1996. An assessment of mollusks as indicators of environmental change in Florida Bay. Pp. 52-54 in: Program and abstracts, Florida Bay Science Conference, December 10-12, 1996, Key Largo, Florida.

Lyons, W. G. In press. Changes in benthic molluscan assemblages in Florida Bay, 1994-1996. Proceedings, Conference on Ecological and Hydrological Assessment of the 1994-95 High Water Conditions in the Southern Everglades, Miami, Florida, 22-23 August 1996.

Turney, W. J., and B. F. Perkins. 1972. Molluscan distribution in Florida Bay. Sedimenta III. University of Miami, Fisher Island Station, Miami Beach, Florida. 37 pp.