Molluscan fauna have been used successfully to reconstruct 200 years of historical information on seagrass distribution, water quality, salinity,

Figure 1. Aquatic vegetation indicators (in percentage abundance) in four cores from Florida Bay. Cores are scaled to years on the right-hand side and depth (cm) on the left. Click for larger image.

biodiversity, and faunal abundance in Florida Bay. Molluscan assemblages identify significant perturbations of an environment and filter out "noise" caused by day-to-day or short-term fluctuations, such as those caused by tropical storms. Estuarine molluscs tolerate a wide range of fluctuating environmental conditions, yet on a scale of seasons, their populations will migrate to more ideal conditions as Lyons (1996, 1998) has demonstrated. Individual molluscs, however, are relatively limited in their movement in post larval stages, and typically provide information about a specific site for the duration of their lifetime. In addition, molluscs occupy a number of trophic levels and represent a diverse, significant component of the biomass of the Florida Bay ecosystem, allowing this single group to serve as a proxy for the trophic structure and ecologic health of the system.

Evidence for significant changes in abundance and distribution of seagrass and aquatic vegetation (SAV) in Florida Bay can be seen by examining the distribution of epiphytic species (fig 1).

Molluscs that live on macro-benthic algae or seagrass have increased during the 20th century in the Bob Allen, Russell Bank, and Taylor cores. Species that live on seagrass exclusively have declined relative to the general SAV dwellers since about 1970 at Bob Allen and Russell Bank, and in the upper portions of Pass and Taylor. While substrate data are site specific, repetition of this trend in these four cores (as well as other cores not reported on here) indicates this may reflect a genuine increase in SAV during the 20th century throughout eastern and central Florida Bay. If, through examination of additional cores, this trend can be substantiated, it could have a significant impact on restoration efforts. These data imply that the abundance of seagrass in Florida Bay in the latter half of the 20th century may represent an anomaly.

Brachidontes exustus is a euryhaline species that can tolerate diminished water quality, and it is nearly ubiquitous at sites in central and eastern Florida Bay today. Beginning about 1980, Brachidontes exustus became the most dominant species at Russell Bank and Bob Allen, and it increased in dominance in the upper portion of the cores from Pass Key and Taylor (fig. 2). The concentration of this euryhaline opportunist species in the upper portion of all the cores implies that the system has been under increasing stress in the last 20 years, either due to increased salinity fluctuations and/or diminished water quality.

Molluscan faunal data illustrates important changes in the distribution of fresh and low salinity water in northern transitional, eastern, and central Florida Bay (Fig. 2). Since

Figure 3. Diversity measures in four cores from Florida Bay. Cores are scaled to years on the right-hand side and depth (cm) on the left. Click for larger image.

about 1900, there has been a steady decline in fresh water and mesohaline (<12 ppt) molluscs at the mouth of Taylor Creek. Changes in salinity at Taylor Creek and Pass Key (southeast of Little Madeira Bay) may be due to water management practices, fluctuations in average annual precipitation, sea level, or a combination of all of these. Russell Bank core, in eastern Florida Bay, shows distinctive changes in the molluscan assemblage between 1910 and 1930, indicating conditions that are more saline. Changes that occur between 1910 and 1920 at Bob Allen and between 1910 and 1930 at Russell indicate a shift toward less stable conditions. During this time period, average rainfall did not fluctuate significantly in southern Florida, but substantial human alteration of the environment occurred with the construction of Flagler Railroad in the Keys.

Several measures of diversity were calculated for the cores: (1) total number of individual molluscan specimens; (2) faunal richness; (3) evenness; and (4) Shannon's diversity index (fig. 3). In general, these measures of diversity all fluctuate over time, which is to be expected in an estuarine environment. However, at Bob Allen and Russell Bank, evenness shows an inverse relationship to the percent abundance of Brachidontes in the upper part of the core. This is consistent with our hypothesis that the increase in Brachidontes in the last two decades indicates a stressed system.

Molluscan data help to clearly define restoration goals on a number of components critical to successful resource management. Information on seagrass distribution, salinity, water quality, and biodiversity, interpreted from molluscan data, can be used in models by water and land managers. By establishing what the system was like prior to significant human influence, and examining how the system has changed during the 20th century, mangers can establish realistic success criteria for restoration efforts. By answering the question "What was the system like 100 years ago?" we can then answer the question "What should the system look like after restoration?" We can also can answer the question "How will the system respond to the changes induced by restoration?"

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report (PDF, 8.7 MB))

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