Abstract Methods Restricted Interior Environment Foraminiferal Distribution Current Transport Conclusions Acknowledgments Literature Cited Appendix

The trend of a foraminiferal population across a platform should be considered as an irregular but gradational organic continuum. In general, the number of individuals usually decreases in an offshore direction, a trend which appears to correlate closely with mud content of bottom sediments in Florida Bay (Figs. 2, 3). Although considerable local variation exists in the bay, the number of species generally increases seaward (Figs. 5, 8) and increases most abruptly in areas transitional between two environments, such as in the passes between the Florida Keys connecting the bay with the inner reef tract of the open interior, and in the outer reef tract (platform margin) that separates the platform interior and slope environments (Fig. 1). Within major environments, the number of species is most consistent when bottom topography is uniform. Where the platform surface is interrupted by sand bars, patch reefs, mudbanks, and islands, the number of individuals tends to vary abruptly and widely.

Approximately 50 foraminiferal species common on the Florida platform are distributed among 32 genera and 20 families (Bock et al., 1971; Rose and Lidz, 1977). Species belonging to the Soritidae and Miliolidae regularly compose 60 to 80% of the population. Next, in order of decreasing abundance, are the Rotaliidae, Elphidiidae, and Amphisteginidae. Assorted arenaceous species and the pelagic Globigerinidae are relatively minor components. Some species are restricted to specific environments and therefore serve as excellent ecologic indicators. Others are less closely limited; thus, their significance as environmental indicators is less precise. Still others are ubiquitous; these are the species that might best serve as time-stratigraphic markers in the geologic record. The platform interior fauna is divided into five transitional subfaunas: open interior, restricted interior (nearshore, mudbank, and "lake"), and brackish and interior (Figs. 2, 3).

Open Interior Fauna. - Populations of open interior foraminifera in Florida Bay are dominated by the Soritidae, which commonly occur in frequencies of as much as 60%, with Miliolidae and Elphidiidae constituting about 30%, and assorted arenaceous species, Rotaliidae, and Discorbidae making up the complement (Fig. 2). No species is restricted to this environment, but Clavulina tricarinata, Sorites marginalis, and Peneroplis proteus are the best monospecific indicators (Figs. 5, 6). Archaias angulatus and Cyclorbiculina compressa reach their greatest abundance in the open interior. Other common species include Triloculina linneiana and Valvulina oviedoiana, although infrequent specimens of the platform margin species Articulina, Borelis pulchra, and Pyrgo nasutus are also characteristic (Fig. 8).

Identification of the open interior fauna also relies on the absence of common margin and restricted interior species, such as Amphistegina "lessonii" (Figs. 4, 7) and Asterigerina carinata (Fig. 8), or low relative frequency of the brackish-water species Ammonia beccarii ornata (Figs. 5, 9). Since bottom topography tends to be fairly uniform, the fauna also tends to be consistent. Small islands do not materially affect the species content of open interior sediments, but patch reefs along the outer parts of the environment include some margin fauna. In terms of test shape, the open interior is characterized by flat, multichambered species (soritids); elongate, fusiform miliolids; elongate, arenaceous species; and thin-walled rotalid forms.

Restricted Interior Fauna. - The restricted interior fauna grades into that of the open interior; the abruptness of the transition depends upon the type of barrier responsible for restriction of circulation (Fig. 3). If complete physical barriers, such as rocky islands (i.e., the Florida Keys) are involved, the boundary is sharp. If submerged sills or mudbanks produce the barrier, the boundary may be a gradation within narrow limits. If, however, the barrier is simply a broad expanse of calm, shallow water (i.e., western Florida Bay), the two environments will merge almost imperceptibly.

Miliolids dominate populations of the restricted interior, constituting 30 to 55% of the fauna (Figs. 2, 3). Soritidae compose about 25%, and Elphidiidae are present in frequencies of about 20%. Only the infrequent species Trochammina inflata and Trichohyalus aguayoi are limited to the restricted interior, but Quinqueloculina tenagos, Miliolinella circularis, and Triloculina rotunda reach their greatest abundance here (Fig. 6). Species from the adjacent open interior environment, such as Borelis pulchra, Rosalina beccarii ornata, and those belonging to Articulina, are present in low frequencies.

The eastern bay is usually defined (Ginsburg, 1956; Wanless and Tagett, 1989) as "starved" due to low biotic productivity resulting from shallow depths, isolation from open-marine water, and lack of nutrients in the fresh waters that flow from the mainland into northeast Florida Bay (Montague et al., 1989). Its fauna can be separated into three gradational subfaunas (Fig. 3). Most of the 'index species' are not restricted to one subenvironment but serve as indicators by relative frequency of occurrence. (1) The proximity of an island may be detected in the molluscan and foraminiferal assemblages (Ginsburg, 1964; Turney and Perkins, 1972). Trichohyalus aguayoi and Trochammina inflata occur only in the immediate vicinity of small islands. The striate miliolid Quingueloculina tenagos is abundant around such islands but also occurs frequently throughout the restricted interior environment. These three species constitute the "nearshore fauna." (2) On the shoal mudbanks, the rotund miliolids Triloculina rotunda and Miliolinella circularis are concentrated, forming the "mudbank fauna." Also abundant is Elphidium discoidale. (3) In the basins between mudbanks, Quingueloculina agglutinans, Q. lamarckiana, and Valvulina oviedoiana tend to congregate. These species make up the "lake fauna." These three subfaunas are most clearly distinguished where bottom topography is clear-cut and distinct. As the bay shoals to the northeast with less relief between banks and basins, the faunas begin to merge. To the west, mudbanks become less well defined topographically in a broad expanse of shallow water. The basins and banks are carpeted by dense grass and have a relatively large and varied biota, and again the distinctions between faunas become more subtle.

Shapes of dominant species of the restricted interior are rotund miliolids, elongate fusiform miliolids, flat multichambered species (Archaias spp.), rounded and ornamented (beaded, frosted, and so on) rotaliform species (Trichohyalus, Trochammina, and Ammonia spp.), and rounded, thin-walled, planispiral species (Elphidium spp.).

Brackish Interior Fauna. - The brackish interior fauna is the most distinctive of the platform interior faunas and is found in mangrove swamps and small bays immediately adjacent to the Florida mainland, where subject to freshwater Everglades runoff, as well as in the interiors of many low islands during periods of rainfall. The bays and island interiors are subject to pronounced freshwater runoff during the wet season and high evaporation during the dry season; therefore, the fauna is controlled predominantly by salinity. Even at its outer limit, where its fauna grades into other faunas, the brackish influence can still be recognized. Brackish populations are dominated by Ammonia beccarii ornata, which constitutes 20 to 60% of the species, and Elphidium discoidale, present in frequencies of 15 to 40% (Figs. 2, 3). Soritids, so prevalent in other parts of the interior, are sparse or absent, and of the miliolids, only the inflated species Quingueloculina tenagos, Miliolinella circularis, and Triloculina rotunda are occasionally present (2-14%). Sediments farther inland contain only species of Ammonia and Elphidium. Although species of Ammonia and Elphidium are present in other groups, they never occur in large percentages other than in the brackish-water areas, where they may be the only species present. The brackish fauna tends to contain few taxa but large numbers of individuals. Typical test shapes of dominant forms are rotund miliolids (when present), rounded and ornamented rotaliform species (Ammonia beccarri), and rounded, thin-walled, planispiral species (Elphidium discoidale).