Development of Water-Management System and Impact on the Hydrology of Southeastern Florida : Changes in the Marine Ecosystem of Biscayne Bay

Shallow-water marine organisms are very sensitive to environmental changes in salinity, temperature, nutrient input, and dissolved oxygen. Temporal and spatial salinity patterns in Biscayne Bay have profoundly affected the marine ecosystem caused by water-management driven changes in surface- and ground-water discharge. In addition to those changes caused by natural events, long-term change in land and water uses during the 20th century in the bay watershed contributed greatly to the deterioration of marine conditions. Water quality has been greatly degraded by increased nutrient loads, trace metals, and other pollutants. Dredging and artificial channels that were cut within the bay affected the water quality by removing natural seagrass beds and increasing turbidity (Wanless and others, 1984).

Although predevelopment spring discharge diminished considerably from pre-1907 flow conditions, Kohout and Kolipinski (1967) determined that the effect of coastal ground-water discharge on bay salinity was apparent in the broad zone of diffusion where chloride levels ranged from slightly saline (400 mg/L) near the shoreline to concentrations that were equivalent to native seawater farther offshore. According to Kohout and Kolipinski (1967), the upward movement of ground water in the littoral zone was manifest as salinity-controlled biological zonation of fauna and flora (fig. 58). On a more localized scale, Kohout and Kolipinski (1967, p. 491) observed that brackish, shallow water, nearshore spring discharge in Biscayne Bay was characterized by subcircular “ponds,” formed by the restricted growth of seagrass sensitive to changes in salinity.

Figure 58. Distribution of selected fauna related to underlying hydrologic factors in the Cutler Ridge area. From Kohout and Kolipinski (1967). Impact of Anthropogenic Development on Coastal Ground-Water Hydrology in Southeastern Florida, 1900-2000 [larger image]

Sediment core profiling in 150 years of deposition in Biscayne Bay has revealed distinct changes in the marine ecosystem, which was different in the early to mid-1850s than today. The salinity of bay coastal areas was much lower than the normal marine salinities of modern times (fig. 59). The water in Manatee Bay at the extreme southern end of Biscayne Bay was very nearly fresh. The central bay also was lower in salinity, but cores indicate lower magnitudes of change from normal marine salinity. Faunal evidence indicates possible lowered oxygen or higher organic concentrations in the surficial sediments by the late 1800s, which may have had a substantial effect on the abundance or density of seagrass present during that time.

Figure 59. Salinity estimates in Biscayne Bay during (A) 1850-1900 and (B) August 1996. Modified from Ishman and others (1998). [larger image]

Sediment from the early 1900s reflect the first marked change in salinity, which reached levels close to normal seawater. This increase is reflected by a substantial increase in the abundance of typical Atlantic Continental Shelf faunal components. Fossil evidence indicates that seagrass became more abundant from the early 1900s to present in the central bay (fig. 60). A similar record of ecosystem change is recorded in Manatee Bay (fig. 59), with a substantial change in salinity to mesohaline (5 to 18 parts per thousand) conditions occurring in the early 1900s (Ishman and others, 1998). Coastal vegetation changes during this time, such as the first appearance of red mangrove in this region, are consistent with marine records and indicate increased salinity (Ishman and others, 1998).

Figure 60. Relative abundance of three benthic foraminifera (Articulina mucronata, Archaias angulatus, and Miliolinella circularis) in cores FB1 and MB1 from Biscayne Bay. Significant shifts in their relative abundance in the cores are highlighted in blue to illustrate major changes in salinity and seagrass abundance/density from the late 1800s to present. Location of cores FB1 and MB1 are shown in figure 59. Modified from Ishman and others (1998). [larger image]

Biscayne Bay salinity remained relatively stable until the early 1940s when it increased from about 18 to 40 parts per thousand on average, but was probably subject to broad fluctuations. The occurrence and increasing abundance of epiphytal and macroalgal habitat dwelling organisms indicate a change in substrate conditions and increased seagrass abundance during this time (Ishman and others, 1998). These latter changes appear to coincide with the construction of an enhanced canal, levee, and water-conservation area system designed to better manage water resources in the urban corridor and the Everglades. From the late 1980s to present, salinity decreased slightly in Manatee Bay, and field observations in this region suggest deteriorating conditions in the health of the seagrass (Ishman and others, 1998).

The freshwater-saltwater interface in southernmost Miami-Dade County is well inland of Florida Bay (fig. 50), according to Fitterman and Deszcz-Pan (1999), and ground-water discharge to overlying surface flows is considered to be relatively small compared to surface-water outflows (U.S. Army Corps of Engineers and South Florida Water Management District, 1999). The lowering of the water table associated with the construction of an extensive drainage network helped to reduce surface flow in the lower part of the Everglades as experienced along the southeastern coast; such reductions in flow contributed to, and appeared to be closely timed with, the increase in salinity in Florida Bay (Halley and others, 1998).