Comparisons among Springs

       The springs sampled during this study are representative of the wide range of characteristics exhibited by springs in central Florida. This diversity reinforces the conclusion that study of individual springs is desirable to provide water managers with adequate data to support their decisions.

Figure 30. Relative proportions of cations and anions in spring-water samples, 2004.

Hydrology and Water Chemistry

       The springs studied range from the first-magnitude Silver Springs group, with an average discharge of about 780 ft³/s to Green Spring, a third magnitude spring, with an average discharge of about 2 ft³/s. Silver Springs has the least disturbed natural flow system, whereas the spring runs at De Leon Spring, Gemini Springs, and Green Spring have been modified by water-control structures.

       Differences in water chemistry among the springs sampled reflect local differences in water chemistry in the Upper Floridan aquifer. Total dissolved solids concentrations ranged from about 270 mg/L at the Silver Springs group and 450 mg/L at De Leon Spring to about 1,500 to 1,550 mg/L at Gemini Springs and Green Spring. The three springs sampled in the Silver Springs group (the Main Spring, Blue Grotto, and Abyss) have similar proportions of cations and anions (fig. 30). The water from Gemini Springs and Green Spring has higher proportions of sodium and chloride. The water from De Leon Spring has higher proportions of sodium and chloride than water from Silver Springs, but lower proportions of calcium and bicarbonate than Silver Springs.

       All of the springs studied are affected by human activities, based on the presence of organic compounds found in wastewater in all the spring-water samples, as well as qualitative observations of algae and periphyton. The most commonly detected compound was DEET, which was found in all the springs sampled except De Leon Spring. The pesticide atrazine and its degradate CIAT were detected in water from Silver Springs and in both vents at Gemini Springs. No pesticides were detected in water samples from De Leon Spring and Green Spring. Levels of wastewater compounds and pesticides were low and likely not high enough to cause direct harm to organisms (U.S. Environmental Protection Agency, 2004), but little information exists about possible effects of mixtures of various chemicals.

Aquatic Communities

       Available habitats for macroinvertebrates varied greatly from site to site. Green Spring run contained nearly all leaf pack and snag material with a thin band of water pennywort (Hydrocotyle sp.) and an abundance of detritus. Gemini Springs contained areas of bare sand, muck, extensive beds of algae, and submersed and emergent vegetation along the margins of the run. De Leon Spring run contained an abundance of submersed and emergent vegetation and an artificial waterfall at the outflow of the spring pool that was covered with algae.

       The macroinvertebrate data indicated some differences among De Leon Spring, Gemini Springs, and Green Spring. None of the five odonate taxa collected at De Leon Spring were collected at either Gemini Springs or Green Spring (table 5). One species each of an odonate (Telebasis byersi) and a trichopteran (Neotrichia sp.) was collected from Gemini Springs, but not from the other springs. No plecopteran taxa were collected during this study. Of the three springs sampled, De Leon Spring had the highest overall species richness, EPTr, and ETOr, and had the most disturbance-intolerant assemblage (FI = 4) (table 6). Gemini Springs had high relative abundances of oligochaetes, chironomids, amphipods, hydrobiids, and ostracods (fig. 29). The impounded pool and presence of zooplankton, phytoplankton, and attached algae at Gemini Springs are somewhat indicative of eutrophic conditions in comparison to most relatively oligotrophic spring runs in close proximity to source vents. It is also notable that during visits to Gemini Springs in 2004, the impounded pool was closed to swimming and a sign was posted warning of high coliform bacteria levels in the water. Green Spring had the lowest species richness, no EPT or ETO taxa, and a community dominated by three organisms (collectively comprising 88 percent of all animals collected): oligochaetes, amphipods, and chironomids. The unusual water chemistry of Green Spring and the spring morphology (deep vent and shallow, short run modified by an impoundment) are hypothesized to be limiting factors in terms of richness and abundance of the benthic fauna.

       Amphipods are commonly associated with karst habitats of Florida and may often occur in great abundance (Woodruff, 1993; Mattson and others, 1995; Walsh, 2001). In De Leon Spring and Gemini Springs, amphipods made up the largest percentage of any invertebrate group collected (fig. 31) and were consistently dominant in all months of the study except for a shift in dipteran abundance at Gemini Springs in spring (May) (tables 6 and 8). At De Leon Spring, both amphipod species peaked in abundance in summer (August), but the lowest abundance of Gammarus sp. occurred in winter (February) whereas Hyalella azteca was least abundant in spring (fig. 32). Only one amphipod species (H. azteca) was identified in samples from Green Spring.

Figure 31. Total number of all amphipods collected by season in De Leon Spring, Gemini Springs, and Green Spring, 2004.

Figure 32. Total number of Gammarus sp. and Hyallela azteca by season at De Leon Spring, 2004.

       Sample sizes of invertebrates collected in this study were too limited to allow for detailed quantitative or statistical analysis of trends or correlations with water-quality properties; however, some qualitative observations suggested possible associations between biological metrics and water chemistry. Species richness appeared to be negatively related to magnesium, potassium, sodium, and specific conductance. More invertebrates were collected when DO and nitrate-N were high but phosphorus and potassium concentrations were low. The relative abundance of dipterans was positively associated with specific conductance and TOC but negatively associated with nitrate-N.

       Invertebrate assemblages observed in springs of the St. Johns River were consistent with communities that have been studied in other north-central springs and spring-fed streams or rivers. Beck (1965) characterized benthic communities of calcareous streams in Florida as consisting primarily of mollusks, chironomids, ephemeropterans (e.g., baetids and heptageniids), trichopterans (e.g., Cheumatopsyche), and crustaceans (e.g., Palaemonetes paludosus and Hyalella azteca). Woodruff (1993) found that benthic sediments of Manatee Spring (lower Suwannee River drainage) had a community dominated by oligochaetes, amphipods, leeches, and isopods. The study by Mattson and others (1995) provided the most detailed tabulation of aquatic invertebrate communities in Florida karst habitats. Mattson and others (1995) noted that species richness and/or relative abundance generally increased in association with hard, alkaline spring water. Moreover, substrata of limestone outcrops, wood, and submerged macrophytes provide important habitat for benthic invertebrates in these systems. In the present study, sampling of rock and wood habitats was not feasible; thus, it is likely that species richness of the aquatic communities at De Leon, Gemini, and Green Springs is greater than reported herein.

       Fish assemblages in Silver Springs and De Leon Spring had the greatest species richness of the springs surveyed. Centrarchids and poeciliids generally were in greatest abundance during surveys by USGS personnel (Walsh and Williams 2003) (tables 3, 7). Silver Springs had centrarchids and poeciliids in greatest abundance (32.1 percent and 22.8 percent composition, respectively), followed by cyprinids (16.1 percent) and fundulids (13.0 percent) (table 3). Centrarchids (56 percent) dominated in De Leon Spring and were followed by poeciliids and fundulids in numerical abundance (15.8 percent) (table 7). Fish collections at Gemini Springs and Green Spring were limited to the use of crayfish traps, minnow traps, and visual surveys. Taxonomic composition of the fishes in Gemini Springs was similar to the fauna of De Leon Spring. The fish assemblage at Green Spring was dominated by poeciliids (possibly the only family represented), presumably because of the water chemistry (low DO and presence of H₂S), small size, and isolation from Lake Monroe. The presence of nonindigenous fishes in three of the four springs surveyed is of particular concern. Three nonindigenous fish species were observed in Gemini Springs, two in De Leon Spring (and a third reported by Davis and Herring, 2005), and one in Silver Springs.