The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
The FWC using a modified stratified sampling design to allocate samples in the two
dominant benthic habitats in the spring run; bare sediments and filamentous algae.
Although historic quantitative data are not available, it appeared that filamentous algal
mats first became common in the spring run during the winter and spring of 2000, when
withdrawals reduced spring flow to zero or low rates of flow for successive months in
the dry season. These withdrawals greatly reduced current velocities and allowed water
from the Hillsborough River to back into the spring run. Algae coverage averaged 38
percent bottom coverage at sites visited in 2000 by Allen et al., with no bare sediments
reported in the most downstream sampling zone. However, the return of normal flows
has reduced the abundance of filamentous algae in the spring. Benthic algal coverage
during December 2003 reported by the University of Florida averaged 6 percent, with
coverage exceeding 20 percent at only two of the twenty sites sampled.
Abundance values from the three quantitative collections are summarized in Appendix
E. Tarebia granifera was by far the most abundant species in all habitats sampled in the
2000 collection, accounting for 84.4 percent of all organisms in the quantitative
samples. Nematodes and the crownsnail Pyrgophorus platyrachis were the second and
third most abundant taxa. Aside from T. granifera, Nematoda, and P. platyrachis, no
other invertebrate taxon accounted for more than two percent of the total organisms in
any sample. Pyrgophorus platyrachis was the only taxon that had a statistically
significant difference in abundance between habitats, being abundant in algal mats than
on bare sediment.
Based on evenness (Pielou 1969) and diversity (Krebs 1999) values calculated for the
combined habitats, the FWC concluded that the invertebrate community in 2000 was
characterized by low species richness and extreme dominance by one species. Even
when the qualitative collections were included, the FWC noted (in Allen et al. 2001) that
the species composition in 2000 was very different from that reported by the District and
FDEP in 1997. The FWC also reported that that invertebrate community evaluations of
other coastal spring runs on Florida's west coast (Homosassa and Weeki Wachee)
were indicative of more evenly distributed populations and the presence of many more
euryhaline species (Sloan 1954; 1956). Data from low salinity zones of the Weeki
Wachee and Crystal Rivers sampled by Mote Marine Laboratory (Culter 1996) also
support this statement.
Compared to the 2000 collection, quantitative sampling in November 2001 found
marked changes in the abundance of a number of taxa (Appendix E). Although
differences in abundance between these collections have not been statistically tested,
some changes seem apparent. The mean density of oligochaetes in combined habitats
increased by over a factor of eight between the two periods, from 1,172 to 9,779
number per square meter (/m 2 ), with large increases for members of both the families
Naididae and Tubificidae. The total number of mollusks decreased between 2000 and
2001, due largely to decreases in Tarebia granifera. Tarebia decreased from 47,839 to
8,766 numbers/m 2 in the combined habitats. In contrast, the crownsnail Pyrgophorus
platyachis increased by over a factor of six, from 3,058 to 19,458 number/m 2 .
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