The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
The HBMP project (PBSJ 2003) mapped the linear and areal extent of various
vegetation communities along the lower river. The lower river contained approximately
2.03 hectares of emergent wetlands vegetation, with areas of the most wetlands
occurring between kms 6 and 8.5 and upstream of Sulphur Springs. Nine vegetation
associations were identified, which were aggregated to seven groups for purposes of
analysis. Leatherfern and mixed herbaceous wetlands were the dominant groups
immediately below the dam. Much or the area from km 12 to 15 has sharply incised
banks that do not support much vegetation, although leatherfern was found there.
Cattails spanned much of the river, comprising much of the emergent vegetation from
km 4 to the region below the dam.
3.10.2 Benthic macroinvertebrates
Both the WAR/SDI and HBMP studies found spatial and temporal patterns in the
species composition of macroinvertebrate communities that corresponded to salinity
gradients in the lower river. Both studies found freshwater and estuarine species in the
lower river. As expected, freshwater species were most abundant in the upper reaches
of the river and increased their distribution after prolonged flow events. Using a fixed
station design in which the same sites in the river were samples repeatedly, WAR/SDI
(1995) reported that freshwater species were periodically found at stations 3 and 5, the
latter of which is located about 3 km downstream of Sulphur Springs. During periods of
no discharge from the dam, these sites were increasingly colonized by estuarine
species.
WAR/SDI also pointed out that there were frequently low bottom dissolved oxygen
concentrations at sites 3 and 5 during periods of no discharge from the dam, which
contributed to low abundances of invertebrates in deep mid-channel areas. The
pronounced density stratification that occurs in the upper river during periods of no flow
from the dam is a major factor contributing to this hypoxia. Benthic macroinvertebrates
were collected in shallow waters at sites 3 and 5 in the second year of the WAR/SDI
study and much greater invertebrate abundances were found, apparently due to
improved dissolved oxygen concentrations at the shallower depths.
Using a probabilistic design, PBSJ (2003) presented macroinvertebrate data for upper
and lower sections of the river, including graphics for various parameters in one
kilometer intervals. Several taxa showed gradients in their distribution in the river, either
increasing in an upstream or downstream direction, or with peak abundances in the
middle reaches of the river. There was a disproportionate number of species which
showed either their first or last occurrence in the most downstream 3 kilometers in the
river, indicative of a progression to a more saline, bay-like fauna in that reach of the
river. Abundances of more freshwater organisms (e.g. chironomids) increased toward
the dam. Like WAR/SDI, PBSJ found a zone of minimum organism abundance and
diversity just downstream of Sulphur Springs near kilometers 10 and 11, with hypoxia
being the likely causative factor.
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