Intensive Survey of the Guadalupe River Segment 1806

mayflies were well represented (11 species; 9.6% of the community). The
standing crop was large and reflected an abundant food supply. Filterers of
fine particulate organic matter (FPOM) were the dominant functional feeding
group (Table 12), indicating an abundance of suspended FPOM, probably
mostly in the form of phytoplankton produced upstream in Ingram Lake. The
three groups that utilize suspended or sedimented FPOM (gatherers, filterers,
miners) cumulatively comprised 75.0 percent of the community, reflecting the
importance of FPOM as the primary food material at the site.
At Old Medina Road in Kerrville, 1806.0237 (Station F), diversity and
equitability were higher than upstream, and pollution-sensitive mayflies were
fairly well represented (6 species; 11.7% of the community), indicating
relatively stable environmental conditions. However, species richness and
standing crop were appreciably lower than at any other riverine station,
possibly due to the effects of siltation observed at the site. Urban runoff
from Kerrville is the most likely contributor of silt, since there are no point
source discharges upstream. Compared to upstream, filter feeders were less
significant, while miners increased in importance, further reflecting increased
sedimentation of FPOM. However, in absolute terms miners were not
especially abundant, and all users of FPOM cumulatively decreased in
significance, indicating that much of the silt was inorganic in nature, as
might be expected if urban runoff were the major contributor.
The headwaters of Flat Rock Lake, 1806.0233 (Station G), exhibited extra
ordinarily high species richness, diversity, and equitability compared to
previous studies on small Texas reservoirs, and mayflies were well
represented (3 species; 39.5% of the community), indicating stable environ
mental conditions. FPOM feeders were predominant (76.8% of the community),
as might be expected in a lentic, depositional habitat of this type.
Slightly less favorable environmental conditions were evident in Flat Rock
Lake 200 m below Third Creek confluence, 1806.0228 (Station H), where
species richness declined and sensitive mayflies were not as common (1
species; 12.3% of the community). Hov/ever, diversity and equitability
remained high and indicated generally stable environmental conditions. The
standing crop was considerably smaller and FPOM feeders decreased in
importance (61.5% of the community), indicating a longitudinal decrease in the
amount of organic matter available as food material, and that no appreciable
degree of organic enrichment results from Third Creek inflow.
Immediately above Flat Pock Dam, 1806.0227 (Station I), diversity and
equitability were low due to the abundance of two predatory species
(Chaoborus punctipennis, which feeds on zooplankton; Coelotanypus
scapularis, which was probably feeding on Chaoborus). Mayflies were
absent. However, species richness was comparable to the mid-lake station,
and diversity and equitability were relatively high when Chaoborus and
Coelotanypus were excluded from the calculations (2.87 and 0.83,
respectively). Chaoborus and Coelotanypus are not normally associated with
organic pollution, and their abundance and resultant depressed diversity were
not considered indicative of water quality degradation. Also, there was no
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