Patuxent River Basin
In the 1999-2003 assessment for the Tributary Teams,
benthic community condition in the Patuxent River was
best in the oligohaline portion of the river and worst in the tidal fresh and
mesohaline regions (Table 1). Benthic degradation in
the mesohaline region appears to be related to adverse effects from low dissolved
oxygen. The intensity of summer hypoxic events varies annually, and this
variability is reflected in the B-IBI, as suggested by a positive relationship
between
the percentage of samples failing the Restoration Goals (B-IBI < 3) and the
frequency of low dissolved oxygen observations (DO < 2 mg/L; see
Figure 1). Degradation was due primarily to low
abundance, biomass, and species diversity, and to low biomass of species
sensitive to pollution, which are typically representative of mature communities
in the absence of low dissolved oxygen stress. One factor linked to hypoxia
is the amount of decaying organic matter from algal blooms. Large algal
concentrations are likely to result in more extensive hypoxia and increased
benthic degradation. Consistent with the poor water clarity and the high
algal concentrations measured by the Water Quality Monitoring Program, the
lower Patuxent River shows a positive relationship between
the percentage of samples with severely degraded condition and average
chlorophyll concentrations below the pycnocline(Figure 2).
Benthic community status at Patuxent River fixed monitoring stations showed
degraded conditions at Lyons Creek (tidal fresh) and Broomes Island in 2003,
and good condition at Holland Cliff and Chalk Point (stations 079, 071, 077,
and 074, on map). At Holland Cliff, the magnitude of a
degrading trend in the B-IBI
continued to diminish with the 2003 data, suggesting recovery at this
station. Chalk Point is under the thermal influence of the Chalk Point
power plant. However, no significant impacts on benthos from the thermal
discharge have been documented to date. Likewise, an oil spill in nearby
Swanson Creek in April 2000 did not show impacts at the Chalk Point fixed station.
(Llansó and Volstad, 2001)
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Water Quality Map
Segment Analysis
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Benthic community condition for Patuxent River Basin sites.
All sites sampled to date (1994-2003) shown.
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