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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Benthic community condition for Patuxent River Basin sites.
All sites sampled to date (1994-2003) shown.

Figure 1.  Relationship of benthic index of biotic integrity to percent dissolved oxygen observations below 2 mg/L (June-September) in the mesohaline Patuxent River.  Each point represents a different year, 1995-2003.  Dissolved oxygen data are fortnightly near-bottom observations from Chesapeake Bay Water Quality Monitoring Program stations RET1.1 and LE1.1 through LE1.4.

Figure 2.  Relationship of benthic index of biotic integrity to average Chlorophyll a concentration in the mesohaline Patuxent River.  Each point represents a different year, 1995-2003.  Chlorophyll data are below pycnocline, April through June fornightly observations from Chesapeake Bay Water Quality Monitoring Program Stations RET1.1 and LE1.1 through LE1.4.