Potomac River Basin
For the period 1999-2003, benthic community condition was poor throughout the
Potomac River estuary, but more so in the lower mesohaline than in the upper
regions (Table 1). The probability of observing
degraded benthos in the mesohaline region was 71% with good confidence. In
the oligohaline and tidal fresh portions of the river, probabilities were 36%
and 47%, although for a majority of samples benthic condition was classified
as indeterminate (Table 1).
Much of the problem in the Potomac River is
severe oxygen depletion in the lower deep mainstem. Unlike the Patuxent
River, no significant relationship is observed when the percent samples failing
the Restoration Goals is plotted against the frequency of low dissolved oxygen
observations below 2 mg/L. This is because hypoxia in the Potomac River
is a perennial problem that affects waters below the pycnocline, with little
inter-annual variability. Relationships between the B-IBI and dissolved
oxygen in the Potomac River are best explored as a function of depth. The
frequency of low dissolved oxygen events in the Potomac River is strongly
associated with water depth, and so is the probability of observing severely
degraded benthos (Llansó et al. 2003).
Of the seven fixed monitoring stations in the Potomac River (stations 036, 040,
043, 044, 047, 051, 052 on map), only two showed significant trends in the B-IBI
in 2003. Station 44 at Morgantown exhibited a degrading trend, and Station
51 in shallow water near St. Clements Island exhibited an improving
trend. Degrading trends in abundance, biomass, and diversity contributed
to the observed B-IBI trend at Station 44. This station is on the slope
of the deep channel of the Potomac River and may be affected by tilts of the
pycnocline bringing episodic fluctuations in dissolved oxygen and salinity,
which are likely to exert stress on the benthic community during the
reproductive season. The improving trend at Station 51 is due to significant
increases in diversity and the abundance of carnivore and omnivores and species
sensitive to pollution. These changes may be indicative of improving water
quality conditions in the shallow flanks of the lower Potomac River.
We're working on the following...Please visit again to see the results
Water Quality Map
Segment Analysis
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Benthic community condition for Potomac River Basin sites.
Red = Severely Degraded; Black = Degraded;
Violet = Marginal; Green = Meets Goals
All sites sampled to date (1994-2003) shown.
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