NEW: Annual Report - George Mason University

Isopods (#/m2)
100
Station 9
10
1980 1985 1990 1995 2000 2005
Year
Figure 188. Long term trends in Benthos: Isopods. River mainstem.
H. Synthesis to Date
139
Isopods, also called
aquatic sow bugs, are
crustaceans which are
about 5-10 mm long.
They have never been
found in the cove so no
graph is presented. Like
amphipods, their absence
in the cove is probably
due to the homogeneous
substrate. Their densities
have gradually increased
during the study to current
levels of about 300/m 2 .
Important trends in water quality and biotic variables for those subjected to both linear
regression and Lowess analysis are summarized in Table 21. These results point to a
general improvement in water quality in the cove and some improvements in the river
mainstem. Plankton variables also show a shift to a more desirable state with a decrease
in phytoplankton and an increase in zooplankton in the cove. The river exhibited less of a
change.
Water quality and aquatic biota have been measured in Gunston Cove using a generally
consistent monitoring protocol since the early 1980's. Major changes in the protocol
include: (1) decrease in the number of stations monitored for water quality and plankton,
(2) increased emphasis on biweekly/semimonthly sampling, (3) change in method for
zooplankton. The current sampling protocol for plankton utilizes only two stations which
are monitored on a biweekly/semimonthly basis. This sampling regime was based on the
similarities observed between stations in the cove on any given date and the differences
observed, especially in plankton, between monthly sampling dates. Three stations have
been maintained for both trawl and seine sampling given the variability that is found
between stations and the importance of fish to the management of the ecosystem. Since
water quality, plankton, and fish sampling have been maintained at a consistent core of
sampling sites using consistent methods, long term trends can be assessed over the entire
sampling period. The exception to this is zooplankton which was originally sampled only
by sieving 96 L with a 73 µm mesh net. This protocol was changed in 1990 to one in
which a 202 µm tow net sample was collected for larger zooplankton (macrozooplankton)
and a 44 µm sieve was instituted for smaller zooplankton (microzooplankton). This made