NUTRIENT MONITORING IN THE WATER COLUMN.
Much of the VCR/LTER site is covered by water, and the primary cause for
progressive state change in the system is rising sea level and the impacts
of increased inundation. Because of the dominance of estuarine waters in
our system and the commitment to intersystem studies, we have monitored
dissolved nutrient concentrations and other water quality indices within
the VCR/LTER. This is our preliminary evaluation of results from summer
1992 until the end of 1993.
Monthly samples were taken from ten sites representing a variety of
environments: mouths and heads of tidal creeks from both mainland and
island sides of the lagoon, different locations within the lagoon, and a
nearby culturally eutrophied enbayment. At each site hydrological data
were collected, and water returned to the laboratory for analysis of
ammonium, nitrate plus nitrite, filterable reactive phosphorus, total
suspended solids, chlorophyll a, and biochemical oxygen demand.
Temporal trends have begun to become evident for nutrient concentrations.
The strongest trend is for filterable reactive phosphorus
(Fig.1).
Concentrations tend to be highest in warmer months and least in colder
months. Ammonium 
(Fig.2) and nitrate plus nitrite
(Fig.3) show greater
nonseasonal variation. Both have distinct peaks and nadirs throughout the
year that may be related to weather patterns and storms. Interestingly,
the peaks for ammonium often are offset from those of nitrate plus nitrite.
Thus the mechanisms responsible for the rises and falls in concentration
appear to be different for the oxidized and reduced nitrogen species.
Spatial patterns have also begun to emerge. The concentrations and
patterns of nutrients for stations near Hog Island are different from those
for the sites nearest the mainland. "Island sites" have somewhat lower
concentrations, presumably reflecting the importance of the more oligo-
trophic ocean input and/or lower relative contribution from barrier island
uplands. Concentrations in both areas tend to be less than 10 uM for each
nutrient. Filterable reactive phosphorus is most often less than 1 uM
(Fig.1), and both ammonium
(Fig.2) and nitrate plus nitrite
(Fig.3) are generally less than 5 uM
except during peaks.
An interesting pattern of difference has emerged for nitrate peaks at the
mainland and island stations: peak concentrations were found during the
winter for the two groups of stations but in different months (Fig.3).
Moreover, high nitrate concentrations are correlated with low salinity
registered at the same date on the mainland side
(Fig.4), but not on the
island. Here, nitrate plus nitrite peaks seem to occur in consort with
high turbidity (Fig.5) and relatively high
salinity.
The eutophied embayment of Oyster Harbor often has the highest concen-
trations for nitrogen species. Extremely high peaks of nitrate plus
nitrite are noted here. Filterable reactive phosphorus is comparable to
the less impacted sites. Interestingly, the temporal patterns of filter-
able reactive phosphorus and nitrate plus nitrite at this site are very
similar to those at the mouth of Phillips Creek taken at the most similar
time of day.
Overall, nutrients within the LTER water column are as one might expect
for a light-limited lagoonal system relatively unimpacted by man. The
cause of light limitation through high turbidity may be the result of man's
activities; but culturally eutrophication, as increased nutrient concen-
trations, appears minimal. Concentrations are high relative to clearer
water systems, especially at comparable salinities. But compared to other
temperate estuaries these concentrations are low.
(Fig.1).
Concentrations tend to be highest in warmer months and least in colder
months. Ammonium 
(Fig.2) and nitrate plus nitrite
(Fig.3) show greater
nonseasonal variation. Both have distinct peaks and nadirs throughout the
year that may be related to weather patterns and storms. Interestingly,
the peaks for ammonium often are offset from those of nitrate plus nitrite.
Thus the mechanisms responsible for the rises and falls in concentration
appear to be different for the oxidized and reduced nitrogen species.
(Fig.4)