Cassondra Thomas James Spittler Linda Blum Rebecca Tarnowski Robert Christian

Since the summer of 1992, monthly water quality samples have been collected from ten sites around the VCR/LTER. These sites represent a variety of environments including the mouths and heads of tidal creeks on the mainland and Hog Island, Quinby Inlet and Oyster Harbor. Oyster Harbor is anthropogenically eutrophic. Water samples are analyzed for ammonium, phosphate, nitrate and nitrite, total suspended solids, and chlorophyll a. Hydrological data are also collected at each site. The following is an evaluation of results from July 1992 through February 1997. The data points represent the averages from four mainland sites, three Hog Island sites, one Quinby Inlet site and one Oyster Harbor site.

Phosphate shows a distinct seasonal trend with phosphate increasing during the warmer months and decreasing during the colder months (Fig 1). The peaks also appear to increase in height over the years, but no statistical tests have been run to verify this. Ammonium and nitrate/nitrite do not appear to fluctuate seasonally (Fig 2, 3a & b). Ammonium appears to fluctuate at a greater frequency than nitrate/nitrite. This suggests that the mechanisms responsible for the variations in concentrations of the two nitrogen species are different.

Spatial patterns can also be detected from the data. The mainland creeks and Oyster Harbor tend to have higher concentrations of nitrate/nitrite and ammonium than do the Island creeks and Quinby Inlet (Fig 1, 2, 3a & b). This may reflect the ocean’s more oligotrophic input and smaller nutrient contributions of barrier island uplands. Phosphate did not show any distinct spatial pattern. However, there is an unusual peak for mainland and island creeks in July 1994.

Oyster Harbor on average had the highest concentrations of both ammonium and nitrate/nitrite. Three peaks exceeded 60 M nitrate/nitrate while the average for the other sites hovered around 1.7 M nitrate/nitrite. The highest peak for Oyster Harbor may correspond to an agricultural fertilization event. Nitrate/nitrite concentrations seem to fluctuate more widely in the mainland creeks than the Island creeks or Quinby Inlet. This may be a reflection of surface and groundwater runoff from agricultural uplands. The average ammonium concentration at Oyster Harbor was just slightly higher than the mainland creeks, but were notably higher that the Island creeks and Quinby Inlet. Phosphate was highest in the Island creeks but not by any notable amount.

These results are very similar to the trends reported by Castaldelli et al. in 1994 (www.vcrlter.virginia.edu/1994ASC/Christian.html) for the same sites. The data are typical of temperate light limited lagoonal systems that have minimal anthropogenic influences. One possible trend in the data that needs further investigation is the increase in phosphate over time.