Chemistry of the Offshore Surface Waters of Lake Erie: Pre- and Post-Dreissena Introduction (1983–1993)

Major changes in ambient surface nutrient chemistry were observed after the introduction of Dreissena to Lake Erie. For example, statistically significant increases in spring soluble reactive phosphorus (SRP) (180%, 1.0 to 2.8 μg P/L), nitrate+nitrite (40%, 0.57 to 0.80 mg N/L), ammonia (131%, 15.1 to 34.9 μg N/L), silica (75%, 0.8 to 1.4 mg/L), N:P ratio and turbidity and a significant decrease in total Kjeldahl nitrogen (TKN) (25%, 0.24 to 0.18 μg N/L) were observed in the western basin from the 1983 to 1987 pre-Dreissena baseline period to the 1989 to 1993 Post-Dreissena period. In the summer, total phosphorus (TP) (13%, 20.1 to 17.5 μg P/L) and TKN (27%, 0.30 to 0.22 μg N/L) decreased, while nitrate+nitrite (122%, 0.18 to 0.40 mg N/L) and the N:P ratio increased significantly. Fewer chemical parameters changed significantly in the central and eastern basins, but major changes were observed. For example, spring SRP concentrations in the central and eastern basins increased 250% (0.8 to 2.8 μg P/L) and 92% (2.4 to 4.6 μg P/L), respectively. Silica in these basins increased 300% (0.1 to 0.4 mg/L) and 250% (0.2 to 0.7 mg/L), respectively. TKN decreased in all basins in both the spring and summer (range = 22 to 27%), while TP decreased in all basins in the summer (range = 13 to 24%) but not in the spring. Spatially, spring Post-Dreissena (1989 to 1993) ammonia, TP, and nitrate+nitrite concentrations were high in the western basin and decreased easterly, while chloride concentrations were variable with no downward or upward trend. In the central basin and eastward through the eastern basin, concentrations of ammonia, chloride, nitrate+nitrite, and total phosphorus were remarkably consistent during and between the pre- and Post-Dreissena periods. After the Dreissena invasion, a different spatial pattern of SRP, silica and phytoplankton biomass was observed. SRP and silica concentrations were high in the western basin and decreased into the central basin as in the pre-Dreissena period. Similarly, Post-Dreissena SRP and silica concentrations were low in the western portion of the central basin but then unexpectedly increased easterly by > 250% and > 1,000%, respectively, over the pre-Dreissena period. Phytoplankton biomass increased from within the west end of the western basin to a peak about halfway into the central basin, after which biomass decreased into the eastern basin. The increase in the dissolved fraction of nutrients in the western basin can be attributed to the excretion of dissolved fractions by Dreissena spp. after digestion of particulate matter, the remineralization of surficial organic sediments containing nitrogen and phosphorus-rich feces and pseudofeces and to a decrease in uptake of SRP by less abundant populations of phytoplankton in the western basin. In the western portion of the central basin, it is possible that SRP is being carried by the prevailing westerly current into the central basin stimulating phytoplankton population growth combined with minimal Dreissena grazing causing a peak in phytoplankton abundance. There does not appear to be a satisfactory explanation for the simultaneous increase in SRP and the lack of any change in phytoplankton pre- and Post-Dreissena in the eastern portion of Lake Erie.