BENTHIC MACROINVERTEBRATE WATER QUALITY SURVEY OF THE INFLOW AND OUTFLOW OF A NEW YORK CITY DRINKING WATER RESERVOIR

The Ashokan Reservoir, one of the largest drinking water sources for New York City, and its watershed are managed by New York City’s Department of Environmental Protection (DEP) to ensure that city residents are supplied with clean drinking water. Significant fine particulate turbidity associated with high flow events in the Upper Esopus watershed are a huge problem for the reservoir, while authorized release outflows and spillovers from the reservoir cause large discharge and turbidity releases to the lower Esopus Creek. Recent large storm events such as Hurricane Irene and Tropical Storm Lee of 2011 severely impacted the watershed.

In an effort to assess the water quality of the streams before and after these storms, we compared Benthic Macroinvertebrate (BMI) studies that were conducted in the summers of 2007, 2011, and 2012 along 13 sites in the both the Upper and Lower Esopus Creek as well as some of its tributaries. BMI are useful for biological monitoring because the presence of certain turbidity-sensitive or tolerant species can indicate the level of impact on the water quality. Major ion concentrations and physical parameters were also measured in each year. Large variations in discharge throughout the sampling years and the subsequent changes in turbidity levels from erosion of clay-rich soils in the watershed did not significantly influence the BMI population. The main differences in data from 2011 to 2012 can be explained by the modification of the habitat from the storm events, making the stream bed a less suitable environment for macro-invertebrates. Likewise, comparisons of 2007 to 2011 data suggest that local anthropogenic and natural effects may be the significant drivers of changes in BMI population; BMI were generally not impacted by turbidity, so long as dissolved oxygen content was not affected. Although turbid baseflow and even more turbid storm flow conditions are a concern when complying with drinking water standards, these conditions may be a natural characteristic of this area and have not impacted the water quality significantly enough to threaten macroinvertebrate diversity.