Long-Term Study of Soluble Reactive Phosphorus Concentration in Fall Creek and Comparison to Northeastern Tributaries of Cayuga Lake, NY: Implications for Watershed Monitoring and Management

Abstract

This study focuses on soluble reactive phosphorus (SRP), a key driver of eutrophication worldwide and a potential contributor to the emerging global environmental problem of harmful algal blooms (HABs). Two studies of tributary SRP concentrations were undertaken in sub-watersheds of Cayuga Lake, NY, the subject of a total maximum daily load (TMDL) development process, due to phosphorus impairment of its southern shelf. The long-term study compared SRP concentration in Fall Creek in the 1970s with that in the first decade of the 2000s, thus spanning a period of change in phosphorus sources, as well as in regional climate. The spatial study used data collected between 2009 and 2018 and compared SRP concentrations in Fall Creek to levels in northeastern tributaries that flow into the lake close to areas where HABs have been problematic. SRP was measured using standard procedures. Flow-weighted mean SRP concentration ranged between 15.0 µg/L and 30.0 µg/L in all years studied in both the 1970s and 2000s, with the exception of 2010. Annual discharge in Fall Creek showed no trend between 1970 and 2018, but a higher proportion of high streamflow samples was captured in the 2000s compared to the 1970s, which resulted in proportionally increased SRP concentration in the latter time period. There was no significant difference in the SRP concentration—flow rate relationship between the two time periods. Adjusted for flow rate, SRP concentrations in Fall Creek have not changed over many decades. Increasing phosphorus contributions from growing population and urbanization since the 1970s may have been counterbalanced by improvements in wastewater treatment and agricultural practices. Mean SRP concentration in northeastern tributaries was significantly (p < 0.001) higher than in Fall Creek, likely reflecting more intense agricultural use and higher septic system density in the watersheds of the former. This finding justifies continued monitoring of minor northern tributaries. Future monitoring must emphasize the capture of high flow conditions. Historical stability and highly variable hydrology will slow the watershed response to management and confound the ability to detect changes attributable to decreased phosphorus inputs. Large scale monitoring on decadal timescales will be necessary to facilitate watershed management.

Water quality data for Fall Creek, New York, USA: 1972–1995

“…It has already served as an important baseline, and will continue to do so in the future. Combined with additional data collected after 1995 it has demonstrated the stability of the Fall Creek watershed over long periods of time and under changing conditions, with implications for expected response to management decisions in the broader region (O'Leary et al, 2019). The motivation for the first work on this data set, where particular analytes were selected in collaboration with limnologists studying lake responses to tributary inputs in the early 1970s, is still relevant today as new concerns about the proliferation of harmful algal blooms drive additional watershed studies with the goal of developing predictive models and effective mitigation strategies.…”

“…Schaffner and Oglesby (1978) examined P loading to lakes and used Fall Creek data to argue that the best calculations of loading reflect forms of P with the most biological significance. Bouldin et al (1975) hypothesized on the relative bioavailability of different forms of P. Their hypothesis was later supported by Prestigiacomo et al (2016) who demonstrated that biologically available P in the region is a composite of completely bioavailable soluble reactive P, mostly bioavailable soluble unreactive P, and less bioavailable particulate P. Haith et al (2012) used work derived from the data set to develop and test a model of phosphorus loading in the Cayuga Lake watershed. The data set was augmented with the results of additional sampling in the 2000's, which led to the finding of no significant change in flow rate‐adjusted soluble reactive P concentration in Fall Creek over many decades. As a result, inter‐annual variation in soluble reactive P loading to Cayuga Lake was determined to be primarily a function of variation in discharge (O'Leary et al, 2019). Seasonal variation in nitrate export was documented by Johnson et al (1976b), and the Fall Creek data also contributed to a comprehensive study of long‐term nitrate dynamics by M. S. Johnson et al (2007).…”