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NET ANTHROPOGENIC PHOSPHORUS INPUTS; SPATIAL AND TEMPORAL VARIABILITY IN THE CHESAPEAKE BAY REGION
Citation:
RUSSELL, M. J., D. E. WELLER, T. E. JORDAN, AND K. SIGWART. NET ANTHROPOGENIC PHOSPHORUS INPUTS; SPATIAL AND TEMPORAL VARIABILITY IN THE CHESAPEAKE BAY REGION. BIOGEOCHEMISTRY. Springer, New York, NY, 88(3):285-304, (2008).
Impact/Purpose:
Measurement of anthropogenic phosphorus input variability in Chesapeake Bay region.
Description:
Coastal watershed eutrophication has increasingly become a regional and global issue as larger proportions of the earth’s human population settle in coastal areas. Human activities on the land have severely impacted the water resources of the Chesapeake Bay, one of the world’s largest and best studied coastal regions. We estimate net anthropogenic phosphorus inputs (NAPI) in the Chesapeake Bay region using a budgeting approach similar to that used to estimate net anthropogenic nitrogen inputs (NANI) in previous research. NAPI is an index of phosphorus pollution potential. NAPI was estimated by quantifying all phosphorus inputs and outputs for each county. Inputs include fertilizer applications and non-food phosphorus uses while trade of food and feed can be an input or an output. The average of 1987, 1992, 1997, and 2002 NAPI for individual counties ranged from -0.7 to 78.8 kg P ha-1. The overall area-weighted average NAPI for 266 counties in the region was 4.75 kg P ha-1, indicating a positive net phosphorus input that can accumulate in the landscape or can pollute the water. The influences of particular components on NAPI are quantified after grouping counties by dominant land use activities. Large positive NAPI were associated with agricultural and developed land cover. Temporal trends in NAPI between 1987 and 2002 were assessed. County average NAPI increased from 4.13 kg P ha-1 to 4.94 kg P ha-1 between 1987 and 1997 but decreased slightly to 4.86 kg P ha-1 by 2002. A few easily calculated proxy variables including human population density, livestock unit density, percent low intensity development, and percent row crop land explained 93% of the variability in NAPI among counties and may be of use in geographic areas where it is not possible to complete the more complicated NAPI calculation. We compare the average NAPI within the Chesapeake Bay watershed to estimates of total phosphorus discharge into Chesapeake Bay. We estimate that around 9% of total NAPI is discharged into Chesapeake Bay. We also compare USGS’s river monitoring stations to their respective watershed’s NAPI. NAPI explained 31% of the variability in river monitoring watershed phosphorus discharges. The developed land component of NAPI, which takes into account both the presence and intensity of NAPI within developed land cover, had a much stronger direct correlation with total P discharges (R2 = 0.82). At the relatively large scale of these watersheds, however, the same amount of variability in total P discharges was explained by percent developed land without accounting for the intensity of NAPI (R2 = 0.82). A smaller percent of NAPI was calculated to be discharged from the river monitoring watersheds than the entire Chesapeake Bay possibly because these watersheds do not extend into the Coastal Plain where much of the developed land cover is located. We estimate that about 2% of NAPI is being discharged from the river monitoring watersheds, which suggests that the extensive developed land in the unmonitored Coastal Plain could be a major P source area. This work provides an understanding of the sources of P in the landscape, and illustrates the use of NAPI as a predictor of water quality discharges to the Chesapeake Bay.