Long‐Term Trends of Nutrients and Sediment from the Nontidal Chesapeake Watershed: An Assessment of Progress by River and Season

Abstract

To assess historical loads of nitrogen (N), phosphorus (P), and suspended sediment (SS) from the nontidal Chesapeake Bay watershed (NTCBW), we analyzed decadal seasonal trends of flow-normalized loads at the fall-line of nine major rivers that account for >90% of NTCBW flow. Evaluations of loads by season revealed N, P, and SS load magnitudes have been highest in January-March and lowest in July-September, but the temporal trends have followed similar decadal-scale patterns in all seasons, with notable exceptions. Generally, total N (TN) load has dropped since the late 1980s, but particulate nutrients and SS have risen since the mid-1990s. The majority of these rises were from Susquehanna River and relate to diminished net trapping at the Conowingo Reservoir. Substantial rises in SS were also observed, however, in other rivers. Moreover, the summed rise in particulate P load from other rivers is of similar magnitude as from Susquehanna. Dissolved nutrient loads have dropped in the upland (Piedmont and above) rivers, but risen in two small rivers in the Coastal Plain affected by lagged groundwater input. In addition, analysis of fractional contributions revealed consistent N trends across the upland watersheds. Finally, total N:total P ratios have declined in most rivers, suggesting the potential for changes in nutrient limitation. Overall, this integrated study of historical data highlights the value of maintaining long-term monitoring at multiple watershed locations.(KEY TERMS: nutrient and sediment loadings; watershed monitoring; nonpoint source pollution; watershed management; weighted regressions on time, discharge, and season; Chesapeake Bay; long-term seasonal trends; water quality modeling.)

Evaluation of statistical methods for quantifying fractal scaling in water-quality time series with irregular sampling

“…The mean parameter µ varies between 9.5 and 19.6 for TN and between 13.4 and 24.4 for TP in the Chesapeake monitoring network, corresponding to t average of 10.5-20.6 days for TN and 14.4-25.4 days for TP, respectively. This is consistent with the fact that these sites have typically been sampled roughly once or twice monthly, along with additional sampling during storm flows (Langland et al, 2012;Zhang et al, 2015).…”

“…One such network is the Chesapeake Bay River Input Monitoring Program, which typically samples streams roughly once or twice monthly, accompanied with additional sampling during storm flows (Langland et al, 2012;Zhang et al, 2015). These data were obtained from the US Geological Survey National Water Information System (http://doi.org/ 10.5066/F7P55KJN).…”

Season‐specific trends and linkages of nitrogen and oxygen cycles in Chesapeake Bay

“…These trends correspond to changes in the net biogeochemical production and net transport of NH 4 + and NO 2+3 − that operate over seasonal and regional scales in Chesapeake Bay and other comparable estuaries (Cloern and Jassby ). The reductions in NH 4 + and increases in dissolved O 2 also correspond to recent, but modest declines in nitrogen input from the Susquehanna River (Murphy et al ; Zhang et al ; Harding et al b ; Testa et al ).…”

An improved method for interpretation of riverine concentration‐discharge relationships indicates long‐term shifts in reservoir sediment trapping

“…And although our case study data set does not contain any censored values-i.e., concentrations reported as less than detection limit (typically 0.5 mg/L for SS)-there are published approaches for handling such data [e.g., Tobin, 1958], and others have shown how WRTDS can be implemented in such cases [Chanat et al, 2016;Moyer et al, 2012;Zhang et al, 2015]. In fact, WRTDS has already been adopted by many investigators to estimate concentration and flux for a variety of constituents, including nitrogen, phosphorus, chloride, and dissolved organic carbon.…”

“…The validity of such relationships is confounded, however, by other factors that affect concentrations in natural rivers. This site lies downstream of Conowingo Reservoir, where prior studies have suggested long-term decline in the reservoir's ability to trap sediment before it reaches Chesapeake Bay [Hirsch, 2012;The Lower Susquehanna River Watershed Assessment Team, 2015;Zhang et al, 2013Zhang et al, , 2015Zhang et al, , 2016b. This site lies downstream of Conowingo Reservoir, where prior studies have suggested long-term decline in the reservoir's ability to trap sediment before it reaches Chesapeake Bay [Hirsch, 2012;The Lower Susquehanna River Watershed Assessment Team, 2015;Zhang et al, 2013Zhang et al, , 2015Zhang et al, , 2016b.…”