P. A. Bukaveckas & W. N. Isenberg

Transcription

1 Loading, Transformation, and Retention of Nitrogen and Phosphorus in the Tidal Freshwater James River (Virginia) P. A. Bukaveckas & W. N. Isenberg Estuaries and Coasts Journal of the Coastal and Estuarine Research Federation ISSN Volume 36 Number 6 Estuaries and Coasts (213) 36: DOI 1.17/s x 1 23

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3 Estuaries and Coasts (213) 36: DOI 1.17/s x Loading, Transformation, and Retention of Nitrogen and Phosphorus in the Tidal Freshwater James River (Virginia) P. A. Bukaveckas & W. N. Isenberg Received: 13 August 212 /Revised: 18 April 213 /Accepted: 1 May 213 /Published online: 29 May 213 # Coastal and Estuarine Research Federation 213 Abstract A nutrient mass balance for the tidal freshwater segment of the James River was used to assess sources of nutrients supporting phytoplankton production and the importance of the tidal freshwater zone in mitigating nutrient transport to marine waters. Monthly mass balances for were based on riverine inputs, local point sources (including sewer overflow events), ungauged inputs, riverine outputs, and tidal exchange. The tidal freshwater James River received exceptionally high areal loads (446 mg TN m 2 day 1 and 55 mg TP m 2 day 1 ) compared to other estuaries in the region and elsewhere. P inputs were principally from riverine sources (84 %) whereas point sources contributed appreciably (54 %) to high N loads. Despite high loading rates and short water residence time, areal mass retention was high (143 mg TN m 2 day 1 and 33 mg TP m 2 day 1 ). Retention of particulate fractions occurred during high discharge, whereas dissolved inorganic fractions were retained during low discharge when chlorophyll-a concentrations were high. On an annualized basis, P was retained more effectively (59 %) than N (32 %). P was retained by abiotic mechanisms via trapping of particulate forms, whereas N was retained through biological assimilation of dissolved inorganic forms. Results from a limited suite of stable isotope determinations suggest that DIN from point sources was preferentially retained. Combined inputs from diffuse and point sources accounted for only 2 % and 36 % (respectively) of estimated algal N and P demand, indicating that internal nutrient recycling was important to sustaining high rates of phytoplankton production in the tidal freshwater zone. P. A. Bukaveckas (*) : W. N. Isenberg Center for Environmental Studies, Department of Biology, Virginia Commonwealth University, 1 West Cary Street, Richmond, VA 23284, USA pabukaveckas@vcu.edu Keywords Nutrient mass balance. Eutrophication. Rivers. Estuaries. Algal blooms. James River. Chesapeake Bay Introduction Tidal freshwaters occur at the river estuarine interface and are characterized by the presence of bi-directional tidal forces in the absence of salinity (<.5 ppt). Their presence is a common feature in estuaries along the US Atlantic and Gulf coasts, as well as in Europe and the southern hemisphere (Baldwin et al. 29). These systems typically experience high nutrient loads due to their proximity to riverine inputs, and in some cases, from point sources in urbanized coastal areas. Tidal freshwaters are not well-studied in comparison to saline estuaries, but are known to be highly productive which, in combination with high nutrient loads, could result in high rates of nutrient assimilation, regeneration, and retention (Schuchardt et al. 1993; Lampman et al. 1999; Domingues et al. 211; Ensign et al. 212). Their role in the retention of nutrients may be particularly important to mitigating anthropogenic effects on marine environments (Nixon 1995; Howarth et al. 2; NRC 2; Walker and Rabalais 26). Nutrient retention is governed by riverine inputs from the watershed and by residence time within the estuary (Nixon et al. 1996; Tobias et al. 23). Abiotic mechanisms of retention include sedimentation of suspended particulate matter, which is particularly important to P because a large proportion is transported as particulate matter (Froelich 1988; Jarvie et al. 26). Reductions in fluvial velocity at the river estuarine ecotone enhance water residence time thereby allowing greater sedimentation and potential burial of P. Biotic mechanisms of retention are autotrophic and heterotrophic assimilation which convert dissolved, principally inorganic, nutrient fractions into particulate organic forms. This conversion facilitates retention through deposition of cellular

4 122 Estuaries and Coasts (213) 36: remains and subsequent burial-sequestration or loss of N via denitrification (Seitzinger 1988; Nedwell et al. 1999). Chlorophyll-a maxima are a common feature of tidal freshwaters in the Chesapeake region (Anderson 1986; Bukaveckas et al. 211), though their importance to nutrient retention is not well known. A key issue is the seasonality of nutrient inputs in relation to patterns of algal production (Ensing et al. 26; Jarvie et al. 26; Murrell et al. 27). In temperate regions, high loading rates are associated with high discharge during colder months when assimilative uptake is limited. Higher rates of autotrophic and heterotrophic activity in warmer months, coupled with low discharge, allow for high proportional retention. But as this occurs during a period of low nutrient loads, its significance to annual retention may be small. Seasonal variation in nutrient delivery, utilization, and retention has important implications for nutrient management (Boynton et al. 28; Arndt et al. 29). Nutrient inputs from diffuse watershed sources are tied to seasonal variation in runoff, whereas point source inputs are relatively constant year round. As a result, point source inputs account for a greater fraction of nutrient loads during warmer months when algal blooms are more likely to occur. Despite seasonal differences in the timing and lability of nutrient inputs, mass balance assessments of nutrient sources and fate are often based on annualized data which largely reflect processes occurring during high discharge conditions. The objectives of this study were to characterize seasonal and inter-annual variations in nutrient inputs, outputs, and retention in the tidal freshwater James River using a mass balance approach. The tidal freshwater segment of the James River exhibits high phytoplankton production and among the highest annual average chlorophyll-a concentrations throughout Chesapeake Bay and its tributaries. High autotrophic production in this region has been attributed to favorable light and water residence conditions where the channel transitions from a riverine (deep, narrow) morphometry to a wider channel with extensive shallow areas (Bukaveckas et al. 211). Chlorophyll-a concentrations exceed numeric water quality standards for the James and this has led to efforts to reduce nutrient loads. Quantifying rates of nutrient delivery, transformation and retention provides a basis for assessing riverine and local point source contributions to phytoplankton production and the importance of the tidal freshwater zone in mitigating nutrient transport to saline segments of the estuary. We begin with a historical context based on a 25-year record ( ) of nutrient loading at this site, then focus on the fate of nutrient inputs by presenting a mass balance for the period 27 21, and conclude by considering variation in estuarine nutrient concentrations arising from seasonally and spatially variable nutrient inputs and retention. Methods Study Site The James is an Atlantic Coastal river and the third largest tributary of Chesapeake Bay by discharge and nutrient loads (Belval and Sprague 1999). The James River watershed (26,164 km 2 ) is predominantly forested (71 %) with the remaining land use being agricultural (23 %) and urban (6 %; Smock et al. 25). The non-tidal segment flows 368 km eastward to the Fall Line at Richmond, VA; the tidal segment extends a further 177 km to its confluence with Chesapeake Bay. The James is a river-dominated estuary in which the tidal freshwater (salinity <.5) and oligohaline (salinity <5) zones comprise >7 % of the tidal segment. Major urban centers are located at the Fall Line (Richmond Metro area; population=1,258,) and near the confluence with Chesapeake Bay (Virginia Beach-Norfolk- Newport News Metro area; population=1,649,). A nutrient mass balance was constructed for a 61-km segment in the tidal freshwater zone which extended from the Fall Line to a point mid-way between two long-term monitoring stations located near Hopewell, VA (JMS75 and JMS69; Fig. 1). The study reach included the site of the CHLa maximum (JMS75; Bukaveckas et al. 211). The contributing area to the study reach represented 87 % of the James River watershed of which 82 % was gauged (James and Appomattox Rivers) and 5 % was local ungauged inputs. Long-term average discharge of the James and Appomattox Rivers is 213 and 38 m 3 s 1 (respectively); corresponding freshwater replacement time for the study reach is 4 days. The study reach receives point source inputs of N and P from 4 industrial facilities and 6 municipal WWTPs (total combined average discharge=13 m 3 s 1 ). Point sources include combined sewer overflow (CSO) from the City of Richmond. The study reach experiences semi-diurnal tides of.78 m in amplitude resulting in a large tidal prism (32,991, m 3 ) relative to the storage volume (8,793, m 3 ). Owing to the strong tidal forces, the James is well-mixed both vertically and laterally. Nutrient Budgets Overview The historical analyses of nutrient loads was based on data obtained through the USGS River Input Monitoring Program and the Chesapeake Bay Program database on point source inputs. The mass balance for was constructed using a variety of data from sources described below. Supplemental data were collected in 211 to support the Cl budget analysis, characterize longitudinal gradients in suspended particulate matter (SPM) during high discharge events, and assess seasonal changes in δ 15 Nof SPM. Stable isotope data are included here as a qualitative indicator for phytoplankton utilization of point source N

5 Estuaries and Coasts (213) 36: km Fig. 1 Map of the tidal freshwater James River illustrating the boundaries of the study reach and the location of sampling stations. Station designations follow Virginia DEQ Chesapeake Bay Monitoring Program conventions and indicate distance in river miles from the confluence with Chesapeake Bay inputs based on SPM collected at JMS11 and JMS75 (see Fig. 1) during high and low discharge conditions. Budgets were constructed taking into account riverine inputs (James and Appomattox River watersheds), local point sources (municipal WWTPs, industry, and CSO), ungauged inputs, riverine outputs, tidal exchange, and the mass of nutrients stored within the study reach. Direct atmospheric inputs to the surface of the estuary were small (<1 % of total N inputs based on local deposition values; Moore et al. 211) and therefore were not included in the budget. Nutrient retention was estimated by difference from inputs and outputs taking into account changes in storage: Retention ¼ IN riv þ IN point OUT riv TE ΔStorage ð1þ where IN riv represents riverine and ungauged inputs, IN point represents the local point source inputs, OUT riv represents riverine outputs, TE represents net tidal exchange, and

6 1222 Estuaries and Coasts (213) 36: ΔStorage represents the change in storage. To assess changes in storage, the study reach was sub-divided into five zones based on historical sampling locations; the concentration and volume of each zone was used to determine the stored mass of Cl, N, and P (Fig. 1). Budgets were constructed for total nitrogen (TN), total phosphorus (TP), ammonia (NH 4 ), nitrate/nitrite (NO x ), and phosphate (PO 4 ). Due to constraints of data availability, all budget terms were derived at monthly time steps. Data from various sources were used for the mass balance including USGS (river discharge and chemistry), EPA (point source discharge and chemistry), NOAA (tidal amplitudes), and Virginia Department of Environmental Quality (VADEQ) Chesapeake Bay Monitoring Program (see Table 1 for summary). We collected supplemental water chemistry data in 27 (bi-monthly, April November), 29 (weekly, August October), and (weekly, year round) at nine stations within the mainstem James River (Fig. 1). Water samples were collected 1 m below the surface and analyzed for chlorophyll-a, TSS, POC, and total and dissolved fractions of N and P. Analytical methods were described in Bukaveckas et al. (211) except stable isotope analyses for which we followed sample preparation protocols of the UC-Davis Stable Isotope Lab. Stable nitrogen isotope ratios of suspended particulate matter were measured at UC Davis using a FiniganMAT Delta plus dualinlet continuous flow isotope ratio mass spectrometer with on-line sample combustion. Riverine Inputs and Outputs Nutrient inputs from the James and Appomattox watersheds were calculated as the product of average daily discharge and measured nutrient concentrations (N=17 23 per year) obtained from the USGS River Input Monitoring Program (Table 1). Regressions relating concentration to discharge of the James showed significant relationships for TN and TP (R 2 =.61 and.83, respectively; p<.1), but weak relationships for inorganic nutrient fractions (R 2 <.3). For TN and TP, concentration discharge relationships were used in combination with discharge measurements to derive riverine fluxes. For inorganic fractions, concentrations on dates inbetween measurements were set equal to the closest sampling date. No significant concentration discharge relationships were found for the Appomattox and therefore concentrations were set equal to those of the proximal sampling date. We assumed that nutrient inputs from ungauged areas were proportionally equivalent to gauged areas (Boynton et al. 1995; Robson et al. 28). Output fluxes due to displacement by riverine inputs were estimated as the product of river discharge (including ungauged inputs) and measured nutrient concentrations at JMS75. Data from JMS75 were used to estimate nutrient export to the lower estuary because it is the most downstream sampling station within the study reach. Point Source Inputs Municipal WWTPs and industrial dischargers report monthly effluent discharge and nutrient concentrations to the EPA National Pollutant Discharge Elimination System (NPDES; Table 1) database. Monthly nutrient fluxes for each point source were derived as the product of mean effluent discharge and mean nutrient concentrations. Individual point source fluxes were summed to derive the total monthly input. Nutrient inputs from Richmond CSO events were included with other point sources. Due to the unpredictable, event-based nature of CSO events, monitoring of effluent discharge and concentration is lacking. However, modelderived estimates of CSO discharges were available based on continuous monitoring of selected outfall locations (City of Richmond Department of Public Utilities). Data for the three largest outfalls (representing 92 % of total CSO discharge) were available for all four years. Nutrient concentrations were measured by the City of Richmond at the largest CSO outfall (Shockoe) during four events in 29 (N=15 25 samples per event). Concentrations of NO x (mean=.6 mg L 1 )andpo 4 (mean=.4 mg L 1 ) were similar among the 4 events (CV=9 % and 26 %), whereas concentrations of NH 4 (mean=3.7 mg L 1 ; range= mg L 1 ), TN (mean=7.9 mg L 1 ; range= mg L 1 ), and TP (mean=1. mg L 1 ; range= mg L 1 were more variable. Average values for the four events were used in conjunction with the monthly outfall estimates to determine nutrient loads associated with CSO inputs throughout the period of study. Tidal Exchange Tidal exchange was estimated using a chloride (Cl) mass balance approach (Robson et al. 28). As Cl behaves conservatively, retention is assumed to be negligible and the terms of the mass balance equation are re-arranged to solve for net tidal exchange based on measured changes in the mass of Cl in the estuary and measured Cl inputs and outputs. Weekly data were available for a 12-month period (July 21 June 211) during which Cl concentrations were measured for river inputs, 7 stations within the study reach and one station located below the study reach (JMS69). Data from the seven stations within the study reach were used to calculate the mass of Cl stored within the reach. The predicted change in this mass was derived from Cl concentrations in inflow and Cl concentrations at stations located above and below the reach boundary (to represent tidal inflow and outflow). We compared the single-station value to a reach-scale average Cl concentration and found that

7 Estuaries and Coasts (213) 36: Table 1 Data sources used to construct a nutrient mass balance for the tidal freshwater James River Estuarine water chemistry Riverine inputs Point source inputs Tides VCU VaDEQ USGS WWTPs and industry Richmond CSO NOAA Chemistry Discharge Chemistry Discharge Chemistry Discharge Chemistry Tidal amplitude Sample frequency Weekly monthly Monthly Daily Monthly and events Monthly Monthly Monthly Events Daily Sampling dates Time period Apr Nov 27, Aug Oct 29, Jul 21 Jun 211 Sampling locations Parameters measured Dec 26 Nov 21 Jan 27 Dec 21 Jan 27 Jun 211 Jan 27 Jun 211 Sep Nov 29 Jan 27 Jun 211, Jul 21 Sep 21 JMS11 JMS11 USGS #2375 VA63177 VA63177 NOAA # JMS17 JMS14 USGS #24165 VA24996 NOAA # JMS14 JMS99 VA6194 JMS99 JMS87 VA6663 JMS94 JMS75 VA6369 JMS87 JMS69 VA25437 JMS79 APP1.5 VA278 JMS75 VA26557 JMS69 VA4669 VA5291 TN, NH 3,NO x,tp, PO 4, CHLa, TSS, and Cl Discharge TN, NH 3,NO x, Discharge TN, NH 3,NO x, Discharge TN, NH 3,NO x, Water Elevation TP, and PO 4 TP, and PO 4 TP, and PO 4 VCU Virginia Commonwealth University, VaDEQ Virginia Department of Environmental Quality, USGS United States Geological Survey, WWTP waste water treatment plants, CSO combined sewage overflow, NOAA National Oceanographic and Atmospheric Administration

8 1224 Estuaries and Coasts (213) 36: these yielded similar estimates of tidal outflow chemistry because the single station represented a large proportion of the reach volume. By solving for differences between observed and predicted Cl mass, we determined that net tidal exchange was on average 2.5 % of the tidal prism. This value was used to infer tidal exchange throughout the budget period based on measured tidal amplitudes (NOAA; Table 1). Residual error between observed and predicted volumeweighted Cl concentrations averaged 6 % over the 12-month calibration period, corresponding to a mean difference in Cl of 5.4 mg L 1 over an observed range of 6.5 to mg L 1. Given this margin of error in the Cl budgets, we assumed that nutrient retention estimates exceeding 6 % were indicative of source or sink effects within the study reach. In addition, we performed a sensitivity analysis whereby net tidal exchange was increased from 2.5 % to 5 %, 1 %, and 2 % of the tidal prism to assess the sensitivity of retention estimates. Error Analyses As fluxes were the product of nutrient concentrations and discharge, error was calculated using the equation from Eyre et al. (211): :5 Flux Error ¼ ðmean c *error d Þ 2 þ ðmean d *error c Þ 2 þ ðerror c *error d Þ ð2þ where mean c is the mean nutrient concentration, mean d is the mean discharge, error c is the standard error for nutrient concentrations, and error d is the standard error of discharge. Error analyses yielded low estimates of uncertainty for point source contributions (<5 % of mass flux for all fractions) and higher estimates for riverine inputs (1 15 %) and riverine outputs (15 25 %). In order to directly measure the propagation of error in retention estimates, flux errors were added in quadrature (Lehrter and Cebrian 21): :5 RetentionError ¼ ðerror RI Þ 2 þ ðerror PS Þ 2 þ ðerror RO Þ 2 þ ðerror TE Þ 2 ð3þ where error RI is the riverine input standard error, error PS is the point source standard error, error RO is the riverine output standard error, and error TE is the tidal exchange standard error. Error estimates for retention values (as mass) ranged from 15 % (PO 4 ) to 45 % (TN and NO x ; Table 2). Results Riverine and Point Source Inputs The record revealed two historical phases of nutrient loading to the tidal freshwater James River (Fig. 2). Higher loading rates were evident in the early record ( ) when point source inputs were 2-fold (TN) and 4-fold (TP) above current (post-2) values. Average daily TN loads from point sources declined from 21,22 ( ) to 1,71 kg day 1 (27 21) while TP inputs from point sources declined from 1,71 to 46 kg day 1 over the same period. For TN, the reduction in point source inputs resulted in a decline in their relative contribution to total inputs from 59 % ( ) to 46 % (27 21). Point sources constituted a smaller fraction of TP inputs and their proportional contribution declined from 29 % to 16 % over this period. Riverine inputs of TN and TP were highly variable from year-to-year but did not exhibit any consistent long-term trends (mean=16,6 kg TN day 1 and 3,78 kg TP day 1 ). The large declines in TN from point source inputs coupled with their greater proportional contribution resulted in a statistically significant trend of declining total inputs from point and riverine sources (R 2 =.26, p=.7). For TP, declines in point source inputs were insufficient to drive a decreasing trend in combined point and riverine inputs because of their small contribution to total inputs. As a result of declining trends in total inputs of N, but not P, the ratio of TN:TP in inputs to the James increased from 12.9 ( , as molar) to 16.2 (27 21). Seasonal patterns in riverine nutrient inputs followed variation in discharge with highest fluxes occurring in colder months (Fig. 3). As TN and TP concentrations were positively correlated with discharge, high discharge periods accounted for a disproportionately large fraction of annual loads. For the period of the mass balance analyses (27 21), TN inputs were 4-fold higher (2,5 vs. 5,3 kg day 1 ) during high discharge months (Nov April; mean=296 m 3 s 1 ) compared to low discharge months (May Oct; mean=12 m 3 s 1 ). Seasonal differences were larger for TP with average daily loads 6-fold higher in November April (4,2 kg day 1 ) compared to May October (7 kg day 1 ). Highest nutrient fluxes occurred during November 29 to March 21 when discharge averaged 616 m 3 s 1. During 27 21, average discharge (198 m 3 s 1 ) was 2 % below the 4-year mean (25 m 3 s 1 ) and nutrient inputs averaged 23,49 kg TN day 1 and 2,89 kg TP day 1 (Table 2). Nutrient inputs corresponded to areal loading rates of 446 mg TN m 2 day 1 and 55 mg TP m 2 day 1. Point sources contributed 16 % of TP inputs and 47 % of TN inputs but accounted for a greater proportion of dissolved inorganic nutrients due to their high concentrations in effluent. Point sources contributed 89 % of NH 4,53% of NO x, and 64 % of PO 4 inputs on an annual basis. During May October, these proportions increased to 93 % for NH 4 and 75 % for both NO x and PO 4 inputs. Inorganic fractions accounted for 52 % and 19 % of N and P inputs, respectively. Annual average areal loading rates from riverine and point source inputs were 175 mg NO x m 2 day 1, 55 mg

9 Estuaries and Coasts (213) 36: Table 2 Average daily input and output fluxes (±SE) for the tidal freshwater James River during Budget term TN NH 4 NO x (kg day 1 ) TP PO4 Riverine inputs 12,778±1, ±4 4,251±513 2,432± ±34 Point source inputs 1,712±254 2,63±139 4,975± ±15 363±14 Riverine outputs 15,886±2,182 1,63±397 6,987±794 1,187± ±32 Tidal exchange 82±23 23±6 47±1 1±2 ± Retention 7,522±2,636 1,261±422 2,192±952 1,71±34 337±49 Quadrature addition was used to derive the standard error for retention NH 4 m 2 day 1, and 11 mg PO 4 m 2 day 1. Over the 4-year period, there was a statistically significant trend of declining monthly point source inputs of PO 4 (R 2 =.29; p<.2). This was principally due to reductions in effluent concentrations at the Richmond WWTP. CSO events occurred in every month but varied widely in discharge ( ,687 m 3 month 1 ) accounting for up to 12 % of TN (September 21) and 3 % of TP (November 29) in monthly point source inputs. On an annualized basis, CSO inputs were a minor component of nutrient fluxes contributing 4.6 mg TN m 2 day 1 and.6 mg TP m 2 day 1 ( 1 % of inputs from point and riverine sources). Export and Retention Variation in nutrient outputs closely followed patterns in river discharge (Fig. 4). During low discharge (May October), riverine outputs averaged 8,2 kg TN day 1 and 72 kg TP day 1 whereas during high discharge (November April) outputs were 24,4 kg TN day 1 and 1,7 kg TP day 1. Similar patterns were observed for inorganic fractions with 3 4-fold greater fluxes during high vs. low discharge periods. Tidal exchange and storage were minor components of the nutrient budgets (Fig. 4 and Table 2). On an annual basis, tidal exchange resulted in a net loss of nutrients from the study reach, though differences between input and output fluxes were small (<1 %). Simulations using higher estimates for the volume of net tidal exchange (from 2.5 % of tidal prism to 5 %, 1 %, and 2 %) had little effect on the associated fluxes due to small differences between in-coming (JMS69) and out-going (JMS75) nutrient concentrations in tidal waters. Similarly, monthly changes in the mass of nutrients stored within the study reach had little influence on the budget as these corresponded to <1 % of input fluxes. 3, 2,5 25, Point Sources 2, Point Sources TN 2, 15, 1, TP 1,5 1, 5, 5 4, 1, 3, Riverine Inputs 8, Riverine Inputs TN 2, TP 6, 4, 1, 2, Fig. 2 Average daily loads of TN and TP from point sources and riverine inputs to the tidal freshwater James River from 1985 to 21

10 1226 Estuaries and Coasts (213) 36: , 6, TN Discharge 45, 3, Riverine Point Source m 3 s , 175 7,5 6, NH 3 2, 16, TP 4,5 3, 12, 8, 1,5 4, 25, 2, NO x 75 6 PO 4 15, 1, , 15 Jan-7 May-7 Sep-7 Jan-8 May-8 Sep-8 Jan-9 May-9 Sep-9 Jan-1 May-1 Sep-1 Jan-7 May-7 Sep-7 Jan-8 May-8 Sep-8 Jan-9 May-9 Sep-9 Jan-1 May-1 Sep-1 Fig. 3 Riverine and point source inputs of water and nutrients to the tidal freshwater James River during Inputs exceeded export for all nutrient fractions resulting in positive retention estimates in each year (Fig. 5). Proportional retention exceeded the 6 % threshold based on the Cl budget analyses for all fractions in all years. Annual retention was 7,52±2,64 kg TN day 1 and 1,7±34 kg TP day 1, with inter-annual variation ranging from 4,48 to 11,9 kg day 1 for TN and 534 to 2,63 kg day 1 for TP. Retention of TN averaged 32±4 % (range=24 4 %) of inputs whereas retention of TP averaged 59±7 % (range=36 68 %) of inputs. The mass of TN and TP retained was positively related to the magnitude of inputs with highest inputs and retention occurring in 21. Retention of NH 4 (1,26 kg day 1 )andno x (2,19 kg day 1 ) accounted for 46 % of total N retained (Table 2). Among inorganic fractions, the proportion of NH 4,retained(mean= 43±6 %; range=35 61 %) was higher than TN, whereas the proportion of NO x retained (mean=24±2 %; range=2 28 %) was lower. PO 4 retention averaged 34 kg day 1 corresponding to 6±5 %, of PO 4 inputs (range=46 72 %) and accounting for 2 % of total P retained. The mass of PO 4 retained declined with decreasing input loads over the 4-year period. Strong seasonal patterns were observed in retention of TN as well as DIN and PO 4, but not TP (Fig. 6). Retention of inorganic fractions was highest during May to October when CHLa and water residence time were also greatest. Proportional retention of inorganic fractions approached 1 % during this period. By comparison, proportional retention during winter months was <5 % for PO 4, <25 % for NH 4, and near-zero for NO x. Greater proportional retention in summer was a result of reduced nutrient loads and higher rates of retention. The mass of PO 4,NH 4, and NO x retained was 3-fold higher in summer than in winter such that the period from May to October accounted for

11 Estuaries and Coasts (213) 36: , 45, TN OUTriv TE 1, Discharge 3, 5 25 m 3 s , , 5,25 NH , 4,5 TP 1 5 3,5 1, , 1, , 4 1,2 2 NO x PO 4 21, , 7, Jan-7 May-7 Sep-7 Jan-8 May-8 Sep-8 Jan-9 May-9 Sep-9 Jan-1 May-1 Sep-1 68 % of annual NH 4 retention, 9 % of annual NO x retention, and 62 % of PO 4 retention, despite lower loading rates during this period. Negative retention of NO x (net export) occurred in 2 3 months of each year, typically during January to March. For TN, the mass retained was relatively constant year round with higher proportional retention in summer driven by smaller input loads. Seasonal patterns of TP retention were opposite to those observed for TN and inorganic fractions as mass retention was highest in winter months and lowest in summer. The period from November to April accounted for 86 % of annual TP retention. Because seasonal patterns in retention followed seasonal patterns in loading, the proportion retained was relatively constant year round. Seasonal and Longitudinal Patterns in Nutrient Concentrations -2 Point source inputs and estuarine retention gave rise to consistent longitudinal patterns of nutrient concentrations 6 3 Jan-7 May-7 Sep-7 Jan-8 May-8 Sep-8 Jan-9 May-9 Sep-9 Jan-1 May-1 Sep-1 Fig. 4 Outflow and tidal exchange fluxes of water and nutrients from the tidal freshwater James River during Note difference in y-axis scales for riverine (at left) and tidal exchange (at right) fluxes within the study reach (Fig. 7). During summer, concentrations of dissolved inorganic nutrients peaked near the top of the study reach and declined seaward. NO x and PO 4 concentrations were 3-fold higher below the Richmond WWTP/CSO (river mile 19), which accounted for 46 % and 39 % of NO x and PO 4 point source inputs, respectively. Retention effects were observed in the lower half of the study reach (river miles 7 9) where NO x and PO 4 declined to values observed upstream of point sources. This effect was particularly notable for NH 4 with concentrations decreasing near the bottom of the study reach despite two point sources at river mile 76.5 which contributed 76 % of point source NH 4 inputs. Decreases in dissolved inorganic fractions coincided with increasing concentrations of chlorophyll-a in the lower half of the study reach. TN and TP also increased below the Richmond WWTP/CSO, but showed little change in the lower portion of the study reach. In winter, elevated nutrient concentrations were apparent below point source locations, though these were not -1

12 1228 Estuaries and Coasts (213) 36: , 24, TN Retention 4,4 3,3 TP 16, 2,2 8, 1,1 4, 8 NH 3 PO 4 3, 6 2, 4 1, 2 12, NO x Inputs Outputs Inputs Outputs Inputs Outputs Inputs Outputs 9, , 3, Outputs Inputs Outputs Inputs Outputs Inputs Outputs Inputs Fig. 5 Interannual variation in nutrient inputs, outputs, and retention in the tidal freshwater James River during accompanied by downstream declines in concentration as were observed in summer. Retention effects on suspended particulate matter and TP were evident during high discharge events (Fig. 8). A high discharge event in April 211 (>1, m 3 s 1 ) was associated with elevated TSS (>2 mg L 1 ) and TP (>.4 mg L 1 )inthe upper portion of the study reach. Concentrations declined to <5 mg TSS L 1 and <.1 mg TP L 1 at the bottom of the reach. A similar trend, though with smaller changes in concentration, was observed during a December event (discharge=64 m 3 s 1 ). Decreases in TSS and TP coincided with longitudinal declines in fluvial velocity from 1 to <.5 m s 1, whereas conductivity exhibited little change along the length of the study reach. Samples of suspended particulate matter obtained during the April high discharge event and a similar event in March were depleted in 15 N(δ 15 N<3 ; Fig. 9). In contrast, samples collected during summer, low discharge conditions were elevated in 15 N(δ 15 N=3 8 ) suggesting that phytoplankton utilized 15 N-rich wastewater inputs. Discussion The 25-year record of nutrient inputs to the tidal freshwater James River revealed large reductions in point sources contributions of N and P. These were sufficient to drive a decreasing trend in total inputs of TN, but not TP. Despite recent declines from point sources, the tidal freshwater segment of the James River experiences exceptionally high nutrient loads. Areal loads for this segment of the James (446 mg TN m 2 day 1 and 55 mg TP m 2 day 1 ) were high compared to other systems in the region such as Little Narragansett Bay (128 mg TN m 2 day 1 and 12 mg TP m 2 day 1 ; Fulweiler and Nixon 25) and the Upper Patuxent Estuary (29 mg TN m 2 day 1 and 18 mg TP m 2 day 1 ; Boynton et al. 28). Arealloadingrateswerealsohighincomparisontovalues compiled from the literature for both TN (N=2; mean= 13 mg m 2 day 1 ;range=1 411mgm 2 day 1 )andtp(n= 16; mean=17 mg TP m 2 day 1,range=1 88 mg m 2 day 1 ; Nixon et al. 1996; Granger et al. 2; Robson et al. 28; Ferguson and Eyre 21; Devlin et al. 211). High areal P loads

13 Estuaries and Coasts (213) 36: Retention (%) 1% 75% 5% 25% TN Retention % Retention kg d-1 3, 22,5 15, 7,5 Retention ( ) CHLa (µg L -1 ) CHLa & FRT CHLa FRT Replacement Time (d) 5 Retention (%) % 1% 75% 5% 25% NH 3 2,5 2, 1,5 1, 5 Retention ( ) Retention (%) 1% 75% 5% 25% TP 1, 7,5 5, 2,5 Retention ( ) Retention (%) % 1% 75% 5% 25% % NO x 15, 12, 9, 6, 3, Retention ( ) Retention % % 1% 75% 5% 25% PO Retention ( ) -25% J F M A M J J A S O N D -3, % J F M A M J J A S O N D Fig. 6 Seasonal variation in mass retention (kg day 1 ), proportional retention (% of inputs), chlorophyll-a, and water residence time in the tidal freshwater James River. Water residence time was derived from river to the James are principally due to hydrologic loading effects as riverine sources contributed 84 % of annual inputs. The tidal freshwater zone intercepts 87 % of runoff for the entire James basin but accounts for only 1 % of the surface area of the estuary resulting in a loading factor 43 (m 2 :m 2 ). For nitrogen, both riverine and point source inputs contributed equally to high areal loads (see also Moore et al. 211). Hydrologic loading effects were partially offset by low N export from the James watershed in comparison to other Atlantic Coast rivers (Boyer et al. 22; Howarth et al. 26). Despite high loading rates, the tidal freshwater segment of the James retained a large fraction of dissolved inorganic N and P inputs. During low-discharge conditions, virtually all PO 4,NH 4, and NO x were retained. NH 4 was preferentially retained over NO x despite the fact that NO x inputs occurred at the top of the study reach and NH 4 entered near the bottom. This finding is consistent with results from prior studies showing high NH 4 utilization from wastewater sources (Bukaveckas et al. 25). For all three inorganic discharge and volume of the study reach. Monthly average values (±SE) are based on data for fractions, high proportional retention was not simply a result of low loading rates during periods of reduced discharge, but rather, a result of 3-fold higher areal retention in summer. In addition to elevated water temperature, a likely factor contributing to greater retention is that point sources account for the bulk of nutrient inputs in summer, and these were dominated by dissolved inorganic fractions (e.g., 57 % of N inputs were DIN and 46 % of P inputs were PO 4 ). By comparison, riverine inputs were comprised of 36 % DIN and 8 % PO 4. Jarvie et al. (26) observed similar patterns among rivers of the United Kingdom in which seasonal declines in river discharge resulted in greater proportional contributions of dissolved inorganic fractions due to wastewater sources. Our findings suggest that differences in lability of riverine vs. point source nutrient inputs, coupled with higher rates of autotrophic and heterotrophic assimilation in summer, contributed to seasonal variation in retention. This view is consistent with results from a limited suite of stable isotope determinations showing high δ 15 N of

14 123 Estuaries and Coasts (213) 36: mg L -1 mg L -1 mg L -1 mg L -1 mg L -1 µg L River kilometer Fig. 7 Longitudinal patterns in nutrient concentrations and chlorophyll-a in relation to point source inputs. Data are summer (May October) and winter (November April) average values for (±SE). Bars denote proportional contributions by individual point River kilometer sources, with the exception of the Hopewell WWTP and Honeywell Inc., which are both located at river mile 76.5 and shown as a single combined value suspended particulate matter during summer (Fig. 9). As wastewater sources of DIN are typically enriched in 15 N (Wayland and Hobson 21), a seasonal shift in the signature of SPM suggests that summer phytoplankton production was dependent in part on DIN from point sources. Sediment denitrification and subsequent re-suspension may also contribute to higher δ 15 N of SPM in summer. However, chlorophyll-a concentrations were found to be significantly correlated with monthly retention of NH 4,(R 2 =.55) and NO x (R 2 =.69; both p<.1) suggesting that autotrophic assimilation is important. A prior study (Anderson 1986) reported the coincident chlorophyll maxima and dissolved nutrient minima occurring in the James and nearby York and Rappahannock Rivers. These were attributed to diatom production, though in our study we observed high areal retention through the period when spring diatoms give way to summer cyanobacteria (Marshall et al. 26). Also, we do not discount the importance of heterotrophic assimilation, particularly ammonium oxidation as mechanisms of DIN retention (Seitzinger 1988; Arndt et al. 29). Likely these act in concert whereby autotrophic production provides labile C for heterotrophs and deposition of phytodetritus enhances N delivery to sediments and denitrification. Although little DIN escapes the tidal freshwater zone during low discharge conditions, on an annualized basis, DIN accounts for only half of N retained, indicating that there are important sinks for dissolved and particulate organic N. A number of recent studies have documented the bioavailability of various forms of DON, including urea (Finlay et al. 21; Filippino et al. 211). Retention of

15 Estuaries and Coasts (213) 36: Fig. 8 Longitudinal patterns in TSS, TP, specific conductance, and fluvial velocity during high discharge events in April 211 (1,36 m 3 s 1 ) and December 211 (64 m 3 s 1 ). Fluvial velocity was determined from river discharge and cross-sectional area TSS (mg L -1 ) December 211 April ) TP (mg L -1 ) Fluvial Velocity (m s -1 ) River Kilometer River Kilometer 12 1 DON via autotrophic and heterotrophic assimilation may be important in the James given the large inputs of N from wastewater. If biological assimilation of DON was the dominant mechanism of ON retention, variation in retention should track that of inorganic fractions and seasonal patterns in temperature and chlorophyll-a. Though we are unable to partition retention of DON vs. PON with the available data, we estimated retention of TON by difference from TN and DIN. TON retention was lower during May October (34 mg m 2 day 1 ) compared to November April (128 mg m 2 day 1 )withthe latter period accounting for the bulk of annual retention (81 %). These patterns were similar to those observed for TP (see below) and suggest that retention of PON, not assimilatory uptake of DON, was the principal mechanism of TON retention. Overall, 32 % of TN inputs were retained; this figure agrees with expected values based on the water residence time of the James (Fig. 1). Nixon et al. (1996) reported a relationship between N export and water residence time for diverse estuaries and our estimates match predicted values at the low end of the range for residence time and proportional retention. It should be noted however, that while proportional retention was low, mass retention in the James (143 mg TN m 2 day 1 ) exceeded values in the Nixon et al. compilation (range=8 48 mg TN m 2 day 1 ) with the exception of the Scheldt (247 mg TN m 2 day 1 ). Thus high loading rates resulted in exceptionally high areal retention but low proportional retention of N in the tidal freshwater zone. For dissolved inorganic nutrients, the bulk of retention occurred during low discharge conditions, whereas the opposite pattern was observed for TP. Riverine inputs accounted for a large fraction of P loading (84 %) and these were principally in the form of OP (only 19 % as PO 4 ). The bulk of TP retention occurred during periods of high discharge and elevated loading when water temperature and chlorophyll-a concentrations were low. These findings suggest that the primary mechanism of P retention is abiotic, likely due to trapping of sediment-bound P (Schuchardt et al. 1993; Boynton et al. 1995). This inference is consistent with results from longitudinal surveys showing large declines in TSS and TP during transport through the study reach. During high discharge conditions, large differences arose between input and output fluxes because TP concentrations at the top of the reach were greatly elevated, whereas concentrations at the bottom of the reach were not. Proportional retention of TP in the tidal freshwater James (annual range=36 68 %) was considerably higher than would be expected based on water residence time (Fig. 1). Areal retention (33 mg m 2 day 1 ) was 1-fold higher than values compiled by Nixon et al. (1996; range=.3 3 mgm 2 day 1 ) with the exception of the Scheldt (46 mg m 2 day 1 ). Boynton et al. (28) attributed high proportional retention in the Patuxent to its sediment-rich, eutrophic conditions and to the presence of tidal marshes. While tidal marshes are uncommon in the upper James Estuary, backwater areas that were formed by the construction of channel cutoffs (for navigation) may be important sites for sediment trapping. Another important aspect of geomorphology of the James is the transition in channel form from a constricted, riverine morphology to a broader, estuarine morphology which acts to substantially reduce fluvial forces (Shen and Lin 26; Bukaveckas et al. 211). Shallower depths result in shorter settling time and distance (McNair and Newbold 21) which, in combination with reduced fluvial velocity, may enhance sedimentation of SPM and sediment-bound P. We hypothesize that because of these geomorphic factors, the tidal freshwater zone accounts for a disproportionately large fraction of P retention within the James River Estuary, thereby resulting in unusually high areal and proportional retention for this segment. Of considerable interest is the long-term fate of P stored in sediments as release of this material may delay recovery

16 1232 Estuaries and Coasts (213) 36: Fig. 9 Nitrogen isotope signatures of suspended particulate matter in the tidal freshwater James River in relation to seasonal variation in dissolved inorganic N (DIN), chlorophyll-a (CHLa), and discharge DIN (mg L -1 ) N CHLa (µg L -1 ) N Discharge (m 3 s -1 ) N 1 1. M A M J J A S O. from eutrophication in response to reductions in nutrient loads (Neal et al. 21; Stutter et al. 21). In the James, the P content of suspended particulate matter (TP:TSS=.6 mg:mg) is substantially higher than the P content of sedimented particulate matter (.1 mg:mg; Meyers 1994) suggesting that appreciable re-mineralization occurs pre- or post-deposition. Because of well-mixed hydrologic conditions, the James does not experience hypoxia events which enhance release of P from sediments. However, shallow conditions at the lower end of our study reach would enhance sedimentation. P recycling may be large due to the low water to sediment area ratio (see below), but high retention within the study reach shows that P sequestration greatly exceeds re-mineralization. Removal of sediments by dredging may be a potential loss mechanism. To assess its importance, we estimated the average annual rate of sediment removal as 74, m 3 year 1 based on 1,55,16 m 3 removed between 199 and 211 (USACE, pers.comm.). From this value, measurements of sediment bulk density (Schlegel 211), and the previously cited measurement of sediment P, we estimate that sediment removal is equivalent to 17 % of annual TP retention. This analysis suggests that

17 Estuaries and Coasts (213) 36: TN Export (%) TP Export (%) 1% 75% 5% 25% % 1% 75% 5% 25% % R² =.88 R² = Residence Time (mo) Fig. 1 TN and TP export as a function of estuarine water residence time. Closed circles and regression lines are from Nixon et al. 1996; open circles are for the tidal freshwater James River (27 21). Estuaries from Nixon et al include the Baltic Sea, Chesapeake Bay (TN only), Delaware Bay, Narragansett Bay, Guadalupe Estuary in a dry (1984) and wet (1987) year, Potomac Estuary (TN only), Ochlockonee Bay (TN only), Boston Harbor, and Scheldt Estuary the majority of sediment P is retained within the system. The ability to trap sediment-bound P during high discharge conditions may be an important and underappreciated difference in the functioning of estuaries relative to rivers. In rivers, particularly those that are naturally constricted or disconnected from their floodplain, high discharge conditions are unfavorable for deposition in the channel resulting in minimal P retention. In estuaries, increasing crosssectional area and opposing tidal forces dissipate fluvial forces and allow for enhanced sediment deposition and P retention even during high discharge conditions. While some budget components were well-resolved by frequent measurement of concentration and flux (e.g., river and point source inputs), other terms were subject to greater uncertainty due to inherent difficulties in quantifying tidal exchange and sewer overflow events. Nutrient inputs from sewer overflow events were important in a few months though on an annualized basis they were not a significant component of input fluxes ( 1 % of inputs from riverine and point sources). Similarly, tidal fluxes were not an important component of the mass balance for this system despite the large tidal prism. Where estuaries undergo tidal exchange with coastal marine waters, strong gradients in salinity and nutrient concentrations arise due to the large reservoir of marine water which is not appreciably diluted by fluvial inputs. In these cases, tidal exchange drives large imbalances between input and output fluxes resulting in appreciable loss (dilution) of nutrients from the estuary (e.g., Robson et al. 28; Arndt et al. 29). As our system is located at the head of the estuary, concentration differences across the bottom of the study reach (i.e., with the oligohaline segment) were small, and therefore input and output fluxes were closely balanced. The effect of tidal exchange on nutrient fluxes was small and sensitivity and error propagation analyses suggest that retention estimates were robust with respect to uncertainty in tidal exchange. Retention in the tidal freshwater segment affected not only the mass and form of N and P exported to the saline estuarine but also their relative proportions. Inputs to the James were low in N (molar TN:TP=18) compared to other systems such Little Narragansett Bay (23; Fulweiler and Nixon 25), the Upper Patuxent Estuary (26; Boynton et al. 28) and the compiled database (22; see above). On an annualized basis, retention of P (59 %) was greater than N (32 %) such that exports from the reach were depleted in P (N:P=3). Robson et al. (28) reported similar findings for an urbanized estuary in Western Australia where annual retention of P (75 %) was greater than N (43 %). They attributed the preferential retention of P to a N-saturation effect as water entering the estuary contained an excess of N (TN:TP=29). We observed a similar pattern in the James despite input ratios that were lower and similar to Redfield. We hypothesize that abiotic mechanisms of retention via sediment trapping were more effective in attenuating nutrient export than biotic mechanisms via assimilatory uptake because the latter are ineffective during periods of high discharge which accounted for the bulk of annual fluxes. Our annualized values showing low retention of N relative to P reflect processes occurring during high discharge when P inputs were dominated by particulate forms and subject to sedimentation losses. During summer, when phytoplankton nutrient limitation would be most prevalent, N was preferentially retained within the reach (N:P retained=35) resulting in export of N-poor water. Thus seasonal patterns whereby P is retained during high discharge and N, particularly the inorganic fractions, is retained during low discharge promote summer N limitation in the saline estuary (Bala Krishna Prasad et al. 21). Lastly, we consider input and output fluxes in the context of nutrient requirements to support phytoplankton production. We had previously measured water column primary production at stations located in the upper, constricted (JMS99) and lower, shallower (JMS75) segments of our study reach (bimonthly in 27; Bukaveckas et al. 211). Based on the volume-weighted average production (3.46 mg C L 1 day 1 ) and the Redfield ratio, we estimate reach-scale phytoplankton demand to be.61 mg N L 1 day 1 and.84 mg P L 1 day 1.