Modeling phosphorus cycling in a well-mixed coastal plain estuary

Estuarine, Coastal and Shelf Science (1992) 35, 235-252

M o d e l i n g P h o s p h o r u s Cycling in a W e l l - M i x e d Coastal P l a i n Estuary

M a r t i n E. L e b o ~ a n d J o n a t h a n H. S h a r p College of Marine Studies, University of Delaware, Lewes, DE 19958 Received I0 March 1991 and in revised form 12 March 1992

Keywords: phosptkorus; models; estuaries; mass balance; regeneration; Delaware Bay Phosphorus cycling in the Delaware Estuary was examined during 1986-88 using numerical methods to calculate fluxes and estimate net regeneration. In the tidal river, the flux of total phosphorus (TP) increased threefold compared to Delaware River due to municipal inputs, with most of T P flux as dissolved inorganic phosphate (DIP, 56-59%). This observed increase in T P flux in the river, however, is much lower than expected, given known inputs. A mass balance for T P in the river indicated that a large fraction (44-67%) of T P inputs was retained mainly through geochemical processes. Phosphorus cycling in Delaware Bay, in contrast, was dominated by biological uptake and recycling. Within the bay, D I P input from the tidal river was transformed into phytoplankton biomass; the relative flux of particulate phosphorus (PP) increased from 35% of total flux into the bay to 62% at the month due to a 52% increase in mean PP flux. The biological dominance of P cycling in the bay resulted in strong seasonal variations in T P flux; during the annual winter-spring bloom, T P was retained in the bay (53-56%), while the bay was a source ( + 5 2 % ) of T P during the fall. Throughout the year, except during the spring, regenerated D I P was sufficient to supply 102 + 19, 94+ 10, and 95 + 6% of phytoplankton P-demand in the upper, middle, and lower bay respectively. During the spring, there was an imbalance between uptake and regeneration resulting in D I P depletion. A comparison of processes contributing to total regeneration indicated that 71-97 % of apparent regeneration occurred within the water column, with sediment D I P release contributing an additional 3-25%. On an annual basis, 84% of T P entering the salinity gradient of the estuary was exported to coastal waters.

Introduction M a n i m p a c t s p h o s p h o r u s (P) t r a n s p o r t to the oceans t h r o u g h e n r i c h m e n t o f r i v e r d i s s o l v e d i n o r g a n i c p h o s p h a t e ( D I P ) c o n c e n t r a t i o n ; in d e v e l o p e d countries, m a n y rivers are h i g h l y e n r i c h e d in P m a i n l y as D I P ( S i m p s o n et at., 1975; M a c K a y & L e a t h e r l a n d , 1976; C o n o m o s et al., 1979; F r a s e r & W i l c o x , 1981; M e y b e c k , 1982; F r o e l i c h et al., 1985; R e h m , 1985). T h e r e m o v a l o f this excess P w i t h i n t h e estuaries o f u r b a n i z e d rivers w o u l d r e d u c e the influence o f a n t h r o p o g e n i c P i n p u t s on a d j a c e n t coastal waters. H o w e v e r , r e c e n t m e s o c o s m e x p e r i m e n t s i n d i c a t e t h a t w e l l - m i x e d e s t u a r i n e systems m a y f u n c t i o n °Present Address: Division of Environmental Studies, University of California, Davis, California 95616, U.S.A. 0272-7714/92/090235 + 18 $03.00/0

© 1992 Academic Press Limited

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M. E. Lebo & J. H. Sharp

as conduits for P transport to coastal waters, exporting the majority of P they receive (Nowicki & Oviatt, 1990). Traditionally, DIP-salinity diagrams have been used to infer estuarine reactivity (Boyle et al., 1974; Liss, 1976; Officer, 1979). However, this simple approach to understanding P cycling in estuaries is complicated by variations in fluvial D I P concentrations. Non-linear property-salinity distributions can be produced by varying the river concentration of a property of interest (Loder & Reichard, 1981; Officer & Lynch, 1981). In a small Florida estuary, Kaul and Froelich (1984) reported that up to 10% of the observed estuarine reactivity of nutrients could be attributed to variations in river concentrations. Simulated estuarine mixing using advection-dispersion equations to describe the transport of salt and other chemical tracers (Stommel, 1953) has been used to account for fluvial variations. In Delaware and San Francisco Bays, simulated mixing was used to examine seasonal changes in nitrate concentration and alkalinity (Sharp et al., 1986; Cifuentes et al., 1990). T h e application of simulation models to study P cycling in estuaries is complicated by rapid biological and geochemical reactivity. In addition to physical mixing, D I P is removed geochemically, removed biologically, released from seston, released from bottom sediments, and regenerated biologically. T h e balance between these different processes is determined by environmental factors such as the concentration of suspended sediment, water temperature, and pH. In turbid estuaries, P cycling is controlled by particle interactions with D I P concentrations apparently ' buffered' by particle-bound phosphorus (PP, Carritt & Goodgal, 1954; Pomeroy et al., 1965; Butler & Tibbitts, 1972; Fox et al., 1985, 1986, 1987; Froelich, 1988; Fox, 1989). Phosphorus cycling in highly productive estuaries, in contrast, is dominated by biological processes. In many cases, D I P uptake by plankton is sufficiently rapid to completely utilize D I P many times during estuarine mixing (Kuenzler et al., 1979; Sharp et al., 1984; Rehm, 1985; Fisher et al., 1988). When biological P-demand is high, geochemical D I P control mechanisms are overwhelmed by biological demand, and D I P supplied to plankton by geochemical processes is small. In these cases, the principal source of D I P for phytoplankton production is recycled D I P (Fisher et al., 1988). This study uses the large database that is available for the Delaware Estuary to examine several aspects of P cycling. Using P concentrations and river discharge data, the fluxes of DIP, dissolved organic P (DOP), and PP were estimated for 23 river samplings between April 1986--July 1988. River samplings were grouped by season to examine seasonal variations in P fluxes and to determine seasonal mass balances. In addition, we used a modified advection-dispersion model to estimate the rate of D I P regeneration along the salinity gradient of the estuary; reactive terms were added to advection-dispersion equations to include D I P uptake and total regeneration. The goal of this study was to combine knowledge of P distributions with D I P uptake rates and PP distributions to provide an integrated description of P cycling in Delaware Estuary.

Methods and study area

Description of the Delaware Estuary T h e Delaware Estuary is a shallow (mean depth < 10 m), well-mixed coastal plain estuary in the mid-Atlantic region of the United States which is heavily urbanized; several midsized cities (Trenton, Camden and Wilmington) and the metropolis of Philadelphia are located along its banks. It is a relatively simple estuary with Delaware River accounting for

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Figure 1. Delaware Estuary shown with sampling stations along the main axis. T h e river is tidal from the mouth of the bay up to the fall line at Trenton, N e w Jersey (240 km). Salinity increases from 0 ppt at station 14 (130 km) to 30 ppt at the mouth of the bay.

3 2 0 m 3 s -I (58%) of a mean annual discharge of 5 5 0 m 3 s - ] . There is one main subtributary, the Schuylkill River, that contributes an additional 78 m 3 s - 1 into the tidal river (Smullen et al., 1983). T h e tidal river region of the estuary receives effluents from many industrial and municipal facilities (Albert & Kausch, 1988) causing pH and oxygen to decrease and nutrients to increase (Sharp, 1988). At low salinity, a pronounced turbidity m a x i m u m forms with seston concentrations > 30 m g 1- x (Biggs et al., 1983; Lebo, 1990a). D o w n s t r e a m of this turbidity maximum, phytoplankton productivity increases reaching a m a x i m u m in the lower estuary (Pennock & Sharp, 1986). T h e entire tidal region of the estuary was sampled with stations located spatially along the main channel of the estuary (Figure 1). In this study, the term' estuary' refers to the entire tidal region from the m o u t h of Delaware Bay upstream to the fall line near Trenton, N e w Jersey. T h e estuary is subdivided into two main regions: the tidal fresh water' river' (stations 1-9, 11-14) and the ' b a y ' (stations 14-26). Station locations are listed as distances upstream from the m o u t h of Delaware Bay along the main channel of the estuary according to a standard stream mileage system ( D R B C , 1988).

Sample collection and chemical analyses Samples were collected on a series of 23 cruises in the Delaware Estuary between April 1986 and July 1988 on the R]V Cape Henlopen. Stations were located spatially along the main channel of the estuary at intervals of 7-15 km (Figure 1) requiring two days to

238

M. E. Lebo &ft. H. Sharp

complete each transect. Although samples were not collected under constant hydrodynamic conditions (i.e. tide phase), the sampling strategy was designed to separate larger spatial differences ( > 20 kin). At each station, water samples for chemical and biological analyses were collected from the upper meter of the water column in 10-liter Niskin bottles on a C T D rosette system. Sampling was confined to the upper meter since vertical gradients in P are usually small in Delaware Estuary; even during partial stratification in the spring, horizontal gradients in P concentrations are much larger than vertical ones. Salinity was measured on board ship with a bench-type salinometer for all samples with a salinity > 0-1 ppt. When salinity was < 0" 1 ppt, it was estimated with a flow through S T D system. Dissolved P fractions were measured in sample water filtered through pre-baked (450 °C for 2 h ) Whatman G F / C filters. Dissolved inorganic phosphate (DIP) was measured as soluble reactive P (SRP) by the molybdenum blue method (Murphy & Riley, 1962; Strickland & Parsons, 1972) modified for 10-ml aliquots (Sharp et al., 1982). Total dissolved P was measured as SRP (Solorzano & Sharp, 1980) as modified by Lebo (1991). Samples were heated at 450 °C for 2 h and then rehydrated and hydrolyzed with dilute HCI. Dissolved organic P (DOP) was calculated by subtracting SRP from total dissolved P. Samples to determine particulate P (PP) and chlorophyll a were collected on pre-baked (450 °C for 2 h) Whatman G F / C filters. Particulate phosphorous was measured as SRP after samples were heated at 450 °C for 2 h and then hydrated and hydrolyzed with dilute HC1 (Solorzano & Sharp, 1980). Chlorophyll a was extracted overnight in cold (4 °C) 90% acetone in the dark and read at sea by fluorometry (Strickland & Parsons, 1972). Phytoplankton primary production was measured by the incorporation of 14Cbicarbonate into particulate matter (Pennock & Sharp, 1986). Phytoplankton P-demand was estimated from carbon production using the Redfield ratio of 106 for C:P (Redfield et al., 1963). When sample averages are displayed, the sample mean is listed__+one standard deviation of the mean. Sample means are compared using the Student's t-test. Seasonal averages are computed by calendar seasons (e.g. winter is 21 December-22 March). Delaware River discharge is measured by the United States Geological Survey (USGS) at Trenton, New Jersey. Daily averaged discharge is used in this study. Flux estimates

Phosphorus fluxes were calculated at three locations along the estuary to estimate the transport of P into the river (station 1), from the river to the bay (station 14), and out of the bay (station 26). Fluxes were calculated for D I P , D O P , and PP using measured concentrations of each fraction, river discharge data, and the apparent concentration gradients according to Officer (1979): Flux = R(x). C(x) - K(x)" A(x)-

dC(x) dx

,

(1)

where R(x) is river discharge (m 3 s-l), K(x) is the longitudinal dispersion coefficient (m z s-1), C(x) is the concentration of D I P , D O P , or PP (mmol m-3), A(x) is the crosssectional area (m2), and dC(x)/dx is the horizontal concentration gradient of C(x) ( m m o l m - 3 m - 1 ) . For stations 14 and 26, flux calculations included terms for both advection and eddy diffusion. At station 1, diffusion was assumed to be negligible since this station represented the input of the Delaware River to the estuary.

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Modeling phosphorus cycling

T h e parameters used to calculate fluxes with equation I were taken from field measurements, nautical charts, or published data for the Delaware Estuary. Cross-sectional areas were estimated from nautical charts. Concentration gradients were calculated using concentration data for three stations (12, 14 and 15) for station 14 and 3-5 stations (24-28, depending on stations sampled) for station 26. Early in our sampling programme, we discontinued data collection at station 13 due to its proximity to station 14, and, thus, data were not available at that station for most cruises• T o calculate the gradients, C(x) for each location was determined by selecting the polynomial function (lst-4th order) that best described the spatial distribution, and the derivative of C(x) was taken. A simple linear approximation was used for all dates without a clear monotonic trend. Total flow at stations 14 and 26 was estimated from Delaware River discharge averaged for three days prior to each cruise by including additional flow from smaller tributaries; Delaware River discharge was multiplied by: 1.25 for station 14 and 1.72 for station 26 (Smullen et al., 1983). T h e dispersion coefficient at station 26 was estimated from the salinity distribution and river discharge according to Cifuentes et al. (1990): R(x). S(x) K(x) = A(x). dS(x)/dx'

(2)

where S(x) is salinity (ppt) and dS(x)/dx is the horizontal salinity gradient (ppt m-1). All other variables are as defined in equation 1. T h e salinity gradient at station 26 was calculated in the same manner as P concentration gradients. For station 14, a value of 200 m 2 s- 1 was used due to low salinity. Cifuentes et al. (1990) reported an average value of 209 m 2 s- i for the Delaware during 1981-83, and our value was chosen to be similar to that previous estimate. Reactive mixing model D I P regeneration within the bay (0-130 km) was examined for 12 cruises between January 1987 and January 1988 using a one-dimensional advection-dispersion model in which reactive terms were included. Salt transport was described by an advection-dispersion equation (Stommel, 1953; Helder & Ruardij, 1982; Cifuentes et al., 1990):

A(x)• OS(x,t) -o JR(t) S(x,t) A(x) K(x) OS(x't) 1 a'----~ = a x ~x _

_

.

.

_ _

•

_

_

,

(3)

where A(x) is cross-sectional area (m2), S(x,t) is cross-sectionally and tidally averaged salinity (ppt), t is time (s), x is longitudinal distance along the estuary (m), R(t) is river flow (m 3 s-1), and K(x) is the longitudinal dispersion coefficient (m 2 s-t). In addition to advection-dispersion terms, the equation for D I P transport included a reactive term: A(x)" aV(x,t________)-at -axa . J R ( t ) .. V(x,t) . - A(x). K(x) aV(x,t)dx Rem(x)],

(4)

where P(x,t) is D I P (mmol m-3) and Rem(x) is the net rate of D I P reaction during mixing (mmol m-3 s- 1). Rem(x), as defined here, includes plankton uptake, geochemical removal, benthic release, particle release, and water column regeneration of DIP. Equations 3 and 4 were solved numerically according to finite-difference approximations (Cifuentes et al., 1990): aC(xi,tj)= C(xi,ti+l) -- C(xi,ti) at

At

,

(5)

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M. E. Lebo & J. H. Sharp

OC(xi'tj) = C(xi+l'tJ) - C(xi_l,tj) ax

,

(6)

2- Ax

a2C(xi,tj) _ C(Xi+l,tj) - 2 • C(xi,tj) + C(xi_pt i) OX2 AX 2 '

(7)

where C is either salinity or D I P . Following Cifuentes et al. (1990), At (0"05 days) was chosen to be < 0"2 days, and cross-sectional areas were from a salinity intrusion model (Thatcher & Harleman, 1978). Longitudinal dispersion coefficients were estimated from salinity distributions and river flow with equation 2 using average values over each simulation period for R(x), S(x), and dS(x)/dx (Cifuentes et al., 1990). S(x) was determined using a 4th order polynomial fit of measured salinity distributions at the beginning and end of the simulation period. F o r stations with an average salinity < 1 ppt, the dispersion coefficient for the I ppt station was used. Dispersion coefficients during the study period varied from < 50 to 700 m 2 s-1 depending on seasonal changes in river flow. In model runs, the reactive term in equation 4 was separated into two components: uptake by plankton and regeneration. As defined here, regeneration includes all input and removal except D I P uptake by plankton. Simulated stations in the model were located every 5 k m with initial values for D I P and salinity determined from measured profiles by linear interpolation. Fluctuations in salinity and D I P concentration at both the low and high salinity end m e m b e r s were determined using polynomial curvefits (2nd-6th order) of field data. T o obtain a good fit to the data, several (5-7) different lines were used to describe variations in a given parameter at each end member. Linear interpolation was used between daily concentrations. River flow entering the salinity gradient was estimated from Delaware River discharge by including flow from the Schuylkill River; Delaware River discharge was multiplied by 1.25 (Smullen et al., 1983). Average D I P regeneration was determined by solving the modified advection-dispersion equation for D I P (equation 4) with D I P uptake, river discharge, and river D I P concentration specified. For each simulation period, D I P uptake at the beginning and end was averaged, and it was assumed that uptake was constant over the period. T o estimate D I P regeneration, the model was solved by iteration; the rates of D I P regeneration used as model input were adjusted until model output reproduced field survey data for the end of the simulation period. Model runs were terminated when the difference between simulated and field results was < 0.03 ~M for all stations (precision for D I P is 0.05 ~tM). T h e solution achieved in this fashion is, therefore, not unique, but it is one of a family of solutions since adjustment of regeneration at any station affects the values required upstream and downstream. T h e envelope for these solutions probably varies by 5% for each simulation station. Even with this uncertainly, the average regeneration rates derived here provide a good first approximation. Model verification T h e reactive mixing model was tested without reactive terms before estimating regeneration rates (Lebo, 1990a). First, the stability and response time of the model were tested by simulating mixing under constant river discharge and D IP concentration. After starting with a non-linear DIP-salinity distribution, the model produced a linear D I P salinity distribution within 50 days. I n fact, the DIP-salinity distribution was nearly linear after only 20 days. Next, the effect of varying the river D I P concentration on D I P salinity distributions was examined. River D I P was varied between 1-3 I~M using a cosine

Modelingphosphoruscycling

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Figure 2. Model predicted salinity distribution for February 1987 compared with measured distributions. The model run began with the January salinity distribution and simulated forward to February 1987. • , January 1987; ©, February 1987; --, Model.

function with a period of 50 days while river discharge was held constant. As river D I P fluctuated, DIP-salinity distributions were produced that suggested both input and removal during mixing. Lebo (1990a) provides a more extensive discussion of model verification. As a final test, a salinity distribution predicted by the model was compared with field data for the simulation.period (January 13-February 18, 1987). Beginning with the January salinity distribution, the model was simulated forward to February. T h e salinity distribution predicted by the model for February was very similar to the measured distribution (Figure 2). With the reactive terms included, the sensitivity of the model was tested on the value of dispersion coefficients and river flow entered. T o examine the influence of the dispersion coefficients entered on model results, average D I P regeneration was determined using K(x) calculated with equation 8, a 3-fold increase in K(x), and a 3-fold decrease in K(x) for June-July 1987 (Lebo, 1990a). A factor of three was chosen since K(x) varied by about an order of magnitude during the study period. T h e 9-fold variation in K(x) caused estimated D I P regeneration to vary by only 2.9 + 2 . 0 % (n=25). T h e sensitivity of model results to river flow was tested by running the model for J u n e - J u l y 1987 at 80%, 100%, and 120% of the recorded flow. Varying river flow by + 20% caused estimated regeneration to vary by 0"8 + 0 " 6 % ( n = 25). T h e s e small variations in model results due to variations in the dispersion coefficient or river flow are within the probable uncertainty of 5 % in estimating D I P regeneration by the iterative solution technique used.

Results and discussion

Phosphorus fluxes Anthropogenic inputs to the tidal river increased the flux of total P (TP) relative to Delaware River. Although total flux was highly variable during 1986-88, mean T P flux increased from 0.79 + 0.33 mol s - 1(n = 18) at station 1 to 2-46 + 2.12 mol s - t (n = 22) at the head of the salinity gradient (station 14, Figure 3). M e a n values, in this case, are the n o n -

242

M. E. Lebo & J. H. Sharp

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Figure 3. Mean P fluxes for Delaware River (1), beginning of the salinity gradient (14), and mouth of the bay (26) for 1986-88. Error bars indicate one standard deviation.

r-l, DIP; ~, DOP; IN, PP; II, TP. weighted average of all available data for 1986-88. This 3-fold increase in the tidal river is highly significant ( P < 0-01) and is due to major municipal inputs near Philadelphia (140180 km, Frake et al., 1983). Similar to other urbanized estuaries (Simpson et al., 1975; M a c K a y & Leatherland, 1976; Conomos et al., 1979; Fraser & Wilcox, 1981; Froelich et al., 1985; Rehm, 1985), P flux in the river was predominantly as dissolved P (66-72%, Figure 3), with D I P contributing 56-59% of the total. T h i s is in contrast to systems uncontaminated by m a n where the bulk of P transport is as PP (Turekian, 1971; Meybeck, 1982). In Delaware Bay, the mean flux of T P entering (14) and leaving (26) the bay during 1986-88 was nearly equivalent (Figure 3). At the m o u t h of the bay, T P flux was I" 95 ± 2.99 ml s - i (n = 21 ) as compared with 2-46 + 2.12 mol s - i (n = 22) at the head of the salinity gradient. However, there was a change in the composition of T P flux within the bay. Despite the similarity in mean total flux, P flux at station 26 was predominantly particulate (PP) rather than D I P . T h e flux of D I P decreased in the bay from 1.37 ± 1- I0 mol s - 1 at station 14 to 0.42 _ 1-46 mol s - i at the mouth. T h i s highly significant decrease (P = 0-02) was mainly offset by an increase in the flux of PP at the mouth; PP increased from 0.85 ± 0-89 mol s - I at station 14 to 1.29 ± 2.01 mol s - i at station 26. In constrast to D I P and PP, the relative contribution of dissolved organic P ( D O P ) to mean total flux was small and relatively constant throughout the estuary. During 1986-88, D O P accounted for 10-13% of mean T P flux in both the river and the bay (Figure 3). Since average D O P concentrations are relatively constant throughout the estuary (Lebo, 1990a), similar relative fluxes in the river and bay are not surprising. Seasonal variations Seasonal variations in P fluxes were examined at each location by grouping flux estimates for individual cruises by season (Figure 4). Averaging flux estimates by season reduces the statistical power to differentiate differences between values. Consequently, no differences between seasons for 1986-88 (n = 5-8) were significant ( P < 0.05). However, examination of interseasonal variability allowed us to look for mechanisms driving variability. At station 1, T P flux was relatively constant fall-spring (0-57 + 0-14 tool s - 1), but it was higher in the s u m m e r (1-01 + 0.54 mol s - 1). Higher s u m m e r T P flux was due to an increase

Modeling phosphorus cycling

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Figure 4. Seasonally averaged mean P fluxes for Delaware River (1), beginning of the salinity gradient (14), and the mouth of the bay (26) for 1986-88. Error bars indicate one standard deviation. Note change in scale for TF. [], Spring; I~, Summer; II, Fall;

[], Winter.

in D I P during s u m m e r (0.61 mol s - 1) relative to other seasons (0.33 -t- 0.04 mol s - 1); flux of D O P and PP at station 1 were relatively constant at 0-08 +__0.03 and 0-20___0.09 mol s - l respectively during all seasons. Phosphorus fluxes at the beginning of the salinity gradient (station 14) demonstrated more seasonal variability than at station 1 (Figure 4). T P flux was higher and more variable in spring (2.69-t-2.83 mol s - l ) and fall (3.05-t-2"08 mol s -1) than during winter and s u m m e r (2.06 __+0'94 and 1.78 + 1-03 mol s - l respectively). Higher and more variable flux during spring and fall can be attributed to variations in river flow during these periods. M e a n Delaware River discharge, averaged for three days prior to each sampling, was 2 7 5 + 154 ( n = 5 ) , 5 4 9 + 6 1 3 ( n = 8 ) , 2 1 1 + 114 ( n = 5 ) , and 422+261 ( n = 5 ) for winter, spring, summer, and fall respectively. Figure 5 compares T P flux with discharge. For data collected during 1986-88, T P flux was proportional to discharge. Th:s relationship is highly significant (r = 0.92; n = 22; P < 0"001) supporting the importance of river flow variations to T P flux. Highest total flux occurred in April 1987 when flow was > 1700 m 3 s -l. T h e correlation between total flux and discharge was due primarily to seasonal variations in PP (0-39 to 1-31 tool s - ~) and D O P (0-06 to 0-31 mol s - l) rather than D I P (1.16 to 1.50 tool s-1 Figure 4). This constancy of seasonal D I P flux from the river to the bay can be attributed to an inverse relationship between D I P concentration and river flow (Lebo, 1990a); as river flow increases, D I P concentration decreases, thereby, maintaining a relatively constant flux of D I P to the bay. Phosphorus fluxes were most variable at the m o u t h of the bay with large interseasonal and intraseasonal differences (station 26, Figure 4). Variability of flux estimates within seasons was high throughout the year with seasonal mean fluxes of T P , D I P , PP, and D O P not significantly different (P < 0.05) f r o m zero due to large variations in flux estimates for individual cruises. When mean flux of T P for different seasons was compared, there was a large increase in fall (4.62 mol s - l ) relative to other seasons (1-18___0-12 mol s-l). This increase in T P flux during fall was due to increases in both D I P and PP fluxes, with

244

M. E. Lebo & J. H. Sharp

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F i g u r e 5. T P flux

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Delaware River discharge.

seasonal maxima occurring for these fractions during the fall. Seasonal variations in the flux of PP and D I P at the mouth of the bay were large, with D I P flux actually into the bay from coastal waters during the spring. This additional input of D I P into the bay during spring was partially compensated for by moderately high PP flux out of the bay (1.29 __ 1.47 mol s- 1) Phosphorus balances

Retention of P within the river and bay can be estimated by combining seasonally averaged flux estimates with known P inputs. In this simplified two-box representation of the Delaware, the difference between total inputs and exports of P indicates net retention within each region during the period of interest. T h e balance for P in the river includes inputs from the Delaware River, municipal inputs (3-68 mol s -~, Frake et al., 1983), and the Schuylkill River while the only output is transport to the bay. Phosphorus flux from the Schuylkill River into the river was estimated from average concentrations of P fractions at station 10 (located in the Schuylkill River) and river flow estimated from Delaware River discharge. Schuylkill River flow was determined by multiplying Delaware River discharge by the average annual ratio of Schuylkill discharge to Delaware discharge (0.25, Smullen et al., 1983). In the bay, the balance is much simpler with single major input (river flow) and output (coastal waters); subtributaries within the bay accounted for only a small fraction ( < 4% ) of T P inputs. T h e retention of P within the river and bay regions of the estuary was very different. In the river, an average o f 5 5 % of T P inputs were retained as compared with only 16% in the bay (Table 1). Annual averages used in this comparison are seasonally weighted means to eliminate sampling bias due to higher sampling frequency in the spring (8 cruises as compared with 5 cruises during other seasons). Despite minimal removal of T P , the bay was relatively efficient (61%) at removing DIP. This consumption of D I P was, however, offset by a large increase ( + 51%) in the export of PP. T h e bay, therefore, functions more as a P transformer converting D I P into particulate matter rather than as a sink. T h e annual transport of a majority (84%) of river T P inputs through the bay to coastal waters is consistent with mesocosm experiments conducted with Narragansett Bay waters

Modeling phosphorus cycling

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TABLE1. Phosphorus mass balances for river and bay. Values are seasonal averages of flux estimates in mol-P s-~. Phosphorus retention is divided by inputs to determine the percentage Fraction

Season

Inputs

Outputs

River TP

Winter Spring Summer Fall Average Winter Spring Summer Fall Average Winter Spring Summer Fall Average Winter Spring Summer Fall Average

4-92 4.93 5.39 5.49 5.18 1.96 2.55 1-78 3,05 2.34 1.16 1,48 1-25 1.50 1.35 0.74 0,76 0.39 1.31 0.80

1-96 2-55 1.78 3.05 2.34 0-92 1.13 1.13 4.63 1.95 0.29 -0.52 0-37 1.99 0.53 0.52 1,29 0-61 2.42 1.21

Bay TP

Bay DIP

Bay PP

Retention 2.96 2.38 3.61 2.44 2.84 1.04 1.42 0.65 - 1-58 0.38 0.87 2-00 0-88 -0.49 0.82 0-22 -0.53 -0.22 - 1,11 -0-41

Percent 60 48 67 44 55 53 56 37 -- 52 16 75 135 70 - 33 61 30 -70 -56 -85 - 51

(Nowicki & Oviatt, 1990) and nutrient budgets for Chesapeake Bay (Nixon, 1987). Similar balances for D I P and PP in the river cannot be constructed since municipal P inputs are r e p o r t e d as T P . Phosphorus was t r a p p e d within the bay on a seasonal basis. D u r i n g the w i n t e r - s p r i n g , D I P was completely removed from solution in the lower bay (Lebo, 1990a) with a retention of > 75% of river D I P inputs ( T a b l e 1). Even though there was a net export of PP d u r i n g these seasons, 5 3 - 5 6 % of T P was retained within the bay. T h i s retention of P was only t e m p o r a r y with P r e m o v e d during the spring bloom remineralized during the s u m m e r (Lebo, 1990a) and exported from the system during the fall; the bay was a source ( + 52 %) for T P during the fall. T h e three m o n t h time lag between s u m m e r remineralization and fall export is due to the relatively long h y d r o d y n a m i c residence time of water in the bay during the s u m m e r (150 days, Cifuentes et al., 1990).

Phosphate regeneration T h e reactive mixing model was used to estimate apparent D I P regeneration along the bay. F i g u r e 6 shows average p h y t o p l a n k t o n P - d e m a n d and regeneration of D I P for the u p p e r (100-130), m i d d l e (65-100), and lower (0-65) bay during 1987. In all regions of the bay, there was a tight coupling between P - d e m a n d by plankton and mean D I P regeneration. Except for early spring ( F e b r u a r y - A p r i l ) in the m i d d l e bay (65-100 km), average D I P regeneration completely fulfilled p h y t o p l a n k t o n P - d e m a n d ; D I P regeneration p r o v i d e d 102 + 19 (n = 7), 94 + 10 (n = 7), and 95 + 6% (n = 10) of p h y t o p l a n k t o n P - d e m a n d in the upper, middle, and lower bay respectively. In this comparison, sampling dates with low p h y t o p l a n k t o n p r o d u c t i o n ( P - d e m a n d < 0.1 I~mol-P 1- l d a y - l) were excluded. T h i s close coupling between D I P uptake and regeneration indicates a rapid recycling of P. I t was only d u r i n g the early spring ( F e b r u a r y - A p r i l ) in the m i d d l e bay (65-100 km) that there was a disparity between P - d e m a n d and regeneration (Figure 6). D u r i n g this period,

246

M. E. Lebo & J. H. Sharp

1.5 100-

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Figure 6. Average D I P regeneration along the upper (100-130km), middle (65100 km), and lower (0--65 kin) bay. Biotic P-demand, • , (uptake) is shown as a reference. Regeneration rates, ©, were determined from simulated estuarine mixing using measured D I P distributions as the beginning and endpoints (see text).

phytoplankton P-demand increased rapidly as the spring bloom developed in the bay in February. However, this increase in demand was not balanced by a simultaneous increase in regeneration. T h e rate of D I P regeneration remained low until March indicating a large deficit in P cycling. During this period of P-consumption, there was an accumulation of phytoplankton production as biomass. Figure 7 shows average chlorophyll concentrations and the difference between P-demand and regeneration in the middle bay. Clearly, there was an accumulation of phytoplankton biomass during the period when there was a deficit in D I P regeneration relative to phytoplankton demand. Pennock (1987) reported a similar imbalance for nitrogen in the bay during the early spring. T h e in situ regeneration of D I P in the bay was estimated by constructing seasonal mass balances of processes contributing to the total regeneration rate determined with the reactive simulation model (Table 2). T o estimate water column regeneration, geochemical removal, benthic release, and seston release of D I P were estimated. T h e rate of water column regeneration was calculated from total D I P released by subtracting D I P released from particles and benthic sediments. In the balance, total D I P released was determined by adding D I P removed geochemically (Lebo, 1990a) to the estimated regeneration rate. T o extend benthic D I P release data measured during summer months (Seitzinger, 1988)

Modeling phosphorus cycling

247

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TABLE 2. Seasonal P fluxes (106 moles) for the bay with D I P regeneration separated into components. Benthic flux data in parentheses were estimated from the summer rates using the temperature dependence of sediment D I P release reported by Matisoff et al. (1981) Process

Winter

Spring

Summer

Fall

15.4

20.0

14.0

24.0

2.0 (1.5) - 1.9 45.2 46.8

1.6 (4.2) -- 2.3 74.5 78-0

1.5 19.0 J - 2.4 119-1 137-2

3.5 (5.3) - 2.7 14-8 20.9

7.2 61.9

8"9 80-2

8.9 136.7

36.4 25.4

Sources:

River Inputs" Regeneration b Seston Release ~ Benthic Release Geochemical' Water Columre' Total Removal: Export e Phytoplankton h P-demand

*Transport into salinity gradient from the river; bestimated from biotic uptake using the ratio of regeneration/uptake during each season; 'estimated from particle-bound P distributions (Lebo, 1991); aestimated from Seitzinger (1988); 'estimated from January DIP-salinity distributions; Jbalance of regeneration terms; texport to coastal waters; hestimated from areal phytoplankton carbon production.

to other times of the year, summer release rates were multiplied by a factor taking into account an assumed effect of temperature variations on release. The factor for each season was determined by comparing the relative release of DIP from sediments at the average temperature of that season (average from 5-8 cruises) to release during the summer at 25 °C. A v e r a g e t e m p e r a t u r e s u s e d w e r e 2 ( w i n t e r ) , 11 ( s p r i n g ) , a n d 13 ° C (fall). T h e e n e r g y o f a c t i v a t i o n f o r D I P r e l e a s e f r o m s e d i m e n t s ( 1 8 . 1 7 k c a l m o l -~) w a s t a k e n f r o m M a t i s o f f e t al. ( 1 9 8 1 ) .

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M. E. Lebo & ft. H. Sharp

TABLE3. Seasonal P fluxes (10 ° moles) for the river. In the balance, abiotic removal is defined as the difference between total input and removal (biological uptake and transport into the bay)

Season

Inputsa

Uptakeb

Winter Spring Summer Fall Total

38.7 38.8 42-4 43-2 163-1

0.2 14.1 30-8 9-5 54.6

Export" 15-4 20.0 14-0 24-0 73-4

Abiotic Removal 23.1 4.7 - 2.4 9-7 35-1

P-PO4 a no data 8.6 23.1 7.7

aIncludes Delaware R., Schuylkill R., and municipalities; bestimated from measured biotic uptake rates (Lebo, 1990b); 'transport into the bay; aestimated from poisoned samples (Lebo, 1990a). D I P regeneration was dominated by water column processes. In all seasons, regeneration within the water column was the dominant source of regenerated D I P accounting for 71-97% of the total (Table 2). T h e greatest uncertainty in this balance is associated with sediment D I P release since we have assumed a specific temperature relationship taken from another system. However, even a 50°/0 uncertainty in sediment release would not change the overall conclusions of the balance. Water column regeneration was highest during spring-summer with 74-5-119.1 x 106 moles of D I P released. This is equivalent to 87-93% of phytoplankton P-demand. T h e balance of D I P regeneration was supplied mainly through sediment release (3-25 % ). T h e release of D I P from seston or geochemical removal were always small compared with phytoplankton P-demand.

River phosphorus cycling Phosphorus cycling in the river was dominated by antropogenic inputs, biological uptake, and geochemical removal. Processes contributing to river P cycling are compared in Table 3. T h e importance of anthropogenic inputs to P cycling was evident in the large increase in T P flux (Figures 3 and 4). Even with this large increase in T P flux, a large fraction of T P inputs to the river were apparently removed to the sediment. Although biotic uptake of D I P can account for a large fraction of this removal, especially during high biological activity in summer, large deficits in P-removal occurred during fall-spring. Abiotic removal in Table 3 represents the difference between net removal and uptake. This simple mass balance, therefore, suggests that abiotic processes are important in P cycling within the tidal river. It is unlikely that biotic D I P uptake completely accounts for all T P removal even during summer. T h e regeneration of even 50% of P incorporated into plankton during summer would leave a deficit between exports and imports of approximately 12 x 106 moles. Geochemical D I P removal within the river is supported by radiotracer studies (Lebo, 1990a). Comparing abiotic D I P removal, as determined by azide poisoned 32p-PO 4 removal, with apparent non-biological removal, abiotic removal can complete seasonal balances for P in the river (Table 3). In fact, abiotic removal was highest during summer, when no deficit was indicated. This suggests that a large fraction of summer biological production was remineralized and not retained in the sediments. During spring-fall, 32p_ PO 4 experiments indicate that geochemical D I P removal accounted for up to 59% of T P removal. This estimate must, however, be viewed cautiously since 32P - P O 4 removal rates

Modeling phosphorus cycling

249

include both isotopic exchange and net removal (Lebo, 1990b). T h e similarity between spring and fall abiotic removal determined from mass balances and 32P-PO 4 methodology, however, suggests the 32P-PO 4 removal rates may be fairly close to net removal in the tidal river. T h e enrichment of particles with P in the river near Philadelphia corroborates geochemical removal of P as a dominant sink of T P inputs. As D I P concentration increased in the Philadelphia region (140-180km), inorganic P-content of particles also increased from 150 ~tmol g - ~at stations 1-3 to > 190 ~tmol g - ~(Lebo, 1991). Sequential extractions of suspended particles revealed a highly significant ( P < 0.02) increase in PP associated with aluminium and iron oxyhydroxide phases in this region. One consequence of high sediment P-content is that sufficient T P is present in the system to maintain relatively high D I P concentration for several years without any sources of T P to the river. Release of large amounts of sediment P has occurred in several systems after P abatement strategies have been employed (Bjork, 1972; Larsen et al., 1975). T h e removal of dredge spoils from the tidal river of Delaware Estuary for land disposal may help to reduce some of this buildup of P in the sediments. B a y phosphorus cycling Phosphorus cycling in the bay, in contrast, is linked strongly to biological production cycles. Table 2 compares rates of P cycling in the bay to river input and export to coastal waters. In all seasons, P-demand by phytoplankton was greater than the T P flux into the bay. During winter-summer, biological P-demand was highest and 4-10 times greater than total fluvial inputs to the bay. T h e dominant source of D I P to meet phytoplankton P-demand was always D I P regeneration within the water column. This simple picture of. P cycling in the bay is, however, incomplete. Although geochemical reactions do not control P cycling, P geochemistry also is important. Phosphorus is released and removed geochemically during mixing. As suspended material is transported into the bay, D I P is released from amorphous iron and aluminium oxides (Lebo, 1991). Conversely, flocculation of colloidal material removes PP associated with iron colloids (Sharp et al., 1984; Church, 1986). A complete description of P cycling in the bay must include these geochemical reactions. High biological production within the bay transforms D I P into PP. Despite the large input of D I P (58%) into the bay, PP was the dominant form (62%) of P exported (Figure 3). During estuarine mixing, there was a biologically mediated transfer of D I P into particulate matter. Changes in the organic-P content of particles during mixing corroborate this transfer of D I P into cells. Lebo (1991) reported that the P-content of particles was relatively constant in the bay despite a decrease in inorganic PP during estuarine mixing. D I P released from iron and aluminium oxides was offset by increases in PP bound in organic phases; PP associated with cells increased from 52 + 6% in the river to 69 + 6% within the lower bay which is highly significant (P ~ 0.01). This ' repackaging' of D I P during biological cycling temporarily traps P within the bay. D I P uptake by plankton was temporarily out Of balance with regenerative processes in the spring (Figure 6) causing D I P to be completely removed in the bay, and D I P was imported from coastal waters (Figure 4). Although part of the PP produced was exported during the spring, a large fraction of T P was temporarily retained within the bay (Table 1). Phytoplankton blooms during other seasons should similarly enhance temporary P retention. Since D I P removal by phytoplankton is important in many estuaries (Kuenzler et al., 1979; Peterson et al., 1985; Rehm, 1985; Fisher et al., 1988), P retention through

250

M. E. Lebo & J. H. Sharp

biological r e p a c k a g i n g is p o t e n t i a l l y a w i d e s p r e a d sink for P. T h e p e r m a n e n c e o f this b i o t i c P sink d e p e n d s o n r e g e n e r a t i v e processes w i t h i n t h e system. I n the D e l a w a r e , r e g e n e r a t i o n was r a p i d , a n d m o s t o f t h e P r e m o v e d was r e l e a s e d w i t h i n t h e s a m e year.

Conclusions P h o s p h o r u s cycling in t h e D e l a w a r e E s t u a r y s h o w e d v e r y different p a t t e r n s b e t w e e n the r i v e r a n d t h e b a y . I n t h e river, P c y c l i n g was s t r o n g l y influenced b y a n t h r o p o g e n i c i n p u t s w i t h a large increase ( 2 8 0 % ) in T P t r a n s p o r t . D I P c o n c e n t r a t i o n s w o u l d b e s u b s t a n t i a l l y h i g h e r , b u t 4 4 - 6 7 % o f i n p u t s were r e m o v e d , p r e d o m i n a n t l y t h r o u g h g e o c h e m i c a l p r o cesses. O n e o f the c o n s e q u e n c e s o f t h e h i g h g e o c h e m i c a l r e a c t i v i t y in the r i v e r is P - s t o r a g e w i t h i n r i v e r s e d i m e n t s . I n contrast, P c y c l i n g in the b a y is d o m i n a t e d b y biological p r o cesses w i t h D I P r e m o v e d d u r i n g the s p r i n g a n d r e g e n e r a t e d d u r i n g the s u m m e r . A l t h o u g h g e o c h e m i c a l reactions are n e c e s s a r y to d e s c r i b e P c y c l i n g in t h e bay, fluxes are small in c o m p a r i s o n to biological P - d e m a n d . T h e s t r o n g seasonal differences in P t r a n s p o r t in the b a y reinforces t h e n e e d for c o m p r e h e n s i v e s a m p l i n g at all t i m e s o f t h e y e a r to o b t a i n an accurate e s t i m a t e o f n e t T P t r a n s p o r t . O v e r a l l , t h e D e l a w a r e E s t u a r y retains only a small fraction (16 o/0) o f fluvial T P i n p u t s a n d is a large source o f P to coastal waters.

Acknowledgements T h i s m a n u s c r i p t is a p o r t i o n o f a d i s s e r t a t i o n p r e p a r e d for the U n i v e r s i t y o f D e l a w a r e p u b l i s h e d t h r o u g h U n i v e r s i t y M i c r o f i l m s I n t e r n a t i o n a l , P.O. Box 1764, A n n A r b o r , M I 48106-1346, U . S . A . T h e r e s e a r c h was s u p p o r t e d b y g r a n t s to J. H . S h a r p f r o m the Office o f Sea G r a n t ( N A 8 6 A A - D - S G 0 4 0 ) o f the U . S . N a t i o n a l O c e a n i c a n d A t m o s p h e r i c A d m i n i s t r a t i o n . W e t h a n k W . J. U l l m a n , D . L . K i r c h m a n , P. N . F r o e l i c h , C. P. H u a n g , A. D . Jassby, a n d two a n o n y m o u s reviewers for h e l p f u l c o m m e n t s .

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Fox, L. E. 1989 A model for inorganic control of phosphate concentrations in rivers. Geochimica et Cosmochimica Acta 53,417-428. Fox, L. E., Sager, S. L. & Wofsy, S. C. 1985 Factors controlling the concentrations of soluble phosphorus in the Mississippi estuary. Lbnnology and Oceanography 30, 826-832. Fox, L. E., Sager, S. L. & Wofsy, S. C. 1986 The chemical control of soluble phosphorus in the Amazon River and estuary. Geochimica et Cosmoehimica Acta 50, 783-794. Fox, L. E., Lipschultz, F., Kerkhof, L. & Wofsy, S. C. 1987 A chemical survey of the Mississippi estuary. Estuaries 10, 1-12. Frake, A. C., Sharp, J. H., Pike, S. E., Pennock, J. R., Culberson, C. H. & Canzonier, W. J. 1983 Nutrients (nitrogen, phosphorus, silicon). In The Delaware estuary: Research background for estuarine management and development (Sharp, J. H., ed.). A report to the Delaware River and Bay Authority, pp. 65-78. Fraser, T. H. & Wilcox, W. J. 1981 Enrichment of a subtropical estuary with nitrogen, phosphorus and silica. In Estuaries and Nutrients (Neilson, B. J. & Cronin, L. E., eds). Academic, pp. 481-498. Froelich, P. N. 1988 Kinetic control of dissolved phosphate in natural rivers and estuaries: A primer on the phosphate buffer mechanism. Limnology and Oceanography 33~ 649-668. Froelich, P. N., Kaul, L. W., Byrd, J. T., Andreae, M. O. & Roe, K. K. 1985 Arsenic, barium, germanium, tin, dimethylsulphide and nutrient biogeochemistry in Charlotte Harbor, Florida, a phosphorusenriched estuary. Estuarine, Coastal and Shelf Science 20, 239-264. Helder, W. & Ruardij, P. 1982 A one-dimensional mixing and flushing model of the Ems-Dollard estuary: Calculation of time scale at different river flows. NetherlandJournal of Sea Research 15, 293-312. Kaul, L. W. & Froelich, P. N. 1984 Mode'ling estuarine and nutrient geochemistry in a simple system. Geochimica et Cosmochimica Acta 48~ 1417-1433. Kuenzler, E. J., Stanley, D. W. & Koenings, J. P. 1979 Nutrient kinetics ofphytoplankton in the Pamlico River, North Carolina. Water Resources Institute of the University of North Carolina. Report 139. 163 pp. Larsen, D. P., Malueg, K. W., Schults, D. W. & Brice, R. M. 1975 Response of eutrophic Shagawa Lake, Minnesota, U.S.A., to point-source phosphorus reduction. Verhandlungen/Internationale Vereinigung ffir Theoretische und Angewandte Limnologie 19, 884-892. Lebo, M. E. 1990a Biogeochemical phosphorus cycling in the Delaware, an urbanized coastal plain estuary. Ph.D. dissertation, Univ. of Delaware, Newark, Delaware, 255 pp. Lebo, M. E. 1990b Phosphate uptake along a coastal plain estuary. Limnology and Oceanography 35, 1279-1289. Lebo, M. E. 1991 Particle-bound phosphorus along an urbanized coastal plain estuary. Marine Chemistry 34, 225-246. Liss, P. S. 1976 Conservative and non-conservative behaviour of dissolved constitutents during estuarine mixing. In Estuarine chemistry (Burton, J. D. & Liss, P. S., eds). Academic, London, pp. 93-126. Loder, T. C. & Reichard, R. P. 1981 T h e dynamics of conservative mixing in estuaries. Estuaries 4, 64-69. MacKay, D. W. & Leatherland, T. M. 1976 Chemical processes in an estuary receiving major inputs and domestic wastes. In Estuarine chemistry (Burton, J. D. & Liss, P. S., eds). Academic, London, pp. 185-218. Matisoff, G., Fisher, J. B. & McCall, P. L. 1981 Kinetics of nutrient and metal release from decomposing lake sediments. Geochimica et Cosmochimica Acta 45, 2333-2347. Meybeck, M. 1982 Carbon, nitrogen, and phosphorus transport by world rivers. American Journal of Science 282, 401-450. Murphy, J. & Riley, J. P. 1962 A modified single solution method for the determination of phosphate in natural waters. Analytica Chimica Acta 27, 31-36. Nixon, S. W. 1987 Chesapeake Bay nutrient budgets--a reassessment. Biogeochemistry 4, 77-90. Nowicki, B. L. & Oviatt, C. A. 1990 Are estuaries traps for anthropogenic nutrients? 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