A comparison of residence time calculations using simple compartment models of the Altamaha River estuary, Georgia

The residence and flushing times of an estuary are two different concepts that are often confused. Flushing time is the time required for the freshwater inflow to equal the amount of freshwater originally present in the estuary. It is specific to freshwater (or materials dissolved in it) and represents the transit time through the entire system (e.g., from head of tide to the mouth). Residence time is the average time particles take to escape the estuary. It can be calculated for any type of material and will vary depending on the starting location of the material. In the literature, the term residence time is often used to refer to the average freshwater transit time and is calculated as such. Freshwater transit time is a more precise term for a type of residence time (that of freshwater, starting from the head of the estuary), whereas residence time is a more general term that must be clarified by specifying the material and starting distribution. We explored these two mixing time scales in the context of the Altmaha River estuary, Georgia, and present a comparison of techniques for their calculation (fraction of freshwater models and variations of box models). Segmented tidal prism models, another common approach, have data requirements similar to other models but can be cumbersome to implement properly. Freshwater transit time estimates from simple steady-state box models were virtually, identical to flushing times for four river-flow cases, as long as boxes were scaled appropriately to river flow, and residence time estimates from different box models were also in good agreement. Mixing time estimates from box models, were incorrect when boxes were imporperly scaled. Mixing time scales vary nonlinearly with river flow, so characterizing the range as well as the mean or median is important for a thorough understanding of the potential for within-estuary processing. We are now developing an imporved box model that will allow the calculation of a variety of mixing time scales using simulations with daily variable river discharge.     

Temporal and spatial patterns of trace elements in the Patuxent River: A whole watershed approach

Trace element distributions, partitioning, and speciation were examined at 15 sites in the Patuxent River watershed from May 1995 through October 1997 to determine possible sources of trace elements to the river and estuary, to examine the relationship of the trace element discharges to freshwater discharges as well as to land use and geographic region, to validate previous estimates of loadings to the river, and to provide baseline data for trace elements in the Patuxent River watershed and estuary. Six freshwater sites were examined, representing different basins and geographic provinces, and nine sites along the estuarine salinity gradient. Subregions within the watershed varied considerably in concentrations and areal yields for some elements. Concentrations of As, Cd, Ni, Pb, and Zn were elevated in the Coastal Plain sites compared to the Piedmont sites, while Cu and Hg were more evenly distributed. Cadmium, Cu, Hg, Ni, Pb, and Zn showed overall positive correlations with river flow while As and methylHg (meHg) showed negative correlations with river flow. Concentrations of trace elements in the estuarine portion of the river were generally low, and consistent with mixing between Patuxent River water with elevated concentrations and the lower concentrations of the Chesapeake Bay. Interesting features included a local Cd maximum in the low salinity region of the estuary, probably caused by desorption from suspended sediments, and a significant input of water containing high As concentrations from the Chesapeake Bay and from As being released from bottom sediments in summer. Comparisons between the estimated annual flux of trace elements and the estimates of suspected source terms (atmospheric deposition, urban runoff, and known point sources) suggest that, except for Hg, direct atmospheric deposition is small compared to fluvial loads. Current estimates of trace element inputs from point sources or from urban runoff are inadequate for comparison with other sources, because of inappropriate techniques and/or unacceptably high detection limits. A complete examination of trace element dynamics in the Patuxent River (and in other coastal systems) will require better data for these potential sources.     

Comparison of the Salinity Structure of the Chesapeake Bay,the Delaware Bay and Long Island Sound Using a Linearly Tapered Advection-Dispersion Model

Long records of monthly salinity observations along the axis of Chesapeake Bay, Delaware Bay, and Long Island Sound are used to test a simple advection–dispersion model of the salt distribution in linearly tapered estuaries developed in a previous paper. We subdivide each estuary into three to five segments, each with linear taper allowing a distributed input of fresh water, and evaluate the dispersion in each segment. While Delaware Bay has weak dispersion and a classical sigmoidal salinity structure, Long Island Sound and Chesapeake Bay are more dispersive and have relatively small gradients in the central stretches. Long Island Sound is distinguished by having a net volume and salt flux out of its low-salinity end resulting in a smaller range of salinity and increasing axial gradients at its head rather than the usual asymptotic approach to zero salinity. Estimates of residence times based on model transport coefficients show that Long Island Sound has the most rapid response to fresh-water flux variations. It also has the largest amplitude cycle in river discharge fluctuation. In combination, these cause the large seasonal variation in the salinity structure relative to interannual variability in Long Island Sound as compared with Chesapeake Bay and Delaware Bay.     

Phosphorus Burial in Sediments Along the Salinity Gradient of the Patuxent River,a Subestuary of the Chesapeake Bay (USA)

We used a sequential extraction technique and ²¹⁰Pb dating to determine the chemical form and amount of particulate phosphorus (PP) that is retained during burial in 1-m-long sediment cores collected along a salinity gradient from tidal freshwater to the mesohaline waters of the Patuxent River, a subestuary of the Chesapeake Bay. PP buried in the study sites with salinity values ≤3 was similar in concentration and form to PP entering the Patuxent from the watershed, suggesting efficient sequestration by the sediments at these low-salinity sites. PP extracted with citrate–dithionite–bicarbonate was the dominant form of PP at all salinities and all depths, and organic-P was the second most abundant fraction. We estimated that 81% of PP entering from the watershed is trapped in the sediments of the upper Patuxent subestuary and that the subtidal sediments retain three times as much PP as the marshes adjacent to the study sites.     

Standing crops of marsh vegetation of two tributaries of Chesapeake Bay

A comparative study of the standing crop of marsh vegetation was made of the Patuxent River and Parker Creek, two tributaries of Chesapeake Bay. The biomass of marsh vegetation in the tidal freshwater and brackish regions of the Patuxent was relatively uniform with regard to salinity, seasonally high concentrations of dissolved nitrogen, and phosphorus and nutrient gradient. Maximum values of biomass occurred in the tidal freshwater and slightly brackish water region of Parker Creek, a system whose nutrient concentrations approximated 20% of those of Patuxent River. Biomass values for the Patuxent River and Parker Creek averaged about 1417 and 895 g m^?2 dry weight, respectively. Estimates of total annual marsh production based on the maximum standing crop was 27×10³ and 519 metric tons, respectively, for the Patuxent River and Parker Creek.     

Long-Term Changes in Water Quality and Productivity in the Patuxent River Estuary: 1985 to 2003

We conducted a quantitative assessment of estuarine ecosystem responses to reduced phosphorus and nitrogen loading from sewage treatment facilities and to variability in freshwater flow and nonpoint nutrient inputs to the Patuxent River estuary. We analyzed a 19-year dataset of water quality conditions, nutrient loading, and climatic forcing for three estuarine regions and also computed monthly rates of net production of dissolved O₂ and physical transport of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) using a salt- and water-balance model. Point-source loading of DIN and DIP to the estuary declined by 40–60% following upgrades to sewage treatment plants and correlated with parallel decreases in DIN and DIP concentrations throughout the Patuxent. Reduced point-source nutrient loading and concentration resulted in declines in phytoplankton chlorophyll-a (chl-a) and light-saturated carbon fixation, as well as in bottom-layer O₂ consumption for upper regions of the estuary. Despite significant reductions in seaward N transport from the middle to lower estuary, chl-a, turbidity, and surface-layer net O₂ production increased in the lower estuary, especially during summer. This degradation of water quality in the lower estuary appears to be linked to a trend of increasing net inputs of DIN into the estuary from Chesapeake Bay and to above-average river flow during the mid-1990s. In addition, increased abundance of Mnemiopsis leidyi significantly reduced copepod abundance during summer from 1990 to 2002, which favored increases in chl-a and allowed a shift in total N partitioning from DIN to particulate organic nitrogen. These analyses illustrate (1) the value of long-term monitoring data, (2) the need for regional scale nutrient management that includes integrated estuarine systems, and (3) the potential water quality impacts of altered coastal food webs.     

Responses of upper Chesapeake Bay to variations in discharge of the Susquehanna River

Upper Chesapeake Bay can be defined as the segment of the bay extending from its head seaward to the mouth of the Potomac Estuary. The Susquehanna River Estuary provides approximately 87% of the total freshwater input to this reach of the bay. With infrequent exceptions, resulting from summer and early fall hurricanes, the Susquehanna River has a well-defined seasonal flow pattern typical of mid-latitude rivers: high discharge in late winter and early spring, followed by low-to-moderate discharge throughout the summer and autumn. Within this general seasonal framework, there are large year-to-year variations in the magnitude of the early spring freshet and of the summer-fall period of low flow. The large seasonal variations in river flow produce significant responses to this fluctuating signal which are manifested in variations in the milieu of upper Chesapeake Bay. Stratification, circulation, sedimentation, nutrient levels, dissolved oxygen levels, and a number of other important and characteristic properties and processes of the upper bay all are closely coupled to variations in discharge of the Susquehanna River. Effects are not limited to the bay proper. Renewal of the tributary estuaries of the upper bay is controlled by variations in flow of the Susquehanna River.     

The application of a segmented tidal mixing model to the Great Bay Estuary,N.H.

Measurements show that in general salt is vertically well-mixed everywhere in the Great Bay Estuary, New Hampshire except near the river entrances at the head of the estuary. Dyer and Taylor’s (1973) modified version of Ketchum’s segmented tidal prism model has been applied to the Great Bay Estuarine System in order to predict high and low water salinity distribution for a specified river flow. The theory has been modified here to account for the mixing which occurs at the junction of two branches of an estuary. The mixing parameter, which in this model is related to the tidal excursion of water in the estuary, has been determined for different segments in the estuary on the basis of a comparison between predictions and a comprehensive data set obtained for a low river flow period. Using a mixing parameter distribution based on the low river flow calibration procedure the salinity distribution has been predicted for high river flow. The resulting salinity distribution compares favorably with observations for most of the estuary. The corresponding flushing times for water parcels entering at the head of the estuary during periods of low and high river flow is 54.5 and 45.9 tidal cycles respectively.     

External nutrient sources,internal nutrient pools,and phytoplankton production in Chesapeake Bay

External nutrient loadings, internal nutrient pools, and phytoplankton production were examined for three major subsystems of the Chesapeake Bay Estuary—the upper Mainstem, the Patuxent Estuary, and the 01 Potomac Estuary—during 1985–1989. The atomic nitrogen to phosphorus ratios (TN:TP) of total loads to the 01 Mainstem, Patuxent, and the Potomac were 51, 29 and 35, respectively. Most of these loads entered at the head of the estuaries from riverine sources and major wastewater treatment plants. Approximately 7–16% for the nitrogen load entered the head of each estuary as particulate matter in contrast to 48–69% for phosphorus. This difference is hypothesized to favor a greater loss of phosphorus than nitrogen through sedimentation and burial. This process could be important in driving estuarine nitrogen to phosphorus ratios above those of inputs. Water column TN: TP ratios in the tidal fresh, oligohaline, and mesohaline salinity zones of each estuary ranged from 56 to 82 in the Mainstem, 27 to 48 in the Patuxent, and 72 to 126 in the Potomac. A major storm event in the Potomac watershed was shown to greatly increase the particulate fraction of nitrogen and phosphorus and lower the TN:TP in the river-borne loads. The load during the month that contained this storm (November 1985) accounted for 11% of the nitrogen and 31% of the phosphorus that was delivered to the estuary by the Potomac River during the entire 60-month period examined here. Within the Mainstem estuary, salinity dilution plots revealed strong net sources of ammonium and phosphate in the oligohaline to upper mesohaline region, indicating that these areas were sites of considerable internal recycling of nutrients to surface waters. The sedimentation of particulate nutrient loads in the upper reaches of the estuary is probably a major source of these recycled nutrients. A net sink of nitrate was indicated during summer. A combination of inputs and these internal recycling processes caused dissolved inorganic N to P ratios to approach 16:1 in the mesohaline zone of the Mainstem during late summer; this ratio was much higher at other times and in the lower salinity zones. Phytoplankton biomass in the mesohaline Mainstem reached a peak in spring and was relatively constant throughout the other seasons. Productivity was highest in spring and summer, accounting for approximately 33% and 44%, respectively, of the total annual productivity in this region. In the Patuxent and Potomac, the TN:TP ratios of external loads documented here are 2–4 times higher than those observed over the previous two decades. These changes are attributed to point-source phosphorus controls and the likelihood that nitrogen-rich nonpoint source inputs, including contributions from the atmosphere, have increased. These higher N:P ratios relative to Redfield proportions (16:1) now suggest a greater overall potential for phosphorus-limitation rather than nitrogen-limitation of phytoplankton in the areas studied.     

Oceanic and Hydrologic Influences on Flow and Water Properties in the St. Jones River Estuary,Delaware

Influences of tides, freshwater discharge, and winds on water properties in the St. Jones River estuary (USA), a Delaware National Estuarine Research Reserve, were investigated using multiyear records of sea level, salinity, and turbidity, supplemented by a current profiler time series in 2007. Results demonstrate that instantaneous properties fluctuate with semidiurnal tides and resonant overtides, whereas tidal mean variations are forced by seasonal freshwater inflow and offshore winds. Mean sea level and salinity are highest in summer and vary with seasonal water temperature and rainfall, whereas sea level variability and turbidity are highest in winter on account of storm effects. Salinity and discharge modeling suggest that much (43–65%) of the freshwater resident in the estuary is derived from non-point sources below the head of tide. This diffuse freshwater inflow produces a seaward surface slope and weak mean current, which temporarily reverses under the influence of storm–wind setup within Delaware Bay.     

A resource-based framework for establishing freshwater inflow requirements for the Suwannee River estuary

The availability of methods for establishing freshwater inflow requirements for estuaries lags behind those for establishing flow requirements in riverine ecosystems. Some of the basic principles and approaches for establishing riverine flow requirements may be applicable to estuaries. An emerging approach for establishing freshwater inflow needs for the Suwannee River estuary involves maintaining a natural inflow regime (in terms of magnitude, frequency, duration, and timing of freshwater flows) and identifying important habitat targets to be protected. The salinity-river flow conditions needed to sustain the habitat targets in their existing condition are then identified. A variety of tools are employed, such as salinity metrics, biological metrics, limits of distribution of communities or habitats, and landscape-scale characteristics to define the salinity and corresponding flow ranges needed to protect and maintain the resource targets. With this information, combined with use of models to evaluate flow-salinity relationships and various withdrawal scenarios, river flow criteria can be set which address the freshwater inflow requirements to maintain these ranges. Subsequentmonitoring and research is undertaken to evaluate the effectiveness of the river flow criteria in protecting the estuarine resource targets. This information can be used to subsequently confirm, refine, or modity the flow criteria.     

A Model Study on the Summertime Distribution of River Waters in Long Island Sound

Long Island Sound (LIS), a large urban estuary in the northeastern USA, receives freshwater from many rivers along its northern shore. The size of these rivers varies widely in terms of basin area and discharge. The Regional Ocean Modeling System (ROMS) was applied with conservative passive tracers to identify the distribution, mixing, freshwater residence times, and storm response for all of LIS’s river systems during the summer of 2013. A watershed model was applied to overcome the lack of adequate river discharge observations for coastal watersheds. The Connecticut River was the largest contributor to riverine freshwater throughout the estuary despite its entry point near the mouth. The Connecticut River strengthened bulk stratification in the eastern LIS the most but acted to weaken stratification near the mouths of other rivers and in far western LIS by freshening waters at depth. The Housatonic and Hudson Rivers had the strongest influence on stratification in central and western LIS, respectively. Smaller coastal rivers were the most influential in strengthening stratification near the southwestern Connecticut shoreline. The influence of small coastal rivers was amplified after a major storm due to shorter storm response times relative to the larger rivers. Overall, river water was close to a well-mixed state throughout LIS, but more stratified near river mouths. Freshwater residence time estimates, meanwhile, indicated monthly to multi-seasonal time scales (43 to 180 days) and grew longer with greater distance from the LIS mouth.     

Nutrient Budgets and Management Actions in the Patuxent River Estuary,Maryland

Multi-year nitrogen (N) and phosphorus (P) budgets were developed for the Patuxent River estuary, a seasonally stratified and moderately eutrophic tributary of Chesapeake Bay. Major inputs (point, diffuse, septic, and direct atmospheric) were measured for 13 years during which, large reductions in P and then lesser reductions in N-loading occurred due to wastewater treatment plant improvements. Internal nutrient losses (denitrification and long-term burial of particulate N and P) were measured in tidal marshes and sub-tidal sediments throughout the estuary as were nutrient storage in the water column, sediments, and biota. Nutrient transport between the oligohaline and mesohaline zones and between the Patuxent and Chesapeake Bay was estimated using a salt and water balance model. Several major nutrient recycling terms were directly and indirectly evaluated and compared to new N and P inputs on seasonal and annual time-scales. Major findings included: (1) average terrestrial and atmospheric inputs of N and P were very close to the sum of internal losses plus export, suggesting that dominant processes are captured in these budgets; (2) both N and P export were a small fraction (13% and 28%, respectively) of inputs, about half of that expected for N based on water residence times, and almost all exported N and P were in organic forms; (3) the tidal marsh-oligohaline estuary, which by area comprised ~27% of the full estuarine system, removed about 46% and 74% of total annual upland N and P inputs, respectively; (4) recycled N and P were much larger sources of inorganic nutrients than new inputs during warm seasons and were similar in magnitude even during cold seasons; (5) there was clear evidence that major estuarine processes responded rapidly to inter-annual nutrient input variations; (6) historical nutrient input data and nutrient budget data from drought periods indicated that diffuse nutrient sources were dominant and that N loads need to be reduced by about 50% to restore water quality conditions to pre-eutrophic levels.     

Spatial and temporal variabilities of hypoxia in the Rappahannock River,Virginia

Hypoxia/anoxia in bottom waters of the Rappahannock River, a tributary estuary of Chesapeake Bay, was observed to persist throughout the summer in the deep basin near the river mouth; periodic reoxygenation of bottom water occurred on the shallower sill at the river mouth. The reoxygenation events were closely related to spring tide mixing. The dissolved oxygen (DO) in surface waters was always near or at the saturation level, while that of bottom waters exhibited a characteristic spatial pattern. The bottom DO decreased upriver from river mouth, reaching a minimum upriver of the deepest point of the river and increasing as the water becaume shallower further upriver. A model was formulated to describe the longitudinal distribution of DO in bottom waters. The model is based on Lagrangian concept—following a water parcel as it travels upriver along the estuarine bottom. The model successfully describes the characteristic distribution of DO and also explains the shifting of the minimum DO location in response to spring-neap cycling. A diagnostic study with the model provided insight into relationships between the bottom DO and the competing factors that contribute to the DO budget of bottom waters. The study reveals that both oxygen demand, either benthic or water column demand, and vertical mixing have a promounced effect on the severity of hypoxia in bottom waters of an estary. However, it is the vertical mixing which controls the longitudinal location of the minimum DO. The strength of gravitational circulation is also shown to affect the occurrence of hypoxia. An estuary with stronger circulation tends to have less chance for hypoxia to occur. The initial DO deficit of bottom water entering an estuary has a strong effect on DO concentration near the river mouth, but its effect diminishes in the upriver direction.     

Response times of salinity in relation to changes in freshwater inflows in the Lower Hillsborough River, Florida

The Lower Hillsborough River, Florida is a short (16 km) riverine estuary which has a dam located at its upstream end. Salinity below the dam is influenced by freshwater that flows over or through the structure. Depending on location in the estuary, the response of salinity to changes in upstream freshwater inflows is normally not instantaneous, but lags behind the freshwater release. An analytical approach and a laterally averaged two-dimensional hydrodynamic model were used to examine the response time of salinity in the Lower Hillsborough River to changes in freshwater inflows from the upstream reservoir. A series of case studies were conducted using the model to determine how salinity in the river within one kilometer below the dam would respond to changes in freshwater inflows. The model results suggest that the time lag of salinity in the river depends on whether the upstream freshwater inflows are increasing or decreasing, as well as their magnitude. While the time lag for salinity is about six to eight days for decreasing inflows, it is much shorter for increasing inflows depending on the magnitude of the flow release.     

The Influence of Storms on Water Quality and Phytoplankton Dynamics in the Tidal James River

Due to the unpredictable nature of intense storms and logistical constraints of sampling during storms, little is known about their immediate and long-term impacts on water quality in adjacent aquatic ecosystems. By combining targeted experiments with routine monitoring, we evaluated immediate impacts of two successive storm events on water quality and phytoplankton community response in the tidal James River and compared these findings to a non-storm year. The James River is a subestuary of the Chesapeake Bay and sampling was conducted before, during, and after Hurricane Irene and Tropical Storm (TS) Lee in 2011 and during the same time period (late summer/early fall) in 2012 when there were no storms. We collected and compiled data on nutrient and chlorophyll a concentrations, phytoplankton abundance, nitrogen uptake, primary productivity rates, and surface salinity, temperature, and turbidity in the meso- and polyhaline segments of the James River. Hurricane Irene introduced significant amounts of freshwater over the entire James River and Chesapeake Bay watersheds, while rainfall from TS Lee fell primarily on the tidal fresh region of the James River and headwaters of the Chesapeake Bay. Dinoflagellates dominated the algal community in the meso- and polyhaline segments prior to the storms in 2011, and a mixed diatom community emerged after the storms. In the mesohaline river segment, cyanobacteria abundance increased after TS Lee when salinities were depressed, likely due to washout from the oligohaline and tidal fresh regions of the river. In 2012, dinoflagellates dominated the community in both segments of the river during late summer but diatoms were also abundant and their biomass fluctuated throughout the summer and fall. Cyanobacteria were not present in either segment. Overall, we observed that the high-intensity rainfall from Hurricane Irene combined with high flushing in the headwaters as a result of TS Lee likely reduced primary productivity and altered community composition in the mesohaline segment but not the more estuarine-influenced polyhaline segment. Understanding the influence of high freshwater flow with a short residence time associated with storms is key to the planning and management of estuarine restoration as such disturbances are projected to increase as a result of climate change.     

The sedimentary record of climatic and anthropogenic influence on the Patuxent estuary and Chesapeake Bay ecosystems

Ecological and paleoecological studies from the Patuxent River mouth reveal dynamic variations in benthic ostracode assemblages over the past 600 years due to climatic and anthropogenic factors. Prior to the late 20th century, centennial-scale changes in species dominance were influenced by climatic and hydrological factors that primarily affected salinity and at times led to oxygen depletion. Decadal-scale droughts also occurred resulting in higher salinities and migration of ostracode species from the deep chanel (Loxoconcha sp.,Cytheromorpha newportensis) into shallower water along the flanks of the bay. During the 19th century the abundance ofLeptocythere nikraveshae andPerissocytheridea brachyforma suggest increased turbidity and decreased salinity. Unprecedented changes in benthic ostracodes at the Patuxent mouth and in the deep channel of the bay occurred after the 1960s whenCythermorpha curta became the dominant species, reflecting seasonal anoxia. The change in benthic assemblages coicided with the appearance of deformities in foraminifers. A combination of increased nitrate loading due to greater fertilize use and increased fresh-water flow explains this shift. A review of the geochemical and paleoecological evidence for dissolved oxygen indicates that seasonal oxygen depletion in the main channel of Chesapeake Bay varies over centennial and decadal timescales. Prior to 1700 AD, a relatively wet climate and high freshwater runoff led to oxygen depletion but rarely anoxia. Between 1700 and 1700, progressive eutrophication occurred related to land clearance and increased sedimentation, but this was superimposed on the oscillatory pattern of oxygen depleton most likely driven by climatological and hydrological factors. It also seems probable that the four-to five-fold increase in sedimentation due to agricultural and timber activity could have contributed to an increased natural nutrient load, likely fueling the early periods (1700–1900) of hypoxia prior to widespread fertilizer use. Twentieth-century anoxia worsened in the late 1930s–1940s and again around 1970, reaching unprecedented levels in the past few decades. Decadal and interannual variability in oxygen depletion even in the 20th century is still strongly influenced by climatic processes influencing precipitation and freshwater runoff.     

Impacts of hydrological changes on phytoplankton succession in the Swan River, Western Australia

The Swan River estuary, Western Australia, has undergone substantial hydrological modifications since pre-European settlement. Land clearing has increased discharge from some major tributaries roughly 5-fold, while weirs and reservoirs for water supply have mitigated this increase and reduced the duration of discharge to the estuary. Nutrient loads have increased disproportionately with flow and are now approximately 20-times higher than pre-European levels. We explore the individual and collective impacts of these hydrological changes on the Swan River estuary using a coupled hydrodynamic-ecological numerical model. The simulation results indicate that despite increased hydraulic flushing and reduced residence times, increases in nutrient loads are the dominant perturbation producing increases in the incidence and peak biomass of blooms of both estuarine and freshwater phytoplankton. Changes in salinity associated with altered seasonal freshwater discharge have a limited impact on phytoplankton dynamics.     

The calculation of estuarine turnover times using freshwater fraction and tidal prism models: A critical evaluation

Freshwater fraction and tidal prism models are simple methods for estimating the turnover time of estuarine water. The freshwater fraction method prominently features flushing by freshwater inflow and has sometimes been criticized because it appears not to include flushing by seawater, but this is accounted for implicitly because the average estuary salinity used in the calculation reflects all the processes that bring seawater into the estuary, including gravitational circulation and tidal processes. The model relies on measurable salinity differences among water masses and so must be used for estuaries with substantial freshwater inflow. Tidal prism models are based on flushing by flood tide inflow and ignore seawater inflow due to gravitational circulation. These models should only be applied to estuaries with weak or nonexistent gravitational circulation, which are generally those with little freshwater inflow. Using a framework that is less ambioguous and more directly applicable to the estimation of turnover times than those used previously, this paper critically examines the application of tidal prism models in well-mixed estuaries with complete tidal exchange, partial ebb return, or incomplete flood mixing and in partially mixed estuaries. Problems with self-consistency in earlier versions of these models also apply to the budgeting procedure used by the LOICZ (Land-Ocean Interactions in the Coastal Zone) program. Although freshwater fraction and tidal prism models are different approaches to estimating turnover times in systems with very different characteristics, consistent derivation shows that these models have much in common with each other and that they yield equivalent values that can be used to make comparisons across systems.     

A watershed perspective on nutrient enrichment,science, and policy in the Patuxent River,Maryland: 1960–2000

The Patuxent River, Maryland, is a nutrient-overenriched tributary of the Chesapeake Bay. Nutrient inputs from sewage outfalls and nonpoint sources (NPS) have grown substantially during the last four decades, and chlorophylla levels have increased markedly with concomitant reductions in water quality and dissolved oxygen concentrations. The Patuxent has gained national attention because it was one of the first river basins in the U.S. for which basin-wide nutrient control standards were developed. These included a reduction in NPS inputs and a limit on both nitrogen (N) and phosphorus (P) loadings in sewage discharges intended to return the river to 1950s conditions. Full implementation of point source controls occurred by 1994, but population growth and land-use changes continue to increase total nutrient loadings to the river. The present paper provides the perspectives of scientists who participated in studies of the Patuxent River and its estuary over the last three decades, and who interacted with policy makers as decisions were made to develop a dual nutrient control strategy. Although nutrient control measures have not yet resulted in dramatic increases in water quality, we believe that without them, more extensive declines in water quality would have occurred. Future reductions will have to come from more effective NPS controls since future point source loading will be difficult to further reduce with present technology. Changing land use will present a challenge to policy makers faced with sprawling population growth and accelerated deforestation.