Final Report: The Choptank River: A Mid-Chesapeake Bay Index Site for Evaluating Ecosystems Responses to Nutrient Management

EPA Grant Number: R826941
Title: The Choptank River: A Mid-Chesapeake Bay Index Site for Evaluating Ecosystems Responses to Nutrient Management
Investigators: Malone, Thomas C. , Cornwell, Jeffrey C. , Boicourt, William C. , Stevenson, J. Court , Harding Jr., Lawrence W.
Institution: Horn Point Laboratory
EPA Project Officer: Packard, Benjamin H
Project Period: September 15, 1998 through September 14, 2001 (Extended to September 14, 2002)
Project Amount: $596,097
RFA: Ecological Effects of Environmental Stressors Using Coastal Intensive Sites (1998) RFA Text |  Recipients Lists
Research Category: Ecosystems , Ecological Indicators/Assessment/Restoration , Environmental Statistics


As part of the U.S. Environmental Protection Agency, Science to Achieve Results (STAR) Coastal Intensive Site Network (CISNet) program, the Choptank River watershed and estuary was proposed as an attractive and representative system to examine the effects of human-induced stress on coastal ecosystems. Primary interests were the impacts of meteorological fluctuations and nutrient management activities on water quality and living resources in the Choptank River estuarine system. The Choptank watershed is a low-gradient coastal plain with characteristic agricultural and forested landscape, which is typical of many coastal plain systems, both in the region and beyond. Excess nutrient introduction from municipal point sources and agricultural nonpoint sources were identified as the primary ecosystem stresses.

The primary objectives of this research project were to develop and examine methods for detecting responses to anthropogenic stresses in the Choptank River Index Site and to establish the Site as a sentinel of change for a broader domain of coastal plain ecosystems. Of particular interest were the impacts of meteorological fluctuations and nutrient management in the Choptank drainage basin on water quality and living resources in the estuary. The intent was to resolve responses caused by human activities from the variability imposed by nature, to develop key indices of ecosystem change, and to predict trends and their consequences.

The Choptank River Index Site encompassed three related components: (1) the development of high resolution, long-term time-series through in situ sensing and real-time data transmission; (2) documentation of spatial patterns through aircraft remote sensing, shipboard observation, rainfall and air sampling in the watershed, and special-focus studies; and (3) integration and synthesis of these results with those from established monitoring and research programs. Within the spatial pattern investigations, special-focus studies were conducted to address key, long-standing questions identified at the outset of the program. One set of questions dealt with nutrient dynamics along the margins of the estuary, in the marshes and the shallows, and their effects on the estuary as a whole, which traditionally has been sampled only along the center channel. The other key set of questions concern the relative role of local nutrient inputs into the Choptank tributary compared with inputs from the Chesapeake Bay proper. Answers to these questions bear heavily on the strategy and conduct of nutrient management efforts for not only the Choptank system, but also for the Bay as a whole.

Summary/Accomplishments (Outputs/Outcomes):

The Choptank River Index Site program set out to establish a system for gauging not only the health of a coastal plain estuarine ecosystem, but also trends in the ecosystem's health influenced by human activity. In addition, the Choptank River Index Site was intended to represent a broad class of agriculturally dominated coastal plain systems. To achieve these goals, we also needed to learn more about the workings of the estuary and its watershed. The strategy at the outset was to intensely study the Choptank River system over a 3-year interval to relate observed trends and variability to the longer term, but sparse monitoring programs, to assay the most cost-effective set of limited, key measurements necessary for tracking these changes, and to judge the representativeness of this site within its class of estuaries. In the endeavor to establish this broader applicability, we were greatly aided by concurrent studies in other similar systems such as the Harmful Algal Bloom program in the Pocomoke River Estuary. We also were fortunate to have experienced a broad range of atmospheric forcings during the study, including wet years and drought years, and strong events such as Hurricane Floyd.

Of primary interest in this study was the transport and fate of nutrients. Key questions were addressed as to the role of lateral marshes in the middle and upper reaches of the estuary and the relative contributions of local sources versus inputs from the Chesapeake Bay proper. For both questions, an improved understanding of the estuarine circulation and a quantitative description of the transport processes were deemed necessary. Shipboard river surveys and moored instrumentation revealed that the Choptank Estuary, as well as the Pocomoke River Estuary, are less a smooth continuum from the head to the mouth, but rather a connected series of nearly distinct provinces separated by abrupt transitions controlled by topography. In the upper reaches, above the limit of salt, currents ebb and flow with the tide, but the net nontidal circulation is seaward throughout the water column. Seaward of this region, the circulation alters slightly when salt is encountered and stratification is low, but finite, and a 1-dimensional advective-diffusive salt balance is maintained by river flow and tidal mixing. When the estuarine cross-sectional area increases to what appears to be a critical value, the circulation abruptly changes to a robust, two-layer gravitational flow. We did not expect this strong, two-layer circulation to be spatially confined or it to diminish rapidly over the broad, shallower reaches of the lower Choptank River. CISNet measurements showed that the strength of this two-layer circulation can vary significantly throughout the annual cycle of freshwater inflow. In addition, episodic pulses occurred during Hurricane Floyd that could rapidly spin up this flow in the middle reaches of the subestuary. The episodic nature of the exchange between the Main Stem Chesapeake Bay and the Choptank River tributary estuary was known prior to the CISNet program. However, our measurements revealed that the high-salinity intrusions from the Bay to the Choptank are a predictable combination of both upwelling in the Bay proper and two-layer meteorological flow in the approach channel seaward of the entrance sill.

Two modeling efforts were successful in showing that a simple advective-diffusive balance can be applied to the Choptank River Estuary. The first model provided a sense of the balance between buoyancy fluxes and mixing, but it was neither physics based nor time dependent. Therefore, a second model was constructed based on first principles and including time dependence. The calibration process has yielded interesting results, showing the spatial structure of the transport and exchange processes and providing a quantitative measure of the seasonal progression of salt storage and flushing and exchange with the Chesapeake Bay proper. This model is sufficiently successful; a new, 2-dimensional model is presently being implemented.

Aircraft Remote Sensing provided a synoptic spatial coverage of primary productivity (PP) estimates that are not possible through shipboard techniques. From the periodic overflights (approximately monthly), models were constructed to produce these estimates from ocean color detected by SAS-III radiometers. These models were validated with 1999-2000 data that were not used in calibration. We developed a separate model for optimal biomass-normalized photosynthesis (PBopt) and embedded this model in CBPM-1 to produce CBPM-2, obviating the need for in situ physiological data and allowing application of the model to remotely sensed data. CBPM-2 was applied to remotely sensed data from the Choptank and Patuxent for all flights from 1999-2002 to generate time-series of PP for each tributary.

An evaluation of marsh nutrient burial was conducted in the oligohaline part of the Choptank River, in an extensive marsh area adjacent to a section of the river characterized by high water column nutrient and suspended sediment concentrations. In an analysis of 23 marsh cores for sedimentation rates, N burial and P burial showed moderate to high levels of spatial variability. Overall rates were quite high; virtually all watershed P retained in the marsh and a high proportion of N retained. The accretion of sediment required to match relative sea level rise provides a substantial capacity for nutrient burial, both as inorganic particulates and as organic matter. Our data analysis suggests that a large number of cores are required for a reasonable assessment of nutrient burial.

Comparisons of CISNet measurements with long-term and historical data sets were made in the attempt to detect changes in the state of ecosystem status in the Choptank River Index Site. When these CISNet measurements were compared with the 1986-1991 data reported by Stevenson et al. (1993) and Staver (1997), it appears that there is more total nitrogen (TN) in the Upper Choptank in later years compared to a decade earlier. Conversely, it appears that TN has decreased slightly in the lower Choptank. We hypothesize that several factors may interplay to produce this picture. Most importantly, mainstem bay intrusions most likely have lower concentrations of TN than in previous years, helping to dilute upstream inputs. Another possible factor is that downstream, there has been considerable acreage of farmland converted to low-density residential homes. Thus, the actual fertilization in these areas most likely has decreased from the 150-200 lbs N per acre normally applied to agricultural fields in the region. Also, as the town of Easton has expanded rapidly over the last several decades, more adjacent areas have access to its wastewater collection system, which actually puts effluents into the Choptank at a more upstream location than previously. The other important limiting factor in terms of eutrophication is phosphorus (P), which consists primarily of orthophosphate ions (PO4-3) in the estuary. The occurrence of PO4-3 is more limited in the environment than nitrate. However, it can be even more potent as a pollutant because even small quantities can induce algal blooms and help induce nitrogen fixation. Agriculture has been supplemented by PO4-3 fertilizers for over 150 years and for the last 50 years superphosphate, Ca(H2PO4)2, is most commonly applied to fields. Because PO4-3 binds readily to clay particles, large stores of it are often bound to the soil. Although there is slow leakage of PO4-3 in runoff, large amounts of it can reach the estuary during sedimentation events (e.g., following Hurricane Floyd). In addition, over the last decade, large amounts of PO4-3 have been introduced to the estuary because of the disposal of chicken manure over the landscape. Although this practice is now more closely controlled, new regulations have not fully been implemented. Our 1999-2001 data indicate that, unlike nitrate, which clearly emanates in the upper watershed and propagates downstream, the PO4-3 maximum is often not in the fresh water, but down the salinity gradient. This usually is attributed to release from desorption processes induced as clay particles hit a more reduced environment (especially in anoxic or hypoxic environments). The 1999 PO4-3 maximum we observed in the Choptank clearly was associated with the Hurricane Floyd event. Elevated PO4-3 concentrations (> 2 µM) are maximal at the Tuckahoe and Upper Choptank Confluence and propagate downstream into the Lower Choptank before dissipating in the vicinity of Horn Point Laboratory. In 2000, there were twin peaks (> 2 µM) in July and September in Tuckahoe Creek, but downstream at the Easton Sewage Treatment Plant (STP), PO4-3 was above 1 µM from March until August, suggesting more widespread inputs. In 2001 inputs into this area were very high (> 2 µM) in June and did not get below 1 µM until October. The position of the PO4-3 appeared to be related to the Easton STP, but when the total phosphorus (TP) is considered (panel below) this seemed even more likely. The TP fraction includes organic P, most of which was incorporated into algae, which are episodically flushed out of the Easton STP.

The Choptank River Index Site was established to not only detect ecosystem changes, but also to assess the adequacy of monitoring techniques and the representativeness of this system for the broad class of agriculturally dominated coastal plain estuaries. Although, the 3-4 year CISNet program was not a sufficient interval to detect changes in eutrophic status, when CISNet data were combined with historical data, trends emerged. Furthermore, the CISNet monitoring program provided greater insight into the circulation and nutrient dynamics of the Choptank River estuarine system, which, in turn, increases the power of monitoring to guide nutrient management efforts. A prime example is the ability of this sampling program to identify the contribution of an individual STP. Nutrient dynamics were not only spatially detailed, as revealed by our CISNet program, but also temporally episodic. This episodic character consists of more than the obvious extreme events such as the runoff surge observed during Hurricane Floyd. The observed pulsed nature of exchange of the Choptank River with the Mainstem Bay is a more subtle, but important episodic process. When these more intense observations are combined with the transport models, we expect not only to detect these signals, but also to quantitatively separate the influences of agricultural, STP, and Mainstem Chesapeake Bay sources.

Having intensely focused on the Choptank River system for 4 years, we can now say that, for timescales beyond 1 decade, long-term measurements from a only a few monthly sampling stations can provide an indication of trends in the system, trends that can not be detected over the shorter timescale of this program. However, such limited data provide little insight into nutrient dynamics and perhaps more importantly, little insight into how nutrient management techniques might effectively be applied. For instance, such data do not provide a separation of the effects of local nutrient sources from remote sources in the adjacent mainstem estuary or from its primary river input. This uncertainty leads to significant quandaries in nutrient abatement decisionmaking. With the addition of the modern, high-resolution techniques from remote, continuous in situ, and shipboard sampling, and from modern geochemical analyses of the sedimentary record, we gain the ability to support cost-effective local management efforts, as well as determine how much of the problem is imported from outside the system. In conjunction with the integrating power of models, a distilled and directed monitoring program for the Choptank River Index Site could provide this necessary support.

Comparison of the Pocomoke River reveals important differences with the Choptank River system. The Pocomoke watershed has different soils, and its waters are black, humic, and turbid. The proportion of local versus remote nutrient sources is greater, because the Pocomoke communicates at its seaward boundary with nearly undiluted continental shelf water. However, the basic ecosystem problem is similar-overenrichment by delivery of excess nutrients-as are its nutrient delivery systems. The four circulation domains identified in the Choptank River also occur in the Pocomoke. As a result, the monitoring and modeling techniques that proved successful in the Choptank also are applicable to the Pocomoke.

The broad context for remotely sensed observations we have made in the Choptank River during the past 3-4 years is ecosystem response to nutrient loading. Data we collected during CISNet detailed phytoplankton dynamics in the Choptank (and Patuxent) and revealed the predominant role of regional precipitation and freshwater flow in regulation phytoplankton biomass as chlorophyll-a. The contemporary use of these data is to extend monitoring data from shipboard surveys by providing higher spatial and temporal resolution. Prior to CISNet, for example, only three stations were regularly sampled in the Choptank River and one in the adjacent mesohaline waters of the main stem Bay. The low spatial resolution afforded by this approach precluded the detection of ephemeral events such as algal blooms and served only to approximate gross ecosystem differences between years. In 2000, during CISNet, the Choptank River experienced a relatively strong bloom of the dinoflagellate species, Prorocentrum minimum, that persisted for about 2 weeks and discolored the water at concentrations exceeding 10,000 cells mL-1. Aircraft remote sensing gave a synoptic view of the developing bloom, whereas boat surveys were not suited to resolve the high variability of chlorophyll-a distributions that accompanied the bloom. Data from CISNet and the main stem Chesapeake Bay flights greatly augment the existing chlorophyll-a data that are being used to gauge trends in the Bay and its tributaries.

In assessing the ability of geochemical analyses to provide integrated estimates of nutrient dynamics in marshes, we note that the variability of N and P burial is relatively high, suggesting that studies that use few cores have the potential to provide erroneous data. The variability at the km scale does not appear to be much different than variability at the <100 m scale. The rates of N and P burial in Chesapeake Bay marshes in general are quite high. High rates of sea-level rise, combined with a requirement that marshes accrete sediment to survive, result in a large capacity to bury both organic and inorganic materials. In tidal Chesapeake estuaries, tidal marshes represent a large and previously ignored sink for N and P. Although the absence of marshes fringing the Chesapeake mainstem may limit wetland retention of N and P in the whole system, these wetland ecosystems have a critical water column value to subestuaries. Recommendations for assessing tidal marsh nutrient retention include: (1) a need to use transects at a number of sites to better characterize sites; and (2) a strong suggestion that when considering limited resources, fewer vertical sections in each core may provide the potential for a better assessment of spatial variability.

The detected increases in nutrient concentrations in the Choptank estuary suggest that, although there has been considerable effort to reduce N inputs by using Best Management Practices on farmland and in improving the efficiency of sewage treatment, the loads appear to have increased going into the 21st century. However, with the CISNet program identifying some individual sources, we can now justify specific nutrient management actions. CISNet data were made available at local public hearings during Easton Maryland’s STP license renewal process. These data supported the decision by the Town of Easton to implement a much more efficient (and costly) system of P removal. Another application of the CISNet monitoring and modeling approach would be to provide support for the establishment of Total Maximum Daily Loads (TMDLs) for nutrient delivery to the watershed and estuary.

Journal Articles on this Report : 3 Displayed | Download in RIS Format

Other project views: All 22 publications 6 publications in selected types All 3 journal articles
Type Citation Project Document Sources
Journal Article Glenn SM, Dickey TD, Parker B, Boicourt W. Long-term real-time coastal ocean observation networks. Oceanography 2000;13(1):24-34. R826941 (Final)
  • Full-text: Oceanography - PDF
  • Abstract:
  • Journal Article Glenn SM, Boicourt W, Parker B, Dickey TD. Operational observation networks for ports, a large estuary, and an open shelf. Oceanography 2000;13(1):12-23. R826941 (Final)
  • Full-text: Oceanography-Full Text PDF
  • Abstract: Oceanography-Abstract
  • Journal Article Harding Jr. LW, Mallonee ME, Perry ES. Toward a predictive understanding of primary productivity in a temperate, partially stratified estuary. Estuarine, Coastal and Shelf Science 2002;55(3):437-463. R826941 (Final)
    R828677 (2001)
    R828677 (Final)
    R828677C002 (2002)
    R828677C002 (Final)
  • Abstract: ScienceDirect-Abstract
  • Other: ScienceDirect-PDF
  • Supplemental Keywords:

    atmosphere, watersheds, land, sediments, marine, estuary, ecological effects, particulates, discharge, ecosystem, indicators, restoration, scaling, integrated assessment, decisionmaking, environmental chemistry, marine science, biology, physics, ecology, hydrology, modeling, monitoring, analytical, surveys, measurement methods, remote sensing, Chesapeake Bay., RFA, Scientific Discipline, Geographic Area, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Hydrology, Nutrients, Ecology, Water & Watershed, Contaminated Sediments, Physics, Ecosystem/Assessment/Indicators, Ecosystem Protection, Chemistry, State, Monitoring/Modeling, Ecological Effects - Environmental Exposure & Risk, Air Deposition, Watersheds, Ecological Indicators, anthropogenic stresses, nutrient supply, coastal ecosystem, nutrient transport, agricultural runoff, aquatic ecosystem, anthropogenic stress, basin hydrology, agriculturally impacted watershed, hydrological stability, ecological exposure, CISNet, meteorology, watershed management, remote sensing data, coastal zone, contaminated sediment, nutrient flux, Choptank River, CISNet Program, coastal plain ecosystems, soil, Maryland (MD), predictive model, shipboard sampling, aquatic ecosystems, water quality, discharge, nutrient transport model, stress responses, atmospheric deposition, Chesapeake Bay, ecological response, nitrogen

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    Progress and Final Reports:

    Original Abstract
  • 1999 Progress Report
  • 2000 Progress Report
  • 2001 Progress Report