Neponset River Estuary Assessment Report

Ficarra1

Neponset River Estuary Assessment Report

Lynn Ficarra

Introduction

The Neponset River Estuary is a small, tidally dominated urban estuary located in the Dorchester Bay region of the Boston Harbor, Massachusetts(Gardner, Chen, and Berry 2005). The Neponset River is the second largest river flowing into the Boston Harbor, draining a 340 km2 watershed (Zhu and Olsen 2009, Gardner, Chen, and Berry 2005).While the watershed encompasses 14 towns and cities, the estuary itself is located primarily in Boston and Quincy (Cooke et al. 2003).The estuary is separated from the river by the Walter Baker dam and its mouth meets Dorchester Bay 7 km from the dam (Figure 1, Breault 2011).

While organizations such as the Neponset River Watershed Association (nepRWA) have extensively studied the freshwater components of the watershed, little is known about the Neponset River Estuary. To assess the restoration needs of the estuary and to propose a management plan, factors influencing the health of the estuary must be evaluated. The general characteristics of the estuary (size, volume, tidal oscillations, etc.), dominant habitat types, residence time, and nutrient limitation status are important factors that structure the estuarine system. Understanding these features will help provide the tools to manage this delicate ecosystem.

General Characteristics

The Neponset River Estuary is approximately 7 km long and ranges from 25-250 m wide at low tide and up to 1 km at high tide (Gardner, Chen, and Berry 2005). The depth of the main channel near the mouth is about 4 m at low tide, but may be less than 2 m in some locations (Gardner, Chen, and Berry 2005). The tidal range is approximately 3 m, thus a large portion of the water in the estuary is exchanged at each tide(Zhu and Olsen 2009). The surface area is 2.17 km2 at high tide and 0.88 km2 at low tide (Gardner, Chen, and Berry 2005). River flow through the Walter Baker dam and into the estuary ranges from 0.6 to 3.1 m3/s (Breault 2002). This information is useful in categorizing the estuary based on depth, tidal versus river flow, and salinity stratification.

As a shallow, tidally dominated estuary, the Neponset estuary is either a partially or well-mixed estuary with the latter being most likely. Though the estuary type is not explicitly stated in the literature, NOAA Ocean Service Education characterizes a well-mixed estuary as shallow with low river flow, moderate to strong tidal currents, and no vertical stratification. The Neponset estuary is shallow, river water inflow to the estuary is slowed by the Baker dam, and tidal currents are strong. These provide support for the well-mixed theory; however, the vertical stratification of the estuary is not well known. To determine the extent to which the estuary is vertically stratified, salinity measurements could be taken at varying depths at either end of the estuary and in the center. This may be complicated by seasonal variation in stratification, thus year-round sampling will provide a clearer picture of salinity stratification in the estuary.

To the best of my researching capabilities, the total volume of water in the estuary is unknown. Due to variations in tidal flow, river flow, watershed inputs, estuary depth, estuary width, etc., mathematically estimating the volume of water within the estuary at any given time is very complicated and beyond the scope of my abilities. However, it would beworthwhile to create a model estimating the volume of water in the estuary through time. This would involve several measurements including mapping the bottom topography, plotting the dimensions of the estuary, and charting tidal and river flow variations. Knowing the volume of water is useful for estimating residence time and quantifying waterexchangeduring each tidal cycle. This has implications for nutrient loading and the potential for phytoplankton blooms.

According to Gardner, Chen, and Berry (2005), ice covers the estuary during the winter. This is an example of a seasonal variation in the condition of the estuary. Ice cover can alter many characteristics of an estuary such as salinity, temperature, wave action, dissolved oxygen concentrations, etc. Seasonal variability in the estuary would be a useful factor to study because management strategies may need to be adjusted seasonally with changing ecosystem dynamics.

Habitats

The Neponset River Estuary contains the largest remaining salt marsh habitat in Boston Harbor (Berault 2002). In addition to salt marshes, the Neponset estuary contains shallow mud flats and large stretches of intertidal zone with approximately 29% of the intertidal surface area being non-vegetated (Figure 2, Gardner, Chen, and Berry 2005). Historically, the salt marshes were dominated by native Smooth cordgrass (Spartinaalterniflora) and Saltmeadowcordgrass (Spartina patens), but the invasive Common reed (Phragmitesaustralis) has taken over and caused a decline in Spartina(NepRWA 2006). In the past, wastes from dredging the river river were dumped into salt marshes causing the loss of native organisms and an opportunity for invasive species to move in (Tineret al. 1998).

In 1991, the Neponset River Estuary had 311.64 acres of wetlands comprised of 25.92 acres of non-vegetated wetland, 265.67 acres of salt marsh, and 20.05 acres of common reed marsh (Tineret al. 1998). According to the 2004 nepRWA Assessment Report, all of the remaining salt marshes within the estuary have been severely degraded. It would be very helpful to actually measure the amount of wetlands that currently exist in the estuary to determine how much it has changed since the 1990s. This will indicate the scopeof thedamage that has occurred to wetland ecosystems over the past few decades.

Other than the presence of mud in the mudflats, the sediment characteristics of the Neponset estuary are not well known. While there have been some sediment sampling used to measure nutrients and bacteria, there is no literature describing the characteristics of the bottom substrate. Sediment size and composition as well as levels of turbidity and sedimentation are important habitat descriptors. Sediment particle size and composition may affect which organisms can live within or closely associated with the substrate. Certain particle sizes are more or less likely to be disturbed by turbidity. Sedimentation is important for water clarity which relates to primary production. I theorize that sedimentation rates are low near the river because the dam reduces the flow of sediments into the estuary. Likewise, I theorize that turbidity and sedimentation are increased near Dorchester Bay because of tidal currents. Sediments in shallow areas are more likely to be affected by wind-driven surface waves which mix sediment into suspension and reduce water clarity. Sediment studies are important for understanding water clarity issues.

The Neponset River Estuary is home to a diverse group of organisms. In addition to the previously mentioned salt marsh plants, the estuary is an important habitat for several anadromous fish species. Historically, rainbow smelt, herring, and American shad used the estuary and Neponset River for fish runs which supplied fisheries (nepRWA 2004). Unfortunately, river dams have greatly shortened the distance the fish can travel. This along with pollution has caused the fish populations to decline (Massachusetts Government 1998). The Neponset estuary is also, an important nursery habitat for young commercial species such as winter flounder, herring, clams, bay scallops, conch, dogfish sharks, spiny dogfish sharks, skates (nepRWA 2006). As a result, predators such as bluefish and striped bass use the estuary for hunting (nepRWA 2006). Estuaries provide many ecosystem services which is why management and restoration is crucial to marine and freshwater ecosystems.

Residence Time

According to the NOAA Office of Coast Survey, residence time is the average amount of time a substance remains within a physical system. In the case of an estuary, residence time (or flushing time) refers to the amount of time a drop of water remains within an estuary before being flushed out into the ocean. Flushing time of an estuary is important because it influences the concentrations of nutrients and pollutants in the estuary which directly affects the health of estuarine organisms (Aikman and Lanerolle 2004). In an estuary with a short residence time, nutrients and pollutants within the water column will be quickly removed from the system. Longer residence times allow for increased concentrations of chemicals as water is retained in the system for longer periods of time.

Geographic features contribute to the residence time of an estuary. The Neponset River Estuary is shallow and tidally dominated (tidal exchange may involve the entire estuary) which results in a short time scale for mixing and transport within the estuary(Gardner, Chen, and Berry 2005). Gardner, Chen, and Berry (2005) describe the estuary as a microestuary which they define as having a total length of less than 10 km and having a short residence time of one day or less. The literature does not, however, provide a more precise calculation of residence time for the Neponset estuary.

Nutrient Limitation

A water quality assessment of one sampling station within the Neponset estuary yielded high levels of bacteria, nitrogen, and orthophosphate (Cooke et al. 2003). While this study was lacking in number of sampling sites and was vague about what exactly constituted a “high” level, it is useful in suggesting that nutrient loading and bacteria levels may be a concern. To understand the water quality of the estuary, further studies must be conducted.According to the Massachusetts Government (1998), the estuary is included in a 1998 list of impaired waters due to organic enrichment, low dissolved oxygen, pathogens, and turbidity. This report is outdated, though, and should be updated.

According to the nepRWA 2004 Assessment Report, the estuary is generally limited by nitrogen. It is difficult to quantifyreasonable nutrient levels, though, because the Massachusetts Surface Water Quality Standards do not include specific thresholds for nutrient concentrations (nepRWA 2004). Instead, nutrient level standards are described via vague qualitative guidelines. This makes it nearly impossible to regulate nutrients in the estuary. Potential sources of nitrogen include atmospheric deposition, sewage, livestock waste, and fertilizer runoff (Castro et al. 2003).Nutrient inputs will not be controlled unless government can set up very specific regulations.

Conclusions

There are many gaps in our knowledge about the Neponset River Estuary. These gaps include the volume of water in the estuary, sediment characteristics, salinity stratification, and residence time estimates.The literature tends to focus primarily on the rivers in the Neponset watershed rather than the estuary. Much of the information needed to properly assess the status of the estuary and propose a management plan is unknown. There is a miniscule amount of current literature discussing the estuary and scientific research is especially scarce. In general, available information about the Neponset estuary is outdated and new studies should be initiated.

Some projects have been created to improve conditions in the Neponset River Estuary. The Neponset River Watershed Association and other organizations are researching the potential environmental effects of removing some of the river dams. Eliminating dams will help anadromous fish populations recover (Breault 2002). Another project that nepRWA is working on is the restoration of salt marshes in the estuary (nepRWA 2006). Though these projects are a great start, the estuarine ecosystem is complex and requires workers to continue thinking outside of the box in terms of management strategies. Two strategies that warrant investigation is whether eelgrass and/or oysters may benefit the ecosystem. In addition, how much eelgrass or how many oysters will it take to clean up the estuary?

Figures

References

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