Natural and Anthropogenic Influences on the Mount Hope Bay Ecosystem 2006 Northeastern Naturalist 13(Special Issue 4):199–204 Natural and Anthropogenic Influences on the Mount Hope Bay Ecosystem: Concluding Remarks Daniel G. MacDonald1 and Rodney A. Rountree1,2 The papers in this volume have provided a detailed focus on various aspects of the Mount Hope Bay ecosystem, from the local heat budget to Pseudopleuronectus americanus Waldbaum (winter flounder) stock declines. In this conclusion, we attempt to place these individual studies within the broader context of research performed in Mount Hope Bay over the last several decades. Physical Processes in Mount Hope Bay The cornerstone of any comprehensive interdisciplinary understanding of ecosystem change within Mount Hope Bay is a thorough understanding of the physical processes affecting the Bay. Here we provide a brief review of our current understanding of physical oceanography associated with Mount Hope Bay, as context for the detailed papers on this topic contained within the volume (Kincaid 2006, Fan and Brown 2006, Swanson and Kim 2006) and elsewhere (Mustard et al. 1999). The stratification of Mount Hope Bay varies seasonally, and is dominated by freshwater discharge from the Taunton River, as described in the introduction. Early studies of physical parameters in the region were performed by Hicks (1959) as part of a Narragansett Bay-wide study of temperature and salinity. A long time series of similar measurements conducted by Marine Research, Inc. (MRI), in conjunction with Brayton Point Power Station (BPPS), provides a more recent assessment of temperature and salinity, spanning the years 1972 to the present. Based on this data set, the annual cycle of stratification peaks during the winter and spring, with less stratified conditions during late summer and early fall, consistent with the Taunton River discharge records. It is also apparent from an inspection of these data that the stratification of Mount Hope Bay is controlled primarily by salinity differences, suggesting that the heated power plant discharge may not significantly alter the physical structure of the Bay, and that the excess heat may be carried through the system primarily as a passive component. This conclusion is consistent with the lack of any significant difference in overall stratification between the Hicks (1959) data, which represents pre-discharge conditions, and the long-term MRI profile (MacDonald et al. 2003). 1School for Marine Science and Technology, University of Massachusetts Dartmouth, New Bedford, MA. 2Current address - Marine Ecology and Technology Applications, Inc. 23 Joshua Lane, Waquoit, MA 02536. *Corresponding author - dmacdonald@umassd.edu. 200 Northeastern Naturalist Vol. 13, Special Issue 4 The persistence of salinity driven stratification throughout the Bay at all times of the year sets up some interesting issues with respect to the heated water discharge. The intake for the Brayton Point cooling water is located at approximately a 6-m depth, implying that denser bottom water is drawn into the plant. This water is then heated within the plant and ultimately discharged to the surface. The MRI observations suggest that salinity controls stratification in the water column, which would indicate that the discharged water should sink to somewhere near its original depth of 6 m, despite its additional heat load. Although data from several deployments of thermistor chains in the vicinity of Brayton Point by Applied Science Associates, Inc. (ASA) indicate that the discharge plume sometimes sinks during winter, the plume is confined to the upper portion of the water column most of the time (USGEN 2001). This suggests rapid mixing of the discharge with surrounding surface waters. The fate of the excess heat produced by the Brayton Point discharge across the Bay as a whole was the focus of the heat budget study discussed by Fan and Brown (2006) earlier in this volume. Few studies have directly addressed circulation patterns within the Bay through observations. Several current meters were deployed by Spaulding and White (1990) near the mouth of the Taunton River, the entrance of the Sakonnet River, and the narrows connecting Mount Hope Bay to the East Passage. This study indicated that currents in the Bay are primarily driven by tidal motion, with little influence from the wind, and that tidal currents are generally stronger between Mount Hope Bay and East Passage than in the Sakonnet River. Evidence of net estuarine flow was observed in the study near the mouth of the Taunton River. More recent and detailed observations, performed by Kincaid and discussed earlier in this volume, in the Sakonnet River and East Passage entrances, as well as the Taunton River, have added much needed detail to the original studies. Rounding out the physical contributions to this volume, Swanson et al. (2006) have presented a detailed numerical modeling study describing the physics and circulation of the Bay. This paper complements the observational work well, and provides a more detailed framework for further assessment of the local heat budget. Recent Ecosystem Changes in Mount Hope Bay Much attention has been focused recently on the declining stocks of winter flounder in Mount Hope Bay (e.g., Gibson 1996, Rountree et al. 2003). Although National Marine Fisheries Service (NMFS) records of commercial and recreational landings of winter flounder in the Rhode Island region over the last half century show several periods of stock building and decline, the most recent and most severe has occurred over the last 15 to 20 years. This trend is supported by many other data sets in recent years, including trawl data collected by the Rhode Island Department of Fish and Wildlife (RIDFW) and MRI. The timing of the beginning of this decline coincides with an increase in discharge volume and total heat output from BPPS, causing Gibson (1996, 1998, 2000a, 2000b) to claim a cause-and-effect relationship between these 2006 D.G. MacDonald and R.A. Rountree 201 two events. However, a review of data from various sources presented in this volume weakens this claim. Although analysis by Delong and Collie (symposium presentation, and Collie and Delong 2001) provide strong evidence for some impact of the Brayton Point Power Plant on winter flounder stocks in Mount Hope Bay, other studies provide equally strong contradictory analysis (DeAlteris et al. 2006, Englert et al. 2006, O’Neill et al. 2006, Rountree and Witting symposium presentation). As pointed out by Cichetti (2006) in his summary of discussion at the symposium, there is no clear scientific consensus on this issue, and it represents the most significant issue in the controversy surrounding BPPS. Several studies examined evidence for changes in natural mortality of winter flounder and other fishes due to changes in predator abundances and/ or behavior. As predator populations are affected through various environmental and management mechanisms, these changes can be passed on to prey populations, such as winter flounder. An example is the local cormorant Phalacrocoraz auritus Lesson population, which has increased exponentially over the last few decades. Because cormorants prey directly on winter flounder and other fish species, the large increases in its population may have caused increased natural mortality for some fishes in Mount Hope Bay as suggested by McCay and Rowe (symposium presentation). The shrimp Crangon septemspinosa Say is also a major predator of winter flounder, directly affecting winter flounder eggs, as described by Taylor's symposium presentation. An interesting dynamic of shrimp predation is its interaction with temperature, where increasing water temperatures brought on by climate change and/or thermal effluent results in dramatic increases in the mortality of winter flounder eggs and larvae due to shrimp predation. Webb's presentation commented on the potential impact of the increasing seal population on winter flounder stocks. Many other significant changes have also occurred within the Mount Hope Bay ecosystem during the same time frame. Zostera marina L. (eelgrass) beds, once prevalent throughout the Narragansett Bay region were decimated by a fungal blight in the 1930s, which may have reduced its presence by up to 90% (Short et al. 1996). Further declines of this important fish habitat during the last several decades, presumably due to nutrient loading (Short et al. 1996), have resulted in the complete loss of eelgrass from Mount Hope Bay (Rines 1999, cited in USGEN 2001). The potential importance of increased nutrient loading on the Mount Hope Bay ecosystem was frequently discussed at the symposium. Although, unfortunately, there is little direct measurement data on nutrient loading in the Bay, several studies did point to indirect evidence of nutrient loading effects (Deacutis et al. 2006; Howes and Schlezinger, and Rountree and Witting symposium presentations). Increased nutrient loading from point and nonpoint sources throughout the watershed can contribute to low dissolved oxygen in bottom waters as shown in presentations by Howes and Schlezinger as well as Deacutis et al. Both of these talks described data sets 202 Northeastern Naturalist Vol. 13, Special Issue 4 identifying periods of critically low dissolved oxygen in bottom waters, a condition that can lead to severe effects for local biological populations. As mentioned in the introduction, Rountree and Witting's presentation suggested that the observed shift in Mount Hope Bay fish assemblages from benthic to pelagic species is often a symptom of eutrophication effects due to increased nutrient loading. The phytoplankton and zooplankton communities have also undergone recent changes, although a significant gap in the collection of plankton data between 1986 and 1996 has prevented detailed analyses (Rountree et al. 2003). Analysis of samples collected in Mount Hope Bay for one year in 1997 and 1998 suggest an increase in winter dinoflagellates and winter zooplankton, as compared to mean data from the period 1972 through 1985. These changes were coupled with a decrease in chlorophyll by a factor of 2 to 4, and an increase in ammonia by a factor of 2 (MRI 1999). A mooring deployed in Mount Hope Bay south of Spar Island by the School for Marine Science and Technology (SMAST) at the University of Massachusetts Dartmouth during the summer of 2001 indicates that summertime stratification and nutrient loading was sufficient to contribute to periods of critically low dissolved oxygen concentrations in bottom water (Rountree et al. 2003). A more comprehensive survey of dissolved oxygen throughout the upper portion of Narragansett Bay, including Mount Hope Bay, was presented in the paper by Deacutis et al. (2006). The potential for certain regions of the Bay to become hypoxic can sometimes be enhanced directly through the presence of various physical man-made structures, such as culverts, dams, and bridge crossings, that can alter the natural tidal flow of the system. In the final paper in this volume, Barrett et al. (2006) documented recent efforts to produce an atlas of these tidal restrictions, which can be of use to local municipalities and regulators as they develop management plans for the future. Regional warming trends have also been at work in Narragansett Bay and Mount Hope Bay. Between 1960 and 1990, the average wintertime water temperature in Narragansett Bay increased by nearly 3 °C (Walker 2001). This is set within the context of a 1.8 °C average wintertime increase for terrestrial temperatures across New England and upstate New York between 1895 and 1999 (Keim and Rock 2001). Clearly, the local ecosystem within Mount Hope Bay is affected by both natural and anthropogenic changes occurring at many different scales. This brief assessment of ecosystem changes in Mount Hope Bay is by no means comprehensive, but is included here to provide context with respect to the wide range of papers contained within this volume. Conclusion Collectively, the papers presented at the symposium and the resulting papers published in this volume provide the state of the art in terms of what is known about the Mount Hope Bay ecosystem, and can begin to provide a framework for unraveling the complex web of natural and anthropogenic 2006 D.G. MacDonald and R.A. Rountree 203 interactions responsible for the issues we observe today, not only in Mount Hope Bay, but in other regions as well. Mount Hope Bay represents a complex ecosystem that has been impacted by over 200 years of human activity in the region. The symposium and resulting proceedings demonstrate the need for regional academic institutions, resource managers, conservation groups, and resource users to work together to study Mount Hope Bay from an ecosystem perspective in order to resolve the controversial issues surrounding the Bay and to develop workable management plans for the system in the future. In addition, it is clear that Mount Hope Bay’s ecosystem cannot be studied in isolation from the greater Narragansett Bay system. Significant exchange of water between Mount Hope Bay and Narragansett Bay strongly influence local water quality. The degree of isolation of fish populations between Mount Hope Bay and Narragansett Bay must be determined before proper assessment of Mount Hope Bay fish stocks can be made. Mount Hope Bay can be viewed as a natural laboratory where regional scientists and managers are attempting to determine the relative contributions of natural and anthropogenic, local and regional, influences on an estuarine ecosystem. We believe that the lessons learned, and questions, both asked and unanswered, in this “laboratory” are applicable to many estuaries around the world. Literature Cited Barrett, S.B., B.C. Graves, and B. Blumeris. 2006. 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