This contribution represents a summary of talks presented during the afternoon session of the Mount Hope Bay Symposium, focused directly on issues surrounding observed winter flounder populations, as prepared by the session moderator.

For nearly a decade, Massachusetts resource managers have been systematically inventorying, assessing and restoring coastal wetlands degraded by infrastructure crossings such as bridges and culverts, which, unless properly designed and constructed, restrict tidal flow to upstream areas. These crossings—known as tidal restrictions—alter the natural flooding and flushing dynamics of coastal estuarine and wetland habitats, causing damage to salt marshes, eelgrass beds, and other important shellfish and finfish habitats. The Mount Hope Bay Tidal Restriction Atlas, undertaken by the Massachusetts Wetlands Restoration Program and the US Army Corps of Engineers, is the most recent addition to the statewide inventory effort.

The Atlas lists a total of 74 potential tidal restrictions that were initially identified on maps and aerial photographs, of which 25 sites were documented using rapid field assessment techniques. Each field-visited site was assessed for severity of restriction and habitat impacts, potential environmental benefits of restoration, and logistical feasibility for implementing restoration. Using these factors, sites were prioritized using a high/medium/low scale. A three-page Site Assessment Report, which includes data, maps, and photographs, was generated for each field-visited site. The final Atlas was presented in an interactive digital format to allow users to query its database and readily move between data fields, maps, and photographs. The Atlas was distributed to stakeholders as a tool for use in restoration planning and implementation.

A simple heat budget has been constructed for Mount Hope Bay (MHB) for two one-month periods: winter 1999 (February–March) and summer 1997 (August–September). The box model considered here includes the heat contributions to MHB from the Brayton Point Power Station (BPPS), the exchange across the air–water interface, the Taunton River, and the tidal exchange between MHB and both Narragansett Bay and the Sakonnet River (NB/SR). Comprehensive measurements of MHB temperature fields by Applied Science Associates, Inc., and meteorological data from T.F. Green Airport (Warwick, RI) were used to estimate the different heat flux component contributions. The box model results for winter show that the BPPS heating is balanced (within the uncertainty of the estimates) by air–water cooling alone. The simple winter balance does not hold during the summer, when heat losses due to tidal exchanges between MHB and NB/SR are important. The summer heat budget of MHB—including BPPS heating, air–water cooling and tidal exchange cooling—can be balanced (within the uncertainty of the estimates) by assuming that 3% of the colder NB/SR tidal input water is exchanged with the warmer MHB water during each tidal cycle. The air–water cooling accounts for 84.4% of the total cooling, and the tidal exchange accounts for 15.6% of the total cooling. Taunton River contributions to the heat budget were negligible in both seasons. Analyses show that the model temperature is most sensitive to uncertainty in the measurements used to estimate the air–water heat fluxes—the relative humidity in particular. Thus, local MHB measurements are important for accurate monitoring of the MHB heat budget in the future.

On behalf of Brayton Point Station, an electrical generating plant located on the shore of Mount Hope Bay, the authors performed an innovative biothermal modeling assessment to evaluate effects of heat load from the Station on 10 bay-resident fish and shellfish species. The assessment linked several biological functions (growth, reproduction, avoidance, migratory blockage, and thermal mortality) to hydrothermal simulations of the Station's thermal plume under two plant-operating scenarios and the no-plant scenario. The assessment methodology is described, and results are presented for Pseudopleuronectes americanus (winter flounder), the species with the lowest thermal tolerance temperatures of those studied. Based on the modeling approach and input assumptions, the effects of the Station's thermal discharge on the winter flounder life stages and functions studied were found to be negligible, especially when compared to other effects such as fishing pressure. The largest plant effect observed was only 3.9 percentage points more than for the no-plant scenario (compared to a fishing effect of approximately 40–50%). Limitations of the model and potential future refinements to address additional biological effects are discussed and evaluated. The modeling methodology used to complete this study represents a novel and scientifically grounded approach to quantifying the Station's thermal impacts on the biota of Mount Hope Bay.

Trends in abundance for winter flounder (Pseudopleuronectes americanus), windowpane (Scophthalmus aquosus), hogchoker (Trinectes maculatus), tautog (Tautoga onitis), and scup (Stenotomus chrysops) in upper and lower Mount Hope Bay were compared to trends in Narragansett Bay to assess the effect of natural and anthropogenic stressors, including Brayton Point Power Station, on Mount Hope Bay fishes from 1972 to 2001. Sources of data included the Rhode Island Division of Fish and Wildlife trawl survey for Narragansett Bay and lower Mount Hope Bay, the University of Rhode Island Graduate School of Oceanography trawl survey for Narragansett Bay, and the Marine Research, Inc. trawl and Brayton Point Station impingement surveys for upper Mount Hope Bay. Analysis of covariance and Tukey-Kramer multiple comparison tests were used to evaluate differences in the slopes of transformed abundance indices from 1972–2001 and for two subsets of years, 1972 to 1985 and 1986 to 2001, periods of lower and higher power plant cooling water withdrawals, respectively. Trends in abundance of these species in both upper and lower Mount Hope Bay are not substantively different from those in Narragansett Bay during any of the three time periods evaluated. This is evident through either a high-level visual inspection of the slopes measured for each species, time period, and area or a more detailed inspection of the analysis of covariance results and Tukey-Kramer confidence intervals associated with each slope estimate. Natural and anthropogenic stressors unique to Mount Hope Bay, including Brayton Point Station, have not caused Mount Hope Bay fish stocks to change at rates different from those observed for the same stocks in Narragansett Bay. This supports the conclusion that large-scale factors such as overfishing, climate change, and increased predator abundance are more likely to be the cause of the observed declines in important species such as winter flounder in Mount Hope Bay and Narragansett Bay.

Results are presented from a set of hydrographic surveys conducted within Mount Hope Bay, RI, during the summer of August, 1996. This sub-system of Narragansett Bay is interesting because it has two connections to the ocean and it has a source of thermal energy from the Brayton Point Power Plant. Data was collected on water velocity, salinity and temperature on days with relatively high (≈ 2 m range) and relatively low (≈ 1 m range) tidal forcing. Velocity data were collected along fixed transect lines defining the boundaries of the estuary and at fixed stations. Results show that flow through each of the oceanward entrances has significant horizontal and vertical structure. The source of fresh water is the Taunton River to the north, and at times, exchange through this interface exhibits vertically sheared flow. Exchange is dominated by flow through the interface with Narragansett Bay, where transports reach 3000 m³/s and 6000 m³/s under conditions of low and high amplitude tidal forcing, respectively. Peak velocities exceed 100 cm/s. Values for transport though the smaller of the two salt water connections, with the Sakonnet River, and the fresh water entrance, at the interface with the Taunton River, were ≈ 10– 20% of those through the interface with Narragansett Bay. Velocities are relatively sluggish in the shallow northern shelf region of the estuary, peaking at < 10 cm/s and ≈ 20 cm/s for the low and high tidal amplitude sampling periods, respectively. Temperature and salinity data reveal significant levels of stratification and suggest three end-member water sources including a deep Narragansett Bay source (cold, salty), a shallow river source (warm, fresh) and a source of water from the Brayton Point region (hot, intermediate salinity). A plug of warm water that evolves on the northern shelf over the ebb cycle of the tide is advected to the east–northeast into the shipping channel during the flood. Phase differences in total instantaneous transport through the two mouths of the system suggest that interactions with the Sakonnet River are dominated by the greater volume and efficiency of exchange with the East Passage of Narragansett Bay. Lateral variations in residual transport show East Passage water entering Mount Hope Bay through the deep central portion of the cross-section and exiting through confined regions along the edges of the interface. The pattern in residual exchange with the Sakonnet River shows water exiting and entering Mount Hope Bay through the western and eastern portions of the cross section, respectively. A conceptual model is suggested in which these lateral flow patterns combine with strong vertical mixing in the Sakonnet River Narrows to pump thermal energy downward in the water column and back northward into the bottom waters of Mount Hope Bay.

Brayton Point Station is a 1600-MW electrical generating station located on Brayton Point, in Somerset, MA. The Station draws water from Mount Hope Bay at the Taunton and Lee Rivers for cooling purposes, and discharges the water back into the Bay, through a discharge canal. Mount Hope Bay is a shallow estuary located on the boundary between Rhode Island and Massachusetts. In connection with the renewal of the permit authorizing the withdrawal and discharge of cooling water, a series of studies on Mount Hope Bay were initiated by the owners of Brayton Point Station. These studies included both field and computer modeling components. A hydrothermal model capable of simulating the effects of Brayton Point Station on the Mount Hope Bay waters under a variety of operating scenarios was calibrated using the observed data. Additional cases were run to evaluate the effects of reduced discharges of heated effluent incorporating a cooling tower (enhanced multi mode operation) as well as the case of no discharge. Model results indicated that the temporal temperature variations occur over tidal to annual time scales. Seasonal variations were most discernible in the shallow upper reaches of the Bay, showing warmer than average temperatures during summer and cooler during winter. The calibrated hydrothermal model was also used to estimate the bottom area and water column volume coverage versus temperatures, which helps to quantify the effects of station heat load on the biological functions of winter flounder in Mount Hope Bay.

Narragansett Bay, RI, is considered to be a relatively well-mixed estuary not subject to extensive seasonal stratification and hypoxia. However, results of surveys of dissolved oxygen (DO) for the upper half of Narragansett Bay on August 15, 2001 and on August 6, 2002 have documented evidence of wide-area intermittent subpycnoclinal hypoxia (≤ 3 mg l⁻¹). For the August 2001 survey, severe hypoxic to near-anoxic levels were confined to the Providence River, the western side of Greenwich Bay, and a small area of Mount Hope Bay, but hypoxic levels below 2 mg l⁻¹ were also experienced on the western side of the Upper Bay in an extensive, shallow oxygen minimum. Hypoxic bottom waters (≤ 3 mg l⁻¹) extended from the Upper Bay into the upper West Passage. Hypoxic waters covered approximately 66 km² (36%) of the survey area for August 15, 2001. A more extensive and severe hypoxic event occurred during the August 2002 survey, when near-bottom waters of the entire Providence River and a large area of the Upper Bay and upper East Passage were severely hypoxic to near-anoxic, while other parts of the Upper Bay, upper East Passage and upper West Passage were hypoxic at depths greater than 5 m. Limited data for Mount Hope Bay in August 2002 documented small hypoxic areas of the southern end of that subembayment. The total hypoxic area for August 6, 2002 was approximately 93 km² (65%) of the total area surveyed. Decreased estuarine circulation due to a severe drought may have contributed to the wider extent of hypoxic and near-anoxic waters in large areas of the upper half of Narragansett Bay recorded in the August 6, 2002 survey as compared with the August 15, 2001 survey. Results of the oxygen surveys affirm sediment profile camera work and limited benthic studies that previously suggested parts of the Mid Bay have become subject to increased organic loading impacts. These impacts can take place even under drought conditions, when only point source nutrients are the major contributors to nutrient loadings entering the upper half of Narragansett Bay.

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.