Natural and Anthropogenic Influences on the Mount Hope Bay Ecosystem
2006 Northeastern Naturalist 13(Special Issue 4):71–94
Effects of Brayton Point Station’s Thermal Discharge on
Mount Hope Bay Winter Flounder
Robert J. O’Neill1,*, Thomas L. Englert1, and Jee K. Ko1
Abstract - 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 plantoperating
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.
Introduction
A number of independent trawl and beach-seine surveys of finfish
populations have been conducted in the Narragansett Bay complex over the
past several decades (Collie and Delong 2002, Lynch 2000, MRI 2000).
They indicate that catches of several commercially and recreationally important
species, including Pseudopleuronectes americanus Waldbaum
(winter flounder), have declined region-wide since the late 1970s. Potential
causes suggested for the decline include overfishing, increased abundance
of finfish predators, and habitat degradation due to natural and anthropogenic
effects. In the Mount Hope Bay part of the Narragansett Bay
complex, Brayton Point Station operations have been cited as a possible
stressor on the local finfish populations.
Located in Somerset, MA, Brayton Point Station is a 1600-megawatt
electrical generating station that uses Mount Hope Bay for withdrawal
and discharge of condenser cooling water and plant-service water. Cooling
water is heated as it circulates through condensers that cool spent
steam, and thus its discharge from the Station introduces heat into the
1HDR|LMS, 1 Blue Hill Plaza, PO Box 1509, Pearl River, NY 10965. Corresponding
author - Robert.ONeill@hdrinc.com.
72 Northeastern Naturalist Vol. 13, Special Issue 4
Bay. The heated discharge mixes with ambient bay water, expands into a
thermal plume, and eventually dissipates to nearly ambient temperature
(USGenNE 2001).
To determine the potential for any direct impacts of Brayton Point
Station’s thermal plume on biological functions of finfish in Mount Hope
Bay, a bay-wide biothermal assessment was performed by HDR|LMS. The
assessment effort was funded by Brayton Point Station in preparation for
completing its NPDES permit renewal application. The purpose of the study
was to quantify the direct effects of Station heat load on the biological
functions of nine vertebrate (winter flounder, Cynoscion regalis Block and
Schneider [weakfish], Anchoa mitchilli Valenciennes [bay anchovy],
Pomatomus saltatrix L. [bluefish], Brevoortia tyrannus Latrobe [Atlantic
menhaden], Menidia menidia L. [Atlantic silverside], Alosa pseudoharengus
Wilson [alewife], Morone saxatilis Waldbaum [striped bass], and Morone
Americana Gmelin [white perch]) and one invertebrate (Mercenaria
mercenaria L. [quahog]) species that reside for all or part of the year in
Mount Hope Bay. This paper describes the assessment process and presents
results for winter flounder. Of the 10 species evaluated, winter flounder is
the most thermally sensitive (USGenNE 2001) and has been an important
part of the commercial and recreational fisheries in the region. Results for
the other nine species are provided in Appendix B of USGenNE (2001).
The biothermal assessment model is an innovative tool designed to
mathematically link biological functions of selected species to hydrothermal
simulations of the Station’s thermal plume under various operating
scenarios. Hydrothermal simulations of the Station’s thermal plume were
provided by Applied Science Associates, Inc. (ASA). ASA divided Mount
Hope Bay into 1049 three-dimensional grid cells and modeled temperatures
within each cell. ASA’s thermal data are presented in USGenNE (2001).
Essential to this biological/thermal (thus “biothermal”) synthesis was the
ability to portray thermal effects on biological functions mathematically—
for example, to quantify the level of mortality that would result from a
prolonged exposure over a continuum of possible temperatures. There was
no comprehensive “cookbook” approach to this process in the scientific
literature. Therefore, HDR|LMS developed a series of mathematical
algorithms, each designed to determine temperature effects on a specific
biological function. While modeling components were developed from various
sources in the scientific literature, their subsequent organization and
application in the biothermal model are unique.
Model Description
Assumptions
An overarching goal of the assessment was to take a conservative approach—
that is, to model “worst-case” effects. With respect to selecting
temperatures for modeling, this goal translated into identifying a year with
unusually warm temperatures. The year selected for modeling was 1999,
2006 R.J. O’Neill, T.L. Englert, and J.K. Ko 73
identified as a “reasonable worst-case” warm year based on hydrothermal
modeling of water temperatures and a review of local air-temperature
records. The hydrothermal model and recorded weather information showed
that: (1) 1999 ranked as the second warmest with respect to water temperatures
among 40 consecutive years for which ASA simulated ambient (i.e.,
no-plant) water temperatures, and (2) during the same 40-year period, 1999
ranked as the fifth warmest year with respect to average air temperature
reported at Rhode Island’s T.F. Green County Airport, located 12 miles
southwest of Brayton Point Station.
Bay-water temperature sets under three different heat loads were evaluated
for biothermal effects. The first set was predicted temperatures under
current Station operation (average heat load of 3.18 tBTU per month, which
was the actual monthly average during the 1999 model year). The second set
was predicted temperatures under a hypothetical Station operating scenario
that would yield a 33% reduction in heat (average heat load of 2.25 tBTU per
month) compared to the currently permitted monthly average of 3.50 tBTU.
Finally, as a baseline condition, effects of the ambient temperature regime
were also determined (heat load of 0.0 tBTU per month) for the third set.
Ambient temperatures are those that would occur naturally in the Bay under
the no-plant scenario. Each temperature set contained ASA-simulated water
temperatures at every hour of the 1999 year in each of the 1040 Mount Hope
Bay grid cells.
An intensive search of the scientific literature was conducted to identify
those winter flounder biological functions for which temperature tolerance
data were available. Based on this search, the following functions were
selected for assessment: growth, reproduction, avoidance, migratory blockage,
and chronic (72-hour) thermal mortality (heat shock and cold shock
were evaluated in earlier assessments and shown to have no potential for
harm under either the current or reduced-heat-load Station operating
scenario [USGenNE 2001]). The corresponding winter flounder habitat interactions
were examined for all life stages except larvae (due to insufficient
data). More information about these functions is provided in the “Methods
and Results” section of this paper.
Approach
The biothermal assessment consisted of six steps:
1. Map the distribution and temperatures of the thermal plume over time.
ASA made extensive temperature readings in Mount Hope Bay over several
years. These field observations were used to calibrate WQMAP, a hydrothermal
modeling system developed by ASA and the University of Rhode
Island to model the circulation and water quality of estuarine and coastal
waters (Spaulding et al. 1999). ASA then used WQMAP to simulate temperature
data within each of the Bay’s 1049 grid cells.
As can be seen in Figure 1, grid-cell sizes varied. Typical cell dimensions
were 200 by 300 m (650 by 1000 ft) in the southern portion of Mount Hope
Bay and 50 by 100 m (160 by 330 ft) in the northern portion. The grid was
74 Northeastern Naturalist Vol. 13, Special Issue 4
finer in the area near the Station’s discharge venturi to better resolve the
complex hydrodynamics affecting the recently discharged thermal plume.
Variations in cell size made it necessary to area-weight assessment findings.
This task was accomplished using the following equation:
×
=
=
=
n
i
i
n
i
i i
Area
Bt Area
1
1
( )
Area - Weighted Average (1)
where Bti = predicted biothermal effect for habitat grid cell i (n = total
number of habitat cells), and Areai =area of habitat grid cell i.
The assessment time-step is the amount of time that temperature conditions
within a grid cell were assumed to be constant. Although ASA simulated
hourly baywater temperatures, a 24-hour time-step was used for the
biothermal assessment, resulting in a single averaged temperature value for
each day. Several shorter time-steps were considered—including a one-hour
Figure 1. Mount Hope Bay hydrothermal grid cells.
2006 R.J. O’Neill, T.L. Englert, and J.K. Ko 75
interval—to explore whether plume location and dissipation (i.e., mixing,
which is directly related to predicted temperature) depend on changing phases
of the tide. However, examination of ASA’s hydrothermal model results
revealed little intra-day variation (during the summer period, the hourly
bottom water temperatures characteristically oscillated within just ± 0.5 °C of
the daily mean). In addition, the 24-hour interval is shorter than the 72-hour
duration used in examining chronic exposure. Finally, use of a 24-hour timestep
instead of a 1-hour interval reduced the number of records requiring
processing from 246 million to 10.25 million.
For one of the biological functions measured—migratory blockage of
upper-bay tributaries—a one-hour time-step was used. This time-step is
appropriate because juveniles moving up the river may pass through the
tested hypothetical “door” at each tributary mouth within a relatively short
period of time. From a data-processing standpoint, the one-hour time-step
for migratory blockage was facilitated by the fact that the mouths of the
tributaries are represented by relatively small numbers of grid cells in the
hydrothermal model.
2. Determine acclimation temperatures within all grid cells. Ambient
temperatures in Mount Hope Bay vary substantially from one season to the
next. In response to such temperature changes, fish undergo physiological
changes that alter their thermal preferences and tolerances. This adjustment
process is called acclimation.2 Acclimation temperature is the temperature
to which a fish has been exposed for a period of time sufficient to allow
adjustment of physiological processes, e.g., metabolic rates (Brett 1956,
Coutant 1972). The time required varies from several days to more than a
week (Fry 1971).
The acclimation process was incorporated into the winter flounder
biothermal assessment. For each grid cell for each day of the 1999 year
under the two Station operating scenarios and the no-plant scenario, an
acclimation temperature was developed. It was calculated by averaging the
temperatures within a given grid cell over the seven days prior to the day
evaluated. This methodology is consistent with two key facts: (1) the home
range of winter flounder juveniles is quite small, with 98% residing within a
100-m (328-ft) radius over a 1-week time frame (Saucerman and Deegan
1991), and (2) typical ASA grid-cell dimensions (200 by 300 m) accommodate
a 100-m home-range radius.
3. Determine winter flounder’s temperature tolerance for each
biothermal function evaluated. Acclimation/exposure temperature relationships
were defined for winter flounder growth, avoidance, reproduction
inhibition, and chronic thermal mortality. They are plotted in Figure 2, a
temperature-tolerance polygon that shows how different combinations of
exposure and acclimation temperatures affect winter flounder biological
functioning. Use of the polygon makes it possible to depict a continuum of
acclimation/exposure temperature relationships for multiple biological functions
in a single graphic display.
76 Northeastern Naturalist Vol. 13, Special Issue 4
For each function assessed, the scientific literature reports a range of
temperature tolerances (Beitinger and Bennett 2000). Consideration of the
full range of responses is essential in a biothermal assessment because
reliance on a single thermal threshold could grossly oversimplify the
findings. For example, it could lead to the conclusion that an exposure
temperature 0.1 °C above a single threshold would cause the entire population
to be adversely affected or, conversely, that an exposure temperature 0.1
°C below the threshold would cause no discernable effect. Throughout the
assessment of biothermal effects on the Mount Hope Bay RIS, variable
response was considered in evaluating the effect of acclimation/exposure
temperatures at each time-step in each bay grid cell.
The temperature tolerance polygon reflects a synthesis of the range of
population responses found for each function examined. For example, Figure
2 shows mean chronic thermal mortality thresholds—i.e., acclimation/
exposure-temperature combinations at which 50% mortality would occur.
(The full range of reported threshold mortality temperatures above and
below the mean are depicted in Figure 3).
An explanation of the data presented in Figure 2 follows:
Chronic (72-hour) thermal mortality under a prolonged exposure (dark
blue line)—biothermal assessment of winter flounder chronic mortality is
relevant because not all winter flounder avoid elevated temperatures. The
scientific literature suggests that some portion of the winter flounder
Figure 2. Temperature tolerance polygon for juvenile and adult winter flounder: how
winter flounder temperature tolerances change in response to changing combinations
of acclimation and exposure temperatures.
2006 R.J. O’Neill, T.L. Englert, and J.K. Ko 77
population might burrow in the bottom substrate rather than leave or avoid
an area of elevated temperature, and thus be exposed for a prolonged period
(Klein-MacPhee 1978, Olla et al. 1969).
The 72-hr duration was chosen because it is essentially the longest
laboratory test period for chronic mortality found in the scientific literature.
Typically, no additional mortality is expected, even if the testing period
were to be extended. To test this supposition, we developed a multiple
regression model using the equations from Kilgour et al. (1985) to evaluate
winter flounder mortality beyond the range of exposure durations available
in the scientific literature (i.e., 1, 3, 6, 24, 48, and 72 hours). It showed that
the predicted adult winter flounder TL50 for an extrapolated 25-day exposure
of juveniles fully acclimated to 28.0 °C was 29.0 °C. The value used in the
biothermal model (for winter flounder fully acclimated to 28 °C) was
29.1 °C. Thus, the effect of increasing exposure duration beyond 72 hours
was found to be negligible. It should be noted that the 1972 USEPA Water
Quality Criteria (NAS/NAE, 1973) indicate that chronic assessments should
be done using weekly average temperatures. Since that approach would only
serve to dampen the effect of the peak values used in the biothermal assessment,
and therefore result in less potential for thresholds to be exceeded, it
was rejected in favor of the 72-hour exposure duration.
Data for the development of the chronic mortality line came from Hoff
and Westman (1966). As can be seen in Figure 2, in general, the higher the
acclimation temperature, the higher the exposure temperature that can be
tolerated—until a maximum limit is reached, which is the point at which no
Figure 3. Variable response of winter flounder to exposure temperatures around the
mean chronic threshold.
78 Northeastern Naturalist Vol. 13, Special Issue 4
further increase in thermal tolerance is possible via acclimation. This limit is
the point where the chronic mortality line plateaus (acclimation temperature
of approximately 28 °C).
Based on Coutant (1972) and USEPA (1976), and as depicted in Figure 3,
the temperature at which winter flounder chronic thermal mortality approaches
zero was set at 2 °C lower than the mean tolerance line (TL5072 hr)
shown in the polygon. By extension, assuming a normal distribution, winter
flounder chronic thermal mortality would effectively be 100% at 2 °C higher
than the TL50, and at 0.5 °C above the mean temperature, approximately
75% of the population would respond.
Avoidance (red line)—a thermal avoidance response occurs when mobile
species evade stressful high temperatures by moving to water with lower, more
acceptable temperatures (Meldrim et al. 1974). The avoidance response can
deter a species from occupying otherwise useful habitat in the vicinity of a
thermal plume. Because the percentage of winter flounder that burrow rather
than avoid elevated temperature is unclear, both avoidance and chronic mortality
were assessed. It is important to not sum the predicted results for these two
biological functions—an action that would represent double-counting. Clearly,
a fish can either burrow or avoid in response to elevated temperature.
As shown in Figure 2 for chronic mortality, in general, the higher the
acclimation temperature, the higher the exposure temperature that can be
tolerated before avoidance occurs. No further increase in mean thermal
tolerance is possible after the acclimation temperature reaches approximately
28 °C. Lower and upper bounds around the mean were set at ± 5 °C
based on the extensive laboratory avoidance test results reported in Mathur
et al. (1983). Multiple sources were available to support development of the
avoidance line in Figure 2, including Gift and Westman (1971), IA (1978),
Meldrim et al. (1974), and Terpin et al. (1977).
Growth zone (green-blue area)—this area delineates the acclimation/exposure
temperature combinations for which growth is predicted (McCullough
1999). The upper limit of the growth zone was defined as roughly half-way
between the optimal growth temperature and the temperature producing netzero
growth. The 1972 EPA Recommended Water Quality Criteria (NAS/NAE
1973) specify that, in the absence of zero-growth data, upper growth-zone
limits can also be approximated via the following equation:
Critical growthlimit = OT + (UILT - OT) / 3 (2)
where the optimal temperature (OT) is the “temperature preference” of fish
in a thermal gradient—an adaptive mechanism that allows the organisms to
be positioned in an environment where they can achieve optimal physiological
performance (Coutant 1977, Hutchison and Maness 1979), and the upper
incipient lethal temperature (UILT) is the temperature that is survivable by
50% of the population for an extended exposure.
It is relevant to note that maximum growth temperatures are not consistently
maintained in nature on a daily basis, and delineation of a growth
“zone” makes clear the fact that suboptimal temperatures are not necessarily
2006 R.J. O’Neill, T.L. Englert, and J.K. Ko 79
adverse.Data in support of the development of the growth zone depicted in
Figure 2 came from Radle (1971).3
Temperature tolerance zone (yellow area)—this area is outside the
growth zone. It delineates the temperature regime over which winter flounder
can survive, but in which they are stressed and experience near-zero or
negative growth, i.e., weight loss (Beitinger and Bennett 2000).
Reproduction inhibition zone (pink area)—Buckley et al. (1990) note
that adult winter flounder acclimation temperature before spawning is an
important factor in egg thermal mortality. Their research showed that
eggs and subsequent larvae produced by adults acclimated to seasonally
warm temperatures (7 °C) are better adapted for survival at warmer exposure
temperatures (10 °C). Thus, since spawning females are typically
spent after the first three weeks of the spawning period (Stoner et al
1999), the pre-spawning acclimation temperature for adult winter flounder
was approximated as the average over the three weeks (rather than the
seven days) prior to the day evaluated.
The lower and upper limits of the reproduction inhibition zone shown in
Figure 2 depict the various combinations of acclimation and exposure
temperatures for which zero and 100% winter flounder egg mortality, respectively,
is predicted. Within the reproduction inhibition zone, a normal
probability distribution is applied, where the midway point is representative
of 50% egg mortality.
Because Buckley et al. (1990) did not test for adult acclimation temperatures
above 7.0 °C, there was no basis for evaluating acclimation effects on
egg mortality at temperatures higher than that value (Fig. 2). Thus, for adult
acclimation temperatures greater than 7.0 °C, the lower and upper limits of
the reproduction inhibition zone shown at 7.0 °C in Figure 2 were applied
(i.e., the far right-edge of the reproduction inhibition zone).
In addition to Buckley et al. (1990), two other sources were available to
support development of the reproduction inhibition zone: Rogers (1976) and
Williams (1975).
4. Determine winter flounder’s horizontal and vertical habitats in Mount
Hope Bay. Figure 4 delineates the life-stage habitats used in assessing winter
flounder biothermal effects. Winter flounder prefers sand or sand-silt bottoms,
which are common throughout Mount Hope Bay. Based on habitat
data from Stoner et al. (2001) and bathymetric data documenting Mount
Hope Bay depths from Swanson et al. (1998), the winter flounder horizontal
habitat was delineated as < 5 m for juveniles, which occupy shallow areas,
and > 5 m for adults, which occupy the remaining deeper waters.
5. Determine when winter flounder life stages inhabit Mount Hope Bay.
Figure 4 shows the periods of occurrence in Mount Hope Bay for the winter
flounder life stages examined. This information reflects the findings of
decades-long Mount Hope Bay ichthyoplankton and trawl sampling programs
(MRI 2000) combined with period-of-occurrence data found in the
scientific literature (McCracken 1963, Pearcy 1962).
80 Northeastern Naturalist Vol. 13, Special Issue 4
As noted above, the winter flounder larval life stage could not be
examined due to the lack of specific thermal tolerance data (i.e., paired
acclimation/exposure-temperature information) in the scientific literature.
6. Apply the preceding inputs to predict the plume’s effects on winter
flounder biological functions. For each grid cell within winter flounder’s
habitat in Mount Hope Bay, the acclimation temperature (the average of the
cell’s temperatures during the prior seven days) was determined from ASA
hydrothermal modeling output using the model's bottom layer temperatures
(since winter flouder is a benthic species). This was done for every day that
winter flounder reside in the habitat. Then, for each day for each biothermal
function evaluated, the cell’s acclimation temperature and exposure temperature
were evaluated using the thresholds in the temperature tolerance
polygon in order to predict the biothermal effect.4 This process was carried
out for each Station operating scenario using the ASA scenario-dependent
baywater temperature values.
Table 1 summarizes the life stages, habitats, and periods of occurrence
examined for winter flounder. Collectively, these parameters comprised the
inputs to the analytical process.
Figure 4. Juvenile and adult winter flounder habitats in Mount Hope Bay.
2006 R.J. O’Neill, T.L. Englert, and J.K. Ko 81
Table 1. Winter flounder assessment—biothermal metrics evaluated and model inputs applied.
Habitat Period of occurrence in Mount Hope Bay
Biothermal function Life stage (see Fig. 4) Dates Rationale
Reproduction Eggs Adult Jan 25 to May 11 The period of occurrence for winter flounder egg production was
determined from the catch records of MRI’s 1973–1999 Mount
Hope Bay ichthyoplankton sampling program (MRI 2000).
Growth, avoidance, and chronic mortality Juveniles Juvenile Apr 21 to Oct 31 Both MRI data as well as life-history information from the scientific
literature were used. To avoid excluding juveniles whose
larval Stage IV antecedents occurred as early as April 21, the
period of occurrence for juvenile winter flounder was assumed to
be April 21 through October 31. Juvenile winter flounder remain
in the nursery area throughout the summer and move offshore
when water temperatures drop below 8 ºC (Casterlin and
Reynolds 1982).
Growth, avoidance, and chronic mortality Adults Adult Periods when bay- McCracken (1963) notes that adult winter flounder leave bays
water temperatures and estuaries when bottom temperatures exceed 15 ºC.
are below 15 ºC
Blockage Juveniles Juvenile April, October Winter flounder juveniles move into Mount Hope Bay’s tributaries
during April, remain there until the fall, and then in October
return to the Bay’s more saline water to overwinter.
82 Northeastern Naturalist Vol. 13, Special Issue 4
Methods and Results
Results of the Mount Hope Bay winter flounder biothermal assessment
are summarized in Table 2 and discussed below. For each biothermal function
evaluated, details about data processing techniques are included.
Growth
For this assessment, it was assumed that the rate of growth is uniform
throughout the growth zone (Fig. 2) and that sufficient food is available for
growth when exposure temperatures are within the growth-zone range.
The following steps were performed for winter flounder adults and
juveniles:
1. For each day within winter flounder’s period of occurrence, the acclimation
and exposure temperatures for each habitat grid cell were examined
relative to the growth zone shown in the temperature tolerance polygon. If
the acclimation/exposure point of intersection fell within the growth-zone,
that portion of the habitat was found suitable for growth.
2. The areas of the individual grid cells suitable for growth on a given day were
summed to derive the total habitat area available for growth on that day.
3. For each day evaluated, the total area available for growth was divided by
the total habitat area to derive the portion of habitat suitable for growth.
The summation of these values over the applied period of occurrence
yields the cumulative number of days for which growth is expected.
No change in growth of adult winter flounder was predicted (Table 2).
For juveniles, which are present in the Bay 194 days of the year, growth days
lost are 8.6, 6.3, and 1.1 under current Station operation, operation with
reduced heat load, and the no-plant scenario, respectively (Fig. 5).5 Compared
to the no-plant scenario, the effect of current operation is a decrease of
only 3.9%.
Table 2. Winter flounder assessment—summary of biothermal effects found.
Mount Hope Bay environment
Current Station
station operation with No-plant
Biothermal metric Life stage operation reduced heat load effect
Growth Juveniles 4.4% 3.2% 0.6%
(% growth days lost) (8.6 days lost) (6.3 days lost) (1.1 days lost)
Adults None predicted
Reproduction Eggs 5.2% 4.8% 2.5%
(% thermal egg mortality)
Avoidance Juveniles 3.0% 2.8% 1.9%
(% of habitat avoided) Adults None predicted
Potential for blockage at the Juveniles None predicted
entrances of the Mount Hope
Bay tributaries
Chronic mortality Juveniles 4.6% 3.6% 1.5%
(% mortality from Adults None predicted
a 72-hour exposure)
2006 R.J. O’Neill, T.L. Englert, and J.K. Ko 83
Temperature during critical reproductive seasons
The length of incubation is of key importance in assessing biothermal
effects on reproduction—the longer the incubation period, the greater the
likelihood that potentially lethal temperatures could be experienced. Typically,
winter flounder incubate for 18 days. Thus, if a single egg cohort (a
given day’s egg production) is to avoid thermal mortality, non-lethal temperatures
must occur for 18 consecutive days.
Egg catch data from the MRI 1973–1999 ichthyoplankton sampling
program indicate that winter flounder spawn in Mount Hope Bay between
January 25 and May 11. The catch data were used to approximate the number
of eggs in development on a given day and the percentage of annual Baywide
winter flounder egg production represented by that number. The daily
production value is critical input to the biothermal model because it provides
the basis for defining predicted temperature effects relative to the proportion
of the total egg production exposed. For example, when the predicted temperature
effect for a given day is 100% mortality, all the eggs present on that
day, regardless of their stages of development, are lost.
Using the predicted adult-habitat temperatures, the reproduction inhibition
temperature zone shown in Figure 2, and the methods discussed earlier,
the biothermal effect on reproduction was determined as follows:
1. The percentage of thermal egg mortality on a given day was determined
within each adult winter flounder habitat grid cell (Fig. 4).
2. This value was then multiplied by the percentage of the standing egg
density present on that day (i.e., the portion of the total egg production that
could potentially be affected) to yield a daily value.
3. Daily values were then summed to determine the percentage of annual egg
production subject to thermal egg mortality.
Figure 5. Total number of growth days for juvenile winter flounder.
84 Northeastern Naturalist Vol. 13, Special Issue 4
Figure 6 presents the plume’s predicted effect on winter flounder egg
mortality—5.2%, 4.8%, and 2.5% for current Station operation, reducedheat-
load-operation, and the no-plant scenario, respectively. As can be seen,
compared to the no-plant scenario, current operation increases thermal egg
mortality by only 2.7 percentage points (i.e., 5.2% - 2.5%).
Thermal avoidance and habitat loss
Using the mean avoidance line in Figure 2, with the lower and upper
bounds around the mean set at ± 5 °C and a normal distribution assumed, the
percent avoidance in each grid cell within the adult or juvenile winter
flounder habitat was determined for each day. The area-weighted percentage
of total habitat avoided was then determined, per Equation 1, by summing
together the products of predicted avoidance and planar area for all grid cells
in the habitat, and dividing the summed result by the total winter flounder
habitat area and then multiplying by 100.
Thermal avoidance caused by the Station’s plume was predicted to be
zero for adult winter flounder, largely because adults are not present in
Mount Hope Bay during the warmer months of the year, and when adults
are present, temperatures even in the warmest portions of the near-field
thermal plume are below winter flounder avoidance temperatures. It is
well documented that adult winter flounder leave shoreline waters when
the temperatures reach approximately 15 °C (McCracken 1963, USEPA
2002). This is not an avoidance response, since there is no laboratory
correlation between 15 °C and a short-term avoidance response (i.e., no
evidence of thermal discomfort). Instead, the adult winter flounder exodus
Figure 6. Winter flounder thermal egg mortality.
2006 R.J. O’Neill, T.L. Englert, and J.K. Ko 85
reflects an evolutionary adaptation. That is, a temperature of 15 °C acts as
a signal that Mount Hope Bay has begun its normal seasonal warming—a
signal to which adult winter flounder respond by moving into the cooler
ocean waters.6
As shown in Figure 7, for juvenile winter flounder, the predicted maximum
percentage of habitat avoided over the period of occurrence is 2.9%,
2.8%, and 1.9% under current Station operation, reduced-heat-load operation,
and the no-plant scenario, respectively.
Potential for blockage at the entrance of the Mount Hope Bay tributaries
Winter flounder juveniles move into Mount Hope Bay’s tributaries (Fig. 1)
during May and June, remain there until October, and then return to the Bay’s
more saline water to overwinter in November–December. The assessment of
blockage tested whether passage through the “door” at each tributary mouth is
prevented by temperatures that trigger an avoidance response. The blockage
evaluation focused on movement into and through the tributaries in the
northern portion of Mount Hope Bay. Such patterns of movement generally
require passage through Brayton Point Station’s thermal plume.
The potential for winter flounder blockage was quantified by comparing
the mean avoidance temperatures shown in Figure 2 with predicted hourly
temperatures at the mouth of each tributary. If the potential for blockage was
indicated that is, if the mean avoidance temperature was less than the
predicted plume temperatures then the cross-sectional area of the plume
isotherms that exceeded the avoidance temperature was determined. This
area was then compared to the total cross-sectional area available for migration
into a given tributary.
Figure 7. Juvenile winter flounder thermal avoidance.
86 Northeastern Naturalist Vol. 13, Special Issue 4
Because the analysis of migratory blockage accounted for the movement
of winter flounder through the Bay and into the tributaries, the acclimation
temperature was defined as the average temperature at Spar Island (located at
the Bay’s approximate midpoint) over the seven days prior to the day evaluated.
It was assumed that juveniles reside in the open portions of Mount Hope
Bay before they move into the tributaries. In assessing the potential for
blockage, this assumption resulted in the use of conservative acclimation
temperatures (i.e., colder temperatures than might apply if juveniles reside
closer to Brayton Point prior to moving into the tributaries). The acclimation
temperatures at Spar Island and the exposure temperatures in the tributary
mouths were determined using the hourly bottom-layer temperatures calculated
by the ASA hydrothermal model for those locations.
The results of this hourly analysis showed that no avoidance was predicted
over any portion of any tributary mouth cross-section. Thus, passage
for winter flounder was assured for all of the tributaries and operating
scenarios considered.
Potential for chronic thermal mortality due to a prolonged exposure
The assessment tested whether sedentary (i.e., burrowed) winter flounder
might be exposed to temperatures that would be lethal after prolonged (72-
hour) exposure. If the temperatures are below the lethal threshold, then no
effect is predicted.7
The assessment of winter flounder thermal mortality due to a prolonged
exposure followed the same analytical approach used for avoidance. That is,
the population response around the mean (i.e., TL5072 hr) was determined as
shown in Figure 4.
Several assumptions were made in assessing winter flounder chronic
thermal mortality:
Populations are static. The net movement of fish populations into and out
of each grid cell was assumed to be zero. This assumption is conservative,
because it ignores movement out of the cell to avoid elevated temperatures.
Throughout the 72-hour exposure, each grid cell’s maximum daily temperature
applies. This approach is conservative because the average daily
temperature over a 3-day period would always be less than the highest daily
value (assuming the daily values are not identical) and thus less likely to
cause mortality.
The results showed no chronic thermal mortality for adult winter flounder.
Figures 8 through 10 present the average predicted chronic mortality
for juveniles under the three scenarios examined. In general, while these
results show an isolated pocket of elevated mortality in the Lee River
(where the shallow water depths cause the plume temperatures to be higher
than at the point of Station discharge), overall area-weighted mortality is
small (e.g., the difference in area-weighted predicted mortality between
current Station operation and the no-plant scenario is a modest 3.1 percentage
points [4.6% - 1.5%]).
2006 R.J. O’Neill, T.L. Englert, and J.K. Ko 87
Figure 8. Juvenile winter flounder 72-hour chronic mortality under current Station
operation.
Figure 9. Juvenile winter flounder 72-hour chronic mortality under reduced-heat
Station operation.
88 Northeastern Naturalist Vol. 13, Special Issue 4
Figure 10. Juvenile winter flounder 72-hour chronic mortality under the no-plant
scenario.
Summary and Conclusions
Table 2 summarizes the winter flounder biothermal effects found. No
effects were found for adults for any of the biological functions examined.
For eggs and juveniles, effects were found for certain biological functions,
but they were less than 5% in all cases except for thermal egg mortality. For
that case, the effect was 5.2% under current operation, but the difference
between that operating scenario and the no-plant scenario was only 2.7%.
Thus, it is concluded, based on these model predictions, that the thermal
plume occurring under either the current or reduced-heat operating scenario
will not cause any appreciable harm to the Mount Hope Bay winter flounder
population. This conclusion is consistent with findings for the other nine
species evaluated in the biothermal assessment (USGenNE 2001).
Importance of a conservative modeling approach
Because the biothermal model accounts for direct thermal effects only, it
was deemed important to take a conservative approach—that is, to model
“worst-case” effects. This was done to ensure that the direct impact of the
Station’s thermal plume would not be underestimated. As a result, it is likely
that biothermal effects found are to a degree overestimates of the plume’s
direct impact on the species and life stages examined.
Key conservative aspects of the model include the following:
1. Effects for a very warm year were modeled. The year 1999 was chosen
because it had the second highest water temperatures in a 40-year period.
2006 R.J. O’Neill, T.L. Englert, and J.K. Ko 89
2. It was assumed that winter flounder populations are static within grid cells.
The assumption of net-zero movement in and out of grid cells in assessing
chronic thermal mortality ignores the ability of fish to leave areas of
elevated temperatures and thus avoid any thermal effects.
3. In the chronic mortality assessment, each grid cell’s highest daily temperature
was modeled rather than an average of the daily temperatures within
the 72-hour analytical period. Assuming that daily values are not equal, the
average daily temperature would always be less than the temperature
evaluated.
4. The chronic mortality assessment used the longest time frame (72 hours)
found for such assessments in the literature. Chronic thermal assessments
of 1, 3, 6, 24, and 48 hours were also found in the literature.
5. In assessing migratory blockage, the temperature near Spar Island (mid-
Bay) was used in calculating acclimation temperature. If juveniles reside
closer to Brayton Point prior to moving into the tributaries, calculated
acclimated temperatures would be warmer values, which in turn would
result in a lower avoidance response than calculated.
Viewing results within their environmental context
In examining the output of biothermal modeling, it is important to be
mindful of the environmental context, as other direct effects may be operating
within the sphere of study.
In the case of Mount Hope Bay, documented fishing pressure appears to
greatly exceed the modeled effects of the Station’s thermal plume on winter
flounder. For example, the 1990–1998 average fishing exploitation rate for
Narragansett Bay and adjacent coastal waters, deduced from the Report of
the 36th Northeast Regional Stock Assessment Workshop (36th SAW)’s
Stock Assessment Review Committee (SARC 2004), is 55 percent;8 Gibson
(2000) reports a similar rate for this period—53 percent. It should be noted,
however, that recent implementation of tightened commercial and recreational
fishing regulations for Narragansett Bay and Rhode Island coastal
waters has resulted in some success, as shown in SARC (2004), which puts
average exploitation rates for 1999, 2000, and 2001 at 40, 39, and 37
percent, respectively.
It should also be noted that the modeled effects from the Station’s
thermal discharge occur only during the first year of life for each cohort.
Fishing impacts, however, affect the same cohort during each year that is
susceptible to exploitation by the fishery (i.e., from age 2), with a potential
to compound effects over multiple years.
Addressing model limitations
The authors acknowledge two limitations inherent in biothermal assessment
tools: the ability to readily mimic or simulate field conditions with
models populated solely or primarily with laboratory data, and the ability to
address the impacts and interactions of non-thermal environmental influences
potentially affected by thermal inputs, such as competition, predation,
and changes in water quality, food availability, and physiological condition
90 Northeastern Naturalist Vol. 13, Special Issue 4
(Wismer and Christie 1987). This assessment measures only direct thermal
effects. Quantification of indirect thermal effects (e.g., increased predation
or diminished predator avoidance) are currently beyond the reach of
biothermal modeling, however, largely because insufficient data is available
to properly quantify the associated variables and assess their complex interactions
within a large natural ecosystem.
In the future, the ability to improve the precision of in situ effects and
behaviors will likely depend on technologies not yet available or currently in
early stages of development. One possible enhancement to make more field
data available to the modeling process could be the use of data storage tags
(DSTs). DSTs attached directly to the species of concern can record such
parameters as temperature, conductivity, and pressure. DSTs have “recently
emerged as a new area of science that could combine the rigor of controlled
conditions with the acquisition of detailed data measured in situ” (Ropert-
Coudert and Wilson 2004). However, more time is likely needed for further
advancement in miniaturization and sensor resolution in this new field of
data acquisition before it can be applied—at least to the juvenile winter
flounder population of Mount Hope Bay, which could suffer physical impairment
or behavior alterations due to the current size of the archival units.
More work is also needed to enhance the ability to predict the effect of
multiple stimuli acting alone or in concert on the biota of a water body. For
example, in addition to the thermal impact, egg mortality can also be caused
by such environmental factors as predation, changes in salinity and dissolved
oxygen levels, wave action, solar and UV radiation, and pollution.
Inherited genetic flaws or poor parental condition also can influence egg
mortality rates (Bunn et al. 2000).
Nonetheless, the Mount Hope Bay biothermal assessment demonstrates
that information currently available can be used to develop a sophisticated
model of the direct impacts of thermal discharges on species resident in a
water body. When conservative assumptions are incorporated throughout the
analysis, they help to ensure that required approximations do not cause the
analysis to underestimate the potential for thermal effects.
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Notes
1Under current operation of the plant, cooling water for the Station’s four generating
units is withdrawn from and returned to Mount Hope Bay in a once-through cooling
configuration during the warmer months; from October through May the Station
operates in a “piggyback” configuration, in which cooling water from Units 1, 2, and
3 is recirculated to cool Unit 4. In this second set alternative cooling-water configuration,
some of the heated water would be circulated to a mechanical-draft cooling
tower, where it would be cooled with ambient air and then recirculated to condenser
cooling-water inlets. This hypothetical configuration would reduce Station heat load
(and Station flow) by 33% compared to current Station operation.
2The basic reason acclimation occurs is that fish lack the physiological mechanisms
to control tissue temperature, and thus their peripheral body temperature is essentially
the same as the surrounding water. Therefore, as water temperature and thus
fish body temperature change, corresponding changes occur in thermal preference,
avoidance, and mortality thresholds.
3The NAS/NEA 1973 critical growth limit equation was used for 9 of the 10 species
evaluated. For winter flounder, however, an acclimation-preference data set was not
found in the scientific literature. Thus, for this species, the upper limit of the growth
zone was approximated by the field observations in Radle 1971. For example, field
collection temperatures associated with the greater-than-half-full stomach observations
for juvenile winter flounder were conservatively designated as the exposure
“tipping point” at which elevated temperatures yield zero net growth.
4The decision to assess impacts in single cells, rather than in groups of cells, had a
large impact on model run times and post-processing requirements. Nonetheless, it
was pursued so that the precise location in the Bay of any predicted biothermal
effect could be pinpointed.
94 Northeastern Naturalist Vol. 13, Special Issue 4
5For growth and the other biological functions examined, an “effect” is seen for the
no-plant scenario because of natural seasonal variations in Mount Hope Bay ambient
water temperatures in 1999. The no-plant effect demonstrates the model’s
sensitivity to temperature variations.
6As stated in Neill (1979), “predictive thermoregulation comprises directed movements
to a subset of habitat within which the fish “expects,” on the basis of
individual or evolutionary experience, to find acceptable temperatures.” Predictive
thermoregulation, a seasonal evolutionary adaptation, was outside the scope of the
biothermal model.
7The potential for burrowing is supported by winter flounder physiological tolerance
data, which reflect several instances of “no response” when the fish were subjected
to elevated temperatures (Ichthyological Associates, Inc. 1978).
8SARC reports instantaneous fishing mortality rates for the Southern New England–
Mid-Atlantic winter flounder stock complex, which includes Narragansett Bay. The
SARC rates were converted to exploitation rates using the Type II fishery equation
from Ricker (1975) and an instantaneous mortality rate of 0.2.