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The thermal variability of the waters of Fathom Five National Marine Park, Lake Huron
Journal of Great Lakes Research 36 (2010) 570–576
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Journal of Great Lakes Research j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / j g l r
Notes
The thermal variability of the waters of Fathom Five National Marine Park, Lake Huron Mathew Wells a,⁎, Scott Parker b,1 a b
Department of Physical and Environmental Science, University of Toronto at Scarborough, 1265 Military Trail, Toronto, Ontario, Canada Fathom Five National Marine Park, 248 Big Tub Rd., Tobermory, Ontario, Canada
a r t i c l e
i n f o
Article history: Received 21 September 2009 Accepted 22 February 2010 Communicated by David Schwab Index words: Internal wave Thermal variability Fathom Five National Marine Park Cold shock
a b s t r a c t Measurements of the thermal stratification at 3 locations within Fathom Five National Marine Park in Lake Huron, Ontario during the summers of 2006 and 2007 found large oscillations in the position of the thermocline. These oscillations led to considerable variability in the temperature at a given depth, with frequent changes in temperature at a rate of 5 °C per hour, and brief periods where temperatures changed at the rate of 10 °C per hour. The thermal stress due to such fast rates of temperature change has been previously implicated in negative effects on many aquatic organisms. The thermocline was observed to move by as much as 20 m vertically, and had dominant periods of oscillation of 12, 17 and 24 h. The strongest temperature variability occurs in the depth range of 10–20 m, which accounts for 20% of the total lakebed area within Fathom Five. The temperature variability was lowest in deep regions well below the thermocline and at a sheltered area behind a reef. This variability was a ubiquitous feature of the water column of Fathom Five during the summer stratification, and the impact of these frequent short-term thermal fluctuations on benthic and fish habitat is discussed in this note. © 2010 Elsevier B.V. All rights reserved.
Introduction The surface water temperatures of the Great Lakes are subject to variability on a variety of timescales, from decades to hours. In the long-term, climate-change is leading to increases in the surface water temperature on the order of 0.1 °C per year (Austin and Colman, 2007). In the course of a year, the surface water temperatures change by 20 °C (Beletsky et al., 1999). Surface water temperature can change by 1–2 °C over 12 h during a typical diurnal cycle, (Loewen et al., 2007), while water temperatures can drop by up to 10 °C in the space of a few hours during upwelling events (Plattner et al., 2006). The variability of water temperature has often been cited as an important factor in shaping the benthic communities in near shore regions of the Great Lakes (Kilgour et al., 2000; Finlay et al., 2001; Fitzsimons et al., 2002). Most aquatic organisms are ectotherms, so their physiological and behavioural responses are strongly affected by water temperature (Fry, 1947; Smith, 1970). Rapid temperature decreases can cause lethal and sublethal effects in many aquatic organisms (Donaldson et al., 2008), so that sites that experience rapid temperature changes could have lower productivity or growth rates. The thermal variability in the near shore regions of the Great Lakes is likely to be large during the summer stratified period, however this variability is not usually ⁎ Corresponding author. E-mail addresses: [email protected] (M. Wells), [email protected] (S. Parker). 1 Tel.: + 1 519 596 2444x314. 0380-1330/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jglr.2010.04.009
reported as part of biological studies on near shore ecosystem. Here, we report observations of rapid thermal variability from Fathom Five National Marine Park (located in Lake Huron, Fig. 1) and discuss the implications of this variability for aquatic organisms. A major source of temperature variability in the Great Lakes occurs during summer, when large amplitude internal waves lead to oscillations in the depth of the thermocline with frequencies between 12 and 24 h. These large amplitude internal waves have been documented in all the Great Lakes, including Lake Michigan (Hawley and Muzzi, 2003; Hawley, 2004), Lake Huron (Murthy and Dunbar, 1981) and Lake Ontario (Rao and Murthy, 2001). The different timescales over which the thermocline moves depends upon the physical forcing mechanisms. Movements of the thermocline over short periods (b24 h) could be due to (1) Poincare waves (Mortimer, 2006), (2) forcing by diurnal winds or (3) resonance of the thermocline with weak tides. In the first case, a 17 hour period would result. Poincare waves are intrinsically linked to the Coriolis effect whereby the Earth's rotation influences the movement of water in large bodies. The oscillatory motions are completed in an inertial period of 12/(sin θ) hours at latitude θ, which at 45°N is then close to 17 h (Mortimer, 2006). The amplitude of Poincare waves is fairly large, and is of the order of 5 m (Mortimer, 2006). The second case involves a diurnal oscillation (a 24 hour period) in the position of the thermocline resulting from strong diurnal wind patterns. This has been observed in many large lakes such as Lake Kinnerat in Israel (Antenucci et al., 2000), Lake Tahoe in California (Rueda et al., 2003) and Lake Biwa in Japan, (Saggio and Imberger, 1998). Lastly, a semidiurnal period (near 12 h) has been observed
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Fig. 1. a) Location of Fathom Five National Marine Park (FFNMP) on northern tip of the Bruce Peninsula. In Lake Huron b) a map of the bathymetry of the Fathom Five National Marine Park, showing the park boundaries. In 2006 moorings were located at the NW, SW and SE sites in late summer from 19 July and the 19 October 2006. In 2007 moorings were deployed at the Cove Island location (CI) and the SW mooring location, between 16 April and 10 October.
during periods of summer stratification at sites within the Great Lakes, in particular in narrow channels connecting two basins. Such changes are thought to be in response to the 12.4 hour M2 lunar tidal period. For example, in the mouth of Green Bay, Michigan observations by Miller and Saylor (1985) found currents with a peak period near 12 h, Similar observations have also been made in the Straits of Mackinac that connect Lake Huron to Lake Michigan (Sloss and Saylor, 1976). The basic features of the thermal regime of Fathom Five are similar to those reported by Bennett (1988) for the waters of Georgian Bay. The surface water temperature drops to 0 °C in late February for about a month, during which time most of the surface of Fathom Five is frozen. The spring convection period starts in mid-April, and mixing to the bottom continues until temperatures start to rise above 4 °C in mid-May. The water column starts to be strongly stratified by the end of June, with a surface mixed layer to approximately 10 m depth. The maximum surface temperature occurs in early August with surface temperatures in the range 19–22 °C. Below the sharp thermocline at 20 m depth the water remains less than 7 °C, dropping to 4 °C in the deepest areas. After October, the surface water rapidly cools and the whole water column becomes well mixed with a temperature close to 4 °C by December. What is missing from the preceding discussion of the thermal regime of Fathom Five is an understanding of the temperature variability within the water column. The data presented in Bennett (1988) gives the impression of smooth changes in temperature within the water column, even though he also reports some rapid thermal fluctuations. At Fathom Five the typical summer water temperatures in the thermocline change by 10 °C over 20 m, so the ubiquitous internal waves described earlier could result in persistent temperature variability, in particular at the depth where the thermocline intersects with the lakebed. The temperature variability associated with internal waves and coldwater upwelling has a recognized ecological effect (e.g., Spigarelli et al., 1982; Levy et al., 1991; Finlay et al., 2001; Evans et al., 2008; Donaldson et al., 2008). As a rather extreme example, Emery (1970) showed that a coldwater upwelling in Fathom Five caused the mortality of resident demersal fishes and crayfish. Here, we aim to determine the typical rates of temperature variability in the water column at Fathom Five in the summers of 2006 and 2007 summers, with the aim of determining what typical rates of temperature fluctuation are. In particular we would like to ascertain the frequency of extreme temperature changes (≤10 °C/h), which would change the population distribution of benthic organisms. Since the physiology of fish and other aquatic organisms is inextricably linked to water temperature (Finlay et al., 2001; Donaldson et al., 2008), the distribution of organisms at various depths within Fathom Five will likely be influenced by the degree of temperature variability experienced within the park.
Methods and materials Field site description Fathom Five National Marine Park is located near Tobermory at the northern tip of Ontario's 100 km long Bruce Peninsula at 45° 19′ 17″ N, 81° 37′ 34″ W (Fig. 1a) and lies between Georgian Bay (area 15,000 km2) and the main basin of Lake Huron (area 44,000 km2, Bennett, 1988). Fathom Five has an area of 114 km2 and is bisected by the Niagara Escarpment as it submerges off the northern tip of the Bruce Peninsula, emerging periodically to form a series of small islands. The western half of Fathom Five is located atop the escarpment with water depths less than 40 m. The eastern half is located below the escarpment, with water depths greater than 90 m. Fathom Five was established in 1987, as Canada's first National Marine Conservation Area (McBurney, 2001), and has both a cold offshore and a warm inshore component. Fish species typical of the offshore component include lake trout (Salvelinus namaycush), lake whitefish (Coregonus clupeaformis), burbot (Lota lota), and bloater chub (C. hoyi). Fish species typical of the inshore waters include bluntnose minnow (Pimephales notatus), common shiner (Luxilus cornutus), rock bass (Ambloplites rupestris), and yellow perch (Perca flavescens) (Leslie and Timmins, 2001). The macroinvertebrate community of the region is dominated by zebra mussels (Dreissena polymorpha) and quagga mussels (D. bugensis), but also includes chironomids, isopods, gammarid amphipods, and gastropods. Water quality and lower trophic assessments have characterized Fathom Five as being a healthy oligotrophic ecosystem (McCrea et al., 2001; Munawar et al., 2001). The mean water circulation within Fathom Five is dominated by strong exchange flows between the main basin of Lake Huron and Georgian Bay. Measurements made during the International Field Year for the Great Lakes (1974) showed that during summer circulation there is a mean surface flow into Georgian Bay and a deeper (N25 m) return flow into Lake Huron in the Fathom Five area (Schertzer et al., 1979; Bennett, 1988). The average speed of the surface current is on the order 0.05 m/s (Sheng and Rao, 2006), which is among the highest mean speeds observed for summer currents in Lake Huron (Beletsky et al., 1999). This exchange flow is driven by a persistent horizontal temperature difference in the waters of Georgian Bay and Lake Huron. This temperature gradient is set up by the prevailing westerly winds, which lead to the accumulation of warm water on the western side of the Bruce Peninsula (the eastern sides of Lake Huron) and the upwelling of relatively colder waters on the eastern side of the Bruce Peninsula (the western side of Georgian Bay), including the establishment of a semi-permanent upwelling in the coastal boundary of the southern tip of Manitoulin Island (Bennett, 1988). Data from the NOAA coast watch website (http://
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coastwatch.glerl.noaa.gov/) show that the surface temperature differences across the Bruce Peninsula were often of order of 5 °C during the summer, due to the cold water upwelling on the eastern side of the Peninsula. The mean monthly air temperatures at Tobermory closely follow the lake temperatures. For 2006 and 2007 the meteorological records of Environment Canada show that the mean summer air temperature in July and August were 20 °C, and mean winter air temperatures in January and February are close to −5 °C. The wind speeds were highest in winter with means of 4.4 m/ s, and there were lower mean wind speeds of 2.7 m/s during the summer months. Thermistor deployment During periods of summer stratification in 2006 and 2007, three strings of thermistors were deployed at locations within Fathom Five (Fig. 1b). The 2006 deployment was from July 19 to October 19, and the 2007 deployment was from April 16 to October 2. In both years, three moorings were deployed, but in each year a problem occurred with one of the moorings. On each mooring, six or seven thermistors were attached along a rope kept afloat by a surface buoy. The thermistors were arranged so that there was a good resolution where the thermocline was located, at approximately a depth of 15 m. The thermistors used were HOBO Water Temp Pro v2 units, and have an accuracy of ±0.2 °C and recorded temperature every 12 min. In 2006 the moorings were located at three sites of interest near the underwater escarpment that runs through Fathom Five. The mooring at the NW location was to the west of the escarpment and north of Flowerpot Island in 35 m of water, the second mooring at the SW location was located near the top of the escarpment near Middle Island in 40 m of water and the third mooring at the SE location was located east of the escarpment in water 85 m deep. Problems developed with the SW mooring after 3 weeks, so data is not shown from this location. The SE mooring had temperature data-loggers at depths of 5, 10, 15, 20, 40, 60 and 80 m. The NW mooring had temperature data-loggers at depths of 1, 6, 11, 16, 21, 26, 30 and 31 m. In 2007 the three moorings were located at three sites, one at the 80 m deep SE location, one at the 35 m deep NW location and one at a location between a submerged reef and Cove Island (CI) in 35 m of water (Fig. 1b). Problems developed with the deep SE mooring and no data were recovered for 2007. The NW location had 6 loggers at depths of 2, 6, 11, 16, 21, 29 m depths, and the Cove Island mooring had 7 loggers at depths 1, 6, 11, 16, 21, 26 and 30 m depths. Results The 2006 temperature time-series for the NW and SE moorings are plotted in Figs. 2 (a, b) and show strong variability in the position of the thermocline. The surface waters warm to a maximum of 23 °C by early August and then gradually cool to 10 °C by early October. For most of the summer, the mean depth of surface mixed layer was around 10 m. In mid October however, the mixed layer deepens dramatically as the water rapidly cools. Below the depth of 20 m, the water temperatures remain low during the summer stratification, reaching minimum temperatures near 5 °C below the depth of 60 m. During the period from July 19 to October 23, the water column was stratified with surface temperature between 15 and 20 °C. During this period, the water temperature within the epilimnion frequently changed temperature as cold waters rose to the surface, or the warm surface waters were depressed to 20 m depth. These events may have been driven by strong local winds, or by the passage of large amplitude internal waves, such as Kelvin waves, which are caused by distant wind events (Rao and Schwab, 2007). The timing of these upwelling and downwelling events were well correlated between the NW and SE mooring, with a correlation factor of R2 = 0.6 for the temperatures near the main thermocline. Similar dynamics were seen
in the longer 2007 temperature time-series plotted in Figs. 2 (c, d), for the NW and Cove Island moorings. The water column was initially well mixed on May 20 with a temperature just above 1 °C. The thermal stratification started to develop around May 30 as the water column warms past 4 °C. The surface waters reached a peak of 22 °C on August 3 after which the water slowly cooled to 16 °C on October 2. At depths below 20–30 m, the water temperature ranged between 6 and 8 °C. The mean depth of the surface mixed layer was around 10 m as in the previous year. The most interesting feature of the temperature time-series in Fig. 2 is the intense variability in the position of the thermocline, which was normally between 10 and 25 m depth, but could move vertically by as much as 20 m in a 16 hour period. The mean temperature profiles and the range of observed temperatures for each data logger are plotted in Figs. 3 and 4, for the month of August 2006 and August 2007 respectively. There were considerable departures from the mean temperature between depths of 5 and 20 m, while there was very little variability in temperature below depths of 30 m. This temperature variability is indicative of the strong movements of the thermocline associated with large amplitude internal seiches and internal waves, which occurred almost every day. This variability is quantified in Figs. 3 (b–d), where the relative frequency of a given temperature occurring at each data logger is plotted during the period of peak stratification in August. The Fig. 3 shows a tight distribution in temperature variability for the deepest sites. For depths between 10 and 20 m, the distributions are much broader, reflecting the moving thermocline. For example, at the NW mooring the temperature recorded at the 11 m depth shows the percentage of the time that the temperature was at 20 °C is the same as when it was at 6 °C. Similar observations can be seen in the temperature profiles from the 2007 moorings shown in Fig. 4a. There is a difference in the thermal variability at these two sites, with the open water site (Fig. 4c) having higher temperature variability in the deeper waters, than the sheltered Cove Island site (Fig. 4b). The greatest variability in temperature was between depths of 10 and 20 m, due to frequent fluctuations in the thermocline position (Figs. 3 and 4). Based upon the bathymetry shown in Fig. 1b, 60% of lakebed of Fathom Five lies at depths greater than 20 m, another 20% of the lakebed lies between the depth contours of 10 and 20 m, and the remaining 20% is at depths shallower than 10 m. The lakebed at depths between 10 and 20 m is where the strong fluctuations in depth of the thermocline occurred; hence during summer the temperature at the open water sites is expected to fluctuate between 6 °C and 20 °C almost every day. Figs. 5a and b illustrate the rate of temperature change at various depths in August 2007. The median rate of change of temperature was relatively slow at about 0.2 °C per hour. However there were infrequent events where the rate of change of temperature was greater than 5 °C per hour and as much as 10 °C per hour. Temperature changes greater than 5 °C per hour occurred about 2% of the time in the open water site, and less than 1% of the time in the sheltered site. Changes of 2 °C per hour occurred about 10% of the time at both sites. The reduced temperature variability in Cove Island can be attributed to the shallow reef that surrounding the mooring location. As the reef is shallower than the depth of thermocline most of the vertical movements in the thermocline seen at the NW site were not felt in these more protected waters and so the temperature variability was less. The oscillations in the thermocline shown in Fig. 2 have dominant periods between 12 and 24 h. Fig. 5c shows a Fourier transform of the 2006 NW mooring data. There is a peak at a frequency of 1/17.1 h (0.058 cycles per hour), corresponding to the frequency of an inertial oscillation at the 44°N (Mortimer, 2006) as well significant energy at frequencies near diurnal (1/24 h or 0.04 cycles per hour) and semidiurnal (1/12 h or 0.08 cycles per hour) periods. There is also an indication of a timescale greater than 4 days (0.01 cycles per hour),
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Fig. 2. The evolution of the temperature profile against time is shown here for the a) NW and b) SE moorings in 2006, and for 2007 at the c) NW and d) CI location.
representing variability that could be associated with the passage of meteorological systems or the slow passage of Kelvin waves. Similar spectral peaks are seen at the other mooring sites in 2006 and 2007 during the period of strong summer stratification.
Discussion and conclusions Our field work shows that temperature variability is a persistent and ubiquitous feature of the water column at Fathom Five, for depths
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Fig. 3. These plots show the considerable variability experienced at the 3 mooring sites during August 2006. In (a) the mean profile is by the lines, with the range of all data plotted as points. The frequency histograms during the first 3 weeks of August are plotted in b) for the SW mooring, in c) for the NW mooring and in d) for the SE mooring.
between 10 and 20 m. Rapid rates of temperature change, as high as 10 °C per hour were found around 0.1% of the time, with slower temperature changes greater than 2 °C per hour found about 10% of the time. The dominant periods of the internal wave motions were between 12 and 24 h, with mean wave amplitude of order 5 m, although displacements of the thermocline by as much as 20 m were not uncommon. The variability of temperature was least below 50 m depth, and also at a site that was poorly connected to the main waters of Lake Huron due to a surrounding shallow reef. Due to the many reports in the literature of the chronic effects of rapid temperature variability, the spatial and temporal variation in temperature variability within the park may be an abiotic driver of the benthic ecosystem in Fathom Five. Observations of large and persistent movements of the thermocline at periods between 12 and 24 h driven by Poincare waves, internal tides, and diurnal wind forcing have been reported in many large lakes worldwide, such as Loch Ness in Scotland (Mortimer, 1952), Lake Geneva in Switzerland (Lemmin et al., 2005), Lake Biwa in Japan (Saggio and Imberger, 1998) and Lake Kinnerat in Israel (Antenucci et al., 2000). There are also many sites in the Great Lakes where comparable observations of the movement of the thermocline have been reported (Murthy and Dunbar, 1981; Rao and Murthy, 2001; Hawley and Muzzi, 2003; Hawley, 2004; Mortimer, 2006). It is important to know how rapidly temperature changes can occur within the water column, as most animals are most sensitive to rapid
changes in temperature rather than slow changes (Donaldson et al., 2008). With water temperature changing at a rate as much as 5 °C per hour and 10 °C in a 12–24 hour period, particularly in the 10 to 20 m water depths, the offshore aquatic ecosystem of Fathom Five is quite dynamic. These rates of change of water temperature have been previously demonstrated to lead to severe ‘cold shock’ of many fish (Donaldson et al., 2008). The observations of Emery (1970) also suggested that temperature changes of order 10 °C per hour could be lethal to some benthic fauna in Fathom Five, so these infrequent events may have a strong role in controlling the benthic community. The report by Emery (1970) stands as the only direct observation or study of the ecological effects of internal waves in Fathom Five. However, the literature provides some insights and opportunity for discussing potential chronic and acute effects of temperature variability. Below, we discuss the potential biological response of aquatic organisms to these ubiquitous thermal fluctuations. The offshore region of Fathom Five is generally subdivided into upper (b50 m) and deeper (N50 m) water fish communities. The upper group is most likely to interact with the thermocline and includes cisco (Coregonus artedii), lake whitefish, lake trout, Chinook salmon (Oncorhynchus kisutch), longnose sucker (Catostomus catostomus), yellow perch (P. flavescens), walleye (Stizostedion vitreum), emerald shiner (Notropis atherinoides), and rainbow smelt (Osmerus mordax) (Spangler and Collins, 1992; Riley et al., 2008). The deeper group also includes lake
Fig. 4. (a) These images show the considerable variability experienced at the 2 mooring sites during August 2007. The histograms show that at deeper depth in the sheltered site CI behind Cove Island (b) receives less temperature variability that the open water site at the NW mooring shown in (c). Near the water surface the variability is the same at both sites.
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Fig. 5. A histogram of the rate of temperature change, during the period of peak stratification in August 2007, is shown in a) for the open water site at the NW mooring and in b) for the sheltered location behind Cove Island (CI). c) A Fourier Transform of the 2007 data from the NW mooring showing that there is broad peak in spectral energy around the periods of 24 and 12 h, corresponding the diurnal and semidiurnal periods (0.04 and 0.08 cycles per hour). The spectral peak of 17.1 h (0.06 cycles per hour) may be associated with the passage of Poincare wave oscillating near the inertial period.
whitefish and lake trout as well as burbot, deepwater sculpin (Myoxocephalus thompsonii) and historically included of 6 species of deepwater cisco, of which only the bloater chub and shortjaw cisco (C. zenithicus) remain extant (Todd, 2003; Bence and Mohr, 2008). In general, water temperature heterogeneity and mobility provides these fishes with an opportunity to seek preferential thermal habitat, thus these species can easily swim up or down to their preferred temperature regime. For example, coldwater species such as lake trout prefer ∼11 °C and lake whitefish prefer ∼12 °C, whereas coolwater species such as yellow perch prefer ∼ 23 °C and walleye prefer ∼22 °C (Casselman, 2002; Wismer and Christie, 1987). Benthic species, such as mottled sculpin (Cottus bairdii), may be habitat restricted and more exposed to the zone of temperature change. As presented in the review by Donaldson et al. (2008) and others (Brandt, 1980, Spigarelli et al., 1982, Christie and Regier, 1988; Levy et al., 1991), there is a cascade of stress responses associated with temperature shock (e.g., neuroendocrine response, corticosteroid-catecholamine release, osmoregulatory change, immunological change, habitat change, loss of disease resistance and developmental changes). Magnuson et al. (1979) noted that width of the fundamental thermal niche of many fish species is narrow (±2 °C of the mean temperature), so that even the frequent small temperature changes shown in Fig. 5 have potential to significantly impact fish consumption or growth. This optimal temperature range was frequently exceeded by the frequent changes of temperature seen in Fathom Five. Thus while the mean temperature may be favourable to some benthic species, the temperature variability may be a source of stress. The benthic community is more susceptible to the effects of temperature fluctuations because many of the species are less mobile. From studies in Lake Ontario, Wilson et al. (2006) noted that the total mass of quagga mussels was lowest at the sites with high upwelling frequency and Kilgour et al. (2000) found that the frequency of upwelling was an important indicator of the benthic community. Similarly Fitzsimons et al. (2002) attributed the absence of crayfish at sampling sites due to the thermal stress associated with frequent upwelling of cold water. Nalepa et al. (2005) reported greater abundance of the benthic amphipod Diporeia spp. on the west side of Lake Michigan where upwelling brings colder more enriched waters. McCabe and Cyr (2006) found variability in the water temperature promoted increased diversity of algal species in Lake Opeongo, Ontario. The information on the depths affected by variable water temperatures can be used to help characterize and understand aquatic ecosystems and habitats in Fathom Five. For instance, the vulnerability to cold shock from upwellings may explain the reduced fish species richness in Dunks
Bay as compared to other local protected bays (Leslie and Timmins, 2001). The information may also be useful in identifying critical habitat for species at risk, such as the shortjaw cisco (Todd, 2003; Naumann and Crawford, 2009), where deep waters and stable cold temperatures may be important habitat requirements. This work has demonstrated that there is an on-going abiotic process, a potential ecosystem driver, in Fathom Five. An improved understanding of the process and how the temperature variability influences species behaviour, physiological responses, and associated interactions is required. The high rates of temperature variability that were found at Fathom Five are likely to present at many other coastal sites around the Great Lakes. However the vertical amplitude and frequency will depend upon the bathymetry at the region of interest; for instance local bathymetry could lead to resonance and amplification of the internal waves, or it could dampen the amplitude as was seen in the sheltered location behind the reef in our observations. Typically the temperature variability due to internal waves and seiches is overlooked in many field experiments, as frequently only one or two temperature profiles would be made to characterize the temperature of the water column. Given the ubiquity of internal waves in stratified lakes, it would be useful to measure the climatology of the internal wave dynamics around the Great Lakes for future research on the diversity and thermal range of aquatic organisms in the near shore region. Acknowledgments This work would not have been possible without the extensive help of Jeff Truscott of Parks Canada, Paola Travaglini of the Canadian Hydrographic Service, and Lisa Tutty and Alex Ragnosig from the University of Toronto. Conversations with Helene Cyr and Brian Shuter are gratefully acknowledged. Funding for MGW was provided by an NSERC discovery grant. Comments by two anonymous reviewers and the editors were very helpful in focusing the paper. References Antenucci, J.P., Imberger, J., Saggio, A., 2000. Seasonal evolution of the basin-scale internal wave field in a large stratified lake. Limnol. Oceanogr. 45, 1621–1638. Austin, J.A., Colman, S.M., 2007. Lake Superior summer water temperatures are increasing more rapidly than regional air temperatures: a positive ice-albedo feedback. Geophys. Res. Lett. 34, L06604. doi:10.1029/2006GL029021. Beletsky, D., Saylor, J.H., Schwab, D.J., 1999. Mean circulation in the Great Lakes. J. Great Lakes Res. 25, 78–93. Bence, J.R., Mohr, L.C., 2008. The state of Lake Huron in 2004. Great Lakes Fish. Comm. Spec. Pub. 08-01. Bennett, E.B., 1988. On the physical limnology of Georgian Bay. Hydrobiologia 163, 21–34.
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Mortimer, C.H., 2006. Inertial oscillations and related internal beat pulsations and surges in Lakes Michigan and Ontario. Limnol. Oceanogr. 51, 1941–1955. Munawar, M., Munawar, I.F., Dermott, R., Munawar, S.F., Norwood, W., Wenghofer, C., Lynn, D., Johannssen, O.E., Niblock, H., Carou, S., Fitzpatrick, M., Gasenbeek, K., Weisse, T., 2001. Aquatic ecosystem health of Fathom Five National Marine Park: a structural and functional assessment. In: Parker, S., Munawar, M. (Eds.), Ecology, Culture and Conservation of a Protected Area: Fathom Five National Marine Park, Canada. Ecovision World Monograph Series. Backhuys Publishers, Leiden, pp. 99–149. Murthy, C.R., Dunbar, D.S., 1981. Structure of the flow within the coastal boundary layer of the Great Lakes. J. Phys. Oceanogr. 11, 1567–1577. Nalepa, T.F., Fanslow, D.L., Foley III, A.J., 2005. Spatial patterns in population trends of the amphipod Diporeia spp. and Dreissena mussels in Lake Michigan. Verh. Internal. Verein. Limnol. 29, 426–431. Naumann, B.T., Crawford, S.S., 2009. Is it possible to identify habitat for a rare species? Shortjaw Cisco (Coregonus zenithicus) in Lake Huron as a case study. Environ. Biol. Fish. 86, 341–348. Plattner, S., Mason, D.M., Leshkevich, G.A., Schwab, D.J., Rutherford, E.S., 2006. Classifying and forecasting coastal upwellings in lake Michigan using satellite derived temperature images and buoy data. J. Great Lakes Res. 32, 63–76. Rao, Y.R., Murthy, C.R., 2001. Coastal boundary layer characteristics during summer stratification in Lake Ontario. J. Phys. Ocean. 31, 1088–1104. Rao, Y.R., Schwab, D.J., 2007. Transport and mixing between the coastal and offshore waters in the Great Lakes: a review. J. Great Lakes Res. 33, 202–218. Riley, S.C., Roseman, E.F., Nichols, S.J., O'Brien, T.P., Kiley, C.S., Schaeffer, J.S., 2008. Deepwater demersal fish community collapse in Lake Huron. Trans. Am. Fish. Soc. 137, 1879–1890. Rueda, F.J., Schladow, S.G., Palmarsson, S.O., 2003. Basin-scale internal wave dynamics during a winter cooling period in a large lake. J. Geophys. Res. 108, 3097. doi:10.1029/ 2001JC000942. Saggio, A., Imberger, J., 1998. Internal wave weather in a stratified lake. J. Limnol. Oceanogr. 43, 1780–1795. Schertzer, W.M., Bennett, E.B., Chiocchio, F., 1979. Water balance estimate for Georgian Bay in 1974. Water Resour. Res. 15, 77–84. Sheng, J.Y., Rao, Y.R., 2006. Circulation and thermal structure in Lake Huron and Georgian Bay: application of a nested-grid hydrodynamic model. Cont. Shelf Res. 26, 1496–1518. Sloss, P.W., Saylor, J.R., 1976. Large-scale current measurements in Lake Huron. J. Geophys. Res. 81, 3069–3078. Smith, W.E., 1970. Tolerance of Mysis relicta to thermal shock and light. Trans Am. Fish Soc. 99, 418–422. Spangler, G.R., Collins, J.J., 1992. Lake Huron fish community structure based on gill-net catches corrected for selectivity and encounter probability. N. Am. J. Fish Manag. 12, 585–597. Spigarelli, S.A., Thommes, M.M., Prepejchal, W., 1982. Feeding, growth, and fat deposition by brown trout in constant and fluctuating temperatures. Trans. Am. Fish Soc. 111, 199–209. Todd, T.N., 2003. Update COSEWIC status report on the shortjaw cisco Coregonus zenithicus in Canada. COSEWIC Assessment and Update Status Report on the Shortjaw Cisco Coregonus zenithicus in Canada. Committee on the Status of Endangered Wildlife in Canada, Ottawa. 19 pp. Wilson, K.A., Howell, E.T., Jackson, D.A., 2006. Replacement of Zebra mussels by Quagga mussels in the Canadian near shore of Lake Ontario: the importance of substrate, round goby abundance, and upwelling frequency. J. Great Lakes Res. 32, 11–28. Wismer, D.A., Christie, A.E., 1987. Temperature relationships of Great Lakes fishes: a data compilation. Great Lakes Fish. Comm: Spec. Pub., vol. 87-3. 165 pp.
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Journal of Great Lakes Research j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / j g l r
Notes
The thermal variability of the waters of Fathom Five National Marine Park, Lake Huron Mathew Wells a,⁎, Scott Parker b,1 a b
Department of Physical and Environmental Science, University of Toronto at Scarborough, 1265 Military Trail, Toronto, Ontario, Canada Fathom Five National Marine Park, 248 Big Tub Rd., Tobermory, Ontario, Canada
a r t i c l e
i n f o
Article history: Received 21 September 2009 Accepted 22 February 2010 Communicated by David Schwab Index words: Internal wave Thermal variability Fathom Five National Marine Park Cold shock
a b s t r a c t Measurements of the thermal stratification at 3 locations within Fathom Five National Marine Park in Lake Huron, Ontario during the summers of 2006 and 2007 found large oscillations in the position of the thermocline. These oscillations led to considerable variability in the temperature at a given depth, with frequent changes in temperature at a rate of 5 °C per hour, and brief periods where temperatures changed at the rate of 10 °C per hour. The thermal stress due to such fast rates of temperature change has been previously implicated in negative effects on many aquatic organisms. The thermocline was observed to move by as much as 20 m vertically, and had dominant periods of oscillation of 12, 17 and 24 h. The strongest temperature variability occurs in the depth range of 10–20 m, which accounts for 20% of the total lakebed area within Fathom Five. The temperature variability was lowest in deep regions well below the thermocline and at a sheltered area behind a reef. This variability was a ubiquitous feature of the water column of Fathom Five during the summer stratification, and the impact of these frequent short-term thermal fluctuations on benthic and fish habitat is discussed in this note. © 2010 Elsevier B.V. All rights reserved.
Introduction The surface water temperatures of the Great Lakes are subject to variability on a variety of timescales, from decades to hours. In the long-term, climate-change is leading to increases in the surface water temperature on the order of 0.1 °C per year (Austin and Colman, 2007). In the course of a year, the surface water temperatures change by 20 °C (Beletsky et al., 1999). Surface water temperature can change by 1–2 °C over 12 h during a typical diurnal cycle, (Loewen et al., 2007), while water temperatures can drop by up to 10 °C in the space of a few hours during upwelling events (Plattner et al., 2006). The variability of water temperature has often been cited as an important factor in shaping the benthic communities in near shore regions of the Great Lakes (Kilgour et al., 2000; Finlay et al., 2001; Fitzsimons et al., 2002). Most aquatic organisms are ectotherms, so their physiological and behavioural responses are strongly affected by water temperature (Fry, 1947; Smith, 1970). Rapid temperature decreases can cause lethal and sublethal effects in many aquatic organisms (Donaldson et al., 2008), so that sites that experience rapid temperature changes could have lower productivity or growth rates. The thermal variability in the near shore regions of the Great Lakes is likely to be large during the summer stratified period, however this variability is not usually ⁎ Corresponding author. E-mail addresses: [email protected] (M. Wells), [email protected] (S. Parker). 1 Tel.: + 1 519 596 2444x314. 0380-1330/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jglr.2010.04.009
reported as part of biological studies on near shore ecosystem. Here, we report observations of rapid thermal variability from Fathom Five National Marine Park (located in Lake Huron, Fig. 1) and discuss the implications of this variability for aquatic organisms. A major source of temperature variability in the Great Lakes occurs during summer, when large amplitude internal waves lead to oscillations in the depth of the thermocline with frequencies between 12 and 24 h. These large amplitude internal waves have been documented in all the Great Lakes, including Lake Michigan (Hawley and Muzzi, 2003; Hawley, 2004), Lake Huron (Murthy and Dunbar, 1981) and Lake Ontario (Rao and Murthy, 2001). The different timescales over which the thermocline moves depends upon the physical forcing mechanisms. Movements of the thermocline over short periods (b24 h) could be due to (1) Poincare waves (Mortimer, 2006), (2) forcing by diurnal winds or (3) resonance of the thermocline with weak tides. In the first case, a 17 hour period would result. Poincare waves are intrinsically linked to the Coriolis effect whereby the Earth's rotation influences the movement of water in large bodies. The oscillatory motions are completed in an inertial period of 12/(sin θ) hours at latitude θ, which at 45°N is then close to 17 h (Mortimer, 2006). The amplitude of Poincare waves is fairly large, and is of the order of 5 m (Mortimer, 2006). The second case involves a diurnal oscillation (a 24 hour period) in the position of the thermocline resulting from strong diurnal wind patterns. This has been observed in many large lakes such as Lake Kinnerat in Israel (Antenucci et al., 2000), Lake Tahoe in California (Rueda et al., 2003) and Lake Biwa in Japan, (Saggio and Imberger, 1998). Lastly, a semidiurnal period (near 12 h) has been observed
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Fig. 1. a) Location of Fathom Five National Marine Park (FFNMP) on northern tip of the Bruce Peninsula. In Lake Huron b) a map of the bathymetry of the Fathom Five National Marine Park, showing the park boundaries. In 2006 moorings were located at the NW, SW and SE sites in late summer from 19 July and the 19 October 2006. In 2007 moorings were deployed at the Cove Island location (CI) and the SW mooring location, between 16 April and 10 October.
during periods of summer stratification at sites within the Great Lakes, in particular in narrow channels connecting two basins. Such changes are thought to be in response to the 12.4 hour M2 lunar tidal period. For example, in the mouth of Green Bay, Michigan observations by Miller and Saylor (1985) found currents with a peak period near 12 h, Similar observations have also been made in the Straits of Mackinac that connect Lake Huron to Lake Michigan (Sloss and Saylor, 1976). The basic features of the thermal regime of Fathom Five are similar to those reported by Bennett (1988) for the waters of Georgian Bay. The surface water temperature drops to 0 °C in late February for about a month, during which time most of the surface of Fathom Five is frozen. The spring convection period starts in mid-April, and mixing to the bottom continues until temperatures start to rise above 4 °C in mid-May. The water column starts to be strongly stratified by the end of June, with a surface mixed layer to approximately 10 m depth. The maximum surface temperature occurs in early August with surface temperatures in the range 19–22 °C. Below the sharp thermocline at 20 m depth the water remains less than 7 °C, dropping to 4 °C in the deepest areas. After October, the surface water rapidly cools and the whole water column becomes well mixed with a temperature close to 4 °C by December. What is missing from the preceding discussion of the thermal regime of Fathom Five is an understanding of the temperature variability within the water column. The data presented in Bennett (1988) gives the impression of smooth changes in temperature within the water column, even though he also reports some rapid thermal fluctuations. At Fathom Five the typical summer water temperatures in the thermocline change by 10 °C over 20 m, so the ubiquitous internal waves described earlier could result in persistent temperature variability, in particular at the depth where the thermocline intersects with the lakebed. The temperature variability associated with internal waves and coldwater upwelling has a recognized ecological effect (e.g., Spigarelli et al., 1982; Levy et al., 1991; Finlay et al., 2001; Evans et al., 2008; Donaldson et al., 2008). As a rather extreme example, Emery (1970) showed that a coldwater upwelling in Fathom Five caused the mortality of resident demersal fishes and crayfish. Here, we aim to determine the typical rates of temperature variability in the water column at Fathom Five in the summers of 2006 and 2007 summers, with the aim of determining what typical rates of temperature fluctuation are. In particular we would like to ascertain the frequency of extreme temperature changes (≤10 °C/h), which would change the population distribution of benthic organisms. Since the physiology of fish and other aquatic organisms is inextricably linked to water temperature (Finlay et al., 2001; Donaldson et al., 2008), the distribution of organisms at various depths within Fathom Five will likely be influenced by the degree of temperature variability experienced within the park.
Methods and materials Field site description Fathom Five National Marine Park is located near Tobermory at the northern tip of Ontario's 100 km long Bruce Peninsula at 45° 19′ 17″ N, 81° 37′ 34″ W (Fig. 1a) and lies between Georgian Bay (area 15,000 km2) and the main basin of Lake Huron (area 44,000 km2, Bennett, 1988). Fathom Five has an area of 114 km2 and is bisected by the Niagara Escarpment as it submerges off the northern tip of the Bruce Peninsula, emerging periodically to form a series of small islands. The western half of Fathom Five is located atop the escarpment with water depths less than 40 m. The eastern half is located below the escarpment, with water depths greater than 90 m. Fathom Five was established in 1987, as Canada's first National Marine Conservation Area (McBurney, 2001), and has both a cold offshore and a warm inshore component. Fish species typical of the offshore component include lake trout (Salvelinus namaycush), lake whitefish (Coregonus clupeaformis), burbot (Lota lota), and bloater chub (C. hoyi). Fish species typical of the inshore waters include bluntnose minnow (Pimephales notatus), common shiner (Luxilus cornutus), rock bass (Ambloplites rupestris), and yellow perch (Perca flavescens) (Leslie and Timmins, 2001). The macroinvertebrate community of the region is dominated by zebra mussels (Dreissena polymorpha) and quagga mussels (D. bugensis), but also includes chironomids, isopods, gammarid amphipods, and gastropods. Water quality and lower trophic assessments have characterized Fathom Five as being a healthy oligotrophic ecosystem (McCrea et al., 2001; Munawar et al., 2001). The mean water circulation within Fathom Five is dominated by strong exchange flows between the main basin of Lake Huron and Georgian Bay. Measurements made during the International Field Year for the Great Lakes (1974) showed that during summer circulation there is a mean surface flow into Georgian Bay and a deeper (N25 m) return flow into Lake Huron in the Fathom Five area (Schertzer et al., 1979; Bennett, 1988). The average speed of the surface current is on the order 0.05 m/s (Sheng and Rao, 2006), which is among the highest mean speeds observed for summer currents in Lake Huron (Beletsky et al., 1999). This exchange flow is driven by a persistent horizontal temperature difference in the waters of Georgian Bay and Lake Huron. This temperature gradient is set up by the prevailing westerly winds, which lead to the accumulation of warm water on the western side of the Bruce Peninsula (the eastern sides of Lake Huron) and the upwelling of relatively colder waters on the eastern side of the Bruce Peninsula (the western side of Georgian Bay), including the establishment of a semi-permanent upwelling in the coastal boundary of the southern tip of Manitoulin Island (Bennett, 1988). Data from the NOAA coast watch website (http://
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coastwatch.glerl.noaa.gov/) show that the surface temperature differences across the Bruce Peninsula were often of order of 5 °C during the summer, due to the cold water upwelling on the eastern side of the Peninsula. The mean monthly air temperatures at Tobermory closely follow the lake temperatures. For 2006 and 2007 the meteorological records of Environment Canada show that the mean summer air temperature in July and August were 20 °C, and mean winter air temperatures in January and February are close to −5 °C. The wind speeds were highest in winter with means of 4.4 m/ s, and there were lower mean wind speeds of 2.7 m/s during the summer months. Thermistor deployment During periods of summer stratification in 2006 and 2007, three strings of thermistors were deployed at locations within Fathom Five (Fig. 1b). The 2006 deployment was from July 19 to October 19, and the 2007 deployment was from April 16 to October 2. In both years, three moorings were deployed, but in each year a problem occurred with one of the moorings. On each mooring, six or seven thermistors were attached along a rope kept afloat by a surface buoy. The thermistors were arranged so that there was a good resolution where the thermocline was located, at approximately a depth of 15 m. The thermistors used were HOBO Water Temp Pro v2 units, and have an accuracy of ±0.2 °C and recorded temperature every 12 min. In 2006 the moorings were located at three sites of interest near the underwater escarpment that runs through Fathom Five. The mooring at the NW location was to the west of the escarpment and north of Flowerpot Island in 35 m of water, the second mooring at the SW location was located near the top of the escarpment near Middle Island in 40 m of water and the third mooring at the SE location was located east of the escarpment in water 85 m deep. Problems developed with the SW mooring after 3 weeks, so data is not shown from this location. The SE mooring had temperature data-loggers at depths of 5, 10, 15, 20, 40, 60 and 80 m. The NW mooring had temperature data-loggers at depths of 1, 6, 11, 16, 21, 26, 30 and 31 m. In 2007 the three moorings were located at three sites, one at the 80 m deep SE location, one at the 35 m deep NW location and one at a location between a submerged reef and Cove Island (CI) in 35 m of water (Fig. 1b). Problems developed with the deep SE mooring and no data were recovered for 2007. The NW location had 6 loggers at depths of 2, 6, 11, 16, 21, 29 m depths, and the Cove Island mooring had 7 loggers at depths 1, 6, 11, 16, 21, 26 and 30 m depths. Results The 2006 temperature time-series for the NW and SE moorings are plotted in Figs. 2 (a, b) and show strong variability in the position of the thermocline. The surface waters warm to a maximum of 23 °C by early August and then gradually cool to 10 °C by early October. For most of the summer, the mean depth of surface mixed layer was around 10 m. In mid October however, the mixed layer deepens dramatically as the water rapidly cools. Below the depth of 20 m, the water temperatures remain low during the summer stratification, reaching minimum temperatures near 5 °C below the depth of 60 m. During the period from July 19 to October 23, the water column was stratified with surface temperature between 15 and 20 °C. During this period, the water temperature within the epilimnion frequently changed temperature as cold waters rose to the surface, or the warm surface waters were depressed to 20 m depth. These events may have been driven by strong local winds, or by the passage of large amplitude internal waves, such as Kelvin waves, which are caused by distant wind events (Rao and Schwab, 2007). The timing of these upwelling and downwelling events were well correlated between the NW and SE mooring, with a correlation factor of R2 = 0.6 for the temperatures near the main thermocline. Similar dynamics were seen
in the longer 2007 temperature time-series plotted in Figs. 2 (c, d), for the NW and Cove Island moorings. The water column was initially well mixed on May 20 with a temperature just above 1 °C. The thermal stratification started to develop around May 30 as the water column warms past 4 °C. The surface waters reached a peak of 22 °C on August 3 after which the water slowly cooled to 16 °C on October 2. At depths below 20–30 m, the water temperature ranged between 6 and 8 °C. The mean depth of the surface mixed layer was around 10 m as in the previous year. The most interesting feature of the temperature time-series in Fig. 2 is the intense variability in the position of the thermocline, which was normally between 10 and 25 m depth, but could move vertically by as much as 20 m in a 16 hour period. The mean temperature profiles and the range of observed temperatures for each data logger are plotted in Figs. 3 and 4, for the month of August 2006 and August 2007 respectively. There were considerable departures from the mean temperature between depths of 5 and 20 m, while there was very little variability in temperature below depths of 30 m. This temperature variability is indicative of the strong movements of the thermocline associated with large amplitude internal seiches and internal waves, which occurred almost every day. This variability is quantified in Figs. 3 (b–d), where the relative frequency of a given temperature occurring at each data logger is plotted during the period of peak stratification in August. The Fig. 3 shows a tight distribution in temperature variability for the deepest sites. For depths between 10 and 20 m, the distributions are much broader, reflecting the moving thermocline. For example, at the NW mooring the temperature recorded at the 11 m depth shows the percentage of the time that the temperature was at 20 °C is the same as when it was at 6 °C. Similar observations can be seen in the temperature profiles from the 2007 moorings shown in Fig. 4a. There is a difference in the thermal variability at these two sites, with the open water site (Fig. 4c) having higher temperature variability in the deeper waters, than the sheltered Cove Island site (Fig. 4b). The greatest variability in temperature was between depths of 10 and 20 m, due to frequent fluctuations in the thermocline position (Figs. 3 and 4). Based upon the bathymetry shown in Fig. 1b, 60% of lakebed of Fathom Five lies at depths greater than 20 m, another 20% of the lakebed lies between the depth contours of 10 and 20 m, and the remaining 20% is at depths shallower than 10 m. The lakebed at depths between 10 and 20 m is where the strong fluctuations in depth of the thermocline occurred; hence during summer the temperature at the open water sites is expected to fluctuate between 6 °C and 20 °C almost every day. Figs. 5a and b illustrate the rate of temperature change at various depths in August 2007. The median rate of change of temperature was relatively slow at about 0.2 °C per hour. However there were infrequent events where the rate of change of temperature was greater than 5 °C per hour and as much as 10 °C per hour. Temperature changes greater than 5 °C per hour occurred about 2% of the time in the open water site, and less than 1% of the time in the sheltered site. Changes of 2 °C per hour occurred about 10% of the time at both sites. The reduced temperature variability in Cove Island can be attributed to the shallow reef that surrounding the mooring location. As the reef is shallower than the depth of thermocline most of the vertical movements in the thermocline seen at the NW site were not felt in these more protected waters and so the temperature variability was less. The oscillations in the thermocline shown in Fig. 2 have dominant periods between 12 and 24 h. Fig. 5c shows a Fourier transform of the 2006 NW mooring data. There is a peak at a frequency of 1/17.1 h (0.058 cycles per hour), corresponding to the frequency of an inertial oscillation at the 44°N (Mortimer, 2006) as well significant energy at frequencies near diurnal (1/24 h or 0.04 cycles per hour) and semidiurnal (1/12 h or 0.08 cycles per hour) periods. There is also an indication of a timescale greater than 4 days (0.01 cycles per hour),
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Fig. 2. The evolution of the temperature profile against time is shown here for the a) NW and b) SE moorings in 2006, and for 2007 at the c) NW and d) CI location.
representing variability that could be associated with the passage of meteorological systems or the slow passage of Kelvin waves. Similar spectral peaks are seen at the other mooring sites in 2006 and 2007 during the period of strong summer stratification.
Discussion and conclusions Our field work shows that temperature variability is a persistent and ubiquitous feature of the water column at Fathom Five, for depths
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Fig. 3. These plots show the considerable variability experienced at the 3 mooring sites during August 2006. In (a) the mean profile is by the lines, with the range of all data plotted as points. The frequency histograms during the first 3 weeks of August are plotted in b) for the SW mooring, in c) for the NW mooring and in d) for the SE mooring.
between 10 and 20 m. Rapid rates of temperature change, as high as 10 °C per hour were found around 0.1% of the time, with slower temperature changes greater than 2 °C per hour found about 10% of the time. The dominant periods of the internal wave motions were between 12 and 24 h, with mean wave amplitude of order 5 m, although displacements of the thermocline by as much as 20 m were not uncommon. The variability of temperature was least below 50 m depth, and also at a site that was poorly connected to the main waters of Lake Huron due to a surrounding shallow reef. Due to the many reports in the literature of the chronic effects of rapid temperature variability, the spatial and temporal variation in temperature variability within the park may be an abiotic driver of the benthic ecosystem in Fathom Five. Observations of large and persistent movements of the thermocline at periods between 12 and 24 h driven by Poincare waves, internal tides, and diurnal wind forcing have been reported in many large lakes worldwide, such as Loch Ness in Scotland (Mortimer, 1952), Lake Geneva in Switzerland (Lemmin et al., 2005), Lake Biwa in Japan (Saggio and Imberger, 1998) and Lake Kinnerat in Israel (Antenucci et al., 2000). There are also many sites in the Great Lakes where comparable observations of the movement of the thermocline have been reported (Murthy and Dunbar, 1981; Rao and Murthy, 2001; Hawley and Muzzi, 2003; Hawley, 2004; Mortimer, 2006). It is important to know how rapidly temperature changes can occur within the water column, as most animals are most sensitive to rapid
changes in temperature rather than slow changes (Donaldson et al., 2008). With water temperature changing at a rate as much as 5 °C per hour and 10 °C in a 12–24 hour period, particularly in the 10 to 20 m water depths, the offshore aquatic ecosystem of Fathom Five is quite dynamic. These rates of change of water temperature have been previously demonstrated to lead to severe ‘cold shock’ of many fish (Donaldson et al., 2008). The observations of Emery (1970) also suggested that temperature changes of order 10 °C per hour could be lethal to some benthic fauna in Fathom Five, so these infrequent events may have a strong role in controlling the benthic community. The report by Emery (1970) stands as the only direct observation or study of the ecological effects of internal waves in Fathom Five. However, the literature provides some insights and opportunity for discussing potential chronic and acute effects of temperature variability. Below, we discuss the potential biological response of aquatic organisms to these ubiquitous thermal fluctuations. The offshore region of Fathom Five is generally subdivided into upper (b50 m) and deeper (N50 m) water fish communities. The upper group is most likely to interact with the thermocline and includes cisco (Coregonus artedii), lake whitefish, lake trout, Chinook salmon (Oncorhynchus kisutch), longnose sucker (Catostomus catostomus), yellow perch (P. flavescens), walleye (Stizostedion vitreum), emerald shiner (Notropis atherinoides), and rainbow smelt (Osmerus mordax) (Spangler and Collins, 1992; Riley et al., 2008). The deeper group also includes lake
Fig. 4. (a) These images show the considerable variability experienced at the 2 mooring sites during August 2007. The histograms show that at deeper depth in the sheltered site CI behind Cove Island (b) receives less temperature variability that the open water site at the NW mooring shown in (c). Near the water surface the variability is the same at both sites.
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Fig. 5. A histogram of the rate of temperature change, during the period of peak stratification in August 2007, is shown in a) for the open water site at the NW mooring and in b) for the sheltered location behind Cove Island (CI). c) A Fourier Transform of the 2007 data from the NW mooring showing that there is broad peak in spectral energy around the periods of 24 and 12 h, corresponding the diurnal and semidiurnal periods (0.04 and 0.08 cycles per hour). The spectral peak of 17.1 h (0.06 cycles per hour) may be associated with the passage of Poincare wave oscillating near the inertial period.
whitefish and lake trout as well as burbot, deepwater sculpin (Myoxocephalus thompsonii) and historically included of 6 species of deepwater cisco, of which only the bloater chub and shortjaw cisco (C. zenithicus) remain extant (Todd, 2003; Bence and Mohr, 2008). In general, water temperature heterogeneity and mobility provides these fishes with an opportunity to seek preferential thermal habitat, thus these species can easily swim up or down to their preferred temperature regime. For example, coldwater species such as lake trout prefer ∼11 °C and lake whitefish prefer ∼12 °C, whereas coolwater species such as yellow perch prefer ∼ 23 °C and walleye prefer ∼22 °C (Casselman, 2002; Wismer and Christie, 1987). Benthic species, such as mottled sculpin (Cottus bairdii), may be habitat restricted and more exposed to the zone of temperature change. As presented in the review by Donaldson et al. (2008) and others (Brandt, 1980, Spigarelli et al., 1982, Christie and Regier, 1988; Levy et al., 1991), there is a cascade of stress responses associated with temperature shock (e.g., neuroendocrine response, corticosteroid-catecholamine release, osmoregulatory change, immunological change, habitat change, loss of disease resistance and developmental changes). Magnuson et al. (1979) noted that width of the fundamental thermal niche of many fish species is narrow (±2 °C of the mean temperature), so that even the frequent small temperature changes shown in Fig. 5 have potential to significantly impact fish consumption or growth. This optimal temperature range was frequently exceeded by the frequent changes of temperature seen in Fathom Five. Thus while the mean temperature may be favourable to some benthic species, the temperature variability may be a source of stress. The benthic community is more susceptible to the effects of temperature fluctuations because many of the species are less mobile. From studies in Lake Ontario, Wilson et al. (2006) noted that the total mass of quagga mussels was lowest at the sites with high upwelling frequency and Kilgour et al. (2000) found that the frequency of upwelling was an important indicator of the benthic community. Similarly Fitzsimons et al. (2002) attributed the absence of crayfish at sampling sites due to the thermal stress associated with frequent upwelling of cold water. Nalepa et al. (2005) reported greater abundance of the benthic amphipod Diporeia spp. on the west side of Lake Michigan where upwelling brings colder more enriched waters. McCabe and Cyr (2006) found variability in the water temperature promoted increased diversity of algal species in Lake Opeongo, Ontario. The information on the depths affected by variable water temperatures can be used to help characterize and understand aquatic ecosystems and habitats in Fathom Five. For instance, the vulnerability to cold shock from upwellings may explain the reduced fish species richness in Dunks
Bay as compared to other local protected bays (Leslie and Timmins, 2001). The information may also be useful in identifying critical habitat for species at risk, such as the shortjaw cisco (Todd, 2003; Naumann and Crawford, 2009), where deep waters and stable cold temperatures may be important habitat requirements. This work has demonstrated that there is an on-going abiotic process, a potential ecosystem driver, in Fathom Five. An improved understanding of the process and how the temperature variability influences species behaviour, physiological responses, and associated interactions is required. The high rates of temperature variability that were found at Fathom Five are likely to present at many other coastal sites around the Great Lakes. However the vertical amplitude and frequency will depend upon the bathymetry at the region of interest; for instance local bathymetry could lead to resonance and amplification of the internal waves, or it could dampen the amplitude as was seen in the sheltered location behind the reef in our observations. Typically the temperature variability due to internal waves and seiches is overlooked in many field experiments, as frequently only one or two temperature profiles would be made to characterize the temperature of the water column. Given the ubiquity of internal waves in stratified lakes, it would be useful to measure the climatology of the internal wave dynamics around the Great Lakes for future research on the diversity and thermal range of aquatic organisms in the near shore region. Acknowledgments This work would not have been possible without the extensive help of Jeff Truscott of Parks Canada, Paola Travaglini of the Canadian Hydrographic Service, and Lisa Tutty and Alex Ragnosig from the University of Toronto. Conversations with Helene Cyr and Brian Shuter are gratefully acknowledged. Funding for MGW was provided by an NSERC discovery grant. Comments by two anonymous reviewers and the editors were very helpful in focusing the paper. References Antenucci, J.P., Imberger, J., Saggio, A., 2000. Seasonal evolution of the basin-scale internal wave field in a large stratified lake. Limnol. Oceanogr. 45, 1621–1638. Austin, J.A., Colman, S.M., 2007. Lake Superior summer water temperatures are increasing more rapidly than regional air temperatures: a positive ice-albedo feedback. Geophys. Res. Lett. 34, L06604. doi:10.1029/2006GL029021. Beletsky, D., Saylor, J.H., Schwab, D.J., 1999. Mean circulation in the Great Lakes. J. Great Lakes Res. 25, 78–93. Bence, J.R., Mohr, L.C., 2008. The state of Lake Huron in 2004. Great Lakes Fish. Comm. Spec. Pub. 08-01. Bennett, E.B., 1988. On the physical limnology of Georgian Bay. Hydrobiologia 163, 21–34.
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