The Hamilton Harbour Remedial Action Plan has adopted a dissolved oxygen goal for restoring habitat in the pelagic portion of Hamilton Harbour based on the ecological needs of Cisco (Coregonus artedii), a fish formerly abundant in Hamilton Harbour. The goal for dissolved oxygen is based on retaining an adequate volume of optimum Cisco habitat characterized as temperature <20°C and dissolved oxygen >6 mg l−1 during the June to September period. The goal also specifies minimum habitat requirements for when optimum conditions are not achieved, that being a smaller volume of refuge habitat with temperature <20°C and dissolved oxygen >3 mg l−1 for no more than 2 weeks per year. Weekly temperature and dissolved oxygen profiles during May to October, 1987 to 2012 in the center of Hamilton Harbour were assessed to evaluate optimum and refuge Cisco habitat relative to the Remedial Action Plan goal for dissolved oxygen. This goal was met only in 2009. However, this was fortuitous, based on a combination of cooler water temperatures in May and June and exchanges of cool oxygenated water from Lake Ontario. From 1987 to 2002 optimum habitat was estimated to be absent at least one week and up to seven weeks during June to September. Cisco could not have survived in Hamilton Harbour during six of these years when refuge habitat was absent for one or two weeks. Since 2003, Cisco habitat in Hamilton Harbour improved markedly, as some refuge habitat was always present. As well, the number of weeks with inadequate refuge habitat, and with no optimum habitat has declined. These improvements in Cisco habitat since 2003 were related to higher dissolved oxygen in the mid-depths of Hamilton Harbour.

Introduction

Hamilton Harbour was declared an Area of Concern (AOC) in 1987 due, in part, to anoxia and the consequential loss of biota in the open water habitat of Hamilton Harbour (IJC, 1987). In the late 1800s municipal sewage input and declining water quality were implicated in the decline of Cisco (Coregonus artedii) in Hamilton Harbour, and by the mid-1930s they had been eliminated (Whillans, 1979), despite good catches at the time in western Lake Ontario (Christie, 1973). This degradation was long-standing, as a number of other coldwater and coolwater fishes including Lake Whitefish (C. clupeaformis), Walleye (Sander vitreus), Lake Trout (Salvelinus namaycush), and Lake Sturgeon (Acipenser fulvescens) had disappeared from Hamilton Harbour by the mid-1930s due to anoxia (Holmes and Whillans, 1984). Following the declaration of an AOC a Hamilton Harbour Remedial Action Plan (HHRAP) was developed to restore the ecosystem. The 2002 RAP goal for dissolved oxygen (DO) in Hamilton Harbour was established as >4 mg l−1 (O'Connor, 2003) based on the Province of Ontario's DO criterion for warmwater biota (OMOE, 1994). However, the open water habitat of Hamilton Harbour is more suitable for coolwater and coldwater fish. The Hamilton Harbour and Watershed Fisheries Management Plan (HHWFMP) recognized the need for restoration of the coolwater and coldwater fish communities in Hamilton Harbour (Bowlby et al., 2010), and this led to a re-evaluation of the RAP goal for DO.

An updated HHRAP goal for DO (HHRAP Office, 2012) was developed based on the habitat requirements for Cisco (Bowlby and Richards, 2008). Cisco and Walleye are the HHWFMP target species for open water habitat (Bowlby et al., 2010). Relative to walleye, Cisco are more susceptible to anoxic conditions in Hamilton Harbour, as it occupies cooler, deeper water, where hypoxia tends to develop first. As a pelagic species, Cisco would be unaffected by a limited zone of hypoxia in the deepest part of Hamilton Harbour, which may have always been subject to hypoxia (O'Connor, 2003). Accordingly, deeper profundal species such as Lake Trout were not considered for either the open water fish community of Hamilton Harbour (Bowlby et al., 2010) or for development of a RAP goal for DO. Here, we summarize the development of the new RAP goal for DO, and describe the trend towards attainment of this goal during 1987–2012.

Updated HHRAP goal for dissolved oxygen

The updated HHRAP goal for DO is based on creating suitable habitat for Cisco. Due to the conservative nature of habitat needs of Cisco, this goal will also protect other pelagic coldwater and coolwater species:

During June to September inclusive, the water column at center station should have a minimum 4 meter thick layer of water with a temperature <20°C and a DO >6 mg l−1. Compliance with this goal is to occur in at least 15 of 17 profiles measured weekly, and during any exceedance episode, the water column at center station should still have a minimum 2 meter thick layer of water with a temperature <20°C and a DO >3 mg l−1. (HHRAP Office, 2012)

We have proposed a DO goal that sets minimum habitat requirements for Cisco when optimal conditions are not achieved in Hamilton Harbour. Although mortality of Cisco is not acceptable, under minimum habitat conditions, growth may be impacted. The RAP goal for DO is based on measurement during the usual period of hypoxia during June to September. The period of sampling for this goal is based on a proposed future sampling schedule that might be limited to this period. However the growth period for Cisco is longer, and so the DO goal would allow growth impairment of Cisco for up to two out of 26 weeks (8%) of the May-October growing season. This relies on the formation of a habitat zone (sometimes called “thermal habitat volume” which has been described for several species, including Cisco, Walleye, and Lake Trout (Frey, 1955; Christie and Regier, 1987). For our purposes, the habitat zone for Cisco in Hamilton Harbour is the zone of usable habitat limited by warmer water near the surface and hypoxic water at greater depth. The rate of oxygen depletion depends on several factors including productivity of the system, water temperature, morphometry and turbulent mixing. Hamilton Harbour has a significant area where the water depth is greater than 20 m, which is deep enough to stratify during the summer. At depth, bacterial activity degrades the organic material settling out from the upper layers, and vertical mixing is reduced. These factors make Hamilton Harbour very susceptible to the development of hypoxia/anoxia.

Cisco occupy the cooler pelagic habitat of lakes during summer (Scott and Crossman, 1973). During summer in Lake Ontario, Cisco occupy a wide range of temperatures, generally below 17°C with a mean of 11.6°C (Hoyle, 2015). Older Cisco are rarely found above 20°C (Frey, 1955; Rudstam and Magnuson, 1985). Nevertheless, they grow well at higher temperature, and the optimum temperature for growth of Cisco is 18.1°C (Jobling, 1981). Younger Cisco can tolerate warmer water, and the upper incipient lethal temperature for young-of-the-year Cisco is 26°C (Edsall and Colby, 1970).

Rudstam and Magnuson (1985) modelled DO thresholds for mortality and impairment of growth in relation to temperature for Cisco based on laboratory studies (Figure 1). The lower DO threshold between 100% mortality and impaired growth increases from 1.9 mg l−1 DO at 12°C to 3.3 mg l−1 at 20°C. The upper DO threshold is from 4.4 mg l−1 DO at 12°C to 5.8 mg−1 at 20°C, above which growth of Cisco is unimpaired. These results are consistent with field studies. In particular, in 27 Indiana lakes, Frey (1955) observed that Cisco persisted with an anoxic hypolimnion where a “Cisco layer,” or refuge habitat, was available with DO >3 mg l−1 and temperature <20°C. The Cisco layer is consistent with the region of impaired growth (Figure 1), particularly at 20°C. The updated HHRAP goal for DO incorporates the Cisco layer DO and temperature thresholds for Cisco refuge habitat.

The required thickness of Cisco refuge habitat has not been well documented. The Cisco layer in 27 Indiana lakes ranged 0.1–21.6 m thick with about one-half measuring 1–4 m thick (Frey, 1955). The catch of Cisco in test gillnets in seven of these lakes increased with Cisco layer thickness (Figure 2). Although these data were sparse an optimum thickness for Cisco refuge habitat may be >4 m. In five Ontario lakes similar in size to Hamilton Harbour, Cisco schools occupied a zone 1.2–1.7 m thick in the presence of Lake Trout predators (Vascotto, 2006). In lakes without predators Cisco schools occupied a thinner depth zone, as less water column is required to escape predation. Walleye are the dominant open water predator in Hamilton Harbour with habitat requirements likely to overlap with Cisco, especially as the oxygen becomes lower in the hypolimnion. Accordingly, the updated HHRAP goal for DO incorporates a 4-m thick unimpaired habitat for 92% of the growing season and a 2-m thick refuge habitat when optimum habitat is unavailable.

Methods

Hamilton Harbour is located in the western end of Lake Ontario, Canada. Water quality has been monitored in Hamilton Harbour since the late 1960s, and in 1987 a comprehensive sampling program with regular sampling of Center Station (43° 17' 21“ N, 79° 50' 09” W) began during ice-free months (Hiriart-Baer et al., 2009; 2016). For the purposes of this analysis, data from May to October during 1987 to 2012, inclusive, were used to evaluate DO and temperature relative to Cisco habitat requirements. Exception years were 1992, 1993, and 1996, during which time few or no samples were available. Dissolved oxygen and temperature profiles were recorded on about a bi-weekly basis until 1995, and on a weekly basis since 1997. Weekly calibrations of profiling equipment and method validation and comparison have been carried out over the period of analysis, thereby maintaining the integrity of the data. Profiles were usually 22–24 m to the bottom of Hamilton Harbour, and data here are truncated at 23 m to minimize complications arising from interactions at the sediment-water column interface.

The quality of pelagic habitat for Cisco was determined at depth for each sample day based on temperature and DO defining these zones: (1) unsuitable due to high temperature (>20°C), (2) unsuitable due to low DO (<3 mg l−1), (3) refuge habitat (<20°C and >3 mg l−1) and (4) optimum habitat (<20°C and >6 mg l−1). Sometimes samples were collected about 2–3 weeks apart, but the RAP goal for DO is based on weekly sampling. Estimates for unsampled weeks during 1 June to 30 September were interpolated. The number of interpolated weekly estimates during 1987 to 1995 ranged from 9–10, and during 1997 to 2012 ranged from 1–6.

Results

Hamilton Harbour has a long history of summertime hypoxic conditions. At 10 m depth, the minimum annual DO has ranged from 0.2–4.0 mg l−1 since 1987 (Figure 3). This depth was chosen to illustrate long-term trends of the DO dynamics of the zone which typically has temperatures suitable for Cisco and other coldwater species. Since 2004, DO at 10 m has tended to be higher than in previous years, except for 2007.

Detailed monitoring results for temperature and DO in the entire water column for 2009 and 2010 show current conditions in Hamilton Harbour with contrasting extremes of Cisco habitat quality (see below). For conciseness only these years are presented here. During May and October temperature usually ranged from 10–16°C in all years, and was often isothermal, consistent with spring and fall turnovers (Figure 4). Similarly, DO in May and October was more uniform through the water column, generally 6–10 mg l−1 (Figure 5). Accordingly, June to September was an appropriate choice of season for the RAP goal for DO. Water temperature profiles show that thermal stratification generally forms by the end of May, and by late June a well-developed thermocline ranging from 5–10 m below the surface is established in the deeper parts of the harbor. In concert, DO declined in the deeper layers during summer, typical of hypereutrophic systems (Figure 5). The initial decline in DO around the beginning of June was generally in the lower 10–15 m, and occurred quickly, consistent with a large oxygen demand in Hamilton Harbour. Summer temperatures were variable between and within years due to initial temperature at the start of the summer (Figure 4), and to exchanges with Lake Ontario (Yerubandi et al., 2009). For example, in July 2009 exchanges with Lake Ontario resulted in a 2°C drop in temperature of the surface and bottom layers of Hamilton Harbour (Figure 4). In contrast, exchanges in July 2010 resulted in a drop in temperature in only the bottom layers of Hamilton Harbour. These exchanges between Hamilton Harbour and Lake Ontario contributed to corresponding increases in DO.

Cisco habitat in Hamilton Harbour became constricted at the beginning of June by both increasing temperature in the surface layer and hypoxia in the lower layer (Figure 6). Decreased Cisco habitat persisted through the summer until surface temperatures declined in early September, returning the upper levels to Cisco habitat. DO increased in lower levels through September following temperature declines in the upper levels of Hamilton Harbour. The amount of optimum Cisco habitat and refuge habitat in Hamilton Harbour was variable through the summer and from year to year mainly due to exchanges of cooler oxygen-rich water with Lake Ontario. Exchanges with Lake Ontario replenished cooler temperatures and DO, and ameliorated seasonal trends toward hypoxia in Cisco habitat. Accordingly, Cisco habitat was impaired to a far greater extent in 2010 than 2009 (Figure 6). In 2009, optimum Cisco habitat was present through the summer, and thickness remained >4 m during all but 2 weeks (Figure 6). Moreover, refuge habitat was present during these two weeks, and so the RAP goal for DO was met. In contrast, optimum Cisco habitat was absent during three weeks in 2010, and <4 m thick during another seven weeks. Moreover refuge habitat was <2 m during three weeks, two of which coincided with absence of optimum habitat (Figure 6).

The HHRAP goal for DO was met only in 2009. From 1987 to 2002 optimum habitat was estimated to be absent at least one week and up to seven weeks during June to September (Figure 7). In most of the remaining weeks during these months and years, optimum Cisco habitat remained impaired. Additionally, Cisco could not have survived in Hamilton Harbour during six of these years when refuge habitat was absent for one or two weeks. Since 2003, Cisco habitat in Hamilton Harbour has improved markedly (Figure 7): Refuge habitat was always present; the number of weeks of impairment of refuge habitat has declined, and the number of weeks of absence of optimum habitat has declined. These improvements in Cisco habitat since 2003 are clearly related to higher DO in the mid-depths of Hamilton Harbour (Figure 3).

Discussion

Although the HHRAP goal for DO was met in 2009, this was fortuitous, based on a combination of cooler water temperatures in May and June and relatively frequent exchanges of cool oxygenated water from Lake Ontario (Figures 4 and 5). These exchanges are dependent on meteorological forcings, for instance, wind driven currents and other physical processes (Yerubundi et al., 2009). These forcings are highly variable both intra- and inter-annually and cannot be relied upon to maintain DO at levels required by Cisco in Hamilton Harbour. Climate change may further modify the predictability of these events, and so reliance on exchange with Lake Ontario for oxygenating Hamilton Harbour would be inadvisable.

During 2003–2006 Cisco refuge habitat was not impaired and some optimum habitat was always present (Figure 7). Other than in 2009, these years had the highest quality of Cisco habitat and matched a period of higher hypolimnetic DO in Hamilton Harbour (Hiriart-Baer et al., 2016). Increases in DO and Cisco habitat since 2003 are consistent with long-term trends in several key water quality parameters. Hiriart-Baer et al., (2009; 2016) reported longer term (1987–2007) significant improvements in DO and parameters associated with eutrophication-driven oxygen demand including phosphorus, chlorophyll, and Secchi disc depth, as well as abiotic parameters such as ammonia and nitrate/nitrite. However, only these abiotic parameters improved over a shorter term (2000–2007) corresponding more closely in time with observations of improved Cisco habitat in this study. In addition, Hiriart-Baer, et al. (2016) observed a significant positive correlation between springtime dissolved organic carbon concentrations in Hamilton Harbour and DO depletion rates since 2007, apparently due to respiration of bacteria feeding on DOC. The role of DOC in the longer term dynamics of DO and Cisco habitat are not yet clear. The HHRAP set goals for phosphorus, chlorophyll a, Secchi disc depth, and ammonia, in part to meet the RAP goal for DO and improve habitat for the aquatic community (HHRAP Office, 2012). Attainment towards meeting the HHRAP goal for DO will require continued reduction in phosphorus and nitrogen compounds to Hamilton Harbour (Hiriart-Baer et al., 2009).

The HHRAP goal for DO was developed specifically for a pelagic fish community. Particularly, Lake Whitefish, Walleye, and Cisco have a pelagic juvenile stage, and are suited to habitat based on the RAP goal for DO. The deepest part of Hamilton Harbour may have always been subject to hypoxia (O'Connor, 2003), as prior to dredging the Burlington Canal between Hamilton Harbour and Lake Ontario the natural opening between Hamilton Harbour and Lake was much shallower. Accordingly, hydraulic exchanges would have been smaller relative to present-day, and limited to the warmer surface layers during summer. Under this scenario, exchanges involving the deeper layers of Hamilton Harbour may have been less common in preventing hypoxia in the deeper layers. This is consistent with the historic fish community of Hamilton Harbour. Historically, Lake Trout left Hamilton Harbour during fall to spawn elsewhere (Holmes and Whillans, 1984). Since juvenile Lake Trout reside near the lake bottom in deeper water (Elrod and Schneider, 1987), rearing habitat may have been unsuitable in Hamilton Harbour. The current scenario of higher hydraulic exchanges with Lake Ontario may have opened the possibility for Hamilton Harbour to support juvenile Lake Trout. If RAP goals for phosphorus, chlorophyll a, Secchi disc depth, and ammonia overachieve with respect to DO goals, then habitat suitability for Lake Trout in Hamilton Harbour could be examined to support the broader objective of Lake Trout restoration in Lake Ontario.

Acknowledgements

We gratefully acknowledge Environment Canada staff and students for collecting and processing water quality data from Hamilton Harbour. Comments from Mike Yuille and two anonymous reviewers improved this manuscript. The encouragement of Vic Cairns was instrumental in the development of the manuscript.

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