Physical Processes & Cultural Influences
Aquatic habitats along the Toronto waterfront are the product of various combinations of physical conditions and processes. In essence, there are three major controlling influences on the location, function, and attributes of shoreline habitats:
The modern shoreline of Lake Ontario is situated between two post-glacial abandoned shorelines. The landward abandoned shoreline originally marked the edge of the higher post-glacial Lake Iroquois, resulting in a stranded shoreline bluff and abundant beach material along the present day tablelands. The Lake Iroquois shoreline influences the morphology of modern streams and focusses the mid-reach recharge of ground water sources. However it has a minor effect on current aquatic habitats.
An off-shore abandoned shoreline created by the lower post-glacial Admiralty Lake has a much greater effect on today’s shoreline. The former Admiralty Lake shoreline has left a variety of submerged features including a prominent off-shore bluff known as the Toronto Scarp that runs parallel to the Toronto Islands and Scarborough shoreline. Admiralty Lake was also the source of relict sand and gravel deposits that can still be found in deep off-shore waters. The most significant surficial geological features that affect and determine current shoreline conditions are found between the abandoned Admiralty Lake shore and the modern shoreline. Most current and historic habitats were created in this inundated area. For example, historically, the dynamic movement of littoral material established the peninsula and lagoons of Toronto Bay. The bulk of this material was supplied from shoreline erosion of the significant deposits of sands found in the Scarborough Bluffs and re-worked beach deposits made available during rising water levels. In addition, the Toronto Scarp at the shoreline of the former Admiralty Lake is an important area of congregation for Salmonid fish.
Western Lake Ontario Bathymetry
The bathymetry of western Lake Ontario displays a number of features that affect aquatic habitats. Lake Ontario is a deep, cold, oligotrophic (nutrient-poor) lake with relatively steep shorelines, particularly on the northern shore. Shale bedrock is apparent along the shorelines of Niagara Region, Halton Region, Mississauga and Etobicoke. A major depositional zone exists at the Hamilton lakehead. There is an underwater bluff, similar to the Scarborough Bluffs, off the Niagara Region shoreline.
The Toronto area shoreline can be described in five zones:
- Etobicoke Shale Outcrop
- Humber Bay Depositional Area
- Toronto Scarp
- Scarborough Sand Plains
- Scarborough Boulder-Laden Till
1. Etobicoke Shale Outcrop
Along the western sector a thin till layer that originally covered the bedrock has been scoured off by glacial action leaving a prominent area of bedrock substrate that extends from the mouth of Mimico Creek westward to Burlington. This bedrock forms a convex shoreline profile consisting predominantly of broken shale boulders on top of bedrock extending into deep water.
Humber Bay Depositional Area
From Humber Bay east to Ontario Place the substrates are dominated by fine material. Humber Bay and Toronto Bay are depositional areas containing recent silt deposits that predominantly come from the suspended sediment loads associated with the Humber and Don Rivers. The depositional area of Humber Bay is thought to be formed as a result of significant fluting in the underlying bedrock which produces the deep basin-like depression of the Bay.
The Toronto Scarp represents the former shoreline of Admiralty Lake about 5km from the existing Lake Ontario shoreline. It is a prominent underwater bluff comprised of extensive sand deposits. The water depth increases abruptly at the edge of the shelf from about 20m to approximately 60m.
Scarborough Sand Plains
An extensive underwater sand plain occurs from the south shore of the Islands to the Toronto scarp and eastward to Bluffers Park. This material is a very thick deposit of sand that is most likely a glacial relict of flooded beaches and eroded material that originated from an interglacial river deposit of deltaic sands derived from the cathedral section of the Scarborough Bluffs. Within these sand substrates there are small pockets of gravels and cobbles, especially in the nearshore areas just west of Bluffers Park. This section of sand-dominated substrates displays a prominent concave shoreline profile.
Scarborough Boulder-laden Till
From the east side of Bluffers Park to the East Point area there is a transition zone from sand to cobble, gravels and boulders. This coarser material originated from the high boulder content of adjacent tills that were eroded from the shore and re-worked as a boulder pavement. The headland created at East Point is a direct result of the high boulder and cobble content of the till, creating an area resistant to erosion. The boulder pavement provides an excellent example of unconsolidated material forming a convex shoreline profile. The extensive quantity of nearshore gravels is thought to provide a high degree of shoreline protection by attenuating waves and providing a dynamic equilibrium between erosion and accretion.
Wave Zone Areas
Along the wave zone area bedload sediments from the major rivers have surcharged the shoreline with sand and helped to establish the barrier beaches associated with local coastal wetlands at the mouths of the Rouge and Highland Rivers.
The boulder-laden till also loaded the wave zone areas with a vast quantity of aggregates. Approximately 1 million cubic metres of stone were historically removed by stone-hooking for use in construction activities.
Toronto Waterfront Substrates and Features
In summary, as shown below, Toronto Waterfront Substrates and Features, the key substrates along the Toronto waterfront are shale bedrock, sand, muds and clay, and boulder, cobble and gravel.
The shoreline profiles vary considerably along the waterfront. For example, in the vicinity of the Toronto islands (section 1 and section 2) the Toronto Scarp appears as a precipitous drop that varies from 15 – 60 metres to the deep lake, with the relatively shallow waters of Toronto Bay being sheltered by the islands. In section 3, there is a gradual slope into the Lake from the base of the Leslie Street Spit, followed by a deep bluff formed by the Toronto Scarp. In section 4, the effects of the Toronto Scarp have almost disappeared, and in section 5 there is the gradually sloping convex shoreline of the Scarborough boulder till.
Meteorological conditions — wind, nearshore wave climate, regional climatic conditions, solar heating, and thermal characteristics — have considerable influence on shoreline conditions and aquatic habitats.
- Wind and Waves
- Littoral Transport
- Thermal Conditions
- Hypolimnetic Upwellings
- Water Levels
Winds and Waves
Winds, in combination with over-water fetch lengths, determine wave conditions across the Toronto waterfront. A high percentage of lake currents and most nearshore waves are induced by wind conditions. Winds are responsible for the lake-wide circulation patterns that create the west-to-east ambient currents throughout the Toronto Waterfront. Although prevailing winds are generally from the west, the much longer eastern fetches produce far more wave energy coming from the east. In the eastern sector of the Toronto Waterfront the predominant eastern wave energy is partially balanced by wave energy from the southwest. In contrast, in the western sector the southwest waves provide less energy because of the much shorter fetches to the southwest.
Balances in the wave energy are important because breaking waves are the driving force that moves sediment and other materials along the shore. This littoral transport is the main mechanism that established the Toronto Islands. It also sorted and piled a variety of aggregates in the wave zone and moved historic and recent deltaic sediments to create our beaches. Sediment eroded from the north shore of Lake Ontario was transported into the Toronto Islands because the net wave energy is directed westward. Changes in net wave energy directions, which can be caused by shoreline features, frequently define the boundaries of littoral cells.
Littoral cells are sections of the shoreline defined so that no input or outflow of sediments take place across their boundaries — see image above. They are important shoreline features because actions taken on the shoreline can have consequences anywhere within their littoral cell but seldom effect the shoreline in other cells.
One of the interesting aspects of littoral transport along the Toronto waterfront is that the potential for material to be moved along the shoreline is limited by sediment supply, as shown in the Potential and Actual Sediment Transport maps on this page.
The volume of littoral drift produced through erosion of the shoreline is less than could actually be carried by the available wave energy. For example, between Bluffers Park and East Point the available wave energy could transport 120,000 cubic metres of sand per year but now only 15,000 cubic metres per year, on average, is produced through erosion.
Historically, about 45,000 cubic metres were produced annually, before significant artificial armouring of the shoreline began in the 1970s. In contrast, stonehooking between 1850 and 1910 increased sand supply through higher toe erosion by removing the stones that naturally armoured the lakebed.
Potential Sediment Transport Map
Actual Sediment Transport Map
Daily and seasonal weather conditions, especially solar heating, play a critical role in the ecology of Lake Ontario. The lake waters stratify according to temperature in the summer and winter. The amount and intensity of solar heating defines the scope and extent of this thermal stratification and the subsequent aquatic habitat conditions. Two additional temperature-induced conditions that dramatically affect nearshore habitats are the formation of a thermal bar and hypolimnetic upwellings.
Early in the spring the nearshore waters of the lake heat up and form a band of warm water that is held in place by a thermal bar consisting of colder, denser off-shore water (water is at its maximum density at 4 degrees Celsius; represented by the light blue band on the mid-May diagram).
Thermal Bar Spring Progression (see Figure 1)
The warmer water (shown in yellow and red) builds in depth and concentrates warm water discharges from rivers, creeks and storm drains within the nearshore area. This phenomenon typically lasts until mid June, and surcharges the nearshore area with warm, nutrient-rich water. The early season influx of nutrients has a profound effect on aquatic life by promoting primary production and accelerating the establishment of warm, eutrophic conditions along the shoreline of the oligotrophic Lake Ontario. The thermal bar dissipates into full stratification in the early summer and under the appropriate wind conditions is vulnerable to hypolimnetic upwellings of deep cold lake water.
Thermal Bar Spring Progression
Ontario Mid-lake Temperature Sections
The prevailing north-west winds and the location of the Toronto waterfront on the north-west coast of Lake Ontario make this area vulnerable to the displacement of relatively warm surface water by cold hypolimnetic upwellings. Dramatic temperature changes occur quickly during an upwelling event and can be lethal to fish. Upwellings have the opposite affect of the thermal bar in that they can reduce productivity, limit the growth and survival of aquatic organisms, and disperse offshore the warmer water associated with river discharges and point sources. Alewife, a relatively new species to Lake Ontario, has not adapted to hypolimnetic upwellings and is prone to massive die-off each spring because of the dramatic change in water temperatures.
As the last in the chain of Great Lakes, the amount of water flowing into Lake Ontario, and hence the water levels, are greatly influenced by precipitation and evaporation throughout the Great Lakes Basin. Water level fluctuations, both seasonally and from year to year, are a normal occurrence in the Great Lakes. Over the decades, historical records show that Great Lakes water levels tend to follow an irregular cyclical pattern, as shown below. The pattern of annual fluctuations has been dampened since lake-wide regulation of water levels was introduced in the Great Lakes in 1958.
Fluctuating water levels play an important role in the development and maintenance of diverse shoreline ecosystems. They affect currents, wave action, turbidity, pH, temperature and nutrients. Wetland plants and animals are generally adapted to these changes and in many cases depend on them for certain functions (such as germination of seeds from sediments exposed by low water levels).
The Great Lakes system experienced extremely high water levels in the 1870s, early 1950s, early 1970s, mid-1980s and mid-1990s. Extremely low water levels were experienced in the late 1920s, mid-1930s, mid-1960s, and in the late 1990s leading up to today. The recent decline in water levels is due mostly to evaporation during the warmer-than-usual temperatures of the past three years, a series of mild winters, and below-average snowpack in the Lake Superior basin. It is expected that net water supplies to Lake Ontario in 2003 will be significantly lower than the long-term average
Prior to settlement of the Toronto area, the shoreline was very different from the one we know today. The rivers and creeks supplied clear, cool water and provided habitats for river-spawning fish such as salmon. Nutrient-rich estuaries supported wetlands teeming with wildlife. Sandy spits provided protection from winds and wave action. Sheltered stretches of shoreline were lined with lush stands of emergent vegetation. Much of the nearshore was covered with sand, gravel and stonei.
- Forest Clearing
- Sawmills and Gristmills
- Shoreline Alterations
- Toronto and Region Area of Concern
- Wet Weather Flow Management Master
- Invasive Species
- Dredging Activities
- Shoreline Regeneration Initiatives
Colonization of the Toronto watersheds in the late 1700s and early 1800s resulted in profound changes to physical conditions in the rivers and creeks, which in turn affected waterfront habitats, fish and wildlifeii. These changes began with extensive clearing of the dense forest cover that originally blanketed the uplands. As the forest trees and understory plants were removed, and land contours altered by grading, water and sediment runoff to the creeks and rivers increased, resulting in increased flooding and bank erosion downstream. Estuaries and rivermouth wetlands were choked by excessive inputs of sediments
Sawmills and Gristmills
Numerous sawmills and gristmills were built along the banks of the creeks and rivers. They discharged their wastes directly into the watercourses, resulting in water pollution and siltation of fish spawning grounds. The millponds increased water temperatures, trapped sediments and altered flow regimes. The dams created barriers to fish moving upstream. The native salmon populations that were once plentiful in this area declined rapidly, with the last recorded catch in Toronto Bay occurring in 1874iii.
From 1850-1910, stonehooking — the removal of aggregate materials from the lake bottom for use in construction — was a major force in changing physical conditions and shoreline processes. During this time period, 1 million cubic metres of materials were removed from Toronto Harbour alone — enough to cover the entire waterfront from Etobicoke Creek to the Rouge River with a layer 1 metre thick and extending 25 metres offshore, as shown below. As a consequence of the loss of aggregate materials, large amounts of valuable aquatic habitat disappeared, and the shoreline was exposed to accelerated erosion.
In areas that still have an abundant supply of stone material (eg Northumberland County) it is an important component of the physical structure of the shoreline. The movement of stone material along the shoreline forms bays, points and bars which are critical elements of aquatic habitats, as shown in these two photos.
Other early shoreline alterations included weed removal, filling of wetlands and small streams, hardening of the shoreline, and channelization of watercourses. Starting in the 1790s, aquatic plants were frequently removed from Toronto Bay because they impeded navigation. A map of Toronto Bay in 1813 shows early shoreline modifications in the form of docks, jetties and filling of small creeks.
By 1913, further alterations included navigable channels such as the Western and Eastern Gaps and the Keating Cut. Ashbridge’s Bay at the mouth of the Don River became severely polluted by wastes from the growing Town of York, the Gooderham and Worts Distillery, and associated cattle byres.
Alterations to Toronto Bay, 1813
Alterations to Toronto Bay, 1913
Toronto and Region Area of Concern
By 1987, environmental conditions were so badly impaired that the Toronto waterfront was included on the International Joint Commission’s list of 42 Areas of Concern around the Great Lakes requiring remedial actioniv. The impairments noted for Toronto’s waterfront were:
- Restrictions on fish and wildlife consumption
- Degradation of benthos
- Restrictions on dredging
- Eutrophication and undesirable algae
- Beach closures
- Degradation of aesthetics
- Degradation of fish and wildlife populations
- Loss of fish and wildlife habitat
The key factors contributing to these problems are combined sewer overflows, contaminated stormwater, loss of habitats, and degradation of natural landscapes. In the past 25 years, eutrophication has been reduced, sediment quality has improved, and habitat availability and diversity have been increased, but Toronto remains on the list of Areas of Concern.
Wet Weather Flow Management Master Plan
The City of Toronto’s 2002 Wet Weather Flow Management Master Plan (WWFMMP) provides important direction for ongoing improvementsv. The plan proposes a program totalling $1 billion over the next 25 years, including public education, municipal operations, shoreline management, stream restoration, and control measures at the end-of-pipe, during conveyance, and at the source. The shoreline management proposals include structures at the waterfront, near the mouths of Etobicoke Creek and the Humber River, to deflect ongoing inputs of pollutants away from waterfront beaches. These are proposed because the WWFMMP is limited to the City of Toronto, and there will be continued contributions of bacteria, nutrients and sediments into the watercourses from the “905” municipalities north of the City of Toronto.
Over the next 25 years, implementation of the WWFMMP will improve waterfront aquatic habitats by reducing inputs of nutrients, sediments and chemical pollutants to the watercourses and Lake Ontario. It will also improve habitat conditions in the rivers and creeks, with benefits to aquatic species that migrate upstream from the Lake and estuaries
Invasive species have also been responsible for major alterations in aquatic communities. Since the 1800s, more than 140 exotic aquatic organisms of all types — including plants, fish, algae and mollusks — have become established in the Great Lakes. One of the most dramatic recent invasions has been the zebra mussel, which colonizes rocky substrates and other hard surfaces. Zebra mussels are highly efficient filter feeders, removing substantial amounts of phytoplankton and zooplankton from the food chain. They have also caused significant improvements in water clarity, which in turn is increasing the diversity and productivity of aquatic plants in the nearshore zone.
During the industrial period from 1900-1960, extensive lakefilling transformed the 826 hectare Ashbridge’s Bay wetland complex, most of the central waterfront south of Front Street, portions of the Toronto Islands including the airport, the Leslie Street Spit, Ontario Place and the Western Beaches, as seen below.
In the 1970’s, the Metro Toronto and Region Conservation Authority began to develop a series of lakefill parks along the waterfront (Colonel Sam Smith, Humber Bay, Ashbridge’s Bay, and Bluffers Parks) to provide recreation opportunities for a rapidly increasing urban population.
Industrial Period Alterations to the Toronto Bay, 1900 to
The Toronto Port Authority undertakes dredging in the Keating Channel, Inner Harbour, East Gap, Western Channel, Coatsworth Cut and Ashbridges Bay
Each year, 35,000 to 40,000 cubic metres of sediment settle in the Keating Channel. The material originates from run-off and erosion upstream in the Don River. Annual dredging is undertaken in the channel for flood protection and maintenance of navigable water. The channel is dredged to a depth of 5.8 metres below chart datum and the dredged material is transported by tug and barge to the Toronto Port Authority’s Confined Disposal Facility (CDF) within the Leslie Street Endikement. The project is subject to ongoing environmental monitoring by the Port and Conservation Authorities. The dredging operation is jointly funded by the City of Toronto, the Toronto and Region Conservation Authority (TRCA) and the Toronto Port Authority.
Although the majority of sediment from the Don watershed is captured in the Keating Channel, aerial photographs plainly show a plume of sediment that makes its way further, into the Inner Harbour. When needed, small quantities of material are dredged at berths to maintain the required depth for shipping. Quantities in the order of 3,000 cubic metres are dredged every three to five years. Similar to the Keating Channel dredgate, the material is transported to the Toronto Port Authority’s CDF. In general, Berth Nos. 275, 352 and 353 are dredged to 8.2 metres below chart datum while Berth Nos. 243 and 245 are dredged to 5.8 metres below chart datum.
The East Gap represents a portion of the main shipping channel into Toronto Harbour. Prior to the construction of the Leslie Street Spit, regular dredging of the Gap was required to maintain the navigation depth of 8.2 metres below chart datum. Some sediment continues to intrude into the Gap from western littoral drift and erosion off the Centre Islands. The quantity of this sediment is in the order of 3,500 cubic metres per year. The Toronto Port Authority is currently undertaking a five-year program to remove approximately 60,000 cubic metres of material from the Gap. This material consists of clean sand suitable for open water disposal in accordance with MOE guidelines. The Port Authority has worked with TRCA to use the clean material in Embayment “A” of Tommy Thompson Park to improve aquatic habitat conditions and develop an emergent vegetation wetland area.
Similar to the East Gap, erosion of the shoreline of the Toronto Islands results in transportation of material into the Western Channel and restricts navigation. Preliminary work is currently underway to assess possible alternatives for the disposal of the dredged material. An environmental assessment will be undertaken and dredging will probably commence within the next two years. The current design depth of the channel is 8.2 metres below chart datum, but may be reduced as a result of the environmental assessment.
Maintenance dredging is required in the Coatsworth Cut channel in Ashbridge’s Bay every two or three years. The design depth of the channel is 1.8 metres below chart datum. The Toronto Port Authority has permitted this dredge material to be transported and disposed in the Toronto Port Authority’s Confined Disposal Facility. This dredging is funded by the City and coordinated by TRCA.
Shoreline Regeneration Initiatives
Cultural modifications of the shoreline changed dramatically with the advent of the 1967 Waterfront Plan developed by Metro Toronto. Lakefilling activities were directed away form creating port and industrial lands and focussed on creating a series of regional waterfront recreational parks. The parks provided waterfront access, local greenspace, boating facilities, and — most important to this strategy — aquatic habitats. Following is a summary of the key projects.
Sam Smith Waterfront Park incorporates many successful habitat creation projects, including wetlands, coastal meadows, shoals and reefs.
Humber Bay Park Park is the site of a range of intensive habitat restoration works. They include a Ministry of Natural Resources habitat project that placed extensive amounts of woody debris in a sheltered embayment. Test scale wetlands were established in the estuary of Mimico Creek in 1995 and additional wetlands were created in association with the pedestrian bridge over the Creek. The estuary now provides an excellent opportunity to recreate a coastal wetland estuary complex. As part of the development of the Humber Bay Shores area, habitat islands, beaches and shoals have been strategically built along the east side of Humber Bay Park, including one of the largest wetland creation projects to date.
The Toronto Bay area
was the focus of a study by the Toronto Bay Initiative (A Living Place: opportunities for habitat regeneration in Toronto Bay) that outlines many habitat opportunitiesvi. The wetland project and pike spawning habitat at Spadina Quay is an excellent example of created habitats within the harbour and is a useful design template for larger initiatives. The restoration of the lower Don River and the wetland at the mouth of the Don River is one of the largest proposed restoration schemes for the Toronto Waterfront.
Within the Toronto Islands at the trout pond, a large wetland complex was enhanced and reconnected to the lagoons. This lacustrine marsh provides critical habitat functions for the fish and wildlife community of the islands. Works undertaken in the mid 1990’s on the islands focussed on repairing vertical seawalls with a variety of shoals and riparian improvements. Of particular interest is the wetland shoreline that was created at the Queens City Yacht Club that provides vegetated shorelines and improved public access.
The potential for Tommy Thompson Park to act as an aquatic habitat catalyst for the waterfront is based on the habitat opportunities in the 160 hectares of lagoons and bays associated with the park. The Cell One wetland capping project is the single largest wetland gain to date on the waterfront. Additional wetland creation projects in the Park include Triangle Pond, Embayment A, and Embayment C.
Ashbridge’s Bay and Bluffer’s Parks are the location of two of the very first shoal and reef features within a boat basin on Toronto’s waterfront. Both parks have tremendous potential for additional habitats works.
East of Ashbridge’s Bay, the open coast shoreline is characterized by groynes and headland features. Overall these structures function well as aquatic habitat with the best example being the recent headland structure west of the RC Harris Water Filtration Plant. East of Bluffer’s Park, the Sylvan Avenue project is an outstanding example of integrating aquatic habitats into the form and function of an erosion control project. The Port Union Road shoreline improvement project is another example of the integration of aquatic habitats into a shoreline management structure.
i Whillans, T. Waterfront Ecosystems: Restoring is Remembering. In Roots, B.I, Chant, D.A. and Heidenreich, C.E. 1999. Special Places: the changing ecosystems of the Toronto Region. Royal Canadian Institute.
ii Etobicoke and Mimico Creek Watersheds Task Force. 2002. Greening our Watersheds: Revitalization Strategies for Etobicoke and Mimico Creeks. Toronto and Region Conservation Authority.
iii Whillans, T. Waterfront Ecosystems: Restoring is Remembering. In Roots, B.I, Chant, D.A. and Heidenreich, C.E. 1999. Special Places: the changing ecosystems of the Toronto Region. Royal Canadian Institute.
iv Toronto and Region Remedial Action Plan. 2001. Clean Waters, Healthy Habitats. Progress Report 2001. Waterfront Regeneration Trust.
vCity of Toronto. 2003. Wet Weather Flow Management Master Plan. www.city.toronto.on.ca/wes/techservices/involved/wws/wwfmmp
viKidd, J. 1998. A Living Place: Opportunities for Habitat Regeneration in Toronto Bay. Toronto Bay Initiative.
Phytoplankton and Zooplankton
Phytoplankton and zooplankton provide the principal forage base for many life stages of aquatic organisms. With less eutrophication due to reduced nutrient inputs, plankton productivity has returned to more normal levels in recent years. The degradation of phytoplankton and zooplankton is listed as a potentially impaired use in the Toronto and Region Area of Concern that requires further assessment (studies are planned by TRCA).
image from SOLEC 2000
Suitable conditions for the growth of attached algae include the availability of hard substrates (such as Etobicoke shale), high levels of phosphorus, and the nearshore thermal bar that forms in spring and early summer. Increased water clarity also boosts algae growth in deeper water.
Attached algae form important habitat for benthic invertebrates, which in turn provide a food source for larger invertebrates, fish, migratory shorebirds and aquatic mammals. However, when algae become detached from their substrate, wash up on the shoreline and decay, they create foul odours that become a nuisance to waterfront residents, particularly in the Etobicoke portion of the Toronto waterfront.
The taste and odour problems in Toronto’s water supply are due to free-floating algae that increase rapidly during warm weather, particularly in waters with a high organic content. The cause of the taste and odour impairments is geosmin, a naturally occurring chemical that is created during the metabolisation of the algae as it decays. This problem should be reduced as water quality is improved. In the meantime, Toronto’s water treatment plants have now installed powder activated carbon feed systems to control this problem.
Invertebrates associated with aquatic habitats include a wide range, from tiny plankton to larger insects, mollusks, crayfish and snails. Many of them have two distinct life stages: a larval aquatic one, and an adult one that may be aquatic, aerial or terrestrial. Many larvae and some adults are benthic, or bottom-dwellers, feeding on decaying plant material and bacteria.
The benthic invertebrate communities in depositional areas such as Toronto Bay and the Lower Don River are dominated by pollution-tolerant species such as worms. In other areas, for example away from the influence of the Don River, the densities of pollution-tolerant species are considerably lower. It is expected that implementation of the City of Toronto’s Wet Weather Flow Management Master Plan, which will reduce the loadings of organic-rich sediments from combined sewer overflows and storm sewers, will result in increases in the diversity of benthic invertebrates in the Toronto Bay area.
In contrast, areas with hard, rocky substrates and/or plentiful aquatic macrophytes support more diverse and self-sustaining communities of benthic invertebrates which in turn support communities of fish, amphibians, reptiles, birds and mammals.
Recent bioassay tests show that in many places, sediment quality is now good enough to support sensitive species like Hexagenia (mayflies). The limiting factors include quantity, quality and location of substrates, particularly in depositional areas. Hexagenia larvae create burrows in silty sand, but these are easily collapsed in areas with high silt loadings.
One of the best-known invertebrates associated with aquatic communities is the mosquito. There are several different species of mosquito that occur in the Toronto area. West Nile Virus is primarily transmitted by species of mosquito that breed in sheltered, stagnant water in urban areas. The mosquito species found in natural ecosystems such as wetlands and estuaries tend not to be the ones that carry the virus. In addition, complex wetland ecosystems include predatory fish, birds, frogs and insects that help control mosquito populations. For more information on transmission of West Nile Virus, visit www.trca.on.ca.
Aquatic and Riparian Vegetation
Submerged, Aquatic Plants
Submerged, rooted aquatic plants are extremely important in creating good habitat conditions for fish and other aquatic organisms. Plants slow currents, hold substrate, fix carbon dioxide, support invertebrates and provide shelter for fish and wildlife. A study undertaken by TRCA in summer 2002 demonstrated a significant improvement in the extent and diversity of submerged vegetation in the Toronto Bay area since 1995.:
The image to the right shows the location of Macrophytes in Toronto Bay and Outer Harbour in Summer 2002.
This trend is also apparent in other areas of the waterfront, particularly in the sheltered embayments. Water celery (Vallisneria sp), a good indicator of improved conditions that provides excellent aquatic habitat, is becoming a principal component of the plant community.
Wild celery vallisineria spp
Emergent vegetation grows near the shore in shallow zones, particularly in the estuaries, sheltered embayments of the lakefill parks, and the north shore of the Toronto Islands and its lagoons. Areas of deep water with steep, hard-edged dock walls support very little emergent vegetation.
A limiting factor for the growth of emergent and submerged vegetation in many barren areas of the waterfront may be the presence of carp, which uproot or consume the plants, as well indirectly restrict their growth by stirring up the bottom substrates during feeding, which increases turbidity and reduces the light available for photosynthesis. Other limiting factors, particularly in estuaries and areas near storm outfalls, are inputs of organic pollution, high densities of suspended solids and excessive sedimentation.
Emergent vegetation is also vulnerable to persistent high water levels and to major flooding episodes, such as Hurricane Hazel. In healthy aquatic ecosystems, the vegetation is resilient and regenerates after such natural events, but recovery may be severely impeded in systems that are degraded by such factors as poor water quality, sedimentation and carp.
For example, comparisons of the Rouge Estuary Marsh in 1954 and 1999 (see aerial photographs of Rouge Estuary in 1954 and 1999) illustrate a dramatic loss of emergent vegetation, probably due to a combination of disturbance by carp, high water levels and watershed impacts.
Rouge Marsh and Estuary 1958
Rouge Marsh and Estuary 1999
In contrast, excellent results are being achieved in wetland creation projects on the waterfront such as embayment as in Tommy Thompson Park, which mimics a backwater lagoon and is developing excellent stands of submerged and emergent vegetation. See plan and photograph of TTP embayment A below.
TTP Embayment A
Riparian vegetation occurs at the interface of the land and water. It can be lowland (seasonally/permanently flooded) or upland (on drier ground). See photographs of riparian vegetation.
Riparian vegetation has numerous ecological functions. It can filter pollutants, nutrients and sediments from incoming water; detain flows and evaporate water; provide organic material to watercourses; moderate water temperatures by providing shade; and reduce bank erosion. Pike spawn in suitable lowland areas. Many forms of wildlife such as frogs, turtles, mink, muskrat, coyote, herons and colonial birds also inhabit the riparian zone.
Reptiles and Amphibians
Reptiles and amphibians (herptiles) are some of the most environmentally sensitive species associated with aquatic and terrestrial near-shore habitats. They depend on healthy, functional wetland and shoreline habitats that are typically found in estuaries, coastal marshes, and vegetated sheltered embayments. Unfortunately, there is very little historical data on reptiles and amphibians on Toronto’s waterfront, and their long term population trends are poorly understood.
Over the last several years, scientists, naturalists, and other wildlife watchers have become more concerned about these habitat-dependent herptile species. This has generated numerous amphibian and reptile monitoring programs in attempts to empirically document changes in these populations, and correlate them to environmental conditions.
For example, the TRCA has been participating in the Marsh Monitoring Program (MMP), which was established by Bird Studies Canada and Environment Canada in 1994, and includes a variety of sites across the Toronto waterfront. Herptile abundance and diversity is very low across the Toronto waterfront, which is likely attributed to the physical and biological degradation of waterfront habitats. Populations are primarily restricted to the significant estuary habitats and the remnant coastal marshes.
Monitoring has also shown that these populations have great resilience and quickly respond to improvements in their habitat. The restoration of aquatic habitats, particularly productive emergent marsh habitats, can result in great improvements in coastal herptile communities. The TRCA and other organizations have had great success in improving herptile communities when restoration projects incorporate critical habitat features such as basking/sunning logs, rock piles, hibernacula, isolated ponds, protected nesting sites, deep water over-wintering sites, and vegetated corridors.
There are eight species of amphibians which are commonly found in Lake Ontario, and these include; Northern Leopard Frog, Wood Frog, Green Frog, Bullfrog, Chorus Frog, Spring Peeper, Grey Tree Frog, and American Toad.
The Toronto Waterfront currently supports only three of the eight common species including Northern leopard frog, green frog, and American toad. Two other species, Chorus Frog, and Grey Tree Frog have been listed as probable, but have not been confirmed.
The importance of shorelines and associated aquatic habitats to birds and avian communities is well documented, and has been the subject of considerable study in along the Toronto waterfront. The presence of self-sustaining, diverse, aquatic communities is not only necessary to the bird species that live and reproduce on the waterfront year-round, but are also critical for other birds that forage and migrate through waterfront areas.
The shorelines of large waterbodies like Lake Ontario are documented biological centres of organization which support high diversities of bird species, act as fall-out and staging areas during migration, and provide corridors which facilitate regional movement of species.
The dependence of avian communities on aquatic habitats can be generally categorized into the following groups:
- Dependent on wetland habitat for all stages of lifecycle eg. Virginia rail
- Migrational stopover and staging species eg. Canada warbler
- Seasonally-dependent (eg over-wintering) species eg. Common loon
- Colonial waterbirds eg. Caspian tern
The value of diverse aquatic habitats to bird life on the Toronto waterfront is probably best described through the example of Tommy Thompson Park. Tommy Thompson Park (TTP), also known as the Leslie Street Spit, has been formally designated as an Important Bird Area (IBA) of global significance by Birdlife International. The designation is based on a variety of criteria including:
- Occurrence of breeding populations of colonial waterbirds
- Value of the area for migratory waterfowl
- Value of the area for both migratory and resident songbirds
- Value of the area for migratory owl and raptor species
The existence and persistence of the avian communities associated with TTP can be attributed to the complex of natural and created habitats that exist within the park. In addition, the biological value of these habitats is greatly increased by their location or proximity on the north shoreline of Lake Ontario.
There is no definitive count of the number of avian species which use the Toronto Waterfront, although local naturalist groups, agencies and bird professionals suggest a number of just over 300 species. That is higher than other well known natural areas on the north shore of Lake Ontario, such as Second Marsh (288), and just below other IBAs such as Presquille Point (320).
As an indication of good general health one of the factors to consider is the presence of a well balanced and functioning mammal population. The distribution of mammal species can vary greatly and are usually regulated by several environmental factors. The factors can be grouped into four major categories: weather/ climate, food, other animals and disease, habitat. (Dobbyn,1994). The complex of aquatic, wetland and terrestrial habitats currently found along the waterfront should attract a wide range of mammal species. However, currently waterfront sites support relatively low numbers of mammal species in comparison with less urban sites. Smaller, less mobile species such as the rodents are more likely to remain isolated in small pockets of habitats and are physically unable to disperse do to development barriers, roads, houses etc. The lack of connecting corridors between habitat blocks is one major factor. Larger more mobile species such as coyote and raccoons tend to move more freely through the developed areas and use all types of natural blocks, parks, brown fields and habitat nodes for foraging and habitation. Habitat quality is impacted by invasive species, chemical contamination and urban population influences. Waterfront aquatic and near-shore terrestrial habitats could, through enhancement, provide areas for resident wildlife while connecting corridors between isolated habitats located along the waterfront and those running north south along watershed green space.
Mammals commonly found on the Toronto waterfront include several species of bat, red fox, eastern cottontail, groundhog, eastern grey squirrel, meadow vole, raccoon, opossum, mink, weasel species, striped skunk, red squirrel, eastern chipmunk, shrews, mole, white footed mouse and muskrat. Less common are beaver, coyote and white-tailed deer. (Dobbyn, 1994)
Small mammals perform a notable role in wetland, nearshore and terrestrial ecosystems and are considered keystones to these systems while serving as a food source to larger mammals (eg coyote) and predatory birds (eg owls and hawks). Some of the many small mammals that take advantage of waterfront habitats include mice, muskrats, fox and rabbits.
The relatively small size of most waterfront habitat areas limits their value to large mammals such as deer. The small blocks are most likely used to provide migration routes, temporary cover, and occasional forage.
Beavers living in urbanized areas can disrupt parks and naturalized areas by girdling, cutting or felling trees onto pathways and roads. Dams built by beavers may cause flooding, alter watercourses and have a potential negative effect on fish habitat. They can also damage or kill newly planted trees. Both the beaver and coyote take advantage of large isolated blocks of natural areas such as Tommy Thompson Park and these areas need to be properly managed to strike a balance between natural predator and prey to keep the population of these species at a desirable level.