Long-term changes in water quality
The Wastewater Treatment Facility for the town of Dundas is located at the extreme west end of Cootes Paradise Marsh. Sewage from the facility has been discharged into the marsh since 1919. Until 1962, the sewage only received primary treatment (meaning that only large bits in the effluent were removed). In 1962, the facility was upgraded to a secondary-treatment plant; this process removed some of the nutrients, primarily phosphorus, which is deemed to be the most-limiting nutrient in freshwater ecosystems. Finally in 1978, the wastewater treatment plant became a tertiary-treatment facility. With tertiary treatment, most of the phosphorus is removed from the effluent before it is discharged into the marsh. In 1987, another improvement was implemented that removed sediment from the effluent prior to release.
Consequently, the total loading of phosphorus from sewage had dropped almost 5-fold between the 1970s and the 1990s, but despite this drop in the amount of primary nutrient, there was no corresponding drop in algal biomass. If phosphorus were the primary factor controlling the growth of algae, we should have seen a direct response. On the contrary, what we saw was a continued deterioration in water quality over those years.
There was also a notable drop in biodiversity in Cootes Paradise Marsh over those years, especially with respect to the submergent and emergent plant, fish, aquatic insects and zooplankton communities, indicating that Cootes Paradise Marsh had been in declining ecological health.
When a marsh ecosystem is healthy, the water is usually very clear. The amount of algae in the water is usually minimal because most of the nutrients in the ecosystem are tied up in the aquatic macrophytes such as the cattails and pondweeds. The aquatic plants are so diverse and abundant that they usually block out the light that is necessary for algae to grow. By contrast, in a degraded marsh, algae dominate and block out light that is necessary for the aquatic plants to grow. The algae may be in the form of “phytolankton” (floating in the water) or “epiphyton” (growing on top of plant leaves, stems, etc.).
Scientists measure the clarity of the water in a number of ways, and a common method is to quantify the turbidity in the water. The higher the turbidity, the more murky the water. As a reference, most healthy marshes of Lake Ontario, water turbidity rarely exceed 5-10 NTU units, except after storm events. In the table below, you can see the large difference in water turbidity between the vegetated and open-water areas during the 1948 survey. This comparison also indicates that both the open-water and vegetated areas continued to get more turbid over the next 50 years.
Submergent aquatic plants
A diverse community of submergent plants provides critical spawning and nursery habitat for warm-water fish such as largemouth bass and northern pike, two species that used to be abundant in the marsh and harbour.
When water in the marsh becomes turbid, there is usually insufficient light to support a diversity of submergent plants such as pondweed and waterweed. A plant survey conducted during 1946 noted 16 species of vascular plants, compared with a survey 50 years later that noted only 1 pollution-tolerant species:
- In 1946
- 16 species were common: Ceratophylum, Utricularia, Myriophylum and Potamogeton
- In 1995
- 1 species primarily, Potamogeton pectinatus, accounting for only 1% of the aquatic vegetative cover.
Emergent aquatic plants
The health of the emergent plant community is not necessarily dependent on water clarity, although nutrient enrichment (also called eutrophication) could still affect the kind of species found there. According to an archealogical study, wild rice (Zizania aquatica) was one of the dominant wetland grasses present in Cootes Paradise Marsh 1000 years ago, when the marsh was presumably deeper, because wild rice is an annual that grows in relatively deep water, where the submersed juvenile stage requires good light penetration.
Supporting evidence that the marsh had been deeper is that ample pollen from floating species such as white and yellow water lilies (Nuphar and Nymphaea) had also been found there during that time, and these are usually found in deeper waters. Wild rice was eventually replaced by two other species of emergent plants, cattail and burreed (Typha latifolia, Sparganium sp. ) and this occurred at roughly the time of European settlement, presumably because deforestation in the watershed accelerated sediment accretion (accumulation) and nutrient enrichment in the marsh. This proposed succession is consistent with observations that cattail and burreed require shallower water and greater organic content. In particular, the dominance ofTypha latifolia is predictable because of its competitive superiority in shallow water and in relatively fertile soils.
It is important to note, however, that even by the late 18th century, wild rice had still been part of the pollen record, and according to accepted folk lore, aquatic plants, including wild rice, used to line the main channel in the marsh during the 1800s and prevented boat from accessing the growing town of Dundas. Therefore, loss of plant diversity in the marsh accelerated in the 19th century, but was not severe until almost the 20th century when the surrounding watershed had become heavily urbanized.
Compared with information regarding the submergent plant community, there is much more available for the emergent plant community. Close to a dozen plant surveys had been conducted by scientists from McMaster University and the Royal Botanical Gardens from 1934 to 1999. A comparison of emergent species found in the 1946 survey with that found in the 1995 survey reveal a similar decline in biodiversity:
- In 1946
- 22 species accounting for 46% vegetative cover, dominated by Typha latifolia and Glyceria maxima and Sparganium and Sagittaria were very common and dense in areas.
- In 1995
- 8 species accounting for 10%, primarily Typha Xglauca and Glyceria maxima; exotic species such as Phragmites and Lythrum are now well established.
Natives displaced by Exotics
During the early plant surveys (prior to 1970s), the dominant cattail species was T. latifolia, the species native to N. America, and which is known to be relatively intolerant of deep water. About 3 decades ago, T. angustifolia also became established, a native of Europe which is more tolerant of deep water. Currently, both species as well as the putative hybrid, T. Xglauca can be found in Cootes Paradise, and this species has been known to tolerate fluctuating water levels.
Tracking the decline of emergent cover
According to the literature, 76% of the area cover for emergent vegetation present in the 1800s had disappeared by the mid-1970s. We have evidence from aerial photos and plant surveys that this decline began shortly after 1934 when there was still about 85% cover.
By assembling all available historic information from 1934 to 1993, we can see there is a statistically significant inverse relationship between percent cover of emergent vegetation and water level in the marsh. This relationship was able to predict quite accurately the amount of emergent vegetation that would be present in 1999 based on the given water level (shown as an open circle in the graph below).
The marsh ecosystem is a very dynamic environment that can support a very large foodweb, consisting of different primary producers such as the emergent, floating and submergent plants mentioned earlier, as well as many different animals that spend their lives inside the sediment, at the water-sediment interface, or in the water column among the plants. The type of animal community that is associated with a healthy marsh tends to be very different from that associated with a degraded marsh. That’s because a healthy marsh has abundant macrophyte, and these are used by certain juvenile and sub-adult stages of aquatic insects both for food and for protection from predators (i.e. fish and other invertebrate predators). Many insects are intolerant of low oxygen content or excessive nutrients, while others are very tolerant.
In 1948, scientists found a very diverse community of larval insects that included 57 genera, belonging to 23 families and 6 orders (trichopterans, dipterans, ephemeropterans, odonates, etc.)
By 1976, there were only 9 genera, belonging to 6 families and 3 orders (ephemeropterans,dipterans, hemipterans).
In the most recent survey in 1995, there were only 5 genera, belonging to 3 families and 2 orders (dipterans and odonates).
Change in trophic structure: fish community
Fish can be classified according to their “trophic” characteristic, that is the type of food that they eat: piscivores are those that eat other fish, planktivores are those that eat zooplankton (ie. water fleas) primarily, benthivores are those that feed on animals in the sediment or at the water-sediment interface. Omnivores are those that eat both plant and animal diet items.
There were several obvious changes in the trophic strucutre of the fish community in Cootes Paradise Marsh over the 6 decades, as the marsh became degraded. During the 1940s, there were 35 species of fish species that included piscivores (e.g. northern pike, largemouth bass), planktivores (e.g. alewife, gizzard shad) as well as benthivores (e.g. common carp, white suckers). Although the common carp were present in the marsh at that time, they did not dominate and were restricted to areas of the marsh where there was low dissolved oxygen concentration.
In the 1995 survey, the total number of species dropped slightly to 31, but the fish commmunity was dominated by common carp, which accounted for most of the fish biomass. The other fish that were abundant were planktivores, including alewife, white perch and pumpkinseed. Piscivores such as largemouth bass and northern pike were very rare.
Change in size structure: zooplankton
The plants are known as the primary producers of the marsh, and the name given to the trophic level that feeds directly on the primary producers is “herbivores”. The primary herbivores of Cootes Paradise Marsh are the zooplankton, that includes species commonly called water fleas (Daphnia ). These water fleas tend to be relatively large, and when they are present, these efficient grazers of the system can keep the water cleared of algae.
During the 1949 survey, they were found in moderate numbers in the marsh, but in the three subsequent surveys (1970s and 1990s), Daphnia virtually disappeared, being found only in areas where there were still some aquatic macrophytes. When they were present in samples, they had large “helmets” on their heads, indicating that there were many planktivores around (water fleas grow these helmets to make it more difficult for fish to eat them). In most of the other areas in the marsh, the much smaller species, Bosmina , as well as rotifers dominated.