A conceptual ecological model to aid restoration of Cootes Paradise Marsh, a degraded coastal wetland of Lake Ontario, Canada

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Abstract

An ecological model is derived from recent studies, based on 60 years of empirical observations and experimental data, that conceptualizes how Cootes Paradise Marsh was transformed from a lush emergent marsh with considerable ecological diversity in all trophic levels, to one that is currently turbid, devoid of vegetation, and dominated by a few exotic plant and fish species. This conceptual model contains 17 key components that interact and contribute to the overall unhealthy state of the marsh. The most influential component is high water level which caused the initial loss of emergent vegetation in the 1940s and 1950s. In the absence of plants to attenuate sediment and assimilate nutrients, the marsh became turbid and windswept, and this led to the disappearance of submergent vegetation over the next two decades. Currently, high water turbidity is being maintained by wind re-suspension, high sediment loading from the watershed during the summer, high algal biomass resulting from excessive nutrient loads from sewage effluent and surface runoff, and the feeding and spawning activities of a very large population of common carp ( Cyprinus carpio). Due to vegetation loss, the substrate has become mostly loose sediment that is no longer suitable for the diverse assemblage of aquatic insect larvae that lived on the plants and detrital material in the 1940s. Benthic grazers have been kept in low abundances due to predation by benthivorous carp; consequently, epiphytic algae have proliferated and further contribute to light limitation of macrophytes. High nutrient loadings contribute to high diurnal fluxes in dissolved oxygen levels that tend to select against less tolerant organisms such as insect larvae (other than chironomids) and piscivores (northern pike and largemouth bass). Without piscivores in the marsh, the planktivores have become dominant and have virtually eliminated all of the large herbivorous zooplankton (e.g., Daphnia), except for a few pockets in the marsh inlets close to residual macrophyte beds. Because of the dominance of small-bodied inefficient grazers (rotifers and small cladocerans), algal biomass is high, and the community has a large proportion of heterotrophic forms that tolerate low light environments. This ecological model suggests that the current turbid un-vegetated state of Cootes Paradise may be very stable. It will persist as long as water levels remain unfavorable for natural re-colonization by the emergent flora, and/or water turbidities remain sufficiently high to suppress the growth of submergent vegetation. Using this conceptual model, I developed a model of how Cootes Paradise Marsh may have functioned as a healthy marsh prior to the 1940s, and use these models as a basis to explore a number of restoration and management options and discuss their implications on the aquatic foodweb.

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