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by Cameron Smith
Six or seven years ago, Grenadier Pond in Toronto’s High Park was sickly. It was choking on algae; its waters were turgid and oxygen depleted; pike could no longer live there, and largemouth bass were barely hanging on.
There were no frogs; bulrushes had given way to mud flats; many bird species had disappeared; at least 25 species of wetland plants had died out; and a thick layer of sediment covered the pond’s bottom, contributing to oxygen deprivation, and partially blocking groundwater from entering.
The story of the pond’s recovering is a chronicle of interdependence, a continuing example of the linking that goes on within nature.
Key to almost everything that was wrong with the pond was the high level of nutrients, especially phosphorus and nitrogen. They encouraged the growth of algae. As it spread across the top of the pond, it shaded lower waters so that other plants couldn’t grow, depriving fish and other aquatic animals of a friendly habitat. And since it was constantly dying and being replaced, there was a steady supply of dead algae sinking to the bottom of the pond and decaying.
The decaying process consumed oxygen, making the pond’s deeper waters eutrophic -- short on oxygen. Only the top three metres could support fish. That made the pond an unfriendly habitat for big fish, because they need deeper water in summer for its coolness, and in winter for a resting habitat.
Without big fish to prey on them, sunfish proliferated. Their main source of food were zooplankton which feed on phytoplankton -- microscopic algae. With so many sunfish, not enough zooplankton were surviving to keep the algae under control. It was a vicious, downward spiral: too much algae being produced and decaying; too few zooplankton to control it.
The phosphorus and nitrogen nutrients came from three main sources: from the droppings of geese and other waterfowl (accounting for 41 per cent); from new and old algae sediment in the pond -- when algae decays in oxygen-deprived water, it releases phosphorus (32 per cent); and from storm sewers carrying runoff from lawns and gardens that had been heavily fertilized (24 per cent).
The city’s strategy aimed at reducing the amount of phosphorus and nitrogen in the pond, and taking the pressure off zooplankton so they could control the algae.
This was begun by making the pond more attractive to pike and bass, and less attractive to geese. And by public education campaigns urging nearby residents not to use so much fertilizer.
Concrete curbs that that stretched along almost half of the 2.9 kilometres of shoreline were removed to create transitional wetlands that would provide habitat for fish. And shrubs and plants were introduced to create barriers to geese.
In addition, water levels were regulated to rise in the spring and drop in the summer. Pike need this, because they spawn on matted grasses at the water’s edge during spring flooding.
With geese droppings reduced, and the pond stocked with pike and largemouth bass that began dining royally on sunfish -- the amount of algae began declining, the water got clearer, and shorelines began to flourish with wetland plants.
But only part of the shoreline has flourished. Not all the concrete curbs have been removed. And at the northern end of the pond, so much silt has been deposited that what had been a wetland sits perched above water. It needs to be dredged, as does loose sediment on the bottom of the pond which hampers groundwater from entering and, when stirred up by winds and waves, decays further, releasing more phosphorus and consuming more oxygen.
These are tough economic times for the city. When children are going homeless and hungry, it’s hard to argue money should be spent on dredging. But Grenadier Pond is one of the brightest jewels in the city’s crown. A place of respite, a cause for celebration -- for everyone. If there’s a private benefactor who wants to create a gift for all future generations, what could be better than completing work on the pond?
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