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What’s in the Water at Cootes Paradise Marsh

April 23, 2021

By Tys Theysmeyer, Head of Natural Lands, Royal Botanical Gardens.

What’s in the water? The answer is an ever-changing array of things. That said, most visible for the spring of 2021 will be decaying blobs of green algae. Shallower water in Lake Ontario is changing up the nature of algae growth as we move into warmer weather. Driving this growth is the fact that Cootes Paradise Marsh is extremely overfertilized, and as a result the ecology and plant community are out of balance, with the most successful plant group being algae species.

Remediating the sources of this excessive supply of nutrients has been a primary goal of the Hamilton Harbour Remedial Action Plan and each area resident has a role to play to keep “plant food” out of the system, as right now, things are too enriched (the scientific term for water in this condition is hypereutrophic).

Cootes Paradise is a large marsh with areas of clean water and areas that are challenged by human pressure. The most challenged areas are the bay at the mouth of Chedoke Creek (Princess Point Bay) and, secondarily, the very west end in West Pond, downstream of the Dundas Wastewater Plant. Nutrient supplies in these areas are several times what the natural balance can withstand. Waters from West Pond then flow downstream through much of Cootes Paradise before exiting to Burlington Bay.  In contrast, the waters in the southwest interior bay of the marsh and associated floodplain ponds are quite clean and clear.

Phosphorus is the nutrient that drives plant growth. In nature it is a rare element, and it is now used quite sparingly in agricultural practices, so its main sources are sewer overflows, parking lot runoff, sewer cross-connections (where a toilet is hooked into an overflow drain), and even treated sewage, which is periodically deposited in substantial amounts.

Water depth and temperature in nutrient-enriched water affects which groups of algae grow. This year, below-average water levels have changed up the algae species to those that grow on the bottom. Virtually all of Cootes Paradise is currently less than 60 cm (2’) deep. This means sunlight can now penetrate through the water column and the population of suspended single-cell microscopic algae (phytoplankton), and reach all the way to the bottom. The algae overgrowth smothers the other plants (e.g. pondweeds and waterlilies) and as they decay, they become a source of new nutrients that fuel the growth of the dominant algae species. The shallower water is also easily warmed, which accelerates this process. In deeper water, the sunlight can’t fully penetrate the water column’s single-celled algae. In this scenario, the algae grows so dense that it gives the water a “pea soup” colour and shades the bottom.

Filamentous green algae blobs from the bottom
Filamentous green algae blobs from the bottom, Cootes Paradise
Green water
Single-celled algae (phytoplankton) in the water column

As growth of algae progresses under shallow water conditions, algae growing on the bottom of the marsh eventually tears off into chunks that float to the surface, buoyed by gas bubbles derived from decomposition. The visible result is shorelines littered with rotting bits of algae that have washed ashore. The decomposition process also robs the water of some of its oxygen and what usually follows are fish kills of various severity. The success of invasive fish like Common Carp and Goldfish is tied to their ability to survive during these periods of very low oxygen in the water. On a more positive note, the growth of algae is a sign of life and the marsh attempting to heal itself again and grow a new plant community.

Where does water go after a big rain fall? RBG Ecologist Jenn Bowman teaches us about rainfall and runoff and how rain gardens can help with water absorption.

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