Hill's pondweed (Potamogeton hillii) COSEWIC assessment and status report: chapter 5

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Habitat

Habitat requirements

Hill’s pondweed is found in cold, clear, slow-moving, calcareous streams, ditches, and ponds with a muddy substrate. Rarely is it in turbid or polluted waters, in open lakes (Hellquist, 1984), or fast moving streams (personal observation by the report writer). It is often found on the upstream side of road culverts, among stumps and fallen trees, or in shallow water among rushes and sedges (Hellquist, 1984). It was typically observed during field visits in shallow channels in open marshes dominated by grasses or sedges (most often Phalaris arundinacea) (Figure 4). Hill’s pondweed is associated with P. foliosus, P. natans, P. pusillus, P. amplifolius and P. gramineus (Hellquist, 1984). It occurs in successional communities, and does not appear to persist in one locality over a long period (Mitchell and Sheviak, 1981 in Brownell, 1986).

Hill’s pondweed is most closely associated with high alkalinity. It has been found in water with 53.0 to 316.7 mg/l HCO₃^- (Hellquist, 1980, 1984). About 79 % of known Hill’s pondweed locations coincide with dolomitic limestone (Hellquist, 1984).

Trends

The parameters associated with the species’ habitat have only recently become better understood, which makes it difficult to determine habitat trends. While it is known that Hill’s pondweed occurs in calcareous alkaline waters, no information was found regarding changes in the number or quality of calcareous streams or wetlands in Ontario. Almost two-thirds of southern Ontario’s wetlands have been lost, though this has had less of an impact on the Bruce Peninsula where most Hill’s pondweed sites are found (Snell, 1988).

Protection/ownership

Five of the 22 extant sites reported in the Ontario Natural Heritage Information Centre (NHIC) database are on protected lands (national park) or in areas designated as comprising significant natural areas: two sites are in Bruce Peninsula National Park, and three in Areas of Natural and Scientific Interest (ANSI) at Cabot Head, Scott Point, and Caledon Lake Forest. Designation as an ANSI, however, does not confer protection unless the property is included under specific protective legislation. One other site is on Saugeen First Nations’ Territories. All other populations are believed to be on private lands (NHIC database).