Ogden's Pondweed (Potamogeton ogdenii) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

According to the Ontario Natural Heritage Information Centre, Ogden’s pondweed has been found at three sites (Appendix 1). The oldest collection is from Hastings County but due to its age (1873) and vague locality data, it is considered to be historic. The other two sites (dated 1974 and 1987) are recent enough to be considered at least potentially extant.

Search effort

Field visits were made in 2005 to the 1974 and 1987 sites. Additional fieldwork was done elsewhere within the presumed range of the species in the area between the two recent sites and central Hastings County. Based on a consideration of the dates of the two recent Ontario collections, and the mid-summer time of fruit production recorded in Hellquist & Mertinooke-Jongkind (2003), fieldwork was conducted in mid-August of 2005. In mid-July of 2006, additional areas were searched in Murphys Point Provincial Park.

The most recent Ontario collection of Ogden’s pondweed was made by K.W. Spicer on 27 June 1987 at Davis Lock at the west end of Sand Lake, which is on the Rideau Canal. The specimen was originally identified as P. strictifolius but was revised to P. ogdenii in 1995 by R.R. Haynes. This site was searched by the author on 11 August 2005. Most of the day was spent paddling in the area described on the herbarium label. The area consists of extensive quiet shallow bays with diverse aquatic vegetation including numerous species of Potamogeton. Several specimens of possible P. ogdenii were collected but were later determined to be mainly P. strictifolius.

The area is probably little changed from 1987 with no recent development and little evidence of disturbance to the aquatic vegetation in the sheltered bays adjacent to the canal. Although the wake from passing boats using the canal could disrupt the aquatic communities, there was little evidence of such disturbance away from the channel. There was considerable Eurasian water-milfoil (Myriophyllum spicatum) in the main channel but little of this invasive species was established in the quiet bays. Although Ogden’s pondweed was not found at Davis Lock in 2005, it should be presumed to still be extant but probably in small numbers that would be easily overlooked amongst several similar species that were common there.

In 1974, Kathy Lindsay, while conducting a life science survey of Murphys Point Park Reserve (later regulated as Murphys Point Provincial Park; Lindsay, 1974), collected Ogden’s pondweed in the shallow water of Black Creek near a beaver dam. This area was searched by the author on 17 August 2005. Most of the day was spent walking nearly the entire length (over two km) of the creek in the park. In the past, the creek within the park has consisted of a series of interconnected beaver ponds that provided much aquatic habitat; however, in the summer of 2005, most of the ponds were drained. In the last few years, beaver ponds have been somewhat erratic in this section of Black Creek (T. Kiesewalter, pers. com., 2006). Since the mid-1990s the beaver populations had been low in the area and the dams had not been maintained (M. Ogilvie, Park Superintendent, pers. com., 2007). In 2005, the area consisted of a narrow, rocky stream (with very little flow), passing through a series of meadow marsh communities. The only habitat suitable for pondweeds was scattered, small, shallow pools within the creek bed. Several Potamogeton collections were taken from these pools but none were ultimately identified as Ogden’s pondweed. Considering the limited habitat and thorough search made by the author, it would seem that Ogden’s pondweed no longer occurs in Black Creek in Murphys Point Provincial Park.

There are, however, areas of diverse aquatics in the protected bays of Big Rideau Lake within and adjacent to the park, especially Hoggs Bay into which Black Creek empties. Some of these areas (including parts of Hoggs Bay) were examined briefly for Ogden’s pondweed on 17 August 2005 but no plants resembling Ogden’s pondweed were found. On 14 July 2006, a more extensive survey of the Hoggs Bay area was undertaken. The author spent most of the day paddling in this large shallow bay and several small nearby bays in Big Rideau Lake. Hoggs Bay supports extensive shallow aquatic zones dominated by a diverse collection of pondweeds. During the survey, eleven species of Potamogeton were noted in Hoggs Bay, including lots of P. zosteriformis. Unfortunately, no plants of Ogden’s pondweed were found. There was considerable Eurasian water-milfoil established in the bays. Despite the additional 2006 survey of Hoggs Bay, much more extensive inventories would be needed to determine whether the species still occurs in the park or nearby. Ogden’s pondweed seems to have disappeared from Black Creek, but it should be presumed to be extant in the park or in nearby wetlands but perhaps in small numbers that would be easily overlooked amongst several similar species that are common in the area.

On 18 August 2005, the author spent the day examining a series of seven wetlands in Hastings County and Lennox & Addington County with similar environmental characteristics to the Murphys Point and Davis Lock sites. The sites visited in Lennox and Addington County are: wetlands along an unnamed stream just southeast of Mitten Lake, adjacent to County Road 41; the northeast end of Little Mellon Lake; and the east end of Haley Lake. In Hastings County, the sites are: Beaver Creek just east of Crowe Lake, adjacent to County Road 33; Beaver Creek adjacent to Beaver Creek Road; Beaver Creek adjacent to Shanick Road, and the Moira River adjacent to Highway 7. One hour or less was spent at each of these locations. Specimens of pondweeds were collected from most of these sites and compared with material at DAO that had been verified by either R.R. Haynes or C.B. Hellquist; however, none of the author’s collections were Ogden’s pondweed.

Given the results of the field study, and examination of Potamogeton specimens at DAO, it would seem that Ogden’s pondweed is very rare in the province. The lack of confirmation in 2005 of the two most recent sites (Murphys Point and Davis Lock) does not mean the species has disappeared from those locations. The difficulty in identifying the species in the field, coupled with the fact that several similar species are quite common within its apparent range, indicates that Ogden’s pondweed could be very easily overlooked if present in small numbers and intermingled with more common species. As with other species of Potamogeton that produce turions, populations of Ogden’s pondweed have been found to vary greatly from year to year at individual sites (Hellquist & Mertinooke-Jongkind, 2003). This yearly variability may have affected the outcome of the 2005 field study.

Abundance

Nothing can be reported on the species’ abundance since no Ogden’s pondweed was found in 2005. None of the Ontario specimen labels give any indication of abundance.

Fluctuations and trends

No information is available on the fluctuations or trends on the species since no Ogden’s pondweed was found in 2005 and none of the Ontario specimen labels give any indication of abundance.

Rescue effect

Considering the endangered or threatened status of Ogden’s pondweed in adjacent states--and its overall rarity in the United States--it is unlikely that Ontario populations could be replenished by propagules from the US.

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