Rapids clubtail (Gomphus quadricolor) COSEWIC assessment and status report: chapter 5

Habitat

Habitat requirements

Gomphus quadricolor inhabits medium to large streams and rivers. The four Ontario rivers where this species has been recorded have mean annual discharge levels of 2.5 to 31 m³/sec (Figure 5, Table 1). The Mississippi is the largest and shows much more year to year variability than the other rivers. The rivers are typically clear and cool, with gravel and cobble riffles and projecting boulders interspersed with muddy pools (Walker 1958, Cuthrell 2000, Tim Cashatt pers. comm. 2007). The species occurs in some sluggish mud-bottomed rivers in Wisconsin (Wisconsin Odonata Survey 2007). The river channels at the Credit, Mississippi, and Thames River sites are 30 to 50 m wide, while the Humber site is about 20 m wide (Figure 6) (Catling and Brownell 2002, Harris and Foster 2006). The Mississippi River has the clearest water (lowest turbidity and suspended solids) and the lowest biological oxygen demand and chloride concentrations (Table 1). Mean July water temperature of the four rivers is 21 to 23 °C (Table 1).

Figure 5. Mean annual discharge of four Ontario rivers where Gomphus quadricolor has been recorded (data from Water Survey of Canada 2007). Monitoring stations are as follows: Credit River at Erindale, Humber River at Eden Mills, Mississippi River at Appleton, Thames River at Ingersoll.

Figure 5. Mean annual discharge of four Ontariorivers where Gomphus quadricolor has been recorded (data from Water Survey of Canada 2007).

 

Table 1:Water quality attributes of four Ontario rivers where Gomphus quadricolor has been recorded (data from Water Survey of Canada 2007 and Ontario Ministry of Environment 2007)
Ontario Rivers Mean July Water Temp. (ºC) Mean Annual Discharge (m³/s) Surface Water Biological Oxygen Demand (mg/l) Turbidity (Jackson Turbidity Unit) Suspended Solids (mg/l) Chloride (mg/l)
Credit 23.1 8.1 1.8 20.5 38.5 51.2
Humber 21.8 2.5 1.6 20.3 32.9 23.1
Mississippi 23.3 31.4 0.8 2.8 3.0 6.4
Thames 21.6 5.8 2.7 14.0 29.6 61.1

Interspersion of small rapids or riffles with quiet muddy pools is probably important for Gomphus quadricolor. Oviposition occurs over rapids and eggs or young larvae drift downstream to quiet pools, as is a common pattern among gomphids (Walker 1958). At the Humber River, most male Gomphus quadricolor were observed on exposed boulders in gravel and cobble riffles 10 to 30 cm deep. Riffles were interspersed with pools with a soft, clay substrate (Figure 6) (Harris and Foster 2006). Rapid and riffle habitat on the Mississippi River is isolated by long sections of slow-moving river (Catling and Brownell 2002). The historical sites on the Credit and Thames rivers are similar (Harris and Foster 2006) to the Humber site. All known Ohio sites have steep cliff faces on the stream banks (Larry Rosche pers. comm. 2007).

Adult males preferentially use boulders or bedrock protruding from the river as perches from which they make short flights over the riffle, repeatedly returning to the same stone (Walker 1958, Catling and Brownell 2002, Larry Rosche pers. comm. 2007). Shoreline rocks or vegetation may also be used, particularly where mid-stream boulders are absent.

Larvae are typically found in quiet muddy pools downstream from shallow rapids. Patches of Typha and other emergent vegetation may be present (Walker 1958). Larvae probably burrow into the top few centimetres of the bottom sediment, as do most Gomphidae (Corbet 1999). Larvae crawl into dense grass and other emergent vegetation on the riverbank before moulting into adults (Walker 1958).

Forest cover on the riverbank provides cover for tenerals and adult females, which disperse from the river after emerginG. Female Gomphus quadricolor move as much as 800 m inland from the edge of the river in Ohio (Larry Rosche pers. comm. 2007).

The specific locations (all within a km of the location of the G. quadricolor population) and their durations are: South Thames River downstream from Ingersoll, N of County Rd 9 (1975 to 2006); Credit River at Dundas St. W, E of Mississauga Rd., Erindale (1965 to 1995); Humber River at Caledon King Townline, Bolton (1964 to 1988); Mississippi River at dam, downstream of Pakenham (1970 to 2006).

Figure 6. Gomphus quadricolor habitat on the Humber River, June 2005.

Figure 6. Gomphus quadricolor habitat on the HumberRiver, June 2005

Habitat trends

Much of the Canadian and global range of Gomphus quadricolor has been subject to intensive agricultural and urban development since the early 1800s. Such development can alter the aquatic environment by increasing water temperatures, altering stream chemistry, and increasing sedimentation.

The Credit River watershed is one of the most rapidly urbanizing parts of Canada with the vast majority of the forest cover having been cleared (Credit Valley Conservation 2004). The South Thames River subwatershed has only 11% forest cover (Upper Thames River Conservation Authority 2001), while the Humber River watershed has 17% forest cover (Humber Watershed Alliance 2000). In contrast, the Mississippi River watershed is about 70% forested and forest cover has been increasing since the early 1900s as abandoned farmland reverts to forest (Alex Broadbent pers. comm. 2007). Loss of forest cover on the riverbanks has been extensive. Forest cover at the Credit, Thames, and Humber river Gomphus quadricolor sites is mostly discontinuous and less than 50 m wide.

Dams have been in place on many southern Ontario streams since the mid-1800s for recreation, mills, and hydroelectricity generation. Potential impacts on Gomphus quadricolorhabitat include the loss of riffle habitat in reservoirs, higher water temperatures (as groundwater-fed streams are held in reservoirs), and sediment accumulation resulting from the loss of flushing effect from spring freshets. Alteration of floodplain vegetation could impact habitat for adults. All four Ontario rivers are regulated for flood control, but the effects are most evident on the Credit and Humber where spring peak flows have been truncated to minimize downstream flooding.

Another stream-dwelling gomphid, Ophiogomphus rupinsulensis, is commonly associated with Gomphus quadricolor in Ontario. Ophiogomphus rupinsulensiswas observed historically on the Credit River at Erindale, Streetsville, and Meadowvale, and 50 km upstream at The Forks, but none have been recorded on the Credit River since the 1920s. This is suggestive of habitat deterioration because mostOphiogomphusspecies are intolerant of pollution (Bodeet al. 1996), although O. rupinsulensis is probably the most pollution-tolerant member of the genus (P. Brunelle pers. comm. 2007). Also suggestive of habitat deterioration on both the Credit and Humber Rivers is the decline of certain species of mayflies and other aquatic insects that are now limited to only a few short stretches (H. Frania, Royal Ontario Museum, pers. comm. to P.M. Catling, 2006). Over the past few years local residents have reported a substantial decline in volume of both of these rivers.

Water quality in most southern Ontario streams has been altered due to urban and agricultural runoff, which probably has had impacts on Gomphus quadricolor larvae. Chloride concentrations associated with road salting, sewage treatment plant effluent, and other human sources are increasing in the Credit, Humber, and Thames rivers and probably other southern Ontario rivers. Spikes in chloride levels in the Humber River are high enough to impact sensitive aquatic species (Credit Valley Conservation 2004, Todd and Kaltnecker 2004). Phosphorus levels in the Thames and Humber rivers consistently exceed the provincial water quality objective of 30 ug/l, and Ontario rivers are generally declining in water quality (Todd and Kaltnecker 2004, D’Amelio 2007). Nitrate levels in the Thames River routinely exceed the Canadian water quality guideline (2.9 mg/l) for protection of aquatic species (Todd and Kaltnecker 2004, D’Amelio 2007). Dragonfly larvae are sensitive to Biological Oxygen Demand (BOD) levels in excess of 10 mg/l (Corbet 1999). Surface water BOD in excess of this value has been reported only once at water monitoring stations on the four rivers. This was on the Credit River in March 1989.

Figure 7. Hydrographs for rivers (Water Survey of Canada 2007). Dotted lines = Mean +/- 1 standard devation.

Figure 7. Hydrographs for rivers (Water Survey of Canada 2007).Dotted lines = Mean +/- 1 standard devation.

Habitat protection/ownership

Most of the land surrounding the Humber River site is owned by the Toronto Region Conservation Authority (Humber Watershed Alliance 2000). Lands adjoining the Credit River site include a municipal park and the University of Toronto Erindale campus. The Thames River site is surrounded by private land. The Mississippi River sites are surrounded by private land and municipal parks.

Most of the land upstream of the Humber, Credit and Thames sites is in private ownership. The watershed of the Mississippi River is a mixture of private and crown land.

All sites are potentially at risk from urban development, agricultural runoff and water level regulation in the watershed. The Oak Ridges Moraine Conservation Plan provides some protection of water quality and habitat connectivity for parts of the headwaters of the Humber and Credit rivers (MMAH 2007). Conservation authorities at all sites monitor water quality and invasive species and work with municipalities and landowners to protect the river environment.

Page details

Date modified: