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Sago Pondweed (Potamogeton pectinatus L.):


A Literature Review

Habitat--Wetland area and fetch


Wetland Area and Fetch

When water depths decrease to 0.5 wavelengths, waves begin to feel the bottom and release energy, and larger, more damaging waves are caused by increased wind and fetch (Bird 1968). The depth of the wave-mixed zone is an important factor that determines the depth of water colonizable by macrophytes. The extent of this zone depends on shore slope and aspect in relation to prevailing winds and lake size (Spence 1982). In shallow wetlands, the wave-mixed zone can largely coincide with the macrophyte zone, but if fetches are large, the wave-mixed zone deepens and the effects of wave action on sediment-induced turbidity can prevent growth of sago and other submersed macrophytes (Chamberlain 1948; Spence 1982) or wash plants out of fine-textured bottoms (Carter et al. 1985a).

The data of Anderson and Jones (1976) show that the macrophyte zone in shallow lakes larger than about 500 ha normally occupies < 25% of the lake area. Anderson (1978) was able to predict, with a combination of four maximum tolerance values, the presence or absence of sago in 136 of 140 sample sites in a large Manitoba wetland. No sites with a maximum fetch of 2,300 m, north fetch > 600 m, northwest fetch > 500 m, and water depth > 120 cm supported sago. Lake Ichkeul in Tunisia (90 km2) supports about 20 km2 of sago (Skinner and Smart 1984), but the plant is absent or sparse in the central area and along several long stretches of shoreline, possibly because of excessive fetch. Kantrud (1986a) found a similar absence of sago in the central area of maximum fetch in a large (1,132 ha), shallow North Dakota wetland. Ongoing studies of Butler and Hanson (1985, 1986, 1988, unpublished) show that in a large (1,598 ha), shallow (mostly < 4 m) Minnesota lake, concentrations of suspended solids are much greater (40-100 ppm) in central areas of greater fetch than in deeper, more sheltered sites (10-50 ppm). They used the formulas of Carper and Bachman (1984) to show that even moderate winds could expose large areas of the bottom to wind energy sufficient to cause sediment resuspension and increased turbidity. On large lakes, littoral zones can be barren due to wave action, with sago and other macrophytes restricted to river inlets (Sheffer and Robinson 1939). In other large lakes, wave action can restrict sago beds to deeper (> 3-4 m) sites where plants grow in sparse clumps (Hill et al. 1975). Pip (1987) observed no significant differences in frequency of sago in wetlands larger or smaller than 10 ha in area.


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