Bentshoe Lake

age in years

age in years

Source: From Harvey (1989)

waterways, until the spring melt occurs. At that time they are flushed into the system in concentrations many times higher than normal. Measurements in some Ontario lakes have shown a reduction in pH values of more than two units in a matter of a few days although decreases of the order of one pH unit are more common (Ontario: Ministry of the Environment 1980; Jeffries 1990). This augmented level of acidity may last for several weeks, and, unfortunately, it often coincides with the beginnings of the annual hatch. The recently hatched fry cannot survive the shock, and fish populations in acidic lakes often have reduced or missing age groups which reflect this high mortality (Baker and Schofield 1985). The aquatic ecosystems of lakes which are normally well-buffered are not immune to major damage if the melt is rapid and highly acidic (Ontario: Ministry of the Environment 1980).

Fish populations in many rivers and lakes in eastern North America, Britain and Scandinavia have declined noticeably in the last two to three decades, as a consequence of the effects of acid rain. Surveys of European and North American lakes have established that 50 per cent of lakes with a pH <5 are likely to contain no fish (Mason 1990). More than 300 lakes in Ontario, Canada, mainly in the vicinity of Sudbury, are fishless and many others have experienced a reduction in the variety of species they contain (Ontario: Ministry of the Environment 1980; Park 1991); several rivers in Nova Scotia, once famous for their Atlantic salmon, are now too acidic to support that fish (Israelson 1987). In the Adirondack Mountains of New York State between 200 and 400 lakes have lost some or all of their fish species (Harvey 1989). Damage to fish populations has occurred in an area of 33,000 sq km in southern Norway, with brown trout particularly hard hit (Baker and Schofield 1985). In nearby Sweden, 100 lakes sampled in the mid-1970s had already lost 43 per cent of their minnow, 32 per cent of their roach, 10 per cent of their arctic char and 14 per cent of their brown trout populations as a result of acidification (Almer et al. 1974), and by the end of the decade it was estimated that some 4,000 lakes in Sweden were fishless (LaBastille 1981). In Britain, no obvious problems emerged until the 1970s, when rivers and lakes in southwest Scotland, the English Lake District and Wales showed the first signs of decline in the numbers of trout, salmon and other game fish (Park 1987). Since then a number of studies have shown that the problem is particularly acute in those streams draining forested catchment areas, which are commonly more acidic. Many of these streams are completely devoid of fish, or support only a few species in their lower reaches, where the acidity is usually less (Elwood and Mulholland 1989).

Waterbodies which have lost, or are in the process of losing their fish populations are often described as 'dead' or 'dying'. This is not strictly correct. All aquatic flora and fauna will decline in number and variety during progressive acidification, but, even at pH 3.5, water boatmen and whirligig beetles survive and multiply (LaBastille 1981), and species of protozoans are found at pH levels as low as 2.0 (Hendrey 1985). Phytoplankton will disappear when pH falls below 5.8 (Almer et al. 1974), but acid-tolerant

Sphagnum mosses will colonize the lake bottoms (Pearce 1982d). Rapid Sphagnum growth has been reported in Scandinavia, but in eastern North America large quantities of green algae are more common. The absence of insect larvae and other organisms which normally graze on the algae may in part explain its abundance (Stokes et al. 1989). Leaves or twigs falling into the water will be slow to decompose, because the bacteria which would normally promote decay have been killed by the acidic conditions (Park 1987). The absence of phytoplankton and the general reduction in organic activity allows greater light penetration, which makes acid lakes unnaturally clear and bluish in colour (LaBastille 1981). This ethereal appearance may suggest death, but even the most acid lakes have some life in them.

Survival Basics

Survival Basics

This is common knowledge that disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

Get My Free Ebook


Post a comment