Survey data from 2931 Ontario lakes were analyzed to determine how fish species richness was empirically related to a set of 19 physical and chemical limnological variables. Lake area was the dominant factor, explaining 18% of the variation in species number. Total aluminum, latitude, dissolved organic carbon, and elevation together explained an additional 16%. The strong relationship between species number and lake area was quantified using lakes with pH [Formula: see text] and then applied to ail the surveyed lakes to estimate species number. Deviations from the expected values indicated that species number decreased with decreasing pH below 6.0, resulting in significantly fewer species in lakes with pH < 6.0. In the subset of lakes with pH < 6.0, pH alone explained 21% of the variation in species number; elevation, lake area, and dissolved organic carbon together explained an additional 20%. Interactions between pH and lake area were identified; lake size decreased significantly both at low pH (<6.8) and at high pH [Formula: see text]. An understanding of these interactions was essential in explaining the relative roles of pH and lake area in determining species richness.

Concentrations of mercury in dorsal muscle tissue of lake trout (Salvelinus namaycush) from Ontario lakes were positively correlated with variables indicating lake dystrophy (dissolved organic carbon, colour, iron, transparency) and were also correlated with watershed area and lake area. Stepwise multiple regression selected dissolved organic carbon as the only variable which explained a significant amount of variation (37%) in mercury concentrations in lake trout. The relationship between dissolved organic carbon and mercury appeared to be strongest in the group of lakes with values of dissolved organic carbon less than 4.0 mg∙L−1. In contrast, mercury concentrations in smallmouth bass (Micropterus dolomieui) were correlated with variables reflecting both water hardness (magnesium, calcium, conductivity) and acidity (pH, alkalinity). The relationship was inverse for the water hardness variables and positive for acidity. Stepwise regression identified three variables significant in explaining variation in mercury in smallmouth bass: calcium, dissolved organic carbon, and latitude. Mechanisms that may explain the effects of organic matter, water hardness, and acidity on mercury accumulation by fish are discussed.

Capture of muskellunge by angling resulted in a reduction of blood pH, elevated lactic acid concentrations, and a drop in total carbon dioxide and bicarbonate concentrations. The acidaemia was most severe immediately after capture and began to decline well before the blood lactate levels rose. Blood lactate levels were not as high as those characterizing fatigue in most other species. Recovery from the acidosis required 12 to 18 h and was accompanied by declines of 22% and 4Ou/o in haemoglobin and haematocrit levels respectively. With the exception of dying fish, there were only slight fluctuations in plasma sodium and potassium levels during recovery, indicating that there was no severe ionoregulatory dysfunction.Thirty per cent of all angled muskellunge died. The last stages immediately preceding death were characterized by declining blood pH and elevated potassium levels.

Bowland Lake, an acidified lake (pH 4.8–5.2), was treated with calcite (CaCO3) in 1983. Neutralization allowed for successful reproduction by reintroduced lake trout (Salvelinus namaycush). Mortality of lake trout embryos and juveniles in field toxicity tests decreased from 52–99% preneutralization to 0–30% postneutralization. The resident yellow perch (Perca flavescens) appeared unaffected by the chemical treatment. Both inter- and intra-specific competition were evident in the growth and body condition of perch and stocked lake trout in the years after neutralization. Springtime acid episodes continued to occur in the nearshore areas after the lake was neutralized, but no adverse effects on fish species were detected.

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