[Map,32K,GIF]
In general, dissolved-oxygen (DO) profiles during thermal stratification in lakes in the blast zone were altered from a positive, heterograde curve to a clinograde curve. The new lakes, Castle and Coldwater Lakes, also developed clinograde DO curves during summers 1981 and 1982. The change in curve shape, characterized by the absence of DO in the hypolimnion, may be due in part to chemically reduced materials in the lakes. Deadmans Lake retained a heterograde curve throughout the study. Spirit Lake was virtually devoid of DO into fall 1980, but my May 1981 the lake DO concentrations were 4.2 and 3.2 milligrams per liter near the surface and bottom, respectively.
Water samples taken in June 1980 from Spirit, St. Helens, and Venus Lakes showed increased concentrations of total and dissolved organic and inorganic constituents. As a result of the eruption, dissolved solids in the study lakes increased by 2 to 30 times preeruption concentrations measured during 1971 and 1974 studies. Increases in concentrations of many constituents in Spirit, St. Helens, Venus, and Fawn Lakes were less as the distance from the volcano increased. The water type in the study lakes, assumed to have previously been a calcium-bicarbonate type, shifted to calcium-sulfate, calcium-sulfate-chloride, or mixed type in all of the lakes except Walupt. Castle Lake formed as a calcium-bicarbonate type and remained so during the study; however, Coldwater Lake formed with a mixed water type. The shift of major-ion compositions in the water chemistry of the lakes was probably caused by water-soluble salts of chloride and sulfate condensed on the surfaces of ash particles, as suggested by other investigators.
Preeruptin nutrient levels were naturally small and primary production was probably limited first by the amount of phosphorus available and then by nitrogen. After the eruption, the lakes were enriched to some extent in inorganic phosphorus, which declined by 1981 to about 0.05 milligrams per liter in Spirit Lake and less in the other lakes. The algae probably were limited more by reduced light, increased trace metal concentrations, and bacterial competition than by small nutrient levels. As the lakes recover, nutrient concentrations probably will again be the first limiting factors to algal primary production.
Concentrations of total organic carbon in Spirit Lake increased about 20 times over preeruption concentrations, and doubled in Venus, Fawn, and St. Helens Lakes. The major source of the organic materials is assumed to be from the pyrolization of the coniferous forest that surrounded the volcano prior to eruption. Concentrations of iron and manganese were substantially increased over preeruption levels, particularly in Spirit Lake. The anoxic conditions in the hypolimnia of the blast-zone lakes mobilized high concentrations of iron and manganese.
Because of the lack of preeruption data for the study lakes, changes in the biology that may have occurred as a result of the eruption can only be surmised. It is assumed that little, if any, of the aquatic life present in Spirit Lake before May 18, 1980, survived the effects of the eruption. By October 1980 a dense population of Rhabdoderma lineare (a bluegreen algal species), numbering 15,000 million organisms per cubic meter, had developed in Spirit Lake. This population was not present in such large numbers either in 1981 or 1982.
Of all the study lakes, newly created Castle Lake had the largest phytoplankton biomass (as approximated by chlorophyll a concentrations). Average summer chlorophyll a in Castle Lake was 6.6 micrograms per liter by 1982; average summer concentrations in the other five lakes were less than 4.0 micrograms per liter. On the basis of concentrations of chlorophyll a, all lakes except Castle Lake were oligotrophic; Castle Lake was mesotrophic.
Phytoplankton concentrations in Castle Lake were higher during 1982 than during 1981 and were generally higher than concentrations in the other lakes. Despite the low light transmission in St. Helens Lake, the concentrations of algae during 1981 and 1982, ranging from 110 to 1,090 million organisms per cubic meter, were not substantially different from the other lakes. The phytoplankton communities in the existing blast-zone lakes in late summer 1980 were composed primarily of green and bluegreen algae. During most of summer 1981, the phyto-flagellates were the most numerous forms identified in most of the lakes. Diatoms were relatively unimportant in all the blast-zone lakes, existing and new, until summer 1982.
After the eruption, concentrations of zooplankton in Spirit, St. Helens, and Venus Lakes were more than 100 times less than populations in Walupt Lake and 1,000 times less than those in Fawn Lake. This may indicate some degree of mortality during the eruption in lakes close to the volcano. The total number of zooplankton increased from 7 to about 12,000 organisms per cubic meter in Spirit Lake between May 1981 and August 1982. Of all the lakes, Deadmans Lake had the largest concentrations ranging from about 23,000 to 111,000 organisms per cubic meter, of zooplankton in 1981 and 1982; St. Helens Lake generally had the smallest (less than 70 organisms per cubic meter). By summer 1982, zooplankton concentrations in Coldwater Lake were higher than in Castle Lake. Community compositions within the lakes were variable and many reflect changes due to season or food supply. The zooplankton population in Deadmans Lake was continuously dominated by the rotifer Kellicottia longispina during the study. Rotifers were absent from all the study lakes in the blast zone except Fawn Lake in 1980, but were present in all the lakes by 1981. Most zooplankton species identified in the study lakes may be found in nearly all parts of the world.
The time required for the recovery of the lakes to preeruption conditions depends on the degree of stabilization of the volcano and lake watersheds, on the biological processes within the lakes, and the rates of dilution and water exchange for each lake. Information from this study suggests that, excluding Spirit Lake, St. Helens Lake will recover the slowest and Venus Lake the fastest. An estimate of recovery time for Spirit Lake was not possible because of the extraordinary quantity of debris added to the basin and the threat of future mud and pyroclastic flows into the lake.
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