68 A new paradigm for the aphanizomenon bloom problem in upper klamath lake

Abstract

Upper Klamath Lake (UKL) is the largest lake in Oregon (area 287 km2, avg. depth 4.2 m). It is naturally eutrophic and regularly suffers nuisance summer blooms of cyanobacteria, principally Aphanizomenon flos‐aquae (AFA). Sediment coring studies show that AFA was absent or minimal until about 1880 when a steady increase began, culminating in the blooms of recent decades. These studies show concomitant increases in sediment N (∼20%) and P (∼50%) along with shifts in the algal flora indicating increased eutrophication. These changes correlate with increased human impacts, such as deforestation, construction, roadbuilding etc., and especially the ditching, diking and draining of adjacent wetlands for conversion to agriculture. Agricultural nutrient runoff, especially P, has been often cited as the cause of the AFA blooms, and most attention has been focused on the dynamics of UKL during the summer bloom. We propose that a more significant factor may be the loss of early‐season suppression of AFA because of the loss of the lake‐associated wetlands, which originally constituted 42% of the lake area, and which have declined in area by 66.3% since the late 1800's. The melting of snow and ice in the spring would flush into the lake a surge of wetland plant decomposition products, most significantly organic acids and humic substances. We propose that formerly these wetland effluents caused a complex of effects on lake pH, solar UV transparency, photochemical interactions, nutrient availability, and Daphnia grazing dynamics, which would have combined to prevent the development of any AFA bloom.