Microcystins, nutrient dynamics, and other environmental factors during blooms of non-microcystin-producing Aphanizomenon flos-aquae in Upper Klamath Lake, Oregon, 2009

Abstract

Seasonal phytoplankton blooms dominated by cyanobacteria in Upper Klamath Lake, Oregon, produce toxic microcystins at concentrations that may be detrimental to local wildlife. In 2009, water column, sediment trap, and surficial sediment samples were collected to relate the occurrences of microcystins in these samples to other environmental variables. Microcystin concentrations in sediment trap and surficial sediment samples were lower than in water column samples, and sediment trap samples contained higher concentrations than surficial sediment samples, with a peak in one sample (1107 μg/g) that exceeded the maximum concentration measured in any other water or sediment sample from this area. Concentrations of microcystins and cells of Microcystis aeruginosa increased following the decline of the first bloom dominated by non-microcystin-producing Aphanizomenon flos-aquae in response to an increase in nitrogen and phosphorus concentrations. Results of this study also show that nitrogen fixation by A. flos-aquae during spring and early summer provided nitrogen for growth of toxigenic M. aeruginosa and that phosphorus availability may have played a greater role overall in determining the pattern of microcystin occurrence in the lake, given the apparent influence of phosphorus on growth and decline of the A. flos-aquae bloom. This study is the first report of microcystins in sediments of Upper Klamath Lake, the largest lake (by surface area) in Oregon, and contributes to understanding the conditions under which elevated microcystin concentrations occur. [Supplementary materials are available for this article. Go to the publisher's online edition of Lake and Reservoir Management for the following free supplemental resources: microscope images of major phytoplankton species, microcystin concentrations in surficial sediment samples from randomly selected sites, loadings of the first 2 principal components from PCA analysis of microcystin and water quality data, and the medians and ranges of measured limnological variables.]