Microcystin distribution in physical size class separations of natural plankton communities

Graham, J.L. and J.R. Jones. 2007. Microcystin distribution in physical size class separations of natural plankton communities. Lake and Reserv. Manage. 23:161-168. Phytoplankton communities in 30 northern Missouri and Iowa lakes were physically separated into 5 size classes (>100 μm, 53-100 μm, 35-53 μm, 10-35 μm, 1-10 μm) during 15-21 August 2004 to determine the distribution of microcystin (MC) in size fractionated lake samples and assess how net collections influence estimates of MC concentration. MC was detected in whole water (total) from 83% of lakes sampled, and total MC values ranged from 0.1-7.0 μg/L (mean = 0.8 μg/L). On average, MC in the >100 μm size class comprised ~40% of total MC, while other individual size classes contributed 9-20% to total MC. MC values decreased with size class and were significantly greater in the >100 μm size class (mean = 0.5 μg/L) than the 35-53 μm (mean = 0.1 μg/L), 10-35 μm (mean = 0.0 μg/L), and 1-10 μm (mean = 0.0 μg/L) size classes (p < 0.01). MC values in nets with 100-μm, 53-μm, 35-μm, and 10-μm mesh were cumulatively summed to simulate the potential bias of measuring MC with various size plankton nets. On average, a 100-μm net underestimated total MC by 51%, compared to 37% for a 53-μm net, 28% for a 35μm net, and 17% for a 10-μm net. While plankton nets consistently underestimated total MC, concentration of algae with net sieves allowed detection of MC at low levels (≤0.01 μg/L); 93% of lakes had detectable levels of MC in concentrated samples. Thus, small mesh plankton nets are an option for documenting MC occurrence, but whole water samples should be collected to characterize total MC concentrations.