Correlations Between a Cyanobacteria Bloom's Decline and Environmental Dynamics.

Abstract

Lake Kainui is an ombrogenous peat lake that is eutrophic and has frequent cyanobacterial blooms. This research aimed to elucidate the factors that correlated with cyanobacterial bloom decline in Lake Kainui. Abiotic and biotic variables in Lake Kainui were monitored weekly across the bloom decline phase. Physicochemical parameters measured included nutrients, micronutrients, water temperature, dissolved oxygen (DO), pH and meteorological variables. Planktonic and particle-associated bacterial community composition (BCC) was assessed through time using molecular fingerprinting (ARISA). Total algal, cyanobacterial, planktonic bacteria, planktonic viruses and planktonic crustacean abundances were determined using microscopy.

The bloom decline coincided with multiple factors including; an increasing ratio of TN:TP, changes in the ratio of mixing depth (z¬mix) to euphotic depth (zeu) and pH. The lake was stratified for several weeks before the decline and the resulting lake stability potentially created an adverse environment for cyanobacterial dominance. Changes in planktonic biota were various. Virus dynamics were positively correlated with phytoplankton dynamics. If virus abundance was negatively correlated to phytoplankton diversity, it would potentially mean that viruses were controlling the phytoplankton assemblage, however virus abundance was positively correlated to phytoplankton diversity, which suggests that viruses had a positive impact on the bloom. The positive effect was most likely due to to viruses upregulating the amount of dissolved organic matter through lysis. Bacterial abundance showed no clear correlation with other variables, but bacterial community composition (BCC) shifted as the bloom declined. Planktonic Cladocera spiked just after the bloom decline. Bosmina was the dominant species of Cladocera, but was probably too small to ingest colonial Microcystis, which suggests that the Bosmina may be consuming degradation products produced by Microcystis. These observations indicate that if planktonic biota played a role in bloom dynamics it was most likely due to shifts in the biota causing shifts in trophic cycling that alternately favour or disfavour blooming.