Modeling water quality and cyanobacteria blooms in Lake Okeechobee: II. Dynamics of diurnal cycles and impacts of cyanobacteria diel vertical migration

As an important shallow subtropical lake for Florida agriculture and the Everglades, Lake Okeechobee has experienced strong phytoplankton blooms, often dominated by cyanobacteria, over the past several decades. It is well-known that some cyanobacteria, including Microcystis aeruginosa, may migrate vertically over a day-night cycle due to changes of cell and colony buoyancy. Yet, the implications of this migrating behavior to overall bloom dynamics in the lake remain unclear. Using a newly developed coupled physical-biogeochemical model, this study examined the diurnal cycle of cyanobacteria blooms during summertime in the lake, focusing on the interactions between physical processes (e.g., currents, vertical mixing) and diel vertical migration (DVM) of cyanobacteria. Limited field evidence of cyanobacteria DVM, based on in situ phytoplankton biovolume at the surface and the daily optical observations by a Sea Photometer Revision for Incident Surface Measurements (SeaPrism) radiometer, were also presented, which were largely consistent with modeled results. Model results indicate that in the central lake, upward migration of cyanobacteria in the morning allowed cells to concentrate at the surface layer, alleviating light limitation and promoting cell growth. At the same time, prevailing southerly or southeasterly winds moved the cells toward the northern and northwestern lake. At night, strong vertical mixing due to surface cooling and wind mixing re-distributed the cells vertically. Over diurnal cycles, however, vertical processes (DVM, mixing, surface growth) were dominant over the horizontal transport. As a result, surface cyanobacteria biomass were highest in late morning and around noontime but decreased rapidly in the afternoon. Nevertheless, horizontal transport of cells would repeat almost every day from late spring throughout the summer, leading to accumulations of cyanobacteria cells often in long, narrow bands (< 2 km wide) along the northwestern and northern lake. In addition, the seasonality of cyanobacteria blooms appears to be dominated by the seasonal changes of water temperature and winds. This new understanding of cyanobacteria bloom dynamics may have significant implications to field monitoring, bloom forecasting, and water management decisions.