US Long-Term Ecological Research Network

Harmful algal blooms are a globally pervasive problem, and with a changing climate are expected to be an increasing concern. Because of this rising concern, scientific understanding is becoming ever more important to improve prediction and inform management decisions. Spatio-temporal dynamics of algal blooms remain poorly understood, largely due to multiple drivers, sampling limitations, and spatial variability itself. Observations of near shore blooms or scums can differ widely from pelagic areas of a lake, where most sampling occurs. With the advent of new rapid sampling technologies, sampling regimes can be augmented with rapid fluorescence sampling of algal pigments (chlorophyll and phycocyanin) that can substantially improve spatial and temporal sampling scopes.

To understand the variability of the fluorescence in Lake Mendota, we sampled a transect covering near shore, bay, and pelagic areas for 20 weeks during the open water system using a sonde to measure algal fluorescence. Transect results indicated seasonal differences in near shore, pelagic, and bay areas that should be considered in future sampling efforts (Fig. left). Near shore observations were frequently outliers from the remaining pelagic sampling locations. Additionally, different regions of the pelagic zone experienced overall seasonal differences in fluorescence. Patches of fluorescence fell into two patterns based on transect sampling: patches smaller than 700 meters or large areas of elevated fluorescence covering approximately half of the lake. Horizontal variability, based on modeling results, was correlated with precipitation, lowered water column stability, solar flux, and wind speed and direction (Fig. below).

We found drastically different patterns for chlorophyll and phycocyanin (Fig. left); however, temporal patterns of water clarity and algal biomass better matched phycocyanin measurements, as 74% of the total biomass for 2010 was blue-green algae.

Past studies have concluded that while many drivers can impact timing and intensity of algal blooms, weather played a crucial role in algal dynamics. Epilimnetic fluorescence was modeled using a suite of variables to determine potential drivers of algal blooms. Major drivers of epilimnetic fluorescence were seasonal succession, stability, elevated winds over several days, and calm winds on the sampling day; these results suggest the importance of internal mixing of nutrients and a stable water column for buoyancy regulation of algal bloom development in Lake Mendota (Fig. below).  Modeling efforts indicated that phycocyanin was better predicted than chlorophyll. These results suggest that use of rapid sensor technology may aide in better understanding variability and drivers of algal blooms in a eutrophic system, and perhaps improve our ability to accurately model and predict algal blooms.