Spatial structure and temporal dynamics in Clear Lake, CA: The role of wind in promoting and sustaining harmful cyanobacterial blooms.

Clear Lake in Lake County, CA, USA has experienced highly toxic cyanobacterial blooms for more than a decade, with multiple cyanobacterial taxa and cyanotoxins appearing sporadically, typically throughout much of the early-spring to late-fall seasons. Recurring blooms have been attributed to high internal nutrient loads within the lake, with hydrography and hydrology playing important but still poorly documented roles in controlling the availability of growth-limiting elements to the phytoplankton community. The lake is approximately 180 km2 in areal extent and composed of three somewhat disjointed lobes, or ‘Arms’. The large size of the lake presents a formidable task for synoptic lakewide surveys, and for understanding the specific features that stimulate the development and magnitude of harmful blooms. We conducted a study in August of 2020 that involved the use of an autonomous underwater vehicle and deployment of a hand-held water column profiler to describe the lakewide status of various biological, chemical and physical features. Discrete water samples were also collected from ten stations located throughout the lake to produce a near-synoptic depiction of lake status. Additionally, a mechanically-driven, continuously-monitoring water column profiler was deployed at a central lake location to document short-term temporal (minutes to month) changes in water column structure and chemistry. Wind was a dominant feature affecting the lake’s chemistry and biology during the study, resulting in massive concentrations and dramatic spatial heterogeneity of phytoplankton biomass and cyanotoxin in the eastern and southeastern Arms of the lake, and confirmed by the analysis of discrete water samples. Unique insight into the processes leading to or prolonging blooms was revealed by the water column profiler which demonstrated rapid development (within a few hours) of suboxic conditions during periods of calm winds. We speculate that these quiescent periods are fundamental events in the lake’s ecology, resulting in episodic ‘pulses’ of nutrient release from the sediments which can stimulate or refuel blooms of cyanobacteria in the water column.