The Great Midwest Flood of 2008 Reflected in Great Lake Michigan Biogeochemistry and Phytoplankton Sequences: Resurgence of Diatoms Provided Beneficial Consequences for a Ravaged Great Lake Food Web

Abstract:

The upper Midwest has borne the marks of extreme climate events frequently during the last 20 years of our time series research on the 58,000 km² Great Lake Michigan. These include two "Godzilla" El Niño events (1997-8, 2015-6); periods of strong drought; a hundred-year Midwest Flood in 2008 (19 inches over 4 days); and an extensive and sustained upwelling in 2015. Each of these isolated events had dramatic and measurable effects on physical hydrodynamics (e.g., transport and thermal structure), plankton, and fisheries biology (e.g., distributional ecology). They are superimposed upon regular mid-latitude (43°N) seasonal progressions from isothermal near-freezing winter to strongly stratified summer conditions. Episodic biological alterations (e.g., invasive mussel establishment) changed optical and heat transfer characteristics in addition to decimating fisheries resources. The extreme climate events and biological invasions play off each other. Here we focus on the Great Flood of 2008. River floodwater injection of phytoplankton themselves, not limiting nutrients, carried organisms out onto pristine waters to bloom. The figure shows satellite-derived chlorophyll (W. Balch, Bigelow) for the 5 Great Lakes in early July for the year prior to the flood (2007) and the same Julian date one month after the flood (2008). Combined with ship-based offshore transects and repeated profiling stations, remote and empirical in situ sensing were corroborative. Rapid offshore movement (ca. 1 km/day) of a surficial lens produced an unseasonably strong 2-month near-surface phytoplankton bloom over deep water that was resistant to mussel grazing, leading to improved recruitment of planktivorous forage fish. Dissolved silicate depletion in surface water, silicate increases in particulate material, and extreme inhibition of Deep Cyanobacterial Maxima showed diatoms to have reappeared after 5 years of paucity. Diatom resurgence occurred at least as far as midlake, over 90 km offshore, confirming the intensity of the event. Sequential transects to further offshore stations validated the wave concept, as surface biomass enrichment occurred further offshore over time. The Flood was one of several extreme climate events punctuating a "Mauna Loa CO₂-like" sequence of seasonal cycles influenced by long-term change.