The Massive Upwelling of 2015 Reflected in Great Lake Michigan Biogeochemistry and Phytoplankton Sequences: Resurgence of Diatoms Provided Beneficial Consequences for a Ravaged Great Lake Food Web

Abstract:

As an Inland Sea, the Laurentian Great Lake Michigan is large enough (58,000 km²) to have oceanographic-scale circulation. Because it spans 4° of temperate latitude and is subject to dominant westerly winds, both seasonality (northern cooling) and Eckman transport result in a persistent southerly current along the western basin. Most summers short bursts of southerly wind initiate brief (days) very nearshore upwelling well-known to beach and diving recreationalists (last map on top panel for 2012; NOAA-GLERL). In 2015, more consistent southerlies were associated with a massive upwelling event (Figure, bottom panels) that spread eastward to encompass nearly 25% of western Lake Michigan (15,000 km²) during September-October. This event had dramatic and measurable effects on physical hydrodynamics (e.g., transport and thermal structure), plankton, and fisheries biology (e.g., distributional ecology). Episodic biological alterations (e.g., invasive mussel establishment) have created new “normal” optical and heat transfer characteristics in addition to decimating fisheries resources. These were offset briefly by this phenomenon. The extent of upwelling (green) was greater than any recorded since satellite sensing began in 1975. Ship-based offshore transects and repeated profiling stations confirmed links between remote and empirical in situ sensing. Unusually high concentrations of ammonium in surface waters (up to 2µM), derived from excretion by prolific benthic mussels, contrasted both with earlier expeditions and previous years. Circulation-induced upwelling of coastal bottom water thus produced an unseasonably strong month-long near-surface phytoplankton bloom over deep water, leading to improved recruitment of planktivorous forage fish. Dissolved silicate depletion in surface water, silicate increases in particulate material, and biogeochemical signals of bottom water showed that this event, differing in mechanism but leading to the same outcome as the Great Flood of 2008, could produce again the diatom productivity desired by the middle food web. The Upwelling was one of several extreme climate events punctuating a "Mauna Loa CO₂-like" sequence of seasonal cycles influenced by long-term change identified in our 20-year time series.