Doughnut in the desert: Late-winter production pulse in southern Lake Michigan

Contrary to expectations of fairly uniform and unstratified waters, sea-viewing wide field-of-view sensor (SeaWiFS) and moderate-resolution imaging spectroradiometer (MODIS) imagery revealed a spatially complex chlorophyll a pattern, termed the "doughnut," in southern Lake Michigan during March to April. Phosphorus-rich coastal waters and sediments are entrained along gyre convergence zones and moved into deeper waters, stimulating a ring of production. Cross-lake surveys (April 2001 and April 2006) with two separate profiling instruments (vertical Seabird CTD casts, horizontal Acrobat tows) uncovered columnar patterns for Chl a, temperature, and other variables (colored dissolved organic matter, transmissivity) consistent with a spatially complex, rotating gyre structure. Optical plankton counter transects, plankton net tows, and sonar documented that the spatial heterogeneity extends to higher levels of food webs (zooplankton). The horizontal and vertical patterns suggest a previously unrecognized coupling between late-winter storm-induced gyre formation, coastal water plus sediment capture, and deep-water productivity. The pulse may explain how certain zooplankton species characteristic of the Great Lakes can successfully overwinter through what was previously perceived as a very unproductive and resource-stressful period. The magnitude of the winter pulse may be linked to climate change, as higher temperatures and more frequent winter storms suppress coastal ice formation and encourage movement of nutrient-enriched waters and sediments into deeper waters.