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Occurrence of Key Members in Lake Michigan’s Microbial Food Web: Contribution to a Changing Ecosystem

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Monday, October 27, 2014

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Hunter Carrick
Central Michigan University

The Lake Michigan ecosystem has undergone unprecedented changes in the open water, whereby the annual, winter-spring phytoplankton bloom is now gone (2-fold decline after 2005), following the invasion of non-native, zebra and quagga mussels. That said, little is known about the impact mussels have on components of the microbial food web, MFW (bacteria, pico-algae, and phototrophic and heterotrophic protists). We measured the abundance of the entire MFW among three lake regions (Lake Superior; northern Lake Michigan, LMN; southern Lake Michigan, LMS) using complimentary epifluorescence and light microscopy analyses. Identical methods had been used to estimate the MFW in 1987 at the southern Lake Michigan station, and these data served as a historic comparison. Size-fractionated chlorophyll-a concentrations was measured to estimate phytoplankton size structure by passing water through series of screens (2-um, 20-um, and whole water). Nearly all the microbial food web (MFW) components in Lake Michigan measured here declined in abundance relative to their levels in 1987, such that the biological community is more similar to that in Lake Superior compared with the historical conditions that once existed in Lake Michigan. The modest reductions in Hpico numbers reflected the resilience of bacteria populations to the changes that have occurred in Lake Michigan over the past 27 years (range 239 to 1.36 x 103 cells • mL-1). The abundance of Ppico at both LMN and LMS declined significantly since the 1980’s, yet the percent contribution of the <2µm fraction increased and now constituted > 50% of pelagic chlorophyll in Lake Michigan. The results seem to indicate an enhanced role for bacteria and pico-size algae in the lake’s food web.

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