THE ROLE OF EPHEMERAL STRATIFICATION, ANOXIA, AND ENTRAINMENT IN MEDIATING SPATIOTEMPORAL TROPHIC STATE DYNAMICS IN A LAKE MICHIGAN DROWNED RIVER MOUTH SYSTEM (MONA LAKE, MI)

Abstract

Mona Lake, MI (a drowned river mouth system) has become eutrophic as result of cultural eutrophication. The integrated monitoring effort and subsequent modeling (LAKE2K) reported on here has shifted the management focus to internal phosphorus loads (60 percent of annual load, 90 percent of load during the stratified and anoxic period) as a necessary precursor to trophic state change. Sediment phosphorus release can yield extreme elevations (> 1 mgSRP/L) of bottom water soluble reactive phosphorus (SRP), with blooms of potentially toxic cyanobacteria (largely Microcystis) occurring annually. Such blooms are ascribable to stochastic mixing and phosphorus entrainment to the surface waters, with entrainment forces shown to be significant as a result of this lakes geographic proximity to large fetch events across Lake Michigan. Intrusion events from Lake Michigan are shown to strengthen stratification in Mona Lake, increasing hypolimnetic phosphorus accumulation prior to mixing events. Hypothetical phosphorus reduction strategies applied to the calibrated model indicate treatment of internal loading and a 25 percent reduction in external loading would allow Mona Lake to remain below 20 ug/L total phosphorus (eutrophic threshold).