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Abstract

We chronicled the seasonally recurring hypolimnetic hypoxia in Muskegon Lake - a Great Lakes estuary over 3 years, and examined its causes and consequences. Muskegon Lake is a mesotrophic drowned river mouth that drains Michigan's 2nd largest watershed into Lake Michigan. A buoy observatory tracked ecosystem changes in the Muskegon Lake Area of Concern (AOC), gathering vital time-series data on the lake's water quality from early summer through late fall from 2011 to 2013 (www.gvsu.edu/buoy). Observatory-based measurements of dissolved oxygen (DO) tracked the gradual development, intensification and breakdown of hypoxia (mild hypoxia <4 mg DO/L, and severe hypoxia <2 mg DO/L) below the similar to 6 m thermocline in the lake, occurring in synchrony with changes in temperature and phytoplankton biomass in the water column during July-October. Time-series data suggest that proximal causes of the observed seasonal hypolimnetic DO dynamics are stratified summer water-column, reduced wind-driven mixing, longer summer residence time, episodic intrusions of cold DO-rich nearshore lake Michigan water, nutrient run off from watershed, and phytoplankton blooms. Additional basin-wide water-column profiling (2011-2012) and ship-based seasonal surveys (2003-2013) confirmed that bottom water hypoxia is an annually recurring lake-wide condition. Volumetric hypolimnetic oxygen demand was high (0.07-0.15 m, DO/Liter/day) and comparable to other temperate eutrophic lakes. Over 3 years of intense monitoring, similar to 9-24% of Muskegon Lake's volume experienced hypoxia for similar to 29-85 days/year - with the potential for hypolimnetic habitat degradation and sediment phosphorus release leading to further eutrophication. Thus, time-series observatories can provide penetrating insights into the inner workings of ecosystems and their external drivers. (C) 2018 The Authors. Published by Elsevier B.V. on behalf of International Association for Great Lakes Research.