Nate Dugener¹ and Bopi Biddanda, Annis Water Resources Institute, Grand Valley State University.
¹Present address: Office of Emergency Response, Illinois Environmental Protection Agency.

Summary: The availability of oxygen is a major factor determining the abundance and distribution of life on Earth. Two recent peer-reviewed journal articles by GV students and researchers report on the annually recurring hypoxia (low oxygen conditions) in an urbanized Great Lakes estuary in West Michigan and deliberate on its implications for aquatic ecosystem health. These studies provide insight into the role of temperature, precipitation, and phytoplankton production in driving the dynamics of bottom water hypoxia where colder and drier years over the watershed result in milder hypoxia, and warmer and wetter years result in severe hypoxia. Findings also have implications for aquatic habitat degradation via release of legacy phosphorus and the generation of greenhouse gasses under a warming climate and continued anthropogenic stress.

Background to problem

As we dig up ancient fossil carbon and burn them to fuel our modern lifestyle, carbon dioxide is noticeably rising in the atmosphere. Less noticeable is the fact that there is a corresponding equimolar drop in atmospheric oxygen that is going on. Fortunately, this drop in oxygen concentration is negligible because the atmosphere is nearly 21% oxygen whereas carbon dioxide is only ~0.3%. Well-oxygenated aquatic water bodies, however, hold orders of magnitude less oxygen than the atmosphere – only ~0.001% by volume or weight. Both warming that reduces the capacity of water to hold dissolved gasses and enhanced biological respiration under warmer conditions can collude to substantially lower aquatic oxygen concentrations daily (think night when there is no photosynthesis producing oxygen) or seasonally (think of the dark winter months). Indeed, deoxygenation is widespread in the hydrosphere today. Since the industrial revolution about 250 years ago, the oceans have lost around 2% of their oxygen content, and freshwater systems (that are smaller and warming disproportionately faster relative to the oceans) are even more impacted.

Availability of oxygen sets the stage for most aerobic life forms including us. Bottom water hypoxia, or low oxygen conditions (usual concentrations of dissolved oxygen in healthy waters is around 8-12 mg/L hypoxic levels are <4-0 mg/L; <4 mg/L makes the habitat unsuitable for most fishes, at <2 mg/L few invertebrates survive, and at 0 mg/L only anaerobic microbes occur), is expanding as the Earth continues to warm (warmer water can hold less dissolved gasses than colder water) and anthropogenic disturbance of the landscape and ensuing release of nutrients to the downstream aquatic ecosystems continues unabated. Muskegon Lake, a Great Lakes estuary, experiences hypoxia on a yearly basis. With increasing precipitation and warming, as well as increased eutrophication (the process wherein a body of water becomes enriched with nutrients and stimulates excessive algal growth), hypoxia incidence and intensity are increasing around the world. Stratification, or the separation of the warmer and lighter surface water (epilimnion) from colder and denser bottom water (hypolimnion) of a lake, is a necessary condition for the development of hypoxia. As the hypolimnion becomes hypoxic, sediment-bound legacy phosphorus is released into the water column. This positive feedback loop results in increased nutrient input to the ecosystem and further stimulates algal growth. For example, water-column mixing due to large storm events brings these nutrients to the surface, initiating harmful algal blooms. However, water column mixing (or intrusions of cold and oxygenated upwelled water from coastal Lake Michigan as in the case of Muskegon Lake) can also intermittently disrupt hypoxia as strong winds circulate oxygenated water from the surface into deoxygenated zone below, briefly breaking up the hypoxia. With temperatures rising and eutrophication ongoing, the expectation is that hypoxia will expand and intensify in the world’s waters. As hypoxia gains greater stronghold in our natural waters, fish habitat degrades, smaller fish populations begin to decline as predation increases with lack of bottom water habitat, and socio-economic aspects that rely on coastal fisheries and tourism weaken as fisheries decline.

Schematic conceptual diagram of the dynamics of bottom water (hypolimnion) hypoxia within Muskegon Lake. The schematic illustrates the importance of inorganic nutrients (IN) and organic matter (OM) loading into both surface and bottom water. Cold water intrusions from Lake Michigan (LM), internal phosphorus loading (IPL), and water column mixing are shown to further illustrate the dynamics of hypoxia in Muskegon Lake as revealed by the high-frequency time-series Muskegon Lake Observatory buoy (www.gvsu.edu/buoy/).

This study

Using high-frequency, time-series data from the Muskegon Lake Observatory buoy (MLO) operated by the Biddanda Lab at the Water Resources Institute (https://www.gvsu.edu/buoy/), the two studies at hand examined the causes and consequences of hypoxia in Muskegon Lake from 2011-2021. In addition to tracking changes in dissolved oxygen at various depths in the water column with sensors from the MLO to assess the dynamics of hypoxia throughout the growing season, we also conducted biweekly nutrient sampling, and seasonal respiration (oxygen consumption) experiments during 2021. The goal of the seasonal respiration experiments was to determine the nutrient or carbon input most relevant to oxygen decline in the bottom waters. To examine hypoxic conditions over a longer period of time, a hypoxia severity index was developed to determine the differences in hypoxia severity from 2011-2021. Utilizing USGS public data, we examined the relationship between winter and spring temperatures and precipitation to summer hypoxia to determine a correlation between the two. This would allow us to anticipate if a severe or mild hypoxic summer were to occur based on the prior winter and spring observations.

Bird’s eye view of Muskegon Lake and the location of the Muskegon Lake Observatory buoy (MLO, orange circle). Other locations listed include the Muskegon River and Bear Lake as inflows into Muskegon Lake, the outflow of Muskegon Lake into Lake Michigan via the navigational channel, and the location of GVSU’s Annis Water Resources Institute (GVSU-AWRI) in relation to the MLO. The inset in the lower left represents Muskegon Lake within the Great Lakes basin.

Key findings and their significance

While water-column stratification set the stage for bottom water hypoxia, frequent wind-mixing events intermittently reduced the thickness or intensity of the hypoxic zone in Muskegon Lake. In the 2021 study, respiration experiments revealed that riverine and surface organic matter inputs contributed most to bottom water hypoxia in the spring, whereas surface inputs did so during summer, and riverine inputs during fall, indicating seasonally variable sources drive hypoxia. In 2021, the hypoxic zone in Muskegon Lake persisted from early-June until mid-September. A warm spring allowed for an early onset of summer stratification that gained strength throughout the summer as surface temperatures did not cool throughout the season. A relatively calm summer further limited mixing events from occurring due to storms and precipitation events. Biweekly measurements indicated increased phosphorus in the bottom water during anoxia (complete lack of oxygen) via internal phosphorus loading from the sediment with the potential for fueling subsequent surface blooms. Our findings on the role of seasonally changing temperature, loading, phytoplankton production, bottom water respiration, and internal phosphorus loading in shaping hypoxia dynamics have relevance to similarly afflicted ecosystems in the Great Lakes basin. 

Time-series heat maps of 2012, 2014, 2019, and 2021 thermal stratification and dissolved oxygen from the Muskegon Lake Observatory (MLO) buoy. Additionally, 2012 and 2021 were selected as they were the most severe on the hypoxia severity index, while 2015 and 2019 were the mildest years. Each graph is scaled to the earliest deployment and the latest retrieval of the MLO, leading to a lack of data when the buoy was not yet deployed in other years. Black lines were drawn at DO concentrations of 4 mg/L to signify mild hypoxia and 2 mg/L to signify severe hypoxia. Black brackets were drawn to highlight wind mixing events (top) and cold, oxygenated upwelled water intrusions (bottom), respectively. Missing data throughout the heat maps are attributed to malfunctioning sensors and extreme biofouling.

Our hypoxia severity index determined that years 2012 and 2021 had the most severe hypoxia while 2015 and 2019 had the mildest hypoxia. The severity of hypoxia did not have a statistically discernible trend over the decade, and we are unable to say if it was worsening over the years without additional years of data. A driving factor of hypoxia severity was the temperature and amount of Muskegon River water entering Muskegon Lake during the winter and spring and spring surface temperatures of Muskegon Lake wherein warmer water temperatures corresponded to severe hypoxia while colder temperatures resulted in mild hypoxia. Thus, knowledge of the environmental conditions prior to the onset of hypoxia can be useful in predicting the potential severity of hypoxia for any particular year.

A schematic diagram of a positive feedback loop of hypoxia, internal phosphorus loading, and eutrophication occurring in Muskegon Lake.

Broader ecosystem-level and societal implications

Overall, our findings in Muskegon Lake suggest that variable temperature and river flow are major drivers of hypoxia and warming surface waters will lead to the further deoxygenation of the interior of Earth’s waters, with relevance to streams, lakes, rivers, reservoirs, estuaries, and coastal waters everywhere. Seasonal differences in organic matter input into the bottom water changes the rate of respiration where both surface and riverine inputs were equally influential in the spring, surface inputs were most influential in the summer, and riverine inputs were most influential in the fall. As hypoxia persists throughout the summer, bottom water phosphorus increases, which creates a delayed positive feedback loop of harmful algal blooms that further deplete the bottom water of oxygen upon their decay in bottom water. Furthermore, expanding hypoxic and anoxic zones may be potential sources of potent greenhouse gasses such as nitrous oxide and methane as well – fueling another potential positive feedback loop. In a fast-warming world already experiencing an intensified hydrological cycle, changing weather patterns, increasing anthropogenic pressure, and an uncertain future in the face of climate change, understanding the causes and consequences of ongoing deoxygenation of natural waters may provide clues to how water bodies will be impacted, and how they in turn will affect us.

Bottom-water hypoxia is rapidly expanding in marine and freshwater ecosystems as the Earth continues to warm and eutrophication thrives as land use changes continue to discharge nutrients. As the global water cycle amplifies under a warming climate with an increased incidence of extreme weather, consequences to consider for the future of hypoxia globally include effects on the food web and the carbon cycle, as well as impacts on recreational and social reliance upon these freshwater resources. Muskegon Lake, a model urbanized Great Lakes estuary with detailed chronicles of hypoxia over multiple years (www.gvsu.edu/buoy/), can serve as a sentinel for deoxygenation studies. With this year culminating in record-breaking high summer temperatures across the world, it remains to be seen what next year – year with El Nino and the likelihood of even warmer conditions – will bring.

Published peer-reviewed source literature with free open access:

Dugener, N. M., A. D. Weinke, I. P. Stone and B. A. Biddanda. 2023. Recurringly hypoxic: Bottom water oxygen depletion is linked to temperature and precipitation in a Great Lakes Estuary. Hydrobiology, 2, 410–430. https://doi.org/10.3390/hydrobiology2020027

Dugener, N. M., I.P. Stone, A. D. Weinke, and B. A. Biddanda. 2023. Out of oxygen: Temperature and loading drove hypoxia during a warm, wet, and productive year in a Great Lakes Estuary. J. Great Lakes Res, 49 (10): 1015-1028. https://doi.org/10.1016/j.jglr.2023.06.007