Green Bay, the largest freshwater estuary in the U.S., is a shallow and highly productive arm of Lake Michigan. As was the case in many other sites following industrialization of waterways in the early-mid 20th century, lower Green Bay faced heavy chemical contamination and nutrient loading. Excessive primary production in the water column drove up oxygen demand at the benthos, resulting in frequent seasonal hypoxia (i.e. “dead zones”) in many areas, which (among other pollution-related causes) lead to marked changes in benthic faunal communities. Remediation efforts have been underway since the 1970s, with both positive and seemingly unchanging results. My work as an M.S. student at the University of Wisconsin-Milwaukee School of Freshwater Sciences focused on the case for re-establishment of the extirpated Hexagenia mayfly population and comparing meiobenthos communities between more and less organically enriched locations.
Hexagenia mayflies are among the most prolific and ecologically valuable taxa to make temperate freshwaters their home. Spending 1-2 years as substrate-burrowing nymphs and roughly 24 hours as winged, reproductive adults, a “hatch” can produce trillions of individuals and introduce over 100-thousand tons of protein into a system overnight where populations are healthy (e.g., the Upper Mississippi River). With each reproductive female harboring some 8,000 eggs, swarms of adults can become so large that they are confused with storm fronts on DOPLAR radar. As nymphs, they are also an important food source for dozens of fish species, and, given their relative intolerance of organic pollution, a valuable indicator species for water quality.
Publications & Media:
UWM Press (2014) - “How Healthy is Green Bay? The Mayflies Know”
Green Bay Press Gazette (2015) - “Mayflies: Messy nuisance or economic boon?”
2019 seminar poster: “The mud may not be the problem for mayflies in lower Green Bay”
Hexagenia in lower Green Bay: on the way to recovery?
Hexagenia were once one of the most abundant benthic macroinvertebrates in the lower Green Bay system—earning the local namesake “the Green Bay fly”. These large insects provided a robust food source for thriving perch and walleye populations, and processed an immense volume of accumulating detrital organic matter at the bay floor. Ecological degradation lead to their officially declared extirpation in 1955, and they have yet to make a full recovery in Green Bay (as they have in other formerly degraded Great Lakes sites—e.g., Lake Erie’s western basin).
Nymph survival, growth, secondary production in lower Green Bay substrates (blue) vs Upper Mississippi substrates (red).
Why have Hexagenia yet to return? One initial hypothesis suggested that the consistency of lower Green Bay sediment was to blame. An overaccumulation of organic matter at the benthos has resulted in a highly-fluidized “gyttja” substrate—potentially too non-cohesive to support the construction of burrows, critical to nymphs’ life cycles. I tested this hypothesis in a 2014 laboratory microcosm experiment (see publication in Journal of Great Lakes Research) using Hex. nymphs collected from the Upper Mississippi River, subjected to either their native substrate or that from lower Green Bay in oxygenated aquaria. Nymph survival, growth, and secondary production (i.e. tissue elaboration over time) actually appeared to be higher in lower Green Bay substrates (left)—thus, sediment fluidity could likely be ruled out as a factor limiting the mayflies’ recovery in lower Green Bay. Likely causes still at play: prevalence of seasonal hypoxic zones and/or a reproductive density effect (e.g., Allee’s principle).
Hexagenia nymphs (“neonates”), newly hatched in situ in lower Green Bay (2015)
Can Hex. eggs hatch & nymphs survive in situ in lower Green Bay?
Field-based mesocosm studies in 2014 and 2015 revealed that eggs collected from adult Hexagenia in Western Lake Erie were capable of hatching into viable nymphs, which survived (and grew) through at least one fall-spring season at Longtail Point, lower Green Bay. This was especially significant, as the site is still heavily organic-rich and remains within the U.S.-Canada International Joint Commission’s designated ‘Area of Concern’.
For more details, see publication in JGLR.
En masse Hex. egg stocking
From 2014-2018, close to one billion Hexagenia eggs were deployed to several nearshore sites in lower Green Bay and connected adjacent bays in an effort to determine whether or not noticeable adult emergences would occur, and to overcome the potential reproductive density effect still limiting Hex. recovery. Reports are anecdotal—but a number now exist from local residents and business operators near our sites, indicating that, although hypoxia likely still represents a substantial barrier to a full-scale recovery, a recovery of a certain degree may be in the works.
Meiobenthos Community Comparisons
Copepod (harpacticoid, calenoid, cyclopoid) densities. Dark grey bars represent sites within the AOC, light grey represent sites outside. P-values denote significant differences at the 0.05 level.
Meiobenthos can be generally defined as invertebrates small enough to pass through 1mm, but be retained by 45 μm mesh. Similar forms exist in and on most coastal substrates worldwide, and community metrics (e.g., density, diversity) have been suggested in some cases to be used as indicators of habitat quality. In 2015, I conducted a study comparing these metrics for total meiobenthos, copepoda, and ostracoda at several sites within and outside the IJC ‘Area of concern’ (AOC) (sites within are still considered degraded).
Resulting densities (for total meiobenthos, copepods, and ostracods) were, in many cases, significantly higher within the AOC, possibly due to a higher abundance of detrital food. Community diversity (Shannon-Wiener, Simpson’s Index) did not differ substantially within vs outside the AOC, aside from one site outside being significantly lower for both indices.
Additional research on meiobenthos community metrics along this organic pollution gradient, among others in the region, is needed to determine whether or not these taxa/communities can be reliably used as habitat quality indicators—namely, additional sites, collection seasons, and higher taxonomic specificity.
Find out more about this study in our JGLR (2018) paper.
This work was funded in large part by the National Fish and Wildlife Foundation.