Trophic Status of Southeastern Lake Michigan Based on the Chironomidae (Diptera)

Chironomids and oligochaetes were identified from a total of 2,432 samples collected at depths <42 m over a 9-year period in southeastern Lake Michigan. Each of 13 chironomid taxa was assigned a trophic indicator status based on density trends among samples characterized by oligochaete trophic indices in the ranges < 0.50 (Type 0), 0.50–1.50 (Type 1), and >1.50 (Type 2). A trophic index based on the chironomid fauna was generated for year, depth intervals, and combinations of years and depths. When averaged over all years, the trophic index at the 8- to 24-m depth interval was 1.26, indicating mesotrophic conditions in this portion of the littoral and sublittoral of southeastern Lake Michigan. Similarly, at depths >28 m the trophic index value was 0.41, suggesting oligotrophic conditions in the profundal. No annual trophic status trends were evident.     

Long-term trends of Lake Michigan benthos with emphasis on the southern basin

Lake Michigan benthic macrofauna have been studied for almost a century, allowing for a unique analysis of long-term changes in community structure. We examined changes in abundances of three major taxonomic groups of benthic macroinvertebrates (Diporeia, Oligochaeta, and Sphaeriidae) in southern Lake Michigan from 1931 to 2015 and identified the most likely causes for these changes. Abundances of all three groups increased during 1931–1980 with the bulk of these increases occurring in nearshore (≤50 m) waters and coincident with increased loading of phosphorus (P) and subsequent increased primary production. Abundances of all three taxa declined during 1980–2000 again mostly in nearshore waters and coincident with decreased P loading. The quagga mussel (Dreissena rostriformis bugensis) invasion was associated with a further decline in phytoplankton primary production during 2000–2015. Both Diporeia and Sphaeriidae declined in abundance during that time, with Diporeia exhibiting the more pronounced decrease of the two groups. In contrast, Oligochaeta increased in abundance during 2000–2015. The quagga mussel has become, by far, the most abundant benthic macroinvertebrate species in terms of density and biomass. Overall, the primary driver of changes in the abundances of the three major taxa during this 85-year period appeared to be changes in phytoplankton primary production due to changing P loadings and, later in the time series, Dreissena filtering. The dreissenid mussels invasions coincided with a rapid decline of Diporeia abundance, but the mechanism of this negative effect remains unidentified. In contrast, Oligochaeta likely benefited from the quagga mussel invasion, perhaps via quagga-generated food supply.     

Survival,growth, and production of Hexagenia bilineata mayflies in fluidized sediment from lower Green Bay,Lake Michigan

Organic pollution in lower Green Bay, Lake Michigan over the past century was accompanied by the local extirpation of Hexagenia (primarily H. limbata) mayflies. Recoveries were made in other systems where population crashes had occurred (e.g., western Lake Erie); however, an active recovery does not appear to be taking place in Green Bay. Excessive primary production has caused substantial benthic organic matter accumulation resulting in a fluidized “sludge-like” substrate as the majority of the sub-littoral habitat. Fluidized substrate potentially hinders Hexagenia nymphs' abilities to construct and maintain burrows critical to their life cycles. In this study, Hexagenia bilineata nymphs were collected from an Upper Mississippi River backwater where their presence at high densities in relatively fluid sediment had been observed, and reared in oxygenated aquaria containing lower Green Bay or Upper Mississippi River sediment. Their survival, growth, secondary production, and biomass turnover were calculated for a 166 day period. Seventy-five percent of nymphs survived or metamorphosed into winged sub-imagos in lower Green Bay substrates compared to 40.6% in Upper Mississippi River substrates. Nymph dry weight more than tripled in Green Bay substrates and more than doubled on Upper Mississippi River substrates. Production was notably higher in lower Green Bay substrates. Differences in survival and production between the two treatments were statistically significant (P < 0.05), while differences in growth and biomass turnover were not (P > 0.05). Based on these results, the high fluidity of lower Green Bay substrates did not appear to hinder burrow construction or maintenance.     

Occurrence of the Asiatic Clam Corbicula Fluminea in the Maumee River and Western Lake Erie

The Asiatic clam was found in only two locations along the southern shore of the western basin of Lake Erie: the thermal plume areas of two coal-fired power plants with once-through condenser cooling systems. Three sewage treatment plant outfalls were sampled, but Corbicula were found only in the sewage treatment plant outfall that was within the thermal plume of a power plant. No Corbicula were found in the vicinity of the nuclear power plant which has a closed-cycle natural draft cooling tower, and hence no significant thermal plume.From the age of some of the specimens, this clam has probably resided in the region since 1978. The Great Lakes are farther north than the natural range of this clam, and prolonged cold temperatures in this region may be responsible for the confinement of this clam to baseload power plants with continuous thermal plumes.     

Maturity Schedules of Lake Trout in Lake Michigan

We determined maturity schedules of male and female lake trout (Salvelinus namaycush) in Lake Michigan from nearshore populations and from an offshore population on Sheboygan Reef, which is located in midlake. Gill nets and bottom trawls were used to catch lake trout in fall 1994 and 1995 from two nearshore sites and Sheboygan Reef. Each lake trout was judged immature or mature, based on visual examination of gonads. Probit analysis, coupled with relative potency testing, revealed that age-at-maturity and length-at-maturity were similar at the two nearshore sites, but that lake trout from the nearshore sites matured at a significantly earlier age than lake trout from Sheboygan Reef. However, length at maturity for the nearshore populations was nearly identical to that for the offshore population, suggesting that rate of lake trout maturation in Lake Michigan was governed by growth rather than age. Half of the lake trout males reached maturity at a total length of 580 mm, whereas half of the females were mature at a length of about 640 mm. Over half of nearshore males were mature by age 5, and over half the nearshore females matured by age 6. Due to a slower growth rate, maturity was delayed by 2 years on Sheboygan Reef compared with the nearshore populations. Documentation of this delay in maturation may be useful in deciding stocking allocations for lake trout rehabilitation in Lake Michigan.     

Lake-wide Distribution of Dreissena in Lake Michigan, 1999

The Great Lakes Science Center has conducted lake-wide bottom trawl surveys of the fish community in Lake Michigan each fall since 1973. These systematic surveys are performed at depths of 9 to 110 m at each of seven index sites around Lake Michigan. Zebra mussel (Dreissena polymorpha) populations have expanded to all survey locations and at a level to sufficiently contribute to the bottom trawl catches. The quagga (Dreissena bugensis), recently reported in Lake Michigan, was likely in the catches though not recognized. Dreissena spp. biomass ranged from about 0.6 to 15 kg/ha at the various sites in 1999. Dreissenid mussels were found at depths of 9 to 82 m, with their peak biomass at 27 to 46 m. The colonization of these exotic mussels has ecological implications as well as potential ramifications on the ability to sample fish consistently and effectively with bottom trawls in Lake Michigan.     

Current Status and Trends in Muskegon Lake,Michigan

A long-term monitoring program was initiated in 2003 to determine the ecological status of Muskegon Lake, a Great Lakes Area of Concern. This paper presents data generated from the first 3 years of the monitoring program, discusses how the data are being used to establish and justify lake restoration targets, and assesses how water quality conditions have changed over time. Between 1972 and 2005, lake-wide averages of total phosphorus and soluble reactive phosphorus from the water surface have declined from 68 to 27 μg/L and from 20 to 5 μg/L, respectively. In addition, average chlorophyll a concentrations have declined from 25 to 6 μg/L over this period, while Secchi disk depths have increased from 1.5 to 2.2 m. Wastewater diversion, and perhaps dreissenid filtering activity, is most likely responsible for these changes. However, nitrate concentrations have increased from 70 to 270 μg/L over the same time period. During 2003–2005, phytoplankton abundance and fish catch were lower in the spring compared to the summer and fall. Microcystis was the most abundant phytoplankton genus; the fish community generally was dominated by round goby (Neogobius melanostomus) in spring and summer, and sunfishes (Centrarchidae) in the fall. Dreissenid abundance was highly variable over time, but densities were low relative to Saginaw Bay. Approximately 65% of the Muskegon Lake shoreline has been hardened (i.e., physically altered). Overall, the water quality of Muskegon Lake has improved over the past 30 years, but environmental challenges still exist, including contaminated sediments, loss of natural habitat, and invasive species.     

Resurrection of the Lake Michigan Eutrophication Model,MICH1

The Lake Michigan model, MICH1, was developed more than 30 years ago. This framework was evaluated using field data collected in 1976 and was later applied to predict total phosphorus and phytoplankton concentrations in Lake Michigan during the 1980s and early 1990s. With a renewed interest in the interaction of phytoplankton with toxics and the applicability to Total Maximum Daily Load studies, several new models have been developed and older models have been revived. As part of our interest in plankton dynamics in Lake Michigan, the MICH1 model was resurrected. The model was evaluated over the 1976–1995 period, with a surprisingly good model fit to lake-wide average total phosphorus (TP) field data. However, the model was less successful in mimicking the chlorophyll-a measurements, especially in the hypolimnion. Given the results, the model was applied to perform a few long-term TP model simulations. Using the model with average 1994–95 phosphorus loadings, a steady state was reached within approximately 20 years, and the lakewide phosphorus concentration was below the International Joint Commission water quality guideline of 7 μg/L. This exercise demonstrated that a relatively simple, four-segment model was able to mimic the TP lake-wide data well. However, this model was less suitable to predict future chlorophyll-a concentrations due to the limitation in the representation of the foodchain and the difficulty of the coarse segmentation of the model to capture the deep chlorophyll-a layer. Strengths and limitations of this model can guide future development of eutrophication models for Lake Michigan and the other Great Lakes.     

Currents and Temperatures in Green Bay,Lake Michigan

Current velocities and water temperatures were measured in the four main passages between Green Bay and Lake Michigan and at several sites within the bay during summer and fall 1977. Monthly resultant currents indicate there is anticlockwise circulation in the bay during dominant southwesterly wind and a reversal of this pattern during episodes of northeasterly wind. It is common for two layers to flow through the mouth of the bay in opposite directions during the stratified season. Cold hypolimnetic lake water entering through the mouth and extending far into the bay maintains stratification and promotes flushing. The effects of resonance of forced and free long wave disturbances are prominent in current records; these oscillations are coherent and in phase across the mouth.     

Satellite-Tracked Current Drifters in Lake Michigan

Satellite-tracked current drifters are being used to monitor near-surface currents in Lake Michigan. These drifters are now commercially available, and preliminary tests show their satellite-determined positions to be within 0.5 km. The drifters appear to be ideal for monitoring near-surface lake currents and testing hydrodynamic lake models.     

Spatial Patterns in PCB Concentrations of Lake Michigan Lake Trout

Most of the PCB body burden in lake trout (Salvelinus namaycush) of the Great Lakes is from their food. PCB concentrations were determined in lake trout from three different locations in Lake Michigan during 1994–1995, and lake trout diets were analyzed at all three locations. The PCB concentrations were also determined in alewife (Alosa pseudoharengus), rainbow smelt (Osmerus mordax), bloater (Coregonus hoyi), slimy sculpin (Cottus cognatus), and deepwater sculpin (Myoxocephalus thompsoni), five species of prey fish eaten by lake trout in Lake Michigan, at three nearshore sites in the lake. Despite the lack of significant differences in the PCB concentrations of alewife, rainbow smelt, bloater, slimy sculpin, and deepwater sculpin from the southeastern nearshore site near Saugatuck (Michigan) compared with the corresponding PCB concentrations from the northwestern nearshore site near Sturgeon Bay (Wisconsin), PCB concentrations in lake trout at Saugatuck were significantly higher than those at Sturgeon Bay. The difference in the lake trout PCB concentrations between Saugatuck and Sturgeon Bay could be explained by diet differences. The diet of lake trout at Saugatuck was more concentrated in PCBs than the diet of Sturgeon Bay lake trout, and therefore lake trout at Saugatuck were more contaminated in PCBs than Sturgeon Bay lake trout. These findings were useful in interpreting the long-term monitoring series for contaminants in lake trout at both Saugatuck and the Wisconsin side of the lake.     

Potential Sources of Asbestos in Lake Michigan

Chrysotile and amphibole mineral fibers, recognized carcinogens, have been found in Lake Michigan water. Since the sources of these fibers in Lake Michigan are unknown, a study was undertaken to determine potential sources. Information was gathered by contacting governmental agencies and reviewing literature. Two industries that used to discharge asbestos-containing effluents are located in the Lake Michigan drainage basin. Geologic deposits of mineral fibers occur in Marinette County, Wisconsin and the Upper Peninsula of Michigan. Currents and Army Corps of Engineers dredging and disposal activities were investigated as potential mechanisms of asbestos distribution in Lake Michigan. Limited environmental monitoring indicates that atmospheric loading and wash-out from numerous point and non-point sources may be an important mechanism of asbestos contamination of Lake Michigan. More environmental monitoring will be required in order to determine the relative importance of these potential sources.     

Age truncation of alewife in Lake Michigan

Empirical evidence has shown increased variability in harvest and recruitment of exploited fish populations, which can result directly from exploitation or indirectly from interactions between external drivers and the internal dynamics of age-structured populations. We investigated whether predation in a freshwater system could affect a prey fish population, in the same way fishing affects targeted populations. Using fishery-independent trawl survey data and a suite of quantitative indicators, we evaluated changes in the alewife population in Lake Michigan. Our results provide evidence for a reduction in the mean spawner age, a reduction in the diversity of age classes and the distribution of biomass across them, and increased variability in the proportion of first time spawners in the spawning stock. We used wavelet analysis and estimates of lifetime egg production to demonstrate how the alewife population displays behaviors of instability as the overall biomass declines. Our results provide evidence that predation pressure can influence prey fish populations in a similar manner to fishing on harvested populations, and that conservation of a broad reproducing age structure is likely to be important for buffering against adverse environmental fluctuations and for sustainable management of fish populations.     

Population Characteristics of Bythotrephes in Lake Michigan

The population characteristics of Bythotrephes were evaluated at seven nearshore (45 m) and eight offshore (80 to 150 m) stations in Lake Michigan during July and September 2000. Bythotrephes was generally most abundant at offshore stations, but mean density was patchy (4 to 1,326/m²) among locations. During the year, there was a shift from reproduction by mainly instar III females to reproduction by instar II females. The shift generally reflected a change in the population structure of Bythotrephes at most sites during the same period. Bythotrephes populations in July were generally characterized by small body size at reproduction (instar II and III), large clutch size, and small neonates. Later in the season, body size at reproduction and neonates were larger and clutch size was smaller. Most growth (body length) of Bythotrephes occurred between instar I and II whereas little growth occurred between instar II and III. Spine length of Bythotrephes increased between July and September. The population characteristics and reproductive strategies of Bythotrephes appear to be adaptations to fish predation and food limitations.     

Deglacial and Lake Level Fluctuation History Recorded in Cores,Beaver Lake,Upper Peninsula,Michigan

Sediment cores collected from the littoral and pelagic zones of Beaver Lake, Michigan record fluctuations in the water level of Lake Superior. Beaver Lake is a small 300 ha lake in Pictured Rocks National Lakeshore (PRNL) now separated from Lake Superior by a dune-capped barrier bar. Cores were collected using a vibracorer from a lake-ice platform in February 1997. A 2.85 m long core in 10 m of water contains well-sorted sand, rhythmites, peat, interbedded sand and gyttja, and is capped with 1 m of massive gyttja. A 9480 BP AMS age from the basal sand provides a minimum deglacial date for the area. Further analysis indicates a sand-dominated depositional environment from a low lake stand at approximately 8500 BP to present. An approximate 8800 BP red to gray sediment color transition records either the cessation of meltwater input from Lake Agassiz or receding ice, while a younger similarly colored transition, 6600 BP in age, likely records sediment reworking in the coastal zone.Four AMS ages on peat range from 8520 to 7340 BP and are indicative of the Houghton low phase. Burial of the peat by stratified sand and gyttja after 7340 BP indicates a rising lake level. Peat at a higher level in the lake basin, encountered in shallow littoral cores, ranges in age from 6800 to 6420 BP, which estimates a 0.91 m rise/century in lake level to the Nipissing level by 5000 BP.     

Evidence of Natural Reproduction by Planted Lake Trout in Lake Michigan

Lake trout (Salvelinus namaycush) fry were captured in southeastern Lake Michigan for the first time since the species was reintroduced from hatchery stocks in 1965. Spawning apparently occurred in fall 1979 on newly placed limestone riprap covering recently constructed power plant intake and discharge pipelines. Eggs presumably hatched in late February–March, 1980, and 57 fry (22–43 mm total length) were collected April–June, 1980, and three fry (55–62 mm) were collected in August, 1980.     

Lake-wide,annual status of the Mysis diluviana population in Lake Michigan in 2015

Mysis diluviana is one of the most abundant zooplankton by biomass in the Laurentian Great Lakes of North America, a predator of other zooplankton and an important prey for fishes. Studies of long-term trends in Lake Michigan have shown 2005–2016 densities to be 50–80% lower than 1990s densities, but these observations have been based on annual monitoring that is either spatially or seasonally limited. We combined Lake Michigan Mysis data from three annual programs and the 2015 Cooperative Science and Monitoring Initiative to achieve broad spatial coverage during spring, summer, and fall of 2015 and broad depth coverage during spring 2016. Lake-wide, annual density and biomass were 82 (SE: 10) Mysis/m² and 200 (SE: 36) mg dry mass/m². Density and biomass estimates were highest offshore, generally higher in the north basin, and seasonally highest in summer. Annual lake-wide averages for depths >30 m were better captured by seasonally-extensive annual programs than spatially-extensive annual programs, although spring sampling may bias annual values low. Mysis cohorts grew 0.026 mm/d (age-0) and 0.007 to 0.027 mm/d (age-1). Annual mortality was 81–98%. Reproduction was fall-spring and seasonal lake-wide estimates ranged from 0.6 to 19.1% females brooding, 13–20 embryos/brood, and 3–46 embryos/m². Annual production (423 mg dry mass/m²/yr, SE: 31) was lower than all but one previous estimate from lakes Michigan, Huron, and Ontario. While Mysis tend to persist, low Mysis production may be a concern for prey fishes that feed on Mysis.     

Spatial,seasonal, and historical variation of phytoplankton production in Lake Michigan

There have been few direct measurements of phytoplankton production made in Lake Michigan since invasive dreissenid mussels became established in the lake. Here we report the results of 64 measurements of phytoplankton primary production made in Lake Michigan during 2016 and 2017. We conducted two lake-wide surveys, one in the spring 2016 isothermal period and one after summer stratification in 2017 and examined seasonal production with bi-weekly sampling between May and November 2017 at an offshore station in the southwestern part of the lake. We assessed nearshore-offshore gradients by sampling at three transect locations on three occasions in 2017. Spring 2016 production and production:biomass (P:B) ratios (reflective of growth rates) were similar across the lake and were higher than those reported before dreissenid mussels became established, suggesting that despite decreases in phytoplankton biomass, growth rates remain high. Summer 2017 production and growth rates increased from south to north. Areal production in 2017 peaked in late summer. Mean 2017 summer production (499 ± 129 mg C m^?2 day^?1) was lower than values reported prior to the mussel invasion, and the fraction of total production occurring in the deep chlorophyll layer was about half that measured pre-mussels. At the offshore site picoplankton accounted for almost 50 % of the chlorophyll. As spring P:B ratios have increased and summer P:B and seston carbon:phosphorus ratios have not changed, we conclude that the decrease in phytoplankton production in Lake Michigan is due primarily to grazing by mussels rather than to stronger nutrient limitation.     

Evidence of persistent,recurring summertime hypoxia in Green Bay,Lake Michigan

Six years (2009–2015) of temperature and dissolved oxygen profile data show hypoxic conditions are common in the bottom waters of southern Green Bay, Lake Michigan during the summer. Depleted oxygen concentrations (<5?mg?L^?1) affect nearly 70% of the 38 stations sampled representing an area of ~500–600?km². Stratification typically lasts 2+?months, from late June to early September, and some stations exhibit bottom water hypoxia (<2?mg?L^?1) at a frequency of nearly 25% when sampled during this period. A monitoring program initiated in 1986 by the Green Bay Metropolitan Sewerage District has provided a 23?year, recreational season record (May–September) of continuous (15?min interval) in situ bottom water oxygen and temperature measurements at the Entrance Light station of the Green Bay navigational channel. The duration of the hypoxic season ranges from 2?weeks to over 3?months at this shallow 7?m offshore site. This variability likely results from a combination of thermal stratification, oxygen consumption in deeper waters of the bay, and physical forcing mechanisms that drive cool, oxygen depleted, bottom waters on a southerly trajectory across this sensor. These data suggest the duration of hypoxic conditions may have increased during the stratified season in recent years. Hypoxia in the bay would also appear to be sensitive to relatively small changes in these forces, particularly changes in organic carbon loading and the duration of stratification.     

White and longnose sucker diet composition in Lake Michigan and Saginaw Bay,Lake Huron

Sucker species (Catostomidae) are a common benthic invertivore in North American waterbodies yet are understudied in the Laurentian Great Lakes. Despite large biomass and potential competition with more economically valuable fish species, the diets of Great Lakes suckers are poorly described. We explored the gut contents of adult white suckers (Catostomus commersonii) in Lake Michigan and Saginaw Bay, Lake Huron, and longnose suckers (Catostomus catostomus) in Lake Michigan. Chironomidae was a primary prey for white suckers in both lakes, along with Amphipoda in Saginaw Bay. For longnose suckers in Lake Michigan, gut contents were dominated by Amphipoda and Isopoda. Relative to other benthic invertebrate taxa, white suckers positively selected Chironomidae as preferred prey (indexed via Chesson’s alpha). Moreover, the average length of consumed Chironomidae and Amphipoda increased with white sucker length, suggesting preferential size selection of prey as suckers grow. Although dreissenid mussels are overwhelmingly abundant in the benthos of Lake Michigan and Lake Huron, relatively few dreissenid mussels were consumed by either sucker species. Thus, suckers are unlikely to contribute to the control of these invasive invertebrates.