Lake Michigan has a long history of non-indigenous introductions that have caused significant ecological change. Here we present a summary of eight papers that document recent changes and the current state of the lower food web of southern Lake Michigan after the establishment of large dreissenid populations. Results are based on long-term data sets collected by federal and academic research and monitoring programs that place recent changes into a historic context. Dramatic and significant changes in the lower food web, such as the loss of the spring diatom bloom, large declines in phytoplankton productivity, and a decline of Mysis populations, were directly or indirectly attributed to the expansion of Dreissena rostriformis bugensis. Total phosphorus concentrations and loadings also have decreased in the last 20 years. Changes in the Lake Michigan ecosystem induced by D. r. bugensis have produced conditions in the offshore pelagic region that are similar to oligotrophic Lake Superior. The future state of the lower food web in southern Lake Michigan is difficult to predict, mainly because population trends of D.r. bugensis in cold, offshore regions are unknown. Hence. monitoring programs designed to collect long-term, consistent data on the lower food web of Lake Michigan are essential.

Trends in density, biomass, population structure, and nutritional state of Dreissena polymorpha and Dreissena rostriformis bugensis were examined in southern Lake Michigan between the 1990s and 2008. Density and biomass of D. polymorpha increased to a peak in the early 2000s and then declined. In contrast, D. rostriformis bugensis was first found in the southern basin in 2001 and has continued to increase in density or biomass at all depths ever since. In 2008, maximum mean density of D. rostriformis bugensis occurred at 16–30m (19,000/m²), but maximum biomass (AFDW) occurred at 31–50 m (43.9 g/m²). D. rostriformis bugensis has only recently (since 2005) began to increase at depths >50 m. When both species were present in 2004 at depths <50 m, a condition index (CI) for D. rostriformis bugensis was 27% higher, and shell weight per shell length was 48% lower compared to D. polymorpha. For D. rostriformis bugensis, CI decreased in 2008 compared to 2004 at 25 m and 45 m, but biochemical content (lipid, glycogen) did not. Seasonal changes in both RNA/DNA ratio (growth) and ETS (metabolic activity) in D. rostriformis bugensis were unaffected by reproductive activity, and only ETS appeared to change seasonally relative to bottom temperatures. Spawning of D. rostriformis bugensis occurred in late summer at 25 m, but occurred in spring at 45 m. Veliger densities peaked in both spring and late summer at both depths. Future population expansion (biomass) is expected to be most rapid at depths >50 m.

Phytoplankton abundance, composition, and productivity were monitored on a bi-weekly basis from March/ April through November/December at two offshore stations in southeastern Lake Michigan in 1983–1987, 1995– 1998 and 2007–2008 (exception 1983–1984 which were sampled from May to August). During the spring isothermal mixing period, surface-mixed layer (SML) chlorophyll a and phytoplankton biomass (carbon) and water column primary productivity decreased substantially in 2007–2008 as compared to 1995–1998 (66%, 87%, and 70% decrease, respectively). Smaller or no decreases were noted between 1983–1987 and 1995–1998 (chlorophyll a 23% decrease, phytoplankton biomass 5% increase, and production 22% decrease). Phytoplankton composition also changed during the spring isothermal mixing period in 2007–2008 as compared to 1983–1987 and 1995–1998; all phytoplankton groups with the exception of cyanobacteria and chlorophytes exhibited dramatic reductions in 2007–2008. The pronounced changes in phytoplankton properties during spring mixing in 2007–2008 were attributed to the filtering activities of the quagga mussel (Dreissena rostriformis bugensis). During mid- and late thermal stratification periods, SML phytoplankton chlorophyll a and phytoplankton carbon and water column primary production exhibited only one significant change across all decades (mid-stratification production in 2007–2008 as compared to 1995–1998 and 1983–1987). Phytoplankton compositional changes in the SML also were limited during thermal stratification. The size of the deep chlorophyll layer (DCL) in 2007–2008 was similar to or smaller than those in 1983–1987 and 1995–1998. However, phytoplankton composition in the DCL changed as net diatoms constituted < 5% of total phytoplankton in the 2007–2008 DCL but over 50% in 1983–1987 and 1995–1998.

Between 1990 and 2001, late-winter phytoplankton blooms were common in parts of the lower Great Lakes (southern Lake Michigan, Saginaw Bay and southern Lake Huron, and western Lake Erie), providing resources for over-wintering Zooplankton. In Lake Michigan up to 2001, detailed remote sensing and ship studies documented well-developed late-winter blooms in the southern gyre (circular bloom termed the ‘doughnut’). However, from 2001 to 2008, the winter blooms in Lake Michigan also supported early season veliger larvae from the introduced, cold-water adapted “profunda” morph of quagga mussels (Dreissena rostriformis bugensis). Remote sensing and ship studies revealed that settled mussels caused an extraordinary increase in water transparency and a simultaneous decrease of Chl a in the late-winter bloom. Before quagga mussels in 2001, water transparency was 74–85% at deep-water sites, whereas it increased progressively to 89% by 2006 and 94–96% by 2008. Chlorophyll a concentrations in the gyre rings were 1.1–2.6 µg/L in 2001, declining to 0.5–1.7 µg/L by 2006 and 0.4–1.5 µg/L by 2008. The reduction of Chl a in the winter bloom rings from 2001 to 2008 was 56–78% for the western limb and 74–75% for the eastern limb. Zooplankton species abundance, composition and abundance also changed, as cyclopoid copepods became very scarce and overwintering omnivorous calanoid copepods declined. Reduction in late-winter phytoplankton and Zooplankton poses a serious threat to open-water food webs.

Southern Lake Michigan has changed in response to alterations in nutrients and invasive species. NOAA and EPA monitoring results are used to examine those changes. NOAA provides detailed seasonal resolution, but limited spatial coverage, whereas the EPA provides more spatial coverage, but limited seasonal resolution. We compare changes in total phosphorus (TP), silica, nitrate plus nitrite, and chlorophyll concentrations from before and after the invasion by the quagga mussel (Dreissena rostriformis bugensis). Although TP at NOAA stations was consistently higher than at EPA stations, both confirm declines in spring and summer surface mixed layer (SML) conditions. Chlorophyll differed at EPA and NOAA stations before quagga mussel invasion, but not after the invasion. Spring chlorophyll decreased at NOAA stations after the invasion, but summer conditions did not change at either set of stations. Pre-invasion silica at NOAA stations was slightly higher than at EPA stations, and the lake's Si reservoir increased over the study period. Basin-scale spring Si increased gradually, whereas summer SML Si increased dramatically after 2003, likely reflecting reduced diatom production. Basin-scale nitrate increased significantly from pre- to post-invasion in both spring and summer. Summer nitrate utilization declined drastically in recent years, likely reflecting reduced phytoplankton production. TP loads decreased; however, the timing of changes in chlorophyll and Si and nitrate utilization suggest the recent increase in dreissenid filtering dramatically reduced spring phytoplankton abundance and production across the entire southern basin. The offshore pelagic zone of the historically mesotrophic southern Lake Michigan is now similar to oligotrophic Lake Superior.

We determined the clearance rates of the profunda morph of the quagga mussel (Dreissena bugensis) using seston and Cryptomonas ozolini, a high-quality algal food, for the temperature range 1–7°C, which is the full temperature range this morph is likely to experience during isothermal conditions or in the hypolimnion of deep lakes. Experiments at 3 °C with the shallow-water morph of the quagga and the zebra mussel provided very similar results. The clearance rates were combined with dreissenid abundance in 0–30 m, 30–50 m, 50–90 m, and >90 m depth zones of the southern basin of Lake Michigan to calculate a maximum (using Cryptomonas) and minimum (using seston) fraction of the water column cleared (FC) per day in the different depth zones at 3 °C to determine dreissenid impact on the spring phytoplankton bloom from 1994 to 2008. Starting in 2003 or 2004 with the replacement of zebra mussels by quagga mussels in shallow water and expansion of quagga mussel biomass in deep water, FC began to exceed likely phytoplankton growth in the 30–50 m zone. In 2007–2008, FC greatly exceeded likely phytoplankton growth by a factor of about 5 in the 30- to 50-m depth zone, where dreissenids were extremely abundant. Low FC in the offshore region led to the hypothesis of a mid-depth carbon (C) and phosphorous (P) sink caused by mussel uptake of seston-associated C and P that affected not only the mid-depth region, but also the offshore region “downstream” of the mid-depth zone.

The density and life history characteristics of Mysis relicta were evaluated at a 110-m and 45-m station in southeast Lake Michigan during spring, summer, and fall for two time periods, 1995–2002 and 2007–2008. Mysis were more abundant during 1995–2002 than 2007–2008 for all seasons and depths, with average declines across seasons of 81% at 45 m and 70% at 110 m. Offshore densities of Mysis in 2007–2008 were similar to published densities within the same region during 1985–1989, but under differing ecosystem conditions (e.g. higher fish biomass and primary production in the 1980s). Growth averaged 0.032± 0.002 mm/day in both 1995–2002 and 2007–2008, and the proportion of females with broods (overall 7%) did not differ between time periods. Mean brood size adjusted for length did not differ between 1995–2002 (17 ± 0.6) and 2007–2008 (15 ± 1.3). The mean length of reproductive females was higher in 2007–2008 (17 ± 0.2) than during 1995–2002 (16 ± 0.1). New recruits (≤4 mm) were collected during each season for each time period, but were lower in abundance in 2007–2008 than in 1995–2002. During spring, there was a significant relationship between offshore water column chlorophyll concentration and Mysis recruit abundance, but not during summer/fall. Declining spring chlorophyll levels may be altering food availability for small mysids in spring, and the decline of the benthic macroinvertebrate Diporeia may be increasing fish predation pressure on Mysis despite declining planktivore abundance.

The predatory cladocerans, Leptodora kindtii, Bythotrephes longimanus, and Cercopagis pengoi coexist in the waters of southeastern Lake Michigan near Muskegon, Michigan. Leptodora is indigenous, whereas Bythotrephes and Cercopagis are nonindigenous and became established in 1986 and 2000, respectively. To observe seasonal changes in their abundances, and relationships to each other, cladocerans were collected from 1994 to 2008 at an offshore (110-m) site, from 1998 to 2008 at a transitional (45-m) site and from 1999 to 2008 at a nearshore (15-m) site. Bythotrephes was most abundant at the offshore site compared to Leptodora and Cercopagis. Bythotrephes peak abundances usually occurred in autumn at all sites. Cercopagis tended to be more abundant at the nearshore site, and peak densities occurred in summer. At the mid-depth site, similar abundances occurred for all three predatory cladocerans, however, the date of peak abundance was usually earliest for Cercopagis, followed by Leptodora, and latest for Bythotrephes. In recent years, 2007 and 2008, densities of all three predatory cladocerans have increased. Temperature preference, fish predation, and competition between the invertebrate predators may all be important in allowing the dominance of one species over the other seasonally or spatially.

Bythotrephes were collected on a regular basis at a 110 m deep reference station in Lake Michigan over a 10-year period 1994–2003. The measured population structure in conjunction with an updated bioenergetic model was used to estimate daily predation demands by Bythotrephes on the Zooplankton community. The bioenergetic model incorporated the effect of temperature on growth and respiration and used a scalable size structure to adjust for a dynamic range in size across the season. A general linear model was developed to apply the bioenergetic results to routinely collected field data for estimating predation needs. Daily population consumption needs were estimated to be approximately equal to Bythotrephes standing biomass but varied as a function of water temperature and percent instar composition. At a temperature of 18 °C the predation needs of the population were equal to the population biomass. At warmer temperatures (22– 24 °C) the daily needs were up to 35% above the population biomass. Within and across years the population was variable, while trend lines from the long-term data indicated biomass and predation needs had an initial peak in mid-August followed by a plateau period with a seasonal high peak mid-October. A decrease in the midseason long-term average size structure suggests that Bythotrephes may experience prey limitation during this time period of the year. Over the course of the 10-year period population cycles and peak biomass were fairly stable with no indication of a change in predation needs.