Thomas F. Nalepa

Long-term Trends in Benthic Macroinvertebrate Populations in Lake Huron over the Past Four Decades

ABSTRACT Surveys of the benthic macroinvertebrate community were conducted in the main basin of Lake Huron in 2000 and 2003, and in Georgian Bay and North Channel in 2002. Results were compared to surveys conducted in the 1960s and early 1970s. Although data of earlier surveys were inconsistent, our best estimates suggest that total density of the four major benthic taxa (Diporeia spp., Oligochaeta, Sphaeriidae, and Chironomidae) in the main basin declined dramatically between the early 1970s and 2000. Populations of all major taxa continued to decline between 2000 and 2003, particularly Diporeia and Sphaeriidae. Diporeia was rare or absent in the southern end of the lake and in some nearshore areas in 2000, and by 2003 was not found at depths < 50 m except in the far northeastern end of the lake. Densities of the major taxa in Georgian Bay and North Channel in 2002 were not different from densities in 1973 despite differences in survey methods. A limited study in southern Georgian Bay, however, found that densities of both Diporeia and Sphaeriidae declined to zero at most sites between 2000 and 2004. The population of Dreissena polymorpha was stable in all lake areas, but Dreissena bugensis increased, particularly at the 31–50 m depth interval in the main basin. Since there were no extensive surveys in Lake Huron in the period between nutrient abatement (late 1970s) and the establishment of Dreissena (early 1990s), it is difficult to determine relative roles of these events on observed declines. However, since phosphorus loads have been stable since the early 1980s, declines between 2000 and 2003 can likely be attributed to Dreissena.

Recent changes in density, biomass, recruitment, size structure, and nutritional state of Dreissena populations in southern Lake Michigan

ABSTRACT 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.

Initial Colonization of the Zebra Mussel (Dreissena polymorpha) in Saginaw Bay, Lake Huron: Population Recruitment, Density, and Size Structure

The various life stages of the zebra mussel (Dreissena polymorpha) were examined during the initial years (1991–93) of the mussels invasion into Saginaw Bay, Lake Huron. Yearly trends in densities of larvae, newly-settled juveniles, and adults were poorly related. Larval densities were lowest in 1991 and increased each year, but the number of settled juveniles was highest in 1991. Adults increased between 1991 and 1992 and then declined in 1993. Mean adult densities at sites with hard substrates were 11,700, 33,200, and 4,100/m2 in each of the 3 years, respectively. Year-to-year variation at individual sites was high and likely a result of recruitment dynamics and spatial patchiness of available substrate. By 1993, densities on hard substrates were generally similar throughout the bay, but length-frequency distributions in the inner and outer bay were quite different. The 1991-cohort was not distinguishable in the inner bay in 1993 either because of poor growth or a limited life span, but this cohort was readily distinguishable in the outer bay. In addition, ash-free dry weight of a standard 15-mm mussel in the inner bay declined 65% between 1991 and 1993. Although food concentrations (chlorophyll and particulate organic carbon) declined to low levels in 1993 and both densities and soft-tissue weight of Dreissena declined, it is not clear whether populations in the bay have peaked and are now at equilibrium with the surrounding environment.

Nutrient Changes in Saginaw Bay, Lake Huron, After the Establishment of the Zebra Mussel (Dreissena polymorpha)

Concentrations of particulate and dissolved nutrients in Saginaw Bay, Lake Huron, were examined relative to zebra mussel colonization which occurred summer 1991. The magnitude and spatial pattern of changes indicate that mussels had a significant impact on nutrients in Saginaw Bay. Annual means for total suspended solids, particulate organic carbon, particulate phosphorus, and particulate silica in the inner bay were significantly lower in 1992 and 1993 (post-zebra mussel) than in 1991 (pre-zebra mussel). Annual means decreased from 11.5 mg L−l, 1.45 mg C L−l (121 μM), 20.4 μg P L−1(0.66 μM), and 1.52 mg SiO2 L−1 (24 μM) respectively in 1991 to 4.4 mg L−1, 0.79 mg C L−1(66 μM), 11.2 μg P L−l (0.36 μM), and 0.77 mg SiO2 L−1(12 μM) in 1993. In contrast, there were no significant differences among years for these parameters at control stations, which were located in the outer bay and had no known populations of mussels. Annual means for nitrate, ammonium, and silica were significantly higher in the inner bay in 1992 than in 1991, but not significantly different in 1993. Means increased from 0.39 mg N L−1, 21.0 μg N L−], and 1.11 mg SiO2 L−1 respectively in 1991 to 0.47 mg N L−1, 30.9 μg N L−1, and 1.71 mg SiO2 L−1 in 1992. No significant differences were observed for these parameters in the control group. Differences between 1992 and 1993 may reflect differences in the amount of runoff and circulation between Saginaw Bay and Lake Huron. n nA phosphorus budget indicated that zebra mussels were a significant sink for phosphorus. Mussels from the inner bay accumulated 108, 682, and 52 t respectively in 1991, 1992, and 1993. Comparatively, the annual pool of phosphorus in the water column of the inner bay decreased from a pre-zebra mussel (1979–1980) average of 712 t to 421 and 382 t in 1992 and 1993 respectively.

Changes in the Freshwater Mussel Community of Lake St. Clair: from Unionidae to Dreissena polymorpha in Eight Years

To determine density changes in both the zebra mussel, Dreissena polymorpha, and native mussels, Unionidae, in Lake St. Clair, surveys were conducted in 1990, 1992, and 1994 and compared to a similar survey in 1986 when no D. polymorpha was found. Collection methods were the same each year; divers used the quadrat method to collect 10 replicate samples at 29 sites located throughout the lake. The total number of unionids collected declined from 281 in 1986, to 248 in 1990, 99 in 1992, and 6 in 1994, while the number of species collected in each of the four respective years was 18, 17, 12, and 5. The decline in the unionid community occurred gradually over this time period as the D. polymorpha population expanded from the southeast region of the lake to the northwest region. Mean density and biomass of D. polymorpha throughout the lake was 1,700 m−2 and 4.7 gDW m−2 in 1990, 1,500 m−2 and 3.5 gDW m−2in 1992, and 3,200 m−2 and 3.1 gDW m−2 in 1994. The density increase can be attributed to the expansion of the population into the northwest region, while the decrease in biomass was mostly a result of a decline in the weight per unit length. Mean biomass of the D. polymorpha population in 1994 was actually lower than the mean biomass of unionids in 1986; however, based on literature-derived filtering rates, the filtering capacity of the D. polymorpha population in 1994 was 12 times greater than the filtering capacity of the unionid community in 1986. This increase has likely led to reported changes in the Lake St. Clair ecosystem (increased water clarity, increased plant growth, and shifts in fish communities).

Trends in the Benthic Macroinvertebrate Community of Saginaw Bay, Lake Huron, 1987 to 1996: Responses to Phosphorus Abatement and the Zebra Mussel, Dreissena polymorpha

Trends in benthic macroinvertebrate populations were examined in inner and outer Saginaw Bay, Lake Huron, from 1987 to 1996. These years represent the time period after phosphorus abatement, but immediately before (1987 to 1990) and after (1991 to 1996) colonization of the bay by the zebra mussel, Dreissena polymorpha. In 1987 to 1990, densities of the major macroinvertebrate groups in the inner and outer bay were not significantly different from, or were greater than, densities reported just prior to abatement efforts in the early 1970s. Oligochaete densities in the deepwater/silt region of the inner bay were trending downward between 1988 and 1991, but pollution-tolerant forms dominated the community, indicating the system was eutrophic just prior to Dreissena colonization. Dreissena impacts on the macroinvertebrate community varied depending on the particular habitat. At shallow-water/sand sites in the inner bay, Gammarus increased, and sphaeriids declined after Dreissena colonization, but no changes were observed in oligochaetes and chironomids, and overall species diversity showed little change. At deepwater/silt sites in the inner bay, densities of oligochaetes and chironomids declined just after the peak in Dreissena, but then returned to levels generally similar to those found prior to Dreissena. The oligochaete trophic index at deepwater/silt sites indicated a shift from eutrophic to more oligotrophic indicator species after Dreissena became established, and species diversity increased. In the outer bay, Diporeia and sphaeriids declined after Dreissena peaked, but few other changes were observed. Total non-dreissenid macroinvertebrate biomass (AFDW) in the inner bay, and in shallow areas of the outer bay, did not change as a result of Dreissena colonization. On the other hand, biomass in the deeper regions of the outer bay decreased because of the loss of Diporeia. Changes in the inner and outer bay typify the growing dichotomy between nearshore and offshore in the Great Lakes since Dreissena became established.

Seasonal and Annual Variation in Weight and Biochemical Content of the Zebra Mussel, Dreissena polymorpha, in Lake St. Clair

Abstract Zebra mussels, Dreissena polymorpha, were collected monthly from April/May to November in 1990 and 1991 from two sites in Lake St. Clair. The sites were characterized by relatively high and low mussel densities. The following variables were measured: ash-free dry weight (AFDW) per unit shell length (SL), lipid content and classes, carbon content, and nitrogen content. Mussels from the high-density site had a lower AFDW:SL relationship, lower lipid content, and a lower C:N ratio than mussels from the low-density site. Seasonal trends in these variables were consistent between sites and years. AFDW:SL, lipid, and carbon content were highest in the spring and then declined to minimum levels in late summer/fall. The mean seasonal decline in weight from spring to late summer for a standard 15-mm mussel was 60%. This decline was greater than might be expected from gamete release alone and was likely a result of nutritional stress from warm summer temperatures and limited food supplies. Between 1990 and 1991, the mean AFDW of a 15-mm mussel declined 34% and 50% at the high- and low-density site, respectively. However, when C:N ratios and lipid levels in 1990 and 1991 were compared, C:N ratios were only lower in fall 1991 compared to fall 1990, and lipid levels for the 2 years were generally similar.

First Finding of the Amphipod Echinogammarus ischnus and the Mussel Dreissena bugensis in Lake Michigan

The first finding of the amphipod Echinogammarus ischnus and the mussel Dreissena bugensis in Lake Michigan is documented. These two species are widespread and abundant in the lower lakes, but had not yet been reported from Lake Michigan. E. ischnus is generally considered a warmwater form that is typically associated with hard substrates and Dreissena clusters in the nearshore zone. Along the eastern shoreline of Lake Michigan, this species was present at rocky, breakwall habitats along the entire north-south axis of the lake. Although not abundant, this species was also found at soft-bottomed sites as deep as 94 m in the southern basin. The finding of this species in deep offshore waters apparently extends the known habitat range for this species in the Great Lakes, but it is found in deep water areas within its native range (Caspian Sea). D. bugensis was not abundant, but was present in both the southern and northern portions of the lake. Individuals of up to 36 mm in length were collected, indicating that it had probably been present in the lake for 2 or more years. Also presented are depth-defined densities of D. polymorpha at 37 sites in the Straits of Mackinac in 1997, and densities at up to 55 sites in the southern basin in 1992/93 and 1998/99. Mean densities decreased with increased water depth in both regions. Maximum mean density in the Straits in 1997 was 13,700/m2 (≤ 10 m), and maximum density in the southern basin in 1999 was 2,100/m2 (≤ 30 m). Mean densities at the ≤ 30-m interval in the southern basin remained relatively unchanged between 1993 and 1999, but increased from 25/m2 to 1,100/m2 at the 31 to 50 m interval over the same time period. D. polymorpha was rare at sites > 50 m. The presence of E. ischnus and the expected population expansion of D. bugensis will likely contribute to further foodweb changes in the lake.

Recent changes in benthic macroinvertebrate populations in Lake Huron and impact on the diet of lake whitefish (coregonus clupeaformis)

Surveys of the benthic macroinvertebrate community were conducted in the main basin of Lake Huron in 2000 and 2003, and corresponding studies of lake whitefish diets were conducted in 2002-2004. Populations of three major benthic taxa, Diporeia spp., Sphaeriidae, and Chironomidae, declined dramatically between 2000 and 2003, with densities declining 57%, 74%, and 75% over this 3-year period. By 2003, Diporeia, an important food source for lake whitefish, was gone or rare at depths < 50 m except in the far northeastern portion of the lake. In contrast, densities of the Dreissena bugensis (quagga mussel) increased between 2000 and 2003, particularly at the 31-50 m depth interval, while densities of the zebra mussel Dreissena polymorpha remained stable. As expected, the diet of lake whitefish varied with fish size. Age-0 lake whitefish fed mainly zooplankton, most of which were Daphnia (98%). Medium lake whitefish (< 350 mm excluding age-0 fish) fed mainly on zooplankton, Chironomidae, and Dreissena bugensis, and large lake whitefish (350 to 688 mm) fed mainly on D. bugensis and Gastropoda. The diet of medium and large lake whitefish reflected the changing nature of the benthic community; that is, Diporeia was rarely found in the diet while D. bugensis played a prominent role. Since Diporeia has a much higher energy content than D. bugensis, contrasting density trends in the two organisms will have long term consequences to the relative health of lake whitefish populations in the lake.

Long-Term Decline in Freshwater Mussels (Bivalvia: Unionidae) of the Western Basin of Lake Erie

Long-term trends in the abundance of unionids in the western basin of Lake Erie were examined from data collected at 17 stations in 1961, 1972, and 1982. The mean number of unionids at these stations declined over this time period, decreasing from 10 m−2 in 1961, to 6 m−2 in 1972, down to 4 m−2 in 1982. This decline in abundance was reflected in the decrease in the number of stations where mussels were found; unionids were found at 16 of the 17 stations in 1961, but at only 6 stations in 1982. Reasons for the decrease in the unionid population are not generally apparent, but are probably related to the decline in water quality and periods of low oxygen levels over the time period of the surveys.