One of the effects of the dreissenid invasion into the Laurentian Great Lakes appears to be a resurgence in the abundance of the nuisance alga Cladophora glomerata which experienced a marked decline following phosphorus abatement in the late 1970s and early 1980s. A subsidy of bioavailable phosphorus excreted by dreissenid mussels could be an important mechanism facilitating the growth of C. glomerata. To assess the importance of phosphorus released by mussels to C. glomerata growth in the nearshore, we conducted a survey of mussel distribution and abundance followed by in situ experiments designed to measure dreissenid phosphorus excretion rates. Average dreissenid mussel abundance in our study area was 3674 individuals/m², with an average biomass of 52.2 g of shell free dry mass/m². The mussels excreted bioavailable soluble reactive phosphorus at an average rate of 7.02 µg SRP/g shell free dry mass/hour, contributing about 11 t of soluble reactive phosphorus to our study area over the C. glomerata growing season. Dreissenids appear to be an important source of recycled bioavailable phosphorus to the nearshore, supplying more soluble reactive phosphorus to our study area than local watercourses and WWTPs, and more phosphorus than is required to sustain local C. glomerata growth.

Mercury cycling in Lake Superior and Lake Michigan was evaluated based on measurements of mercury levels, modeling of evasional fluxes, and development of first-order mass balance models. Total mercury, methylmercury, and dissolved gaseous mercury were measured on sampling cruises in Lake Michigan (2005) and Lake Superior (2006). Average total mercury concentrations in unfiltered surface water were higher in Lake Michigan (420 ± 40 pg/L) compared to Lake Superior (210 ± 20 pg/L). Methylmercury levels were below the detection limit in Lake Michigan. Larger sample volumes were collected to lower detection limits in Lake Superior in 2006 and methylmercury levels averaged 7 ± 6 pg/L. Dissolved gaseous mercury concentrations were also higher in Lake Michigan (27 ± 7 pg/L) compared to Lake Superior (14 ± 8 pg/L). Evasional fluxes were estimated using a two-film model for air-water exchange. The annual evasional flux in Lake Michigan was determined to be ∼380 kg/yr from Lake Michigan and ∼160 kg/yr from Lake Superior. Total mercury burdens in each lake were estimated to be ∼2500 kg in Superior and ∼2100 kg in Lake Michigan demonstrating that evasional fluxes play an important role in the mass balance of each lake, particularly Lake Michigan. A simple first-order mass balance model demonstrates the importance of air-water exchange and sedimentation as primary removal processes for Hg in each lake. Uncertainties in the mass balance model are highlighted due to lack of key data, particularly in Lake Superior.

We conducted a probability-based net tow sampling of Lake Superior in 2006 and compared the zooplankton biomass estimate with an estimate from laser optical plankton counter (LOPC) surveys. The net survey consisted of 52 sites stratified across three depth zones (0–30, 30–150, >150 m). The LOPC tow surveys were extensive and spatially covered much of Lake Superior (>1300 km of towing). The LOPC was field calibrated to Lake Superior zooplankton samples collected across the years of 2004 to 2006. The volume-weighted lakewide zooplankton biomass determined by traditional net tows to 100-m sample depth was 20.1 (±7.8 SD n = 52) mg dry-weight m^-3. The estimates varied by depth zones within the lake, where nearshore (0–30 m) estimates were highest and highly variable. Net sites for the LOPC field calibration were removed to allow for LOPC validation with independent nets; the resulting net-based estimate 20.0 (± 9.3 SD n = 38) mg dry-weight m^-3 and LOPC lake-wide estimate 19.1 (±3.3 SD) mg dry-weight m^-3 agreed well. Consistency across survey methods for lake-wide estimates suggested that LOPC survey data provides a comparable assessment tool to traditional nets for collecting zooplankton biomass data. We briefly compare our results with some observed historical patterns. Onshore—offshore trends in zooplankton biomass concentrations were similar to the last major lake-wide survey in 1973. The LOPC provided high resolution data on Zooplankton biomass distribution. Using simultaneously collected in situ sensor data, the LOPC zooplankton biomass distributions over horizontal and vertical space can be modeled as a function of temperature and fluorescence.

Eleven male walleyes (Sander vitreus) and 10 female walleyes from the Saginaw Bay (Lake Huron) population were caught during the spawning run at Dow Dam (Midland, Michigan) in the Tittabawassee River during April 1996, and individual whole-fish polychlorinated biphenyl (PCB) determinations were made. Total PCB concentrations averaged 7.95 and 3.17 mg/kg for males and females, respectively. As part of the Natural Resource Damage Assessment remediation process, contaminated sediments from the Saginaw River, the main tributary to Saginaw Bay, were removed during 2000 and 2001. Total PCB concentrations of 10 male and 10 female walleyes caught at Dow Dam during April 2007 averaged 1.58 and 0.55 mg/kg, respectively. Thus, dredging of the Saginaw River appeared to be effective in reducing PCB concentrations of Saginaw Bay adult walleyes, as both males and females decreased in PCB concentration by more than 80% between 1996 and 2007. However, the ratio of male PCB concentration to female PCB concentration did not decline between 1996 and 2007. This persistent gender difference in PCB concentrations was apparently due to a gender difference in habitat utilization coupled with a persistent spatial gradient in prey fish PCB concentrations from the Saginaw River to Lake Huron.

Knowledge of stock—recruitment dynamics is as important for control of pest species such as the sea lamprey (Petromyzon marinus) as it is for sustainable harvest management of exploited fish species. A better understanding of spatial and temporal variation in recruitment of pest populations may inform managers on where and when to effectively apply different control methods. Sea lamprey stock—recruitment data combined from streams across the Great Lakes basin into a Ricker stock—recruitment model indicated both compensation (density-dependent survival) and a large amount of density-independent recruitment variation. We evaluated the use of a mixed-effects model to look at common year effects and stream-level variables that could affect productivity and growing season length, with the objective of identifying factors that may explain this density-independent variation in recruitment. After selecting the “best model”, we tested factors that might affect recruitment variation, using a Great Lakes dataset comprising 97 stream— years. Lake Superior tributaries, streams with larger numbers of lamprey competitors, and streams regularly requiring lampricide treatment showed significantly higher recruitment. Alkalinity and thermal regulation did not affect the observed recruitment pattern among streams. In four long-term study streams we observed significant variation among streams, tested as a fixed effect, but no evidence suggested a common pattern of variation among years. Differences in recruitment among streams were consistent with evidence of quality of spawning and larval habitat among streams. Our findings suggest that management models should account for differences in recruitment dynamics among sea lamprey-producing streams, but not common year effects.

The feasibility of satellite-based monitoring of phytoplankton chlorophyll a concentrations in Lake Erie is assessed by applying globally calibrated, ocean-derived color algorithms to spatially and temporally collocated measurements of SeaWiFS remote sensing reflectance. Satellite-based chlorophyll a retrievals were compared with fluorescence-based measurements of chlorophyll a from 68 field samples collected across the lake between 1998 and 2002. Twelve ocean-derived color algorithms, one regional algorithm derived for the Baltic Sea's Case 2 waters, and a set of regional algorithms developed for the western, central and eastern basins of Lake Erie were considered. While none of the ocean-derived algorithms performed adequately, the outlook for the success of regionally calibrated and validated algorithms, with forms similar to the ocean-derived algorithms, is promising over the eastern basin and possibly the central basin of the lake. In the western basin, each of the regional algorithms considered performed poorly, indicating that alternative approaches to algorithm development, or to satellite data screening and analysis procedures will be needed.

In recent decades, three important events have likely played a role in changing the water temperature and clarity of the Laurentian Great Lakes: 1) warmer climate, 2) reduced phosphorus loading, and 3) invasion by European Dreissenid mussels. This paper compiled environmental data from government agencies monitoring the middle and lower portions of the Great Lakes basin (lakes Huron, Erie and Ontario) to document changes in aquatic environments between 1968 and 2002. Over this 34-year period, mean annual air temperature increased at an average rate of 0.037 °C/y, resulting in a 1.3 °C increase. Surface water temperature during August has been rising at annual rates of 0.084 °C (Lake Huron) and 0.048 °C (Lake Ontario) resulting in increases of 2.9 °C and 1.6 °C, respectively. In Lake Erie, the trend was also positive, but it was smaller and not significant. Water clarity, measured here by August Secchi depth, increased in all lakes. Secchi depth increased 1.7 m in Lake Huron, 3.1 m in Lake Ontario and 2.4 m in Lake Erie. Prior to the invasion of Dreissenid mussels, increases in Secchi depth were significant (p < 0.05) in lakes Erie and Ontario, suggesting that phosphorus abatement aided water clarity. After Dreissenid mussel invasion, significant increases in Secchi depth were detected in lakes Ontario and Huron.

Brook trout (Salvelinus fontinalis) are found throughout Lake Superior, Lake Nipigon, and their tributaries. Lacustrine and adfluvial life history variants were historically popular with anglers and were called coasters; coaster brook trout populations are now severely reduced and are of conservation concern. Coasters were known to grow larger and mature later than their stream resident counterparts. This study compared movement patterns, age, size, condition, and relative weight of wild coaster and resident brook trout from the Hurricane River, Pictured Rocks National Lakeshore, Michigan. Wild brook trout ≥ 100 mm from the Hurricane River downstream from Hurricane Falls were tagged with passive integrated transponder tags and monitored for stream-lake movement behavior from May 2003 to November 2007. During 2006 and 2007, brook trout were scale sampled and aged to construct a regression that was then used to calculate the age of all brook trout tagged from 2003 to 2007. Most brook trout movement took place in the fall with October the peak month of emigration with a secondary peak in late spring/early summer and some activity nearly year round. There were no differences found in age structure, size or condition between coasters and residents while in the stream. Our data suggest that a priori growth differences are not determining the expression of coaster outmigration and that stream-lake movements made by coasters, likely driven by habitat requirements, may be highly flexible and facultative.

In Lake Erie and Lake Ontario, the Ponto-Caspian amphipod Echinogammarus ischnus has replaced the native amphipod Gammarus fasciatus on rocky substrates colonized by dreissenid mussels, which provide interstitial refugia for small invertebrates. Based on the premise that an invader's vulnerability to predation is influenced by its evolutionary experience with the predator and its ability to compete for refugia, we hypothesized that amphipod species replacement is facilitated through selective predation by the round goby Neogobius melanostomus, a Ponto-Caspian fish that invaded the Great Lakes in the early 1990s and is now colonizing the St. Lawrence River. In laboratory experiments, we determined if E. ischnus excludes G. fasciatus from mussel patches, and if the vulnerability of G. fasciatus to predation by gobies is increased in the presence of the invasive amphipod. E. ischnus and G. fasciatus did not differ in their use of mussel patches, either when alone or in each other's presence. Both species were equally vulnerable to predation by the round goby. In field experiments, we determined if the round goby exerts a stronger impact than native predators on the relative abundance of amphipod species. Our results suggest that E. ischnus is more vulnerable to native predators, but the round goby does not have a differential impact on the native amphipod. We conclude that competition with E. ischnus does not increase the vulnerability of G. fasciatus to goby predation, and that the round goby does not promote the replacement of G. fasciatus by E. ischnus in the St. Lawrence River. The outcome of antagonistic interactions between exotic and native amphipods is mediated more by abiotic factors than by shared evolutionary history with other co-occurring exotic species.

In 2007 archived great blue heron (Ardea herodias) eggs collected from Indiana Dunes National Lakeshore, IN, (Indiana Dunes) in 1993 were analyzed for 11 perfluorinated compounds (PFCs) and 7 polybrominated diphenyl ethers (PBDEs). Concentrations of perfluorooctane sulfonate, the major contributor to total PFC concentrations, were below the toxicity thresholds estimated for bobwhite quail (Colinus virginianus) and mallards (Anas platyrhynchos), but within the toxicity threshold estimated for white leghorn chickens (Gallus domesticus). The ranking of PBDE congener concentrations by percent concentration (PBDE-47>-99>-100>153>-154>-28>-183) was consistent with the Penta-PBDE formulation. Total PBDE concentrations in great blue heron eggs from Indiana Dunes were elevated and probably reflect local contamination from highly urbanized and industrialized inputs into Lake Michigan. Polybrominated diphenyl ether concentrations were within levels associated with altered reproductive behavior in other avian species and based on trends in other Great Lakes birds are probably higher today.

The primary production in meromictic Lake Kivu is sustained by external nutrient inputs and by internal loading due to upwelling caused by sub-aquatic sources. We present here the results of external loading of phosphorus (P), nitrogen (N) and silica (Si) by rivers and atmospheric deposition measured from 2006 to 2008. These external inputs are compared to internal loading. The input of soluble-reactive P (SRP), supplied in equal parts from rivers and atmospheric deposition, adds up to 230 t P yr^-1, 20 times less than total P load. Ammonium (mainly via rainwater) and nitrate (mainly via rivers) are primary sources of the dissolved N load (5400 t N yr^-1), with both species contributing ∼50%. Dissolved Si input (40,000 t Si yr^-1) is unique in that only ∼60% enters by rivers, while the remaining ∼40% comes from sub-aquatic sources and atmospheric deposition is negligible. Based on the molar nutrient ratios, we identify P as the limiting factor for algae production. Despite the strong anthropogenic impact on the catchment and the high particle erosion (74 t km^-2 yr^-1), the area-specific nutrient mobilization is rather low. The external nutrient input is therefore not the cause for the reported increase of methane production in the last decades. External loading to the epilimnion plays a lesser role for all three nutrients (∼10% for SRP, ∼25% for dissolved N and ∼45% for dissolved Si), as compared to the lake-internal loading by upwelling (90%, 75% and 55%, respectively). Lake Kivu, therefore, is similar to other East African large lakes in that the internal loading exceeds the external loading. Despite the substantial uncertainty of the load estimates of up to 50%, we can conclude that the observed nutrient input is consistent with the primary production of 260 g C m^-2 yr^-1 recently measured by Sarmento et al. (2006) and also consistent with the lake-internal fluxes established by Pasche et al. (in press).

Stocking of hatchery-raised Chinook salmon has been the principal tool utilized by fishery managers for controlling alewives in Lake Ontario and elsewhere in the Great Lakes. Stocked Chinook salmon are also often viewed by anglers as the principal source of maintaining catch rates. Stocking levels are often controversial and set with limited information about the relative contribution of wild fish to lake-wide populations. Recent research documenting large numbers of age-0 fish in tributaries suggested that wild reproduction was increasing and greater than previously thought. Estimating the contribution of wild Chinook salmon is imperative for successful management of this economically important recreational fishery. To differentiate wild from hatchery-derived Chinook salmon, we developed and validated a classification rule from scale pattern analysis of known-origin fish that was based on the area of the scale focus and the distance between the scale focus and the first circulus. We used this technique to determine the annual proportion of anglercaught, age-3 wild Chinook salmon in Lake Ontario from 1992 to 2005. On average over 14 years, the annual proportion of wild age-3 Chinook salmon was 62% (±13.6%, 95% CI), but has varied between 24% (±9.4%) and 82% (±11.2%). Wild fish have been a high proportion of the Chinook salmon population in Lake Ontario since the late 1980s throughout a period when the lake underwent considerable changes, suggesting that wild and hatchery-origin Chinook salmon are both important components for managing the predator-prey dynamics in Lake Ontario and maintaining angler catch rates.

We estimate the economic benefits of Saginaw Bay coastal marsh with the travel cost and contingent valuation methods. The travel cost method is based on revealed preferences: actual recreation behavior. Using a sample of the general population of Michigan and Michigan hunting and fishing license holders we find that Saginaw Bay recreation site selection is negatively related to travel cost and positively related to wetland acreage. The contingent valuation method is based on stated preferences: answers to hypothetical survey questions. We find that willingness-to-pay is negatively related to marsh protection cost and positively related to income and environmental organization membership. Using a combination of theory and empirical results we argue that revealed and stated preference methods are complementary when estimating the total value of coastal marsh. The present value of each acre of coastal marsh is $1870 for the purpose of recreation. The present value to recreation nonusers adds $551 per acre. The total present value of each acre of coastal marsh could be as high as $2421.

As part of the Lake Michigan Mass Balance Project, total and methyl mercury were determined for lake trout (Salvelinus namaycush) and five forage fish species collected from Lake Michigan near Saugatuck, Michigan, and Port Washington, Sheboygan Reef, and Sturgeon Bay, Wisconsin, between 1994 and 1995. With a mean concentration of 179 ng/g wet wt., whole lake trout total mercury (Hg_T) concentrations ranged between 27.6 and 348 ng/g wet wt. For combined sites, 1–4 yrs, 5–6 yrs, 7–11 yrs, and 12–15 yrs lake trout mean Hg_T concentrations were 73.7, 130, 212, and 280 ng/g, respectively. Forage fish species alewife (Alosa pseudoharengus), bloater (Coregonus hoyi), slimy sculpin (Cottus cognatus), deepwater sculpin (Myoxocephalus thompsoni), and rainbow smelt (Osmerus mordax) had mean Hg_T concentrations of 63.8, 55.3, 36.7, 51.4, and 35.2 ng/g wet wt., respectively. With the exception of alewife, bloater, and slimy sculpin, all fish species contained approximately 100% methyl mercury (MeHg). Field bioaccumulation factors (BAF) were consistent with a Lake Michigan food chain that is more efficient at transferring MeHg to higher trophic levels than some inland lakes. This and other studies of lake trout from Lake Michigan document decreasing Hg_T concentrations in lake trout from 1971 to 1985 and constant or increasing concentrations between 1985 and 2000. These observations were supported by a similar trend in Lake Michigan Hg sediment fluxes. To our knowledge, this is the most intense two year study of mercury in fish for any Great Lake or other large fresh water system and is one of the most complete studies of mercury cycling in the Lake Michigan food chain.

Toxin-producing Cyanobacteria have been documented in Lake Erie and Ontario in the last several years. We developed algorithms to discriminate potentially toxic cyanobacterial blooms from other harmless phytoplankton blooms and to extract relative phycocyanin abundances from Moderate Resolution Imaging Spectrometer (MODIS) satellite data. Lee's quasi-analytical algorithm was used to calculate total absorption and backscatter from the 250 m, 500 m and 1 km bands of MODIS scenes. A non-negative least square algorithm was then utilized to discern relative concentrations of Chlorophyta (green algae), phycocyanin-rich Cyanobacteria (blue-green algae), and colored dissolved organic matter and suspended sediments combined in lake waters using published absorption spectra for these components. MODIS-derived cyanobacterial concentrations and/or bloom distributions from 10 scenes acquired in the summers of 2004 and 2005 were successfully verified against contemporaneous calibrated measurements of pigments that were acquired from measurements made using continuous fluorimetric measurements of surface water (1 m depth) from six cruises, and three additional cyanobacterial blooms reported in the scientific literature between 2002 and 2006. These results demonstrate that this methodology could be used to develop a cost-effective practical screening method for rapid detection and warning of potentially toxic cyanobacterial blooms in the lower Great Lakes.

Temperature influences the rates of many ecosystem processes. A number of recent studies have found evidence of systematic increases in Great Lakes surface water temperatures. Our study aims to construct empirical relationships between surface water temperatures and local air temperatures that can be used to estimate future water temperatures using future air temperatures generated by global climate models. Remotely sensed data were used to model lake-wide average surface water temperature patterns during the open-water period in Lakes Superior, Huron, Erie, and Ontario. Surface water temperatures typically exhibit linear warming through the spring, form a plateau in mid-summer and then exhibit linear cooling in fall. Lake-specific warming and cooling rates vary little from year to year while plateau values vary substantially across years. These findings were used to construct a set of lake-specific empirical models linking surface water temperatures to local air temperatures for the period 1995–2006. Hindcasted whole-lake water temperatures from these models compare favourably to independently collected offshore water temperatures for the period 1968–2002. Relationships linking offshore water temperatures to inshore water temperatures at specific sites are also described. Predictions of future climates generated by the Canadian Global Climate Model Version 2 (CGCM2) under two future greenhouse gas emission scenarios are used to scope future Great Lakes surface water temperatures: substantial increases are expected, along with increases in the duration of summer stratification.

Deepwater sculpin Myoxocephalus thompsonii are an important component of Great Lake's offshore benthic food webs. Recent declines in deepwater sculpin abundance and changes in bathymetric distribution may be associated with changes in the deepwater food web of Lake Huron, particularly, decreased abundance of benthic invertebrates such as Diporeia. To assess how deepwater sculpins have responded to recent changes, we examined a fifteen-year time series of spatial and temporal patterns in abundance as well as the diets of fish collected in bottom trawls during fall of 2003, 2004, and 2005. During 1992–2007, deepwater sculpin abundance declined on a lake-wide scale but the decline in abundance at shallower depths and in the southern portion of Lake Huron was more pronounced. Of the 534 fish examined for diet analysis, 97% had food in the stomach. Mysis, Diporeia, and Chironomidae were consumed frequently, while sphaerid clams, ostracods, fish eggs, and small fish were found in only low numbers. We found an inverse relationship between prevalence of Mysis and Diporeia in diets that reflected geographic and temporal trends in abundance of these invertebrates in Lake Huron. Because deepwater sculpins are an important trophic link in offshore benthic food webs, declines in population abundance and changes in distribution may cascade throughout the food web and impede fish community restoration goals.

Although mass mortality of alewives (Alosa pseudoharengus) exposed to cold temperatures can be a destabilizing force in the interactions between salmonines and alewives in the Great Lakes, the physiological basis of these events is poorly understood. To examine the influence of diet on acute exposure to cold temperatures, we fed alewives two commercial frozen foods (Artemia and Daphnia) which differed in nutritional composition. Compared to Daphnia, the Artemia contained higher levels of key nutrients (polyunsaturated fatty acids, carnitine, and vitamin E) that have been implicated in cold tolerance of fishes. When exposed to a laboratory cold challenge, the alewives maintained on frozen Artemia exhibited significantly lower mortality rates than those fed frozen Daphnia. Of the three nutrients examined, vitamin E may be most responsible for enhancing cold tolerance since levels of this nutrient were significantly elevated in the Artemia-fed fish. More information regarding the nutritional composition of alewife diets would improve our understanding of alewife cold-temperature mortality and may eventually lead to better management of salmon and trout populations in the Great Lakes.