Rapid water level rise drives unprecedented coastal habitat loss along the Great Lakes of North America

Lake Michigan rose to record high water levels in the 2010s; during this time, some coastal sites experienced habitat loss rates an order of magnitude higher than during previous high water periods throughout the 20th century. The high magnitude and rapid rate of rise observed during the 2012–2020 period in combination with a slight increase in the percentage of storm waves likely accelerated habitat loss rates beyond levels that were observed over the past century. Our data suggest that rapid and relatively large changes from low water levels to high water levels are the main driver of large erosional losses, as the coastal system shifts abruptly from one water-level regime to another. One likely impact of climate change on Great Lakes’ water level is an increase in the variability of fluctuations, thus more scenarios of abrupt and rapid water-level rise and associated habitat loss are expected in the future. We propose that the unprecedented habitat loss observed during the 2012–2020 timeframe will become the new normal in the coming century as enhanced variability in water levels facilitates sustained coastal land loss.     

Great Lakes coastal wetlands as suitable habitat for invasive mute swans in Michigan

Great Lakes coastal wetlands provide critical habitat and food resources for more species than any other Great Lakes ecosystem. Due to past and current anthropogenic disturbances, coastal wetland area has been reduced by >50% while remaining habitat is frequently degraded. Invasive mute swans have contributed to the degradation of coastal wetlands by removing submergent vegetation and competitively excluding native species from breeding areas and food resources. Despite current control practices, mute swan population estimates in Michigan are ~8000, comparable to population estimates in the entire Atlantic Flyway of North America. We collected local abiotic data and adjacent land cover data at 3 scales from 51 sites during 2010 and 2011 and conducted 2 mute swan detection surveys each year during the summer and fall. We developed a single-species, single-season occupancy-based habitat suitability model to determine current and potential mute swan habitat among Great Lakes coastal wetlands. We found mute swans occupied heterotrophic coastal wetlands adjacent to urban areas, which were high in ammonium and oxidation-reduction potential and low in nitrates, dissolved oxygen, and turbidity. Our model provides managers with a valuable tool for rapidly identifying mute swan habitat areas for control efforts, particularly the need for targeting mute swan populations in or near urbanized areas. Our model will also aid managers in monitoring areas that mute swans may invade and prioritizing coastal wetland areas for restoration efforts.     

Suitable habitat model for walleye (Sander vitreus) in Lake Erie: Implications for inter-jurisdictional harvest quota allocations

Models predicting habitat distributions can give insight into species–habitat requirements and anticipate how populations respond to environmental change. Despite the economic and ecological importance of walleye (Sander vitreus) in Lake Erie, no preferred-habitat model exists and the spatial extent of suitable habitat is poorly understood. Empirical species-habitat models for three groups of walleye (juveniles, adults, and all walleye) was developed using records from a long term gill net data base (21 years). We examined the degree to which habitat suitability varies with vertical stratum for each group and whether the new model yields different estimates of available walleye habitat when compared to the current depth-based approach. Walleye occurrence in gill nets was positively related to water temperature, negatively related to water depth and water clarity, and unrelated to dissolved oxygen concentration. A model that incorporated interaction terms among the independent variables performed better than the linear, quadratic, and cubic generalized linear models (GLMs) for all three groups. Our results indicate that the extent of suitable habitat varies spatially in Lake Erie and is greatest in the West basin. Weighted Habitat Suitability Areas (WHSA), a combination of habitat quality and quantity, differed significantly among basins and vertical strata in Lake Erie. The current quota allocation strategy for Lake Erie walleye is based on the proportional amount of preferred habitat by jurisdiction. However, the current depth-based definition of preferred habitat may not be an adequate representation of walleye suitable habitat shared by each jurisdiction.     

Distribution and abundance of freshwater polychaetes, Manayunkia speciosa (Polychaeta), in the Great Lakes with a 70-year case history for western Lake Erie

Manayunkia speciosa has been a taxonomic curiosity for 150 years with little interest until 1977 when it was identified as an intermediate host of a fish parasite (Ceratomyxa shasta) responsible for fish mortalities (e.g., chinook salmon). Manayunkia was first reported in the Great Lakes in 1929. Since its discovery, the taxon has been reported in 50% (20 of 40 studies) of benthos studies published between 1960 and 2007. When found, Manayunkia comprised < 1% of benthos in 70% of examined studies. In one extensive study, Manayunkia occurred in only 26% of 378 sampled events (1991–2009). The taxon was found at higher densities in one area of Lake Erie (mean = 3658/m²) and Georgian Bay (1790/m²) than in five other areas (mean = 60 to 553/m²) of the lakes. A 70-year history of Manayunkia in western Lake Erie indicates it was not found in 1930, was most abundant in 1961 (mean = 8039, maximum = 67,748/m²), and decreased in successive periods of 1982 (3529, 49,639/m²), 1993 (1876, 25,332/m²), and 2003 (79, 2583/m²). It occurred at 48% of stations in 1961, 58% in 1982, 52% in 1993, and 6% of stations in 2003. In all years, Manayunkia was distributed primarily near the mouth of the Detroit River. Causes for declines in distribution and abundance are unknown, but may be related to pollution-abatement programs that began in the 1970s, and invasion of dreissenid mussels in the late-1980s which contributed to de-eutrophication of western Lake Erie. At present, importance of the long-term decline of Manayunkia in Lake Erie is unknown.     

Geophysical benthic habitat mapping in Lake Tanganyika (Tanzania): Implications for spatial planning of small-scale coastal protected areas

Optimizing community-based fisheries management to enhance both food resource and biodiversity conservation in large lakes requires detailed knowledge of benthic habitats, which determines suitability for fish breeding sites. This information is unavailable for much of Lake Tanganyika, whose fisheries are threatened by a warming climate, destructive harvesting practices, and sediment pollution. Lake Tanganyika possesses a remarkably diverse fish population. Much of this is concentrated in areas with water depths less than 30 m and on rocky substrate. Here, geophysical tools were used to map benthic habitats in a 21 km² co-management area of the lake in western Tanzania. Echosounding defined the position of the 30-m isobath, which varies with proximity to deltas and rift-related faults. Side-scan sonar discriminated among four unique substrates: crystalline bedrock, calcite-cemented sandstones, mixed siliciclastic sediments, and shell-rich sediments. Unlithified mixed silts and sands constitute over 91% of the study area. Rocky substrate composed of crystalline basement and calcite-cemented sandstone make up the less than 9% of the substrate in the study area. Crystalline bedrock was present from 0 to 30 m water depth, whereas the calcite-cemented sandstones were encountered in water less than 5 m deep. The spatial organization of rocky substrates is interpreted to be controlled by basin structure and lake level history; these habitats make ideal targets for establishing new small-scale protected areas. The techniques illustrated in this study are broadly applicable elsewhere in Lake Tanganyika, and to other large lakes where data needs for placing conservation reserves are lacking.     

Effects of using air temperature as a proxy for potential evapotranspiration in climate change scenarios of Great Lakes basin hydrology

Hydrologic impacts of climate change are regularly assessed with hydrologic models that use air temperature as a proxy to compute potential evapotranspiration (PET). This approach is taken in the Large Basin Runoff Model (LBRM), which has been used several times for calculation of the runoff from the terrestrial part of the Great Lakes basin under climate change scenarios, with the results widely cited. However, a balance between incoming and outgoing energy, including the latent heat of evaporation, is a fundamental requirement for a land surface, and is not enforced under this approach. For calculating PET and evapotranspiration (ET) in climate change scenarios, we use an energy budget-based approach to adjusting the PET as an alternative that better satisfies conservation of energy. Using this new method, the increase in ET under enhanced greenhouse gas concentrations has reduced magnitude compared to that projected using the air temperature proxy. This results in either a smaller decrease in net basin supply and smaller drop in lake levels than using the temperature proxy, or a reversal to increased net basin supply and higher lake levels. An additional reason not to rely on a temperature proxy relation is that observational evidence demonstrates that the correlation between air temperature and ET (or PET) is restricted to the mean annual cycle of these variables. This brings into question the validity of air temperature as a proxy for PET when considering non-annual variability and secular changes in the climate regime.