A Comparative Examination of Recent Changes in Nutrients and Lower Food Web Structure in Lake Michigan and Lake Huron.

The lower food webs of Lake Huron and Lake Michigan have experienced similar reductions in the spring phytoplankton bloom and summer populations of Diporeia and cladocerans since the early 2000s. At the same time phosphorus concentrations have decreased and water clarity and silica concentrations have increased. Key periods of change, identified by using a method based on sequential t-tests, were 2003-2005 (Huron) and 2004-2006 (Michigan). Estimated filtration capacity suggests that dreissenid grazing would have been insufficient to directly impact phytoplankton in the deeper waters of either lake by this time (mid 2000s). Despite some evidence of decreased chlorophyll:TP ratios, consistent with grazing limitation of phytoplankton, the main impact of dreissenids on the offshore waters was probably remote, e.g., through interception of nutrients by nearshore populations. A mass balance model indicates that decreased phosphorus loading could not account for observed in-lake phosphorus declines. However, model-inferred internal phosphorus dynamics were strongly correlated between the lakes, with periods of increased internal loading in the 1990s, and increased phosphorus loss starting in 2000 in Lake Michigan and 2003 in Lake Huron, prior to dreissenid expansion into deep water of both lakes. This suggests a limited role for deep populations of dreissenids in the initial phosphorus declines in the lakes, and also suggests a role for meteorological influence on phosphorus dynamics. The high synchrony in lower trophic level changes between Lake Michigan and Lake Huron suggests that both lakes should be considered when investigating underlying causal factors of these changes.

Using Satellite Observations to Assess the Spatial Representativeness of the GLNPO Water Quality Monitoring Program.

The U.S. EPA's Great Lakes National Program Office (GLNPO) annual water quality survey (WQS) collects data at a relatively small number of stations in each lake. The survey was designed to measure conditions in the open-water regions of the lakes where an assumption of spatial homogeneity was thought likely to be met and the measured variables could be characterized by simple statistics. Here we use satellite observations to assess how well statistics based on samples collected in the GLNPO sampling network represent the lake-wide values of two variables, surface chlorophyll concentration and Secchi depth. We find strong linear relationships between the mean values calculated from the samples and the corresponding averages based on the subsets of the full satellite images. Although overall the means of the values from the sample locations agree well with means calculated from most of the non-coastal regions of the lakes, in terms of water depth, the GLNPO station averages best represent the regions of Lake Huron deeper than 30 m, of Lakes Michigan and Superior deeper than 90 m, and of Lake Ontario deeper than 60 m. When the lake regions are defined by distance offshore rather than by depth, the GLNPO station chlorophyll means in Lakes Huron, Ontario, and Superior are closest to the means for the area of the lakes > 10 km offshore. In Lake Michigan the closest correspondence is with the > 20 km offshore region. On a whole-lake basis in Lake Erie the GLNPO station chlorophyll averages are closest to the average calculated from the entire lake.

Estimation of the width of the nearshore zone in Lake Michigan using eleven years of MODIS satellite imagery.

The nearshore zone, that region of water directly influenced by its proximity to the coast, has received increasing attention in recent years. The extent of the nearshore zone has been defined by some constant descriptive feature: e.g., a specific depth or a particular distance offshore. This type of definition does not allow for the dynamic nature of the relationship between the land and water and how it may be influenced by local, seasonal, or transient effects. Here satellite observations examined evaluate how the width of the nearshore zone in Lake Michigan varies with position along the coastline and with time. Satellite-derived estimates of chlorophyll concentration along seventy-one shore-normal transects spaced approximately 10 km apart around the lake were used to determine the width of the nearshore zone, defined as the point at which the estimated chlorophyll concentration close to the shore approaches the more-uniform offshore concentration. Of a total of 23,807 transects extracted from MODIS observations made between 2003 and 2013, we successfully fit a bi-linear model relating chlorophyll concentration to distance offshore to 15,996. We found that the width of the nearshore zone is variable, both seasonally and spatially. Although the overall median width of 4.5 km (mean width 5.3 km) closely corresponds to the 5 km value used in a number of Great Lakes studies including Lake Michigan, ten percent of the estimates are greater than 8.9 km, likely representing times of enhanced mixing and transport of nearshore waters into the offshore.