3.3 Appendices

 

Criteria for establishing lake depth zones and embayments in the Great Lakes

 

Offshore/Nearshore boundary

This was established specifically at the whole depth interval closet to the median depth, so ~½ lake is deeper and ½ shallower.  This depth is generally close to the mean depth of the lake a previous definition used to define nearshore (EPA 1992). 

 

Shallow nearshore/Medium Nearshore boundary

This was established at 30 m water depth, or less, as defined below.  This is an ecologically-based criterion.  It constrains to a zone where a bottom thermocline may be present in summer but the depth is not deep enough have a hypolimnion well represented.   It also defines a zone where benthic-pelagic coupling is intensified and the nature of the food web differs from offshore.  The maximum depth of this zone is limited in some lakes to less than 30 m to maintain this zone as near-coastal (generally, but not always within 10 km of coastline), but more importantly so as not to exceed about 25% of the whole lake area.   Across the lakes, this strategy provides a shallow nearshore zone representing a minimum of 10% of the area of a lake and a maximum near 25% (see below).

 

In Lake Superior, we have rejected sites drawn in the shallow nearshore category that were not adjacent to the main coast -- i.e., those which fell along island shorelines or represented topographic shoals within the body of the lake, rather than being part of a transition from shore to deeper waters, were rejected.  This is an effective strategy across the Great Lakes for defining this shallow nearshore category.

 

Lake

Shallow nearshore boundary

Nearshore/Offshore Boundary

Mean Depth

 

Depth

% of area1

Depth

% of area1

(EPA 1992)

 

 

 

 

(within nearshore)

 

Superior

30 m

10.0%

150 m

50.0%

149

Michigan

30 m

25.9%

80 m

51.4%

85

Huron

20 m

25.1%

50 m

51.9%

59

Erie

10 m

19.5%

20 m

54.8%

19

Ontario

30 m

24.0%

80 m

50.4%

86

1 Defined using hypsographic curves derived digitized bathymetry from NOAA and other sources, with resolution depending on the lake. The area of the lake represented by depth intervals (1, 5, or 10 m depth bins, depending on the lake) was calculated to produce the hypsographic curve and % of area was then used as part of the criteria for setting depth boundaries (as above).

 

Embayment population: definition and criteria

The embayment population was defined using morphological criteria defined in EPA (1992).  Embayments are semi-enclosed; in simple terms, the shoreline indentation and thus the length of the embayment are longer than the width of the mouth opening to the lake.  The embayment population was further restricted to those larger than 1 km2 in area but no greater than ~50 km2, and no embayments could have more than 2 sub-embayments that otherwise meet the criteria..  The criteria therefore excluded larger Bays, like Green Bay and Saginaw Bay, which are included in the nearshore of the lakewide design.  Existing embayment lists were refined and supplemented from previous studies (EPA 1992) using GIS investigations to apply the selection criteria to the US shoreline through the Great Lake Environmental Indicators (GLEI) project (see Danz et al. 2005a and http://glei.nrri.umn.edu).  The final selection of 50 embayments was done by random draw.

 

 

References

 

Danz, N., R.R. Regal, G.J. Niemi, V.J. Brady, T. Hollenhorst, L. Johnson, G.E. Host, J. Hanowski, C. Johnston, T. Brown, J. Kingston, and J.R. Kelly. 2005a. Environmentally stratified sampling design for the development of Great Lake environmental indicators. Environmental Monitoring and Assessment 102:41-65.

 

Danz, N.P., G.J. Niemi, R.R. Regal, T. Hollenhorst, L. B. Johnson, J.M. Hanowksi, R.P. Axler, J.H. Ciborowski, T. Hrabik, V. J. Brady, J.R. Kelly, J.C. Brazner, R.W. Howe, and G.E. Host.  2005b.   Integrated measures of anthropogenic stress in the U.S.Great Lakes basin. Enviornmental Management (In press)   

 

EPA. 1992.   Great Lakes monitoring and research strategy.   U.S. EPA Report, EPA/620/R-92/001, Environmental Monitoring and Assessment Program, Office of Research and Development, Washington DC 20480