log ( SD )= -0.69 log ( E )+ 0.26 (3-3)
and light attenuation coefficient (E) can be predicted from SD (Figure 3-2B) using the
equation:
log (E)= -0.96 log ( SD)+ 0.30 (3-4)
where SD is in meters and E is per meter.
Although E and SD are highly correlated, the large 95% confidence limit (46-
236%) associated with the MDC-SD model published by Canfield et al. (1985) has lead
to speculation that the use of light meter readings could lead to the development of a
more robust model. The MDC of macrophytes in the 32-lake study was negatively
related to the mean light attenuation coefficient (Figure 3-1B) and the relationship was
represented by the following equation:
log (MDC)= -0.51 log (E)+ 0.48 (3-5)
where MDC is in meters and E is per meter. Light attenuation, however, did not predict
MDC any better than SD transparency and actually had a slightly lower coefficient of
determination (R2 = 0.41) than SD readings (R2 = 0.46). This finding demonstrated SD,
an easily measured and inexpensive index of water transparency, is as useful for
assessing MDC as E values that require the use of complex and expensive equipment.
Canfield et al. (1985) suggested the major factor contributing to the variability in
the MDC-Secchi relationship is the type of plant colonizing the lake bottom because
different species of plants have different light requirements. The amount of surface light
penetrating at the maximum depth at which submersed aquatic macrophytes colonized in
the 32 study lakes ranged from < 1% to 47%. The mean percent of incident light at the
maximum depth of colonization was 11%, which was in agreement with much of the