Northern Prairie Wildlife Research Center

RICH KAHL

*Wisconsin Department of Natural Resources, Wildlife Research, 1350 Femrite
Drive, Monona, WI 53716*

Effective management and restoration of submerged macrophyte communities often depends on an accurate and meaningful estimate of water clarity. Most studies report water clarity as light attenuation coefficients (AC's) derived from measuring light availability at a single depth or as Secchi transparencies. A few studies use these parameters to indirectly estimate the 1% photic zone based on previously documented relationships between these parameters. Yet submerged macrophytes may require 5-10% of surface light (i.e., the 5% or 10% photic zone), not the 1% commonly used to define the photic zone. I investigated the feasibility of using a regression equation based on the empirical equation for light attenuation in pure water to estimate the 5% and 10% photic zones from light availability measured at regular intervals through the water column. The regression equation used 'ln(Iz/Io)' and 'z' as the dependent and independent variables, respectively, from the empirical equation:

where,

n = AC,

Io = light intensity at water surface,

Iz = light intensity at depth z,

z = depth in meters.

By setting the intercept = 0, the AC (n) equals the slope of the regression equation and any given photic zone depth can be estimated by:

I measured percent of surface light available at 15 cm intervals through
the water column for three sampling locations on each of four-six study lakes
biweekly from April-August, 1986-91. From these, four site-year combinations
were randomly selected for the following analyses. For each sampling site
and date, the regression equation was used to predict the 5% and 10% photic
zones and test the strength of the regression based on the empirical light
attenuation equation. I then investigated whether AC's and estimates of the
5% and 10% photic zones calculated for the 45, 105, and 150 cm sampling depths
differed among these depths. The regression analysis yielded r^{2}'s
>0.98 (*P* = 0.0001) for all sampling sites and dates. AC's and photic
zones varied among the three depths for the four sampling sites pooled (*P* = 0.0001),
but the AC's and photic zones differed for only two of the sites individually
(*P* <0.05). For the pooled data, mean estimates of the 5% and 10%
photic zones from the AC's differed by 28 and 22 cm, respectively, between
the 45 and 150 cm sampling depths. For sites and dates with known 5% or 10%
photic zone depths, estimates from the regression equation and from the AC's
for the 105 cm sampling depth predicted these photic zones with equal accuracy.
However, estimates from AC's for 45 and 150 cm sampling depths were not as
reliable, with the 45 cm sampling depth occasionally leading to large errors.
Therefore, AC's derived from light availability at a single depth are not
representative of the entire water column and probably are not comparable
among studies, especially if sampled at different depths or sites with considerably
different water clarity. The 5-10% submerged macrophyte photic zones estimated
by the regression equation provide wetland managers with more consistent and
meaningful assessments of water clarity conditions, expected maximum depth
of colonization and abundance of submerged macrophytes, and suitability for
transplanting various species.

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