Linear Regression: EPT Taxa Richness vs. Percent Sand and Fines for Minnesota Streams

Plot

Analysis: Linear regression
Independent variable: Percent (%) sand and fines
Dependent variable: EPT (Ephemeroptera, Plecoptera and Trichoptera) taxa richness
Classification: Using Omernik level II Ecoregions with dummy variable = 0 for Ecoregions 46, 47, and 51 and dummy variable = 1 for Ecoregions 50 and 52
Key: symbols = observations, solid line = mean line, dashed line = 95% prediction limit, black lines and symbols = Ecoregions 46, 47 and 51, blue lines and symbols = Ecoregions 50 and 52
Sample size: n = 267

Variable	Estimate	t value	p
Intercept	14.473	15.53
Ecoregion class	3.472	2.89	0.004
% Sand & fines	-0.0763	-5.37	<0.0001
% Sand & fines x Ecoregion class	-0.0112	-0.51	0.61

Model generated using SAS Proc GLM with the solution option. The 95% prediction limits were calculated using the equation in step 9 of box 14.3 from Sokal and Rohlf (1995). The intercept, slope, grand mean of % sand & fines, and corrected sums of squares were calculated using Proc REG and Proc UNIVARIATE.
The ecoregions were aggregated into two classes because a preliminary ANOVA indicated that the base material for bedded sediments, % coarse substrates, was significantly greater in Ecoregions 50 and 52 than in Ecoregions 46, 47, and 51. Percent coarse substrates, however, did not significantly differ within either group.
Replicate visits to a site were excluded from the data set.

Agency: Minnesota Pollution Control Agency, St. Paul, Minnesota
Location: Minnesota - analyses restricted to wadeable sites (52 km² < watershed area < 523 km²) from the five ecoregions indicated below.
Omernik Level III Ecoregions examined include: 46 (Northern Glaciated Plains), 47 (Western Corn Belt Plains), 50 (Northern Lakes and Forests), 51 (North Central Hardwoods), and 52 (Driftless Area).

Michael B. Griffith, griffith.michael@epa.gov, (513) 569-7034