## Linear Regression: EPT Taxa Richness vs. Percent Sands 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

### Model

- Ecoregions 50 and 52: EPT richness = 14.473 - 0.0763 (% sand & fines)
- Ecoregions 46, 47 and 51: EPT richness = 17.945 - 0.0875 (% sand & fines)
- r
^{2}= 0.304 - Date: 6 October, 2006

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 |

### Data Analysis

- 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.

### Data Origin

- Agency: Minnesota Pollution Control Agency , St. Paul, Minnesota
- Location: Minnesota - analyses restricted to wadeable sites (52 km
^{2}< watershed area < 523 km^{2}) 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).

### Sampling Design

- Data collected 1996 through 2004.
- Sampling protocols:
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### Contact

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