Proposed Break of LDI ≤ 2.0 for Determining Minimal ly
Affected Reference Conditions for Water Bodies

Mark T Brown and Kelly C Reiss

October 18, 2005

To avoid an arbitrary break for determining sites t hat are minimally affected by human influences versus those sites actually affected by human activities, we propose using a Landscape Development Intensity (LDI) index break o f less than or equal to 2.0 for sites minimally affected and greater than 2 to designate nonreference sites. This break point of 2.0 has been supported by analyzing a data set of 1 93 depressional palustrine wetlands in Florida, including 75 emergent and 118 forested wet lands. The study set of wetlands includes wetlands spatially located throughout the state and embedded within a variety of land uses from undeveloped lands to row crops, past ure, residential, and commercial.

We have broken the study wetlands into four groups for analysis, including Group 1 – LDI = 1.0; Group 2 – 1.0 < LDI ≤ 2.0; Group 3 – 2.0 < LDI ≤ 3.0; and Group 4 – LDI > 3.0. Using the Florida Wetland Condition Index (FW CI) scaled from 0-100 and its three separate species assemblages (diatoms, macrophytes, macroinvertebrates) (Lane et al. 2003; Reiss and Brown 2005; Reiss, 2006), we have c ompared the mean group scores for Groups 1 and 2 using a 2-sample t-test (Table 1) an d all groups using Fisher’s one way multiple comparison test (Table 2).

Both statistical comparisons show that there is no significant difference between diatom, macrophyte, or macroinvertebrate FWCI scores among LDI Groups 1 and 2 (p < 0.05), suggesting that the variation in scores for palustr ine wetlands with LDI scores ≤ 2.0 may represent the ecological “noise” of minimally affec ted wetlands. Figure 1 shows a box plot example of the scores for the four LDI Groups for the macroinvertebrate FWCI (n = 149). Using Fisher’s one way multiple comparison test we found that macroinvertebrate FWCI scores for Group 1 (LDI = 1) and Group 2 (1.0 < LDI ≤ 2.0) were not statistically different; Group 3 (2.0 < LDI ≤ 3.0) and Group 4 (L DI > 3.0) were not statistically different (p < 0.05); and Groups 1 and 2 were stati stically different from Groups 3 and 4 (Table 2).

Table 1. Comparison of Florida Wetland Condition I ndex (FWCI) scores using a 2- sample t-test for three separate species assemblage s (diatoms, macrophytes, and macroinvertebrates) for depressional palustrine wet lands belonging to low LDI groups, including Group 1 (LDI = 1.0) and Group 2 (1.0 < LD I ≤ 2.0).

Diatom FWCI Macrophyte

FWCI Macroinvertebrate

FWCI Group 1 - LDI = 1.0 84.9 (15.2) 85.1 (9.1) 66.8 (17 .1) Group 2 - 1.0 < LDI ≤ 2.0 80.4 (17.8) 80.3 (13.4) 6 5.9 (18.0)
T-value 0.93 1.82 0.19 p-value 0.36 0.07 0.85

DF 34 52 32 Group values represent mean (standard deviation). DF = degrees of freedom.Table 2. Comparison of Florida Wetland Condition I ndex (FWCI) scores using Fisher’s

one way multiple comparison test for three separate species assemblages (diatoms, macrophytes, and macroinvertebrates) for depression al palustrine wetlands.

Diatom

FWCI Macrophyte

FWCI Macroinvertebrate

FWCI Group 1 LDI = 1.0 84.9 (15.2) a 85.1 (9.1) a 66.8 (17.1) a Group 2 1.0 < LDI ≤ 2.0 80.4 (17.8) a 80.3 (13.4) a 65.9 (18.0) a Group 3 2.0 < LDI ≤ 3.0 64.4 (18.9) b 56.8 (20.3) b 44.3 (22.1) b Group 4 LDI > 3.0 35.5 (25.9) c 26.5 (21.7) c 34.0 (18.4) b Group values represent mean (standard deviation). Means in the same column followed by the same lette r are not significantly different at α = 0.05 using
Fisher’s LSD.Macroinvertebrate FWCI

4 3 2 1 100

80

60

40

20

0

Figure 1. Box plot comparison of macroinvertebrate FWCI scores among LDI Groups, including Group 1 (LDI = 1.0), Group 2 (1.0 < LDI ≤ 2.0), Group 3 (2.0 < LDI ≤ 3.0), and Group 4 (LDI > 3.0). Groups with same letter a re not significantly different at α = 0.05 using Fisher’s one way multiple comparison. B oxes represent 25 th – 75 th quartile range, circles represent group mean values, horizon tal lines represent median values.

a a

b

bLDI vs Pollutant Load

0 0.5 1 1.5 2 2.5 3 3.5 4

0 2 4 6 8 10

Area Weighted LDI

Figure 2. Area-weighted Landscape Development Inte nsity (LDI) index verses phosphorus pollutant load in 64 hydrologic units (s ub-watersheds) of the St. Marks River watershed. (from Brown and Vivas, 2005)

Not withstanding the forgoing, Brown and Vivas (200 5) suggested there was no observable relationship between LDI and phosphorus and nitrogen concentrations in
watersheds of the St. Marks River below an LDI of 3 .0. The variability in background concentrations of phosphorus evident in watersheds having LDI scores of less than 3.0 (Figure 2) is the result of subtle differences in s oils, eco-hydrology, and surficial geology of undeveloped watersheds. In other words, it is d ue to what we might call the background variation of the environment. In our op inion, the use of an LDI of 2.0 represents a very conservative break between distur bed and reference conditions in light
of this fact.

**Literature Cited** Brown, M.T. and B. Vivas. 2005. Landscape Developm ent Intensity Index. Environmental Monitoring and

Assessment 101:289-309 Lane, C.R., M.T. Brown, M. Murray-Hudson, and M.B. Vivas. 2003. The Wetland Condition Index (WCI): Biological Indicators of Wetland Condition for Isol ated Depressional Herbaceous Wetlands in Florida. A Report to the Florida Department of Environmental P rotection. Howard T. Odum Center for Wetlands, University of Florida, Gainesville, Florida, USA Reiss, K C. 2006. Florida Wetland Condition Index f or depressional wetlands. Ecological Indicators 6:3 37-

352 Reiss, K.C. and M.T. Brown. 2005. The Florida Wetla nd Condition Index (FWCI): developing biological indicators for isolated depressional forested wetla nds. A Report to the Florida Department of
Environmental Protection. Howard T. Odum Center for Wetlands, University of Florida, Gainesville, Florida, USA.