Bioassessment LDI, HDG, BCG

Landscape Development Intensity Index (LDI)

The Landscape Development Intensity index (LDI) is an estimate of how much humans have altered an area of interest around a waterbody. Various land use types (low density residential, row crops, industrial, natural) are assigned coefficients of land use intensity based on estimates of the amount of human energy that is put into those land use types.

The LDI is calculated by multiplying each land use coefficient by the percentage of the area of interest occupied by that land use, and then summing the results. The Florida Department of Environmental Protection (DEP) uses the LDI as a tool to estimate potential land use impacts on streams, lakes and wetlands. For streams and rivers, DEP typically uses a LDI calculated for the 100 m buffer of the waterbody for 10 km upstream of the point of interest. For lakes and isolated wetlands, DEP typically uses a LDI calculated for the 100 m buffer around the waterbody. LDI values less than two (≤2) can be considered minimally disturbed.

Human Disturbance Gradient (HDG)

The Human Disturbance Gradient (HDG) is both a conceptual model and semi-quantitative method for measuring how humans alter and degrade waterbodies (streams, rivers, lakes, wetlands). It includes relative measures of land use disturbance, water quality alteration, hydrologic alteration and habitat alteration.

DEP has used a HDG to develop biological tools, including the Stream Condition Index (SCI), Bio Reconnaissance (BioRecon) and the Lake Vegetation Index (LVI). In the development of these tools, candidate metrics were tested for correlation with the HDG, and metrics with the highest correlations were chosen for the indices.

Biological Condition Gradient (BCG)

The Biological Condition Gradient (BCG) is a conceptual model that relates biological response to increasing levels of stressors. The model describes how attributes of aquatic ecosystems change in response to increasing levels of stressors.

The attributes include several aspects of community structure, organism condition, ecosystem function, and spatial and temporal attributes of stream size and connectivity. The BCG is divided into six tiers of biological condition along the stressor-response curve, ranging from a natural condition to a severely changed condition. The model provides a common framework for interpreting biological information regardless of methodology or geography. It is based in fundamental ecological principles and has been extensively verified by aquatic biologists throughout the United States.

DEP has used the BCG to identify impairment thresholds for its biological tools.

Get a detailed explanation of LDI, HDG and BCG