This study will examine the effects of agricultural activities on
physical, chemical, and biological characteristics (algae, and benthic
invertebrate communities and habitat) over a wide range (gradient) of
agricultural land-use intensity. Gradient analysis is based on the
observation that environmental patterns govern the structure and
function of ecological systems (populations, communities, and
ecosystems). Gradient studies can be applied within and among selected
environmental settings to determine the management implications of the
observed similarities and differences (that is, to answer the question:
for maximum effectiveness, should regulations and management strategies
vary across the country?). The information obtained from this
investigation will help managers and regulators identify factors that
can be manipulated to make cost-effective improvements in water quality
on a regional and national basis.
Specific questions that will be addressed are:
What is the form (e.g., increasing linear, decreasing linear,
non-linear/threshold) and magnitude of the response?
Which landscape features are most important in differentiating
physical, chemical, and biological responses?
Which physical and chemical factors are associated with
biological responses and which are the most important factors in
driving these responses,
At what levels of land-use intensity do physical, chemical, and
biological components express the greatest change (i.e., identification
of "break points" and "thresholds" where the slope
of the response in community condition changes radically?
What are the tolerances of specific organisms to land-use
How do physical, chemical, and biological responses to urban
land-use gradients compare among environmental settings
(e.g. hydroclimatic regions or ecoregions)?
How can the responses along urban land-use gradients be used to
predict future changes (trends) in water quality?
What are the implications of these findings for the management
and regulation of water quality?
physical, chemical, and biological constituents respond to the
agricultural land-use gradient?
Investigation of responses to a gradient of land-use intensity
requires a very specific design in order to succeed. The gradient of
interest must be precisely defined, and then sites must be selected in such
a way as to control for other gradients (e.g., gradients of natural
features such as climate, geography, soil characteristics, stream size,
hydrologic variability) so that they do not overwhelm the land-use
(anthropogenic) gradient of interest. This is particularly important
for ecological studies since many natural gradients (e.g., elevation,
stream size, climate) are known to have dramatic effects on the
distribution of organisms even in the absence of anthropogenic
The objective of gradient analysis is to identify
which land-use features are associated with particular
gradients and which are most important in determining
water-quality conditions as defined by physical, chemical, and
biological characteristics. Features that are strongly
associated with changes in water-quality conditions and that
can be effectively manipulated become candidates for managing
and regulating the resource.
We sampled 24 intermediate-sized Yakima River tributary streams for
macroinvertebrates, algae, chlorophyll a and biomass, habitat,
nutrients, pesticides, suspended sediment, and basic water quality
parameters. These response variables will be related to detailed
landscape variables, including elevation, drainage area, slope,
geology, soil types, crop type, irrigation practices, and others, to
assess the effect of varying agricultural intensity in the Yakima River