environmental setting, water quality, and ecological indicators of

Photograph by Dennis K. Demcheck, U.S. Geological Survey, 2001
Figure 9. Grazing livestock near a surface-water data-collection site in the Mermentau River Basin, southwestern
Louisiana.
Comparisons between thresholds and the concentrations
determined during the sampling period should be
made carefully for the following reasons:
1. Nutrient concentrations greater than the
thresholds might not cause excessive or
nuisance plant growth immediately, because
other factors such as turbidity and stream
shading affect light penetration, and
influence photosynthesis and aquatic-plant
productivity. Downstream, these factors
might become more favorable for eutrophic
conditions to occur.
2. Drought and streamflow conditions that
occurred during the sampling period may not
represent long-term conditions (fig. 6).
3. Nutrient thresholds are probably lower for
subtropical streams, including those found in
the study area, than for the more northern
22
temperate streams for which they were
developed (Walter Dodds, Kansas State
University, written commun., June 6, 2002).
Phosphorus exists in natural water in several forms
but transformation pathways for phosphorus are not as
numerous as those for nitrogen. Most phosphorus in
aquatic systems is not available for biological uptake
because the solubility of phosphorus is relatively low and
phosphorus adsorbs strongly to particles. Concentrations
of dissolved phosphorus and orthophosphorus in
ground water and surface water, and total phosphorus in
surface water were determined for this study and are
listed in table 6.
The median concentration of dissolved phosphorus
was 0.070 mg/L in ground water and 0.094 mg/L in surface
water (table 6). The highest concentrations of phosphorus
in surface water occurred February-June.
Concentrations greater than 0.25 mg/L were detected at