Watershed Protection Plan - Lower Rio Grande Valley Development ...

STATE OF THE WATERSHED
Figure 13. 24-hour Dissolved Oxygen Values for Stations in the Tidal (13072 and 13073) and Above Tidal (13074 and
13081) Segments of the Arroyo Colorado
25
Arroyo Colorado 24-Hour Dissolved Oxygen
20
Dissolved Oxygen (mg/L)
15
10
5
0
May 05
Aug 05
Apr 04
Jul 04
May 05
Aug 05
Aug 98
Jul 02
Oct 02
May 03
Jul 03
Aug 03
Oct 03
Apr 04
Jul 04
May 05
Aug 05
Aug 98
Aug 98
Jul 99
Jun 00
May 02
May 03
Aug 03
Apr 04
Jul 04
May 05
Aug 05
13081 13074
NON-TIDAL
13072 - Zone of Impairment
TIDAL
13073
24-hour average criteria
24-hour minimum criteria
analysis of water samples collected in the Arroyo
Colorado between 2000 and 2002, the average
concentration of fi ve-day biochemical oxygen demand
(BOD5) at the downstream end of the non-tidal segment
of the Arroyo Colorado was 4.7 mg/l (Figure 14).
Average BOD5 concentrations were found to be
slightly higher in the upper portion of the tidal segment
of the Arroyo Colorado (6.7 mg/l). However, this is
thought to be the result of periodically high organic
matter production in-stream. The higher growth of
microscopic algae during algal blooms in the upper
portion of the tidal segment of the Arroyo Colorado
produces suspended organic matter in the form of
millions of algal cells. These cells produce large
amounts of BOD as they die and are decomposed
by bacteria. Filtered water samples from the tidal
segment of the Arroyo Colorado show an average
BOD5 concentration comparable to that of the non-tidal
segment (4.7 mg/l).
Nitrogen
Nitrogen is an essential plant nutrient. Along with
other nutrients such as phosphorus, plants need only
water and light to grow. This includes microscopic
aquatic algae also known as phytoplankton. Low
concentrations of nutrients can reduce plant growth
and therefore impede the production of food for
organisms that are dependent on this growth, while
high concentrations of nutrients can cause excessive
and uncontrolled growth of algae in aquatic systems
(i.e., algal blooms). Nitrogen-containing substances are
often measured to assess water quality on a short-term
(acute) or long-term (chronic) basis. Elevated ammonia
nitrogen levels can contribute to high phytoplankton
growth rates, but high levels of ammonia nitrogen
can also have an acute (toxic) effect on aquatic life.
Elevated levels of nitrate and nitrite nitrogen often
lead to nutrient enrichment concerns, especially in
coastal water bodies. High levels of ammonia and
nitrate nitrogen are often caused by nonpoint source
Arroyo Colorado Watershed Protection Plan 42 January 2007