Middle St. Johns - Florida Department of Environmental Protection

Water Quality Assessment Report: Middle St. Johns 249
EXCEEDANCES OF NUTRIENT THRESHOLDS
The state currently has a narrative nutrient criterion instead of a numeric value for
nutrient thresholds. The narrative criterion states, “In no case shall nutrient
concentrations of a body of water be altered so as to cause an imbalance in natural
populations of aquatic flora or fauna.” The Impaired Surface Waters Rule provides an
interpretation of the narrative nutrient criterion. In general, the Trophic State Index (TSI)
and the annual mean chlorophyll a values are the primary means for assessing whether a
waterbody should be assessed further for nutrient impairment.
The rule also considers other information that might indicate an imbalance in flora or
fauna due to nutrient enrichment, such as algal blooms, excessive macrophyte growth, a
decrease in the distribution (either in density or aerial coverage) of seagrasses or other
submerged aquatic vegetation, changes in algal species richness, and excessive diel
oxygen swings.
Potential nutrient impairment was evaluated by calculating annual mean chlorophyll
a values for estuaries and streams and the TSI for lakes. For lakes, the TSI was
calculated using chlorophyll a, total phosphorus, and total nitrogen measurements. Direct
evidence of imbalances of flora and fauna in waterbodies was also considered in the
evaluation of nutrient impairments.
In estuarine areas, a water was considered nutrient enriched if the annual mean
chlorophyll a values were greater than 11 micrograms per liter (µg/L) or if annual mean
chlorophyll a values increased by more than 50 percent over historical values for at least
two consecutive years. For streams, a water was considered nutrient enriched if the
annual mean chlorophyll a values were greater than 20 µg/L or if the annual mean
increased by more than 50 percent over historical values for at least two consecutive
years.
A lake with a mean color greater than 40 platinum cobalt units (PCUs) was
considered nutrient enriched if the annual mean TSI exceeded 60. A lake with a mean
color less than or equal to 40 PCUs was considered nutrient enriched if the annual mean
TSI exceeded 40. In addition, a lake was considered nutrient enriched if there was an
increase in TSI over the 1989 to 2000 period or if TSI measurements were 10 units higher
than historical values.
EXCEEDANCE SOF BIOLOGICAL THRESHOLDS
Bioassessments were carried out for streams, lakes, canals, and rivers using the
Impaired Surface Waters Rule as guidance and following the Department’s standard
operating procedures, which provide definitions and specific methods for the generation
and analysis of bioassessment data. These are referenced in the individual bioassessment
data tables contained in the Basin Status Reports. The purpose behind using a
bioassessment methodology in surface water characterizations is that biological
components of the environment manifest long-term water quality conditions and thus
provide a better indication of a waterbody’s true health than discrete chemical or physical
measurements alone. Similar to water quality criteria, bioassessment methods involve
the identification of a biological reference condition, based on data from unimpaired or
least impacted waters in a given region.