Lake Production, Succession, and Eutrophication - Hydro-Environmental Analysis: Freshwater Environments - page 463

the state of Florida uses for evaluating lakes in that state. Additionally, the method is not applicable
to macrophyte-dominated lakes.

16.4.5 f LorIda ' S t ropHIc S tate I ndex
The Florida Department of Environmental Protection (FDEP) has commonly used a modiied version
of Carlson's TSI to manage lakes throughout the state (FDEP 2009). The Florida TSI has been used,
for example, to determine lakes that are impaired (e.g., for 305(b) and 303(d) assessment). As with
Carlson's TSI, the Florida TSI is not a direct measure of any single lake characteristic, but combines
TN, TP, and chlorophyll-a on a common scale that can also be related to trophic state categories.
Carlson's TSI was modiied by Florida since Florida lakes do not share some of the character-
istics of northern lakes, such as an oxygenated hypolimnia and cool/cold-water isheries (FDEP
2009). Florida's high frequency of dark or stained waters also precluded use of the Secchi depth as
a measure of trophic state. Florida based the modiied TSI on studies by Salas and Martino (1991,
see Section 16.4.3). Salas and Martino analyzed tropical and subtropical lakes of the Americas
(including lakes in north Florida) to identify trophic states corresponding to TP concentrations,
concluding that tropical-subtropical TP concentrations were higher than the equivalent temperate
concentrations by a factor of approximately 2. The FDEP (2009) determined that the subtropical
lakes of Florida were more similar to the tropical and subtropical lakes analyzed by Salas and
Martino (1991) than to the lakes analyzed by the OECD (1982), resulting in the development of a
modiied Florida TSI. That modiied TSI was based on replacing the Secchi depth, used in Carlson's
TSI, with TN; including chlorophyll-a (Salas and Martino only used TP); and based on an analysis
of data from 313 Florida lakes. The Florida TSI is computed from (FDEP 2009):

( )

CHLTSI Nutrient TSI +

FloridaTSI = (16.3)

2

where

CHLTSI =+× 16 8144 . . ln (Chlorophyll-a) and

ForTN/TP > 30 (phosph orus limited)

Nutrient TSI =× 10 236 [. × ln(TP × 1000 238 ) − . ]

ForTN/T PP(nitrogenlimited) < 10

Nutrient TSI =× 10 596215 [. +× . lnTN ( + 000 .01 )]

For N/TP 10 < < 30 (colimited)

Nutrient TSI TP_TSI TN_TSI) = + / 2 , where

TN_TSI =+ 56 [. 19 8 × ln(TN)]

TP_TSI = [. 18 6 × ln(TP × 1000 )] −18 .

The relationship between the values of chlorophyll-a, TP, and TN are tabulated in Tables 16.6
a nd 16.7.
The values for the TSI were adjusted so that a chlorophyll-a concentration of 20 μg L -1 was equal
to a TSI value of 60, used to represent “fair” lakes, while lakes above 70 were assessed as “poor”
as tabulated in Table 16.7.