LAKES AND RESERVOIRS - Water Quality Engineering in Natural Systems - page 251

TABLE 7.1. Empirical Relations between Biomass and TP
Concentrations

typically the limiting nutrient in inland lakes and
stormwater detention ponds. In marine systems, nitro-
gen is commonly found to be the limiting nutrient.

Formula Reference

Smith and
Shapiro
(1981)

log c b = 1 55 . log TP

10 10

EXAMPLE 7.6

6 404 .

− 1 55 . log

10

0 0204 . ( TN/TP ) + 0 334 .

(a) Water-quality measurements in a lake indicate that
available phosphorus is on the order of 60 µ g/L, and
available nitrogen is about 2 mg/L. Is the lake nitrogen
or phosphorus limited? (b) If the concentration of TP
is 90 µ g/L and the concentration of TN is 4 mg/L, esti-
mate the biomass concentration and trophic state of
the lake.

log 10 c b = 0.807 log 10 TP − 0.194 Bartsch and

Gakstatter
(1978)

log 10 c b = 0.76 log 10 TP − 0.259 Rast and Lee

(1978)

Dillon and
Rigler
(1974)

log 10 c b = 1.449 log 10 TP − 1.136

ln c b = 1.058 lnTP − 0.934 Harper and

Baker
(2007) a

Solution

(a) The ratio of available nitrogen to available phos-
phorus is 2 × 10 3 /60 = 33. Since this ratio is greater
than 10, the lake is phosphorus limited.
(b) From the data given, TP = 90 µ g/L and
TN = 4000 µ g/L, and using the formulas in Table 7.1
yields the following results:

a Florida lakes.

TABLE 7.2. Limiting Nutrients for Various Water Bodies

Nutrient Source Typical N/P Limiting Nutrient

Rivers and streams
Point source dominated:
Without phosphorous
removal

c b
( µ g/L) Formula

<<10 Nitrogen

Smith and Shapiro 52

With phosphorous
removal

>>10 Phosphorous

Bartsch and Gakstatter 24

Rast and Lee 17

Nonpoint source
dominated

>>10 Phosphorous

Dillon and Rigler 50

Harper and Baker 46

Lakes
Large, nonpoint source
dominated

>>10 Phosphorous

Hence, the biomass concentration is estimated to be
in the range 17 to 52 µ g/L. Based on this biomass
concentration (>10 µ g/L) and the concentration of
TP (>20 µ g/L), the

Small, point source
dominated

<<10 Nitrogen

lake can be classified as

eutrophic.

phosphorus concentration does not necessarily result in
a significant decrease in the chlorophyll a concentration.
It has been estimated that the phosphorus concentra-
tion should be below 10-15 µ g/L to limit algal blooms
(vollenweider, 1975). TP concentrations of 10 and
20 µ g/L are generally accepted as the boundaries
between oligotrophic, mesotrophic, and eutrophic states,
respectively.
In municipal wastewaters with activated sludge treat-
ment (without phosphorus removal), nitrogen tends to
be the limiting nutrient, and in mixed agricultural and
urban runoff, phosphorus tends to be the limiting nutri-
ent. The limiting nutrients that are typical for various
water bodies are shown in Table 7.2. Phosphorus is

7.3.2 Measures of Trophic State

Trophic state is not measured in a single, unique, unam-
biguous way. Current methods measure symptoms (e.g.,
phytoplankton concentrations, µ g Chl a /L) or causes
(e.g., TP concentrations). Typical symptomatic criteria
state that oligotrophic water bodies have chlorophyll a
concentrations less 4 µ g Chl a /L, mesotrophic water
bodies have 4-10 µ g Chl a /L, and eutrophic water bodies
have more than 10 µ g Chl a /L. Typical causal criteria are
that oligotrophic water bodies have less than 10 µ g
TP/L, mesotrophic water bodies have TP concentrations
in the range of 10-20 µ g TP/L, and eutrophic water