Environmental Engineering Reference
In-Depth Information
sedimentation). Assumptions include steady-state P concentration, com-
plete mixing of inputs, constant sedimentation, little fluctuation of loading
over time, and limited P input from sediments (internal loading). More
complex relationships are available to deal with exceptions to most of
these assumptions (Cooke et al., 1993). The equation can also be used to
estimate total N (TN). In practice, L is determined by measurements of to-
tal P in inflowing streams; atmospheric deposition and groundwater inputs
are generally ignored. Nutrient input into streams is often heavily depen-
dent on land-use patterns, which will be discussed in the next section.
Groundwater input may be difficult to determine, particularly in heteroge-
neous geological substrata or where septic inflows create areas with ex-
ceptionally high P influx. Determination of mean depth and calculation of
flushing rate require morphological mapping of the lake basin and hydro-
logical measurements. Sedimentation rate can be highly variable between
and within lakes, depending on characteristics such as fetch, epilimnion
depth, and form of P (i.e., considerable variance occurs in the relationship
). Direct determination of sedimentation rates may provide more
accurate estimates of P loss from the epilimnion.
Once the TP in the lake is calculated, the next step is to calculate the
chlorophyll that can be supported by this amount of nutrient. A clear re-
lationship exists between total P and chlorophyll (Figs. 17.6 and 17.7)
when values for many lakes are plotted. Equations can be derived from
such data sets; the following has been proposed by Jones and Bachmann
(1976) using data from 143 lakes:
10/ z
1.09, r 2
log 10 chla
1.46 log 10 TP
0.90
where chla is the summer mean chlorophyll in mg m 3
g liter 1 ), TP is
(
the summer mean total phosphorus in mg m 3
g liter 1 ), and r 2 is the
proportion of the variance that can be described by the relationship.
Use of this and the preceding equation is demonstrated in Example 17.1.
Smith (1982) used a larger and more variable data set and demon-
strated that more variance can be accounted for if TN is considered in ad-
dition to TP. If a plot of these chlorophyll values versus total P is divided
into categories of TN:TP ratios (Fig. 17.7), it shows that chlorophyll per
unit P is lower when the relative amount of N is low. An equation relat-
ing algal biomass to chlorophyll using N and P has been proposed by Smith
(1982; corrected equation, n
(
311 lakes):
0.753, R 2
log 10 chla
0.640 log 10 TP
0.587 log 10 TN
0.75
Units and variables are the same as in the previous equation. This equa-
tion is probably most useful in high P waters (Cooke et al., 1993) and may
not apply to tropical lakes (Sarnelle et al., 1998).
The probability that an algal bloom will occur, particularly a bloom of
cyanobacteria, may be more important than average chlorophyll values. A
21-year data set on P loading to Lake Mendota, Wisconsin, was used to
evaluate the probability of algal blooms (Lathrop et al., 1998). In this
analysis, with no change in current levels of loading there was a 60%
chance of a cyanobacterial bloom on any given summer day. When load-
ing was decreased by half, there was only a 20% chance of a bloom. This
study illustrates that managers deal with variable and unpredictable sys-
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