Swimmable Use Support Classification

FS=Full Support 
MS=Full Support(marginal) 
PS=Partial-Support 
NS=Non-Support

Glossary Term Category

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Yes

Glossary Term Ext Def

Additional information about swimmable use: Swimmable Use
Support - Swimmable use support is routinely assessed for
lakes
in Minnesota's 305 (b) Report to Congress. Minnesota's phosphorus
(P)
criteria provide a sound basis for determining a lake's ability
to
support swimmable uses. For the purposes of Minnesota's 305(b)
reports to Congress we have used the P criteria in conjunction
with
Carlson's TSI (Carlson, 1977) as a means to classify lakes
relative
to support of swimmable use. The following categories of
swimmable
use support and working definitions are as follows: full-support
-
few algal blooms and adequately high transparency exist to
support
swimming throughout the summer; full-support (marginal) -
swimmable
use is still fully supported, but the lake is near the P limit
for
its ecoregion and small increases in in-lake P could result in
increased algal blooms and perceptible decreases in transparency;
partial-support (impaired) - algal blooms and low transparency
may
limit swimming for a significant portion of the summer; and
non-support - severe and frequent algal blooms and low
transparency
will limit swimming for most of the summer. By using Carlson's
TSI we
are able to estimate use support based on chlorophyll-a and
Secchi
transparency in addition to P. The relationships between TP,
chlorophyll-a and Secchi transparency have been well established
for
Minnesota lakes based on data from the ecoregion reference lakes
(Figure 5) and other sources. Use-support thresholds for each
ecoregion are defined in Figure 7.

Figure 7. MPCA's Swimmable Use Support Classification
Relative
to Carlson's Trophic State Index by Ecoregion

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The Northern Lakes and Forests and North Central Hardwood
Forests
ecoregion P criteria levels, 30 micrograms per liter and 40
micrograms per liter, respectively, serve as the upper thresholds
for
full-support (marginal) swimmable use. Those concentrations
correspond to Carlson's TSI values of 53 and 57, respectively.
Full-support of swimmable use is set at slightly lower
concentrations, 25 micrograms per liter and 30 micrograms per
liter,
respectively, which ensure that conditions associated with
"impaired
swimming" would occur less than ten percent of the summer. P
concentrations above criteria levels would result in greater
frequencies of nuisance algal blooms (Figure 5) and increased
frequencies of "impaired swimming." The upper threshold
for
partial-support of swimmable use was set at 60 and 63 Carlson TSI
units, respectively, for these two regions. As P concentrations
increase from about 30 micrograms per liter to 60 micrograms per
liter, summer-mean chlorophyll-a concentrations increase from
about
10 micrograms per liter to 25 micrograms per liter (Figure 5) and
summer-mean Secchi transparency decreases from about 1.7 m to 0.8
m
(Figure 5). Over this range, the frequency of nuisance algal
blooms
(greater than 20 micrograms per liter of chlorophyll-a) increases
from about five percent of the summer to about 60 percent of the
summer (Figure 6).

The increased frequency of nuisance algal blooms and reduced
Secchi transparency results in a high percentage of the summer
(26-50
percent) perceived as "impaired swimming" (Heiskary and
Wilson, 1990).

P concentrations above 50 micrograms per liter (Northern Lakes
and
Forests) and 60 micrograms per liter (North Central Hardwood
Forests)
were associated with non-support of swimmable use. At P
concentrations above 60 micrograms per liter, nuisance algal
blooms
may occur over 60 percent of the summer. This will result in a
high
frequency (greater than 50 percent of summer) of impaired
swimming
and greater than 25 percent as "no swimming."
For the Western Corn Belt Plains and Northern Glaciated Plains
the
upper P thresholds for full-support and full-support (marginal)
are
40 micrograms per liter and 70 micrograms per liter,
respectively,
which correspond to Carlson's TSI units of 57 and 65. At a P
concentration of 70 micrograms per liter, summer-mean
chlorophyll-a
concentrations average 30-35 micrograms per liter and Secchi
transparency is about 0.7 meter. Nuisance algal blooms (greater
than
30 micrograms per liter of chlorophyll-a for these regions) would
occur for approximately 40 to 50 percent of the summer
("severe
nuisance" in Figure 6).

 Figure 6. Chlorophyll-a Interval Frequency as a
Function of
 Summer-Mean Chlorophyll-a

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Note: This graph is derived based on a logarithmic function
developed by Walker (1984). This function relates summer-mean
chlorophyll-a to the underlying chlorophyll-a values that
comprise
the mean. The values have been sorted into four categories that
correspond to varying frequencies of extreme values
("blooms")
of chlorophyll-a. The graph represents the estimated frequency
(expressed as a percent of time) a given bloom level would be
exceeded (experienced) for a given summer-mean chlorophyll-a
concentration. For example at a summer-mean of 10 µg/L the
frequency of chlorophyll-a greater than 10 µg/L would be
about
40 percent and the frequency of chlorophyll-a greater than 20
µg/L
would be less than five percent. In contrast at a summer-mean of
30
µg/L chlorophyll-a greater than 20 µg/L would occur
about
75 percent of the time. These four chlorophyll-a
"classes"
(bloom frequencies) have been generally characterized as follows:
chlorophyll-a greater than 10 µg/L = "mild
blooms;"
chlorophyll-a greater than 20 µg/L = "nuisance
blooms;"
chlorophyll-a greater than 30 µg/L = "severe nuisance
blooms;" and chlorophyll-a greater than 60 µg/L =
"worse
yet." While these characterizations are generally
applicable,
user perceptions will vary between ecoregions (Smeltzer and
Heiskary,
1990) and may even vary between lakes (lake users) within an
ecoregion.
Few lakes in these two ecoregions have P concentrations of 40
micrograms per liter or less (Carlson's TSI of 40, Table 1).
Partial-support, which corresponds to an upper P threshold of 90
micrograms per liter or less (Carlson's TSI = 69), is a more
reasonable goal for the majority of the lakes in these two
ecoregions. TP concentrations greater than 90 micrograms per
liter
are considered not supporting of swimmable use. At TP
concentrations
greater than 90 micrograms per liter, Secchi transparency
averages
0.5 meter or less, summer-mean chlorophyll-a is often 35
µg/L
or greater, and nuisance algal blooms may occur 75 percent of the
summer or more.
Of the 2,245 lakes in this assessment 1,206 (54 %)
fully-support
swimmable use, 257 (11%) fully-support but are marginal, 316 (14
%)
partially-support, and 466 (21 %) do not support swimmable uses
(Figure 8).
Figure 8. Statewide Swimmable Use Assessment and
Support.
 Total of 2,245 lakes assessed (2,235 in these four
ecoregions).
[Note: FS="fully supports," MS="fully-supports
(marginal)," PS="partially supports (impaired),"
and
NS="non supporting."]

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The relative percentage of lakes in each category varies
between
ecoregions and is summarized in Figure 9.

Figure 9. Swimmable Use Support By Ecoregion

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In the NLF ecoregion 81 percent of the assessed lakes
fully-support swimmable use. In the NCHF ecoregion 51 percent
fully-support swimmable use. In the WCBP and NGP ecoregions the
vast
majority (76 and 74 % respectively) do not support swimmable use.
 The reasons for non-support of swimmable use vary between
regions. NLF ecoregion lakes which do not support swimmable use
are
often smaller and shallower than the norm and often have some
past or
present source of excess P loading in their watershed such as a
wastewater treatment plant discharge. NCHF ecoregion lakes that
do
not support swimmable use are often shallower than the norm. Also
they often have a source (or multiple sources) of excessive P
loading
such as: wastewater treatment plant discharge, numerous feedlots,
excessive land application of bio-solids, high percentage of
agricultural land use, or high percentage of impervious area
(receive
large amounts of stormwater runoff) in their watershed. All of
these
sources can contribute high P loading to a lake. In the WCBP and
NGP
ecoregions the vast majority of lakes are quite shallow (Table 1)
and
have highly agricultural watersheds. Runoff from these
agricultural
lands is typically very high in P. This high P loading from the
watershed and shallowness of the lakes (which promotes poor
retention
of P by lake sediments and internal recycling of P) typically
lead to
high in-lake P concentrations and subsequently nuisance algal
blooms
and low transparency. The combination of high watershed P loading
and
the limited assimilative capacity of shallow lakes often limit
the
degree to which water quality of these lakes might be improved.