Is it Important?
refers to how clear the water is. The greater the amount of total
suspended solids (TSS) in the water, the murkier it appears and the
higher the measured turbidity.
The major source of turbidity in the open water zone of most lakes
is typically phytoplankton,
but closer to shore, particulates may also be clays and silts from shoreline erosion,
resuspended bottom sediments (this is what turns the western arm
of Lake Superior near Duluth brown on a windy day), and organic detritus
from stream and/or wastewater discharges. Dredging operations, channelization,
rates, floods, or even too many bottom-feeding fish (such as
carp) may stir up bottom sediments and increase the cloudiness of
of particulate matter can modify light penetration, cause shallow
lakes and bays to fill in faster, and smother benthic habitats
- impacting both organisms and eggs. As particles of silt, clay,
and other organic materials settle to the bottom, they can suffocate
newly hatched larvae and fill in spaces between rocks which could
have been used by aquatic organisms as habitat. Fine particulate
material also can clog or damage sensitive gill structures, decrease
their resistance to disease, prevent proper egg and larval development,
and potentially interfere with particle feeding activities. If light
penetration is reduced significantly, macrophyte growth may be decreased
which would in turn impact the organisms dependent upon them for
food and cover. Reduced photosynthesis can
also result in a lower daytime release of oxygen into
the water. Effects on phytoplankton growth are complex depending
on too many factors to generalize.
levels of turbidity for a short period of time may not be significant
and may even be less of a problem than a lower level that persists
longer. The figure below shows how aquatic organisms are generally
adapted from "Turbidty: A Water Quality Measure", Water
Action Volunteers, Monitoring Factsheet Series,
UW-Extension, Environmental Resources Center. It is a generic, un-calibrated
impact assessment model based on Newcombe, C. P., and J. O. T. Jensen.
1996. Channel suspended sediment and fisheries: a synthesis for quantitative
assessment of risk and impact. North American Journal of Fisheries Management.
for Natural Variation
varies seasonally and with depth in a complex manner as discussed
previously in response to physical, chemical and biological changes
in the lake. Inorganic and detrital particles from the watershed
vary largely in response to hydrological events such as storms and
relatively small amounts of wave action can erode exposed
lakeshore sediments, in this case a minepit lake from northeastern
Minnesota. Can you guess what mineral was mined here?
effect turbidity has on humans might be simply aesthetic - people
don't like the look of dirty water. However, turbidity also adds
real costs to the treatment of surface water supplies used for drinking
water since the turbidity must be virtually eliminated for effective
disinfection (usually by chlorine in a variety of forms) to occur.
Particulates also provide attachment sites for heavy metals such
as cadmium, mercury and lead, and many toxic organic contaminants
such as PCBs, PAHs and many pesticides.
is reported by RUSS in nephelometric units (NTUs) which refers to
the type of instrument (turbidimeter or nephelometer) used for estimating
light scattering from suspended particulate material. Turbidity can
be measured in several ways. Turbidity is most often used to estimate
the TSS (total suspended solids as [mg dry weight]/L) in the lake's
tributaries rather than in the lake itself unless it is subject to
large influxes of sediments. For the WOW project we will attempt
to develop empirical (meaning: based upon direct measurements) relationships
between TSS and turbidity for each system since turbidity is easily
measured and TSS analyses are not very sensitive at the typically
low concentrations found in the middle of most lakes. Also, TSS is
a parameter that
directly relates to land uses in the watershed and is a key parameter
used for modeling efforts and for assessing the success of mitigation
and restoration efforts.
What in the world are Nephelometric Turbidity Units (NTUs)?
the units we use when we measure Turbidity. The term Nephelometric refers
to the way the instrument estimates how light is scattered by suspended
particulate material in the water. The Nephelometer, also
called a turbidimeter, attached to the RUSS unit has the photocell
(similar to the one on your camera or your bathroom nightlight) set
at 90 degrees to the direction of the light beam to estimate scattered rather
than absorbed light. This measurement generally provides a very good
correlation with the concentration of particles in the water that
and streams, there are 3 major types of particles: algae,
detritus (dead organic material), and silt (inorganic, or mineral, suspended
sediment). The algae grow in the water and the detritus comes
from dead algae, higher plants, zooplankton,
bacteria, fungi, etc. produced within the water
column, and from watershed vegetation washed in to the water.
Sediment comes largely from shoreline erosion and from the resuspension
of bottom sediments due to wind mixing.
we measure turbidity to provide a cheap estimate of the total
suspended solids or sediments (TSS) concentration (in
milligrams dry weight/L). TSS measurement requires you to filter
a known volume of water through a pre-weighed filter disc to collect
all the suspended material (greater than about 1 micron in size)
and then re-weigh it after drying it overnight at ~103°C to
remove all water in the residue and filter. This is tedious and difficult
to do accurately for low turbidity water - the reason why a turbidimeter
is often used. Another even cheaper method is to use an inexpensive
devise called a Turbidity Tube. This is a simple adaptation for streams
of the Secchi
disk technique for lakes. It involves looking down a tube at
a black and white disk and recording how much stream water is needed
to make the disk disappear.
This device yields data for streams that is similar to a secchi depth
measurement in lakes. As for secchi measurements are made in the shade
with the sun to your back to make an accurate and reproducible reading
- the shadow of the observer should be adequate.
sample water into the tube until the image at the bottom
of the tube is no longer visible when looking directly
through the water column at the image. Rotate the tube
while looking down at the image to see if the black and
white areas of the decal are distinguishable.
this depth of water on your data sheet to the nearest 1
cm. Different individuals will get different values and
all should be recorded, not just the average. It is a good
idea to have the initials of the observer next to the value
to be able identify systematic errors.
you see the image on the bottom of the tube after filling
it, simply record the depth as > the depth of the tube.
Then construct a longer tube, more appropriate for your
is a standard measurement in stream sampling programs where suspended
sediment is an extremely important parameter to monitor. It may also
be useful for estimating TSS in lakes, particularly reservoirs, since
their useful lifetime depends upon how fast the main basin behind
the dam fills with inflowing sediments from mainstem and tributary streams
and from shoreline erosion. In the WOW lakes, direct inputs of sediments
from tributaries are probably too low to significantly affect the
turbidity of the water column out in the main lake. However, algal
densities, particularly in the more eutrophic
lakes in the Minneapolis Metro area represent enough particulate
material to be easily measureable by the RUSS turbidity sensors.
Although chlorophyll sensors
(fluorometers) would be the best way for us to estimate algal abundance
(we lack the funding at present), in these lakes the turbidity sensors
provide an alternate estimate of algae.
below were developed to show how both organic (algae) and inorganic
(silt and sediment) particulates affect turbidity values. The first
set of images show filter discs prepared by filtering identical volumes
of water from Lake Independence, with their corresponding values
of turbidity and chlorophyll. The second set shows another set of
filters generated using a nearshore water sample from an erodible
The experiment: Filter equal volumes of water from
mid-lake and nearshore at Lake Independence through fine-mesh
Is it Important?
disk depth provides an even lower "tech" method for assessing
the clarity of a lake. A Secchi disk is a circular plate divided
into quarters painted alternately black and white. The disk is
attached to a rope and lowered into the water until it is no longer
visible. Secchi disk depth, then, is a measure of water clarity.
Higher Secchi readings mean more rope was let out before the disk
disappeared from sight and indicates clearer water. Lower readings
indicate turbid or colored water. Clear water lets light penetrate
more deeply into the lake than does murky water. This light allows
photosynthesis to occur and oxygen to be produced. The rule of
thumb is that light can penetrate to a depth of about 2 - 3 times
the Secchi disk depth.
is affected by algae, soil particles, and other materials suspended
in the water. However, Secchi disk depth is primarily used as an
indicator of algal abundance and general lake productivity.
Although it is only an indicator, Secchi disk depth is the simplest
and one of the most effective tools for estimating a lake's productivity.
for Natural Variation
disk readings vary seasonally with changes in photosynthesis and
therefore, algal growth. In most lakes, Secchi disk readings begin
to decrease in the spring, with warmer temperature and increased
growth, and continue decreasing until algal growth peaks in the
summer. As cooler weather sets in and growth decreases, Secchi
disk readings increase again. (However, cooler weather often means
more wind. In a shallow lake, the improved clarity from decreased
algal growth may be partly offset by an increase in concentration
of sediments mixed into the water column by wind.) In lakes that
thermally stratify, Secchi disk readings may decrease again with
As the surface water cools, the thermal
stratification created in summer weakens and the lake mixes.
The nutrients thus released from the bottom layer of water may
cause a fall algae bloom and the resultant decrease in Secchi disk
also may affect readings. Erosion from rainfall, runoff, and high
stream velocities may result in higher concentrations of suspended
particles in inflowing streams and therefore decreases in Secchi
disk readings. On the other hand, temperature and volume of the
incoming water may be sufficient to dilute the lake with cooler,
clearer water and reduce algal growth rates. Both clearer water
and lower growth rates would result in increased Secchi disk readings.
color of the water also affects the readings. In most lakes, the
impact of color may be insignificant. But some lakes are highly
colored. Lakes strongly influenced by bogs, for example, are often
a very dark brown and have low Secchi readings even though they
may have few algae.
Impact of Pollution
tends to reduce water clarity. Watershed development and poor land
use practices cause increases in erosion, organic matter, and nutrients,
all of which cause increases in suspended particulates and algae
disk depth is usually reported in feet to the nearest tenth of
a foot, or meters to the nearest tenth of a meter. Secchi disk
readings can be used to determine a lake's trophic status. Though
trophic status is not related to any water quality standard, it
is a mechanism for "rating" a lake's productive state
since unproductive lakes are usually much clearer than productive
J.P. 1991. A citizen's guide to understanding and monitoring lakes
and streams. Publ. #94-149. Washington State Dept. of Ecology,
Publications Office, Olympia, WA, USA (360) 407-7472.
M.L. 1989. NALMS management guide for lakes and reservoirs. North
American Lake Management Society, P.O. Box 5443, Madison, WI, 53705-5443,