Abstract
Crawford Lake is a meromictic lake, characterized by sedimentary bedrock type. The
purpose of this paper was to investigate the relationship between iron and sulfur, epilimnetic
temperature and thickness, and to determine the trophic status of this particular lake. Only data
collected during the fall season were the focus of the present analysis. The results showed that
the availability of both ferrous ions and hydrogen sulfide varied with depth, but failed to show any
interaction between the two in the water of the hypolimnion, suggesting that ferrous ions were not
available to interact with hydrogen sulfide. In addition, Crawford Lake turned out to be an
oligotrophic lake, characterized by low primary productivity. There is little indication that this lake
has become more productive in the recent past. Finally, the trend toward thinner epilimnion with
higher epilimnetic temperature and vice versa had been observed in Crawford Lake.
Introduction
Crawford Lake is located in Southern Ontario and is characterized by sedimentary
bedrock type. It is a permanently stratified lake, allowing it to be grouped together with other
meromictic lakes (Wetzel, 1983).
This paper intends to investigate some of the aspects of water chemistry and productivity
that are typical of all lakes in the fall and relate them to data collected from Crawford Lake.
Specifically, one of the issues that will be addressed within the body of this paper has to do with
the relationship between iron and sulfur within both the epilimnion and hypolimnion of this lake.
In addition, mean Secchi depth values that were collected from many years will by analyzed to
determine the trophic status of Crawford Lake and to investigate if any changes in the productivity
of this lake had occurred in the recent past. Finally, some analysis and discussion will be
devoted to the relationship between the epilimnetic temperature and epilimnetic thickness of
Crawford Lake, with emphasis on how the former influences the latter. All of these aspects of
water chemistry and productivity are of crucial importance to the study of freshwater communities
because even the slightest changes in the cycling of micronutrients, productivity or temperature
relationships can have profound effects on the structure and functions of lake bodies.
The biochemical cycling of essential micronutrients in lakes is governed primarily by
changes in oxidation-reduction states (Wetzel, 1983). Oxidation is a process in which electrons
are lost; reduction is a process in which electrons are gained (Gillespie et al., 1989). Oxidation
and reduction reactions are collectively referred to as redox reactions and these are determined
by pE, which can be defined as the “activity” of electrons (Wetzel, 1983). In a chemical sense,
this “activity” refers to the “effective” (chemically active) concentration of electrons in a solution
(BIO 332Y lecture notes, 1997). Wetzel (1983) states that when pE is large and positive, the
“activity” of electrons is low, and this results in an oxidizing environment. The opposite holds true
for a reducing environment.
In natural waters, iron is one of the most electroactive redox reactants (Wetzel, 1983).
As it undergoes oxidation, Fe2+ (ferrous ion)  Fe3+ (ferric ion) + e-, and as it undergoes
reduction, Fe3+ + e-  Fe2+ (BIO 332Y lecture notes, 1997).
Oxidized conditions are found in the epilimnia of lakes, because the environment around
the lake is an oxidizing environment (BIO 332Y lecture notes, 1997). When iron is in its oxidized
form in the presence of water, Fe3+ + OH-  Fe(OH)3 (ferric hydroxide) and Fe3+ + PO43- 
FePO4 (ferric phosphate) (BIO 332Y lecture notes, 1997). Both ferric hydroxide and ferric
phosphate are precipitates (Wetzel, 1983). As a result, they fall down the water column until they
reach the hypolimnion (BIO 332Y lecture notes, 1997). In this way, very little iron and
phosphorus is left behind in the top layers of the lake (BIO 332Y lecture notes, 1997). In
hypolimnion, organic matter decomposes, creating a reducing environment (BIO 332Y lecture
notes, 1997). When pE declines to about 200 mV, ferrous ions diffuse readily from the sediments
and become available for interaction (Wetzel, 1983).
In addition, sulfur turns into sulfate (SO42-) as it becomes oxidized (Wetzel, 1983). As the
environment becomes a reducing environment, as is the case in hypolimnia, the sulfate becomes
reduced to sulfide, which in turn reacts with hydrogen ions in the water, forming hydrogen sulfide
(H2S) (Wetzel, 1983). Reduction of sulfate to hydrogen sulfide takes place when pE declines to
less than 100 mV (Wetzel, 1983). Under these conditions, much of the ferrous ions are already
present and ready to react with hydrogen sulfide (Wetzel, 1993). It turns out that especially near
the sediments, much of the hydrogen sulfide reacts with ferrous ions to form ferrous sulfide (FeS),
which is a precipitate (Weztel, 1983). In this way, much of the sulfur returns to the sediments
(Wetzel, 1983).
Besides, the depth of the hypolimnion determines how much oxygen will be used up
during the summer (BIO 332Y lecture notes, 1997). The deeper the hypolimnion, the more
improbable it will be that all of the oxygen will be used up by the end of the summer (BIO 332Y
lecture notes, 1997). As a result, the phosphorus that initially precipitated as ferric phosphate will
undergo little or no regeneration from the sediments (Wetzel, 1983). This is because the unused
oxygen tends to form an oxidized water microzone, which is a layer of sediments into which
oxygen had diffused and kept the pE high (Wetzel, 1983). As soon as Fe3+ and PO43- become
swept into this oxidized layer of sediments, they loose electrons and react with one another,
forming the precipitate ferric phosphate (Wetzel, 1983). As a result, little or no phosphorus will
enter the water body, which translates into low primary productivity of the lake, resulting in
reduced amounts of aquatic algae and macrophytes (Wetzel, 1983). In addition, such a lake is
characterized by an increased light penetration (BIO 332Y lecture notes, 1997). The opposite
holds true for a lake that has a thin hypolimnion.
The transparency of water to light can be evaluated by measuring Secchi depths, a
method devised by an Italian scientist, Secchi (Wetzel, 1983). Secchi depth measurements are
very often employed as indicators of the trophic status of a lake (Henderson-Sellers and
Markland, 1987). According to Harper (1992), a mean Secchi depth of 6 meters or more
characterizes an oligotrophic lake. The mean Secchi depth for a mesotrophic lake ranges
between 6 and 3 meters, whereas the mean Secchi depth for a eutrophic lake falls somewhere
below 3 meters (Harper, 1992).
Finally, the fall season is characterized by lower atmospheric temperatures compared
with the temperatures of the water bodies (Wetzel, 1983). As a result, more heat will be lost from
lakes than will enter them (Wetzel, 1983). As a lake starts to cool, the water at the surface will
become more dense and sink, a process that is primarily governed by gravity (Wetzel, 1983).
This water will be replaced by the warmer water from below (Wetzel, 1983). Further cooling of
the lake will cause the surface waters to penetrate into the metalimnion, causing its erosion
(Wetzel, 1983). The higher the temperature of the epilimnion, the less erosion of the metalimnion
will take place, resulting in a thinner epilimnion (BIO 332Y lecture notes, 1997). The opposite
holds true when lower temperature characterizes the epilimnion.
Until now, those issues that have been addressed with respect to water chemistry and
productivity of lakes during the fall season were looked at from a theoretical point of view. The
purpose of this paper is to see whether any of these theoretical expectations are met in real life
situations, and this is where the data collected from Crawford Lake between the years 1973 and
1997 comes into consideration. In most cases, the data were simply graphed to see whether any
patterns could be observed. The only statistical analysis that was done on the data involved a
calculation of the coefficient of variation, which can be defined as a measure of variability relative
to some mean (Zar, 1996). The coefficient of variation was used in the present analysis to find
out how variable all the mean Secchi depth values were and to determine the reliability of the
estimation of the trophic status of Crawford Lake.
Results
Ferrous Ion and Hydrogen Sulfide
All three graphs that deal with this particular set of data were constructed by taking all the
values collected during the fall seasons between the years 1986 and 1996 and simply plotting
them according to the depths at which they were recorded.
Figure 1 represents the relationship between the availability of ferrous ions with depth.
From this plot it becomes clear that the deeper one goes into the hypolimnion of Crawford Lake,
the more ferrous ions become available. The same type of pattern can be observed in Figure 2,
which represents the relationship between the availability of hydrogen sulfide with depth. Again,
the deeper one goes into the hypolimnion of Crawford Lake, the more hydrogen sulfide becomes
available. Figure 3 simply intends to graph the interaction between ferrous ions and hydrogen
sulfide within the hypolimnion of Crawford Lake with increasing depth. This graph points out that
with increasing depth ferrous ions and hydrogen sulfide fail to react with one another and to form
the precipitate ferrous sulfide.
There were some data points recorded that represented the availability of ferrous ion and
hydrogen sulfide in the surface water of Crawford Lake. These values, however, were so small
that they fell below the detectability limit. It is questionable how those individuals, who recorded
those values, were able to make these minute measurements. Due to the unreliability of these
data points, no graph was constructed using them.
Mean Secchi Depth and Trophic Status
Figure 4 represents the time trend in mean Secchi depth recorded between the years
1973 and 1997, excluding several years for which no data were available. Each point on this
graph represents the mean Secchi depth that was measured during one particular year. This plot
points out that there has been only a very small change in terms of the mean Secchi depth over
time, suggesting that the transparency of the Crawford Lake water to light changed very little
between the years 1973 and 1997. In addition, this graph indicates that the mean Secchi depth
for Crawford Lake lies somewhere between 6 and 7 meters.
Figure 5 can be divided into two parts. The left side of the graph represents the mean
Secchi depths measured between the years 1986 and 1997. The reason why only this set of
data was selected to construct this particular graph had to do with the fact that for only this time
period the individual Secchi depths that comprised each of the mean Secchi depth values were
available. The individual Secchi depths were necessary to calculate the coefficient of variation for
each of the mean Secchi depth values (Appendix), and the coefficient of variation values obtained
from these calculations were plotted on the right side of the graph.
The lines labelled 25%, 50%, and 75% have to do with quartiles (Zar, 1996). The value
that corresponds to the 50% mark is the median or the middle measurement (Zar, 1996). The
label 25% represents 25% of all the measurements that fell below the median, whereas the label
75% represents 25% of all the measurements that fell above the median (Zar, 1996). Table 1
gives the actual values that correspond to the 25%, 50%, and 75% labels on the graph for both
the mean Secchi depths and the corresponding coefficients of variation. The value that needs to
be considered in order to determine the trophic status of Crawford Lake is the median value for
the Secchi depth. Table 1 indicates that this value is equal to 6.765 meters and that it can vary
by 10.7%, as indicated by its corresponding coefficient of variation.
Epilimnetic Temperature and Epilimnetic Thickness
Figure 6 represents the relationship between the temperature at 1meter depth and
epilimnetic thickness in Crawford Lake. Each point on this graph corresponds to a measurement
that was taken in one particular year. It becomes obvious from this figure that the higher the
temperature at 1meter depth, the thinner the epilimnion, and vice versa.
Temperature at 1meter depth is not the same thing as the epilimnetic temperature, but
Figure 7 points out that the actual values for both are similar enough to consider the temperature
at 1meter depth as a substitute for the actual epilimnetic temperature. This assumption had been
made to ensure that more data were available to investigate the relationship between epilimnetic
temperature and thickness in Crawford Lake.
Discussion
With respect to ferrous ions and hydrogen sulfide, they are expected to be absent from
the surface water of any lake (BIO 332Y lecture notes, 1997). The data from Crawford Lake
showed that this was the case. Some data points were obtained, but these had questionable
reliability. The reason why no ferrous ions and hydrogen sulfide were present in the surface
water of Crawford Lake was because the epilimnia of lakes are characterized by an oxidizing
environment and ferrous ions and hydrogen sulfide are reduced forms of iron and sulfur,
respectively.
It turned out, however, that ferrous ions and hydrogen sulfide were present in the
hypolimnion of Crawford Lake and that their availability increased with increasing depth. These
results correspond to what would have been expected from a typical lake. In hypolimnion,
organic materials are decomposing, using up the oxygen (BIO 332Y lecture notes, 1997). This
gives rise to a reducing environment that favours the presence of reduced forms of
micronutrients, such as ferrous ions and hydrogen sulfide (Wetzel, 1983). The deeper one goes
into the hypolimnion, the more decomposition of organic matter goes on, and the more reducing
is the environment (BIO 332Y lecture notes, 1997). Therefore, it is expected that the amounts of
ferrous ions and hydrogen sulfide should increase with depth of the hypolimnion (BIO 332Y
lecture notes, 1997).
The situation, however, becomes more complicated when one does not look at the
availability of ferrous ions and hydrogen sulfide in hypolimnion of Crawford Lake separately, but
rather focuses on the availability of one with respect to the other. It is expected that as the
environment becomes more reducing, the availability of both the ferrous ions and hydrogen
sulfide should increase with increasing depth, but because they are more available, more should
be there to react with one another and precipitate as ferrous sulfide (BIO 332Y lecture notes,
1997). In this way, much of the sulfur should return to the sediments (Wetzel, 1983). As a result,
less ferrous ions and hydrogen sulfide should be left in the water of hypolimnion with increasing
depth (Wetzel, 1983). It turns out, however, that this type of trend is not observed in the
hypolimnion of Crawford Lake. It is true that the deeper one goes into the hypolimnion of this
lake, the more ferrous ions and hydrogen sulfide become available, but their availability does not
decrease with increasing depth as would be expected if they reacted with one another. It is
possible that because the hypolimnion of Crawford Lake did not contain much of ferrous ions to
begin with, all of it could have already been forming other organic complexes and was simply not
available to interact with hydrogen sulfide.
In addition, Crawford Lake is characterized by a deep hypolimnion. As a result, not all of
the oxygen found in hypolimnion of this lake will become used up by the end of the summer (BIO
332Y lecture notes, 1997). This favours the formation of the oxidized water zone, allowing little or
no regeneration of phosphorus, which initially precipitated with ferrous ions as ferric phosphate,
from the sediments (Wetzel, 1983). This translates into low primary productivity, resulting in
reduced amounts of aquatic algae and macrophytes (Wetzel, 1983). A reflection of reduced
presence of phytoplankton and macrophytes is the degree of transparency of water to light, which
is measured using the Secchi depth (Wetzel, 1983). For Crawford Lake, the median Secchi
depth turned out to be 6.765 meters, which according to Harper (1992) qualifies this lake as an
oligotrophic lake and also provides support for the low primary productivity hypothesis for
Crawford Lake based on the depth of its hypolimnion. In addition, the coefficient of variation is an
indication of how variable the mean Secchi depth is (Zar, 1996). The coefficient of variation for
the median Secchi depth of 6.765 meters turned out to be 10.7%, which translates into a value of
0.72 meters (Appendix). If one were to add or subtract this 0.72 meters from the 6.765 meters
(Appendix), the result in either case would still qualify Crawford Lake as an oligotrophic lake.
This can have important implications for anyone who would decide to go out in fall and make a
measurement of Secchi depth. Any unreasonable measurements could simply be rejected.
The fact that Crawford Lake qualifies as an oligotrophic lake suggests that it is an
unproductive lake with clear water, low chlorophyll a concentration and low phytoplankton
biomass (BIO 332Y lecture notes, 1997). It is a wonderful lake for recreation and it receives a lot
of attention from tourists. In addition, the fact that only very little change has occurred over time
in terms of the mean Secchi depth suggests that there is no immediate danger of this lake
becoming more productive as for now, but this does not mean that monitoring its condition should
not be a priority.
Finally, Crawford Lake reflects the typical relationship between epilimnetic
temperature and thickness. All the measurements for epilimnetic temperature had been taken
during the fall season. This is the time when lower temperatures than the water bodies
characterize the atmosphere (Wetzel, 1983). As a result, more heat will leave a lake than will
enter it (Wetzel, 1983). As a lake starts to cool, the water at the surface will become more dense
and sink (Wetzel, 1983). This water will be replaced by warmer water from below (Wetzel, 1983).
Further cooling causes the surface waters to penetrate into the metalimnion, resulting in its
erosion (Wetzel, 1983). The higher the temperature of the epilimnion, the less erosion of
metalimnion will take place, resulting in a thinner epilimnion, and vice versa (BIO 332Y lecture
notes, 1997). Such trends were observed in Crawford Lake.
In conclusion, it is very educational to be able to apply theory to real life situations. Not
only will one become aware of the many changes that might be occurring within a lake over time,
but also pinpoint the reasons why such changes might be taking place.
Water Chemistry of Crawford Lake
by
Jolanta Kruk
# 941684810
Date due: March 23/1997
Date submitted: March 23/1997