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<CENTER TAG INSERTEDEutrophication in Lakeside Lake

By David Walker<P</CENTER TAG INSERTED>

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<BAbstract</I</B

Lakeside lake is an urban lake located in Tucson Arizona. It is currently managed as a sport fishery and recreational lake. Algae blooms are prevalent in mid- late summer. This paper discusses eutrophication in general and how it applies to Lakeside in particular. A diagnosis is given as to why these blooms occur. Specific managerial techniques are discussed to help alleviate the problem of algal blooms and move the lake from a eutrophic to a more mesotrophic state

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<P<BIntroduction</I

Lakeside reservoir is an urban lake in Tucson Arizona currently managed as an urban fishery by the Arizona Department of Game and Fish (AZG&F). Lakeside was included in AZG&F's Urban Fishing Program in 1983. The lake was reconstructed in 1986 and at that time AZG&F established a fishing program that included stocking with rainbow trout during November-March and catfish April-October<A HREF = "#Fitzsimmons">(Fitzsimmons 1992)</A>. In 1986 and 1987 AZG&F stocked the lake with several age classes of largemouth bass<A HREF = "#Fitzsimmons">(Fitzsimmons 1992)</A</A>. Since that time it has been managed as a classic "put and take" type of fishery with frequent restocking and heavy angling pressure.

Lakeside is located on Atterbury Wash which is listed as a navigable water of the U.S. <A HREF = "#Fitzsimmons">(Fitzsimmons </A>1992). The original intent was to maintain the water level in the lake with surface runoff alone. This method proved to be ineffective in maintaining adequate water levels and beginning in June of 1990 reclaimed water was used to supplement runoff <A HREF = "#Fitzsimmons">(Fitzsimmons 1992)</A>.

Lake morphometry is that of a steep-sided impoundment with a substrate of soil cement. The dam is located on the north side of the lake, the inflow of the reclaimed water enters at the northwest end, and the spillway leading into Atterbury Wash is at the south end. There is a berm that crosses the bottom of the lake from east to west and this divides the lake into 2 distinct areas (bays); a north bay and a south bay. At the northwest end there is an "arm" that is a gentler slope, relatively, than the remainder of the lake.

Digitized bathymetric images (using <A HREF= " for Windows™</A>) can be found by clicking on any of the thumbnails below:

<P<A HREF=the1e.gif<IMG SRC=the1tiny.gif ALIGN=MIDDLE>Map 1</A<P<A HREF=lksgry.gif<IMG SRC=grytiny.gif ALIGN=MIDDLE>Map 2</A<P<A HREF=lkstest.gif<IMG SRC=testiny.gif ALIGN=MIDDLE<A NAME="Map 3">Map 3</A>.</A<P<A HREF=lksel.gif<IMG SRC=eltiny.gif ALIGN=MIDDLE>Map 4</A<P<A HREF=lksaer.gif<IMG SRC=lksaertiny.gif ALIGN=MIDDLE<A NAME="Map 5">Map 5</A</A>

Specific data is represented in Table1 and is derived from an average of 1994 and 1995data. This data comes from the city of Tucson, Department of Parks and Recreation.

Table 1.

<TR ALIGN="left"<TD>Surface Acres</TD<TD>9.4</TD</TR>

<TR ALIGN="left"<TD>Volume (acre feet)</TD<TD>99.3</TD</TR>

<TR ALIGN="left"<TD>Depth (ft)</TD<TD>24.4</TD</TR>

</TABLE.

Monthly averages of data obtained from the Tucson Parks and Recreation Department is displayed below:

<A HREF= "lksdata1993.htm">1993 Data</A

<A HREF= "lks1994data.htm">1994 Data</A

<A HREF= "lksdata1995.htm">1995 Data</A

In 1992, an aeration system was installed (see <A HREF = "#Map 5">Map 5</A>). This is in the form of polyethylene within PVC pipes that run from the compressor (on shore) to airlines. These airlines sit on top of the sediment and diffuse air bubbles through the length of the vertical water column. Presumably, the reason for this installation was to increase dissolved oxygen and prevent algal blooms (Pers. comm. Bob Lauderback) Bubbles can be seen diffusing at the water surface. This system effectively stops any seasonal thermal stratification of the area at least directly above it from the bottom to the surface. This is evidenced by data from Tucson Parks and Recreation Department that show little or no difference between temperature readings taken at 5 foot intervals at sites H and I <A HREF = "#Map 3">( see Map3)</A>. Lakeside can be classified as a small, deep lake <A HREF = "#Olem">(Olem & Flock 1990)</A>. Therefore, I believe that thermal stratification would occur without the aeration system currently in place. This will be discussed in more detail later in this paper.

The recent trophic state of Lakeside is unknown. To accurately determine this more data than what is currently available would be needed based on current classification methods. Measurements of total phosphorous, orthophosphate, and chlorophyll a would be needed to assign a trophic state to the lake. While there has been some measurements of phosphorous in the past, there is not enough over a continuous period of time to derive any conclusions from it. These measurements would have needed to been done at least on a monthly basis over a years time to begin to draw on this data to derive averages and form any conclusions. To the best of my knowledge, no chlorophyll a measurements have been performed. It is possible, however, to estimate a trophic state based on previous events. The algal blooms that occur in late summer are at least an indication that phosphorous is available in sufficient amounts to support these blooms for extended periods. Based on anecdotal evidence, it has been suggested that Lakeside is a eutrophic lake and will be referred to as such for the remainder of this paper.

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<P<BGeneralized Ecology of a Temperate Eutrophic Lake</I

In order to assess the health and recommend any restorative actions to a eutrophic lake, a basic understanding of general ecological processes within a eutrophic lake must be understood. The following is provided as a brief overview of these processes. This generalized model assumes there is no artificial circulation or aeration within the lake.

Lakes are very complex and dynamic ecosystems and conditions can vary greatly even within different lakes that are in close proximity to each other. Systems such as, watersheds, anthropocentric activities, regional geography, vegetative cover, hydrology, regional soils, and lake morphometry all exert an influence on the biological productivity, water quality, and trophic status of a given lake.

All lakes are in a continuous state of decay and death. The rate of this decay varies greatly from lake to lake. Where a particular lake exists along the continuum of decay is deemed it's trophic state. These trophic states are listed below <A HREF = "#Olem">(from Olem & Flock 1990)</A>.

<BOligotrophic: </INutrient- poor, biologically unproductive

<BMesotrophic:</I Intermediate nutrient availability and biological productivity

<BEutrophic:</I Nutrient-rich, highly productive

<BHypereutrophic:</I Pea-soup conditions, the extreme end of the eutrophic stage

Changes in trophic status within a lake do not necessarily occur gradually nor do they have to occur in any uniform direction (<A HREF = "#Cooke">Cooke et al 1986</A>) However, this general model of lake succession has been widely accepted. Most restoration efforts of eutrophic lakes are directed toward changing the trophic status closer to a mesotrophic condition. In light of this, it should be one of the first determinants in assessing the overall "health" of a lake ecosystem. Many methods exist for the determination of trophic status <A HREF = "#Mancini">(Mancini et al 1983,<A HREF = "#Reckhow"> Reckhow et al 1993)</A>. The determination of which method to employ depends on local lake conditions. Diagram 1 charts the generalized trend of trophic succession through time. Substance "A" can represent any of a number of properties or substances such as , phosphorous, chlorophyll a, etc…. <A HREF = "#Mancici">(from Mancini et al 1983) </A

<P<A HREF=dia1.gif<IMG SRC=dia1tiny.gif ALIGN=MIDDLE>Diagram 1</A

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<P<BLake Stratification and Mixing</I

The density of water is temperature dependent. As water becomes colder it becomes more dense (down to 4 degrees C) and conversely, as water heats it becomes less dense. Sunlight is the main heat source and therefore waters near the surface heat faster than those at depth. This is the basis of thermal stratification within lakes.

In winter, short photoperiods and decreased temperatures result in more or less constant temperature throughout the water column which results in complete mixing. Sometimes ice (which is less dense than liquid water) forms at the surface. This can result in oxygen depletion within the lake due to little or no atmospheric mixing.

In spring, as the water heats at the surface, it becomes less dense than the cooler waters beneath it. The two layers of water are separated by a region of water where there is a rapid change in temperature and density. The upper layer of less dense warm water is referred as the epilimnion. The

cooler and more dense water beneath this is the hypolimnion. The region between these two, where there is a rapid change in temperature and density, is called the metalimnion. Within the metalimnion there is an area called the thermocline. It is defined as the horizontal plane where there is the greatest degree of temperature change<A HREF = "#Olem "> (Olem & Flock 1990)</A>.

As spring yields to summer, the hypolimnion may become oxygen depleted due to limited mixing with the epilimnion which is exposed to atmospheric oxygen (air)at it's surface. Also, if the thermocline is relatively deep in the water column, then the hypolimnion receives no oxygen from photosynthesis further adding to oxygen depletion within this region <A HREF = "#Jorgensen">(Jorgensen 1980)</A> Nutrients (especially nitrogen and phosphorous) may become concentrated in the hypolimnion<A HREF = "#Cooke"> (Cooke et al 1986)</A>. At low dissolved oxygen, phosphorous (usually the limiting nutrient), may become unbound from it's usually close association with iron (III) <A HREF = "#Cooke">(Cooke et al 1986)</A>. Occasionally, relatively small amounts of nutrients are transported from the hypolimnion vertically through the metalimnion to the better-lit epilimnion where they subsidize algal blooms <A HREF = "#Cooke">(Cooke et al 1986</A>;<A HREF = "#Reckhow"> Reckhow et al 1980</A>; <A HREF = "#Riemann">Riemann & Sondergaard 1986)</A>. These algal blooms usually crash due to nutrient depletion within the epilimnion unless there is some mechanism (disturbance) that keeps nutrients resuspended in this area where sunlight for photosynthesis is readily available. As the dead algal cells sediment (fall) and land on the bottom sediment they are broken down by bacteria and their nutrients are released thereby reconcentrating nutrients within the hypolimnion<A HREF = "#Riemann"> (Riemann & Sondergaard 1986)</A>. During this activity, bacterial respiration during the decomposition process adds to oxygen depletion in the hypolimnion<A HREF = "#Riemann"> (Riemann & Sondergaard 1986)</A>.

During late summer and early fall, the epilimnion begins to cool. As this cooling at the surface progresses to mid- and late-fall, the temperature of the epilimnion matches or can become colder than that in the meta- and hypolimnion. This puts pressure on the now more dense and colder water in the epilimnion to sink and, conversely, the now warmer more dense water in the hypolimnion to replace the epilimnion at the surface. This process is referred as the fall overturn. As the hypolimnion replaces the water at the surface (epilimnion) it can bring substantial amounts of nutrients with it. This may trigger an algal bloom late in the season which usually crashes due to a decrease in temperature and photoperiod <A HREF = "#Riemann">(Riemann & Sandergaard 1986)</A>. As the water becomes the same temperature throughout the water column, the metalimnion and thermocline break down and the water becomes fully circulated leading into the winter.

The process of stratification and mixing is shown pictorially in Diagram 2 below. <A HREF = "#Olem">(from Olem and Flock 1990)</A

<P<A HREF=dia2.gif<IMG SRC=dia2tiny.gif ALIGN=MIDDLE>Diagram 2</A<P<A HREF=dia2cont.gif<IMG SRC=di2continy.gif ALIGN=MIDDLE>Diagram 2 cont.</A

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<BPhotosynthesis and Respiration</I

The flow of energy through aquatic ecosystems starts with the fixation of sunlight by chlorophyll-containing plants. This process is called photosynthesis and also requires the input of a certain amount of energy. The general equation for photosynthesis is as follows:

<B6CO2 + 6H2O + light energy = C6H12O6 + 6O2</B

This accumulation of energy by plants is called primary production because it is the first and most basic form of energy storage. The rate at which this energy accumulates is known as primary productivity. The total amount of sunlight that is assimilated in an ecosystem (i.e. total photosynthesis) is gross primary production. Plants must overcome the tendency of energy to disperse. Free energy must be expended for production as well as for reproduction and maintenance. This energy is provided by the reverse of photosynthesis and is known as metabolic respiration which results in the production of CO2 and O2 and the liberation of energy. Most chlorophyll-containing plants have to contend with photorespiration associated with photosynthesis but not with metabolism. This also acts as a drain on photosynthesis. Energy remaining after respiration and stored as organic matter is net primary productivity. This is described by the following equation:

<BNet primary productivity (NPP) = gross primary production (GPP) - autotrophic respiration ( R )<P

In aquatic ecosystems, the O2 liberated by aquatic plants is usually in a dissolved form and is thus easily measured. During the day, aquatic plants cause a net O2 buildup in the water body in which they reside. In the absence of light, the reverse is said to be true and there is usually a decrease in O2 in the dissolved form (water). Therefore, if we can measure the amount of O2 liberated during respiration, we can extrapolate the amount of CO2 fixed into biomass. We can then predict the amount of "life" that can be supported in any given body of water. The tendency of energy to be dispersed leads to an exponential decrease in the amount of energy transferred from organism to organism. For example; 10 kilos of algae produced leads to approximately 1 kilo of zooplankton, which in turn produces 100 grams of fish which eat zooplankton, which produces only 10 grams of piscivine fish.

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<BPhytoplankton Community Succession</I

The following is supplied as a generalized summary of phytoplankton succession within a eutrophic lake. Whether Lakeside follows this generalized model will require much more data than what is presently available. While I have initiated<A HREF= "algcou.gif"> algal cell counts </A>on a weekly basis at Lakeside, care should be exercised in the extrapolation of any short-term data. Phytoplankton populations can fluctuate dramatically for as of yet unknown reasons. As previously stated, lakes are very complex and dynamic ecosystems and short-term fluctuations should not be construed as any generalized long-term trend.

In winter and early spring phytoplankton biomass is usually very low <A HREF = "#Riemann">(Reimann & Sondergaard 1986)</A>. This also corresponds to a low number of species. This is probably due to decreased temperatures and photoperiods <A HREF = "#Jorgensen">(Jorgensen 1980)</A>. The species of any importance include cryptophytes and chlorophytes (mainly Volvocales).

In spring and early summer, chlorophytes become the species of most importance <A HREF = "#Jorgensen">(Jorgensen 1980)</A>. Of the chlorophytes, the Chlorococcalians' are those usually found in the greatest number during this period being represented by species such as Coelastrum, Ankyra, Sphaerocystis, and Oocystis.Desmids may be noticed now but reach their maximum later in the summer.

Mid- to late-summer corresponds to a marked increase in phytoplankton biomass. Cyanobacteria now almost completely dominate the phytoplankton (up to 90%)<A HREF = "#Riemann">(Riemann & Sondergaard 1986)</A>. If photosynthesis is high, pH may rise. Cyanophtes prefer high pH levels, whereas chlorophytes favor lower pH levels<A HREF = "#Cooke"> (Cooke et al 1986)</A>. In June Anabaena flos-aquae are prevalent these being replaced later by Microcystis species, mainly M. aeruginosa at first and M. wesenbergii later<A HREF = "#Riemann"> (Riemann & Sondergaard 1986)</A>.

There is a gradual transition from cyanobacteria-dominated conditions of late-summer into fall. In late fall, diatoms become prominent and may constitute up to 40% of the biomass <A HREF = "#Riemann">(Riemann & Sondergaard 1986)</A>. As fall yields to winter, biomass levels continue to decrease down to levels typical of this time of year. Cryptophytes are usually the only group to maintain any importance. Most other species of phytoplankton over-winter as resting stages in the sediment <A HREF = "#Riemann">(Riemann & Sondergaard 1986)</A>.

As previously stated, this is a greatly simplified example of phytoplankton succession. Eutrophic lake ecosystems exist that deviate substantially from this example. Lakes in states other than eutrophic will invariably have different phytoplankton communities than those previously described. Examples of the relation of phytoplankton to increasing eutrophication are given in Table 2 below

<A HREF = "#Cooke">(from Cooke et al 1986)</A<A

Table 2:

<TR ALIGN="left"<TD>General trophic state</TD<TD>Water characteristics</TD<TD>Dominant algae</TD</TR>

<TR ALIGN="left"<TD>Oligotrophic</TD<TD>Slightly acidic; very low salinity</TD<TD>Desmids, Staurastrum</TD</TR>

<TR ALIGN="left"<TD>Oligotrophic</TD<TD>Neutral to slightly alkaline; nutrient-poor lakes</TD<TD>Diatoms, especially Cyclotella and Tabellaria</TD</TR>

<TR ALIGN="left"<TD>Oligotrophic</TD<TD>Nutrient to slightly alkaline; nutrient-poor lakes or more productive lakes at seasons of nutrient reduction</TD<TD>Chrysophycean algae, especially Dinobryon</TD</TR>