Causes of Eutrophication and its Effects of on Aquatic Ecosystems

Rachel NashBIOL 271Pr<strong>of</strong>. WagnerT.A. Emily Bernhardt<strong>Eutrophication</strong> Lab Report4/2/2008Abstract<strong>Causes</strong> <strong>of</strong> <strong>Eutrophication</strong> <strong>and</strong> <strong>its</strong> <strong>Effects</strong> <strong>of</strong> on Aquatic Ecosystems<strong>Eutrophication</strong> can occur when inorganic nutrients are introduced into an aquaticecosystem. This study was conducted in order to better underst<strong>and</strong> how run-<strong>of</strong>f containingagricultural fertilizers can cause eutrophication. Using a system <strong>of</strong> aquariums, terrestrial <strong>and</strong>aquatic plants, <strong>and</strong> fertilizer, we measured the dissolved oxygen saturation, temperature, pH,optical density, nitrate/ nitrite, phosphate, <strong>and</strong> ammonia/ammonium <strong>of</strong> tanks subjected t<strong>of</strong>ertilizer <strong>and</strong> control tanks over an experimental period <strong>of</strong> 49 days. We statistically analyzed thedata <strong>and</strong> discovered that the algal density, optical density, <strong>and</strong> dissolved oxygen concentration <strong>of</strong>fertilized tanks suggested that eutrophication occurred. Because the effects <strong>of</strong> eutrophication canhave a negative effect on aquatic ecosystems, <strong>and</strong> fertilizers are widely used in the United Statesto boost crop growth, eutrophication is a serious ecological as well as socio-economic challenge.IntroductionOverproduction <strong>of</strong> organic matter in bodies <strong>of</strong> water is called eutrophication. (Ricklefs,2007). This process commonly occurs when limiting nutrients such as phosphorus <strong>and</strong> nitrogenare introduced into aquatic systems. Phosphorus <strong>and</strong> nitrogen from agricultural fertilizers <strong>of</strong>tenaccumulate in soils <strong>and</strong> are then leached into nearby water sources, causing eutrophication(Stephanou et. al., 2000; Torrent et. al., 2007). <strong>Eutrophication</strong> is characterized by increases inphytoplankton <strong>and</strong> algae biomass as well as changes in species <strong>and</strong> composition <strong>of</strong> macrophytes.These changes can lead to numerous problems such as decreases in water translucence <strong>and</strong>1

saturated oxygen caused by the death <strong>and</strong> decomposition <strong>of</strong> plants, subsequent fish kills, <strong>and</strong> loss<strong>of</strong> aquatic biodiversity. Additionally, blooms <strong>of</strong> algae known as red or brown tides carry toxinsthat are absorbed into animal tissues, such as shellfish, <strong>and</strong> may harm humans if ingested.Cyanobacteria also release neurotoxins that poison livestock <strong>and</strong> threaten humans (Carpenter et.al., 1998). The process <strong>of</strong> reversing eutophication is difficult due to the challenges in trackingpollution to <strong>its</strong> sources as well as the magnitude <strong>of</strong> the affected areas. However, thoughphosphorus has no documented negative effects on humans or animals, nitrate pollution is toxicat high concentrations, <strong>and</strong> can harm infants as well as cattle if levels reach 45 mg/L (Carpenteret. al., 1998). Because <strong>of</strong> this <strong>and</strong> other negative effects associated with eutrophication,numerous papers exist concerning the assessment <strong>and</strong> management <strong>of</strong> eutrophic aquaticecosystems. One such paper describes rating eutrophic levels based on concentrations <strong>of</strong> nitrate,as nitrogen is the most limiting nutrient for phytoplankton growth (Stefanous et. al., 2000).In this study we observed the process <strong>of</strong> eutrophication in a system <strong>of</strong> aquariums bytreating the submerged macrophyte <strong>of</strong> a species <strong>of</strong> Elodea that is common to temperate low-l<strong>and</strong>streams <strong>and</strong> lakes, with fertilizer run-<strong>of</strong>f applied to potted corn plants (Madsen <strong>and</strong> Baattrup-Pedersen, 1995). Our group conducted this experiment in order to better underst<strong>and</strong> the effects <strong>of</strong>fertilizer input <strong>and</strong> eutrophication on aquatic systems. We hypothesized that eutrophicationwould occur in tanks treated with fertilizer <strong>and</strong> that no significant evidence <strong>of</strong> eutrophicationwould occur in our control tanks. Based on our hypothesis we predicted that inputs <strong>of</strong> fertilizershould increase the growth <strong>of</strong> the Elodea plants in our experimental tanks, while the plants in ourcontrol tanks should have a significantly lower increase in growth. Additionally we predictedthat nitrogen, phosphorus, <strong>and</strong> ammonia concentrations as well as optical density <strong>of</strong> treated tanks2

should exceed that <strong>of</strong> our control tanks, while the oxygen saturation <strong>of</strong> water from our controltanks should surpass that <strong>of</strong> our experimental tanks.MethodsOur experiment involved creating a hypothesis, setting up a system <strong>of</strong> plastic tanks to testour hypothesis, testing the water chemistry <strong>of</strong> the aquarium water throughout the experiment,<strong>and</strong> taking final measurements at the end <strong>of</strong> our experiment. In order to test the effects <strong>of</strong>fertilizer addition <strong>and</strong> eutrophication on water quality, we first obtained two plastic aquariumsfilled with 3 liters <strong>of</strong> reverse osmosis water <strong>and</strong> another 3 liters <strong>of</strong> tap water, <strong>and</strong> added 300 mg<strong>of</strong> aquarium salt to each. Next we labeled one tank as our control tank <strong>and</strong> another as ourfertilizer, or experimental, tank. We took two sprigs <strong>of</strong> Elodea, weighed them, <strong>and</strong> added one toeach tank. After this we planted one germinated corn seed each in two plastic pots <strong>of</strong> soil. Wetook initial measurements <strong>of</strong> the water from both tanks including dissolved oxygen saturation,temperature, pH, optical density, nitrate/ nitrite, <strong>and</strong> ammonia/ammonium. After taking thesemeasurements we placed the pots on top <strong>of</strong> the aquariums as shown in Figure 1.TanksPlastic PotsFertilizerControlElodea SprigsFigure 1: Diagram <strong>of</strong> aquarium set-up3

Throughout the experiment we watered the corn on top <strong>of</strong> our experimental tank with watermixed with fertilizer, <strong>and</strong> watered the control corn with plain reverse osmosis water, allowing thewater to drain out <strong>of</strong> the pots <strong>and</strong> through holes in the tops <strong>of</strong> the aquariums. After twenty-eightdays we repeated our initial measurements with the addition <strong>of</strong> a phosphate measurement, <strong>and</strong>also measured the corn height <strong>and</strong> Elodea mass. On day forty-nine <strong>of</strong> our experiment we tookfinal measurements <strong>of</strong> the chemical <strong>and</strong> physical aspects <strong>of</strong> the water <strong>and</strong> weighed the Elodeaalong with the aboveground portion <strong>of</strong> the corn. We also estimated the amount <strong>of</strong> algal growth ineach tank.We completed our three sets <strong>of</strong> measurements using a variety <strong>of</strong> materials. Throughcolorimetric tests using a spectrophotometer we measured phosphate <strong>and</strong> ammonia/ammonium.The reagents included a Nutrafin Phosphate Test <strong>and</strong> a Nutrafin Ammonia Test Kit made byHagen from Montreal, Canada. We also used Quick Dip Nitrate/Nitrite Test Strips made byJungle Laboratories Corp. from Cibolo, Texas, USA. In order to measure optical density we ransamples <strong>of</strong> water from each tank through a spectrophotometer. We measured dissolved oxygen,pH, <strong>and</strong> temperature using specialized digital meters specific to each variable.ResultsControl FertilizedMean 1.44 14.18417Variance 0.142618 30.30072Observations 12 12Pooled Variance 15.22167Hypothesized Mean Difference 0df 22t Stat -8.00122P(T

Figure 2 shows the results <strong>of</strong> a two-tailed t-test assuming equal variance <strong>of</strong> the freshbiomass in grams <strong>of</strong> the corn plants grown with <strong>and</strong> without fertilizer added, where alpha equals0.05. The mean mass <strong>of</strong> the fertilized corn is 14.18 grams while that <strong>of</strong> the control corn is 1.44grams. With a degree <strong>of</strong> freedom <strong>of</strong> 22, P is equal to 5.89E-08, suggesting that the differencebetween the corn grown with <strong>and</strong> without fertilizer is statistically significant.Control FertilizedMean 0.009542 0.033708Variance 8.07E-05 0.001126Observations 12 12Pooled Variance 0.000603Hypothesized Mean Difference 0df 22t Stat -2.41029P(T

Figure 4: Comparative bar graph <strong>of</strong> average nutrientt concentrations (mg/L) <strong>of</strong> waterwithin tanks with <strong>and</strong> without potential fertilizer inputs.Figure 4 illustrates a comparison between the average concentrations <strong>of</strong> phosphate,ammonium, nitrate, <strong>and</strong> nitrite measured in mg/L found in both the experimentalfertilized tanks<strong>and</strong> the control tanks. Though these values are not statistically significant when individualcompounds are compared with a t-test, the bar graph <strong>of</strong> figure 4 shows the differences betweenthe average values <strong>of</strong>the four compounds when compared to one another. As shown, nitrate hasthe highest concentration <strong>of</strong> any <strong>of</strong> the other nutrients, especially when measuredin tanks withfertilizer input, reaching a peak average concentration <strong>of</strong>f 4.17 mg/L while only reaching 1.67mg/L in our control tanks. The second most common compound found in the water samples isphosphate, which is relatively constant between our fertilized <strong>and</strong> control groups, showing only a0.02 mg/ /L differencee between the higher 0.82 mg/L <strong>of</strong> fertilized tanks <strong>and</strong> the 0. .80 mg/L <strong>of</strong> non-anfertilizedtanks. Ammonium is the third highest compound measuredin our tankwater, havingaverage concentration <strong>of</strong> .2 mg/Lfor fertilized tanks <strong>and</strong>d a .1 mg/L for tanks not experiencing6

fertilized run-<strong>of</strong>f. Nitrite, is the least common nutrient found in our aquarium systems having anaverage concentration in fertilized tanks <strong>of</strong> 0.1 mg/L, <strong>and</strong> an average concentration in our controltanks <strong>of</strong> 0.06 mg/L.Control FertilizedMean 0.7175 1.0725Variance 1.639020455 0.950547727Observations 12 12Pooled Variance 1.294784091Hypothesized Mean Difference 0df 22t Stat -0.764196952P(T

abundance, <strong>and</strong> 3 describes high abundance. 67% <strong>of</strong> the control tanks fit the lowest algalcategory, <strong>and</strong> the other 33% were estimated at a category 2 algal abundance. None <strong>of</strong> the controltanks had enough algal growth to fit the high abundance category. 17% <strong>of</strong> the fertilized tankswere at a level 1 algal abundance, 58% at a level two algal abundance, <strong>and</strong> 25% had high enoughalgal growth to be placed in the category 3 <strong>of</strong> high abundance.Control FertilizedMean 8.38875 7.6425Variance 0.07581 0.31387Observations 12 12Pooled Variance 0.19484Hypothesized Mean Difference 0df 22t Stat 4.141149P(T

DiscussionThe results <strong>of</strong> our experiment support our hypothesis that eutrophication should occur intanks treated with fertilizer, <strong>and</strong> that no significant evidence <strong>of</strong> eutrophication should occur inour control tanks. Corn grown with fertilizer had a statistically higher biomass (g) than corngrown without fertilizer, suggesting that nitrogen <strong>and</strong> phosphorus are limiting factors <strong>of</strong> corngrowth, as shown by the results displayed in Figure 2, where P is 5.89E-08. The results shown inFigure 3, where P equals 0.025, support our prediction that the optical density (abs) <strong>of</strong> the waterin control tanks is lower than that in fertilized tanks. This shows that more algae, fungi, bacteria,or plant growth took place in the fertilized tanks than in the control tanks. However, ourprediction that fertilizer should cause the elodea in the experimental group to grow more thanthose in the control tanks was not statistically accurate, as revealed by the values in Figure 5where P equals 0.45. Instead, 25% <strong>of</strong> the fertilized tanks were characterized by high algaldensity, 58% <strong>of</strong> had a middle algal density, 67% <strong>of</strong> the control tanks had a low algal density, <strong>and</strong>none <strong>of</strong> the control tanks had a high algal density (Figure 6). Because the majority <strong>of</strong> the controltanks had low algal growth, <strong>and</strong> none <strong>of</strong> them experienced high algal growth, while one quarter<strong>of</strong> the fertilized tanks had a high abundance <strong>of</strong> algae, <strong>and</strong> the majority had at least a middle levelalgal abundance, we concluded that algae played a major role in the eutrophication processthrough raising the optical density (abs) <strong>of</strong> the water.As predicted, average nitrogen, phosphorus, <strong>and</strong> ammonia concentrations were higher inthose tanks subjected to inputs <strong>of</strong> fertilizer than in the control tanks. However, none <strong>of</strong> thesevalues are statistically significant. Instead, Figure 4 shows that nitrate <strong>and</strong> phosphorus are thenutrients used least by the aquatic life, suggesting that in the fertilized tanks, they were the leastlimiting nutrients. This is also supported by the P-value <strong>of</strong> Figure 5. In testing the differences in9

the growth <strong>of</strong> the Elodea in fertilized <strong>and</strong> unfertilized tanks, P equals 0.45, meaning there is nostatistically significant difference. This means that the fertilizer did not influence the growth <strong>of</strong>the Elodea, though future studies could reveal whether it influenced algal growth to a statisticallysignificant level. The t-test results shown in Figure 7 reveal the final piece <strong>of</strong> evidence insuggesting that eutrophication occurred in the fertilized tanks. In documented cases <strong>of</strong>eutrophication, dissolved oxygen concentration (mg/L) in water is low (McKee et. al., 2003).The dissolved oxygen concentration (mg/L) <strong>of</strong> the water in the control tanks was significantlyhigher than that <strong>of</strong> the tank water that had potential fertilizer inputs, as shown in Figure 7 whereP equals .0004. The ecological implications <strong>of</strong> eutrophication are various when decreases indissolved oxygen (mg/L) are considered on a broad scale. Other studies have found that anoxiacan occur in aquatic ecosystems with intense eutrophication, causing mass fish death, as well asthe death <strong>of</strong> many other aquatic organisms. Reversing the process <strong>of</strong> eutrophication involvesremoving the sources <strong>of</strong> inorganic nutrients such as those that are used on a wide scale in theUnited States for agricultural purposes. The socio-economical <strong>and</strong> even political issues related toeutrophic aquatic environments are complex, <strong>and</strong> difficult to manage, making this study one <strong>of</strong>great ecological importance for the future <strong>of</strong> ecology. Further studies are needed to test methods<strong>of</strong> decreasing agricultural <strong>and</strong> urban run-<strong>of</strong>f <strong>of</strong> inorganic nutrients in order to begin to remedythe negative ecological impacts <strong>of</strong> eutrophication.10

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