Baixe Environmental Modeling-Jerald L Schnoor-Chapter 05 e outras Notas de estudo em PDF para Engenharia Ambiental, somente na Docsity! EUTROPHICATION OF LAKES Like winds and sunsets, wild things were taken for granted until progress began to do away with them. —Aldo Leopold, A Sand County Almanac 5.1 INTRODUCTION Eutrophication refers to the excessive rate of addition of nutrients, usually in refer- ence to anthropogenic activities and the addition of phosphorus and nitrogen to nat- ural waters. Nutrient additions result in the excessive growth of plants including phytoplankton (free-floating algae), periphyton (attached or benthic algae), and macrophytes (rooted, vascular aquatic plants). Eutrophication is a natural process taking place over geologic time that is greatly accelerated by human activities. For example, due to soil erosion and biological pro- duction, lakes normally fill with sediments over thousands of years. As the lakes fill up, the mean depth and detention time of the water body decreases. Sediments are in greater contact with overlying water, and nutrients recycle under anaerobic condi- tions via diffusion. With addition of anthropogenic nutrients, algae growth acceler- ates even further. Eventually a process that would have occurred over geologic time scales is accelerated to decades, and the lake becomes overly productive biological- ly. The process has some undesirable effects on water quality. * Excessive plant growth (green color, decreased transparency, excessive weeds). * Hypolimnetic loss of dissolved oxyg! * Loss of species diversity (loss of fishery). º Taste and odor problems. en (anoxic conditions). Not all eutrophic lakes will exhibit all of these water quality problems, but they are ion, then, is the excessive rate of ad- likely to have one or more of them. Eutrophicat dition of nutrients. Finally, water quality may become so degraded that the lake's Criginal uses are lost. It may no longer be swimmable or fishable. A schematic of a eutrophic lake is shown in Figure 5.1. Of course, the degree of eutrophication is a continuum, and researchers have at- 185 186 Eutrophication of Lakes Nutrient Addition N > 7 Outflow Excessive Algal Growth D.O. Profile Thermocline Nutrient Recycle/ Diffusion , 0.0 mg/L Anoxic Diss. Oxygen Sediments Figure 5.1 Schematic of lake cutrophication and nutrient recycle. Thermal stratification causes a high concentration of oxygen in near-surface waters, but the dissolved oxygen cannot mix vertically, and decp water eventually becomes anoxic. Anoxic conditions in the bottom waters and sediments cause anaero- bic decomposition and the release of nutrients (phosphate, ammonia, dissolved iron). tempted to classify lakes according to their relative extent of eutrophication sino 1939.! It is referred to as the “trophic status” of the water body: * Oligotrophic. * Mesotrophic. * Eutrophic. “Eutrophic” comes from the Greek term meaning well-nourished. Oligotrohio a tems are “undernourished;” that is, biological production is limited by nutr ie vil tions. Eutrophic systems ate overfertilized by anthropogenic nutrient addito concomitant water quality problems. Mesotrophic waters lie somewhere 17 a corr Investigators have based these trophic classification schemes on the ai ing centrations available for algal blooms in the spring and on the areal nutrient rateingm?2y! (based on the lake surface area). 5.2 Stoichiometry 189 30 7 ºo ê 254 o So A o d º é o > or et pésão a. Jº Ss . a E ed e kS Zz 10L eq 16 a 1 5 “ A qu | To 2.0 Phosphate, LM Figure 5.3 Ratio of nitrate to phosphate in surface ocean waters. (Modified from Redfield et al.s) SsugP =0.16ugL'!d! L 31 days 0.16 ug 96 Hg HPOZF E =0.50pgL! d! Ld 30.97 pg P 0.16 ug gmol E = 0.0052 mol L! dl Ld 30.97 pg P 0.0052 mol 16 mol NO; DD RS 0.083 pumol L-! dr! Ld imolP =52ugL'!d! 0.0052 mol 106 mol CO; TD + = 0,55 umol LL! dl! Ld ImolP =AugLid! .0052 umol 1 mol algae TD ++ = 0.0052 umol L'! do! Ld ImolP HPOZ algae 190 Eutrophication of Lakes 0.0052 pumol 3553.26 pg algae ob = 18.5 Lg L! dr! pumol algaç Ld (31 days) (18.5 ES Li d)=573 ug! algas 0.57 mg L! of algae grew during the month of May in Lake Ontario 53 PHOSPHORUS AS A LIMITING NUTRIENT many nutrients could limit algal growth. Parsons and Ts, It is possible that one of being important for photosynthesis, hashi!! list the following micronutrients as Fe Zn Mo Co Mn B cl Cu Na vV Macronutrients include CO» (the carbon source), phosphorus, nitrogen (ammoniaor nitrate), Mg, K, Ca, and dissolved silica for diatom frustule formation. Algae and rooted aquatic plants are photoautotrophs —they usc sunlight as their energy source and CO, for their carbon source. The terminal electron acceptor is usually dissolveé oxygen in the electron trans ded for fer of photosynthesis. Suitable temperature is nee algal growth also. Any of these nutrients could, theoretically, Liebig's law of the minimum from the 19th century: become limiting for growth, as pa Growth of a plant depends on the amount of food-stuff that is presented to it in limit ing quantity” E: 2% : imilar tO Liebig envisioned a saturated response growth curve for each nutrient, simila that shown for Monod!? kinetics (Figure 5.4). p= Fera S é K+S where [max — maximum growth rate, T! S= substrate or nutrient concentration, ML? K, = half-saturation constant, ML? w o . no for ES Of course, it is possible that more than one nutrient will become limita iy and E at the same time. DiToro and co-workers!? have discussed this possib! uated the expressions for an electrical resistance analogue in parallel: 5.3 Phosphorus as a Limiting Nutrient 191 E = Himax SHKS+S) HT)=kT I RD = expr- + +] forT<To ] ks Light, 1 OC Te substrate or Nutrient Conc., S Temperature, T Figure 5.4 Response curves of algal growth rates to limiting nutrients, light, and temperature. et po pf (4) where p= overall limited growth rate, T us limited growth rate due to the ith nutrient, T” and a multiplicative analogue: ul O(S (So (5) é tm 45.) (7515,) (55) The multiplicative analogue is much more limiting to growth (it usually results in a lower growth rate), but it is the one that is recommended for use with phytoplankton populations in the Great Lakes.!* Example 5.2 Limiting Nutrient Concentrations und Overall Growth Rates Estimate the resulting growth rate for diatom phytoplankton in the Great Lakes by three different methods for the following data if the maximum growth rate is 1.0 per day. a. Liebig's law of the minimum. b. Electrical resistance analogy. €. Multiplicative algorithm. NH$ POP Si SmgL! 0.02 0.005 1.6 KomgL! 002 0.020 1.0 Solution: à. Calculation with the Monod kinetic expression yields the following for 194 Eutrophication of Lakes ap É -0-QuPa-OP- k PV a Q; -D 0) If evaporation can be neglected, the inflow rate is approximately equal to the out. flow rate (O,, = 0). Also, we can divide through by the lake volume to express the right-hand side of the equation in terms of the hydraulic detention time (=V0). Po Po 0= TO T kP (8) where + = hydraulic detention time, T After rearranging, the concentration of total phosphorus in the lake is = Pi ka+l 0) The total phosphorus concentration is directly related to the total P concentration in the inflow (Pin), and it is inversely related to the hydraulic detention time and the sedimentation rate constant (the main removal mechanism of total phosphorus from the water column). Thus the fate of total phosphorus in lakes is determined byan important dimensionless number (k,7). There is a trade-off between the detention time of the lake and the sedimentation rate constant—it is the product of these two parameters that determines the ratio of total phosphorus in the lake to the inflowing total P: P 1 o 10 Pa kr+] em The fraction of total phosphorus that is trapped or removed from the water column may be of interest. R = fraction removed = 1 — > = (1) in ki+1 A simple mass balance such as that developed in this section has served as the basis for a number of research papers on the eutrophication of lakes dating back to the work of Vollenweider in 1969.!7 Example 5.3 Mass Balance on Total Phosphorus in Lake Lyndon B. Johnson Lake Lyndon B. Johnson is a flood control and recreational reservoir along à chan of reservoirs in central Texas along the Colorado River. It has an average hydraulic detention time of 80 days, a volume of 1.71 x 108 m?, and a mean depth of ra The ratio of particulate to total phosphorus concentration in the lake is 0.7, and 5.5 Nutrient Loading Criteria 195 ean particle settling velocity is 0.1 m d'!, If the flow-weighted average inflow pcentration of total phosphorus to Lake LBJ is 72 ng L”!, estimate the average co) | phosphorus concentration in the lake annual tota Solution: k = av/H k, = (0.7) (0.1 m d=/(6.7 m) = 0.0104 day ks = (0.0104) (80) = 0.832 p= 7 kr+1 (00104) (80)+ 1 P=39ugL! The answer is a total phosphorus concentration of 39 ug L! in the lake, 70% of material and the remaining 30% is dissolved (most of the dis- - which is particulate s flux of total P to solved P is POZ, available for phytoplankton uptake). The mas the bottom sediment is kP V, a flux rate equal to 69 kg dr, A plot of the fraction of total P remaining in any lake as a function of the dimen- sionless number kt is given by Figure 5.5. The fraction of total P removed to the lake sediments is equal to 0.454 (1 — P/P,); the mass of total phosphorus entering the lake is 152 kg d”!, and the mass outflow is 83 kg d! (Figure 5.6). 55 NUTRIENT LOADING CRITERIA vestigators to consider adoption of a criterion for 118,19 In 1947, Sawyer noted that lakes in New f total phosphorus concentrations in the early Clair Sawyer was one of the first in the classification of eutrophic lakes. England would exhibit algal blooms i spring were greater than 30 pg Lo, Vollenweider argued that it is no much as the nutrient supply rate (the are . yr!),"20 He published “permissible” and “dangerous” loading T on a log-log plot of annual phosphorus loading versus mean depth for about 100 lakes in Europe and North America. These loading rates became widely adopted as nutrient loading criteria by many countries throughout the world. They compriscd à Ciiteria to classify lakes as oligotrophic, mesotrophic, and eutrophic. Vollenweider's Nuttient loading rates are given in Table 5.1. Trophic classifications Were based on Observations of algal blooms and nuisance conditions (an empirical classification Criteria). Actually, both approaches to classify t the nutrient concentration that mattered as al nutrient loading rate expressed in g m? ates for lakes based ing the trophic status of lakes are valid and, 196 Eutrophication of Lakes LO PIPin 0.5 H Lo Ls 5 10 15 20 25 ka 0.0 ! 0 Figure 5.5 Fraction of total P remaining in a lake as a function of the sedimentation coefficient 4, times the detention time 7. 152 kg/day Lake LBJ 69 Figure 5.6 Total phosphorus mass balance for Lake Lyndon B. Johnson in central Texas. EUTROPHICATION OF LAKES Like winds and sunsets, wild things were taken for granted until progress began to do away with them. —Aldo Leopold, A Sand County Almanac 5.1 INTRODUCTION Eutrophication refers to the excessive rate of addition of nutrients, usually in refer- ence to anthropogenic activities and the addition of phosphorus and nitrogen to nat- ural waters. Nutrient additions result in the excessive growth of plants including phytoplankton (free-floating algae), periphyton (attached or benthic algae), and macrophytes (rooted, vascular aquatic plants). Eutrophication is a natural process taking place over geologic time that is greatly accelerated by human activities. For example, due to soil erosion and biological pro- duction, lakes normally fill with sediments over thousands of years. As the lakes fill up, the mean depth and detention time of the water body decreases. Sediments are in greater contact with overlying water, and nutrients recycle under anaerobic condi- tions via diffusion. With addition of anthropogenic nutrients, algae growth acceler- ates even further. Eventually a process that would have occurred over geologic time scales is accelerated to decades, and the lake becomes overly productive biological- ly. The process has some undesirable effects on water quality. * Excessive plant growth (green color, decreased transparency, excessive weeds). * Hypolimnetic loss of dissolved oxyg! * Loss of species diversity (loss of fishery). º Taste and odor problems. en (anoxic conditions). Not all eutrophic lakes will exhibit all of these water quality problems, but they are ion, then, is the excessive rate of ad- likely to have one or more of them. Eutrophicat dition of nutrients. Finally, water quality may become so degraded that the lake's Criginal uses are lost. It may no longer be swimmable or fishable. A schematic of a eutrophic lake is shown in Figure 5.1. Of course, the degree of eutrophication is a continuum, and researchers have at- 185 186 Eutrophication of Lakes Nutrient Addition N > 7 Outflow Excessive Algal Growth D.O. Profile Thermocline Nutrient Recycle/ Diffusion , 0.0 mg/L Anoxic Diss. Oxygen Sediments Figure 5.1 Schematic of lake cutrophication and nutrient recycle. Thermal stratification causes a high concentration of oxygen in near-surface waters, but the dissolved oxygen cannot mix vertically, and decp water eventually becomes anoxic. Anoxic conditions in the bottom waters and sediments cause anaero- bic decomposition and the release of nutrients (phosphate, ammonia, dissolved iron). tempted to classify lakes according to their relative extent of eutrophication sino 1939.! It is referred to as the “trophic status” of the water body: * Oligotrophic. * Mesotrophic. * Eutrophic. “Eutrophic” comes from the Greek term meaning well-nourished. Oligotrohio a tems are “undernourished;” that is, biological production is limited by nutr ie vil tions. Eutrophic systems ate overfertilized by anthropogenic nutrient addito concomitant water quality problems. Mesotrophic waters lie somewhere 17 a corr Investigators have based these trophic classification schemes on the ai ing centrations available for algal blooms in the spring and on the areal nutrient rateingm?2y! (based on the lake surface area). 5.2 Stoichiometry 189 30 7 ºo ê 254 o So A o d º é o > or et pésão a. Jº Ss . a E ed e kS Zz 10L eq 16 a 1 5 “ A qu | To 2.0 Phosphate, LM Figure 5.3 Ratio of nitrate to phosphate in surface ocean waters. (Modified from Redfield et al.s) SsugP =0.16ugL'!d! L 31 days 0.16 ug 96 Hg HPOZF E =0.50pgL! d! Ld 30.97 pg P 0.16 ug gmol E = 0.0052 mol L! dl Ld 30.97 pg P 0.0052 mol 16 mol NO; DD RS 0.083 pumol L-! dr! Ld imolP =52ugL'!d! 0.0052 mol 106 mol CO; TD + = 0,55 umol LL! dl! Ld ImolP =AugLid! .0052 umol 1 mol algae TD ++ = 0.0052 umol L'! do! Ld ImolP HPOZ algae 190 Eutrophication of Lakes 0.0052 pumol 3553.26 pg algae ob = 18.5 Lg L! dr! pumol algaç Ld (31 days) (18.5 ES Li d)=573 ug! algas 0.57 mg L! of algae grew during the month of May in Lake Ontario 53 PHOSPHORUS AS A LIMITING NUTRIENT many nutrients could limit algal growth. Parsons and Ts, It is possible that one of being important for photosynthesis, hashi!! list the following micronutrients as Fe Zn Mo Co Mn B cl Cu Na vV Macronutrients include CO» (the carbon source), phosphorus, nitrogen (ammoniaor nitrate), Mg, K, Ca, and dissolved silica for diatom frustule formation. Algae and rooted aquatic plants are photoautotrophs —they usc sunlight as their energy source and CO, for their carbon source. The terminal electron acceptor is usually dissolveé oxygen in the electron trans ded for fer of photosynthesis. Suitable temperature is nee algal growth also. Any of these nutrients could, theoretically, Liebig's law of the minimum from the 19th century: become limiting for growth, as pa Growth of a plant depends on the amount of food-stuff that is presented to it in limit ing quantity” E: 2% : imilar tO Liebig envisioned a saturated response growth curve for each nutrient, simila that shown for Monod!? kinetics (Figure 5.4). p= Fera S é K+S where [max — maximum growth rate, T! S= substrate or nutrient concentration, ML? K, = half-saturation constant, ML? w o . no for ES Of course, it is possible that more than one nutrient will become limita iy and E at the same time. DiToro and co-workers!? have discussed this possib! uated the expressions for an electrical resistance analogue in parallel: 5.3 Phosphorus as a Limiting Nutrient 191 E = Himax SHKS+S) HT)=kT I RD = expr- + +] forT<To ] ks Light, 1 OC Te substrate or Nutrient Conc., S Temperature, T Figure 5.4 Response curves of algal growth rates to limiting nutrients, light, and temperature. et po pf (4) where p= overall limited growth rate, T us limited growth rate due to the ith nutrient, T” and a multiplicative analogue: ul O(S (So (5) é tm 45.) (7515,) (55) The multiplicative analogue is much more limiting to growth (it usually results in a lower growth rate), but it is the one that is recommended for use with phytoplankton populations in the Great Lakes.!* Example 5.2 Limiting Nutrient Concentrations und Overall Growth Rates Estimate the resulting growth rate for diatom phytoplankton in the Great Lakes by three different methods for the following data if the maximum growth rate is 1.0 per day. a. Liebig's law of the minimum. b. Electrical resistance analogy. €. Multiplicative algorithm. NH$ POP Si SmgL! 0.02 0.005 1.6 KomgL! 002 0.020 1.0 Solution: à. Calculation with the Monod kinetic expression yields the following for 194 Eutrophication of Lakes ap É -0-QuPa-OP- k PV a Q; -D 0) If evaporation can be neglected, the inflow rate is approximately equal to the out. flow rate (O,, = 0). Also, we can divide through by the lake volume to express the right-hand side of the equation in terms of the hydraulic detention time (=V0). Po Po 0= TO T kP (8) where + = hydraulic detention time, T After rearranging, the concentration of total phosphorus in the lake is = Pi ka+l 0) The total phosphorus concentration is directly related to the total P concentration in the inflow (Pin), and it is inversely related to the hydraulic detention time and the sedimentation rate constant (the main removal mechanism of total phosphorus from the water column). Thus the fate of total phosphorus in lakes is determined byan important dimensionless number (k,7). There is a trade-off between the detention time of the lake and the sedimentation rate constant—it is the product of these two parameters that determines the ratio of total phosphorus in the lake to the inflowing total P: P 1 o 10 Pa kr+] em The fraction of total phosphorus that is trapped or removed from the water column may be of interest. R = fraction removed = 1 — > = (1) in ki+1 A simple mass balance such as that developed in this section has served as the basis for a number of research papers on the eutrophication of lakes dating back to the work of Vollenweider in 1969.!7 Example 5.3 Mass Balance on Total Phosphorus in Lake Lyndon B. Johnson Lake Lyndon B. Johnson is a flood control and recreational reservoir along à chan of reservoirs in central Texas along the Colorado River. It has an average hydraulic detention time of 80 days, a volume of 1.71 x 108 m?, and a mean depth of ra The ratio of particulate to total phosphorus concentration in the lake is 0.7, and 5.5 Nutrient Loading Criteria 195 ean particle settling velocity is 0.1 m d'!, If the flow-weighted average inflow pcentration of total phosphorus to Lake LBJ is 72 ng L”!, estimate the average co) | phosphorus concentration in the lake annual tota Solution: k = av/H k, = (0.7) (0.1 m d=/(6.7 m) = 0.0104 day ks = (0.0104) (80) = 0.832 p= 7 kr+1 (00104) (80)+ 1 P=39ugL! The answer is a total phosphorus concentration of 39 ug L! in the lake, 70% of material and the remaining 30% is dissolved (most of the dis- - which is particulate s flux of total P to solved P is POZ, available for phytoplankton uptake). The mas the bottom sediment is kP V, a flux rate equal to 69 kg dr, A plot of the fraction of total P remaining in any lake as a function of the dimen- sionless number kt is given by Figure 5.5. The fraction of total P removed to the lake sediments is equal to 0.454 (1 — P/P,); the mass of total phosphorus entering the lake is 152 kg d”!, and the mass outflow is 83 kg d! (Figure 5.6). 55 NUTRIENT LOADING CRITERIA vestigators to consider adoption of a criterion for 118,19 In 1947, Sawyer noted that lakes in New f total phosphorus concentrations in the early Clair Sawyer was one of the first in the classification of eutrophic lakes. England would exhibit algal blooms i spring were greater than 30 pg Lo, Vollenweider argued that it is no much as the nutrient supply rate (the are . yr!),"20 He published “permissible” and “dangerous” loading T on a log-log plot of annual phosphorus loading versus mean depth for about 100 lakes in Europe and North America. These loading rates became widely adopted as nutrient loading criteria by many countries throughout the world. They compriscd à Ciiteria to classify lakes as oligotrophic, mesotrophic, and eutrophic. Vollenweider's Nuttient loading rates are given in Table 5.1. Trophic classifications Were based on Observations of algal blooms and nuisance conditions (an empirical classification Criteria). Actually, both approaches to classify t the nutrient concentration that mattered as al nutrient loading rate expressed in g m? ates for lakes based ing the trophic status of lakes are valid and, 196 Eutrophication of Lakes LO PIPin 0.5 H Lo Ls 5 10 15 20 25 ka 0.0 ! 0 Figure 5.5 Fraction of total P remaining in a lake as a function of the sedimentation coefficient 4, times the detention time 7. 152 kg/day Lake LBJ 69 Figure 5.6 Total phosphorus mass balance for Lake Lyndon B. Johnson in central Texas. 5.6 i i Relationship to Standing Crop 199 uhere P is the hydraulic flushing rate of the lake joading remaining, L(1 — RY/p, versus mean de a intercept where H=1moflog P. (1/7). A log-lo i f g plot of nutrient Pth (Ff) will have a slope of 1.0 and LU -R) jog (SAD p L0logH+logP (7) By empirical obscrvation of lake characteristics, the li ivi i lakes and eutrophic lakes are at 10 and 20 pg La of co ps os cu ds sen qo sn? phosphorus on an annual Similar simple mass balance models for icti Sim predicting total phosph tions in lakes have been developed by Lorenzen, Larsen, and others 2326 They rey on the concepts of total phosphorus loading on an areal basis and an nt ági tling velocity at steady state. e Pp (18) Multiplying both sides of the equation by the mean depth A, we find o=1-2E sp (19) surf P(gtv)=L (20) a PE qt a) flow rate for the lake). All of these approaches ded on the mass balance principle. In terms of they are nonetheless empirical because the 1 total P are oligotrophic and that lakes where q, = Y/Asut (the surface over are equivalent because they are founi classifying the trophic status of lakes, observation that lakes less than 10 pg L” greater than 20 Hg L”! are eutrophic is empirical. 56 RELATIONSHIP TO STANDING CROP directly by nutrient con- . blooms, decreased transparency, and decay- ing algae in the sediment that consumes oxygen, ruins aquatic habitats, and causes taste and odor problems. The previous mass balance models?!-26 that predict steady- State or annual average total phosphorus concentrations in Jakes do not predict bio- mass or chlorophyll a (phytoplankton of standing crop). Oth- The nuisance conditions of a eutrophie lake are not caused Centrations, rather it is excessive algal pigments as a measure 200 Eutrophication of Lakes er investigators have shown that it is possible to correlate the summer concem Tati tal phosphorus to the chlorophyll a concentration, as shown in Figy Tn of to anidem primarily use ortho-phosphate (POZ) measured as e active phosphorus. Figure 5.8 is valid because total phosphorus is Correlated ee ortho-phosphorus, which is bioavailable to phytoplankton. The ratio of ortho. Vith phate to total phosphorus may be relatively constant for ETOUps Of lakes, My cyclic mineralization of organic particulate phosphorus yields Ortho-phospias the sustain algal growth.?220 e to Other limnological measures can be correlated with chlorophyll a or trophic tus of lakes. Total phosphorus is one of the better correlates, but Secchi disk Ee oxygen consumption rates of bottom sediments, and carbon dioxide fixation Em (primary productivity) have also been utilized. Greater than 50 mg Cm h-tof o mary production is generally considered to be eutrophic, measured by !4C ragi a y bon light and dark bottles, incubated for 4 hours on floating racks in situ. Other indicators of eutrophic conditions are: 1000 E log (Chl a)=- 1.09 + 1.46 log (total P) r = 0.95 o . f : 100> “os , 7 a A o E vs + . oo culo E “mu / = 10 fr 2 RES 4 o . o . .* 5 fo: E + Õ “ Vo .s E da 1+ dra 0.1 e o l 10 100 1000 Total P, ug L-1 Figure 5.8 Relationship b Horus Ê p between summer | total phosP centration for 143 lakes. (From Sakamoto 2?) vels of chlorophyll a and measured 5.7 Land Use and Bioavailability 201 Cyanobacteria (blue-green algae) blooms. Loss of benthic invertebrates such as mayfly larvae (Hexagenia spp.). Secchi disk depths less than 2.0 meters. e º e e Loss of fishery and presence of “rough” fish. e Taste and odor problems. e Aquatic weeds in litoral zones. e Chlorophyll a concentrations greater than 10 ug L!. Some or all of these indicators may be exhibited by any given eutrophic lake. Sedi- ment oxygen depletion rates have been another good indicator of the trophic status of lakes (Table 5.2). Lake Erie was often cited as “dead” in the 1960s and 1970s, but it was not really dead. It was overfertilized by anthropogenic wastes. It was eutrophic. In 1974, the average oxygen depletion rate in the Central Basin was -12 mmol O, m? d"!, which classified it as eutrophic (Table 5.2). A massive reduction of total phosphorus load- ings was undertaken to decrease the inputs from 32,000 metric tonnes per year to 22,000 by requiring effluent limitations on phosphorus discharges from domestic wastewater treatment plants in the watershed and by rudimentary nonpoint source controls. Chloropbyll, total P, and anoxia in the Central Basin of Lake Erie have been much reduced by these actions, and the recycle of phosphate from anoxic sed- iments has also been reduced. The recovery of Lake Erie is an environmental suc- cess story, but further improvement depends on nonpoint source controls. Now, Lake Erie suffers most from toxic organic chemical problems (Chapter 7) and intro- duced species (zebra mussels). 5.7 LAND USE AND BIOAVAILABILITY Nonpoint source runoff, particularly from intensive agriculture, urban stormwater runoff, and combined-sewer overflow are major inputs to streams and lakes that pre- sent a difficult challenge for water quality management and control. Runoff concen- trations of total phosphorus and total nitrogen are shown in Tables 5.3 and 5.4 by land use and region of the United States. In many cases, these are the predominant Table 5.2 Sediment Oxygen Depletion Rates as a Measure of the Trophic State of Lakes (mmol O, md!) Hutchinson Mortimer Classification Criteria Criteria Oligotrophic lakes <-5.3 <77 Mesotrophic lakes -5.3t0-10.3 -71.Tto-172 Eutrophic lakes >-10.3 >-17.2 204 10 qa 10 q co 55H 50 45 40 35 30 25 20 15 10 Figure 5.9 Dynamic model simulation of total Phosphorus in the Great Lakes by Chapra- Eutrophication of Lakes Superior Liv ls 14 1970 80 90 2000 1970 80 90 2000 Western Erie a 1970 80 90 2000 10 40 35 tions are for total phosphorus (in gg L- 1970 values to | mg L-! effluent-P in 1980. Reprinted with permission from Journal Environ. Eng sion. Copyright (1977). American Society of Civil Engineers. mem Boundary between mesotrophy and Euthropy ——— Boundary between oligotrophy and mesotrophy 30 - Michigan 25 Ontario TC MEI=—— 20 - 15 Lili la tis o gsgto- 1970 80 90 2000 SL qua o Central 1970 80 90 2000 — Erie 30 m Eastern 25 Erie 20 15 10-——-—=—07" E 5 | Lili o] ot TO 1970 80 90 2000 1970 80 90 33 Concentit : . o : ings +) in response to a linear decrease in point source loadins pi 5.8 Dynamic Ecosystem Models for Eutrophication Assessments 205 HUMAN-INDUCEI WASTE LOADS NATURAL INPUTS BACTERIA a 8 sa DETRITUS DEAD ORGANISMS ZOOPLANKTON EXCRETA FOOD 6 õ “ap Saca, 7 RO o 'S tg BENTHIC ANIMAL FOOD FISH Figure 5.10 Schematic of an aquatic ecosystem. and are known to be associated with taste and odor problems and toxins during their decay following the bloom. Blue-green algal blooms are sometimes so thick that the floating algal mat may blow onto the shoreline and create a stench due to decay and decomposition. The ability to float is caused by gas vacuoles in the cell that are formed during nitrogen-fixation of some blue-green algae with heterocysts. As a nuisance organism, blue-green algae are difficult to control because they do not need many nutrients and they have very low loss rates (they do not sink).?2 If they are nitrogen-fixing, they may require zero concentrations of nitrate and ammonium 206 Eutrophication of Lakes CARNIVOROUS pag ZOOPLANKTON CARBON Á HERBIVOROUS eg! ZOOPLANKTON CARBON DIATOM OTHERS CHLOROPHYLL CHLOROPHYLL y y UNAVAILABLE AVAILABLE PARTICULATE [pa PHOSPHORUS PHOSPHORUS (ortho-phosphate) : Sedimentation Figure 5.11 Flowchart for phosphorus kinetics in a dynamic ecosystem model. and still grow actively. Various Pphytoplankton taxa are given below in rough order of their seasonal succession. Diatoms [Bisoved silica limiting nutrient (frustules) Cold water, P- or Si-limited Green algae Usually P-limited, summer Dinoflagellates Flagellated, motile, occasional toxins (red tides) Low N requirements (N-fixing) Blue-green algae Low sinking velocities (gas vacuoles) Warm water, late summer or early fall Dinoflagellates can swim to the Photic zone during the day and dive to the nutrient- rich thermocline during the night, demonstrating that luxury uptake of phosphorus or other nutrients may be possible. Growth rates sometimes depend on the nutrient content of the cell rather than the nutrient concentration of the surrounding vate” “Phytoplankton” which translates from Greek roughly as “free-floating plants,” is? misnomer. Sometimes phytoplankton can float or exhibit limited motility. ] ] 5.8 Dynamic Ecosystem Models for Eutrophication Assessments 209 Ej= bulk dispersion coefficient between the jth and the ith adjacent com- partments, L2T- Aj= interfacial area between the jth and the ith adjacent compartments, L? C; = concentration of a conservative tracer in the ith compartment, ML? €;; = half-distance (connecting distance between the middle of the two adja- cent compartments), L Note that the bulk dispersion coefficient is scale dependent as an aggregate for- mulation of all mixing processes. For example, it will not be equal to point mea- surements of dye dispersion in large lakes, and it cannot be determined explicitly. Bulk dispersion coefficients may be thought of as a bulk exchange flow going each way between compartments in Figure 5.13. EA; Q5= (23) y where (9; is the bulk exchange flow with units of LºT-!. The reader is referred to the section on compartmentalization in Chapter 2. Procedures for model calibration and verification are: 1. Calibrate the model using field data for a conservative substance (e.g., chlo- ride) or heat to determine Q, and E, values. 2. Using the same advective flows and bulk dispersion coefficients between compartments, calibrate the model for all water quality constituents for the same set of field data to determine all other adjustable parameters and coeffi- cients (reaction rate constants, stoichiometric coefficients, etc.). 3. Verify the model with an independent set of field data, keeping all transport and reaction coefficients the same. Determine the “goodness of fit” between the model results and field data. A suitable criterion for acceptance of the model calibration and verification should be determined a priori, depending on the use of the model in research or water qual- ity management. A median relative error of 10-20%, model results/field data, is typical in water quality models.*º Ecosystem models consist of a series of mass balance equations within each compartment. The number of compartments will vary depending on the spatial ex- tent of water quality data that is available and the resolution that is needed from the model. A schematic of a typical ecosystem model is shown in Figure 5.14. There are nine state variables shown, but there could be more or less depending on the design of the model. Each of the nine state variables will undergo advection and dispersion às shown in the previous equation; however, fish and zooplankton would not dis- Perse at the same rate as other water quality constituents because they are motile. We assume four limiting nutrients and a multiplicative expression here. Biomass, father than chlorophyll a or carbon, is simulated. Bem 210 Eutrophication of Lakes Es H TT I I I | I A Organic » kton -—> Lo» Phytoplan nico = PO, -P —» Biomass-B [87] ara CNP, Si 4N, P, pra + 1] Ve ko] |ks | NH-N NO3-N eo [Rm | (No) a W VN A 7 AN am Figure 5.14 Ecosystem model for eutrophication assessments. Boxes represent state variables; solid ar- rows are mass fluxes; and dotted lines represent external forcing functions. The model employs a multiplicative Michaelis-Menton expression for phyto- plankton growth rate and linear kinetics (first-order decay) on hydrolysis and death rates of phytoplankton, zooplankton, and fish. Nonlinear second-order kinetic ex- pressions are utilized for grazing rates of zooplankton and fish on their prey. The ! model assumes that phytoplankton fulfill their nitrogen requirement by taking up NH,-N and NO;-N in proportion to their fractional concentration. (This is not strict- 4 ly the case—it is generally accepted that NH,-N is preferred due to its higher ener- gy content.) Temperature affects every rate constant, increasing the rate of the reaction. d[Si) Hs dr uBas+ o + ko(0E =) Dfy (3) dtP) y: “gd To uBapt Fo +k(07-20) Dj, (3) 4INJ = N Wa (ê a tm (x) Í e +(OJ-) Dk (07-20) Ni 29 5.8 Dynamic Ecosystem Models for Eutrophication Assessments 211 Table 5.5 Rate Constants and Stoichiometric Coefficients for Dynamic Ecosystem Models*** Typical Parameter Symbol Value at 20 ºC Range at 20 “ºC Mineralization Organic C ky 01d! 0.001-0.2 d! Organic N kof 0.03 0.001-0.2 Organic P kofe 0.03 0.001-0.2 Organic Si kofsi 0.1 0.001-0.2 Settling velocity Chlorophyll a Voacm O.Imd! 0.005-0.8 m d”! Organic detritus kH 0.1 0.01-0.3 Phytoplankton Vs 0.2 0.00-2.0 Apparent sedimentation, total P k, Olmd! 0.005-0.4m d”! Phyto ratios N:Chlorophyll a 10 5-20 P:Chlorophyll a LO 0.5-2.0 C:Chlorophyll a 65 35-100 Si:Chlorophyll a (diatoms) 40 30-50 Cidry wt 0,4 0.33-0,43 N:dry wt SN 0.05 0,04-0,09 P:dry wt fo 0.01 0.006-0.03 Phytoplankton Death rates e od! 0.05-0.25 d! Maximum growth rates Hemax 1.5 1.0-2.0 Half-saturation constants PO,-P Kp I0ngL'! 625 pg L! NH,-N Ku 10 1-20 NO;-N Ki 15 1-30 Si Kg 75 50-100 Zooplankton Grazing rates 8 04L mg! d! 0.2-20L mg! d! Death + respiration d 0.07 d! 0.02-0.3 dr! Watt AN upar) + O A agr, 07) dt Nr V, aa =+uB-eBoT-0 -g,BZ07-U -goBF BJ O-VIDBIH (28) sa =-eB0T-0 ko] D-K(T) D+ dzo7- 0 (29) 214 Eutrophication of Lakes We have seen how powerful simple mass balance models of Phosphorus Gta limiting nutrient) can be in assessing the eutrop ication status of lakes, In Some cases, the costs of cutrophication are sufficient to cause 8overnments to red point sources and even some nonpoint source discharges of nutriente to wa Dynamic ecosystem modeling is a valuable tool to assess the efficacy of control measures and their benefits. Ervays, Nutrieny 5.9 REFERENCES . Sawyer, CN. JN. Engl. Water Works Assoc., 61, 109 (1947). . Odum, E.P, Fundamentals of Ecology, Saunders, Philadelphia (1961). . Stumm, W., and Morgan, J.J., Aguatic Chemistry, Wiley-Interscience, New York (1981). . Redfield, A.C., James Johnstone Memorial Volume, Liverpool (1934). . Redfield, A.C., Ketchum, B.H., and Richards, EA. in The Sea, Vol, 2, M.N, Hill, Ed, Wi. ley-Interscience, New York (1963). 6. Sigg, L., and Stumm, W., Aquatische Chemie, VDF Publishers, Ziirich, Switzerland (1989). 7. Stumm, W., in Chemisiry of the Solid-Water Interface (Chapter 11), Wiley-Interscience, New York (1992). 8. Martin, J.H., and Fitzwater, S.E., Nature, 331, 341 (1988). 9. Martin, J.H., and Gordon, R.M., Deep Sea Res., 35, 177 (1988). 10. Martin, J.H., Gordon, R.M., Fitzwater, S.E., and Broenkow, WW., Deep Sea Res., 36, 649 (1989). 11, Parsons, T.R., and Takahashi, M., Biological Oceanographic Processes, Pergamon Press, Oxford (1973). 12. Monod, J., The Growth of Bacterial Cultures, Anny. Rev. Microbiol., II (1949), 13. DiToro, D.M., and Matystik, WE, Mathematical Models of Water Quality in Large Lakes, EPA-600/3-80-056, U.S. Environmental Protection Agency, Washington, DC (1980). 14. Thomamn, R.V., and Mueller, JA, Princ Control, Harper & Row, New York (1987). 15. Gordon, L.I., Park, PK., Hager, S.W., and Parsons, (1971). “rm iples of Surface Water Quality Modeling and TR. J Oceanogr Soc. Jpn., 37,81 - US. Environmental Protection Agency, A Compendium of Lake and Reservoir Data Col lected by the National Eutrophication Survey in the Northeast and North-Central United States, Working Paper No. 474, Corvallis, OR (1975). 17. Vollenweider, R.A,, Arch, Hydrobiol., 66,1 (1969). 18. Sawyer, C.N., Sewage Indust. Hastes, 26,317 (1954). - Sawyer, C.N,, Sewage Indust. Wastes, 24, 768 (1952). 20. Vollenweider, R.A,, in Scientific Fundamen he Eutrophicati L d Flow- z ? , tals ofti akes an ing Waters with Particular fhe Eutrophication of ei ari Reference to Nitrogen and Phosphorus as Factors in Eutro- Phication, Organization for Economic Cooperation and Development, Paris (1970). 21. 22. 23. 2. 25. 26. 27. 28. 29. 30. 31. 32. 33. 3. 35. 36. 37, 38. 39, 40. 41, 42. 43. 5.10 Problems 215 Dillon, PJ. and Rigler, EH., J Fish. Res. Bd. Can,, 31, 1771 (1974). Dillon, PJ. and Rigler, EH. Limnol. Oceanogr., 19, 767 (1974). Lorenzen, M.W., in Modeling the Eutrophication Process, E.]. Middlebrooks, Ed., Ann Arbor Science, Ann Arbor, MI, pp. 205-210 (1974). Lorenzen, M.W., Smith, D.J., and Kimmel, L.V, in Modeling Biochemical Processes in Aguatic Ecosystems, R.P. Canale, Ed., Ann Arbor Science, Ann Arbor, MI, pp. 75-91 (1976). Larsen, D.P, Maleug, K.W., Schults, D.W., and Brice, R.M., Verh. Int. Ver. Limnol., 19, 884 (1975). Larsen, D.P. Van Sickle, J., Maleug, K.W., and Smith, PD., Water Res., 13, 1259 (1979). Sakamoto, C., J Water Pollu. Control Fed., 48, 2177 (1979). US. EPA, The Relationships of Phosphorus and Nitrogen to the Trophic State of North- east and North-Central Lakes and Reservoirs, Working Paper No. 23, Corvallis, OR (1974). Lung, W.S., Canale, R.P, and Freedman, PL., Water Res., 10, 1101 (1976). Verhoff, EH., and Hefíher, M.R., Environ. Sei. Technol., 13, 844 (1979). Chapra, S.C., Water Resour. Res., 11, 1033 (1975). Dillon, PJ., and Kirchner, W.B., Water Resour, Res., 11, 1035 (1975). Chapra, S.C., J. Environ. Eng. Div, Am. Soc. Civ. Eng., 103, 147 (1977). Vollenweider, R.A., Schweiz. Z. Hydrol., 37,53 (1975). Thomann, R.V., DiToro, D.M., Winfield, R.P, and O*Connor, D.J., Mathematical Model- ing of Phytoplankton in Lake Ontario—Model Development and Verification, EPA- 660/3-75-005, Washington, DC (1975). DiToro, D.M., and Connolly, J.P, Mathematical Models of Water Quality in Large Lakes, Part 2: Lake Frie. EPA, Washington, DC (1979). Chen, C.W., and Orlob, G.T., Ecologic Simulation for Aquatic Ecosystems, Office of Water Resources Research OWRRC-2044, U.S. Dept. of Interior, Washington, DC (1972). DiToro, D.M., O'Connor, D.J., and Thomann, R.V., in Nonequilibrium Systems in Natur- al Water Chemistry, Advances in Chemistry Series, Vol 106, American Chemical Society, Washington, DC, pp. 132-180 (1971). Thomam, R.YV, DiToro, D.M., and O'Connor, DJ. Environ. Eng. Div, ASCE, 100 (SA3), 699 (1974). DiToro, D.M., O'Connor, D.J., Thomann, R.V. and Mancini, J.L., in Systems Analysis and Simulation in Ecology, Vol. III, Academic Press, New York, pp. 423-474 (1975). Schnoor, I.L., and O'Connor, D.J., Water Res., 14, 1651 (1980). Schnoor, J.L., and DiToro, D.J., Ecol. Modeling, 9, 233 (1980). Thomam, R.V, À Environ. Eng. Div, ASCE, 108, 923 (1982). 5.10 PROBLEMS 1. The following water quality parameters were measured for the nutrient concen- trations in Green Lake near Seattle, Washington, in the spring time. Be 216 Eutrophication of Lakes Ortho-phosphate [POZ] = 10 pg L-! Total phosphorus (dissolved) = 15 gg L”! [NO3-N] = 15 pg L”! [NH;-N] = 10 pg LA a. What is likely to be the limiting nutrient for algal growth in Gree considering the 14:1 stoichiometric guideline for inland lakes? Q Lake b. How much algae in mg L”! biomass would be produced based on lhes; chiometry of equation (2)? tai. Estimate the resulting growth rate for phytoplankton in Lake Erie from the fol lowing data. The maximum growth rate under ideal conditions of light, tempera. ture, and nutrients is 1.3/day. Use three approaches: (a) phosphorus as the limit ing nutrient, (b) the multiplicative Michaelis-Menton expression, and (c) the electrical resistance analogue. nm INH;+NO;]asN — [POF-P] Concentration, ug L! 50 5 K, pg! 25 5 Based on (1) growth rate and (2) stoichiometry, which nutrient is likely to be most limiting to phytoplankton growth? 3. The light extinction coefficient in Lake Erie in 1974 was 1.2 m!. In 1995, it was approximately 0.2 m”!. a. Plot the light intensity in depth for Lake Erie in 1974 and 1995. Surface inten- sity = 400 cal cm? d”! (langleys d”!). b. What do you think are the causes of improvement in light penetration in Lake Erie? (Note: In 1995 there was also a proliferation of zebra mussels, an intro duced species, in Lake Erie.) > If the saturated light intensity that is optimal for growth is 350 cal cm? d what depth was optimal for phytoplankton growth in 1974 versus 1995? Kz)= Lo exp (-k,z) where Jz) = light intensity with depth ho = surface light intensity k. = extinction coefficient, L-! z= depth,L 4. Plot the light intensities versus depth for the following water bodies.