Article pubs.acs.org/est
Accelerated Water Quality Improvement during Oligotrophication in Peri-Alpine Lakes Beat Müller,*,† René Gac̈ hter,† and Alfred Wüest†,‡ †
Eawag, Swiss Federal Institute of Aquatic Science and Technology, CH-6047 Kastanienbaum, Switzerland Physics of Aquatic Systems Laboratory, Margaretha Kamprad Chair, EPFL-ENAC-IIE-APHYS, Swiss Federal Institute of Technology (EPFL), CH-1015 Lausanne, Switzerland
‡
ABSTRACT: Monitoring of four eutrophic Swiss lakes undergoing oligotrophication during more than 25 years (i.e., gradually decreasing nutrient loading, productivity, and associated symptoms of eutrophication) revealed that phosphorus (P) net sedimentation rates (the fraction of a lake’s total P content that is buried within its sediments each year) and P export rates (the fraction of the lakes’ total P content that is exported via the outlet each year) increased as the lakes’ P contents decreased. These findings are of scientific as well as practical interest because they imply that, contrary to the hitherto prevailing view, the P concentration of eutrophic lakes will decrease more than proportional to the reduction of their external P load, and faster than predicted by the linear (eutrophic state-based) models.
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INTRODUCTION Since the publication of the first OECD Report about eutrophication (i.e., increased nutrient loading and productivity) of lakes,1 a large number of studies have identified phosphorus (P) availability as the most important determinant causing excessive productivity and algae-related water quality problems in lakes, e.g., refs 1−14. Symptoms of lake eutrophication include decreased water transparency, water staining by algal blooms, deep-water oxygen depletion, fish kills induced by phyto-toxins, emission of bad odors generated by decaying algae and fish at the shores, impairment as a drinking-water source, and severely decreased recreational value. Hence, excessive anthropogenic eutrophication is a public nuisance and calls for correction (e.g., restriction of P inputs from urban wastewater and nonsustainable agricultural practices). Because such lake-restoration measures are costly, they require careful planning. This implies that project authorities must predict a tolerable P load (LPtol) that is consistent with the desired in-lake steady-state P concentration [TPlake]∞. Because lakes are complex ecosystems, this aim seems to require complex models, e.g., refs 15−19, that mathematically describe all relevant biogeochemical processes, their interactions, and driving forces. Seemingly, such ideal conceptual models are superior to simple one-box models that treat a lake as an open, continuously stirred reactor, fed by one inflow and considering only two overall removal processes: (i) P net sedimentation (NS), and (ii) export of total P via the lake’s outlet (LPout). However, complex mechanistic models are rarely applied in practice, mainly because in most cases they are difficult to calibrate using the available data. In contrast, simplistic mixed-reactor models20−23 are widely applied, because they are easy to use, need little input information, and their results are easy to comprehend. Moreover, © 2014 American Chemical Society
simple models facilitate a conceptual understanding of in-lake processes and comparisons among lakes. For decades, they inspired limnologists worldwide, convinced policy makers of the necessity to restrict external P loads to lakes, and enabled engineers to predict tolerable P loads. Treating a lake as a completely mixed reactor, numerous studies cited by Brett and Benjamin20 attempted to predict inlake total P concentration at lake overturn [TPlake] as a function of (i) the discharge-weighted average total P concentration of all inflows [Pin] and (ii) lake-specific parameters, such as mean lake depth and hydraulic residence time. Best fits to data from 305 lakes20 resulted in the following equations: [TPlake] =
[TPin] 1 + σ × τw
[TPlake] =
[TPin] 1 + c1
[TPlake] =
[TPin] v 1 + z × τw
[TPlake] =
with σ = 0.45 ± 0.04 yr −1
with c1 = 1.06 ± 0.08
(1a)
(1b)
with v = 5.1 ± 0.6 m yr −1 (1c)
[TPin] 1 + c 2 × τwc3
with c 2 = 1.12 ± 0.08 yr 0.53 and c3 = −0.53 ± 0.03 (1d) Received: Revised: Accepted: Published: 6671
September 11, 2013 April 9, 2014 May 21, 2014 May 21, 2014 dx.doi.org/10.1021/es4040304 | Environ. Sci. Technol. 2014, 48, 6671−6677
Environmental Science & Technology
d[TPlake] = Q × [Pin] − β × Q × [TPlake] dt
− σ × V × [TPlake]
(2)
Symbols are explained in the list of symbols at the end of the manuscript. Because the classical one-box model ignores the fact that during stratification the TP concentration decreases in the surface water but increases in the deep water due to redissolution of settling and settled particulate P, Q × [TPlake] tends to overestimate P export. To compensate for this simplification, 6672
Sempachersee
mesotrophic oligotrophic recovering
1986−2012 1987−2008 recovering 1982−2013 1981−2008
1980−2008 1988−2008 eutrophic
692.6 22.8 0.53 45.5 23.2 1.6 4.1 7.3 0.353 83 48.6 6.8 5.2 0.50 0.0065 22 13.1 1.9 168.7 8.5 0.149 33 17.6 1.2
40.0 3.2 0.059 36 18.5 2.2 4.0 91 1972−2012 1981−2012 recovering 128.1 10.2 0.295 47 28.7 3.9 4.2 31 1985−2012 1986−2012 recovering 67.8 5.2 0.174 66 33.4 4.3 4.8 55 1982−2012 1986−2012 recovering 61.9 14.4 0.641 87 45.5 16 8.4 34 1984−2012 1986−2012 recovered
a
V×
Table 1. Physical Characteristics of Lakes Considered in This Studya
METHODS The four investigated peri-alpine lakes (Baldeggersee, Hallwilersee, Pfäffikersee, and Sempachersee), located on the Swiss Plateau in north-central Switzerland, are in various stages of recovery from eutrophication and have been monitored for more than 25 yr (Table 1). The loads of bioavailable P (LP) imported by the most important rivers flowing into these lakes were estimated based on continuous water discharge measurements and established concentration−discharge relationships30 resulting from periodic spot sampling for dissolved P analyses. These results were used to extrapolate to the P load contributed by smaller rivers and streams that were not sampled. Equation 2 presents a slightly modified version of the one-box mass-balance model of the lake’s P budget, as popularized by Vollenweider:22,23
2
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Baldeggersee
Hallwilersee
Pfäffikersee
Greifensee
Türlersee
Aegerisee
where c1 to c6 and v are fitted constants, σ is the net sedimentation rate (yr−1), τw is the hydraulic residence time of the lake (yr), and z is the mean lake depth (m). Bryhn and Håkanson21 listed 12 other equations relating [TPlake] to [Pin], as derived by several authors.24−29 Although eqs 1a to 1e and others listed by Bryhn and Håkanson21 differ, they all suggest that steady-state [TPlake] and [Pin] are basically linearly related. However, on the basis of data from an almost three-decade-long monitoring series of four deep peri-alpine Swiss lakes that have been undergoing oligotrophication (i.e., decreasing nutrient loading, productivity, and associated symptoms of eutrophication): (1) we conclude that the postulate of a constant [TPlake]/[Pin] ratio can no longer be supported if, instead of a large number of lakes with different trophic states, only an individual lake undergoing oligotrophication is considered; (2) we show that the [TPlake]/[Pin] ratio decreases systematically as [TPlake] decreases; (3) we discuss possible underlying processes; and (4) we evaluate the effect of this finding on predictions of the recovery rate and the final steady-state total P concentration at lake overturn [TPlake]∞ following reduction of a lake’s P load. In other words, general patterns identified among a large number of eutrophic lakes do not necessarily apply within an individual eutrophic lake as it undergoes oligotrophication.
catchment area [km ] surface area [km2] total volume [km3] max. depth [m] mean depth, z [m] hydraul. residence time [yr] max. sed. rate, NSmax [t yr−1] [TPlake]crit [mg m−3] data availability tributaries monitoring trophic development
(1f)
lakes
with c6 = 0.71(mg m−3)0.12 yr 0.19
Murtensee
[TPlake] = c6 × τw−0.19 × [TPin]0.88
Most original data are available from the Swiss Federal Office for the Environment FOEN (http://www.bafu.admin.ch/umwelt/12492/12892/index.html?lang=en), and the Office of Waste, Water, Energy and Air (WWEA) of the Canton of Zürich (http://www.awel.zh.ch/internet/baudirektion/awel/de/wasserwirtschaft/messdaten/see_qualitaet.html).
(1e)
1978−2013
with c4 = 0.65 ± 0.03 and c5 = 0.17 ± 0.03 yr −1
1829 66.6 3.34 136 50.6 1.4
c4 × [TPin] 1 + c5 × τw
Zürichsee
[TPlake] =
Article
dx.doi.org/10.1021/es4040304 | Environ. Sci. Technol. 2014, 48, 6671−6677
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Article
Gächter32 introduced the stratification factor β relating the annual average TP concentration in the outlet ([TPout]) to [TPlake] observed at overturn. In the present study, [TPout] was determined on a monthly basis, either measured directly in the outlet or assumed to equal the TP concentration measured in the surface water at the lake sampling station. Because riverine particulate geogenic P adsorbed onto or incorporated in inorganic particles settles to a large extent in the delta region and thus, in the short term, is assumed to not participate in the lake’s biogeochemical P cycling,33 it was neglected in our mass-balance calculations. In other words, the external load of a lake with bioavailable P (LP) is assumed to equal the lake’s total P load minus particulate geogenic P imported by its rivers. At steady-state, eq 2 results in the following: ρ [TPlake]∞ = [Pin] × β×ρ+σ (3)
ic activities (e.g., accidental discharges of liquid manure into rivers) may contribute to additional interannual variability.31 Phosphorus Export. Because monthly determinations of [TPout] and water discharge Q can be recorded precisely, the annual TP export (LPout) presented in Figure 3 can be measured with a higher accuracy than LP. Phosphorus Net-Sedimentation, NS. Despite considerable scatter, NS tends to increase as [TPlake] increases at low [TPlake], but then levels-off at high [TPlake] (Figure 4). The two branches of the red curves were fitted by minimizing the mean squares of the differences after adjusting the critical concentration ([TPlake]crit) and the maximum annual P net sedimentation (NSmax), so that NS was proportional to [TPlake] for [TPlake] ≤ [TPlake]crit but equaled NSmax above [TPlake]crit. The resulting [TPlake]crit concentrations were 34, 55, 31, and 91 mg m−3, and the area-specific NSmax values were 0.6, 0.9, 0.4, and 1.3 t P km−2 yr−1 in Sempachersee, Baldeggersee, Hallwilersee, and Pfäffikersee, respectively. The net-sedimentation rate σ equals NS divided by the lake’s TP content (V × [TPlake]). Therefore, σ is at maximum (σmax) and independent of [TPlake] for [TPlake] ≤ [TPlake]crit, but decreases inversely to the lake’s total P content for [TPlake] ≥ [TPlake]crit. If NSmax is related to phototrophic phytoplankton production, then in stratified lakes likely a larger fraction of V × [TPlake] is annually eliminated in shallow lakes than in deep lakes. In agreement with this expectation, σmax decreases as the lake’s average depth (z) increases (Figure 5). According to Vollenweider22 p 61;23 p 58, Figure 1, a σ of 10 m yr−1/z provided a good fit to his data. Other scientists25,34,35 qualitatively confirmed this inverse relationship between σ and the mean lake depth and estimated the constant parameter to range between 8 and 16 m yr−1. Our data agree well with these parametrizations and show an optimized constant of 14.5 m yr−1 that is at the higher end of the reported range. This is not surprising, because averaging data from lakes in which NS was P-limited (σ = σmax) and lakes in which NS was not P-limited (σ < σmax) would lead to an underestimation of σmax. Stratification Factor. The stratification factor β that relates [TPout] to [TPlake] tends to increase considerably in nine eutrophic Swiss lakes (Table 1) that are undergoing oligotrophication, ranging from ∼0.6 at high [TPlake] to ∼1 when [TPlake] is less than approximately 60 mg m−3 (Figure 6). Because σ and β increased (Figures 4 and 6) when LP decreased (Figure 2), the ratio [TPlake]/[Pin] decreased during the past 25 yr in Sempachersee, Baldeggersee, and Hallwilersee (Figure 7a); whereas in Pfäffikersee, it remained approximately constant because that lake was monitored mostly when [TPlake] < [TPlake]crit (Figure 4d). For the same reason, based on the annually calculated β and σ values (eq 4), the predicted P response time τR decreased in Sempachersee, Baldeggersee, and Hallwilersee during progressive oligotrophication, but not in Pfäffikersee (Figure 7b).
where [TPlake]∞ is the steady-state total P concentration at lake overturn (mg m−3), and ρ is the lake’s flushing rate Q/V (yr−1). The response time (τR) for the lake to reach within 5% of a new steady-state after a change in a parameter (e.g., [Pin]) equals the following:
τR =
3 β×ρ+σ
(4)
P net-sedimentation (NS = σ × V × [TPlake]) was calculated by solving eq 2, after insertion of three-year moving averages of each lake’s P content (Figure 1), external P load (Figure 2a), and P
Figure 1. Total phosphorus concentration [TPlake] observed at spring overturn in the four lakes. Dots are measured concentrations; lines are three-year moving averages.
losses via the effluent (Figure 3). The derivative d[TPlake]/dt was calculated as the slope of the lines in Figure 1 between two consecutive years.
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RESULTS Changes in the Lakes’ TP Concentration, [TPlake]. In all four lakes, [TPlake] was highest in the 1970s or early 1980s. Since then, [TPlake] has decreased up to 25-fold (Figure 1). Annual Phosphorus Load, LP. Figure 2 shows P loads and hydraulic loads to the four investigated lakes. Estimated annual P loads are subject to considerable uncertainty, because major fractions originating from nonpoint sources can be flushed into lakes during only a few short-term flood events and thus may remain undiscovered. Furthermore, unpredictable anthropogen-
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DISCUSSION These results demonstrate that the annual P burial rate σ as well as the annual P export rate β × ρ increased as the lakes’ trophic state improved, thus falsifying the widely accepted assumption that those parameters are lake-specific constants independent of [TPlake]. From a mechanistic perspective, P net-sedimentation NS and hence net sedimentation rate σ must be governed by (1) gross 6673
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Figure 2. (a) Annual loads of bioavailable phosphorus LP and (b) hydraulic loads to the four lakes. Dots are measurements; lines are three-year moving averages. Annual precipitation for the Swiss Plateau (Zürich/Fluntern, www.meteoschweiz.admin.ch) is shaded in gray.
relationship between NSmax and this ratio (Figure 8) supports this hypothesis. According to Figure 4, σ increased in Sempachersee, Baldeggersee, and Hallwilersee as LP gradually decreased (Figure 2). However, it has been observed in other lakes that the annual release of P from a eutrophic lake’s sediments might temporarily remain the same or even increase after a sudden major reduction of the external P load.36−38 A transiently decreasing σ might indeed retard lake water quality improvement (i.e., increase τR); but because the internal P load will slowly decline with time as the sediment equilibrates with the lower [TPlake], temporarily constant or increased internal P loading from the sediment likely will not alter [TPlake]∞ in the long run. Unfortunately, long-term monitoring data from eutrophic lakes requiring restoration are mostly lacking. Consequently, lake management responsibles are forced to guess the tolerable P load that is consistent with the desired steady-state in-lake TP concentration based on little information. Often, the available information may be restricted at best to the average flushing rate ρ, LP0, the corresponding [TPlake,0]∞, NS, and the β0 (where the subscript “0” denotes the condition before LP is decreased). In addition, it is logical to assume that for LP = 0 (a hypothetical boundary condition), [TPlake]∞, NS, and LPout will equal zero. However, that means that, at best, a person responsible for the lake will know only two points of the relationship between LP and [TPlake]∞. Therefore, although the present study has shown that P net sedimentation and export via the effluent are not linearly related to TPlake (Figures 4 and 6), it would be difficult to validly deviate from an assumption of linearity when trying to predict the temporal trend of lake recovery. To overcome this problem, we suggest following the procedure sketched in Figure 9: (1) Assume that NS observed prior to LP reduction equals NSmax. Hence, insert in the graph a straight line parallel to the x-axis at the level of NSmax. (2) Insert another straight line (marked eq (i) through the origin of the graph with the slope σ = 14.5/z (see Figure 5). (3) Superimpose the P net sedimentation curve (drawn in red) on the curve for P export via the effluent, assuming that β = β0 (green curve) or β = 1 (blue curve). Because β increases as [TPlake] decreases, the nonlinear function relating a lake’s TP concentration to its external P load likely lies in the range shaded in blue. After having chosen an acceptable [TPlake], the likely corresponding tolerable range of P loading to the lake (LPtol) can be read from Figure 9. As indicated
Figure 3. Annual export of total phosphorus (LPout) via the outlet as calculated for the four lakes (dots). Lines are three-year moving averages.
sedimentation of P (e.g., affected by phytoplankton density and species composition, the settling velocity and the P content of the various species, their growth and loss rates, etc.) and (2) the sediment’s maximum capacity to retain P (NSmax), which depends on the concentration of P-binding/sorption sites in the sediment and their P binding/sorption affinity.32 Therefore, our schematic model of lake oligotrophication described below assumes the following: (1) at low, growth-limiting P concentrations, gross sedimentation of P increases in proportion to [TPlake]; 2) net P sedimentation equals approximately gross P sedimentation, as long as the sediment’s P-binding capacity is not reached; and (3) net P sedimentation equals NSmax regardless of the absolute amount of settled P if the sediment’s P-binding capacity is exceeded. Currently, we can only speculate about the processes that determine NSmax. As an initial working hypothesis, we assume that NSmax depends on the quality and the lake surface areaspecific load (g m−2 yr−1) of weathering products capable of forming particulate P compounds in the sediment. Accordingly, provided identical geology, morphology, precipitation, temperature, etc. in a number of catchments draining into lakes, we might expect that NSmax increases as the ratio drainage basin area/lake surface area increases. Although the drainage basins of the four studied lakes are qualitatively not identical, the positive 6674
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Figure 4. Annual phosphorus net sedimentation NS (marked in red) and net sedimentation rate (σ) (marked in blue) versus ([TPlake]) in (a) Sempachersee (1986−2012), (b) Baldeggersee (1986−2012), (c) Hallwilersee (1986−2012), and (d) Pfäffikersee (1981−2012). The lines are fits to the data (see text); the error bars are based on an assumed relative error (±20%) of the annual phosphorus loads in Figure 2a.
Figure 6. Stratification factor β increased from ∼0.5−0.6 to ∼1.0 as [TP lake] decreased in nine eutrophic Swiss lakes undergoing oligotrophication. Characteristics of the lakes are listed in Table 1
Figure 5. Maximum phosphorus net sedimentation rate (σmax) decreases as mean lake depth (z) increases. The solid curve represents σmax = k/z, with k = 14.5 ± 2 m yr−1 for the four lakes. The dashed line is the relationship suggested by Vollenweider.22,23
external P loading is required, and a lake can be expected to recover faster from eutrophication than estimated based on hitherto linear lake models. In conclusion, this study demonstrates that in an individual lake undergoing oligotrophication, the steady-state lake concentration [TPlake]∞ decreases more than proportional to the lake’s external P load, LP, and the reaction time of the system to changes of the input LP decreases as oligotrophication progresses. The net sedimentation rate of P, σ, is only constant as long as P is growth-limiting (i.e., at low [TPlake]); and when P
by the dashed diagonal line, it further follows from Figure 9 that relating [TPlake] linearly to LP represents a worst-case scenario in which a much lower tolerable external P load would apparently be required. After a reduction of the external P load, the new lower steadystate [TPlake] will be approached faster as τR decreases. According to eq 4 and Figure 7b, τR decreases as long as σ and β increase with decreasing [TPlake]. Therefore, a lesser decrease of the 6675
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Figure 7. Temporal progressions of (a) the ratio ρ/(β·ρ + σ) relating the lakes’ TP concentration ([TPlake]) to their inlet concentration ([Pin]), and (b) P response time τR (see eq 4), as the annual phosphorus load decreased gradually (see Figure 2a).
when comparing among lakes, its value within a given lake also depends strongly on the extent of eutrophication of the lake and a variety of associated in-lake processes (such as the light penetration depth for algae growth) that vary as [TPlake] changes.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +41-58-765 21 49; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the following colleagues for providing monitoring data from lakes and tributaries: Peter Keller, Amt für Umweltschutz des Kantons Zug; Pius Niederhauser, Amt für Abfall, Wasser, Energie und Luft (AWEL) des Kantons Zürich; Robert Lovas, Umwelt und Energie (UWE), Kanton Luzern; Arno Stöckli, Abteilung für Umwelt des Kantons Aargau; and Elise Folly, Eaux Superficielles et Souterraines, Service de l’Environnement SEN, Protection des Eaux, Canton de Fribourg. The comments of Joseph Meyer, Martin Schmid, and two anonymous reviewers helped to clarify our thoughts and are very much appreciated.
Figure 8. Relationship between maximum net sedimentation of phosphorus (NSmax) and catchment area/lake surface area ratio, f u [ − ]. NSmax = 0.086 gP m−2 yr−1 × f u.
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LIST OF SYMBOLS LP external load of bioavailable P (t yr−1) LPout P export via the outlet of the lake (t yr−1) LPtol P load (t yr−1) that is consistent with the tolerable inlake steady-state P concentration [TPlake]∞ LP0 P load prior to load reduction (t yr−1) NS net sedimentation of P (t yr−1) NSmax maximum net sedimentation of P (t yr−1) [Pin] concentration of bioavailable P in lake inlet (mg m−3) Q discharge (m3 yr−1) t time (yr) [TPin] annual average total P concentration in lake inlet (mg m−3) [TPlake] total P concentration at lake overturn (mg m−3) [TPlake]crit critical total P concentration at lake overturn (mg m−3) above which NS attains a maximum [TPlake]∞ steady-state total P concentration at lake overturn (mg m−3)
Figure 9. Conceptual prediction of the steady state relationship between total phosphorus concentration in a lake at overturn ([TPlake]) and the external P load with bioavailable phosphorus (LP) following a reduction of LP. NSmax and β0 are available from monitoring data prior to restoration. eq (i): NS = (14.5/z)·[TPlake]·V; eq (ii): LP = (14.5/z + β0·ρ)·[TPlake]·V; eq (iii): LP = (14.5/z + 1.0·ρ)·[TPlake]·V; eq (iv): LP = NSmax + β0·ρ [TPlake]·V; eq (v): LP = NSmax + 1.0·ρ·[TPlake]·V.
is not growth-limiting, σ decreases as [TPlake] concentration increases. Therefore, although σ is strongly related to lake depth 6676
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Environmental Science & Technology [TPout] V z β β0 ρ σ σmax τw τR
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annual average total P concentration in lake outlet (mg m−3) lake volume (m3) mean lake depth (m) [TPout]/[TPlake] = stratification factor (−) stratification factor immediately before decreasing the external P load (−) Q/V = lake flushing rate (yr−1) net sedimentation rate of P (yr−1) (i.e., net proportion of V×[TPlake] that is annually retained in the lake sediment) maximum net sedimentation rate of P (yr−1) V/Q = hydraulic residence time of the lake (yr) response time for a lake to reach within 5% of a new steady state, after a change in a parameter (yr)
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dx.doi.org/10.1021/es4040304 | Environ. Sci. Technol. 2014, 48, 6671−6677
