Anaerobic phosphorus release from sediments: a

0 downloads 0 Views 100KB Size Report
The currently accepted view about the role of lake sediments in the cycling of phosphorus is that they act as a net sink for phosphorus (P). Although gener-.
Verh. Internat. Verein. Limnol.

27

1–8

Stuttgart, September 2001

Anaerobic phosphorus release from sediments: a paradigm revisited Yves T. Prairie, Chantal de Montigny and Paul A. Del Giorgio

Introduction The currently accepted view about the role of lake sediments in the cycling of phosphorus is that they act as a net sink for phosphorus (P). Although generally true on an annual basis, sediment release (internal loading) can represent a substantial part of seasonal P inputs to lakes, particularly those which undergo periodic hypolimnetic anoxia. These are the lakes that typically respond very slowly to reductions of external sources of P (NÜRNBERG & PETERS 1984, SAS 1989). The phenomenon has traditionally been described as the anaerobic release of phosphorus following the chemical reduction of sediment iron (III) when the redox potential drops below 200 mV, with the subsequent dissolution of iron (II) and the associated phosphorus (EINSELE 1936, 1938, MORTIMER 1941, 1942, BOSTRÖM et al. 1988). However, this paradigm has been more successful at explaining P release when it does occur than at predicting when, where and to what extent it will occur. Indeed, the universality of this model is now widely contested. Several lakes “break the rule” and continue to have extremely low P release from sediments even after anoxia, inconsistent with the oxygen control model (CARACO et al. 1991). In the experimentally eutrophied Lake 227, there was no increase in phosphorus in the lake water under anoxic conditions, even under ice (SCHINDLER et al. 1977). Conversely, in the hard waters of Lake Onondaga the P release began 5 weeks before the anoxia and 6 weeks before the presence of a measurable quantity of total Fe (II), and seemed to be controlled by dissolution of Ca-P and decomposition of organic matter (DRISCOLL et al. 1993). In view of the failure of the classical theory to explain the diversity of pattern for phosphorus release, different hypotheses have been proposed. The simplest is that P release should be higher in the more eutrophic lakes, containing a high quantity of P in their sediment (NÜRNBERG et al. 1986). Another hypothesis is that the source of P in anoxic hypolimnia is not the phosphorus bound to iron hydroxides

but from another sediment phosphorus pool, the most plausible of which is the organic matter. Indeed, numerous studies (BOERS & VAN HESE 1988, SINKE et al. 1990, BOERS & DE BLES 1991) suggest that aerobic P release in the Loosdrecht lakes is controlled mainly by mineralization processes. In Mirror Lake, New Hampshire, the P release rate also appears to be related to the organic content of sediments (CARACO et al. 1992), again suggesting the action of a biological rather than a chemical mechanism. Nevertheless, the two are not easily separated as iron reduction is itself driven by microbial processes (LOVLEY et al. 1991). Bacteria act indirectly by consuming oxygen and lowering the redox potential, but also directly by reducing Fe (III), used as a proton sink (JONES et al. 1984, DE MONTIGNY & PRAIRIE 1993). Other hypotheses involving bacterial processes are also thought to operate. A release of orthophosphates in the water can occur after bacterial lysis of strictly aerobic bacteria killed by the lack of oxygen (BOSTRÖM et al. 1988), or after degradation of polyphosphates accumulated during aerobiosis by facultative aerobes (FLEISHER 1983, OHTAKE et al. 1984, GÄCHTER et al. 1988). However, it is very hard to test these last two hypotheses in the absence of techniques to determine the P content of sediment bacteria and the content of poly-P in sediments (GÄCHTER & MEYER 1993). Finally, several other factors have been invoked to help explain the release of P, such as pH (CURTIS 1991, SEITZINGER 1991, JENSEN & ANDERSEN 1992), temperature (JENSEN & ANDERSEN 1992), organic matter content in sediments (REDDY 1983), content of the P linked with iron in sediments (NÜRNBERG 1988, OSTROFSKY et al. 1989), size of sediment particles (HESSE 1973), as well as the concentrations of nitrates (AHL 1979, ANDERSEN 1982, FOY 1986), sulfates (CARACO et al. 1989) and calcium (GOLTERMAN 1984, DRISCOLL et al. 1993) in water. Nevertheless, the generality of any of these mechanisms has never been systematically tested in a series of lakes known to undergo hypolimnetic anoxia. The goal of this study was to test whether any of

0368-0770/01/0027-01 $ 2.00 ©2001 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart

2

P transport and retention in wetlands

the above-mentioned hypotheses could explain a large fraction of the variability in the hypolimnetic P accumulation rates observed among lakes, with particular reference to the iron-redox control and organic matter mineralization models. The results presented herein combine data from a systematic field study of 15 lakes in Québec with analogous data from two other lakes, L. Brome and L. Heney.

The study lakes To examine the potential generality of these various possible mechanisms, we studied a set of 15 lakes located in the Laurentian and Eastern Townships regions of Québec. The geology of these two areas is markedly different. The Laurentians area is underlaid by the Precambrian rocks of the Canadian Shield, mostly gneiss with inclusions of other metamorphic and igneous rocks (LANDRY & MERCIER 1983) while the Eastern Townships area is underlaid by Cambrian and Ordovician rock formations, dominated by shale, mudstone, slate, sandstone and dolomite (ST-JULIEN 1972). These lakes were chosen according to three criteria: they were known to have exhibited severe hypolimnetic oxygen depletion in the past, they covered a wide range of different trophic status, and they were located so as to permit each lake to be visited every 2 weeks. Measurements were performed during the summer of 1992, between 25 May and 27 August. The trophic state of these lakes varied from oligotrophic to eutrophic, and the maximum depth from 3 to 20 m. In addition to this core set of lakes, useful data from Lac Brome (Eastern Township) and Lac Heney (Gatineau region) were added to examine the generality of some of the relationships observed.

Methods For each lake, we measured the temporal evolution of hypolimnetic concentration of several parameters. Sampling was performed twice a month, in rotation. The sampling was performed at the deepest point of each lake. Temperature was measured at each meter of the water column for the determination of the thickness of the hypolimnion. The limits of the hypolimnion were then calculated by the graphical method of LIND & DAVALOS-LIND (1993). At each visit, we collected an integrated sample of the epilimnion water with a tube sampler, while the hypolimnetic water samples were obtained with a Van Dorn bottle at every 1 or 2 m depth of the hypolimnion, depending on the depth of the lake. Sampling of sediment was performed at the same time with a Kajak gravity corer. At each sampling, two cores were sampled. Phosphorus, nitrates and ammonia were analyzed

with an ALPKEM RFA300 autoanalyzer, in triplicate, according to standard analytical procedures (see CATTANEO & PRAIRIE 1995 for details). Iron and calcium were measured by atomic absorption. In all cases, dissolved forms were analyzed after filtration of the sample through a 0.45-µm Millipore filter, and total forms without filtration. Filtration was performed immediately after sampling. Samples of nutrients were preserved with 2 mL/L of concentrated H2SO4. Samples of metals were preserved with the same quantity of concentrated HNO3. Total ferrous iron was fixed in situ according to the 1,10phenanthroline method (APHA 1985) and then measured spectrophotometrically in the laboratory the same day. Dissolved oxygen was determined by the modified method of Winkler (azide) (APHA 1985). We considered water to be anoxic when the oxygen concentration dropped below 2 mg/L 1 m above the surface of the sediments, a value often considered to indicate the limit to the persistence of an oxidized layer in the sediments. Temperature, conductivity and pH were measured electrometrically. Chlorophyll a was analyzed on integrated samples by fluorimetry after extraction by alkaline acetone, following WETZEL & LIKENS (1991). We measured bacterial activity by the INT reduction method (TREVORS 1984) on the day of sampling (five replicates per sample). When the hypolimnion was anoxic, all manipulations were performed in a glove box under a nitrogen atmosphere. Bacterial activity was then expressed as mg of INT-formazan produced per g of sediments (dry weight). The content of phosphorus and iron in the sediment was analyzed by the ignition method of ANDERSEN (1976). Granulometry of sediments was estimated by the modified hydrometric method of BOYOUCOS (1962). Organic matter content was estimated from loss on ignition at 475 °C (BYERS et al. 1978). Fractionation of P in the sediment was performed by the method of BONZONGO et al. (1989), a modification of the method of HIELTJES & LIJKLEMA (1980) where the organic P is extracted first by H2O2 before the extraction of loosely bound, NaOH–extractible-P and HCl–extractible-P by solutions of 1 N NH4Cl, 0.1 N NaOH and 0.5 N HCl, respectively. Refractory-P was measured as the total P remaining in sediments after all extractions. Reducible iron and ferrous iron in sediments were analyzed following LOVLEY & PHILLIPS (1987). We calculated the quantity of elements in the hypolimnion of lakes by multiplication of the water volume of each stratum by the concentration measured at this depth. Volumes were determined from bathymetric maps. In order to facilitate comparison among lakes of different hypolimnetic thickness, the hypolimnetic accumulation rates of phosphorus,

Y. T. Prairie et al., Variability in P accumulation among lakes iron, oxygen and ammonia were expressed on an areal basis, and corresponded to the net flux from the sediments. We calculated phosphorus fluxes both as TDP and TP fluxes, because released dissolved P can be quickly incorporated into bacterial biomass.

Results and discussion Of the 15 lakes we sampled, five had to be eliminated from the data analysis. The three most shallow lakes did not stratify and no anoxia developed during the summer. Oxygen concentration did not drop below 3.8 mg/L in the Boivin Reservoir, 4.0 mg/L in Lake Waterloo and 5.5 mg/L in Roxton Pond. In Davignon and Choinière Reservoirs stratification was only transitory. This fact favors the mixing action of the wind. These lakes were thus excluded from further analysis. The main characteristics of the water and sediments of the 10 remaining lakes are given in Table 1. The sediments from all of the lakes contained very similar quantities of phosphorus (range: 1.4–2.9 mg P/g dw), but a much more variable content of iron (range: 6.7–37.9 mg Fe/g dw). The proportion of organic matter also varied widely among lakes, from 10.0% to 48.2%. The main forms of P in the sediments are organic-P and NaOH-P, corresponding to the two principal views about the origin of released P: organic matter vs. iron hydroxides. In the majority of sediments, these two forms

3

of P are sufficiently important to expect a release from the sediments. All lakes were soft water, with relatively low concentrations of calcium and nitrate and neutral pH. The iron-redox control model of P release The classical model of Einsele–Mortimer predicts that when redox conditions are sufficiently low (below 200 mV), phosphorus-rich iron oxy-hydroxides will be the reduced and the resulting ferrous iron, because of its higher solubility, will return to solution along with the associated phosphorus. Thus when oxygen concentrations become very low, one should observe a release of iron and phosphorus, provided both are present in the sediments. In theory, the releases should be roughly synchronous and the accumulating iron and phosphorus should remain approximately in the same stoichiometric ratio. Of the 10 lakes that became anoxic, only two (Lakes Magog and Maskinonge) displayed the expected trends. In L. Magog, after the average hypolimnetic oxygen dropped below 2 mg/L, the hypolimnetic concentrations of both phosphorus and iron rose in parallel (Fig. 1). The Fe:P ratio remained essentially constant (at around 9 wt:wt) even when [Fe] rose 35-fold from the pre-anoxic conditions. The sediment Fe:P of L. Magog was similar, with a value of about 12. Similarly, L. Maskinonge exhibited

Table 1. Main chemical and physical characteristics of the sediments in the 10 selected lakes. LOI (%)

Selby

Magog

Manitou

10.0±0.5

18.2±0.1

30.4±0.7

Maskinongé Connelly 17.0±0.8

Croche

Dufresne

Écho

Bromont

Dupuis

23.4±1.7

43.4±1.3

31.5±0.9

48.2±1.0

12.0±0.6

38.0±1.2

ETS act. (mg INT-F/g)

1.4±0.3

4.0±1.0

5.2±1.4

4.2±1.0

5.1±0.4

8.2±0.3

9.2±1.6

16.2±2.0

1.3±0.3

5.5±1.6

Total Fe (mg/g)

23.8±0.9

27.2±1.8

15.1±3.1

22.4±1.0

17.4±2.5

6.7±0.6

11.7±0.9

9.8±1.7

37.9±1.7

18.4±2.0

7.9±1.0

23.8±11.7

14.5±4.0

11.2±2.8

11.3±2.5

4.4±0.7

6.7±1.8

5.8±3.2

16.9±1.9

16.8±1.6

Reducible Fe (mg/g) Ferrous Fe (mg/g) Total P (mg/g) Fe/P ratio Organic-P (mg/g)

0.21±0.06 0.33±0.07 0.37±0.09

0.33±0.08 0.20±0.07 0.19±0.02 0.15±0.05 0.28±0.19 0.28±0.08 0.45±0.11

1.6±0.1

1.4±0.4

2.9±0.2

2.0±0.1

2.3±0.2

1.5±0.1

1.8±0.1

2.6±0.2

1.8±0.1

14.9

12.3

5.2

11.2

7.6

4.5

6.5

3.8

21.1

7.7

359±288

540±271

1136±844

185±43

660±69

1683±528

232±166

1083±243

1±0.5

31±15

292±189 1162±171

NH4Cl-P (mg/g)

3±2

13±5

37±24

23±26

NaOH-P (mg/g)

676±48

631±54

1246±325

755±84

8±4

HCl-P (mg/g)

330±96

307±102

453±113

622±99

294±74

Refractory-P (mg/g)

226±44

42±25

123±80

88±22

277±130

Sand (%)

57

22

33

39

Silt (%)

22

61

50

Clay (%)

21

17

17

58±20

2.4±0.2

77±22

83±32

724±83

384±251

1089±223 1186±564

166±32

233±78

125±30

673±188

162±56

22±4

158±41

13±9

345±62

69±35

45

39

27

45

40

41

50

34

43

58

44

28

36

11

21

18

15

11

32

23

1211±322 330±113

4

P transport and retention in wetlands

after the onset of anoxia (from 15.7 to 209.9 mg Fe/m2/day). In our lakes, the iron flux was strongly predictable from the concentration of reducible iron in the sediments (r² = 0.72, Fig. 2). Thus, iron and phosphorus appear largely uncoupled.

Fig. 1. Evolution of iron and phosphorus concentrations in the hypolimnetic water of Lake Magog. Solid and open symbols represent phosphorus and iron, respectively. Squares and circles correspond to total and dissolved forms, respectively.

concomitant phosphorus and iron release although the increase from pre-anoxic conditions was much more modest (4-fold). The hypolimnetic ratio remained high, at about 35, considerably larger than the lake’s sediment Fe:P of 11. Such was not the case for the remaining eight lakes. Lakes Manitou, Bromont and Dupuis did not release any appreciable amounts of any form of phosphorus. The concentrations of dissolved phosphorus did increase in Lakes Croche and Selby but were not reflected in the total phosphorus concentrations. Although this may be an indication of release (albeit presumably counterbalanced by sedimentation), the Fe:TDP ratio increased markedly indicating that the two elements are likely not linked as predicted by the classical model. Total phosphorus did increase also in Lakes Dufresne, Echo and briefly in Connelly but there again the Fe:P ratio of the accumulating material markedly increased, showing that iron and phosphorus are not released in the same proportions. The amounts of phosphorus accumulating in the hypolimnia of these lakes, expressed as a flux from the sediments, were quite small. Thus, eight of the 10 lakes studied here did not conform to the traditional view of anaerobic phosphorus release. However, in all lakes, iron did follow the expected trend in that considerable quantities of iron were released

Amounts and form of phosphorus in sediments In the past, the failure of a lake to fit the ironredox control model has been explained with reference to some chemical characteristics of the sediments, such as phosphorus and iron concentrations, or their chemical speciation of the sediments. Here, we examine the applicability to our lakes of some of these claims. NÜRNBERG (1988) found a highly significant relationship (r2 = 0.83) between P release rates and the total sediment P concentration. This correlation was established with 14 cores from different North American lakes. We did not find such a relationship (P > 0.05) in our study, perhaps because of the much lower flux of P in our lakes. Five of the Nürnberg's lakes exhibited a very strong (>5 mg P/m2/day) TP release, and were largely responsible for the significance of the correlation. Surprisingly, the sediments of L. Magog, characterized by a strong release of P, presented the lowest content in sediment total phosphorus. A number of studies found relationships between the release rates of P and the content of the P linked with iron in sediments (HOSOMI et al. 1981, FURUMAI & OHGAKI 1982, BOS-

Fig. 2. Relationship between TFe flux (mg Fe/m2/ day) and reducible iron in sediments of the 10 oxygen-depleted lakes.

Y. T. Prairie et al., Variability in P accumulation among lakes TRÖM 1984, NÜRNBERG 1988). The NaOH-P fraction of the sediments corresponds roughly to P linked with iron and aluminum. With respect to this fraction, sediments of our study lakes can be separated in three distinct groups. The first was formed by the sediments of Lakes Manitou, Connelly, Dupuis and Bromont, containing a high quantity of NaOH-P, the second by those of Lakes Magog, Dufresne, Selby and Maskinongé, and the last by the organic but iron-poor sediments of Lakes Croche and Écho. Despite the wide range of NaOH-P concentrations in our lakes, no trend existed between the content of NaOH-P in sediments and the observed areal accumulation rate of phosphorus.

Relationship with bacteria and organic matter mineralization Decomposition of sedimented organic matter can regenerate P if the inorganic matrix is saturated with phosphorus. Mineralization processes will or will not permit the release of P depending on the C:P ratio of organic substrates, the bacterial growth rate and the availability of P in the ambient environment (BOSTRÖM & PETTERSSON 1982, BOSTRÖM et al. 1989). However, no correlation appeared between the flux of P and the content of sediment in organic matter or with the sediment C:P ratio (P > 0.05) As a proxy for the metabolic activity of sediment bacteria, we measured the electron transport system’s activity (ETS activity) in sediments, a measure of microbial respiration, both aerobic and anaerobic. We hypothesized that this activity might reflect the potential of sediments to release phosphorus. This systematic measure was never made previously. Bacterial activity in sediment was highly variable among lakes, although very stable within lakes. We found a strong positive relationship between ETS activity and the sediments’ NH4Cl-P concentration (r2 = 0.82) and, to a lesser extent, with the H2O2–P concentration (r2 = 0.64) and organic matter content (r2 = 0.74), although not with total P. Very few studies of P release include the measurement of bacterial activity, with the notable exception of BOSTRÖM et al.

5

(1989). This is probably the reason why the correlation observed here between bacterial activity and the NH4Cl-P content was not observed previously. This correlation suggests perhaps a limitation of bacteria by P, since the NH4Cl-P fraction was the most labile of fractions. However, no relationship emerged between ETS activity and the flux of phosphorus from the sediments nor indeed with the fluxes of any of the elements we measured. SINKE et al. (1990) found strong correlations between fluxes of P, ammonia and methane in an intact core from Loosdrecht lakes. As ammonia and methane are by-products of organic matter mineralization in anoxic conditions, their study pointed to the importance of mineralization processes in the release of P. In our lakes, the hypolimnetic accumulation of ammonia, expressed as a flux from the sediments, is not related to the release of phosphorus (P > 0.05). However, we did find a strong relationship between the phosphorus release rate and the rate at which hypolimnetic oxygen was consumed prior to anoxia (r2 = 0.92, P < .001, Fig. 3). This clearly points to decomposition processes exerting a strong control on P release from sediments. That we did not find a relationship between NH4 and P accumulation rates however is more puzzling. First, it may well be that hypolimnetic NH4 accumulation is not a good measure of total metabolism because it can be rapidly transformed into other nitrogen forms. Second, and more interestingly, it

Fig. 3. Relationship between the total phosphorus release rate (mg P/m2/day) and the rate which oxygen is consumed prior to anoxia (mg O2/m2/day).

6

P transport and retention in wetlands

may suggest a more complex interplay between decomposition, phosphorus release and the iron cycle. When oxygen is depleted, dissimilatory iron reduction should become a dominant pathway for the oxidation of organic matter, since the substrates for the other more thermodynamically favored reactions (reduction nitrates and nitrites) are not in sufficiently large abundance (except perhaps for manganese oxides) to sustain much decomposition. However, assuming that dissimilatory iron reduction can be summarized by the “Redfield ratio OM”:

current large iron release, L. Brome also clearly supports the organic matter decomposition hypothesis because, in addition to releasing phosphorus, a concomitant release of nitrogen was observed with an average molar TN:TP ratio of 22, quite close to Redfield (Fig. 5). Unfortunately, total nitrogen was not measured in the other lakes so the generality of this trend is not known.

C106H263O110N16P1 + 424FeOOH + 862H+ → ← 106CO2 + 424Fe++ + 16NH4+ + HPO42 + 742H2O

iron and ammonia are produced in a molar ratio of 26.5, and one can, therefore, estimate the contribution of iron reducers to total metabolism (all releasing nitrogen as NH4) as: %Fe-based metabolism = [Feflux/26.5]/NH4 flux

Surprisingly for our lakes, this calculation indicates that the contribution of iron reduction to total metabolism varies widely among lakes, from less than 2% to 80%. Given that iron reduction appears to be mainly controlled by the amount of reducible iron in the sediments (Fig. 2) whereas P release is more closely related to the overall decomposition potential, it should not be surprising that Fe reduction and P release are uncoupled in most cases. Only when sediment reducible Fe is abundant and when iron reducers contribute substantially to the oxidation of organic matter should the release of P and Fe be coupled. The case of L. Brome Lac Brome, like L. Magog, is another shallow eutrophic lake in the Eastern Township. It undergoes periodic hypolimnetic anoxia and phosphorus release, which we studied as part of a separate project. The sediments of L. Brome release large amounts of phosphorus (4.3 mgP/ m2/day) when the small hypolimnion is depleted of oxygen (Fig. 4). Although apparently fitting the classical redox model with con-

Fig. 4. Temporal evolution of oxygen and total phosphorus concentrations in the hypolimnion of L. Brome.

Fig. 5. Relationship between the phosphorus and nitrogen concentrations (µg/L) in the hypolimnion of L.

Conclusion In conclusion, internal P loading from the anoxic hypolimnion appears less widespread than currently believed. Although the depletion of oxygen at the bottom was always accompanied by a substantial release of iron, phosphorus was only released in very few cases. This fact must be underlined, because the

Y. T. Prairie et al., Variability in P accumulation among lakes anaerobic release of P is often considered as a nearly inevitable mechanism occurring in all eutrophic lakes with anoxic hypolimnia. In this study, we showed that even P-rich sediments under an anoxic hypolimnia do not necessarily release P. Neither the composition of sediments nor such factors as temperature, pH, calcium or nitrate concentration in the water could explain the differences in P release observed among the lakes. Our strongest predictor of the rate of P release was the rate at which oxygen was consumed prior to anoxia, clearly suggesting a dominating role for decomposition processes. In L. Brome, where a massive release of phosphorus was observed after anoxia, a similar increase was also observed for nitrogen, consistent again with the decomposition hypothesis.

Acknowledgments We wish to thank J. VAILLANCOURT and B. LAQUERRE for field assistance and M. J. CARBONNEAU for nutrient analyses. We thank also D. BIRD and V. MESNAGE for their valuable suggestions on the manuscript. This project was funded by NSERC in the form of an operating grant to Y. T. PRAIRIE, and by ECORESEARCH in the form of a postgraduate scholarship to C. DE MONTIGNY. This study is contribution to the GRIL (Groupe de Recherche Interuniversitaire de Limnologie).

References AHL, T., 1979: Natural and human effects on trophic evolution. – Arch. Hydrobiol. Beih. Ergebn. Limnol. 13: 259–277. ANDERSEN, J. M., 1976: An ignition method for determination of total phosphorus in lake sediments. – Water Res. 10: 329–331. ANDERSEN, J. M., 1982: Effect of nitrate concentration in lake water on phosphate release from the sediment. – Water Res. 16: 1119–1126. APHA, 1985: Standard Methods for the examination of water and wastewater, 16th ed. – American Public Health Association, Washington, D.C. 1268 p. BOERS, P. C. M. & VAN HESE, O., 1988: Phosphorus release from the peaty sediments of the Loosdrecht lakes (The Netherlands). – Water Res. 22: 355–363. BOERS, P. C. M. & DE BLES, F., 1991: Ion concentrations in interstitial water as indicators for phosphorus release processes and reactions. – Water Res. 25: 591–598. BONZONGO, J. C., BERTRU, G. & MARTIN, G., 1989: Les méthodes de spéciation du phosphore dans les sédiments: critiques et propositions pour l'évaluation des fractions minérales et organiques. – Arch. Hydrobiol. 116: 61–69. BOSTRÖM, B., 1984. Potential mobility of phosphorus in different types of lake sediment. – Int. Rev. Gesamten Hydrobiol. 69: 457–474. BOSTRÖM, B. & PETTERSSON, K., 1982: Different patterns of

7

phosphorus release from lake sediments in laboratory experiments. – Hydrobiologia 92: 415–429. BOSTRÖM, B., ANDERSEN, J. M., FLEISCHER, S. & JANSSON, M., 1988: Exchange of phosphorus across the sediment–water interface. – Hydrobiologia 170: 229–244. BOSTRÖM, B., PETTERSSON, A. K. & AHLGREN, I., 1989: Seasonal dynamics of cyanobacteria-dominated microbial community in surface sediments of a shallow, eutrophic lake. – Aquat. Sci. 51: 153–178. BOYOUCOS, G. J., 1962. Hydrometer method improved for making particule size analysis of soils. – Agron. J. 54: 464–465. BYERS, S. C, MILLS, E. L. & STEWART, P. L., 1978: A comparison of methods of determining organic carbon in marine sediments, with suggestions for a standard method. – Hydrobiologia 58: 43–47. CARACO, N. F., COLE, J. J. & LIKENS, G. E., 1989: Evidence for sulphate-controlled phosphorus release from sediments of aquatic systems. – Nature 341: 316–318. CARACO, N. F., COLE, J. J. & LIKENS, G. E., 1991: Phosphorus release from anoxic sediments: lakes that break the rules. – Verh. Internat. Verein. Limnol. 24: 2985–2988. CARACO, N. F., COLE, J. J. & LIKENS, G. E., 1992: New and recycled primary production in an oligotrophic lake: insights for summer phosphorus dynamics. – Limnol. Oceanogr. 37: 590–602. CATTANEO, A. & PRAIRIE, Y. T., 1995: Temporal variability in the chemical characteristics along the Riviere de l'AchiganHow many samples are necessary to describe stream chemistry? – Can. J. Fish. Aquat. Sci. 52: 828–835. CURTIS, P. J., 1991: P and Fe release from anoxic Precambrian Shield lake sediments mediated by addition of Fe(II)-insoluble and Fe(II)-soluble bases. – Verh. Internat. Verein. Limnol. 24: 2976–2979. DE MONTIGNY, C. & PRAIRIE, Y. T., 1993: The relative importance of biological and chemical processes in the release of phosphorus from a highly organic sediment. – Hydrobiologia 253: 141–150. DRISCOLL, C. T., EFFLER, S. W., AUER, M. T., DOERR, S. M. & PENN, M. R., 1993: Supply of phosphorus to the water column of a productive hardwater lake: controlling mechanisms and management considerations. – Hydrobiologia 253: 61–72. EINSELE, W., 1936: Über die Beziehungen des Eisenkreislaufs zum Phosphatkreislauf im eutrophen See. – Arch. Hydrobiol. 29: 664–686. EINSELE, W., 1938: Über Chemische and Kolloidchemische Vorgänge in Eisen-Phosphat-Systemen unter limnochemische und limnogeologischen Gesichtspunkten. – Arch. Hydrobiol. 33: 361–387. FLEISHER, S., 1983: Microbial phosphorus release during enhanced glycolysis. – Naturwissenschaften 70: 415. FOY, R. H., 1986: Suppression of phosphorus release from lake sediments by the addition of nitrate. – Water Res. 20: 1345–1351. FURUMAI, H. & OHGAKI, S., 1982: Fractional composition of

8

P transport and retention in wetlands

phosphorus forms in sediments related to release. – Water Sci. Technol. 14: 215–226. GÄCHTER, R. & MEYER, J. S., 1993: The role of microorganisms in mobilization and fixation of phosphorus in sediments. – Hydrobiologia 253: 103–121. GÄCHTER, R., MEYER, J. S. & MARES, A., 1988: Contribution of bacteria to release and fixation of phosphorus in lake sediments. – Limnol. Oceanogr. 33: 1542–1558. GOLTERMAN, H. L., 1984: Sediments, modifying and equilibrating factors in the chemistry of freshwater. – Verh. Internat. Verein. Limnol. 22: 23–59. HESSE, P. R., 1973: Phosphorus in lake sediments, – In: GRIFFITH, E. J., BEETON, A., SPENCER, J. M. & MITCHELL, D. T. (eds): Environmental Phosphorus Handbook: 573–583. – John Wiley & Sons, Toronto. HIELTJES, A. H. M. & LIJKLEMA, L., 1980: Fractionation of inorganic phosphates in calcareous sediments. – J. Environ. Qual. 9: 405–407. HOSOMI, M., OKADA, M. & SUDO, R., 1981: Release of phosphorus from sediments. – Verh. Internat. Verein. Limnol. 21: 628–633. JENSEN, H. S. & ANDERSEN, F. O., 1992: Importance of temperature, nitrate, and pH for phosphate release from aerobic sediments of four shallow, eutrophic lakes. – Limnol. Oceanogr. 37: 577–589. JENSEN, H. S., KRISTENSEN, P., JEPPESEN, E. & SKYTTHE, A., 1992: Iron:phosphorus ratio in surface sediment as an indicator of phosphate release from aerobic sediments in shallow lakes. – Hydrobiologia 235/236: 731–743. JONES, J. G., GARDENER, S. & SIMON, B. M., 1984. Reduction of ferric iron by heterotrophic bacteria in lakes sediments. – J. Gen. Microbiol. 130: 45–51. LANDRY, B. & MERCIER, M., 1983: Notions de géologie avec exemples du Québec. – Modulo, Montréal. 425 p. LIND, O. T. & DAVALOS-LIND, L., 1993: Detecting the increased eutrophication rate of Douglas lake, Michigan: the relative areal hypolimnetic oxygen deficit method. – Lake Reserv. Manage. 8: 73–76. LOVLEY, D. R. & PHILLIPS, E. J. P., 1987: Rapid assay for microbially reducible ferric iron in aquatic sediments. – Appl. Environ. Microbiol. 53: 1536–1540. LOVLEY, D. R., PHILLIPS, E. J. & LONERGA, D. J., 1991: Enzymatic versus non-enzymatic mechanisms for Fe(III) reduction in aquatic sediments. – Environ. Sci. Technol. 24: 1062–1067. MORTIMER, C. H., 1941: The exchange of dissolved substances between mud and water in lakes. I–II. – J. Ecol. 29: 280–329. MORTIMER, C. H., 1942: The exchange of dissolved substances between mud and water in lakes. III–IV, summary and references. – J. Ecol. 30: 147–201. NÜRNBERG, G., 1988: Prediction of phosphorus release rates from total and reductant-soluble phosphorus in anoxic lake sediments. – Can. J. Fish. Aquat. Sci. 45: 453–462. NÜRNBERG, G. & PETERS, R. H., 1984: The importance of internal phosphorus load to the eutrophication of lakes with

anoxic hypolimnia. – Verh. Internat. Verein. Limnol. 22: 190–194. NÜRNBERG, G. K., SHAW, M., DILLON, P. J. & MCQUEEN. D. J., 1986: Internal phosphorus load in an oligotrophic precambrian shield lake with an anoxic hypolimnion. – Can. J. Fish. Aquat. Sci. 43: 574–580. OHTAKE, H., TAKAHASHI, K., TSUZUKI, Y. & TODA, K., 1984: Phosphorus release from a pure culture of Acinetobacter calcoaceticus under anaerobic conditions. – Environ. Technol. Lett. 5: 417–424. OSTROFSKY, M. L., OSBORNE, D. A. & ZEBULSKE, T. J., 1989: Relationship between anaerobic sediment phosphorus release rates and sedimentary phosphorus species. – Can. J. Fish. Aquat. Sci. 46: 416–419. REDDY, K. R., 1983: Nitrogen and phosphorus interchange between sediments and overlying water of a wastewater retention pond. – Hydrobiologia 98: 237–243. SAS, H., 1989: Lake Restoration by Reduction of Nutrient Loading: expectations, experiences, extrapolations. – Academia-Verlag, Richarz, St. Augustin, 497 p. ISBN 3–88345379–X. SCHINDLER, D. W., HESSLEIN, R. & KIPPHUT. G., 1977: Interactions between sediments and overlying waters in an experimentally eutrophied precambrian shield lake. – In: GOLTERMAN, H. L. (ed.): Interactions Between Sediments and Freshwater: 235–243. – Proc. Symp. Junk, The Hague. SEITZINGER, S. P., 1991: The effect of pH on the release of phosphorus from Potomac Estuary sediments: implications for blue-green algal blooms. – Estuar. Coast. Shelf Sci. 33: 409–418. SINKE, A. J. C., CORNELESE, A. A., KEIZER, P., VAN TONGEREN, O. F. R. & CAPPENBERG, T. E., 1990: Mineralization, pore water chemistry and phosphorus release from peaty sediments in the eutrophic Loosdrecht lakes, The Netherlands. – Freshwater Biol. 23: 587–599. ST-JULIEN, P., 1972. Appalachian tectonics in the eastern townships of Québec. – Intern. Geol. Congress. Montréal. 22 p. TREVORS, J. T., 1984:. The measurement of electron transport system activity in freshwater sediment. – Water Res. 18: 581–584. WETZEL, R. G. & LIKENS, G. E., 1991: Limnological Analyses. – Springer-Verlag, New York. 391 p.

Author’s addresses: Y. T. PRAIRIE, C. DE MONTIGNY, Université du Québec à Montréal, Département des Sciences Biologiques, C. P. 8888 Succ. Centre-ville, Montréal (Québec), H3C 3P8, Canada. P. A. DEL GIORGIO*, Université du Québec à Montréal, Département des Sciences Biologiques, C. P. 8888 Succ. Centre-ville, Montréal (Québec), H3C 3P8, Canada. *Present address: Horn Point Laboratory, University of Maryland Center for Environmental Science P. O. Box 775, Cambridge, MD 21613 USA.