Dissolved silica budget in the North basin of Lake Lugano

Chemical Geology 182 Ž2002. 35–55 www.elsevier.comrlocaterchemgeo

Dissolved silica budget in the North basin of Lake Lugano Annette Hofmann a,1, Didier Roussy b, Montserrat Filella c,) a

b

Institute F.-A. Forel, UniÕersity of GeneÕa, 10 Route de Suisse, CH-1290 Versoix, Switzerland Laboratoire de Chimie Moleculaire et EnÕironnement, ESIGEC, UniÕersite´ de SaÕoie, F-73376 Le Bourget du Lac, France ´ c Department of Inorganic, Analytical and Applied Chemistry, UniÕersity of GeneÕa, 30 Quai Ernest-Ansermet, CH-1211 GeneÕa 4, Switzerland Received 29 August 2000; accepted 23 February 2001

Abstract We studied the dissolved silica cycle in the water column of the North basin of Lake Lugano, SwitzerlandrItaly. Lake Lugano is a meromictic, eutrophic lake, permanently stratified below 100-m depth. A one-box model was used to calculate a silica mass-balance over 1993, based on various lake measurements, such as sediment traps, sediment cores, water analysis and biota countings. We found that the North basin of Lake Lugano is at steady state as far as dissolved silica is concerned. The primary source of dissolved silica in the lake is river input Žabout 80%., with diffusion from bottom sediments and groundwater input also playing a role. Atmospheric input is negligible. The main export of dissolved silica occurs via biogenic uptake by diatoms and final burial of their frustules in the bottom sediment. Loss of dissolved silica through the lake outflow only represents 15% of the total output. Of the total amount of Si exported to the lake bottom through diatom sinking, less than 20% is re-supplied to the surface water by diffusion. Thus, the lake acts as an important permanent sink for silica. The long residence time of dissolved silica in the lake Ž7 years. is related to the strong physical stratification of the lake. Only about 10% of the standing stock are available to phytoplankton uptake. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Lake Lugano; Dissolved silica; Diatom; Silica budget

1. Introduction Compounds of silicon occur in all natural waters. They may be present either as suspended solids or in solution. Silicon occurs in minerals such as quartz,

)

Corresponding author. Fax: q41-22-7026069. E-mail address: [email protected] ŽM. Filella.. 1 Present address: Institute for Terrestrial Ecology, ETHZuerich, Grabenstrasse 3, CH-8952 Schlieren, Switzerland.

feldspars, and clays. Biomineralisation of silicon in the form of opal or amorphous hydrated silicon oxide Žbiogenic silica, SiO 2 P nH 2 O. is performed by a number of aquatic organisms, of which the diatoms are the most widely distributed. In the dissolved form, silicon is present as silicic acid, SiŽOH.4 . The undissociated monomeric form dominates in natural waters at pH below 9 ŽAston, 1983; Stumm and Morgan, 1996.. Silicic acid is derived either from weathering reactions of silicate and aluminosilicate minerals or from dissolution of the biogenic opal following death of the organisms.

0009-2541r02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 0 1 . 0 0 2 7 5 - 3

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A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

For a number of years, oceanographers have been concerned with the silica cycle and the problem of obtaining a silica mass balance for the oceans ŽSillen, ´ 1961; Mackenzie et al., 1967; Calvert, 1968; Gregor, 1968; Burton and Liss, 1973; Wollast, 1974; DeMaster, 1981.. Initial interest arose from the suggested control of seawater composition by reactions of silicate minerals with dissolved species ŽSillen, ´ 1961.. Nowadays, however, research is mostly focused on the inter-relationship of carbon and silicon cycles and their connection with global change ŽLonghurst and Harrison, 1989; Nelson et al., 1996; Dugdale and Wilkerson, 1998.. In this context, the silica balance in the world ocean has recently been the subject of a thorough re-estimation ŽNelson et al., 1995; Treguer et al., 1995.. In comparison, relatively ´ little work has been done on dissolved silica mass balances in lakes ŽDickson, 1975; Bailey-Watts, 1976; Parker et al., 1977a; Johnson and Eisenreich, 1979; Schelske, 1985; Conley et al., 1988; Cornwell and Banahan, 1992; Michard et al., 1994; Callender and Granina, 1997.. Most of the studies exclusively devoted to silica were published in the 1970s as a response to growing concern about lake eutrophication, and they are mainly concerned with the Laurentian Great Lakes. Studies on other lakes are scarce. In this work, we present a budget for dissolved silica in the North basin of Lake Lugano in 1993. The lake has been regularly monitored since the 1980s and physical, chemical and biological data have been published in annual reports by the International Commission for the Protection of the Swiss– Italian Waters ŽCIPAIS.. Studies on bottom sediments, trap sediments and water composition were undertaken by the University of Geneva between 1992 and 1994. Combination of these two sources of information provided the elements needed for the establishment of this dissolved silica budget. The aim guiding this study is twofold. One objective is to identify and quantitatively characterise the main processes involved in silica cycling in the lake. It is well known that silicon cycling in temperate lakes is an annual event controlled by diatom production. Dissolved silica is taken up by diatoms in the epilimnion. As the frustules settle down and the lake stratifies, the planktonic diatom production often becomes limited by silica bioavailability. Diatom production during the following temperature-driven

stratification period is then essentially assured by allochthonous silicic acid input; upward diffusion of silica redissolved from the sediments and the benthic nepheloid layer being a continuous but slow process. However, the exact weight of each one of these processes has never been evaluated in the North basin of Lake Lugano. This deep lake basin became meromictic during the 1960s due to intense eutrophication and is now permanently stratified below 80– 100-m depth. These particular characteristics of the lake drive the second objective of our work, namely, the study of how the permanent stratification of the lake affects the annual cycling of dissolved silica.

2. Methods 2.1. Area description Lake Lugano is situated on the southern side of the Alps, at the Swiss–Italian border ŽFig. 1.. It is subdivided into two main basins. They are separated by a frontal moraine at Melide, on which a dam was built in 1844 ŽBarbieri and Mosello, 1992.. The morphological and physical parameters of the North basin are given in Table 1. All field data used in this work were obtained at a station situated in the pelagic part of the North basin, near the village of Gandria. The northern basin of Lake Lugano has an elongated shape, extending over 20 km. Its average width is just over 1 km, with 3 km at the largest point between Lugano and Caprino. The basin is seated in a geologically complex zone ŽReinhard et al., 1959; Bernoulli et al., 1976.. A schematic geological map of the drainage area is shown in Fig. 2. The main building blocks are crystalline rocks Ž paraand ortho-gneisses. at the north and the west, and sedimentary, mainly calcareous rocks ŽTriassic, Jurassic., at the east and the south. Deep faulting occurs within the Triassic series of dolomites and marls and along the ALugano –Mt. GronaB tectonic line that crosses the North basin at Castagnola and brings into contact crystalline and carbonate rocks. Additionally, the Mesozoic series underwent intensive folding. The Lugano area saw significant anthropogenic changes in the 19th and especially the 20th century

A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

37

Fig. 1. Location of Lake Lugano at the Swiss–Italian border. The situation of the pelagic station where measurements were made is indicated.

with the growth of population and industry. As a result, Lake Lugano became eutrophic. In the North basin, an anoxic layer developed in the 1950s that progressed from the lake bottom up the water column to reach the 80–100-m depth mark in the 1990s. It seems to have stabilized at this level since then. The progression of anoxia was concomitant with an increasing density stratification of the water column.

Table 1 Main characteristics of the North basin of Lake Lugano ŽBarbieri and Polli, 1992. Parameter

Units

0–288 m

0–100 m

Volume Surface Mean depth Maximum depth Drainage area

km3 km2 m m km2

4.69 27.5 171 288 270

2.35 27.5 86

2.2. Mass balance calculations A mass balance for dissolved silica over a 1-year period can be calculated with Eq. Ž1. in which inputs equal losses: AqIqPqSsOqRqD M

Ž 1.

where A s annual atmospheric input, I s annual input via the water inflow, P s amount of silica produced by internal lake processes per year, S s annual release from sediments, O s annual amount of dissolved silica leaving the lake via the outflow, R s amount of dissolved silica removed through internal removal processes per year, D M s annual change in mass of the amount of dissolved silica present in the lake. If the system is at steady state, D M s 0. In the present study, mass balance calculations were performed over the year 1993.

A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

38

Fig. 2. Schematic geological map of the drainage area of the North basin of Lake Lugano Žfrom Monnerat, 1995..

2.3. Experimental methods 2.3.1. DissolÕed silica in the lake and the tributary riÕers Dissolved silica was determined spectrophotometrically by the molybdate-heteropolyblue assay, after filtration of the water samples through 0.4-mm membrane filters. The detection limit of the method is 50 mg Si ly1 . The molybdosilicic method detects only the monomeric and dimeric silicic acid ŽAston, 1983.; polymeric species do not participate in the reaction. Often the terms AreactiveB and Anon-reactiveB are used to classify different fractions of dissolved silica in fresh and marine waters ŽBurton, 1975.. They refer, in essence, to the monomeric and polymeric silicon species in solution. The reactive form will participate in the biological cycling of silicon in the hydrosphere. In contrast, it is assumed that polymeric forms are not available for biological uptake and utilisation ŽLewin, 1962.. The reactive fraction

is the dominant dissolved species in natural waters. In the present paper, we will designate this fraction as Adissolved silicaB or Adissolved SiB. Concentrations will be expressed in elemental Si mass per litre. In the lake, concentration profiles have been measured on a monthly basis close to our sampling station off Gandria, by the Laboratorio Studi Ambientali, Lugano ŽLSA.. River-water samples were taken on a weekly ŽCassarate. to biweekly Žother tributary rivers. basis ŽLaboratori Studi Ambientali ŽLSA.-SPAA, 1991.. Mosello and De Giuli Ž1982. demonstrated that a good correlation existed between dissolved silica concentrations and discharge for three representative South Ticino rivers. Therefore, a concentration–discharge method was applied to calculate the yearly load in the tributary rivers of Lake Lugano. Calculation errors are of the order of 2% ŽMosello and De Giuli, 1982.. Data were published by LSA as mean annual concentrations from 1987 to 1990 and as annual loads since 1991.

A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

Algae and zooplankton have been monitored on a regular, bimonthly to monthly basis, since 1983 by LSA. 2.3.2. Trap sediments Between June 1992 and January 1994, sediment traps were deployed monthly at five depths covering the whole water column: 30, 90, 130, 265 and 278 m ŽHofmann, 1996.. Traps consisted of 11-cm-diameter Plexiglas cylinders, which were composed of a lower collector section with an upper column part extending above. With a length to width ratio of 7r1, these cylinders were shown to ideally sample real particle sedimentation fluxes to a level of 95–100% ŽBlomqvist and Hakanson, 1981.. With the selected ˚ ratio, the traps withstand water turbulences of 20–30 m sy1 horizontal speed without losing sediment through resuspension ŽBloesch and Burns, 1980.. They are thus well adapted to the conditions in the sheltered North basin of Lake Lugano. Sediments were collected over 1-month periods. Upon recovery, the water overlaying the sediment Župper column part. was siphoned off. All samples were subsequently freeze-dried. Major elements, including Si, were analysed by ICP-AES ŽIRISrCID Thermo Jarell Ash. after total mineralisation of the sediment with HF in sealed Teflon containers ŽThompson and Walsh, 1983.. Boric acid was added to avoid the formation of volatile tetrafluorine Si complexes. The detection limit for Si in the analytical solution is 0.44 mg ly1 and the mean precision, 2.2%. Biogenic Si present in trap sediments was calculated as follows: First, the lithogenic Si:Al weight ratio was estimated to be 2.4 " 0.16 from measurements in tributary river suspensions and in two turbidity layers Žfour data sets: Monnerat, 1995; Hofmann, 1996.. Multiplication of the fluxes of particulate Al in traps by this Si:Al ratio gives an estimate of the Si lithogenic fluxes. Biogenic Si is obtained by subtraction of lithogenic fluxes from total fluxes. 2.3.3. Bottom sediments A short sediment core Ž30 cm. was taken in April 1993 using a specially designed corer that allows extraction of sediments without perturbation of the surface structure. After retrieval, the sediment column was fixed inside the core tube with hard sponge

39

material. In the laboratory, the core was sectioned into 0.5- to 1-cm slices by gently extruding the sediment out of the tube using a hydraulic piston. Each section of sediment was well mixed, freezedried and analysed for major elements. The analytical procedure described above for trap sediments was also used for bottom sediments. Biogenic Si was calculated as previously explained. Sedimentation rates were established at the sampling site by close correlation of this core with cores taken earlier at the same site and dated using as a marker a 137Cs peak that originated from the Chernobyl fallout in spring 1986 ŽDominik and Span, 1992; Span et al., 1992.. All sediment cores of the AGandriaB site show a succession of turbidite and authochtonous layers that are easily correlated by colour, thickness, mineralogical and chemical composition and physical parameters such as porosity ŽHofmann, 1996.. The Cs peak is overlain by a characteristic turbidite layer that deposited following immediately the Chernobyl event. By considering the autochthonous sediments above the Cs-marker layer and by excluding the deposits originated from turbidities, a sedimentation rate of 395 " 93 g my2 yeary1 was obtained ŽHofmann, 1996.. The relatively large error associated with the calculated value arises from the uncertainty in the estimation of the true thickness of a fine turbidity layer situated near the sediment surface because relatively large core sectioning steps were used. Sedimentation fluxes of total and biogenic Si were determined from the total sedimentation rate and the concentration of Si present in the sediments.

3. Results 3.1. The standing stock In order to test whether the dissolved Si was at steady-state in the North basin of Lake Lugano, the mean total mass of dissolved Si present in the lake was estimated from monthly dissolved Si profiles for the years 1989 to 1994 ŽFig. 3.. Total quantities were obtained for each month by estimating the water volume between two sampling depths, multiplying this volume by the respective average concentration, and adding up the partial quantities. Annual values were calculated by averaging of monthly data. Re-

40

A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

Fig. 3. Depth profiles of total dissolved silica Žmg Si ly1 . in the North basin of Lake Lugano at the Gandria sampling station in the period 1989–1994 ŽLSA, personal communication.. February profiles Žfull symbols. show the situation at the end of winter, when surface mixing is maximum. In March–April profiles Žempty symbols., the uptake of dissolved Si by diatoms clearly depresses dissolved silica concentrations in the upper 20 m.

sults obtained are fairly constant, ranging between 6695 and 6884 T. The mean standing stock for the years 1989–1994 was 6779 " 138 T; for the year 1993, it was 6884 " 300 T. The results obtained suggest that the rate of depletion of dissolved silica in the northern basin of Lake Lugano approximately equals the rate of supply and that the lake can be considered to be at steady-state. 3.2. Inflow Estimation of the amount of silica entering the lake with the water inflow requires the monitoring of water discharge into the lake as well as regular measurement of the concentration of dissolved silica contained in the inflowing water. Permanent water throughflow monitoring stations only exist for the two main tributary rivers, the River Cassarate at Pregassona ŽLugano. since 1963 ŽASMA, 1989– 1995. and the River Cuccio at Porlezza since 1989 ŽAIT, 1986–1995.. The River Livone was monitored from 1985 to 1988 ŽAIT, 1986–1995.. These three rivers drain 158.5 km2 , which corresponds to 59% of

the total drainage area of the basin. The discharge of the remaining surface can be estimated from the relationship existing between river watershed and discharge. From 1984 to 1986, the LSA established correlations between the flow of all controlled rivers having an annual mean discharge below 5 m3 sy1 Ž10 rivers. and the surface of their respective drainage basins ŽLaboratori Studi Ambientali ŽLSA.-SPAA, 1986, 1987, 1988.. In the present study, we use the same approach to quantify the discharge over the following years, but we include only the rivers with a flow F 3.5 m3 sy1 in the correlation. The precision of our estimation is 15%. Results are shown in Table 2. All Lake Lugano tributary rivers have a pluvionival regime characterised by alternating periods of low water levels and floods. Their water discharge closely follows the regime of rainfall. The largest monthly average flow usually occurs between April and June. Peak flow rates are normally recorded in summer ŽJune–October. during thunderstorms. These sudden and violent rainfalls amplify the normal river flows by several orders of magnitude.

A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

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Table 2 Drainage area, mean annual discharge of tributary rivers, dissolved Si concentrations, and total Si lake input and output for 1993 Rivers

Drainage areaa Žkm2 .

Cassarate Cuccio Livone LagadonerCivagno Solda Rezzo Remaining surface Total river input Direct input Žrain. Output Melide

73.9 54 30.6 27.6 17.6 14.5 51.5

Mean annual discharge Žm3 sy1 .

Dissolved Si concentration Žmg ly1 .

Total Si inputroutput b ŽT yeary1 .

Average

1993

1993

1993

2.35 c 1.90 e 0.49 f 0.94 g 0.60 g 0.50 g 1.7 g 8.03 1.31 j 9.33

2.41c 1.92 e 0.95 g 0.86 g 0.55 g 0.45 g 1.61g 8.76 1.40 j 10.16

3.59 d 3.15d 2.32 d 0.73 h 0.88 h 2.14 h 2.5 i

273 191 70 19.8 15.4 30.8 112 712 " 107 3.1 " 1 154 "90

0.07 k 0.48 l

a

From CIPAIS, campaign 1984 ŽLaboratori Studi Ambientali ŽLSA.-SPAA, 1986.. Calculated from river drainage area, mean annual discharge and concentrations of dissolved Si. c Cassarate, measured 1989–1994 ŽHJS, 1991–1996.. d From CIPAIS, campaign 1993 ŽLaboratori Studi Ambientali ŽLSA.-SPAA, 1996a.. e Cuccio, measured 1989–1994 ŽAIT, 1986–1995.. f Livone, measured 1985–1988 ŽAIT, 1986–1995.. g Lagadone, Solda, Rezzo, remaining surfaces, 1989–1994, calculated estimates. h Data extrapolated from measurements taken in 1996 ŽCIPAIS, campaign 1996 ŽLaboratori Studi Ambientali ŽLSA.-SPAA, 1998... i Estimated from adjacent watershed data and from geological setting. j Calculated from annual precipitation data, 1989–1995 and 1993, respectively ŽASMA, 1989–1995.. k Estimated from Zobrist Žpersonal communication. and Giacomelli et al. Ž1999.. l Annual mean concentration at our sampling station ŽGandria. at 0–5-m depth. b

Water may also be discharged into the lake through groundwater seepage and subsurface karst systems. In particular, a permanent groundwater input may take place from the alluvial plain aquifers of the rivers Cassarate and CucciorRezzo. Sporadic karst water input may be relevant in the other zones. The region is severely affected by faulting. Intense faulting, particularly of Triassic dolomites and the structural ALugano –Mt. GronaB line, may have created deep infiltration zones through which meteoric waters could feed karst aquifers that discharge into the lake. To our knowledge, the groundwater drainage of this area has not been investigated and no information is available. Thus, we had to neglect this potential input of silica into the lake in our calculations. Dissolved silica present in river water depends on the size and morphological characteristics of the drainage basin and on its geological setting ŽFig. 2.. The highest concentrations are carried by the rivers Cassarate and Cuccio that drain essentially crys-

talline rocks. Concentrations up to 6.5 mg ly1 have been recorded at low water regime ŽBarbieri and Righetti, 1978.. In the sedimentary environment, Si concentrations depend on the geochemical characteristics of the outcropping formation. Triassic marls and dolomites are poor in silica content whereas the limestone from the Lower Jurassic ŽAcalcari selciferiB . are enriched due to the abundance of sponge spicules. The microcrystalline silica Žup to 36%. is finely dispersed in the rock matrix or occurs as nodules ŽBernoulli, 1964.. Additionally, some areas are covered by moraine and alluvial sediments of heterogeneous composition. Rivers draining the sedimentary zones show somewhat lower concentrations of dissolved Si. However, due to the complexity of the mesozoic rocks and the quaternary deposits, concentrations vary considerably, from 1.3 mg ly1 ŽSolda. to about 3 mg ly1 ŽRezzo.. The Lagadone channel connects a small reservoir lake on the Civagno River with Lake Lugano. Biogenic uptake of dissolved silica in this water reservoir is probably

42

A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

responsible for the low concentrations of dissolved Si in the Lagadone Channel Žaverage of 0.75 mg ly1 .. Mean annual concentrations of the tributary rivers and the mean lake input and output are given in Table 2. The contribution of particulate biogenic silica carried in suspension by rivers has recently been shown to be an important component in the world ocean Si budget ŽConley, 1997., which had been previously overlooked. However, the importance of this component has shown to be minor in small rivers, just 2–5% of the total dissolved and biogenic pool ŽConley, 1997.. The pluvio-nival character of tributary rivers, the high flow rates, and relatively small drainage basins Ž- 15 to 80 km3 . further reduces the potential for diatom production in the drainage area of Lake Lugano. No benthic diatoms that would suggest fluvial origin have been observed in the trap sediments at our sampling station near Gandria. We therefore consider that the biogenic silica fluxes determined from trap data are not biased by riverine contributions. 3.3. Atmospheric input The silica found in the atmosphere is primarily present in particulate form, essentially of natural, inorganic Žquartz. or biological origin Ždiatoms.. Wind stress on the land surface results in advection of soil grains into the atmosphere, the dust later falls on the water surface as particulate matter in rain or as dry fallout. Saharan dust is a major contributor to atmospheric silica in the Mediterranean region and has been identified in rain water in the northern Apennine ŽFlorence, Italy. by Giacomelli et al. Ž1999.. Atmospheric dissolved Si concentrations are expected to be very low. In the southern alpine regions of Ticino ŽSwitzerland. and Lombardy ŽItaly., atmospheric precipitation has been investigated since the 1980s in relation to atmospheric pollution and acidification of high altitude mountain lakes ŽSchnoor et al., 1983; Barbieri and Righetti, 1987; Stumm et al., 1987; Mosello et al. 1991; Boggero et al., 1996.. However, because silicic acid present in rain water was considered to be negligible and to play no role in the acidity balances, it was not determined during the measurement campaigns. No data are available for the Lugano region and only two data sets could

be obtained for the close vicinity ŽTable 2.. The lack of data on dissolved silica in precipitation seems to be a world-wide problem and this fraction is generally estimated in balance studies ŽTreguer et al., ´ 1995.. For the Si budget in Lake Lugano, a mean concentration in rain water of 70 mg ly1 , based on the AregionalB data of Zobrist Žpersonal communication. and Giacomelli et al. Ž1999. was taken ŽTable 3.. The annual mean precipitation was 1486 mm for the period 1989 to 1995, and 1590 mm in 1993 ŽASMA, 1989–1995.. From the surface area of the lake Ž27.83 km2 , Table 1., the direct input of dissolved Si into the lake was estimated to be 3.1 T in 1993 Žassociated error, ca. 30%.. 3.4. Outflow There is no flow gauging at the outlet of the North basin of Lake Lugano. The value generally reported for the water outflow is 12 m3 sy1 ŽBarbieri and Polli, 1992; Ramisch et al., 1999.. This value is based on the water balance in 1984 and was estimated on the grounds of a correlation between drainage area and mean discharge of tributary rivers ŽLaboratori Studi Ambientali ŽLSA.-SPAA, 1986.. However, precipitation and river discharge can vary dramatically from 1 year to another and the use of a fixed value is not advisable. For 1993, we calculated the discharge at the outflow as the sum of input from rivers Ž8.8 m3 sy1 . and rain Ž1.4 m3 sy1 . ŽTable 2.. Although the lake level increased by 100 cm during intense floods in October 1993, the annual mean level remained constant ŽLaboratori Studi Ambientali ŽLSA.-SPAA, 1996a,b., thus no effect from lake level change was considered. No account was taken of evaporation at the lake surface. No direct data is available for the concentration of silica in the outflow. The mean concentration in the outflow was taken as being equal to the annual mean concentration in the 0–5 m layer, i.e. 0.48 " 0.28 mg ly1 . In 1993, the loss of dissolved silica with the outflowing water is estimated to be 154 " 90 T Si. The fairly high error is related to the high seasonal variability of Si concentrations at the lake surface. 3.5. Lake sinks The main sink for dissolved silica is its uptake by diatoms and subsequent sedimentation of the diatom

Table 3 Literature data on atmospheric fluxes of dissolved silica in the Alpine region and world-wide Quoted quantities

Equivalent flux of dissolved Si a Žmmol my2 yeary1 .

Method of analysis

Location

Reference

Rain water

13–143 mg Si ly1

0.73–8.1

Florence, IT

Giacomelli et al., 1999

Dry and wet deposition

f 0.5 mg H 4 SiO4 , my2 dayy1 for wet and dry deposition combined 10 T mol yeary1 of lithogenic silica on 36=10 13 m2 0.2–67.1 mmol Si my2 yeary1 of lithogenic silica 7.99=10 9 g SiO 2 yeary1 , 0.137 g SiO 2 my2 yeary1 9.2=10 9 g SiO 2 yeary1 0.154 g SiO 2 my2 yeary1 26.1=10 9 g SiO 2 yeary1 , 0.316 g SiO 2 my2 yeary1

1.9

Filtered 0.2-mm spectrophotometric determination with heteropolyblue and 1-amino-2-naphthol-4-sulphonic acid as reductant. spectrophotometric heteropolyblue yearly average 1994–1995

Tuffenwies, ¨ Canton Zurich, CH ¨

Zobrist Žpersonal communication.

dissolved Si estimated at 5–10% of total particulate Si dissolved Si estimated at 5–10% of total particulate Si following estimate from International Joint Commission Ž1976.

world ocean surface, mean value Capo Carvallo, Corsica

Treguer et al., 1995 ´

Lake Michigan

Schelske, 1985

following estimate from International Joint Commission Ž1977.

Lake Superior

Eolian dust Eolian dust Žmainly from Sahara. Bulk precipitationb

Bulk precipitationb

a b

1.4–2.8 0.02–6.7 2.74 3.08 5.25

Losno, 1989

Lake Huron Johnson and Eisenreich, 1979

A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

Medium

Flux calculated for the Lugano area with 1590 mm of precipitation in 1993. Includes dissolved silica in precipitation and silica released to solution from particulate matter derived from dry deposition.

43

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A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

frustules to the lake bottom. Adsorption of silica onto particles ŽSwedlund and Webster, 1999. and authigenic formation of new silicate minerals was not considered. The evolution of the concentration of planktonic diatoms is compared with simultaneous changes in concentrations of dissolved silica in Fig. 4. The commonly observed inverse relationship between the two silica fractions ŽParker et al., 1977b; Reynolds and Wiseman, 1982. is also observed in the present case. Thus, the decrease in dissolved silica concentrations in surface water is mainly due to concurrent silica uptake by diatoms. A detailed description of the planktonic succession in 1993 has been published elsewhere ŽHofmann et al., 2001. and will not be given here. The total amount of silica leaving the water body through diatom sedimentation was estimated from trap sediment data at 30-m depth. A detailed monthly calculation is shown in Table 4 for 1993. The vertical transport of diatoms is in general very fast because diatom frustules tend to aggregate into flocs under the influence of physical aggregation processes. For the 1993 spring diatom bloom, settling rates of at least 8 m dayy1 were

observed in Lake Lugano ŽHofmann et al., 2001.. Similar high sedimentation rates have been observed in other freshwater systems ŽParker et al., 1977a; Grossart and Simon, 1993, 1998.. 3.6. Silica dissolution in the water column The amorphous silica contained in diatoms dissolves five orders of magnitude faster than mineral silicates, at about 2 = 10y9 mol cmy2 sy1 ŽHurd, 1983.. Most of the information available about diatom wall dissolution concerns the marine environment. Oceans are strongly undersaturated with respect to amorphous silica and the silica incorporated into diatom cells is quickly recycled after their death ŽSmetacek, 1999.. Most freshwater systems, like the North basin of Lake Lugano, are also strongly undersaturated. Whereas the saturation value of amorphous silica is 29–56 mg Si ly1 at 258C and neutral pH ŽGardner, 1938; Alexander et al., 1954; Lagerstrom, 1959., the contents in dissolved Si varies between 0.1 and 3 mg ly1 . The rate of dissolution of the diatom skeletons is known to be affected by

Fig. 4. Monthly evolution of Ža. the concentration of dissolved silica Žmg Si ly1 . at 2.5-m depth, and Žb. diatom biomass Žmg ly1 . integrated over 0- and 20-m depth, from 1989 to 1995. Note the coincidence between minimum in dissolved silica and maximum in diatom production ŽLaboratori Studi Ambientali ŽLSA.-SPAA, 1990a,b, 1991, 1992, 1994, 1996a,b and personal communication..

A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

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Table 4 Total amount of biogenic Si sedimented in sediment traps at 30-m depth in 1993 Month

Exposure periods

Si sediment trap flux Žmg my2 dayy1 .

Total Si ŽT periody1 .

January February March April May June July August September October November December Annual

06r01r93–02r02r93 02r02r93–02r03r93 02r03r93–06r04r93 06r04r93–03r05r93 03r05r93–31r05r93 31r05r93–05r07r93 05r07r93–10r08r93 10r08r93–31r08r93 31r08r93–04r10r93 04r10r93–04r11r93 04r11r93–30r11r93 30r11r93–04r01r94

29.7 " 1.7 58.4 " 4.6 108.8 " 2.4 620.6 " 30.4 67.5 " 8.8 38.3 " 5.6 37.8 " 2.0 37.7 " 2.8 54.0 " 14.8 58.4 " 43.9 20.8 " 7.3 58.0 " 2.4

20.3 " 1.2 41.3 " 3.3 96.3 " 2.2 423.5 " 20.8 47.8 " 6.2 33.9 " 5.0 34.4 " 4.7 20.0 " 1.5 46.4 " 12.7 45.7 " 34.4 13.6 " 4.8 51.3 " 2.2 874.6 " 43.7

many factors, namely, the presence of protective organic skins ŽBidle and Azam, 1999., temperature Žrapid dissolution in surface waters; slower dissolution in deep waters where temperature is low ŽLewin, 1961; Werner, 1977; Lawson et al., 1978.., and the presence of salt Ženhances the dissolution rates of diatoms ŽKato and Kitano, 1968; Kamatani, 1971; Hurd and Birdwhistell, 1983; Barker et al., 1994... Silica dissolution has not been very extensively studied in lake water. In particular, there is no agreement as to whether biogenic silica dissolution begins in the water column. Some authors show that the major fraction of SiO 2 produced annually in the form of frustules dissolves before being integrated in the permanent sediment ŽParker et al., 1977a.. However, dissolution in the water column does not seem to be important ŽBailey-Watts, 1976; Schelske et al., 1984.. In the present study, the observed rapid sedimentation of diatoms seems to preclude any significant dissolution taking place in the water column. Annual fluxes of particulate biogenic silica remain practically constant down to a depth of 265 m ŽFig. 5. with a mean value of 35.6 " 1.6 g my2 yeary1 in our sampling station. Because resuspension becomes significant only below this depth, it can be concluded from annual sedimentation fluxes that pelagic dissolution of biogenic silica is negligible between the lake surface and 265-m depth. Turbulence above the lake bottom leads to the formation of a benthic

nepheloid layer ŽBNL. with sediment re-suspension and long particle residence times. Dissolution may play an important role in the water layer close to the sediments. This process is discussed in the next section. Particulate riverine inputs are primarily composed of clay minerals and debris from rock weathering. Silicate minerals dissolve slowly enough for the Si contained as suspended particulate matter in this form to be considered unavailable on the time scale of silicate residence in lakes ŽLerman, 1988..

Fig. 5. Annual sedimentation fluxes of total particulate Si Žfull symbols. and biogenic particulate Si Žempty symbol. Žg my2 yeary1 . in sediment traps in 1993.

46

A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

3.7. Benthic silica diffusion Total Si concentration in sediments reflects allochthonous Si inputs, as verified by the correlation observed between Al and Si Ž r 2 s 0.75.. The biogenic fraction Ž36–48%. reflects autochthonous inputs. Biogenic silica is well correlated with sediment water content Ž r 2 s 0.78. ŽFig. 6. as well as with organic carbon and carbonate associated Ca Ždata not shown.. Sediment fluxes of biogenic silica are shown in Fig. 7. They correspond to the fraction of deposited biogenic Si that escaped fast dissolution at the sediment–water interface and ultimately became buried in the permanent sediments. Fluxes do not change with depth in the sediment, which probably reflects the fact that neither significant changes in allochthonous inputs and diatom production nor in dissolution rates with depth have taken place over the 15 years prior to these measurements. Low silica reactivity in sediments has been observed elsewhere and related to frustule surface aging by Van Cappellen Ž1996..The average flux value, over the top 10.5 cm of the sediment core, was 29 " 8.8 g my2 yeary1 . It should be noted that the rate obtained represents an apparent sedimentation flux. Sediment focussing is a common process in lakes by which particles are

transported from the basin slopes to the deepest part of the lake due either to shear at the sediment–water interface ŽLemmin and Imboden, 1987; Gloor et al., 1994. or to gliding of particles in a denser water layer near the sediment surface induced by diffusion of dissolved matter out of the sediment ŽWieland et al., 1991.. Sediment focussing is likely to play a role in the North basin of Lake Lugano since this lake has a trench-like shape with steep sides. One intense turbidity event was indeed observed in June 1993 ŽHofmann and Filella, 1999.. Therefore, the flux estimate given above should be considered as a maximum value. Enhanced diatom frustule dissolution may occur in the BNL because of the long residence time of resuspended particles in water. However, quantitative estimation of the dissolution is difficult to perform in such a dynamic system. Consideration of the BNL as a box, with an inflowing flux at 265-m depth and an outflowing flux at the sediment surface, allows the calculation of an upward net benthic silica diffusion flux of 6.5 " 8.9 g my2 yeary1 . This diffusive flux should be considered as only an order of magnitude estimation because dating difficulties Žsee Section 2.3. and the unknown contribution of sediment focussing result in a high uncertainty in the sedimentation rates of biogenic Si.

Fig. 6. Percentage of total particulate Si Žthick line. and biogenic particulate Si Žthin line. in the first 10.5 cm of the lake sediments Ža. as compared to the water contents in the same sediments Žb.. t1 and t2 correspond to two turbidity layers.

A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

47

Fig. 7. Annual sedimentation fluxes of total particulate Si Žfull symbols. and biogenic particulate Si Žempty symbols. in the sediment traps Ža. as compared to the bottom lake sediments Žb.. All fluxes expressed as g my2 yeary1 . The discontinuous lines indicate mean values.

The vertical flux, Jz , of a solute in the water column can also be estimated from its vertical gradient and vertical eddy diffusion coefficient, K z Jz s yK z Ž EcrEz . .

Ž 2.

The water column of the North basin of Lake Lugano is permanently stratified and total mixing only occurs in the surface layer, i.e. above approximately 50-m depth. At greater depths, stable conditions exist. A mean vertical K z profile, established by Aeschbach-Hertig Ž1994. for the period 1990– 1992 using the Tritium–Helium method, gives a minimum value of 1 = 10y5 m2 sy1 at 80-m depth ŽFig. 8.. K z then progressively increases up to 1.6 = 10y5 m2 sy1 at 120-m depth. The physical stability

Fig. 8. Depth profile of the vertical diffusion coefficient, K z Žm2 sy1 . in the North basin of Lake Lugano. Data derived from Wuest ¨ et al. Ž1992. and Aeschbach-Hertig Ž1994..

48

Table 5 Published values of silica diffusion from sediments in freshwater systems Lake surface and maximum depth

Diffusion flux Žg my2 yeary1 .

Comment

Techniqueb

Reference

Ursee, GE Furesø, DK Grane Langsø, DK Kvindsø, DK

–, 11 m 9 km2 , 36 m 0.11 km2 , 11.5 m 0.14 km2 , 2.5 m

6.1–12.3 34.6 1.7 7.3–69.3

small bog-lake eutrophic oligotrophic eutrophic flux is T dependent

laboratory cores laboratory cores

Tessenow, 1972 Kamp-Nielsen, 1974 Møller-Andersen, 1974

Kulsø, DK Loch Leven, UK

13.3 km2 , shallow 0.17 km2 , 2.8 m

Lake Ontario

19,010 km2 , 244 m

pore water profiles

Nriagu, 1978

Lake Superior

82,102 km2 , 406 m

182 19.4–144.6 Žmean: 80.5. 1.2–2.3 Žmean: 1.7. 1.2–4.4 Žmean: 2.8.

laboratory cores mass balance mass balance laboratory cores

pore water profiles

Johnson and Eisenreich, 1979

Lake Erie Central basin Eastern basin Western basin Lake Huron, southern Lough Neagh, UK Lake Michigan

2566 km2 , 175 m

laboratory cores

Robbins and Edginton, 1979

59,569 km2 , 228 m

pore water profiles laboratory cores laboratory cores pore water profiles laboratory cores

Robbins, 1980 Rippey, 1983 Quigley and Robbins, 1984

Lake Huron, Saginaw Bay

Lake Michigan

57,756 km2 , 281 m

Jacks Lake, Williams Bay Toolik Lake, Alaska

0.80 km2 , 22.7 m 1.5 km2 , 7r25 m

25 16 48 10.6 6.1–36.8 27.1 2.7 14.6–53.4 Žmean: 30.2. 6.6 10.7–48.1 1.9–24.8 2.1 0.2

Lake Lugano, North basin, CH-I

27.5 km2 , 288 m

21c

a

59,569 km2 , 228 m 383 km2 , 8.9 m 57,756 km2 , 281 m

eutrophic

diffusion related to amount of frustules

eutrophic nearshore sediments

mesotrophic, dimictic ultraoligotrophic ice free 3 months a year eutrophic year mean

Bailey-Watts, 1976

Robbins, 1984

pore water profiles laboratory cores pore water profiles pore water profiles pore water profiles

Conley et al., 1988; Conley and Schelske, 1989 Carignan and Lean, 1991 Cornwell and Banahan, 1992

water column

This work

When country not stated, Laurentian Great Lakes. Laboratory cores: laboratory confinement of sediments in core tubes and measurement of solute concentrations in overlying water over time. Pore water profiles: flux calculation from observed concentration gradients at the sediment–water interface based on Fick’s first law and the molecular self-diffusion coefficient of the solute. Water column: flux calculation from water column profiles by using the Eddy diffusion coefficient, K z , of the water body. c Total flux of 136"31 T yeary1 at 75-m depth converted to the specific sediment flux, considering the total bottom surface area below 265-m depth Ž6.5 km2 .. b

A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

Systema

A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

of the lower hypolimnion Žbelow 200 m. is marked by two opposite effects: vertical mixing stimulated by geothermal heat flux from the sediments, and chemically induced stratification by accumulation of dissolved solids in the deep water. K z values of 5 to 7 = 10y5 m2 sy1 have been measured ŽWuest ¨ et al., 1992.. Above 50-m depth, diffusive fluxes are eclipsed by turbulent mixing of the surface water layer. As discussed earlier ŽSection 3.1., the annual standing stock of dissolved Si has remained constant in recent years. The steady-state situation is reflected by the vertical profiles of dissolved Si concentration being almost invariant, with homogeneous concentrations below 150-m depth, a steep but stable gradient between 50 and 120 m, and seasonal uptake above 50 m ŽFig. 3.. Estimation of Jz between 50 and 100 m depth using K z s 0.13 " 0.03 cm2 sy1 gives an average diffusion flux of 6.9 " 0.7 g m2 yeary1 for the period 1989–1994. In 1993, the calculated flux was 6.4 " 1.5 g m2 yeary1 . When taking into account the lake surface at 75-m depth, 21.1 km2 , the total release amounted to 136 " 31 T in 1993. Table 5 records values of published Si sediment diffusion. Values range between 0.2 and 145 g m2 yeary1 . This wide variability is due to the many factors that affect sediment diffusion, such as production of biogenic silica, seasonal changes, lake morphology and depth, but also oxidising conditions ŽMortimer, 1942. and temperature ŽYamada and D’Elia, 1984.. It also reflects the determination method used. Determinations from pore water gradient data are known to give significantly lower values than laboratory core-based measurements ŽBelzile et al., 1996.. This is confirmed by pore water values in Table 5 being systematically lower than core values. Our value falls well within the range of laboratory core determinations.

4. Discussion Table 6 summarises our current understanding of the dissolved silica budget in the North basin of Lake Lugano. The primary source of dissolved silica in the lake is river input, with diffusion from sediment pore waters also playing a role. Input through atmospheric precipitation is negligible. Of the total

49

Table 6 Dissolved silica budget in the water column of the North basin of Lake Lugano in 1993 Process Inputs Atmospheric precipitation River inflow Sediment diffusion Total Outputs River outflow Diatom sedimentation Total

Dissolved silica ŽT yeary1 . 3"1 712"107 136"31 850"111

154"90 875"44 1029"100

amount of Si exported to the lake bottom through diatom sinking, only 16% is resupplied to the surface waters by benthic diffusion. Thus, it is river supply rather than bottom lake supply that modulates diatom production in the North basin of Lake Lugano, with the lake acting as an efficient Si scavenger. Although we have shown that at present steadystate conditions do occur in the lake, the data calculated for Si sources and losses do not satisfactorily close the Si mass balance ŽTable 6.. We suspect three possible reasons. Ži. Water inflow might be underestimated because neither groundwater from alluvial plains nor subsurface karstic waters have been considered. For example, in the crystalline area drained by the River Cassarate, 65% of the total annual precipitation was discharged via the river to the lake in 1993. The remaining 35% was retained on land and lost through evapo-transpiration, anthropogenic consumption and water deviations, or infiltrated to the groundwater body. No data on groundwater seepage from the alluvial plain to the lake are available. Similarly, the volume of karstic waters reaching the lake subaerially in the carbonate rock environments is not known. Because groundwater contributions to the inflow could not be taken into account in the lake water balance, the amount of inflowing Si given in Table 6 should be considered as a minimum estimate. A maximum estimate may be calculated by considering that the total yearly precipitation enters into the lake and has a silica content equal to the river average Ž1093 T Si yeary1 in 1993, bringing the total input

50

A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

up to 1232 T Si yeary1 and the total export to 1101 T Si yeary1 .. Obviously, the true input value will lie somewhere between the value given in Table 6 and this above maximum estimate. A fairly equilibrated Si budget of 1024 T yeary1 input and 1063 T yeary1 output would be obtained for 1993 by making the hypothesis that 50% of the difference between annual precipitation in the river watersheds and annual river discharge actually reached the lake via groundwaters. Žii. The primary export of dissolved Si occurs via biogenic uptake and final burial of diatom frustules in the bottom sediment. Loss of dissolved Si through the lake outflow represents only approximately 15% of the total export. Our estimation of the loss by sedimentation is based on the measured fluxes of biogenic Si in the 30-m depth sediment traps at the Gandria sampling station. We assumed that sedimentation in this pelagic zone was representative of the whole lake uptakersedimentation regime. However, if biogenic Si production is lower in some areas Žparticularly the shadow-rich southern coast line., the measured fluxes may be overestimated. For example, a reasonable assumption of a 10% excess flux Žor 4 g my2 yeary1 . would lower the total output by 87 T yeary1 . A more accurate output budget would necessitate acquisition of more extensive field data sets. Žiii. An additional contribution to the dissolved silica budget may be the erosion of the steep coastline of siliceous carbonate rock. It is however not possible to give an estimate for this input. On the basis of the present data, we cannot identify conclusively the source of the errorŽs. in the final mass balance. The above considerations show, however, that the most plausible sources of error lie in an underestimation of the groundwater inflow, possibly combined with some overestimation of the sediment fluxes. As expected, Si behaves as a reactive element in the North basin of Lake Lugano. The overall steadystate residence time for Si is equal to the amount of dissolved Si in the lake divided by the input Žor the output. flux, which corresponds to approximately 7 years. This value is about half the water residence time, estimated at 12 years for the whole lake ŽBarbieri and Polli, 1992. and at 16–17.7 years below 80-m depth ŽWuest ¨ et al., 1992; AeschbachHertig, 1994.. The high Si residence time in the lake

may be explained by the strong physical stratification of the lake deep-water. The time necessary for the Si released from bottom sediments and the BNL to reach the lake surface can be approximated by the diffusion time t s L2r2 K z ŽSchwarzenbach et al., 1993.. K z values change with depth ŽFig. 8. with values of 0.55, 0.35, 0.13 and 0.1 = 10y4 m2 sy1 in the depth intervals 281–200, 200–120, 120–80, and 80–50 m, respectively. Calculations show that it takes about 8 years for a solute such as silicic acid to reach the 50-m depth level, where it is rapidly entrained to the lake surface by the late-winter deep surface mixing. This value is a maximum estimation since it does not take into account any corona effect Ži.e. supply of silica from lateral lake sediments. ŽBelzile et al., 1996.. Because the amount of dissolved Si below 50-m depth corresponds to 90% of total dissolved Si, diffusion limited transfer of dissolved Si explains its overall long residence time in the lake. If only the standing stock in the mixed layer Ž0–50 m. is considered in relation with total inroutput, the residence time of dissolved Si reduces to less than 1 year Ž0.7–1 year, depending on budget considerations., confirming intense planktonic uptake in the trophic layer. Silica concentrations in lakes are often linked to the trophic state of the water body. Observed decreases in silica concentrations in lakes have been related to eutrophication leading to a high diatom production ŽSchelske and Stoermer, 1971. and a concomitant dissolved silica consumption. In the North basin of Lake Lugano, dissolved Si has progressively accumulated in a growing anoxic layer of high physical stability that emerged in the 1960s in relation with increasing lake eutrophication. This evolution is shown in Fig. 9, where concentration profiles from October 1960r61 and 1993 are compared. These profiles show that total dissolved Si in the 100–284-m layer has increased by 30% during this period while concentrations evolved inversely in the layer above 100-m depth. Concentrations in the upper 100-m layer were higher in the early 1960s, most probably because Si recharge from depth was not limited by strong density stratification as observed nowadays Ževidenced by the strong concentration gradient existing at 80–100-m depth.. The evolution of Si profiles has run parallel to the evolution of lake diatom assemblages. Although historical

A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

51

5. Conclusions

Fig. 9. Depth profiles of dissolved Si in October 1960 and 1961 ŽVollenweider, 1965. and in October 1993 ŽHofmann, 1996..

records on diatom production are scarce, the species evolution since 1900 ŽPolli and Simona, 1992. indicates an increasingly nutrient-rich lake. Among the centric diatoms, Cyclotella predominated in the early decades of last century but declined since the sixties. Since then, Stephanodiscus hantzschii and more recently S. minutulus have strongly developed. S. minutulus reached 45% of the diatom biomass during the 1993 spring bloom ŽHofmann et al., 2001.. We suspect that the present situation, characterised by a strong spring diatom bloom followed by low diatom production until next winter ŽFig. 4, Hofmann et al., 2001., may be an adaptation to the current slow Si recharge conditions. However, the historical data available do not allow definitive conclusions to be drawn. The North basin of Lake Lugano is an efficient sink for dissolved Si, with only about 1r6 of the incoming load reaching the South basin. Because the North basin represents about 40% of the waters flowing into the South basin, the North basin probably has a strong influence on the productivity in the other basins of the lake. For artificial lakes, it has indeed been observed that the loss of silica in rivers due to trapping in reservoirs results in a shift of phytoplankton composition in downstream waters. The so-called Aartificial lake effectB has notably been shown in the Iron Gate–Danube–Black Sea system ŽHumborg et al., 1997.. In the case of Lake Lugano, such a shift would correspond to a natural, permanent effect that merits further investigation.

A budget for dissolved silica in the North basin of Lake Lugano has been satisfactorily established for 1993. This study has allowed the identification and quantification of the main processes affecting silica cycling in the lake: Ži. input from the watershed Žrivers and groundwater., Žii. uptake of dissolved silica by diatoms in the productive surface layer, Žiii. settling of diatom frustules and burial of silica in the bottom sediment, Živ. dissolution and diffusion from the sediment back to the water column and, finally, Žv. transport of dissolved silica from the North to the South Basin. Among these processes, the riverine input is the major Si source whereas burial in the sediment is the major Si sink. However, the one-box approach somewhat hides the complexity of silica cycling in the North Basin of Lake Lugano. Thus, it is not reflected in the model that the high residence time of dissolved silica in the lake Žabout 7 years. is related to the slow silica exchange between a heavily stratified deep-water body that acts as a reservoir, and the well-mixed surface layer. In the upper layer, the silica residence time is less than 1 year, suggesting silica to be a limiting factor in diatom production. The relationship between dissolved silica and the trophic state of the lake merits further investigation. Better knowledge of dissolved silica input sources, particularly of the role played by groundwater Si fluxes, coupled with a comprehensive monitoring of Si concentration and data on diatom production in the mixed layer would allow silica-linked processes to be better placed in the context of lake eutrophication.

Acknowledgements This study was possible thanks to J. Dominik ŽUniversity of Geneva. who directed the trap research project on Lake Lugano. The study was partly funded by the Swiss National Science Foundation Ž20-33569.92 and 20-37621.93.. Thanks to P. Arpagaus, C. Martin, D. Span and B. Villars ŽInstitute F.-A. Forel, Geneva. and C. Crivelli ŽIstituto Cantonale Batteriosierologico, Lugano. for the help with conducting the boat and sampling on site. R. Peduzzi

52

A. Hofmann et al.r Chemical Geology 182 (2002) 35–55

ŽICB, Lugano. kindly facilitated arrangement of laboratory space in Lugano and P. Arpagaus and P.Y Favarger ŽInstitute F.-A. Forel. assisted with laboratory work. We thank A. Barbieri and M. Simona ŽLSA, Lugano. for providing data on dissolved silica and phytoplankton. We are grateful to J. Zobrist ŽEAWAG, Zurich ¨ . for providing unpublished data on dissolved silica in atmospheric deposits and to W. Aeschbach-Hertig ŽEAWAG. for the data on K z values in the Lake. We would like to thank M. Thuering ŽIstituto di scienze della terra, Cadenazzo. for the assistance with geological and hydrological material of Canton Ticino.

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