Quantifying the impact of freshwater diatom productivity on silicon isotopes and silicon fluxes: Lake Myvatn, Iceland

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Abstract

Diatom productivity in the oceans plays a crucial role in the carbon cycle, but is strongly dependent upon the continental silicon supply. However, the relative influence of weathering and biological processes on continental Si fluxes remains poorly constrained. This study aims to quantify the impact of terrestrial diatom productivity on Si fluxes to the ocean. Lake Myvatn in North Iceland is one of the most productive lakes in the Northern Hemisphere, with nutrient-rich waters almost uniquely sourced by groundwater. The primary production is mainly controlled by diatom growth but also by cyanobacteria, and the lake output is via a single river, thereby providing a relatively simple natural laboratory to quantify the impact of diatom growth on the chemistry and Si budget of lake waters. Silicon stable isotopes (δ30Si) provide a tracer of this biocycling, and have been measured in groundwater inputs to the lake, and in time-series monitoring of waters at the lake outlet. The δ30Si values at the outlet range from + 0.70 ± 0.08 to + 1.42 ± 0.06‰, which is significantly heavier than the groundwater input (average cold and hot springs: + 0.50 ± 0.17‰, 2SD) and consistent with the preferential uptake of light Si isotopes by diatoms. The δ30Si value at the outlet increases by up to 0.9‰ in spring and autumn relative to the Si isotope composition of the inflow. These seasonal diatom blooms can be modeled by an open system of Si uptake and affect Si fluxes at the outlet of the lake by up to 79%, or 53% integrated over the year. In the summer a shift to lighter δ30Si values is correlated with a higher pH, which results in dissolution of diatoms releasing light Si isotopes. From mass balance, this seasonal diatom dissolution affects Si fluxes by up to 33%, but is limited to 3.7% integrated over the year. These results clearly illustrate that biological activity can have a significant impact on both isotope composition and elemental abundance of continental derived Si. They also demonstrate the pH dependency of diatom dissolution and/or preservation, which is likely to affect not only the continental Si fluxes to the ocean but also the Si recycling in the oceans themselves.

Research Highlights

► Freshwater diatoms affect continental silicon isotope signal and Si fluxes to ocean. ► Evidence of the pH dependency of diatom recycling rates in Lake Myvatn, Iceland. ► Si isotope mass balance highlights up to 33% of biogenic Si recycling in the summer. ► pH forcing on BSi recycling rates in lakes is crucial for diatom Si supply in ocean. ► Ocean acidification would reduce diatom productivity and oceanic C storage capacity.

Introduction

On geological timescales the chemical weathering of silicates provides a natural means of regulating global climate (e.g. Berner, 1995, Kump et al., 2000, Walker et al., 1981), by converting atmospheric CO2 to bicarbonate ions, which along with metals, are transported to the oceans by rivers where the CO2 is fixed by formation of carbonates. On millennial timescales changes in the flux of alkalinity and nutrients to the ocean from continental weathering can also affect biological productivity and carbonate sedimentation (e.g. Archer et al., 2000), which can in turn influence atmospheric CO2.

Diatom growth is a major process controlling the capacity of the ocean to store CO2 (Ittekkot et al., 2006, Smetacek, 1999). These are the most prevalent silicifying organisms in oceans, using silica as rigid structural material for their cell walls (frustules). Biogenic silica (BSi) production by diatoms in the ocean has been estimated at around 240 Tmol yr 1 (Tréguer et al., 1995), with diatoms also constituting ~ 45% of total marine primary production. Diatoms are extremely sensitive to Si limitation and consume silicon from both continental input and Si recycling in the ocean (Ragueneau et al., 2006). Recycling of Si in the water column and at the sediment–water interface plays a crucial role in sustaining production (Ragueneau et al., 2006). Globally, ~ 50% of the BSi produced by diatoms redissolves in surface waters (Nelson et al., 1995), and up to ~ 90% may redissolve prior to reaching the seafloor (Nelson and Brzezinski, 1997, Tréguer et al., 1995). The dissolution of biogenic opal is mostly driven by the undersaturation of the ocean with respect to silicic acid concentrations (Brzezinski et al., 2003), and is affected by a number of factors including temperature, pH, specific surface area, aluminum concentrations, and organic material coating (Van Cappellen and Qiu, 1997, Van Cappellen et al., 2002).

Chemical weathering of the continents is thought to contribute some 85% of the dissolved silicon in the oceans (Tréguer et al., 1995), sourced by rivers, groundwaters and aeolian dust. Recent studies have indicated a marked decline in riverine Si fluxes (Humborg et al., 2000, Humborg et al., 2002, Ittekkot et al., 2006, Laruelle et al., 2009) that may have severe consequences for oceanic diatom production. However, the relative influence of inorganic and biological processes on the continental weathering signal remains poorly constrained. Silicon isotopes potentially provide key information on these biogeochemical processes because they are fractionated both by inorganic chemical weathering processes on the continents, and by biological processes, in particular diatom growth in the marine and terrestrial environment. In fresh waters (rivers and lakes), Si stable isotopes have been shown to be a promising tracer for quantifying the impact of biological activity on the Si budget (Alleman et al., 2005, De La Rocha et al., 2000, Ding et al., 2004, Georg et al., 2006a, Georg et al., 2007). Likewise, in the oceans Si isotope fractionation has been related to diatom activity (Cardinal et al., 2005, Cardinal et al., 2007, Fripiat et al., 2007, Reynolds et al., 2006, Varela et al., 2004). Indeed, Si isotopes appear to be strongly fractionated during incorporation into diatoms, with no dependence on temperature or species (De La Rocha et al., 1997), pH (Milligan et al., 2004) or salinity (Alleman et al., 2005).

The sub-Arctic Lake Myvatn is one of the most productive lakes in the Northern Hemisphere. The lake is unique in that almost all of the inflow is supplied through groundwater (both high- and low-temperatures) fed through artesian springs, and negligible surface water enters the lake because the area is covered by young and porous lava fields and cross cut by faults (Kristmannsdóttir and Ármannsson, 2004, Ólafsson, 1979a). Therefore, the inflow of water to the lake and its chemistry is remarkably stable (Kristmannsdóttir and Ármannsson, 2004). Moreover, the residence time of water in the lake is short (27 days) and there is a single outflow from the lake, via three channels to a single major river (Ólafsson, 1979a). Consequently, temporal variations in the chemistry of the waters can be readily monitored and Lake Myvatn, therefore, constitutes a remarkable natural laboratory to study the impact of terrestrial biogeochemical processes on Si fluxes to the ocean. The high productivity of Lake Myvatn is dominated by the growth of diatoms, therefore it can be anticipated that this will result in the preferential incorporation of light Si isotopes into the diatoms imparting a distinctive heavy Si isotope signal to the residual lake waters.

This study aims to assess the impact of diatom productivity on Si stable isotopes in lake waters relative to the composition of groundwater input, the principal source of Si in the waters of Lake Myvatn, and to further quantify the impact of diatom growth on Si fluxes from the lake. Silicon isotope compositions have been measured on the groundwaters and bottom sediments. The impact of seasonal diatom blooms has been assessed through time-series monitoring of Si isotope compositions of the outflow from the lake. These results contribute to a better understanding of the impact that lake biogeochemistry has on continental Si fluxes from rivers to oceans. Since 16% of the gross riverine Si load is delivered to the ocean as BSi (Conley, 1997), diatom uptake and dissolution in continental rivers and lakes have a potentially significant impact on silicon fluxes to the ocean, and hence the carbon storage capacity of the ocean.

Section snippets

Environmental setting

Lake Myvatn is located in northeast Iceland (65°35′N and 17°00′W) just below the Arctic circle, and is at 278 m above sea level. The lake is relatively shallow with a maximum depth of 4.2 m and is well-mixed over the entire water column during the ice free period (Kjaran et al., 2004). The area of the lake is 37 km2 and comprises two basins, Ytrifloi (North Basin) and Sydrifloi (South Basin). Average sediment thickness in the South Basin is about 4.3 m. Diatom frustules comprise about 55% and rock

Groundwaters, lake waters and sediments

Samples were collected on 26–27 August 2009, including groundwater from cold (MY01 to MY07) and hot springs (MY08 to MY11), middle lake water (MY12) and water from the outlet of the lake (MY13) (Fig. 1). Water was collected in acid pre-cleaned polypropylene bottles and stored in the dark at 5 °C. At each sampling site, temperature, pH, and conductivity were measured in situ. Lake and outlet water samples were filtered in the field within 24 h through 0.2 μm cellulose acetate membranes. Filters

Groundwaters: major and trace elements and Si isotope analysis

Two major types of groundwater can be distinguished from the inflow to Lake Myvatn: the cold springs (MY01 to 07) from the south east, and the hot springs (MY08 to 11) from the north east. Despite the significant chemical differences between cold and hot springs, the silicon isotope composition of groundwater is nearly constant among these sources. The former are characterized by a mean temperature of 6.2 ± 1.4 °C, pH of 9.4 ± 0.2, and a low conductivity (134 ± 14 μS cm 1) (Table 1). The latter display

Seasonal variations of chemistry and Si content in Lake Myvatn

The chemistry of the groundwater samples studied here is consistent with previous studies where both the temperatures and pH are similar (Kristmannsdóttir and Ármannsson, 2004, Ólafsson, 1979b). The Si concentrations in the cold groundwaters (0.31 ± 0.02 mM; Table 1) are within the narrow range of published values from 0.30 to 0.37 mM (Ólafsson, 1979b, Kristmannsdóttir and Ármannsson, 2004; respectively) (Fig. 6). In contrast, the Si content of the hot springs displays a much larger range (1.76 ± 0.55

Conclusions

This is the first study using silicon isotopes to estimate biogeochemically relevant rates for Si cycling dynamics. Moreover, this work provides some insight into an important, but largely unquantified, aspect of the global silicon cycle, namely processes that occur at the terrestrial/marine interface. The objective of this study was to assess the impact of diatom productivity on Si stable isotopes in Lake Myvatn relative to the groundwater input, the principal Si source in the lake. The Si

Acknowledgements

We would like to thank F. Mokadem, N. Belshaw, S. Wyatt (University of Oxford, UK) for their help in the completion of this project, A. Iserentant (Université catholique de Louvain, Belgium) for XRD measurements, Sam Hammond (Open University, UK) for ICP-MS analyses, M. Hermoso and N. Charnley (University of Oxford, UK) for their help with SEM images, and D. Cardinal and H. Hughes (Royal Museum for Central Africa, Belgium) for the Quartz Merck. The manuscript has greatly benefited from the

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