## 1 Introduction  
Silicon (Si) is an essential element for the growth of many aquatic primary producers like siliceous algae, especially diatoms, a key component of phytoplankton and phytotbethos communities [1; 2]. Despite its importance, Si biogeochemistry in aquatic environments is still poorly understood compared to N and P, limiting our capacity to explain the mechanisms regulating aquatic food web structure and functioning [3; 4]. The global Si cycle consists of continental and oceanic sub-cycles, which are connected through the hydrographic network [4]. Within this network, the dissolved silica (DSi) delivered to coastal seas is thought to depend mainly on biologically mediated processes through the synthesis and decomposition of biogenic silica (BSi), on interactions with the geosphere and, finally, on hydrology and land use [3; 5]. In this context, fluvial ecosystems connecting the continental and marine domains are not simply water conveyors, but rather comprise of a number of subsystems acting as filters and reactors, which strongly regulate Si transport [6]. These subsystems include lentite waters, especially wetlands and lakes, which may have a major effect on Si fluxes [4; 7; 8; 9]. For example, a relevant reduction of land to sea transport of Si has been measured in dammed rivers, such as the Danube, where up to two-thirds of DSi loading was retained by the Iron-Gate dam [10; 11]. Lake functioning depends on size and morphology, with a first distinction being between shallow and deep lakes. In shallow lakes, there is a prevalence of either microphytoethon's (MPB) or submerged aquatic vegetation (SAV), which control both primary productivity and biogeochemical cycles [12]. Large, deep lakes are dominated by phytoplankton communities, in which siliceous algae play a major role in primary productivity and control of nutrient cycling [13]. However, with regard to deep lakes, a distinction should be made between holomic lakes and oligomeric and meromictic lakes. In the last two categories, water stratification can persist over decades, thus increasing Si retention in the deepest water mass, as was found for other elements [14; 15]. The persistent stratification of the water mass can also impede \"benthic-pelagic coupling\", i.e., avoidance of recycling of the DSi regenerated from sedimentary BSi and amorphous silica to the photic layer. Studies on Si retention by lakes have been mainly concerned with mass balances, taking into account input-output differences over a consistent time scale [4]. This approach assumes that lakes are mixed reactors dominated by open waters, but neglects the role of other sub-system components, such as the littoral zone. This land-water interface is shallow, allowing light to penetrate to the bottom so that benthic primary producers develop [16; 17; 18]. Here, helpbytes and macrophytes synthesize and accumulate BSi, thus controlling both stocks and fluxes of DSi [3; 8]. Helpbytes and macrophytes take up DSi from pore-water and transform it into BSi, which concentrates in plant tissue for up to 3% of the total dry weight [19]. Once fixed in plant biomass, BSi is gradually released back into the abiotic environment as DSi through grazing and biomass decay [5; 8]. Fluxes and export of DSi can be enhanced by fungal and microbial activity, which stimulates BSi mineralisation and DSi release from detritus [20]. In the littoral zone, such Si filtering by primary producers is thought to affect the rate of DSi exchange across the water-sediment interface, with DSi being transported towards open water [3; 21]. DSi retention also depends on seasonal factors, especially the seasonality of growth and decay of littoral primary producers [8; 22]. In bare sediments, DSi fluxes are controlled by benthic diatoms and bioturbation driven by benthic fauna [23; 24]. DSi is assimilated to BSi by diatoms in light conditions, with the benthic system acting as a net DSi trap.