## 1 Introduction
In dark conditions, on the other hand, photosynthesis does not occur; the diatom filter is inactive, so the DSi is released back to the water column from sediments [23]. Exchanges from sediment to the water column can be further accelerated by benthic fauna bioturbation [24]. The littoral zone, either with MPB or SAV and helpbytes, is a biogeochemical hotspot, where a greater amount of primary productivity and nutrient recycling takes place, together with faster rates of mineralization, when compared to the contiguous pelagic and terrestrial areas [25; 26; 27]. The littoral zone, however, is heterogeneous and patchy. In fact, there is a mosaic of diverse conditions underlying multiple and differentiated ecological functions on account of the substrate structure, the physical and chemical conditions in the sediment and water column, disturbance regimes, and inputs of organic matter and nutrients, which change radically over time and space [17]. Whereas bare and soft sediments are largely colonized by benthic microalgae, macroalgal aggregates and phaneroagam meadows, rocky shores, and hard-substrates host epilithic algae, depending on light penetration and nutrient availability. The magnitude of processes driving Si dynamics is thought, therefore, to vary in different microhabitats colonized by different primary producers. As far as we know, there are very few lake studies regarding the direct measurements of BSi accumulation and DSi fluxes across the water-sediment interface, all focusing on bare sediments colonized by MPB [28; 29], while further research on SAV and epilithic macro- and microalgae (EA) needs to be carried out. In littoral areas of lakes, a major focus on the magnitude of DSi exchange is not only necessary given its general importance in ecosystem functioning, but also because of its influence on N:P:Si stoichiometry, as Si shortage can shift phytoplankton composition towards communities dominated by noxious or harmful algae [30]. Unlike other phytoplankton taxa, diatomsand a few other microalgae, such as Crysophyyceans, require proportionally more Si relative to N and P, with Si becoming a frequent factor in growth limitation [1; 2]. Only a few studies have examined the stoichiometry of nutrient fluxes in salt and freshwater marshe [31; 32], but no research has analysed DSi fluxes and BSi accumulation in the littoral zone of lakes in relation to N and P.  
This study focuses on BSi and DSi storage and fluxes in the littoral zone of Lake Iseo, a deep sub-alpine lake located in the north of Italy [13]. The purpose of this research is twofold. First, we aim to analyze stocks, transformations, and fluxes of BSi and DSi during the growth phase of benthic vegetation in different habitats of the littoral zone colonized by diverse primary producer communities. Our second aim is to analyze and compare the stoichiometry of stocks and fluxes in order to understand how different benthic vegetation communities influence N:P:Si ratios. We postulate that the littoral zone of lakes is a natural filter, where DSi is assimilated as BSi by macroalgae and microbytbethos from the water column, and by rhizophytes from pore-water. More specifically, our hypothesis is that such BSi and DSi transformations can significantly modify the relationships between the reactive forms of Si, N, and P, thus affecting their stoichiometry and, ultimately, the responses of primary producer communities. Consequently, the main objectives of this study were: (a) to estimate the extent of BSi accumulation in the biomass of the most representative primary producers at the local level; (b) to compare BSi accumulation in sediments colonized by different benthic vegetation; (c) to quantify DSi fluxes across the water sediment interface in relation to the dominant primary producers; and (d) to evaluate how heterogeneity affects Si dynamics and stoichiometry relative to reactive N and P.