## 4 Discussion
### BSi Storage in Primary Producer Biomass and Surficial Sediment and BSi, N, and P Stoichiometry  
Submerged vegetation in the littoral zone accumulated Si, but differences among species were observed. As far as we know, our study is the first to report data on BSi content in _Vallisneria spiralis_, since previously published data gathered from Lake Maggiore by [53] did not distinguish between BSi and other Si forms. In addition, we only found a single value (BSi = 0.02%) for _Najas marina_, which grew in a turbulent, turbid, and presumably light-limited aquatic system [54]. The BSi content of SAV species measured in this study falls in the lower range of values (0.7-2.8%) detected for phanerogam species in shallow freshwater ecosystems, including running waters [55; 19; 56]. Usually, macrophytes are expected to have a lower BSi content in lakes than in rivers, since physical stressors, e.g., current velocity and turbulence, can stimulate BSi assimilation, thus conferring resistance against drag forces [57]. In _N. marina_, the Si, N, and P stoichiometry in the above- and below-ground biomass was similar. On the contrary, _V. spiralis_ tended to accumulate more BSi in its leaves, also with respect to N and P, than in its roots. The difference between species can be explained by the ability of _V. spiralis_ to exploit deep sediment layers, thanks to its long roots. Studies in the same area of Lake Iseo demonstrate that DSi concentrations are greater in pore waters of bare sediments than in sediments colonized by _V. spiralis_[68]. SAV species can, therefore, increment the transport of Si from sediment to the water mass. This capacity to take up DSi is reduced by organic enrichment of sediment, as demonstrated for N and P [69]. In rocky shores, the strong dominance of diatoms and the high Si:N and Si:P ratios in macroalgae tissues can account for Si accumulation, exceeding N and especially P uptake, despite a lower plant biomass [23; 70; 71; 72]. We found that the P content in epilithic biomass was close to or below the lower limit of the optimal range for growth (0.01-0.02%) reported for epilithic primary producers [73; 74; 75; 76]. The actual molar C:N:P = 443:23:1 at RS also evidenced a P and, to a lesser extent, N shortage, when compared to the optimal ratio for periphyton growth, i.e., C:N:P = 119:17:1 [77]. In contrast with what we found at the RS site, SAV species at VS efficiently accumulated N and P. These elements exhibited a seasonal pattern similar to the BSi one, with a peak in the early phase and a sharp decrease in the later part of the season (Figure 2). Such a pattern can be explained by both leaf detachment [78; 79] and \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Reference** & **System** & \
\begin{tabular}{c} **Depth** \\ **(m)** \\ \end{tabular} & **Condition** &
\begin{tabular}{c} **DSi Flux** \\ **(mg Si m\({}^{-2}\) h\({}^{-1}\))** \\ \end{tabular} & **BSi** & **Method**\({}^{*}\) \\ \hline
[58] & lake & & & & 2.3–4.6 & 10.2–15.9 & 1 \\
[28] & lake & 3 & MPB & \(-\)1.6 to 11.5 & & \\
[59] & lake & & & & 2.6–11.8 & 1.0–22.0 & 2 \\
[60] & river & & & & 1.0–4.0 & & \\
[23] & coastal & & & MPB & light \(-\)0.01 \(\pm\) 0.08 & & \\  & lagoon & & & & dark 1.40 \(\pm\) 0.20 & & \\  & coastal & & & & 1.2–29.9 & & \\
[61] & lagoon & 2–4 & Uptake & 3.8–28.7 & & \\
[62] & lake & 5–10 & & & 3.1 \(\pm\) 0.3 & 1.7 & 1 \\
[29] & lake & 3.9 & MPB & light \(-\)2.3–21.9 & & \\  & & & & dark \(-\)2.0–15.13 & & \\
[63] & lake & 20 & & & 0.05 & 0.4–1.5 & 1 \\
[64] & lake & & & & 0.1–3.4 & 0.2–1.4 & 1 \\
[65] & coastal & 33 & & & 0.28–3.36 & & \\
[66] & coastal & 44–45 & & & 1.1–2.9 & & \\
[67] & costal & 2–7 & & & 0.7–5.7 & & \\ \hline \hline \end{tabular} \({}^{*}\) Method used to analyze BSi: Na\({}_{2}\)CO\({}_{3}\) (1) and NaOH (2).  
\end{table} 
Table 6: Sedimentary DSi fluxes (\(\mu\)g Si m\({}^{-2}\) h\({}^{-1}\)) and BSi content (%) in sediments of different aquatic ecosystems. Legend: MPB = microhypothesis, bare = bare sediment, algal = benthic macroalgae.