## 4 Discussion
### The Fate of DSi in the Littoral Zone in Relation to Submerged Aquatic Vegetation  
Human activity and its impact on lake ecosystems is mainly concentrated along the shore, where the coupling of terrestrial and aquatic systems is particularly evident [86]. Urban development, fluctuation of water levels, tourism, and pollution from diffuse or point sources contribute to the direct or indirect impairment of the structure and composition of benthic primary producer communities. Our results pose the question of whether modification of the littoral zone alters its filtering capacity and ultimately the availability of DSi in the water column.  
The filtering capacity of littoral zones can be impaired by eutrophication and degenerative processes resulting from organic enrichment [69; 87]. Like many other littoral zones, the southern part of Lake Iseo is under the influence of coastal runoff [88]. Such stressors can affect the health status of the extensive SAV meadows, which also undergo a great inter-annual variability [79]. Here, in soft, shallow sediments, meadows of _V. spiralis_, and to a much lesser extent _N. marina_, can produce dense biomass covering up to 100 ha, while zones with sparse SAV usually spread over an additional 300-400 ha [79]. In eutrophic waters, _V. spiralis_ develops a large leaf mass, which is known to detach and drift due to waves and currents [78; 89]. In the River Mincio, close to Lake Iseo, during the maximum growth phase, a very rapid leaf renewal was reported with plastochrone intervals of \(\approx\)3 days [78]. Under these conditions, the standing biomass is affected by a consistent leaf loss, resulting in a floating mass, which can also be detected in satellite images [79]. In addition, _V. spiralis_ often undergoes uprooting, which exposes bare sediments to microhytobenthos colonization. Urrooting can be triggered by the organic enrichment of sediment, which stimulates oxygen consumption, exceeding the capacity of _V. spiralis_ to supply oxygen to the rhizosphere [69]. The oxygen balance in the rhizosphere can be also impaired by epiphytes growing on leaves as they settle, thus accreting organic deposits on the surface sediment [90]. Simultaneously, the epiphyte cover can limit light accessibility to _V. spiralis_, negatively affecting photosynthesis and oxygen transport to roots, which triggers a negative feedback  
Figure 7: Average Si content in sediments and Si benthic fluxes under light and dark conditions in the three sampling sites. Error bars represent standard error.  
loop leading to plant dieback [69; 87]. In 2017, in the littoral meadows of Lake Iseo, a bare sediment zone up to 67 ha was exposed as a result of uprooting, and was probably replaced by MPB [79]. In the light of this development, the question arose as to how this large-scale loss of SAV communities alters Si cycling at the land-water interface. When SAV was active, the DSi flux towards the water column was 12.8 g Si m\({}^{-2}\) during the entire summer, while in the same zone and during the same period, the bare sediment released only 3.9 g Si m\({}^{-2}\). The disappearance of pristine SAV communities could, therefore, potentially decrease DSi recycling and its availability in the water column, with direct repercussions on phytoplankton species composition, as Si shortage strongly impacts phytoplankton, driving the communities towards ones dominated by noxious or harmful algae [30].