Decoupling of silica, nitrogen and phosphorus cycling in a meromictic subalpine lake (Lake Iseo, Italy)

Abstract

Silica (Si), nitrogen (N) and phosphorus (P) loads and stoichiometry are key factors controlling the trophic status of lakes and coastal seas. In the hydrographic network, lakes also act as biogeochemical reactors, controlling both nutrient retention and fluxes. This work aimed to examine the coupling of Si, N and P cycling, together with their stoichiometry in a deep meromictic subalpine lake (Lake Iseo, Northern Italy). Si, N and P mass budgets were calculated by quantifying loads in the inlets and in the outlet over a period of 30 months (May 2016−October 2018), in-lake sedimentation rates and net nutrients accumulation in the water body. Lake Iseo acts as a biogeochemical filter, which differentially retains the external Si, N and P loads. Retention of Si and P was similar (75–79%), but considerably higher than N (45%), evidencing a decoupling of their fate due to in-lake processes. This differential retention is likely to be exacerbated by meromixis which enhances Si and P accumulation in the monimolimnion, while impairing denitrification, thus limiting N removal. Such decoupling resulted in an increase of the N:Si and N:P ratios in both the epilimnion and in the outlet compared to the inlets, whereas the ratios decreased in the monimolimnion. As a result, there may be a stronger Si and P limitation of the photic zone, leading to a shift towards more oligotrophic conditions. This transient equilibrium could be impaired in the case of water overturn produced by extreme climate events—a highly relevant issue, considering that a growing number of deep lakes are turning from holo-oligomictic to meromictic as a result of combined eutrophication and climate change.

Decadal changes in surface CO2 concentrations and CO2 fluxes in a mountain lake

“…Water column mixing does not only transport DO to the hypolimnion (Flaim et al 2020), it also transports deep water CO 2 to the surface (Baehr and DeGrandpre 2004; Kuss et al 2006; Saber et al 2020). Silica is retained in the monimolimnion of meromictic lakes and upwelling can increase its bioavailability (Scibona et al 2022). Thus, we suggest that hypolimnetic DO and silica are proxies for CO 2 upwelling, an internal process determining surface CO 2 .…”

Coping with Stress as a Prelude of Naturalization: Different Responses of Lagarosiphon Species to Water Trophy

“…Controlled amounts of NH 4 Cl and KH 2 PO 4 salts were added to water in order to simulate four different conditions of trophy, based on different concentrations of total N and total P. Reference concentrations for each level of trophy were based on the classification of trophy status by the Organisation for Economic Co-operation and Development (OECD) [36] with a calibration based on the trophy status of different prealpine lakes (Lombardy region, Northern Italy) [37,38]. Reference values were established from water bodies where L. major is established and often invasive (e.g., Maggiore, Iseo, Monate, Garda lake) [39][40][41].…”

Anthropogenic Influence and Climate Change on Water and Nutrient Dynamics in the Kebir-Rhumel Basin, (Northeastern Algeria)

“…The global in ux of mineral nutrients, like phosphate and nitrate, into the oceans has increased by over two times (Ittekkot et al, 2000).The ecological status of lakes and coastal waters is determined by the balance of silicates (Si), nitrogen (N), and phosphorus (P) loads and their relative proportions. These bodies of water act as biogeochemical reactors within the hydrological network, controlling the retention and movement of these essential nutrients (Scibona et al, 2022).Most research on eutrophication causes has centered on nitrogen and phosphorus, primarily because human activities release both nutrients. However, silicate is also crucial for algae growth and species composition.…”