A modelling approach to evaluate the present and future effectiveness of hypolimnetic withdrawal for the restoration of eutrophic Lake Varese (Northern Italy).

Hypolimnetic withdrawal has been applied as a restoration measure in lakes subject to eutrophication together with external load reduction, to decrease internal load by removing limiting nutrient phosphorus (P) from anoxic deep waters and contributing to the unloading of bottom sediments from previously deposited nutrients and organic matter. The aim of this study is to evaluate the effect of hypolimnetic withdrawal on Lake Varese, a 24 m-deep and 14.8 km2-large subalpine lake in North-Western Italy. The lake suffered from extended eutrophication in the second half of the 20th century due to uncontrolled delivery of untreated urban sewage. Several restoration measures have been implemented during the years, including hypolimnetic withdrawal. In 2019, a cooperative programme for the protection and management of the lake and its surroundings was launched, establishing a systematic annual hypolimnetic withdrawal in the stratified season since 2020. In this research, we calibrated a one-dimensional (1D) coupled ecological-hydrodynamic model (General Lake Model/Aquatic EcoDynamics - GLM/AED2) of Lake Varese with data surveyed in the lake in 2019-2021. Model simulations of the period 2020-2021 with and without the performed withdrawal proved the effectiveness of this measure on hypolimnetic P concentration reduction. Then, future simulations of 2023-2085 were carried out to predict the future efficiency of hypolimnetic withdrawal and of reductions in external nutrient loads under climate change scenarios. Results show that the prescribed withdrawal increases hypolimnetic temperatures. This effect, coupled with thermocline deepening due to global warming, will possibly lead to decreasing water mass stability in autumn and shorter stratification in the moderately deep Lake Varese, with an eventual decrease of P concentrations in the water column. The future effectiveness of hypolimnetic withdrawal is further discussed considering the possible role of dry periods.

PCDD/Fs, DL-PCBs and NDL-PCBs in European catfish from a northern Italian lake: the contribution of an alien species to human exposure.

PCDDs/Fs, DL-PCBs and NDL-PCBs are environmentally persistent substances that have been associated with adverse effects on human health. Humans are mainly exposed to these pollutants through ingestion of contaminated fish and fishery products; the consumption of fatty fish such as European catfish can contribute considerably to the intake of dioxins and PCBs. Samples of fish muscle of the top-of-the-food-chain predator Silurus glanis from the northern Italian Lake Varese were analyzed to detect the levels of 17 PCDD/F and 18 PCB congeners. All samples presented detectable levels of PCDD/Fs and PCBs, with concentrations ranging from 0.001 to 1.310pgg(-1) wet weight (w.w.) for Σ PCDD/Fs, 0.031 to 21.000pgg(-1) w. w. for Σ DL-PCBs, and 0.397 to 117.910ngg(-1) w. w. for Σ NDL-PCBs. One sample exceeded the maximum levels of 6.5pgg(-1) w. w. for the sum of PCDD/Fs and DL-PCBs set by European regulations for fish (1259/2011 EU), while other samples exceeded the action levels of 0.75pgg(-1) w. w. for the sum of PCDD/Fs, and 2pgg(-1) w. w. for the sum of DL-PCBs (277/2012 EU). Consuming contaminated catfish may pose a risk for human health, especially for the subpopulation traditionally accustomed to eating this fish, as well as sensitive individuals, such as children and pregnant women.