Most of the silicon (Si) in marine coastal systems is thought to recirculate under the biological control of planktonic diatoms. We challenge this view after comparing the biogenic silica (bSi) standing stocks contributed by communities of planktonic diatoms and benthic sponges in five habitats of an extensive continental shelf area of the Mesoamerican Caribbean. In most habitats (outer reefs, patch reefs, sea grass beds, and mangroves), the sponge bSi stocks surpassed those of diatoms. Collectively, bSi in sponge communities was about 88.6% of the total Si pool. Diatoms represented 4.2% and ambient silicate about 7.2%. Consequently, when constructing future regional Si budgets in coastal areas, the Si standing stocks in sponge populations should be empirically examined before deciding that their contribution to the total is negligible. In order to understand Si fluxes in coastal areas where sponges are relevant, we need additional empirical approaches to set the timescale of sponge bSi turnover, which appears to be substantially slower than that of diatom bSi.Silicate, a dissolved form of silicon (Si), is a major ocean nutrient. It fuels primary production by enhancing growth of diatoms, which require silicate to construct their skeletons of biogenic silica (bSi). Therefore, there is enormous interest in predicting the interplay between silicate and bSi budgets. Ever since the earliest models attempting to establish a general balance for Si in the ocean (Harriss

This study investigated the thermal regime of shallow groundwater in the Turin area (NW Italy), where the large energy demand has driven a new interest for ground-source heat pumps (GSHPs). The strongest vertical variability of groundwater temperature is found within 10-20 m below ground surface. In spring, deeper temperatures are higher than shallow temperatures, while in autumn the trend is reversed. These variations are connected with the heating and cooling cycles of the ground surface due to seasonal air temperature oscillation, propagating into the aquifer.The areal temperature distribution shows an increase from the foothill sectors close to the Alps towards the central Po Plain, driven by the progressive warming along the flow path.In the Turin city aquifer temperatures are 0.6 ÷1.6 °C higher than rural sectors. This groundwater warming is linked to the urban heat island effect, mainly driven by the typical artificial land use. Sparse warmer outliers (16-20 °C) are in some cases connected to documented point heat sources, such as GSHP systems, industrial districts and landfills.

Pump and treat (P&T) systems are still widely employed for the hydraulic containment of contaminated groundwater despite the fact that their usage is decreasing due to their high operational costs. A way to partially mitigate such costs, both in monetary and environmental terms, is to perform heat exchange (directly or with a heat pump) on the groundwater extracted by these systems, thus providing low-carbon and low-cost heating and/or cooling to buildings or industrial processes. This opportunity should be carefully evaluated in view of preserving (or even improving) the removal efficiency of the remediation process. Therefore, the heat exchange should be placed upstream or downstream of all treatments, or in an intermediate position, depending on the effect of water temperature change on the removal efficiency of each treatment step. This article provides an overview of such effects and is meant to serve as a starting reference for a case-by-case evaluation. Finally, the potentiality of geothermal use of P&T systems is assessed in the Italian contaminated Sites of National Interest (SIN), i.e., the 41 priority contaminated sites in Italy. At least 29 of these sites use pumping wells as hydraulic barriers or P&T systems. The total discharge rate treated by these plants exceeds 7000 m3/h and can potentially provide about 33 MW of heating and/or cooling power.

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