Groundwater is a vital ecosystem of the global water cycle, hosting unique biodiversity and providing essential services to societies. Despite being the largest unfrozen freshwater resource, in a period of depletion by extraction and pollution, groundwater environments have been repeatedly overlooked in global biodiversity conservation agendas. Disregarding the importance of groundwater as an ecosystem ignores its critical role in preserving surface biomes. To foster timely global conservation of groundwater, we propose elevating the concept of keystone species into the realm of ecosystems, claiming groundwater as a keystone ecosystem that influences the integrity of many dependent ecosystems. Our global analysis shows that over half of land surface areas (52.6%) has a medium‐to‐high interaction with groundwater, reaching up to 74.9% when deserts and high mountains are excluded. We postulate that the intrinsic transboundary features of groundwater are critical for shifting perspectives towards more holistic approaches in aquatic ecology and beyond. Furthermore, we propose eight key themes to develop a science‐policy integrated groundwater conservation agenda. Given ecosystems above and below the ground intersect at many levels, considering groundwater as an essential component of planetary health is pivotal to reduce biodiversity loss and buffer against climate change.

Coastal lagoons are threatened by natural and anthropogenic factors that have been accelerated in the last decades, with detrimental effects on ecosystem and water quality, especially in sheltered systems. Eutrophication, macroalgal blooms, hypoxia and anoxia are among the main threats, which also impact on human activities, e.g. aquaculture and fishery. Study were performed from late 1980s to date in several Mediterranean lagoons, mainly in the Po river delta, in order to elucidate the main causes of such degenerative processes. Here we review (1) relationships between benthic vegetation and sediment biogeochemistry, with focus on factors determining eutrophication; (2) development of macroalgal blooms, with focus on causes and biogeochemical triggers of bloom onset; (3) perturbation of nitrogen cycling due to changes in benthic vegetation assemblages; (4) relationships between primary producers and benthic fauna, especially farmed species; (5) possible feedback loops among shellfish and primary producers.

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.

We present a band ratio algorithm based on remote sensing reflectance (RRS) data to detect an algal bloom composed of cyanobacteria (Planktothrix spp.) and chrysophytes in Lake Idro, a small meso-eutrophic lake situated in the subalpine region (northern Italy). The bloom started around the first week of September 2010 and persisted for about 1 month, with highest mean chlorophyll-a concentrations (17.5 ± 1.6 mgm⁻³) and phytoplankton cellular density (7,250,000 cell·l⁻¹) measured on September 14, 2010. RRS data obtained from in situ measurements were first investigated to select the diagnostic wavelengths (i.e., 560 and 620 nm) of both phycoerythrin (present in the Planktothrix spp.) and other pigments (e.g., fucoxanthin, common to several species of chrysophyte). Testing the algorithm on RRS data derived from atmospherically corrected image data showed the ability of the medium resolution imaging spectrometer (MERIS) to detect the bloom also. The results demonstrate that a combination of in situ and MERIS data is a valuable tool to monitor the extent and duration of phytoplankton blooms.