Abstract
A small standing-water ecosystem (SWE) is a shallow (<20 m) lentic water body with a surface of a few hectares (≤10 ha). Compared to larger counterparts, they exhibit wider ecotones, sometimes even equaling their whole surface, which maximizes structural heterogeneity, supporting exceptionally high biodiversity, metabolic rates, and functionality. Surprisingly, no binding regulations support global strategies for SWE conservation. This work consists of a literature review performed for the period 2004–2018 to assess the ecological and conservation value of SWEs and the contribution of the Water Framework Directive (WFD) in promoting their conservation. Outcomes from this work open new perspectives on SWEs, which emerge as valuable ecosystems, and confirm their pivotal contribution to watershed biodiversity, resilience, and functionality. Results also suggest clear narrative trends and large knowledge gaps across geographical areas, biological components, and target issues. Additionally, we note that SWEs are under-represented in the frame of WFD implementation, stressing their marginality into assessing procedures. All of this calls for further studies, especially outside Europe and with a global, multi-taxon perspective. These should be devoted to quantitatively assess the roles of SWEs in maintaining global water ecosystem quality, biodiversity, and services, and to prioritize management actions for their conservation.
Highlights
Small standing-water ecosystems (SWEs) contribute substantially to the global functioning of catchments, modulating nutrient retention and recycling along the hydrological transport pathways [1].Compared to larger lentic freshwater ecosystems, SWEs are characterized by a lower area/perimeter ratio that emphasizes the contribution of ecotonal zones to their metabolism and functioning, maximizing the importance of their role as biogeochemical reactors [2,3]
The Water Framework Directive (WFD) leaves Member States with the final decision to include these ecosystems in the national strategies for water body protection [22]. In light of these voluntary strategies, we explored the literature that considered SWEs within national implementation programs of the WFD to highlight the disproportionately small amount of resources invested for marginal and/or shallow water bodies compared to large, deep lentic waters
Little more than a decade has passed since the fundamental work by Oertli et al [54] on the growing interest in pond conservation issues, a topic launched in 2004 and implemented thanks to the development of the EPCN
Summary
Compared to larger lentic freshwater ecosystems, SWEs are characterized by a lower area/perimeter ratio that emphasizes the contribution of ecotonal zones to their metabolism and functioning, maximizing the importance of their role as biogeochemical reactors [2,3] They support high metabolic rates that are often intimately coupled with naturally high levels of nutrients and trophic conditions (i.e., eutrophy, hypertrophy) [4]. Strong evidence indicates that—especially in heavily impacted regions—small habitat patches are essential in sustaining varied and complex communities, such as small man-made (secondary) water bodies for plants and animals (e.g., hydrophytes, Odonata species) [6,7,8,9] In this sense, Hill et al [10] have verified the high taxonomic richness associated to urban ponds, found to be similar to that for nonurban ponds, opening new perspectives in conserving freshwater biodiversity in highly impacted and altered landscapes. Springs were recently confirmed by a study comparing large datasets of spring and stream diatoms to be refuges for sensitive and threatened diatom taxa [11], likely to host least-impaired habitat relicts (LIHRe), especially in densely inhabited and heavily impacted geographic areas
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