Aquatic subterranean food webs: A review

The aquatic subterranean ecosystems represent research frontiers for ecology and conservation biology. The aquatic subterranean fauna and associated microorganisms are organised in food webs that are essential in the nutrients cycling and energy fluxes within habitats generally characterised by low resources. However, the knowledge of how these trophic networks are structured and conditioned by the interaction with surface ecosystems is scarce. Traditionally, subterranean aquatic food webs were regarded as simple and truncated because of low species diversity and abundance compared to surface. The current review provides an updated description of aquatic subterranean food webs, based on the latest findings from various types of habitats: cave streams, the hyporheic zone and phreatic aquifers, with the general conclusion that such trophic networks are much more complex and dynamic as historically believed. The energy-limitation hypothesis and the bottom-up forces are increasingly recognised as the main structuring agents of the aquatic subterranean trophic networks. It is predicted that the bottom-up forces are generated by two interconnected factors: nutrients availability in water and quantity, quality and the types (i.e. surface derived photosynthetic and detrital, in some cases mixed with in situ chemosynthetically based organic matter) of basal energy input. Further recommendations for the integration of groundwater research into the current overarching concepts of surface ecology are made, given this field of science remains currently rather descriptive and less hypothesis-driven.