Little janitor of the springs

North Florida has the world's highest concentration of large freshwater springs. For decades, crystal-clear water bubbling from the ground has driven tourism in the form of scuba divers, canoeists, boaters and swimmers, but today, many of those springs don't bubble like they used to; green scum in form of algae often obliterates the view.

Although the blame for algae-choked springs is often pinned on excess nitrate, new research indicates that the absence of algae-eating native freshwater snails of the genus Elimia may be a key factor, actually so important that Elimia species could serve as algae control agent and thereby restoring the springs' health. The first author of the new study affectionately calls them "little janitor of the springs".

Elimia species are major players in freshwater systems throughout central and southeastern North America. In quite a few streams in the southeastern United States they can represent more than 90% of the total macroinvertebrate biomass. The region is also the geographic centre of diversity of the genus. In this region 76 species of the 83 described occur, 15 have DNA Barcodes as the result of a ten-year old taxonomic study.

But how did the snails disappear from Florida's freshwater springs in the first place? The present study and others cited therein suggest pesticides and herbicides could be partly to blame for the snails’ decline but the researchers also found a connection between snail population size and the level of dissolved oxygen. The amount of dissolved oxygen could be influenced by (i) droughts or anthropogenic use of younger (shallower) aquifer water leading to relative dominance of older deeper flowpaths and (ii) groundwater organic matter enrichment leading to higher microbial respiration and oxygen depletion.

Given that dissolved oxygen appears to influence the grazers abundance it seems wise to ensure adequate flows by changing human water consumption patterns, and to reduce the amount of organic pollutants (e.g. from septic tanks, wastewater sprayfields). The authors also suggest to reintroduce native gastropods wherever they biomass falls below a certain threshold. The conclude that breeding and restocking native grazers, especially Elimia sp., to their historic ranges and densities may be an effective restorative intervention.