The role of shore flies on the preservation of microbialites in saline and highly alkaline lake environments

Reliable reconstruction of depositional environments of lake deposits depends on fossil content among others. However, in the conditions found in saline and/or highly alkaline lakes, the biota is very scarce as only some... [ view full abstract ]

Reliable reconstruction of depositional environments of lake deposits depends on fossil content among others. However, in the conditions found in saline and/or highly alkaline lakes, the biota is very scarce as only some species are adapted to these harsh conditions. This is the case of microorganisms, whether forming or not microbial mats, and some metazoan. The ephydridae (shore flies) are the benthic insect fauna typical of inland saline-water habitats. Both larvae and adults of ephydridae feed on the microbial mats. As a result, the larvae form distinctive gypsiferous meniscate back-filled burrows in gypsum precipitating lakes. Despite the wide distribution of shore flies, the types of traces produced by this insect in other type of lakes have been barely documented. This study is focused in the traces left by the shore flies in four endorheic lakes showing significant differences concerning the origin, substrate, water chemistry, depth and climatic conditions, which provides basis for comparisons and reliability. The lakes are two ephemeral and shallow water bodies in Central Spain. These are: El Longar, a meso to hypersaline lake where the mineral assemblages are dominated by sulphates, and Las Eras, a highly alkaline, brackish to saline water body that contains a mixed carbonate-sulphate mineralogy. The other examples come from two permanent and relatively deep pluvial lakes in USA, where typically precipitate carbonates. These are the Pyramid lake in Nevada (salinity <15 ‰) and the Mono Lake in California, a saline and alkaline lake (salinity 85 ‰; pH 9.8). In all the lakes, Ephydridae appear restricted to the shore from the late spring to early autumn. Their occurrence and abundance is related to environmental variables mainly food (diatoms and cyanobacteria) availability and quality as well as physical conditions mainly including temperature, salinity and lower water levels. The flies produce vertical burrows in the phototrophic layers of the microbial mats and reach the anoxic, sulphidic black substrate, which becomes oxygenated. Burrowing activities start forming isolated, circular pits. Finally, within few days the pits coalesce into large areas of surface modified mats showing an intricate network of burrows that can be full of pupa and puparia. In the alkaline lakes, the larvae are able to excrete calcium carbonate forming tubes in the associated calcareous tufa where some of them find protection.
The recognition of traces left by ephydrida in the rock record would provide valuable environmental information in inland lakes where hard-bodied organisms are not available.