Transparent exopolymer particles (TEP) are driven by chlorophyll a and mainly confined to the euphotic zone in a deep subalpine lake

Microgels such as transparent exopolymer particles (TEP) can be as important in freshwater as they are in the ocean because they constitute microenvironments of unique physical, chemical, and biological properties. Previous studies have shown that TEP concentrations can significantly contribute to the organic carbon pool in both marine and freshwater. Despite the relevance of TEP as a microbial substrate in aggregate formation, their fate within the hypolimnion has not been studied in deep lakes. This work is the first extensive analysis on the TEP space-time distribution and their role in the carbon cycle of Lake Maggiore, a subalpine deep lake where the presence of TEP became macroscopically evident in recent times. Our study demonstrated that in oligotrophic Lake Maggiore, characterized by low total organic carbon (TOC) concentration (summer max ~1.8 mg L-1), TEP can reach 36% of TOC in the photic layer while its weighted average in the hypolimnion below 100 m depth is 1.7%. We therefore hypothesised that most TEP is recycled in the upper layers, and that its contribution as carbon burial in the sediment may be lower than expected. TEP concentration exhibited a clear seasonal variability mirroring that of the autotrophic plankton. The models explain TEP variability as a function of chlorophyll a, a proxy for phytoplankton biomass, and a weaker effect of picocyanobacteria. TOC, but not TEP, influenced the abundance of bacterial aggregates, leaving open the role of bacteria and phytoplankton association in TEP formation.