Seasonally ephemeral streams are common globally, and are inhabited by organisms that can survive long dry periods, then capitalize on resources mobilized as flow resumes. In the McMurdo Dry Valleys of Antarctica, a polar desert, ephemeral streams contain diverse food webs supported by microbial photoautotrophy. Here, benthic microbial mats dominated by nitrogen (N-)fixing Nostoc spp., or “black” mats, provide N for primary production in neighboring Oscillatoriacean dominated, or “orange” mats, and for heterotrophy and N-cycling activity within the mats and underlying hyporheic sediments. This study sought to understand how these communities change upon re-wetting early in the flow season, and as flows decrease through the season’s end. We collected samples from one stable-hydrology stream and two flashier systems, between late-December 2024 and late-January 2025, spanning before and after flow resumed and three more occasions. We quantified organic matter (OM) content of sediments and mats, measured DNA and chl-a (biomass proxies), and extracellular enzyme activities (EEA), which assess microbial investment to access carbon (C) and N. We predicted that EEA would increase upon re-wet, then C availability would increase but N availability decrease as the season progressed.
Interestingly, sediment OM declined as the season progressed, more quickly in black than orange mats, with some recovery at the end of the season (P<0.05). Sediment EEA was not temporally dynamic (P>0.05); however, sediment EEA was higher (P<0.05) and expressed lower N-limitation (P<0.01) under black (N-fixing) mats than under orange mats. In the mats, C-limitation was lower than sediments, and lowest in the stable stream and in black mats. N-limitation was lowest in black mats from the stable stream, and showed contrasting shifts through time in the flashy streams. Overall, patterns suggest that C demand by sediment heterotrophs is strong, could contribute to OM consumption through the flow season, and that N availability may accelerate this process. Concurrently, C is more available in mats, likely due to primary production there, and is promoted by N-fixation and flow stability. Work is in progress to finalize autotroph and heterotroph biomass estimates which will inform ideas about growth-driven nutrient C and N supply and demand.