Intermittent
dryland streams are characterised by cycles of intense but short floods
followed by long periods of drought and evaporative contraction, which in turn
results in cycles of activity in microbial processes and
biogeochemical cycling. We
used stable isotopes (δ18O and δ2H) to determine the origin
of surface water and evaporative changes in pools from four creek systems of the
semi-arid Pilbara between May 2011 and October 2012. Change in
dissolved organic matter (DOM) composition was used to examine relationships between pool biogeochemistry and hydrologic
regime. Pools with groundwater inputs or shallow alluvial water throughflow showed
low evaporative losses, while pools isolated from groundwater were more highly
evaporated. Pool DOM composition was usually dominated by humic-like compounds
derived from terrestrial organic matter. However, highly evaporated pools had
large contributions of protein-like compounds, most likely derived from
microbial turnover of organic matter. Concentrations of both humic and
protein-like compounds, as well as the humic:protein-like ratio, were
positively correlated to δ18O and δ2H values at 2-3
months since the last flood. However, in the absence of substantial rainfall
events this relationship became increasingly decoupled over time, as the
influence of groundwater inflow shifted from direct
effects on concentration and DOM inputs to alleviation of contraction pressures.
We suggest that other factors, including differences in UV exposure and establishment
of aquatic vegetation, are important factors contributing to differences in DOM
among pools with increasing time since flood.