Responses of microbial activity in hyporheic pore water to biogeochemical changes in a drying headwater stream
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1. Microbial heterotrophic activity is a major driver of nutrient and organic matter
processing in the hyporheic zone of headwater streams. Additionally, the hypor‐
heic zone might provide refuge for microbes when surface flow ceases during
drought events.
2. We investigated chemical (organic and inorganic nutrients) and microbiological
parameters (bacterial cell concentration, live–dead ratios, and extracellular en‐
zyme activities) of surface and interstitial pore water in a period of progressive
surface-hyporheic disconnection due to summer drying. The special situation of
the chosen study reach, where groundwater mixing is impeded by the bedrock
forming a natural channel filled with sediment, allowed as to study the transfor‐
mation of these parameters along hyporheic flow paths.
3. The chemical composition of the hyporheic pore water reflected the connectivity
with the surface water, as expressed in the availability of nitrate and oxygen.
Conversely, microbiological parameters in all hyporheic locations were different
from the surface waters, suggesting that the microbial activity in the water
changes rapidly once the water enters the hyporheic zone. This feature was prin‐
cipally manifested in higher live–dead ratios and lower leucine aminopeptidase (an
activity related to nitrogen acquisition) in the hyporheic pore waters.
4. Overall, bacterial cell concentration and extracellular enzyme activities increased
along hyporheic flow paths, with a congruent decrease in inorganic nutrients and
dissolved organic matter quantity and apparent molecular size.
5. Our findings show two important functions of the hyporheic zone during drought: (1)
deeper (−50 cm) water-saturated layers can act as a refuge for microbial activity; and
(2) the hyporheic zone shows high rates of carbon and nitrogen turnover when water
residence times are longer during drought. These rates might be even enhanced by
an increase in living microbes in the remaining moist locations of the hyporheic zone