Impact of drying/rewetting cycles on the bioavailability of dissolved organic matter molecular-weight fractions in a Mediterranean stream

Mediterranean fluvial systems are subject to severe drought/rewetting cycles. These extreme periods modify the direction and magnitude of the water fluxes between the stream surface water and the riparian ground water, enabling exploration of the abrupt changes affecting dissolved organic matter (DOM). We investigated the variability in DOM concentration and composition over a drying and rewetting period in a stream-riparian groundwater system. We analyzed dissolved organic C (DOC) and N (DON), and bioavailable DOC (BDOC) in whole water and 3 molecular-weight (MW) fractions (high [HMW]: >10 KDa, medium: 1-10 KDa, low [LMW]: <1 KDa) in surface water and riparian ground water. The quantitative differences in DOM between surface water and riparian ground water were minimal during the drying period, but were greater during the rewetting period because of notable flushing of DOC (>6 mg/L) and DON (>0.4 mg/L) in stream water. Asynchronous inputs of DOC and DON were detected in stream water during rewetting: an abrupt and brief increase (up to 32%) in DOCHMW contribution, and a strong and persistent increase (up to 70%) in DONLMW contribution. Average BDOC values were 18% in stream water and 13% in riparian ground water. BDOC was not correlated with DOC MW, but was positively correlated with DON (r = 0.59, df = 17, p < 0.01). During rewetting, BDOC increased by up to 49%, DONLMW input occurred, and the DOCLMW:DONLMW ratio decreased to values <15:1, suggesting that small, nitrogenous organic compounds contributed to DOM bioavailability. The rewetting period occurred in the brief span of only 2 wk, but it had major biogeochemical implications for DOM and generated a pulse of potentially bioavailable DOM to the stream-riparian interface and downstream communities.