Interannual drought length governs dissolved organic carbon dynamics in blackwater rivers of the western upper Suwannee River basin

Abstract

[1] The Little River (LR) in southern Georgia, U.S., has experienced lengthening droughts since monitoring began in 1972. We evaluated the impacts of drought on riverine carbon cycling using a 9 year data set of dissolved organic carbon (DOC) coupled with laboratory experiments in the LR, as well as long-term data sets in three additional rivers within the Suwannee River basin. Longer drought periods reduced downstream DOC export but also led to higher DOC concentrations in the following hydroperiod. Within a hydroperiod, DOC concentration was positively correlated with temperature and negatively correlated with river discharge but also negatively correlated with total discharge during the previous hydroperiod. Among hydroperiods, DOC concentration was more strongly correlated with antecedent hydrological conditions than with current hydrological conditions across broad spatial scales: in three additional rivers within the Suwannee River basin (Alapaha, Withlacoochee, and Okapilco) and in headwater and downstream reaches of the LR. Microbial DOC consumption and CO 2 production were elevated when DOC concentration was high. Despite dramatic hydrologic changes, DOC composition appeared stable, with optical analyses suggesting that LR DOC is primarily composed of three terrestrial humic-like fluorescence groups. If the current climatic trend of intensifying droughts, elevated temperatures and decreased discharge continue, our results suggest the net effect may be for a more localized riverine carbon cycle with reduced downstream transport of DOC, but higher local mineralization rates due to elevated DOC concentrations.

Dry‐season length and runoff control annual variability in stream DOC dynamics in a small, shallow groundwater‐dominated agricultural watershed

“…Mehring et al . [] reported similar results in the Suwanee River Basin and suggested that the increase in groundwater level (i.e., decrease in groundwater depth) could rapidly flush a highly labile DOC pool with a low density of aromatic moieties. Lambert et al .…”

“…The control of mean annual DOC concentrations by antecedent seasonal drought supports previous findings. Although performed in ecosystems as different as blackwater rivers in the Suwannee River basin, U.S. [ Mehring et al ., ] and Estonian rivers [ Parn and Mander , ], studies have shown that longer antecedent drought led to higher stream DOC concentrations. The present study demonstrated that in addition to the length of the dry season, the magnitude of the groundwater drawdown controlled mean annual DOC concentrations.…”

“…Mehring et al . [] demonstrated that a longer period of low discharge (i.e., the dry season in temperate watersheds) reduced DOC export downstream and increased DOC concentrations in the subsequent hydroperiod in the Suwannee River basin, U.S. However, the effects of groundwater dynamics on stream DOM features are usually considered at the event scale [e.g., Inamdar et al ., ; McGlynn and McDonnell , ; Morel et al ., ; Turgeon and Courchesne , ] and the seasonal scale [e.g., Lambert et al ., ; Laudon et al ., ; Sanderman et al ., ] but rarely at the annual scale.…”

Contribution of surface leaf-litter breakdown and forest composition to benthic oxygen demand and ecosystem respiration in a South Georgia blackwater river

“…5A). FPOM or dissolved organic matter (DOM) both contribute to ER, but concurrent measurements of DOM-associated respiration rates in the swamp averaged only 4% of leaflitter-associated O 2 demand on an areal (m −2 ) basis (Mehring et al 2013). Therefore, DOM respiration cannot fully explain the discrepancy between different estimates of O 2 demand.…”

Sediment Flux and Its Environmental Implications

“…CO 2 ) between terrestrial ecosystems and the atmosphere, there is also a horizontal pathway of A number of studies have shown that DOC riverine concentrations have been increasing over the past two to three decades, especially over Northern Europe and North America (Filella and Rodriguez-Murillo, 2014;Monteith et al, 2007;Evans et al, 2005;Grieve and Gilvear, 2008;Mehring et al, 2013;Tian et al, 2013;Oni et al, 2014;Sucker and Krause, 2010). Several hypotheses have been tendered as possible explanations for this DOC increase, including: decreasing atmospheric sulphur concentration; climate warming (with seasonal temperature increases); increasing precipitation with increasing annual (e.g.…”