Effects of organic matter processing on oxygen demand in a south Georgia blackwater river
Mehring, Andrew Stephen
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Many rivers periodically experience system-wide hypoxia, sometimes without significant nutrient loading. The primary goal of a long-term project titled “Dissolved Oxygen Dynamics in the Upper Suwannee River Basin” was to determine factors (natural or anthropogenic) contributing to low dissolved oxygen concentrations observed in Georgia’s coastal plain blackwater rivers. As a project component, this study assessed effects of organic matter processing on oxygen demand in third-order-stream and fifth-order-swamp reaches of southern Georgia’s Little River. Differences in microbial biomass explained variation in leaf litter oxygen and nutrient uptake among tree species, but nutrient uptake was additionally affected by aluminum and iron accumulating in litter. Microbial biomass was negatively correlated to litter chemistry parameters such as the lignocellulose index, and while labile litter supports higher microbial oxygen uptake, it is also preferentially colonized by macroinvertebrates and breaks down more rapidly. Therefore, recalcitrant litter may make greater long-term contributions to oxygen demand. Detrital standing stocks consisted primarily of leaf litter, but bald cypress (Taxodium distichum) produced significantly more litter per trunk biomass relative to other species in the swamp. Estimates of leaf litter microbial respiration (5.54 g O2 m-2·day-1) accounted for 89% of sediment oxygen demand (SOD) measured by other researchers (6.20 g O2 m-2·day-1) in the same river, illustrating the importance of leaf litter breakdown to overall SOD. Although breakdown rates were faster in the swamp than in the stream, a greater percentage of initial leaf litter standing stocks were retained (54% vs. 30%) in the swamp, potentially due to lower water velocity. Over a seven-year study period, dissolved organic carbon (DOC) transport decreased substantially as annual dry period length increased, while DOC concentration and mineralization were enhanced at low flows. Although DOC mineralization was a small source of oxygen demand (roughly 4%) compared to leaf litter breakdown, our findings suggest that as droughts intensify, temperatures rise, and discharge decreases, enhanced DOC mineralization and reduced downstream DOC export may occur. Research presented here demonstrates that leaf litter accounts for a large natural source of oxygen demand, and illustrates the complex interactions affecting organic matter processing in a Georgia coastal plain blackwater river.