While organic and inorganic nutrient inputs from land are recognized as a major driver of primary production in estuaries, remarkably little is known about how processes within the tidal freshwater zones (TFZs) of rivers modify these inputs. This study quantifies organic matter (OM) decomposition rates in surface sediment layers in the lower reaches of two south Texas river channels and identifies key parameters that influence sediment decomposition rates. Sediment cores were collected from non-tidal and tidal freshwater sites in the Mission and Aransas rivers during two summers (June 2015 and June 2016) and two winters (February 2016, January 2017). We measured oxygen consumption rates, organic carbon and nitrogen content, stable isotope ratios (δ¹³C and δ¹⁵N of OM), and sediment porosity. O₂ consumption rates in TFZ sediments were 385 ± 88 μmol O₂ m⁻² h⁻¹ (summer) and 349 ± 87 μmol O₂ m⁻² h⁻¹ (winter) in the Aransas River and 767 ± 153 μmol O₂ m⁻² h⁻¹ (summer) and 691 ± 95 μmol O₂ m⁻² h⁻¹ (winter) in the Mission River. These rates in TFZs were similar to rates in estuaries and higher than rates at non-tidal riverine sites. Rates of sediment O₂ consumption were primarily controlled by OM content and temperature. Sediment OM was dominated by algal biomass from in situ production in both TFZs. We hypothesize that algal production and sinking within TFZs is a major pathway for translocation of watershed-derived nutrients from the water column to the sediments within TFZs. Further work is needed to quantify linkages between decomposition, nutrient remineralization, and potential removal through processes such as denitrification.

虽然从河口输入的有机和无机养分被认为是河口初级生产的主要驱动力，但对于河流的潮汐淡水区（TFZs）内的过程如何改变这些投入却知之甚少。这项研究量化了德克萨斯州南部两个河道下游表层沉积物中有机质的分解速率，并确定了影响沉积物分解速率的关键参数。在两个夏季（2015年6月和2016年6月）和两个冬季（2016年2月，2017年1月）从米申河和阿兰萨斯河的非潮汐和潮汐淡水站点收集了沉积物岩心。我们测量氧的消耗率，有机碳和氮含量的，稳定的同位素比率（δ ¹³ C和δ ¹⁵N（OM），以及沉积物孔隙度。TFZ沉积物中O _2的消耗速率为385±88μmolO ₂ m ^-2  h ^-1（夏季）和Aransas河中的349± 87μmolO ₂ m ^-2  h ^-1（冬季）和767±153μmolO ₂米^申河中的m ^-2  h ^-1（夏季）和691±95μmolO ₂ m ^-2  h ^-1（冬季）。TFZ中的这些比率与河口中的比率相似，并且高于非潮汐河岸地区的比率。沉积物O _2的比率消耗量主要受OM含量和温度控制。两种TFZ中原位生产的藻类生物质占沉积物OM的比重。我们假设TFZs内的藻类生产和下沉是流域性营养物从水柱转移到TFZs内沉积物的主要途径。需要进一步的工作来量化分解，营养再矿化和通过反硝化等过程去除潜在物质之间的联系。