Tidal Freshwater Zones as Hotspots for Biogeochemical Cycling: Sediment Organic Matter Decomposition in the Lower Reaches of Two South Texas Rivers

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 (delta C-13 and delta N-15 of OM), and sediment porosity. O(2)consumption rates in TFZ sediments were 385 +/- 88 mu mol O(2)m(-2) h(-1)(summer) and 349 +/- 87 mu mol O(2)m(-2) h(-1)(winter) in the Aransas River and 767 +/- 153 mu mol O(2)m(-2) h(-1)(summer) and 691 +/- 95 mu mol O(2)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 O(2)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.

Automated summary

This study quantified organic matter decomposition rates in surface sediment layers in two south Texas river channels, finding that oxygen consumption rates in TFZ sediments were primarily controlled by organic matter content and temperature. Additionally, sediment organic matter in TFZs was predominantly composed of algal biomass from in situ production.