Denitrification and the Role of Macrofauna Across Estuarine Gradients in Nutrient and Sediment Loading

To cope with increasing nitrogen loading to estuarine systems, the demand for removal via denitrification is rising. Understanding how the connections between biogeochemistry and ecology regulate denitrification rates along gradients of nutrient and sediment loading is necessary to predict ecosystem function. Previous studies have shown that denitrification rates are typically highest in muddy, nutrient-rich sediments and decrease in sandy, nutrient-poor sites. Denitrification rates were measured at four subtidal sites positioned from the upper to lower estuary. Site 1 was closest to the primary riverine nutrient supply with the highest suspended sediment and benthic mud content that decreased with distance downstream. Contrary to expectations, denitrification rates were lowest at the most upstream site (22.35 ± 13.34 μmol N₂ m⁻² h⁻¹) and higher downstream (57.84 ± 5.72 μmol N₂ m⁻² h⁻¹). While respiration, primary production, and sediment characteristics affected net denitrification rates and efficiency, results indicated that macrofaunal abundance (Site 1, 60.8 ± 7.3 individuals/core; Sites 2–4, 177.7 ± 10.8 individuals/core), primarily tube-dwelling polychaetes (Site 1, 0.5 ± 0.5 individuals/core; sites 2–4, 122.5 ± 8.0 individuals/core), played an important role. Their influence was highest in muddier sediments (median grain size < 85 μm) where an increase in pseudopolydorid abundance (0.5 ± 0.5 to 149.5 ± 13.4 individuals per core) resulted in a ~ 3-fold rise in denitrification rate (22.3 ± 13.3 to 62.24 ± 11.1 μmol N₂ m⁻² h⁻¹). The linkage between macrofauna and denitrification demonstrates their role in maintaining ecosystem function and bolstering resilience in sites affected by sediment and nutrient pollution.