MEPS 556:59-77 (2016)  -  DOI: https://doi.org/10.3354/meps11865

ABSTRACT: Denitrification plays a key role in global biogeochemical cycles as the main nitrogen (N) loss process that limits availability of N to primary producers. This study presents the first measurements of sediment denitrification rates for New Zealand estuaries. Spatially explicit rates of direct denitrification (D_w) and coupled nitrification-denitrification (D_n) were quantified using the isotope pairing method in 4 oligotrophic to moderately nutrient-enriched estuaries on the South Island, New Zealand (Avon-Heathcote, Waikouaiti, Tokomairiro, Tautuku). Coupled nitrification-denitrification dominated (>86%), with most sites having low dissolved inorganic nitrogen (DIN) concentrations (<30 µmol l^-1). Rates of total denitrification (D_tot) and D_n were negatively correlated with percent sand and positively related to oxygen penetration depth (OPD). Surface sediment chlorophyll a concentrations and percent carbon (%C) were also identified as significant predictors of D_tot and D_n in the single-factor models. By contrast, D_w was the main nitrogen removal process at moderately nutrient-enriched sites and was positively related to DIN concentrations in the water column and negatively correlated with percent sand. OPD and %C were also identified as significant predictors of D_w. While D_w was positively related to DIN concentrations, overall rates of denitrification were not, demonstrating that predictive relationships between N loading and removal are not always possible in oligotrophic estuaries. Our findings demonstrate the importance of grain size as a driver of denitrification rates in estuaries and imply that an increase in fine-grained sediments from land-use intensification may alter rates of N removal in estuaries through complex direct and indirect processes.

KEY WORDS: Denitrification · Estuaries · New Zealand · Nitrogen · Benthic · Sediment grain size · Isotope pairing method · Oxygen flux