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The biogeochemical conditions at the sediment-water interface and along the water column near the discharge of the Santa Marta sewage outfall (SMSO) were studied during the non upwelling (NUPW) and upwelling (UPW) seasons by sedimentary properties and benthic metabolism measurements, as well as, by the implementation of a coupled 3D hydrodynamic-ecological model (AEM3D). Sediment properties (organic matter quantity, C, N and P pools and δ 13 C, δ 15 N and redox potential) and benthic metabolism (aerobic respiration, denitrification, nitrate ammonification and nutrient recycling) were analyzed in four stations located in the proximity and 100, 750 and 1800 m far from the untreated wastewater effluent discharge in both seasons in the Santa Marta Coastal Area (SMCA). From each site, sediment cores were collected between 20 and 30 m depth. Then, the nutrient fluxes were measured in the laboratory via dark incubations; sequentially to fluxes denitrification and dissimilative nitrate reduction to ammonium were measured via the r-IPT (Isotope Pairing Tecnnique). The results indicate that the sediments trace the impact of the outfall (at 750 m and 1800 m with a contribution of terrestrial organic carbon of ~ 40 and ~ 20%, respectively). The results suggest significantly higher sediment oxygen demands (SOD) in the outfall vicinity, as well as a suppression of denitrification and increments in the ammonia nitrogen release through disassimilatory reduction of nitrate to ammonium (DNRA), which was increased during the UPW season.On the other hand, AEM3D model was applied to analyze the seasonal variations of water physicochemical and biological parameters in SMAC under two different nutrient and organic matter loads from wastewater outfall (flow-rate of 1.0 m 3 s -1 and 2.5 m 3 s -1 ) and along the NUPW and UPW season. The model was set up, calibrated and validated based on benthic metabolic measurements carried out within the simulation period, satellite-derived chlorophyll-a (Chl-a) and sea surface temperature (SST) maps, HYCOM database and field and literature water quality data. The model was able to reproduce the magnitude and timing of complex dynamics and fast transitions of temperature, nutrients, and phytoplankton, including the time and duration of stratification and mixing periods during the NUPW and UPW seasons. The model was also able to capture the effect of fertilization from upwelling and from the outfall plume. The wind field was the main driver of nearshore hydrodynamics and the outfall plume dispersion. The shortest average residence times IV of the outfall plume (3.7 ± 0.4 days) corresponded to the period of highest upwelling intensity.Temperature, light intensity and nutrients were the factors that limited phytoplankton growth. The plume concentrations of TOC, TP and PO4 3increased slightly under two scenarios of different wastewater loading. The phytoplankton growth was limited in both NUPW and UPW seasons due to large changes in temperature and advection and mixing in the coastal area, resulting in lar...

The combination of biogeochemical methods and molecular techniques has the potential to uncover the black-box of the nitrogen (N) cycle in bioturbated sediments. Advanced biogeochemical methods allow the quantification of the process rates of different microbial processes, whereas molecular tools allow the analysis of microbial diversity (16S rRNA metabarcoding) and activity (marker genes and transcripts) in biogeochemical hot-spots such as the burrow wall or macrofauna guts. By combining biogeochemical and molecular techniques, we analyzed the role of tube-dwelling Chironomus plumosus (Insecta, Diptera) larvae on nitrification and nitrate reduction processes in a laboratory experiment with reconstructed sediments. We hypothesized that chironomid larvae stimulate these processes and host bacteria actively involved in N-cycling. Our results suggest that chironomid larvae significantly enhance the recycling of ammonium (80.5 ± 48.7 µmol m −2 h −1 ) and the production of dinitrogen (420.2 ± 21.4 µmol m −2 h −1 ) via coupled nitrification-denitrification and the consumption of water column nitrates. Besides creating oxygen microniches in ammonium-rich subsurface sediments via burrow digging and ventilation, chironomid larvae serve as hot-spots of microbial communities involved in N-cycling. The quantification of functional genes showed a significantly higher potential for microbial denitrification and nitrate ammonification in larvae as compared to surrounding sediments. Future studies may further scrutinize N transformation rates associated with intimate macrofaunal-bacteria associations.

In shallow‐water sediments, the combined action of microphytobenthos and bioturbating fauna may differentially affect benthic nutrient fluxes and exert a bottom‐up control of pelagic primary production. In many cases, the effects of microphytobenthos and macrofauna on nutrient cycling were studied separately, ignoring potential synergistic effects. We measured the combined effects of microphytobenthos and chironomid larvae on sediment–water fluxes of gas (O2, TCO2 and N2) and nutrients (NH4+, NO3−, NO2−, PO43− and SiO2) in shallow‐water sediments of a hypertrophic freshwater lagoon. Fluxes were measured in the light and in the dark in reconstructed sediments with low (L = 600 ind/m2), high (H = 1,800 ind/m2) and no (C) addition of chironomid larvae, after 3 weeks of pre‐incubation under light/dark regime to allow for microalgal growth. Besides flux measurements, pore water nutrient (NH4+, PO43− and SiO2) and dissolved metal concentrations (Fe2+ and Mn2+) were analysed and diffusive fluxes were calculated. Chironomid larvae increased sediment heterotrophy, by augmenting benthic O2 demand and TCO2 and N2 dark production. However, on a daily basis, treatments C and L were net O2 producing and N2 sinks while treatment H was net O2 consuming and N2 producing. All treatments were net C sink regardless of chironomid density. Microphytobenthos always affected benthic nutrient exchange, as significantly higher uptake or lower efflux was measured in the light compared with dark incubations. Theoretical inorganic N, P and Si demand by benthic microalgae largely exceeded both dark effluxes of NH4+, PO43− and SiO2 and their net uptake in the light, suggesting the relevance of N‐fixation, water column NO3− and solid‐phase associated P and Si as nutrient sources to benthic algae. Chironomid larvae had a minor effect on inorganic N and P fluxes while they significantly stimulated inorganic Si regeneration. Their bioturbation activity significantly altered pore water chemistry, with a major reduction in nutrient (highest for NH4+ and lowest for SiO2) and metal concentration. Underlying mechanisms are combinations of burrow ventilation and bioirrigation with stimulation of element‐specific processes as coupled nitrification–denitrification, co‐precipitation and inhibition of anaerobic paths such as Fe3+ or Mn4+ reduction or re‐oxidation of their end products. The combined activity of benthic algae and chironomid larvae may significantly attenuate internal nutrient recycling in shallow eutrophic ecosystems, and contribute to the control of pelagic primary production.

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