Knowledge of how zooplankton can utilize cyanobacteria to sustain their nitrogen (N) demand for essential compounds like amino acids (AAs) is a key to predicting responses of higher trophic levels in terms of production and food web structure to future enhanced water column stratification. We explored the natural abundances of bulk N and AA‐specific nitrogen isotopes (δ15N) in particulate organic matter and mesozooplankton size‐fraction samples from three vertically separated water bodies in the central Baltic Sea during two summertime cyanobacteria blooms. The combination of plankton community and isotope data together with environmental variables helped to identify a mechanism of diazotrophic AA supply (synthesized during N2 fixation) for mesozooplankton, that largely depended on the sea surface temperature which regulated the access to the diazotrophic N‐based food web in the surface water (SW). We found that in the warmer summer, thermophilic cladocerans (e.g., Bosmina spp.) benefited most from diazotrophic AAs in the SW (19.8°C), while only in the colder summer, temperate copepods (e.g., Temora longicornis) ascended from the subjacent winter water into the SW (16.2°C) and incorporated diazotrophic AAs. Trophic position estimates based on the phenylalanine and glutamic acid δ15N signatures revealed that the diazotrophic AA supply into mesozooplankton was mainly indirect via feeding on mixo‐ and heterotrophic diets. Significantly enriched δ15N signatures in phenylalanine in the deep mesozooplankton (mainly copepods of Pseudo‐ and Paracalanus spp.) from the bottom water (BW) that was a region of the suboxic zone point to a reliance on a local food web. Mesozooplankton in the BW was feeding on diets of heterotrophic origin and probably profited from the heterotrophic re‐synthesis of AAs originating from sinking organic matter, as well as from the indirect incorporation of de novo synthesized AAs that most likely originated from chemoautotrophic bacteria or archaea communities in the suboxic zone. Our findings suggest that indirect feeding on diazotrophs and chemoautotrophs will be principal ways of amino acid supply for zooplankton in future enhanced stratified aquatic systems. Only a relatively small increase in temperature may restrict temperate key species from diazotrophic N‐based food webs in the mixed layer.
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...
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