Limnology and Oceanography最新文献

Observations and model studies suggest that front dynamics can enhance phytoplankton productivity. This study tested whether frontal systems also increase the abundance of nitrifying microbes and nitrogen recycling during repeat sampling transects across the Mid-Atlantic Bight shelfbreak in July 2019. We measured ammonium concentrations, nitrate dual isotopes (δ¹⁵N, δ¹⁸O), and ammonia monooxygenase subunit A (amoA) genes of ammonia-oxidizing archaea (AOA) and bacteria (AOB). In subsurface shelf waters, ammonium concentrations exceeded 2 μmol L⁻¹, due to a temporary imbalance in regeneration from sinking particles and subsequent nitrification. The inverse correlation between nitrate δ¹⁵N values and ammonium concentrations confirmed nitrate was partially or entirely from local nitrification on the shelf. In contrast, the shelfbreak frontal zone and slope sea subsurface waters had much lower ammonium concentrations (0.1–0.2 μmol L⁻¹) due to tight coupling between ammonium regeneration and nitrification. The deviation of nitrate δ¹⁵N and δ¹⁸O from algal uptake-driven 1 : 1 ratio suggests concurrent nitrification in the euphotic zone. The shelfbreak front acted as an ecological boundary where AOA and AOB amoA gene numbers were partitioned, with AOAs abounding in slope waters and AOBs in shelf waters, likely due to ammonium availability. At certain slope stations, deep-water nutrient inputs via isopycnal lifting induced by Gulf Stream intrusions caused unexpectedly high phytoplankton biomass, which doubled nitrifier abundance and potentially stimulated both ammonium regeneration and nitrification. These findings demonstrate distinct distributions of nitrifying microbes along the salinity gradient from shelf to slope and highlight the significant influence of coastal ocean-western boundary current interactions on nitrogen biogeochemistry.

This study uses dual-labeled (¹³C and ¹⁵N) stable isotope applications to examine microbial uptake and short-term processing of carbon (C) and nitrogen (N) from organic and inorganic compounds in subtropical intertidal sediment. Four treatment applications were applied: (1) algal dissolved organic matter (DOM), (2) amino acid mixture, (3) glucose and $��{\mathrm{NH}}_4^{+}�$, and (4) NaHCO₃ and $��{\mathrm{NH}}_4^{+}�$ to assess bioavailability effects on processing (1 vs. 2) and short-term processing for OM fixed via microphytobenthos only (pennate diatom dominated) (4) vs. material taken up by the entire microbial community (3) across 24 h. ¹³C from algal-DOM was preferentially used by the microbial community vs. ¹⁵N. At 24 h more ¹³C from algal-DOM remained in microbial biomass indicating use of labeled precursor molecules to form biomass. Conversely, ¹³C from the amino acid treatment was not incorporated into biomass and was either rapidly respired to DIC or discarded as the in situ microbial community preferentially used and retained ¹⁵N from amino acids. Short-term export of ¹³C as CO₂ from glucose was lower than from microphytobenthos-C, while retention of ¹⁵N from $��{\mathrm{NH}}_4^{+}�$ was similar between treatments (3 and 4) despite doubling the application N concentration, suggesting potentially higher glucose-stimulated ¹⁵N export via nitrification–denitrification that was not confirmed via flux measurements in this study. Despite careful site selection for similar tidal exposure and sediment types among the three estuaries, the uptake and processing of labeled substrate varied substantially between replicates and sites which challenged traditional statistical analysis. Disproportionate processing of substrates occurring in sediment hotspots of microbial activity can cause variability spanning orders of magnitude which was found to be widespread through comparison of our results against 19 previous studies in intertidal settings. Development of robust analytical techniques to handle variability from abiotic and biotic factors will allow greater clarity surrounding in situ biogeochemical processing in intertidal environments.

The removal of organic particulate matter, predominantly phytoplankton, in eutrophic coastal seas and estuaries is considered an ecosystem service performed by large bivalve assemblages. Mussel farming has been proposed as a measure to mitigate eutrophication, as filtration directly reduces the concentration of chlorophyll a (Chl a), a primary ecological indicator. Seston depletion is typically assessed by in situ investigation, which generally lacks spatiotemporal coverage of features relative to greater ecosystem dynamics. To assess the scale and structure of this service, the present study couples multiple measurement approaches, including moored stations, synoptic transect surveys, flow cytometry, a preliminary drone survey technique, and satellite remote sensing within and around a large mussel farm. Significant depletion patterns were observed with all methods, and mixing gradients could be detected hundreds of meters beyond the farm, with repeatable patterns but distinct findings between methods. The intensity of the depletion signal was correlated with mussel biomass loads, ambient conditions, and hydrodynamic regimes, ranging from 5% to 91% relative Chl a depletion and Secchi depth increases of up to 2 m. Changes in particle size distributions were impacted in all downstream areas, as well as phytoplankton diversity. Observed depletion gradients with satellite imagery were consistent with other measurements and can be used to complement in situ field measurements. The findings of this study, will inform carrying capacity assessments, farm configuration, and development of impact assessment programs on seston removal for bivalve aquaculture.

Seagrasses are critical coastal habitats that provide numerous ecosystem services. The inherent patchiness within meadows exerts a significant influence on the flow-vegetation interaction, which, in turn, affects the array of services provided by these environments. We present field observations of vegetation distribution and hydrodynamic measurements within and surrounding an approximately 2 m diameter gap (bare sediment) in a fragmented seagrass meadow. We show that variability in mean flows and turbulence is correlated with meadow structure at small (O (10⁰ m)) and large (O (10² m)) spatial scales. Our observations reveal that bare gaps within seagrass meadows lead to faster flows and lower bed elevations. Despite slower flow speeds above the dense seagrass adjacent to the gap, the rates of dissipation of turbulent energy above the vegetation are typically around an order of magnitude larger than above the bare bed. Associated with this enhanced dissipation of turbulent energy, we observed a dominance of down-deceleration events promoting fluid exchange and mixing and driving mass flux into the canopy. Analysis of directional variograms demonstrates that the major continuity axis within NDVI (normalized difference vegetation index) imagery (used as a proxy for vegetation biomass) coincides with the major axis of flow on both gap and meadow scales. Conversely, along the axis of minor flow variance, vegetation remains dense and exhibits greater uniformity. These findings indicate a feedback mechanism between seagrass meadow patchiness and spatial structure through flow modification, which may be beneficial for plant development.

Tidal flats are valuable ecosystems that depend on complex biogeomorphic processes between organisms and sediment transport. Climate change has led to a rise in extreme weather events, such as storms. This, in turn, has increased sediment dynamics and created risks for the benthic communities inhabiting tidal flats. However, replicating sediment disturbances caused by extreme weather is difficult. To overcome this, we used the plow rake to enhance the natural tidal currents and wave conditions to simulate intensified sediment dynamics. The raking disturbance was repeated on two intertidal zones with different inundation frequencies and wind fetch levels to simulate the increasing frequency of storm impact on sediments due to climate change. We compared the measurements of sediment dynamics and macrofauna between plots that were raked and the control plots that were only influenced by natural hydrodynamics. Results showed that tidal flat sediments experienced erosion by 10–20 mm after six times biweekly raking treatments, depending on the site-specific hydrodynamic conditions. Sediment dynamics served as a helpful tool for monitoring the species distribution regarding mobility traits: the high dynamic exposed sites were inhabited by mobile species, while the low dynamic sheltered sites were characterized by less-mobile species. Moreover, the raking treatment decreased the abundance of species with immobile traits, yet the species composition did not experience significant change. Overall, the present findings indicate that tidal flats with low sediment dynamics and immobile macrofauna are at higher risk of declining abundance under intensified sediment disturbances than areas with high sediment dynamics and mobile macrofauna.