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Harmful algal blooms (HABs) are increasing globally, causing economic, human health, and ecosystem harm. In spite of the frequent occurrence of HABs, the mechanisms responsible for their exceptionally high biomass remain imperfectly understood. A 50-y-old hypothesis posits that some dense blooms derive from dinoflagellate motility: organisms swim upward during the day to photosynthesize and downward at night to access deep nutrients. This allows dinoflagellates to outgrow their nonmotile competitors. We tested this hypothesis with in situ data from an autonomous, ocean-wave-powered vertical profiling system. We showed that the dinoflagellate Lingulodinium polyedras vertical migration led to depletion of deep nitrate during a 2020 red tide HAB event. Downward migration began at dusk, with the maximum migration depth determined by local nitrate concentrations. Losses of nitrate at depth were balanced by proportional increases in phytoplankton chlorophyll concentrations and suspended particle load, conclusively linking vertical migration to the access and assimilation of deep nitrate in the ocean environment. Vertical migration during the red tide created anomalous biogeochemical conditions compared to 70 y of climatological data, demonstrating the capacity of these events to temporarily reshape the coastal oceans ecosystem and biogeochemistry. Advances in the understanding of the physiological, behavioral, and metabolic dynamics of HAB-forming organisms from cutting-edge observational techniques will improve our ability to forecast HABs and mitigate their consequences in the future.

In aquatic ecosystems, allochthonous nutrient transport to the euphotic zone is an important process that fuels new production. Here, we use high‐resolution physical and biogeochemical observations from five summers to estimate the mean vertical nitrate flux, and thus new production over the Northeast U.S. Shelf (NES). We find that the summertime nitrate field is primarily controlled by biological uptake and physical advection–diffusion processes, above and below the 1% light level depth, respectively. We estimate the vertical nitrate flux to be 8.2 ± 5.3 × 10−6 mmol N m−2 s−1 for the mid‐shelf and 12.6 ± 8.6 × 10−6 mmol N m−2 s−1 for the outer shelf. Furthermore, we show that the new production to total primary production ratio (i.e., the f‐ratio), consistently ranges between 10% and 15% under summer conditions on the NES. Two independent approaches—nitrate flux‐based new production and O2/Ar‐based net community production—corroborate the robustness of the f‐ratio estimation. Since ~ 85% of the total primary production is fueled by recycled nutrients over sufficiently broad spatial and temporal scales, less than 15% of the organic matter produced in summer is available for export from the NES euphotic zone. Our direct quantification of new production not only provides more precise details about key processes for NES food webs and ecosystem function, but also demonstrates the potential of this approach to be applied to other similar datasets to understand nutrient and carbon cycling in the global ocean. [ABSTRACT FROM AUTHOR]

Measurements by the submersible ultraviolet nitrate analyzer (SUNA) can be used to derive high‐resolution in situ nitrate concentration with reliable accuracy and precision. Here we report our operational practices for SUNA deployment (including pre‐cruise instrument preparation and in‐cruise instrument maintenance) and detailed post‐cruise nitrate quality control procedures for SUNA integrated onto the CTD rosette. This work is based on experiences and findings from over 500 individual SUNA casts collected from 24 cruises (of which 14 cruises have been quality controlled so far) over the past 5 yr. After applying previously published spectral corrections for temperature, salinity, and pressure effects, we found residual biases in SUNA nitrate estimates compared to independently measured discrete samples. We further develop and assess a new two‐step procedure to remove remaining biases: (1) a general temperature‐dependent adjustment at low‐nitrate concentrations; and (2) a cruise‐specific full‐range bias correction. Our final quality‐controlled SUNA nitrate data achieve an accuracy of 0.34–0.78 μM, with a precision of 0.08–0.21 μM, at a vertical resolution of 1 m. Additional comparisons between the nitrate and density data confirm the high quality of the quality‐controlled SUNA data. Although applying spectral correction algorithms increases the accuracy and precision of the instrument‐output nitrate concentration, we emphasize that additional constraints of SUNA measurements against other independent sources (e.g., bottle data, temperature, and density) are irreplaceable to ensure the accuracy of final nitrate data. [ABSTRACT FROM AUTHOR]

• Soil microbial metabolism experienced C and P limitations in Poyang Lake wetland. • Flooding alleviated soil microbial metabolic limitations in the floodplain wetland. • Microbial C limitation was more sensitive to flooding changes than microbial P limitation. • Hydrological regime was the most important driver for soil microbial metabolism. Soil microbial metabolic limitations influence the equilibrium between microbial metabolic requirements and the accessibility of soil substrates and nutrients. Although extensive research has examined the effects of hydrological regime changes on soil microbial activity and community structures, our understanding of their effects on soil microbial metabolic limitations remains inadequate. In this study, we quantified and evaluated soil microbial metabolic limitations along a flooding duration (FD) gradient over a three-year period in the Poyang Lake wetland, China, using vector analysis of extracellular enzyme stoichiometry. Our results revealed that the lengths and angles of the vector analyses ranged from 1.32 to 1.58 and 56.82° to 65.19°, respectively, across all samples, indicating that soil microbial metabolism was primarily limited by C and P. Both the vector length and angle showed negative correlations with FD, suggesting that microbial C and P limitations significantly decreased with increasing FD. Flooding regime primarily affected microbial C and P limitations through direct effects, as well as indirect effects resulting from plant features (e.g., plant community diversity, biomass, and nutrient properties) and soil nutrient ratios. Among hydrological, vegetational, and edaphic variables, flooding was the dominant driver of microbial C and P limitations. Overall, our research highlights the importance of hydrological regimes in regulating soil microbial metabolic limitations and provides valuable perspectives for understanding and predicting soil C and nutrient cycling in floodplain wetlands. [ABSTRACT FROM AUTHOR]