Research

The objective of the project is to gain a better understanding of the vertical and cross-continental shelf nitrate distribution and its ecological importance over the Northeast U.S. Shelf (NES). We use high-resolution physical and biogeochemical observations, especially nitrate profiles, from five summers under the Northeast U.S. Shelf Long-Term Ecological Research (NES-LTER) project. We characterized the shape of the vertical nitrate profile, quantified vertical nutrient flux, and estimated the f-ratio (new production over total primary production) over the summertime NES. We find that the summertime nitrate field is primarily constrained by biological uptake and physical advection-diffusion processes, above and below the 1% light level depth, respectively. The f-ratio consistently ranges between 10% and 15% under summer conditions on the NES, suggesting less than 15% of the organic matter produced in summer is available for export from the NES euphotic zone.

Cross-shelf measurements of a) gross primary production (GPP); b) net community production (NCP); and c) NCP/GPP and New production/GPP (f-ratio). Note that the mid- and outer shelf regions are denoted by the different background gray colors in c).

The objective of the project is to develope novel analytical methods to combine high-resolution nitrate data collected from the SUNA nitrate sensor with discrete bottled data to generate a high vertical resolution nitrate data product of the best quality. Applying previously published spectral corrections as well as a new two-step procedure (i.e., temperature-dependent correction and bottle data-based bias correction) to remove remaining biases, 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. Furthermore, we report our operational practices for SUNA deployment (including pre-cruise instrument preparation and in-cruise instrument maintenance), facilitating a broader-scale adoption of the SUNA for a variety of applications, including gliders, floats, moorings, and ships.

• • Ref. Zheng et al., LOM, 2024

SUNA-CTD rosette integration. a-b) SUNA mounted on the CTD rosette onboard R/V Endeavor, where b) is a zoomed-in view of the yellow box in a). c) SUNA mounted on the CTD rosette onboard R/V Neil Armstrong.

The objective of the project is to study along-isopycnal nitrate change due to physical drivers in the Southern California Bight. With in situ nitrate measurements of high spatial and temporal resolution collected from a vertical profiling nitrate sensor and two nearby moorings with fixed-depth nitrate sensors and other physical and biochemical data, we reveal oceanic biological and physical responses to a submesoscale feature coming from offshore. Furthermore, we find a near-inertial along-isopycnal nitrate fluctuation signal which is possibly induced by the along-isopycnal cross-shore nitrate advection of a positive cross-shore nitrate gradient, suggesting a near-bottom source of nitrate in the shallow nearshore region. Understanding these dynamics which will further help with local ecosystem prediction and conservation.

Along-isopycnal a) PAR; b) chlorophyll fluorescence; c) nitrate; d) east-west velocity and f) north-south velocity.

The objective of the project is to investigate the role of dinoflagellate (here, L. polyedrum) vertical migration on bloom formation in the Southern California coastal region. With novel sensors (specially SUNA - nitrate sensor) and state-of-the-art profiler (the Wirewalker), we are able to collect high spatial-temporal-resolution physical and biogeochemical data (temperature, salinity, ChlorophyII, optical backscatter, irradiance, and nitrate). Our observations show that L. polyedrum’s vertical migration was cued by irradiance and nitrate concentrations, with a maximum migration depth regularly surpassing the depth of the 1% light level by 10s of meters. After migrating into the nitracline, the bloom assimilated an estimated 434 +/- 56 mg N m-2 per day with consequent increases in phytoplankton concentration. Comparing the conditions during the 2020 bloom and 70 years of quarterly data gathered at a nearby CalCOFI sampling station demonstrated that the vertically migrating dinoflagellate bloom modified the local biogeochemical environment by driving down inorganic nutrient availability relative to the climatological conditions. This ‘nitrate deficit’ was enough to support nearly 20 million kilograms of fixed carbon within the 150 km by 10 km footprint of the bloom.

• • Ref. Zheng et al., PNAS, 2023

a), Sea surface 2-m averaged PAR. b-c), along-isopycnal Chl-a, and nitrate concentration from the Wirewalker, respectively. The black contours in b) are iso-nitrate lines with values of 3 uM, 8 uM, and 13 uM respectively, and the black-white arrows represent the migration paths. The contours in c) are nitrate anomalies referenced to the first profile during this period, with positive anomalies in black solid lines and negative anomalies in black dotted lines. Note that the x-axis here is the local time, and the y-axis is the mean depth for each isopycnal.

The objective of this project is to develop a method to observe the oceanic fine-scale velocity field from the Wirewalker profiler using commercially available velocimeters. First, to get rid of velocity contaminations induced by the platform's movement, we developed an algorithm to calculate and further correct platform motion using inertial sensors onboard a Nortek Signature1000 acoustic Doppler current profiler (ADCP). Next, we apply an averaging approach that leverages the vertical movement of the platform and the vertical profiling range of the down-looking ADCP to remove surface wave signals and obtain a background velocity field with a spectrum wavenumber runoff at a scale of ~3m - several times finer than that possible from other velocity measuring platforms with similar measurement ranges.

• • Ref. Zheng et al., JTECH, 2022

Estimated velocity shear fields, with E-W component in the upper panel, N-S compoenent in the middle panel and shear squared in the bottom panel. Black lines are isopycnals with values from 1020.1 kg/m3 to 1025.1 kg/m3 with 0.5 kg/m3 intervals. Vertical resolution here is 1m with a temporal resolution of ~10.5 minutes per profile.

As an undergraduate project, with the goal of improving the texture of artificially-raised fish, we invented a robotic fish that could constantly chase after the real fish to make them swim more. We designed the mechanical structure of the fish and fabricated the fish skeleton with the 3-D printing technique. Vertical motion of the fish is accomplished by changing the weight of the whole body by taking in or out water with a combination of syringes and steering motors. Turning of the fish is achieved by three steering motors along the tail with planned angles and sequences. By calculating the relative position between the real fish and the robotic fish itself through images captured by the camera in the fish head, the motion strategy is determined and controls are sent to motors.

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