NGA LTER Summer Voyage: Biogeochemistry Section Report

Participants: Thomas Kelly

Cruise: SKQ202110S

Optical Instruments

Two rosette-mounted optical instruments were used during the cruise: the underwater vision profiler (Hydroptic UVP5; sn009) and the laser in situ scatterometer and transmissometer (Sequoia LISST-200x; sn2167). Both instruments measure particle abundance and size spectra during the downcast. The UVP5 was used on almost all casts (n = 68) and assesses particles between ~250 – 2500 µm while the LISST-200x was used on shallow casts (600m depth rating; n = 46) and quantifies particles into 36 size classes between 1 – 500 µm. Blanks for the LISST-200x (milli-q water) were consistently low and in line with factory calibration values. The LISST-200x was also used onboard the towed DPI instrument during tow-yo transects.

Nitrogen Uptake

Deckboard incubations (500 ml; n = 108) were conducted at 9 intensive stations to quantify nitrate and ammonium uptake rates at 6 depths through the water column. Briefly, Niskin-collected seawater was added to 2 bottles per depth (for 15N-nitrate and 15N-ammonium, respectively). Bottles were spiked with 100 nM (final concentration) of 15N-labeled nitrate or ammonium. Samples were then gently mixed, placed in screened bags to mimic ambient light conditions, secured within a deckboard incubator, and kept at ambient mixed layer temperatures. Nitrate uptake experiments were conducted for 24 hours while ammonium experiments were terminated after 4 hours to minimize the impact of ammonium recycling. Incubations were filtered onto precombusted GF/F filters (Whatman) and stored in labeled cryovials at -80C for further processing back on land.

Additionally, 2 sets of diel ammonium and nitrate uptake experiments (n = 24) were conducted to assess the diel cycle of nutrient uptake by the phytoplankton community in either the mixed layer (10m) or deep chlorophyll maximum (DCM; typically ~30 m). Experiments were conducted in a manner similar to the standard incubations above but consisting of a series of 4 hour incubations with one additional bottle serving as a 24 hour control.

Surface Tethered Sediment Trap

Five (5) deployments of the surface-tethered sediment trap arrays were completed during SKQ202110S. Deployments lasted for approximately 24 hours (18 – 32 h) and were conducted at a subset of intensive stations (i.e. KOD5, KOD9, MID10, MID5, GAK4, GAK8, and GAK15) with the goal of collecting sinking organic matter. Each array was outfitted with 1-4 cross-frames typically placed (1) near the base of the euphotic zone at ~30 m, (2) 70 m, (3) 110 m, and 180 m (when water depth permitted). Four collection tubes per depth allowed for sub-sampling of sinking matter for pigments (Chl-a, phaeopigments; n = 44), carbon and nitrogen abundance and isotopic composition (POC, PON, PIC; n = 39), biogenic silica abundance (bSi; n = 13), non-contamination prone trace elemental analysis (e.g. P; n = 15), and genetic analysis (n = 13).

Intrinsic Sinking Rates

The intrinsic sinking rates of nano- and microplankton was assessed using SETCOLs (Bienfang, 1981; 4” diameter x 24” tall). Each SETCOL contained approximately 3L of Niskin-collected seawater from either the surface mixed layer (~3m) or the deep chlorophyll maximum (DCM; typically ~30 m). Chlorophyll a (Chl-a) and flow cytometry (FCM) samples were taken to establish the initial concentrations of pigments and nano- and microplankton in the SETCOLs. At various time points, samples for FCM were collected from up to 5 apertures on the sides of each SETCOL: top, 75%, middle, 25%, and bottom. Occasional sampling for Chl-a was also conducted but was restricted to fewer timepoints due to volume requirements. FCM samples were poisoned (glutaraldehyde), flash-frozen in liquid N2, and then stored at -80°C. Chl-a samples were immediately filtered and processed according to standard acidification protocols (Strickland and Parsons, 1972) and measured at sea on a calibrated 10AU Fluorometer (Turner Designs). Three experiments were conducted on board and will be used to design future process studies investigating taxa and size-specific relationships to sinking velocities within the NGA.

Moored Instrumentation

Two moored instruments were deployed at the GEO mooring site with the explicit purpose of providing data on marine particle dynamics within the northern Gulf of Alaska. The in situ marine snow camera system (Sexton Systems) is a downward facing camera system with dual strobe lights, which capture photographs of particulate matter every 30 minutes. Due to a last-minute change to a titanium frame, the geometry of the camera system had to be condensed and the frame modified to accept the camera system. The focal distance of the camera was reduced to compensate.

The GEO mooring sediment trap (Hydrobios) was successfully recovered (2020-2021) and redeployed (2021-2022). Timings for the bottle rotation were synchronized with sediment trap deployed in the Bering and Chuchki Sea with sample collections switching on the 1st and 15th of each month. Recovered samples were prepared for storage and transport for analysis back in the lab. The 2020-2021 marine snow camera system was also successfully recovered.

Back To Top