Week 3: July 5th- July 12th
We finished off week three at Station 10, over 1000 miles away from Station 6! We’re now making our way northwest to Hawaii and are in the North Pacific Subtropical Gyre. Station 10 proved to be an exciting spot for megafauna — we saw both a baby whale shark and a manta ray!
This week, we started processing and visualizing the surface underway data that MIMS (or rather EIMS, equilibrator inlet mass spectrometer) of the Pomona-Pitzer team has been recording, as well as the water column data collected by CTD on stations over the last 22 days. MIMS measures the ratio of O2 and argon in seawater to determine the O2 saturation due to biological O2 production. Because oxygen and argon have very similar solubility properties, using O2/Ar ratio, we can isolate biological processes to calculate the balance between Gross Primary production and respiration. This balance represents Net Community Production (NCP), the fraction of photosynthetic carbon sequestration available for other trophic levels of marine communities.
Rates of NCP are important to quantify as we move into the North Pacific Subtropical gyre. Surface waters in the gyre are oligotrophic (nutrient-poor), and primary production is limited by the availability of macronutrients — especially nitrogen. EIMS indeed has been measuring very low biological O2 supersaturation. It is possible that we’re seeing less productivity than we would last year at this same time, due to the current El Niño event. In an El Niño year the trade winds weaken or even reverse, which causes stratification of the water column and less upwelling of nutrients to the surface, which phytoplankton need to grow. It remains to be determined whether the low biological O2 supersaturation detected by EIMS, is driven by El Nino conditions or typical of the gyre environment.
We also got to see a lot of interesting microbes thanks to Caitlin Parente, PhD student from USC, who’s studying diazotrophs. Diazotrophs are specialized organisms that fix atmospheric nitrogen (N2) into organic nitrogen that through recycling processes becomes biologically available forms, like ammonium and eventually nitrate.. We are very interested in these organisms as they are essential to the nitrogen cycle — the focus of this cruise. Caitlyn is especially interested in trichodesmium and crocosphaera, two species of cyanobacteria diazotrophs. She started spotting the trichodesmium this week as we moved into more nutrient-poor waters. Known as sea sawdust, colonies of these bacteria can make the water appear “shimmery.” On Tuesday, we ran an experiment looking at some of the underway water that we noticed had a lot of particles when we were filtering for POC/PON. Caitlyn taught us how to prepare a microscope slide by pumping a small volume of water through a polycarbonate filter. We then looked at it under her microscope and were able to see what we suspect to be two species of crocosphaera, one rod-shaped and the other spherical.
Next week will be our last week on the cruise! For our last blog, expect more updates from the gyre and reflections on the cruise as a whole!