POM: The living and the dead

Author: Catherine Garcia

POM – What is it? – POM stands for “particulate organic matter”. Sea water particles could be living plankton, dead material, or even plastic bits that have made it out to sea. The dead material sometimes includes old plankton cells, plant matter, fecal pellets, airborne dust particles, river-borne soil particles, etc. The further the Ronald Brown moves away from the coast and into open water, a higher portion of the particles we capture is living phytoplankton, bacteria, and zooplankton.  Jenna and I are looking at the organic portion of the particulate matter, or the living/dead material. When this POM gets dense enough, it sinks out of the sunlit ocean layer and might make it to the ocean bottom if not consumed by bacteria on the way down. We call this the “Biological Carbon Pump”.

POM1
Jenna and Catherine filtering POM samples.

POM – Why and how do scientists measure it? – Scientists care about POM because phytoplankton play a VERY large role in removing carbon dioxide (CO2) from the air in the upper ocean. To understand just how much carbon is being removed, scientists need to measure how much carbon is captured in particles and what percentage sinks into the deep ocean by the Biological Carbon Pump. To know how much is carbon, we measure the elemental composition of particulate organic matter to obtain its concentration. The most abundant elements in organic matter are carbon, hydrogen, nitrogen, phosphorus, oxygen, and sulfur.

In the Western Indian Ocean, we are collecting samples for particulate organic carbon, nitrogen, phosphorus, and oxygen. The NOAA/V Ronald Brown has a sea water pump constantly bringing sea water conveniently to our lab bench.  Once we collect ~6L of sea water, all of it is filtered onto glass fiber filters that let any particles smaller than 0.7um flow though.

POM2
Glass fiber filter (on left) used to collect POM samples.

It really isn’t any different from than filtering the coffee grinds out of your coffee, except we keep what’s on the filter! Because larger plankton are rarer and a small fraction of the sea water in this area, we filter out any particles larger than 30um to avoid the occasional zooplankton. The filters are frozen after collection and stored frozen until analysis at Dr. Adam Martiny’s lab at the University of California-Irvine.  Once in the lab, the filters for carbon and hydrogen are eventually baked at more than 900oC in an Elemental Analyzer to turn all the carbon and hydrogen into a gas that can be measured. The phosphorus and oxygen are measured using chemical assays against a standard concentration curve.

POM – What does it tell us? – So, I mentioned why we cared about carbon, but why capture the other elements? Until recently the science community accepted a phytoplankton recipe of sorts: 106 parts carbon: 16 parts nitrogen: 1 parts phosphorus and so on. Mix together some carbon from photosynthesis and nutrients from the deep ocean to get your typical model phytoplankton. This ratio gives scientists a good estimate of how much POM is composed of carbon. Globally, an average phytoplankton does almost converge on this ratio of elements first described by Alfred Redfield in 1934. But not always.

Like us, plankton are what they eat. Phytoplankton in particular have extremely flexible elemental ratios. Just how flexible, and what causes them to change their ratios remains an open question. There are several theories that link elemental composition to environmental conditions, which group is present, or even how fast they grow. Jenna and I will try to track down this mystery on the IO7 cruise track.

One thought on “POM: The living and the dead

  1. Pingback: Life on the Ronald H. Brown as a First-time Field Scientist – GO-SHIP Indian Ocean Cruise 2018

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