This summer, after a three-week quarantine preceding a six-week research cruise more than 200 miles offshore the United Kingdom, zooplankton ecologist Amy Maas returned to BIOS to await the arrival of more than 800 frozen zooplankton samples she had collected at sea. Preserved in vials and stored on dry ice, she expected them to arrive by expedited air mail in three days. Then she could begin detailed study of the organisms, from calculating their metabolic rates to measuring their individual and community sizes.
Maas’s work would conclude four years of study with the EXPORTS project (EXport Processes in the Ocean from Remote Sensing), which uses ocean data to predict the carbon cycle. Her focus is on zooplankton, small and sometimes microscopic organisms that help to form the base of the marine food web. They also play a critical role in removing carbon dioxide from the atmosphere as part of the ocean’s carbon cycle.
Specifically, Maas and several dozen colleagues work to understand how carbon makes it to the dimly-lit “twilight zone” and deeper ocean interior. They want to know how long it stays there, which is vital to understanding present and future ocean ecosystems and global climate.
Freezing is key
On the ship, Maas and crew collected zooplankton samples with the MOCNESS instrument, an acronym for Multiple Open Closing Net with Environmental Sensing System. This huge system is lowered from the research vessel with 10 long, tapered fine-meshed nets that scientists open and close independently at different depths to collect then compare communities of zooplankton. Electronic sensors on the nets also record temperature, depth and salinity, providing snapshots of the zooplanktons’ external environment.
Flash freezing the samples immediately after collection, then keeping them stored at -112°F (-80°C), typically allows for the most accurate measurements of plankton weight and other physiological data when scientists return to their home labs. Outside of their optimal temperature range, the organisms’ enzymes, or protein components, fall apart and degrade. This ruins analysis of enzyme activity, which Maas uses to estimate the metabolic rate of the community.
Freezing animals also helps bring individual organisms used in shipboard experiments back to the lab where their weight can be precisely measured. Analysis of animal oxygen consumption and waste production are related to animal weight, making this an important step to conclude the more than 200 individual metabolic experiments conducted by Maas and her cruise collaborators.
Headaches from shipping delays
Scientific work has seen no exception when it comes to delays in global shipping related to the COVID-19 pandemic. Instead of the anticipated three days for Maas’s samples to arrive packed in dry ice, it took two weeks. And despite frantic attempts to have the samples kept cold during the shipping delay from the U.K. to Bermuda, the worst happened.
“When they finally arrived, we opened them, and it was just the smell of disaster,” Maas said. “They were literally rotting.”
Maas and BIOS research associate Hannah Gossner immediately placed the samples in a lab freezer and turned their attention to recover at least some data from the samples in hand. All told, Maas estimates tens of thousands of dollars and hundreds of hours were spent during the months of preparation for the cruise (including the required pre-cruise, pandemic-related quarantine period on land).
“It’s not like we could easily head back out to sea and recollect our samples,” Maas said. “We needed to salvage something.”
While they found it impossible to salvage the enzyme samples, they were able to take images of many of the individual organisms used in the metabolic experiments on the ZooScan in Maas’s Invertebrate Physiology Lab at BIOS. The ZooScan takes very high-resolution images of multiple zooplankton at the same time, then provides exact measurements of their dimensions. From this they were able to calculate the size of each individual zooplankton; most are smaller than the period at the end of this sentence.
Using conversions between size and dry weight they recently published in a 2021 paper, they had reasonable estimates of the organisms’ weights. Knowing this information allows them to estimate their oxygen use rates, their waste output, and make comparisons between experiments done over time and in different seasons.
A successful outcome
“We were able to take these smelly, partially-rotted things, take their pictures, and figure out their weight,” Maas said. With some assumptions, and the information on oxygen that she collected while on the ship, she was able to get about 75 percent of the data that she had initially hoped.
In October she presented their results via Zoom at an internal meeting of dozens of members of the EXPORTS project. In February, she will also join colleagues in Hawaii for research presentations during the annual Ocean Sciences Meeting.
“This is the reality of science and life, figuring out different plans and being resilient,” Maas said. “But when you’ve been in quarantine for several weeks and on a ship for six weeks, the stakes feel a little higher.”
Funding for this research was provided by the National Aeronautics and Space Administration (NASA).