How much carbon makes it to the deep sea?

The Pelagic-Benthic Coupling Group at MBARI runs a long-term time-series study at a site called Station M, 200 kilometers off the coast of Santa Barbara, California. Our goal is to understand the supply of carbon—in the form of sinking organic matter (e.g., pieces of dead organisms, fecal material, and mucous)—and how its variation over time affects the deep-sea communities far below the surface.

At Station M, 4,000 meters deep, we use a variety of tools to measure how much carbon settles from overlying waters, how much carbon is consumed once it gets there, and the impacts of changing carbon supply on seafloor communities. The measurements we make are needed to calculate global carbon budgets, estimate carbon storage (also known as “sequestration”) in the deep sea, and understand how the ocean interacts with ever-rising atmospheric carbon dioxide.

Approaches:

Because Station M is 4,000 meters deep and we can only go there one or two times a year, we rely on autonomous instruments to collect time-series data. When we do visit Station M by ship, we divide our time between collecting new biological data and providing engineering support to these instruments, which have been collecting data since our last visit to the site.

Recent findings:

The amount of carbon reaching Station M has increased in the past 10 years compared to the previous two decades. This change is driven by an increase in short-lived periods when very large amounts of carbon-rich, freshly dead phytoplankton or gelatinous animals sink to the seafloor. We are currently trying to figure out what’s driving these events. For example, our collaborators Chris Preston at MBARI and Colleen Durkin at Moss Landing Marine Laboratories are looking at the phytoplankton and microbes sinking to Station M to see if certain communities are associated with mass-sinking events.

Cruise Summary

Before taking off for our expedition, the captain asked whether we brought enough movies to watch –days’ worth— in case the forecasted wind and swell kept us from doing our work. We had windy weather, but luckily we were still able to accomplish everything on our agenda. Some of our highlights are listed below.

Working with the ROV Doc Ricketts pilots, we surveyed seafloor communities 4,000 meters deep, as part of a long-term record of abyssal ecosystem change. Despite the reputation of deep-sea systems being slow to change, Station M is noticeably different every time we survey it. This visit, based on our initial impressions, we saw a reduced number of most species of sea cucumbers. But we did see lots of tiny sea pigs (Scotoplanes)—perhaps the large adults we saw in abundance on the last cruise have since reproduced, and now we’re seeing their offspring settling into the area.

Senior Research Technician Linda Kuhnz will conduct a more detailed analysis of the ROV video back onshore. As more groups show interest in extracting resources from the deep sea, studies like this are critical for our understanding of natural versus anthropogenic (i.e., human-caused) changes in abyssal habitats.

We’re always excited to bring up the sediment traps to get our first glimpse into whether this is a high or low food year for the seafloor community at Station M. We also get to see the time-lapse images from the camera system that is installed at the base of the mooring. The recent sediment trap cups were almost empty. Very small amounts of marine snow fell to station M between November 2017 and April 2018. This isn’t a big surprise, because winds were weak during this period compared to other years. Without wind to drive upwelling and bring nutrients to surface waters, phytoplankton production is limited. But this week the spring winds kicked into high gear, and the redeployed traps might document a switch to another “high-carbon” period. Over time, higher land temperatures are expected to drive stronger offshore winds along California, leading to more upwelling and more food to Station M.

Marine Operations Technician Erich Reinecker helped UC San Diego graduate student Margot White conduct CTD rosette casts to collect water samples at different depths. Margot is studying changes in dissolved organic carbon content in the water column, and will compare her results to data collected from Station M many years ago. To prepare her samples, Margot collected very large volumes of water, which dripped through special filters over the course of two days. This method left Margot with downtime to help us with other tasks! Research Specialist Chris Preston also collected water, but her samples came from Niskin bottles mounted on the ROV. She is looking at the genetic diversity of microbes in the water column.

Using the ROV we also collected sediment using push cores for genetic and chemistry analyses. Sediment was also sieved from the mud retrieved by a set of grab respirometers. Tiny worms will be picked out of these samples for studies linking seafloor animal communities to sediment community respiration.

—Crissy Huffard