Expedition log by MBARI Scientist Colleen Durkin

When designing ocean field observations, we are usually faced with a fundamental decision: Should we stay in one place and observe conditions over time, or should we follow the water as the physical currents transport ocean life across space? These are referred to as “Eulerian” vs. “Lagrangian” reference frames. On this expedition, we are embracing both world views to achieve our goal of observing how carbon-rich marine snow particles are exported into the deep sea in a dynamic coastal environment. 

We have had two instruments moored on the deep seafloor since April, providing sustained observations at one location over time. The SINKER imaging system is plugged in at the main science node at MBARI’s cabled ocean observatory (MARS) just outside Monterey Bay and provides continuous, real-time imagery of marine snow particles as they arrive on the seafloor nearly one kilometer (approximately 3,200 feet) below the ocean’s surface. Simultaneously, the Sedimentation Event Sensor (SES) has been moored farther offshore, approximately three kilometers (almost two miles) underwater. This low-power imager can run on batteries for long time periods, allowing us to expand the range of our in-place “Eulerian” observations. We will recover and redeploy both of these instruments during this cruise, allowing us to continue observing carbon export to the deep sea as this season’s El Niño continues to develop.

At the same time, we will be deploying mobile assets during the cruise to study how particles are exported within a moving water mass. Last week, our team at MBARI deployed two long-range autonomous underwater vehicles (LRAUVs) equipped with imaging instruments that will map the distribution of plankton and particles in the top 200 meters (656 feet) of the ocean. Over the weekend, they motored from the MARS node where SINKER is plugged in to the offshore site of the SES mooring. 

Today, on the first day of our expedition, we will sail on R/V David Packard to meet up with the two LRAUVs. We will deploy a drifting, surface-tethered sediment trap (STT) array. This drifting array carries tubes that collect physical samples of sinking particles, and also time-lapse SnoCam+ cameras that record the timing of their arrival in the trap. The array is outfitted with an acoustic beacon that the LRAUVs can “hear” and follow the STT as it drifts with the currents. The LRAUVs then collect continuous depth profiles of the plankton and particles in the water as we simultaneously measure how much is sinking.