July 17, 2012
This morning we started our dive at 6:30 and searched for Sergestes similis shrimp for a new series of Midwater Respirometry System (MRS) experiments. We rapidly collected the six samples we needed, and flew the remotely operated vehicle (ROV) to the mooring to hang the MRS. We will recover it in 24 hours. The ROV was back on deck around 10:00 a.m. and the pilots prepared the ROV for a new type of mission: video transects. A transect is a term used in ecology that defines a determined path followed by scientists surveying the occurrence of a phenomenon of interest (e.g., the presence of a certain type of animal on a defined path). In our case, the goal is to quantitatively measure the different forms of life encountered at different depths in the water column (pelagic habitat). We ran transects from a depth of 200 meters (650 feet) down to 1,000 meters (3,280 feet), at intervals of 100 meters. The transects are run for 10 minutes at a constant speed.
The Midwater Time Series project has provided us with almost 16 years of distribution data for mesopelagic animals—those living in the ocean at a depth of between 180 meters (600 feet) and 900 meters (3000 feet)—at the Midwater 1 station here in Monterey Bay. This collection of video measurements, merged with environmental data (e.g., oxygen concentration, salinity, and temperature), gives us information about changes in the density of animals over the course of a year or between years, and also changes in their depth distribution over time. For example, we are able to pinpoint when organisms appear that we haven’t seen in the bay for quite some time. The siphonophore Praya was commonly seen on our early transects, but had been absent for several years. In the past few years, we’ve again observed a number of these interesting siphonophores that can grow to be longer than a blue whale.
Our observations help us to determine which organisms are changing their vertical depth distributions in response to changes in the oxygen minimum zone (OMZ), an area of low oxygen between 400 to 800 meters (1,300 to 2,600 feet). As this area expands, oxygen-rich habitat for some organisms is limited to the upper layers of the ocean. In some cases, organisms will move into the euphotic zone, the upper 200 meters (650 feet) of the ocean, and become prey items to animals that inhabit this sunlit layer. The broadening of the OMZ over time could lead to reduced numbers of key pelagic species here in Monterey Bay. Time series are crucial for identifying long-term changes in populations, and in distinguishing anthropogenic (human-caused) effects from natural cycles. Ours is the only mesopelagic time series of its kind in the world.
—Kris Walz and Rob Sherlock
After the transects, we started searching for interesting species and collected Tomopteris worms and an amphipod living on jellyfish. The ROV was back on deck at 6:30 p.m. and, while the scientists took care of the samples, the pilots prepared the ROV for tomorrow, already the last day of this expedition! But the day is not over, tonight we will first launch a third trawl net and, after recovering it, Henk-Jan Hoving will launch his camera system for an overnight midwater observation.