Leg 3

Leg Summary:

Leg 3 will use the ROV to investigate the ecological and physiological adaptations of the Gulf’s midwater fauna to three significant hydrographic features: the region of oceanographic fronts at the mouth of the Gulf; the Gulf’s pronounced oxygen minimum layer; and the hydrothermal plume above the Guaymas Basin vents. This research will use the ROV’s video cameras, instruments, and samplers to examine the composition of the midwater community over the Mazatlan, Farallon, and Guaymas Basins, determine its vertical distribution in the water column, and measure the characteristics of its bioluminescent constituents. Scuba divers will collect open water zooplankton in all three sampling regions. The leg 3 coordinator is Dr. Bruce Robison from MBARI, and the Mexican collaborator is Rebeca Gasca Serrano from ECOSUR-Chetumal. 

History & Purpose: 

Our goals are to investigate the influence of oxygen concentration, oceanic fronts, and hydrothermal plumes on the composition, distribution, and characteristics of the Gulf of California’s midwater fauna. Our approach will be to make detailed measurements of midwater animals relative to these parameters, and to compare them with our reference community in Monterey Bay. 

The water mass within the Gulf is a modified extension of the Eastern Tropical Pacific water mass that has a pronounced oxygen minimum layer. Oxygen concentrations of less than 0.2 ml/l encompass from 40 to 50% of the upper kilometer of the water column. In contrast, the layer in Monterey Bay covers only about 20% of the upper kilometer yet it acts as a lower boundary for the vertical distribution of many midwater species and is a defining parameter of the mesopelagic habitat. Based on our mesopelagic time-series data in Monterey Bay, we expect that with increasing depth, the number of gelatinous zooplankton in the Gulf will increase relative to the number of micronekton (fishes and squid) because of the expanded oxygen minimum layer. 

The mouth of the Gulf of California is a region of well-developed oceanic fronts, which strongly affect the horizontal distribution of epipelagic species. When these features are intersected by the diel vertical migrations of mesopelagic animals, the resulting aggregation patterns reflect the hydrographic preferences of the migratory species. Our studies of fronts in Monterey Bay have shown distinct patterns of aggregation and orientation. However, the ephemeral nature of the local fronts makes them very difficult to study. The persistent fronts in the Gulf offer a very promising venue for investigation. We will test the hypothesis that fronts at the mouth of the Gulf help to define the composition of its midwater fauna by creating spatially stable boundaries of prey abundance. 

Hydrothermal vents have strong biogeochemical effects on the adjacent benthic environment, and they can also influence the overlying water column. Previous work on zooplankton has suggested that vent plumes affect the diversity and abundance of planktonic species above the vent fields. We will investigate the water column around and within the vent plumes in Guaymas Basin to determine if they create atypical distribution patters of gelatinous zooplankton and micronekton when compared with Monterey Bay and with other regions of the Gulf. We will also look for physiological and behavioral adaptations to plume chemistry as well as evidence of trophic links to the vent community. 

Studies of bioluminescence will be an integral part of each of the regional surveys. We will trigger bioluminescence during quantitative transects with a low-light video camera, and determine the luminescent signatures of key species in order to characterize the faunal composition in differing hydrographic conditions and habitats. We will measure emission spectra and the kinetics of light production at depth and in the lab. Given its isolation, we expect to find novel bioluminescent systems in the Gulf’s midwater fauna, particularly among the poorly known gelatinous species. 

We will use the ROV’s main video camera to quantify the Gulf’s midwater fauna and to observe behavior patterns. A low-light video system will be employed to study bioluminescence in situ. Specimens for identification, genetic analyses, and laboratory-based physiology and behavior studies will be collected with the ROV’s suction sampler and detritus samplers. Scuba divers will make additional collections of zooplankton. In situ measurements of environmental parameters such as temperature and oxygen concentration will be made with the ROV’s CTDO instrument.

Click here to visit the logbook for Leg 3.