We investigate the ecological effects of central California’s expanding oxygen minimum zone (OMZ) in two basic ways: by analyzing both archived and newly-acquired data gathered by the midwater time-series project, and by making direct measurements of the oxygen consumption rates of midwater species. With our time-series database we examine the vertical distribution patterns of mesopelagic species, and determine the range of oxygen concentrations they have occupied over the last 25 years. We have found some significant changes in depth distribution and changes in abundance that are associated with decreasing oxygen concentrations at depth.

In conjunction with these efforts we conduct in situ experiments with the Midwater Respirometry System (MRS) to measure the oxygen uptake patterns of targeted mesopelagic and bathypelagic species. These data, coupled with lab-based respiration measurements allow us to determine the critical partial pressures of oxygen for these species and thus to link changes in vertical distribution patterns directly to changes in oxygen concentration at the boundaries of their depth ranges.

We measure the respiration rates of midwater animals as a proxy for metabolism in order to determine the physiological effects of declining oxygen concentrations, higher temperatures, and increasing acidity in their environment. To accomplish this we use a first-of-its-kind instrument, developed here at MBARI that allows us to monitor respiration without hauling the subjects out of the depths and subjecting them to decompression and thermal stress in the process. The ROV takes the instrument to depth and places animals in six of its eight chambers (the other two are controls). We hang the instrument on a deep mooring then fly away to do other work, returning in 24 or 48 hours to recover the system and all of its accumulated samples and data.

One of our current targets for respiration studies is the cranchiid squid Galiteuthis phyllura. This species has a bimodal vertical distribution pattern that has a gap centered about the core of the oxygen minimum zone of Monterey Bay. In the last several years, we have measured respiration rates of a broad range of ecologically significant mesopelagic species. Among these is Bathochordaeus, the giant larvacean that comprises a crucial link in the pelagic food web of deep Monterey Bay by contributing to vertical carbon flux through their discarded food filters. This species appears to be shifting its center of distribution upward, in response to the expanding OMZ. We know from our 2003 and 2015 observations in the Gulf of California, that when these animals are exposed to shear forces, such as those found above their preferred depth range, they build smaller feeding structures. Smaller filters mean that they produce smaller and slower-sinking discards, which means less food supplied to the deep benthos.

A long-term goal of this project has been to gain a predictive capability for the consequences of continuing expansion of the OMZ in Monterey Bay. Toward that end, we have been comparing the vertical distribution patterns and respiratory capabilities of midwater species in Monterey Bay with counterpart species in the Gulf of California, where the OMZ is far more extensive. In many respects, we can regard the Gulf’s OMZ as representing the future of Monterey Bay and the Gulf’s midwater community as a precursor of what the expansion of our local OMZ will mean for the future here.

We are also continuing the deep deployments of the MRS to investigate those species that can only be studied at considerable depth. In our deep (>1000 m) deployments to date, we typically find the specimens (jellies and shrimp) alive and apparently in good shape at depth when we return to pick up the MRS after incubation. However, they almost never survive the subsequent trip to the surface; this is strong evidence that the only way to study them is with the MRS in their natural environment. The bathypelagic shrimp Boreomysis californica will continue to be our principal deep target species so that we can compare its physiology and ecology with the vertically migrating, mesopelagic shrimp, Sergestes similis.