Zooplankton in the Deep Sea
Karen Wishner, Professor
Graduate School of Oceanography
Karen Wishner earned a BA from the University of Chicago and a PhD from Scripps Institution of Oceanography. She came to GSO in 1980. She has studied marine zooplankton ecology in the deep sea, Georges Bank, and other regions of the world and spent many months at sea on research vessels. She teaches an undergraduate course on deep-sea biology and several graduate-level zooplankton courses.
The deep-sea water column (below about 500 meters)
is usually thought to be an environment with extremely harsh conditions and
ill-suited for supporting life. It is cold, the pressure is high (1atm pressure
for each 10m of depth), and because there is no light for photosynthesis so
far beneath the surface, animals depend on the few food particles that sink
from the productive upper ocean. Because this food is consumed as it sinks,
plankton abundance (number of animals per volume of water) and biomass (amount
of carbon in living animals per volume of water) decrease exponentially with
increasing depth. Even with these seemingly harsh conditions, the deep-sea water
column is a complex environment with many interesting and unusual organisms
and processes. Recent work with submersibles, remotely operated vehicles (ROVs),
and deep-sea net tows reveals some of the unusual adaptations of the organisms
and the importance of this community to global biogeochemical cycles in the
present and in the geological past.
My lab and that of my colleague Marcia Gowing, a research scientist at the University of California at Santa Cruz, have been studying a deep-sea pelagic interface, the lower boundary of the oxygen minimum zone (OMZ). The OMZ is a midwater region of low oxygen, a permanent feature of the water column that usually occurs at several hundred meters depth. It is several hundred meters thick (see Fig. 1). In some locations, oxygen levels decline to almost zero in the OMZ, then increase again below about 1,000m. When the oxygen is this low (less than about 0.15ml/l), most of the larger plankton are unable to survive and plankton abundances are very low. In 1988, however, we discovered that the lower interface of this zone, the region where oxygen starts to increase again with depth (about 800m at this location), contained a narrow zone of unusually high zooplankton biomass. Since then, we have studied this phenomenon in two locations, the eastern tropical Pacific and the Arabian Sea. With a variety of tools, ships, and colleagues, we have investigated plankton abundances and distributions, feeding rates and food webs, responses of this deep-sea community to the seasonal cycle of monsoons, and interactions with deep-sea biogeochemical cycles and the benthic (sea floor) community.
Our discovery was serendipitous. The first clue that this oxygen gradient was important biologically came during a series of dives with the submersible Alvin on a seamount (Volcano 7) in the eastern tropical Pacific. We noticed a remarkable benthic phenomenon while we studied (with Lisa Levin, Scripps Institution of Oceanography, and Lauren Mullineaux, Woods Hole Oceanographic Institution) the near-bottom zooplankton along the slope of Volcano 7. Near the seamount summit at about 730m below the sea surface, there were virtually no large benthic animals. Just a few meters deeper however, at about 800m, there was a profuse benthic population of large animals. It turns out that this seamount extends up into the extremely low oxygen water of the eastern tropical Pacific OMZ. Oxygen levels at the summit were apparently too low for the survival of large animals, but where oxygen levels became higher at greater depth, many large animals could exist. Our discovery was made on the next-to-last dive of the cruise. After much post-dive excitement and discussion, we reorganized the last dive to explore the zooplankton response to this new environmental feature. In a long and complicated dive the next day, we sampled zooplankton and particle distributions and performed in situ zooplankton feeding rate incubations at the upper and lower summit depths to obtain good data for these two contrasting environments (see Fig. 2). It worked!
The next opportunity to study the phenomenon was seven years later during the Joint Global Ocean Flux Study (JGOFS) program in the Arabian Sea. JGOFS is a large multidisciplinary, multi-investigator, multinational program that investigates carbon and climate cycles throughout the world's oceans. The Arabian Sea was chosen because of its large annual monsoon cycle and some unusual chemistry that may influence global climate change. The Arabian Sea also has an extensive and pronounced OMZ. We welcomed the opportunity to study the OMZ, its zooplankton, and its lower interface in more detail, in another region of the world (one with a strong surface seasonal signal---the monsoon winds cycle), and in the context of a big scientific program with copious data being collected by many experts. In this program, we used a large opening-closing plankton net deployed from the ship, a double MOCNESS (see Fig. 2). We sampled 16 depth strata from 1,000m to the surface at stations across the Arabian Sea during four, month-long seasonal cruises. These were busy cruises out of Muscat, Oman, on the R/V Thompson from the University of Washington. On each cruise there were about 20 principal investigators with their students and technicians, each with a big program, vying for limited instrument deployment time, and operating on a tight and inflexible schedule. The large amount of high-quality data dealing with every conceivable process, organism, chemical, current, hydrographic variable, and weather feature made this a rich integrative data set, which we continue to analyze, synthesize, and discuss with colleagues from the program.
What have we learned about the lower OMZ interface from our work during the last decade? First, there is a marked zooplankton biomass peak at the base of the OMZ, in the depth zone where oxygen starts to increase. A zooplankton biomass profile from one of the Arabian Sea stations is shown with its associated oxygen profile in Figure 3. Biomass was low where oxygen was low for a substantial vertical distance. Most zooplankton were excluded from this suboxic water. At the stations with very low midwater oxygen, there was typically a secondary zooplankton biomass maximum associated with the zone where oxygen increased from 0.05 to 0.1ml/l. The depth of this feature varied from about 500m to more than 1,000m, depending on the geographic location, and was probably less than 100m thick. Zooplankton biomass in the well-oxygenated surface waters above the OMZ is, of course, at least an order of magnitude higher than these deep-sea biomasses, but the deep subsurface peak is a significant ecological feature within the deep sea. The shapes of the vertical profiles varied with geographic location across the Arabian Sea but were consistent year-round at any particular location. The profiles were also remarkably similar between the Arabian Sea and the eastern tropical Pacific. This suggested long-term stability and an overall worldwide similarity in the zooplankton response to OMZs.
Second, there is an unusual assemblage of species that are abundant here, and some of these species are highly and specifically adapted to the strong oxygen gradients of this environment. Zooplankton from within the OMZ are pale and sparse (see Fig. 4), except for some fish that migrate down into this zone each day but migrate back up to the surface at night. The sample from the subsurface biomass peak at the base of the OMZ is full of bright red shrimp and copepods and black fish that remain there day and night. We have been studying the biology of one of these copepod species, Lucicutia grandis, a common inhabitant which in abundance indicates the lower OMZ interface (see Fig. 5). A vertical distribution of this copepod is shown in Figure 3. Its abundance peak is associated with the same general oxygen boundary as the overall biomass peak but is slightly shallower. In fact, it looks like each species and lifestage probably has its own unique oxygen and depth-related distribution within the overall biomass peak. My graduate student Mary Rapien is presently studying the fish and shrimp fauna of this interface zone.
A third important point is that the lower OMZ interface appears to be a zone of high physiological activity and rates. The permanent residents actively feed and reproduce. We know these copepods are feeding at high rates because we have measured feeding rates of individual zooplankton in the eastern tropical Pacific using the submersible Alvin. The rates were measured in situ at depth in special cod end incubation chambers (see Fig. 2) using a tracer bead method that we developed in the lab, with the help of my former graduate student Carin Ashjian. Marcia Gowing has done extensive transmission electron microscopy of the gut contents of many individual specimens from both the eastern tropical Pacific and the Arabian Sea. Lucicutia is an omnivorous detritivore that feeds on many types of particles and probably preys on other zooplankton. Some of the particles are known to originate at the sea surface, but other material in the guts seems to come from the deep sea. Gowing has also found evidence for a direct feeding link from bacteria to these copepods. The bacteria aggregate and are consumed directly by the copepods; this eliminates several steps on the food chain, and provides an ecological shortcut and an energetic advantage.
We know that these copepods are reproducing because we see individuals with spermatophores, the male sperm sac that is placed on the female during mating. We also see changes in the relative abundance of different lifestages over time, which is indicative of growth and development. There may even be some direct influence of the monsoon seasonal atmospheric cycle. The southwest monsoon (July-August) causes a large increase in the amount of material sinking from the sea surface, and we see indications of increased reproduction in these deep-sea copepods at that time.
With my technician Celia Gelfman, we are continuing to investigate the environmental interactions of the various species in this interface zone and their ecological significance. We think this narrow location in the deep-sea water column is one of enhanced activity and processes that might be important for the deep-sea ecosystem and for oceanic biogeochemical cycles. Many processes in the ocean may be localized in "hot spots" or interfaces such as this one, although many of the other interfaces (such as ocean fronts or swarms of plankton) are probably much more ephemeral than the OMZ. However, over geological time, the extent of OMZs has varied, and extensive global warming may cause changes in the future. An understanding of the deep-sea oxygen phenomenon will help us understand past oceans and climates, and we hope to be able to predict the consequences of global change.
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