“Through both models and observations, we can see that the biology and chemistry of the oceans are responsive to physical disturbances and forcing at all scales and that even relatively subtle variations in the mean state can have profound implications for biogeochemical cycling, ocean fisheries, harmful algal blooms, etc.”

There are probably a couple of explanations for the citation trend. First, I was fortunate to have contributed to new paradigms that have changed the way we look at ocean biology and biogeochemistry—the notion that microbial components of the food web dominate the open oceans, and the idea that a trace element, iron, limits plankton biomass and production over large regions of the oceans. Both came into their own during the past decade or so, and are now widely accepted and frequently cited. Second, the same time period was one of substantial investment in large national and international programs aimed at understanding carbon cycling and related phenomena in ocean ecosystems. My lab has been active, for example, in the major JGOFS (Joint Global Ocean Flux Studies) projects in the equatorial Pacific, the subtropical Pacific, the Arabian Sea, and the Southern Ocean, and that body of work is cited both for its contributions to regional oceanography as well as, more generally, the processes we study. Perhaps as much, if not more, than citation numbers, JGOFS has also influenced the context in which such research is viewed and the journals where it is published—not as pure biology or ecology, but as interdisciplinary and geosciences oriented.

  What are the circumstances which led you to your work?

Like many careers, mine has taken some twists into unplanned directions that make sense in retrospect but which turned on the unique circumstances of specific moments in time. One important moment for me came in late 1977 when I was a post-doc in the Food Chain Research Group at the Scripps Institution of Oceanography. I had recently completed my dissertation at the University of Washington on the population dynamics of a planktonic copepod, and I was studying the selective impacts of small predators that feed on copepods. One day, quite by accident, I got pulled into a conversation that my advisor Mike Mullin was having with Dick Eppley about how one might be able to account for concurrent grazing losses to small protozoans in primary production experiments. I had not thought about that kind of problem before, but it seemed intuitive, given my background, that predator-prey encounter frequencies could be reduced by diluting the plankton sample with filtered water and that this would lead to measurable changes in the net growth rates of phytoplankton, which could be used to compute population growth and death rates. So I offered those ideas and later that evening developed the mathematical construct for how such experiments would work. This amounted to a few hours of work, which I viewed as more of an academic exercise that anything else at the time.

When I finally published the method a few years later, the microbial part of the food web was just beginning to be recognized as significant, so it arrived at just the right time to be useful in some of the initial work verifying high phytoplankton growth rates in the oligotrophic open oceans (PRPOOS) and in testing the grazing control hypothesis for high-nutrient, low-chlorophyll (HNLC) conditions in the subarctic Pacific (SUPER). The dilution method was later adopted as a standard protocol for assessing microzooplankton grazing impacts in the JGOFS carbon cycling program, and my work in JGOFS and SUPER got me involved in field tests of the iron-limitation hypothesis for HNLC regions (IronEx II and SOFeX). Looking back on it, there is a logical flow to this work, which is entirely consistent with my original interest in plankton population and community-level ecology. However, I have often thought that my career would have had an entirely different emphasis had it not been for that chance conversation so many years ago.

  How would you describe the significance of this work for your field?

What we aim to do is to break down the dynamics of complex microplankton communities in terms of growth and death processes. Both processes need to be quantified to understand, at a mechanistic level, the factors that control the increases or decreases of populations and the net changes in community composition, and ultimately to be able to predict future states of the ocean ecosystem. Applications of our experimental approaches throughout the oceans have demonstrated that protistan micrograzers are a ubiquitous and high-dynamic component of food webs that consume on average about two-thirds of ocean primary productivity. More importantly, the methods have been useful for testing hypotheses having to do with phytoplankton growth rates, grazer regulation and iron limitation in the open oceans, and for looking at biological responses to natural physical forcing at varying temporal and spatial scales.

The link between plankton dynamics and carbon cycling was made fairly explicitly in the JGOFS working hypotheses—that variability in physical forcing affects nutrient availability, which determines in turn the structural organization of plankton communities and the partitioning of carbon primary production between recycling and export fates. What this means is that carbon and nutrient recycling in the euphotic zone will be high and less carbon will be available for export to the deep sea when protistan micrograzers consume a high proportion of primary production, as is generally the case. Locally or seasonally, however, one can find hot spots of disproportionately high carbon export under physical and nutrient conditions that decouple production and grazing processes. These same principles apply broadly to richer and poorer regions of the oceans, but there is another wrinkle to consider on the global scale. Large areas of the oceans—most of the Southern Ocean around Antarctica, the eastern equatorial Pacific, and the subarctic Pacific—have high, unutilized concentrations of major nutrients (e.g., nitrate) in the euphotic zone, which represent an untapped reservoir for sequestering more carbon to the deep sea.

Thanks mainly to the vision of John Martin and the persistence of Kenneth Coale and other colleagues at Moss Landing Marine Laboratories and elsewhere who have led open-ocean tests of Martin’s hypothesis, these HNLC regions are now clearly demonstrated to be limited by iron. In essence, however, these seemingly aberrant regions display familiar behaviors with respect to carbon cycling or export, to the extent that iron becomes available by various physical mechanisms. We are still in the early stages of understanding the details of how this works, and how it may have influenced climate on geological time scales.

Over the past 30 years or so, the field has made enormous leaps in understanding the structure and dynamics of pelagic systems. Progress has been particularly revolutionary in the areas of marine microbial ecology, the role of iron as a limiting micronutrient, the recognition of decadal-scale patterns in ecosystem variability and capabilities for investigating the coupling of biological and physical processes from small to large scales. Even into the late 1970s, the microbial portion of the food web was generally ignored, large regions of the central open oceans were viewed as unproductive and invariable biological deserts, and individual expertise tended to be disciplinary-defined and regionally focused. As far as microbes go, the most important primary producer in the oceans, a photosynthetic bacterium, was not even discovered until the late 1980s, and since then new microbes with capabilities previously unknown to science have continued to be discovered at an impressive pace.

At the large scale, the new age of satellite observing systems, international carbon and climate-based programs, and massive personal computing power has brought global, interdisciplinary, and synthetic (modeling) perspectives to the field. Ocean variability is now tangible and easily observed. We better appreciate the organizing principles of marine food webs, how ocean regions are connected ecologically and biogeochemically, and the underlying similarities and differences in their expressed characteristics. Through both models and observations, we can see that the biology and chemistry of the oceans are responsive to physical disturbances and forcing at all scales and that even relatively subtle variations in the mean state can have profound implications for biogeochemical cycling, ocean fisheries, harmful algal blooms, etc.

  Where do you see this research going 10 years from now?

Despite recent advances, our understanding of the details of marine plankton dynamics and their implications for biogeochemical cycles and climate change is still lacking. In the not-to distant future, I believe that we can reasonably look to molecular techniques to provide the means for rapid quantitative analyses of populations by species, group, or function. This ability to analyze concurrent dynamics of many populations will open the door to new opportunities to develop and test our understanding of community responses to natural perturbations and experimental manipulations. Even as we seek new details, however, we will still need to address the challenge of separating the wheat from the chafe so that we can incorporate the results effectively in predictive models. This may require an entirely new theoretical construct, or perhaps just a new perspective on current ideas. I feel, however, that one cannot reasonably arrive at an abridged understanding of pelagic community dynamics without knowing more about the details and complexities of the underlying mechanisms and relationships in natural associations.

Michael Landry, Ph.D.
Scripps Institution of Oceanography
University of California, San Diego
La Jolla, CA, USA