Methods of sampling and analysis and our concepts of ocean dynamics

This post was chosen as an Editor's Selection for ResearchBlogging.orgI read a paper today (actually, more like an essay) by Peter Wangersky, a longtime chemical oceanographer. Titled “Methods of sampling and analysis and our concepts of ocean dynamics,” it is essentially a personable ramble through six decades of marine science, reflecting on the technical capabilities and sampling methods over time and the way those capabilities and methods influenced the assumptions that were made and the questions that were posed—in essence, the working mental picture we have of the ocean. Things have indeed changed a lot since he began during World War II. The paper is full of quotable nuggets:

Adding machines did exist, and some could even be made to multiply, after a fashion. They were strictly mechanical, however, and the wear and tear on various joints, machine and human, coupled with the high frequency of random inputs, discouraged us from the use of any but the most necessary statistical tools. Perhaps this is why most analytical chemists of this vintage believe that if one does the analyses right, there’s no need for statistics. This is a self-reinforcing fallacy; if you don’t do the statistics, you never discover your limitations or the limitations of your methods and the universe you are sampling.

He also talks about the analysis of water samples for salinity, in the days before CTDs could measure it in-situ with electrical conductivity:

A group of female technicians, the salinity girls, ran the Mohr-Knudsen silver nitrate titrations to a chromate endpoint for eight hours a day. Needless to say, there was a considerable turnover in this group. There was always a shortage of salinity girls, and the sample bottles kept coming in from the ships and stacking up in the hallways.

Those were different times…

The most interesting aspect of the paper is his discussion of the shift in emphasis starting to occur from ship-based sampling to automated sampling with collections of stationary instruments. My thesis research is using a stationary echosounder, so I’ve done a fair amount of thinking about the differences between the traditional ship-based approach and the more recent ocean observatory approach. I appreciated hearing about this shift from the perspective of someone who has seen the entire evolution of modern oceanography firsthand. He doesn’t discuss it in the same vocabulary I might, but he comes to essentially similar conclusions.

Wangersky’s point is that our perception of the ocean and its dynamics is hugely dependent on the tools we have available to study it. It will be a very interesting time in the coming years, as more types of sensors and instruments are deployed long-term at more locations around the world. Just by virtue of observing the ocean at a new spatio-temporal scale, we’re likely to find stuff going on that we missed before. In the old days, oceanographers generally assumed that the ocean was in a steady state. When hydrographic samples were few and far between, and you had to bring them back to shore for the “salinity girls” to analyze before you knew what they meant, this assumption was kind of necessary. These assumptions were also kind of wrong, which is the point that Stommel made in his famous 1963 paper (remember Stommel?).

As instruments, methods, and computing power have increased, these types of oversimplifications have retreated, to the point where we now embrace variability and dynamic changes, and try to understand them in and of themselves, rather than as unwanted noise on top of some supposed equilibrium or steady state that isn’t really there. Every advance in instrumentation and every expansion of the scope of our observations has yielded a new perspective on the oceans, adding on to an understanding that is slowly and gradually becoming more complete.

Peter J. Wangersky (2005). Methods of sampling and analysis and our concepts of ocean dynamics Scientia Marina, 69 (S1), 75-84 : 10.3989/scimar.2005.69s175

2010/06/12

Filed under: Research Blogging — Tags: , — Sam @ 3:33 am

Varieties of Oceanographic Experience

This post was chosen as an Editor's Selection for ResearchBlogging.org

I’ve been thinking a lot lately about issues of scale in ecology, both because I’m taking a fascinating seminar on the topic this quarter, and because my particular research is conducive to thinking about them. “Scale” came to the fore as a topic of interest starting in the late 70′s, and is tied up with other concepts that were on the rise at that time, like chaos theory, fractal geometry, and nonlinear dynamics. An example of one of these ties is Benoit Mandelbrot’s famous question, “how long is the coast of Britain?” The answer, as he shows, depends on how long your ruler is. This turns out to be a recurring conundrum: the pattern detected by an ecological study depends very strongly on the scale of the measurements.

In 1963, fifteen years before most other people started thinking about this stuff, a short paper was published in Science that, in just four pages, managed to lay out how and why we need to consider the scale of our measurements when designing an experiment to measure oceanographic phenomena. It was written by Henry Stommel, which is not at all surprising if you know who he was. For those who don’t, Stommel was one of the original badass physical oceanographers. The kind of guy who could sit down with a pen and paper and demonstrate why the upper ocean circulates the way it is observed to. Or correctly derive the circulation of the deep ocean before it was observed. No big deal. But thanks to his 1963 paper, titled “Varieties of Oceanographic Experience,” he is cited far and wide to this day as one of the first scientists to explicitly consider the importance of scale to experimental design.

To illustrate why scale is important, take one of Stommel’s examples: say we want to measure changes in the height of the sea surface from month to month. At first glance, it would appear that only 12 measurements are necessary: go out once a month for a year, and you’re set. At second glance, however, this is ridiculous on several levels. The ocean has tides, for one. Measuring sea level once a month would give you a near-random sample of different parts of the tidal cycle, and you wouldn’t be able to detect any long-term trends. There are also shorter- and longer-term fluctuations: regular wind-driven waves, and sea-level deviations due to large oceanic eddies. To resolve sea-level changes from month to month, you will actually need to measure it every hour or so. Do it less frequently, and you will get results that are inconclusive or just plain wrong.

Choosing the right scale of measurement for your question of interest is very important. It is also not trivial, especially when the measurement is not as straightforward as the water level on a ruler nailed to the dock. Even today some people are prone to testing hypotheses using data collected at a scale inappropriate to the question. If Stommel had stopped with this message, the paper might still have found a fair number of readers. But the coup de grace was the graphic he came up with to make the message explicit:

Figure 1 from Stommel (1963).

Figure 1 from Stommel (1963).

It’s a three-dimensional surface, with time along the x-axis and distance along the y. Both are shown on a logarithmic scale, so that each tick mark is a factor of 10 larger than the one before it. Various phenomena that make the sea level go up and down are located on this surface based on their typical size and duration. The height of the surface at each point represents how much the sea level goes up or down—that is, how much energy or variability is concentrated at that space and time scale.

Gravity waves (aka wind waves, the normal ones that crash on the beach and make you seasick on boats) are typically several meters long and perturb the sea surface for a few seconds. They can therefore be placed in the lower left corner. Tides happen every 12-13 hours and affect the entire ocean basin, so they are located near the middle of the time axis, stretching from about a kilometer up to 10,000 km. And every ten thousand years or so, we hit an ice age that lowers the surface of the ocean everywhere about 100 meters, allowing humans to do things like cross the Bering Strait into North America. This figure is an elegant summary of all the different processes that perturb the sea surface, and of the spatio-temporal scales at which they all take place.

This kind of diagram (now known as a Stommel diagram) has found its way over the years into all kinds of different contexts. One direct descendent near and dear to my own heart is the one drawn up by Haury et al. in 1978. It is along similar lines, but shows variability in zooplankton abundance, not sea level. The possibilities for these diagrams are nearly endless, and not limited to the ocean, or even the natural sciences. Any system that has stuff going on over short and long distances and time spans can be clarified by sketching up a Stommel diagram. Drawing a picture like this can help make it clear how to approach your research question, and will hopefully help you avoid screwing it up by choosing the wrong scale of measurements. The hope, as Stommel put it in the last line of the paper, is to “look forward to a time when theory and observation will at last advance together in a more intimately related way.” Amen.

Stommel, H. (1963). Varieties of Oceanographic Experience: The ocean can be investigated as a hydrodynamical phenomenon as well as explored geographically Science, 139 (3555), 572-576 DOI: 10.1126/science.139.3555.572

2010/05/16

Filed under: Research Blogging — Tags: , , — Sam @ 9:50 pm

Krill v. Salps in the Southern Ocean

Last week, writing about copepods, I mentioned that they make up what is probably the most massive group of animals on earth. I also mentioned the likely runner up: krill. In particular, the Antarctic krill, Euphausia superba.

The Euphausiids are a major group of small, shrimp-like crustaceans found worldwide in the marine plankton. Euphausia superba is probably the best-studied, and certainly the most abundant, of these species. They live all around Antarctica in the Southern Ocean, and are usually the dominant macrozooplankton grazer, occurring in vast, patchy swarms. But they are not the only one out there.

ResearchBlogging.orgTheir main competitor for the king of Antarctic plankton is a particular salp species, Salpa thompsoni. Salps are pelagic tunicates, barrel-shaped gelatinous animals that move and feed using a kind of lethargic jet propulsion, drawing water in one end of their bodies and filtering it for food particles before pushing it out the other. They also reproduce incredibly quickly, thanks to two features:

  1. Their bodies are mostly water, so they do not need to actually grow that much material on their way to full size, and
  2. they alternate sexual and asexual generations, with the asexual generation budding off long chains of self-feeding clones.The sexual clones then release sperm and eggs into the water, where they fertilize and grow into the next generation of asexual salps.

Together, these two facts enable salps populations to respond explosively within a few days of phytoplankton blooms. Krill, on the other hand, live several years, and spawn all at once in the early Austral spring. Their larvae will not reproduce until the next year.

Krill and salps do not tend to do well at the same time. Good years for salps tend to be bad ones for krill, and vice versa. What’s more, abundances of both are related to the extent of winter sea ice—positively for krill, and negatively for salps. This is due to a combination of their different reproductive strategies, as well as different food needs.

During the winter, krill live underneath the sea ice, and feed on ice algae, which grows on the underside of the ice pack. Well-fed krill are better prepared to produce eggs and spawn early in the season. Salps, on the other hand, can’t scrape the algae of the ice, but can respond quickly to open-water phytoplankton blooms. The upshot is that following cold winters with more sea ice, krill spawn more successfully, leading to a bigger year-class the next season. In winters with little sea ice, krill do not spawn as successfully, but salps explode as soon as the ice retreats and the spring phytoplankton bloom begins.

Over the last half-century, temperatures have been trending upwards in the Antarctic, and sea ice extent has been trending down. Along with these changes, “krill years” appear to have become less frequent. This does not augur well for the parts of the Antarctic food web that feed on krill—whales, seabirds, penguins, fish, and many others. Salps, though totally cool and more reproductively energetic than rabbits, just don’t have the crunch, tang, and oily, protein-ey goodness of Euphausia superba, the superb Antarctic Krill.

V Loeb, V Siegel, O Holm-Hansen, R Hewitt, W Fraser, W Trivelpiece, S Trivelpiece (1997). Effects of sea-ice extent and krill or salp dominance on the Antarctic food web Nature, 387, 897-900

2010/03/15

Filed under: Research Blogging — Tags: , , , , , , — Sam @ 7:40 pm

Sustainabewildering Seafood

I just finished reading a new paper from Jennifer Jaquet et al., mostly from Daniel Pauly’s group at UBC. The paper is titled “Conserving wild fish in a sea of market-based efforts,” and it appears in the current issue of the conservation biology journal Oryx. In it, the authors investigate the proliferation and effectiveness of the many consumer-education and sustainability-certification schemes that have proliferated in the past decade or so. I didn’t realize how many there were. Table 1 in the paper shows a list of everything out there, from dolphin-safe tuna in the late 1980′s, and kicking off for real in 1997 with the Marine Stewardship Council and Monterey Bay Aquarium’s Seafood Watch. From there, the next four pages are filled with a list of every market-based sustainable seafood initiative since then.

ResearchBlogging.orgThey conclude, not surprisingly, all these cards and labels can get confusing. Ask any well-meaning seafood shopper who has tried to figure it all out. Alternatively, ask the well-meaning shopper’s marine biologist friend or family member—I’m fairly up to date on these issues, understand the principles of marine ecology, and am conversant in stock assessment science and fishery management (at least as practiced in the good old US of A). And I still need to do research to really figure out what is good to eat and what isn’t. With the proliferation of these market-based initiatives (seafood cards, certification schemes, boycotts, distributor guides) public awareness has been raised, but there is no evidence to indicate that the proliferation has led to a measurable conservation benefit for any threatened species.

They also propose several solutions to this confusion, some of which I think are better than others. The best of them, in my opinion, is directing the conservation message higher than consumers. It is reasonable to expect consumers to give a damn about whether the fish they are buying is riding the road to collapse or not. It is less reasonable to expect them to become experts in worldwide fisheries. The conservation movement would probably do better to to work with a smaller number of distributors, retailers, and chefs. As a former line cook, I can say that chefs—at least the good ones—really give a damn about the food they serve. Read this from my first chef if you need proof (warning: salty language…). There aren’t enough scientists and conservationists to build trusting relationships with every seafood eater, but we can come a bit closer with the next step up the supply chain.

Their other good ideas are eliminating harmful subsidies (that’s more a government policy one than market-driven consumer-education one) and making the seafood supply chain more transparent. The latter is, in my opinion, really important. It won’t change everyone’s behavior on it’s own, but knowing where fish came from (and for that matter, just what kind of fish it actually is…scrod, anyone?1) is a necessary precondition to a truly sustainable system.

They have a few less-good ideas, too. IMO. One is using more negative messaging. I’m fairly skeptical that this is a good strategy in the long run. After a while, people start to tune out criticism and pessimism if it’s arriving in a constant stream. Another is setting seafood consumption targets (in addition to catch quotas). I think this is kind of analagous to an energy tax as opposed to a carbon tax: we don’t actually care if people use more energy or eat more seafood. We care if too much CO2 enters the atmosphere, or if we catch fish faster than they can make babies. In theory, if we can effectively control the negative externality (pollution or resource mining), the rest will take care of itself. In theory, of course, all theories are correct—but I think this one is more or less right.

The fundamental truth of these consumer-education initiatives is that the ecological health of the world’s oceans can’t fit on a wallet card. Any set of information that will fit on a wallet card will necessarily leave out species, unfairly tar some responsible fisherman, give some irresponsible ones a pass, and grossly simplify an incredibly complex issue. I think they’ve done a lot of good—the demand among fisheries for eco-certification is proof that there is consumer demand for sustainable fish. But ultimately, as Jaquet and friends recognize, they aren’t the be-all and end-all. All the bike riders in Seattle won’t stop global warming, and all the wallet cards in the world won’t stop overfishing. To do that we need the really hard stuff: international agreements, transparent and traceable supply chains, and effective, adaptive, ecosystem-oriented, socially-aware, science-based management. I’m not totally on board with all their solutions, but that much I think we’d agree on.

Jacquet, J., Hocevar, J., Lai, S., Majluf, P., Pelletier, N., Pitcher, T., Sala, E., Sumaila, R., & Pauly, D. (2009). Conserving wild fish in a sea of market-based efforts Oryx, 44 (01) DOI: 10.1017/S0030605309990470

1 A businessman comes out of Logan Airport and hails a cab. He gets in and asks the cabbie “Hey, where’s a good place around here to get scrod?” The driver looks at him in the rearview mirror and says, “Buddy, I’ve heard that question many times in many different ways. But this is the first time I’ve heard it in the past pluperfect subjunctive.” {badump-tshhhhhhhhh}

2010/02/04

Filed under: Research Blogging — Tags: , , , , , , — Sam @ 11:25 pm

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