Notes from October 23, 2013 Geological Fluid Mechanics Seminar

Thin Phytoplankton Layers: Characteristics, Mechanisms and Consequences, (2011), William M. Durham and Roman Stoker, Amer. Rev. Mar. Sci.

George started it off with a nice summary of the paper. The paper sections described:

1. Observations of thin phytoplankton layers (TPL) – scientists find them in many coastal and ocean settings with a range of sizes. TPL are important because they aggregate large numbers of phytoplankton in a small space – which has ecosystem consequences.

2. Theoretical mechanisms for how they form and what keeps TPL from breaking apart. Figure 2 describes 6 types of mechanisms (straining, convergent swimming, buoyancy, gyrotactic trapping (voted the class favorite – what’s not to like about gyrotactic!), in situ growth and intrusion). Shear and nutriclines/light abundance are the two major types with the latter being one of ‘choice’ and the former out of the little bugger’s control.

3. The authors ran models on concentrations they would expect to find under theoretical mechanism conditions.

4. Models are insufficient – therefore there is a need for field evidence.

5. The authors then go through field observations

6. They try and link the observations to mechanisms

7. The paper then moves onto trophic/ecosystem observations – with figure 5 nicely summarizing the results.

8. The review finishes with summary points.

We discussed whether the paper was too general, whether it had enough context. Individual reactions to the paper seemed to depend on the reader’s background – definitely suitable for a general audience unfamiliar with the subject matter.

We all agreed the figures were extremely good – particularly Figure 2, which provided a great summary and the details in Figure 6 – though the paper may have gone a bit overboard on the sub- headings with one for every paragraph in some sections.

As a group, we posited that uncertainties regarding species diversity or variability within any given TPL could mean that no single mechanism was responsible for creating or maintaining TPLs – need more data on species types and ecosystem dynamics. Individual TPLs could conceivably exist due to multiple mechanisms/ along with feedbacks from ecosystem dynamics.

We spent some time talking about motility and life at low Reynolds numbers. Several folks hypothesized that the phytoplankton can adjust their speed as necessary – which could explain the discrepancy between measured lab motility values 10x the theoretical TPL velocity.

Alan provided more detail on eddy diffusivity – a measurement of how much things are being stirred apart.