Phytoplankton Ecology

  1. Why people should care about phytoplankton ecology
    1. They provide the basis for much of the aquatic food chain
    2. Substantial impact on global environmental balance
      1. Global phytoplankton primary productivity is estimated at 1015-1016 g carbon/year
    3. They can form nuisance blooms
      1. Sheer abundance can lead to anaerobic conditions, etc.
      2. Toxin production
      3. Toxin production can sometimes be a problem even when a bloom is not present
      4. Pseudonitzschia
    4. Provides an educational complement to terrestrial ecology
  2. What are phytoplankton?
    1. Photoautotrophs adapted to life floating in open water rather than attached to some substrate.
    2. Phytoplankton communities are surprisingly diverse (G. Evelyn Hutchinson's "paradox of the plankton"). A single plankton tow might have as many as50-100 co-occurring species. Apparently there is relatively little competitive exclusion.
    3. May be unicellular or multicellular. Sargassum is a very large and complex member of the phytoplankton.
    4. Size is one important way to classify plankton. Unfortunately, different authors use different break-points for size classes. We have been following your text book, which uses the following scale:
      1. Macroplankton are > 200 µm in diameter.
      2. Microplankton are 20-200 µm (micrometers) dia.
      3. Nannoplankton are 2-20 µm (micrometers) dia.
      4. Picoplankton are 0.2-2 µm (micrometers) dia
      5. The nannoplankton and picoplankton were neglected until recently, and their environmental significance is an area of active research.
    5. Growth rates for smaller plankton can be quite high. Some cyanobacteria are capable of doubling several times in a single day, while even in relatively slow growing organisms (e.g., certain diatoms) can double every week or two.
      1. If you want to follow population dynamics in these organisms, you have to sample frequently, or else you will miss the key events.
    6. Phytoplankton are phylogenetically diverse, and have dramatically different physiological capabilities. To understand the community dynamics, it is necessary to track the diversity of the community.
      1. Important phytoplankton groups include cyanobacteria, cryptomonads, haptophytes, several heterokont groups, dinoflagellates, euglenoids, and green algae.
  3. Distinctive properties of the aquatic environment
    1. No solid substrate
      1. In lakes, streams, and coastal environments, there is typically a bottom, although it may be below the euphotic zone
      2. In the open ocean, and in a few freshwater environments, the bottom sediments are so far away that they are effectively unavailable to the plankton.
    2. Light is an essential nutrient for phototrophs
      1. Available only close to surface (how close is a function of water clarity)
      2. Secchi depth -- the depth at which a secchi disk is no longer visible from the surface, thus clearer waters have greater secchi depths
      3. Euphotic zone -- the region with enough light (PAR) to allow photoautotrophic survival. Note that this is in part defined by the action spectrum for photosynthesis, which varies with taxon
    3. Circulation can move an organism out of the euphotic zone (bummer)
      1. Adaptations to control position in water column can be very important
      2. Size -- small particles sink more slowly than large ones
      3. Density -- low density results in slow sedimentation
      4. Food storage as low-density material -- lipid
      5. Cyanobacteria produce gas vesicles
      6. Marine diatoms replace denser ions with lighter ones to reduce their density
      7. Form resistance -- certain shapes tend to settle slowly. Spines and certain colony shapes can help this (C.S. Reynolds)
    4. In the open ocean, detritus and debris can settle out of the reach of surface circulation. When this happens, their nutrients become unavailable to organisms at the surface.
      1. Thus nutrient availability is often limiting for marine phytoplankton
      2. Many phytoplankton can survive in very low nutrient environments, and some have elaborate adaptations to capturing and sequestering nutrients.
      3. In certain areas upwelling brings bottom waters -- and their dissolved nutrients -- to the surface. Tremendous phytoplankton blooms can occur in these areas. With high primary productivity, the entire food chain benefits; zooplankton and fish multiply.
        1. El Niño (ENSO) was named because a periodic change in circulation in the southern Pacific would cause upwelling off the coast of Chile to fail, which would cause a dramatic drop in primary productivity. The zooplankton would quickly be eaten up by the fish, which would swim off in search of greener pastures, leaving Chilean fishermen without any fish to catch. Because the collapse in primary productivity would typically become noticable around Christmas -- on roughly a seven year cycle -- the fishermen called it El Niño, a reference to Christmas.
      4. Recent research has shown that in much of the open ocean, iron is the limiting nutrient -- it is essential for Ferredoxin, FeS protein. It is also used by cynaobacteria in N fixation.
        1. Some people have even proposed fertilizing the oceans with iron to facilitate algal growth and CO2 uptake as a means of remediating global CO2 production.
        2. Doesn't look practical, and the side effects (unintended consequences) are unknown, and potentially serious. The concept is, however, based on solid science.
    5. The physical properties of water are different from those of air
      1. Denser -- makes it possible to float
      2. Soluability of gasses is relatively low, so both CO2 and O2 may become limiting
        1. There is evidence in the geologic record of periods when oceanic circulation slowed greatly, and huge expanses of the ocean became stratified, with the bottom waters becoming anoxic. Fossil giant clams have been described that are thought to have been dependent upon symbiotic photosynthetic proteobacteria, much as modern giant clams rely on zooxanthellae.
        2. Although the modern ocean is relatively well oxygenated, substantial anoxic regions (sometimes called "dead zones") do exist. Large parts of the Gulf of Mexico, off of the US gulf coast, are anaerobic. Although the cause and extent of these areas is still under investigation, there is reason to think that they may be induced by nutrient-rich runoff from the USA, and that the problem may be increasing.
      3. High thermal mass means that temperature fluctuations tend to occur relatively slowly
      4. Turbulent mixing helps keep phytoplankton in suspension
        1. Various scales of mixing, from small-scale to Langmuir cells.
        2. Mixing time of a basin is a function of the size of the basin. A large lake might circulate once per day, while an entire ocean basin takes many years.
    6. Steady state vs. chaotic environment
    7. Bottom-up vs top-down control
      1. In some environments with very simple trophic systems (i.e., small oligotrophic lakes), introduction of a key predator can influence whole food chain right down to the phytoplankton.
      2. It is unclear what this means for more complex systems.
Required Reading: Just catch up on the heterokont chapters...

Supplementary Reading:

Smith et al., 1992. Science 255:952. [For information on global primary productivity]

Friedmann E.I. (ed.) 1993 Antarctic microbiology. Wiley, New York.

Harris, G. 1986 Phytoplanton Ecology: Structure, Function, and Fluctuation. Chapman and Hall, New York.

Reynolds, C.S. 1984. The Ecology of Freshwater Phytoplankton Cambride University Press, New York.

Sandgren, C.D. 1988. Growth and Reproductive Strategies of Freshwater Phytoplankton. Cambridge University Press, New York.