Plankton: Definition, Physics, and Phytoplankton Overview

Size, Scale, and Units

  • Classroom point of reference: a 1-m (≈3-ft) meter stick.
  • Metric breakdown
    • 1\,\text{m}=10\,\text{dm} (decimeters are rarely used in practice)
    • 1\,\text{m}=100\,\text{cm}
    • 1\,\text{m}=1000\,\text{mm}
  • Visual cue: 1 cm contains 10 mm; a sewing-pin head ≈1 mm.
  • Micron (µm) scale
    • 1\,\text{mm}=1000\,\mu\text{m}
    • Therefore 1\,\mu\text{m}=10^{-6}\,\text{m} (six decimal places smaller than a meter)
    • Scientific-notation recap: 10^{-3}\,\text{m}=1\,\text{mm},\;10^{-6}\,\text{m}=1\,\mu\text{m}

Definition and Etymology of “Plankton”

  • Word origin: Greek planktos → “wanderer” / “drifter”.
  • Operational definition used in oceanography:
    • Any organism unable to swim faster than the ambient water current.
    • May possess limited motility but cannot overcome bulk water motion.
    • Typically (not always) microscopic or at least small.

Reynolds Number & Physical Basis for Planktonic Life

  • Reynolds number (Re) quantifies the balance between an organism’s inertia and the viscosity of its environment:
    • Re=\dfrac{\rho\,v\,L}{\mu}
    • \rho = density of organism (kg m⁻³)
    • v = velocity relative to fluid (m s⁻¹)
    • L = characteristic length/size (m)
    • \mu = dynamic viscosity of the fluid (kg m⁻¹ s⁻¹)
  • Interpretations
    • Numerator ((\rho v L)) ≈ inertia generated by the organism.
    • Denominator ((\mu)) ≈ thickness or “stickiness” of the surrounding fluid.
    • Low Re \rightarrow viscosity dominates → drifting/laminar behavior (typical of plankton).
    • High Re \rightarrow inertia dominates → ability to coast, resist currents (typical of large nekton like whales).
  • You will NOT be asked to calculate Re in this course, but conceptually recognize low-Re vs. high-Re worlds.

Inertia vs. Environmental Viscosity: Illustrative Examples

  • Thought experiment #1: Push a whale vs. push a plankton cell to a marked line.
    • Whale: large \rho, L → high inertia → continues past the line after the push stops.
    • Plankton: tiny \rho, L → negligible inertia → stops almost immediately at the line.
  • Thought experiment #2: Moving water mass (current).
    • If the water parcel shifts a few meters:
    • Whale’s inertia allows it to stay relatively stationary while water flows past (or it can actively swim elsewhere).
    • Plankton are entrained and transported with the parcel—true drifters.

Categories of Plankton

  • Two overarching functional groups (based on trophic strategy):
    1. Phytoplankton (“phyto” = plant-like)
    • Perform autotrophy (mostly photosynthesis).
    • Primary producers & base of marine food webs.
    1. Zooplankton (“zoo” = animal-like)
    • Perform heterotrophy (consume other organisms/particles).
    • Can range from single-celled protozoa to small metazoans.
  • Reminder: the current course unit focuses on phytoplankton.

Phytoplankton Size Classes & Representative Taxa

  • Four principal phytoplankton groups (to be detailed in later lectures):
    1. Cyanobacteria
    • Size: ~<1–\sim10\,\mu\text{m} (10⁰–10¹ µm).
    • Only prokaryotic phytoplankton discussed.
    1. Haptophytes
    • Slightly larger; upper end overlaps large cyanobacteria, generally still <20\,\mu\text{m}.
    1. Dinoflagellates
    • Broad range; can extend from \sim10\,\mu\text{m} up to >200\,\mu\text{m} (approaching >0.5\,\text{mm}).
    1. Diatoms
    • Often the largest single-celled phytoplankton; siliceous frustules.
    • Size overlaps dinoflagellates but many species exceed 200\,\mu\text{m}.
  • Size rule-of-thumb recap:
    • 1\,\mu\text{m}=10^{-6}\,\text{m}=10^{-3}\,\text{mm}
    • 500\,\mu\text{m}\,(=0.5\,\text{mm}) marks transition into objects visible to naked eye.

Cellular Organization: Prokaryotes vs. Eukaryotes

  • Prokaryotes (no membrane-bound nucleus)
    • Only cyanobacteria among phytoplankton fall here.
  • Eukaryotes (true nucleus & organelles)
    • Haptophytes, dinoflagellates, diatoms.
  • Importance: cellular complexity affects metabolic rates, ecological roles, and evolutionary history.

Key Takeaways & Exam Tips

  • Remember the functional definition: plankton = organisms incapable of overcoming currents (low Re context).
  • Distinguish Reynolds number components and why plankton operate in a viscosity-dominated realm.
  • Be fluent with metric prefixes (cm, mm, µm) and scientific notation conversions.
  • Know the two trophic categories (phyto- vs. zoo-plankton) and which metabolic pathways define each.
  • Recognize the four major phytoplankton taxa and the unique status of cyanobacteria as prokaryotes.
  • Expect later lectures to dive into: detailed physiology, ecological significance, and adaptive strategies of each phytoplankton group.