OSX-2007: Plankton

🌱 1. What Is Plankton?

Definition:

A community of plants, animals, and bacteria whose powers of locomotion are insufficient to prevent them from being passively transported by currents.

  • This is a working definition, but it's not absolute:

    • Some plankton can move actively as they grow (e.g., mollusc larvae).

    • Definition is most accurate for early life stages or smaller plankton.

🔬 2. Plankton Classifications

By Life Cycle:

  • Holoplankton: Entire life in planktonic form (e.g., copepods, tunicates).

  • Meroplankton: Only part of life cycle in plankton (e.g., larval stages of crabs, fish, molluscs).

By Size (descending):

  • Megaplankton (>20 mm)

  • Macroplankton

  • Mesoplankton (commonly sampled)

  • Microplankton

  • Nanoplankton

  • Picoplankton (<2 µm)

🧫 3. Major Planktonic Groups

A. Phytoplankton (producers)

  • Diatoms: Single cells or chains, common in samples.

  • Dinoflagellates: Often mobile via flagella.

B. Zooplankton (consumers)

  • Copepods, arrow worms, tunicates

  • Crustacean larvae: Crabs, barnacles, amphipods

  • Mollusc larvae:

    • Gastropod veliger: One shell, ciliated velum

    • Bivalve veliger: Two shells, limited locomotion

  • Fish larvae and eggs (rare in samples)

🧬 4. Why Is Plankton Important?

  • Drives global biogeochemical cycles

  • Base of the marine food web

  • Produces oxygen

  • Mediates organic matter flow

  • Impacts ecosystem stability

  • Sensitive to environmental changes (climate change, pollution)

⚠ 5. Harmful Algal Blooms (HABs)

  • Toxin-producing plankton (e.g., Alexandrium → PSP via saxitoxin)

  • Trends: PSP incidents have increased significantly since the 1970s

  • Causes:

    • Rising awareness and improved detection

    • Real increase in harmful blooms due to nutrient enrichment and climate change

  • Consequences:

    • Human poisoning (PSP, DSP, ASP)

    • Anoxia and hypoxia → fish kills

🧪 6. Recent Research Projects

A. Irish & Celtic Sea Surveys

  • Tracked bivalve larvae using genetic primers (species-level ID)

  • Key for mussel farming: timing rope deployment to larvae presence

  • Monitored bacteria and pathogens in plankton

  • Used plankton nets, genetic tools, and oceanographic modelling

🧬 7. Plankton and Disease Transmission

  • Plankton = disease vectors:

    • Transport pathogens over large distances (e.g., Vibrio, cholera)

    • Affect human health through shellfish consumption

  • Irish Sea: critical area with known pathogenic species thresholds

🚢 8. Fieldwork & Sampling Methods

Sampling Tools:

  • Plankton Net (used on RV Prince Madog):

    • Length: ~2 m

    • Opening: 0.5 m diameter

    • Mesh: 0.25 mm (250 µm)

    • Tow speed: ~1–2 knots

    • Tow time: 5–10 minutes

    • Sample collector at bottom

Types of Tows:

  • Vertical, horizontal, and oblique

  • Oblique tow preferred for depth-integrated samples

Net Efficiency:

  • Filtration efficiency = volume filtered / expected volume

  • Influenced by:

    • Mesh size

    • Net shape

    • Tow speed

    • Amount of debris (clogging risk)

🧮 9. Calculating Plankton Density

Volume of water sampled (V):

  • Modeled as a cylinder:

    V=πr2⋅dV = \pi r^2 \cdot dV=πr2⋅d

    • rrr: radius of net opening

    • ddd: distance net travelled (from flow meter)

Distance (d):

  • d=K×nd = K \times nd=K×n

    • KKK: constant (m/rev) — e.g., 0.3 m/rev

    • nnn: number of propeller revolutions (final – initial)

Density:

  • Count individuals in known subsample volume (e.g., 5 mL)

  • Scale to entire tow volume

🔬 10. Lab Analysis

Procedure:

  1. Filter sample, rinse with seawater

  2. Fix in preservative → transferred to ethanol

  3. Subsample 5 mL → add to Bogorov counting chamber

  4. Use microscope to identify/count:

    • Copepods, crab larvae, diatoms, etc.

Tools:

  • Bogorov chamber preferred over petri dish to immobilise plankton

  • Microscope scans full tray area methodically

Data Entry:

  • Include:

    • Date

    • Flow meter start/end values

    • Counts for each plankton group

  • Use class spreadsheet to calculate final densities

🌊 11. Module Objectives

  • Evaluate relationships between plankton and physical parameters:

    • Salinity: copepod density increases with rising salinity

    • Temperature

    • Wind

    • Turbidity (SPM)

    • Chlorophyll-a

  • Combine plankton data with physical measurements to produce your final report.

📷 12. Visual Identification & Common Species

  • Expect to see:

    • Diatoms (very common, hundreds per sample)

    • Dinoflagellates

    • Crustacean larvae

    • Arrow worms

    • Possibly fish eggs/larvae

  • Remember: orientation under microscope may vary

🛠 13. Additional Equipment

  • Bongo nets: two plankton nets side by side

  • Multi-nets: various mesh sizes, selective depth sampling

    • Often problematic due to clogging

  • CTD and SPM sensors: used for physical parameter collection

💡 Field Tips

  • Securely attach sample collectors

  • Label everything clearly

  • Handle nets carefully – avoid damage from overloading

  • Rinse nets thoroughly to recover organisms

🌟 Final Thoughts

  • Plankton are critical for marine ecosystems, fisheries, and human health

  • Sampling & identifying plankton helps track ecosystem changes

  • Your field and labwork will contribute to real-world environmental understanding