Biophysical mechanisms

1. Introduction

  • First of three lectures on biophysical interactions: how physical oceanography shapes animal distribution, especially seabirds.

  • Today’s focus:

    1. Biophysical coupling – how physics (currents, tides, topography) interacts with biology (prey, predators).

    2. Topography + flow – how seabed features and water movement create predictable foraging hotspots.

    3. How these processes create predictable, accessible, prevalent, and abundant prey patches.

2. Predator Foraging Requirements

A seabird searching for prey in a seemingly featureless ocean needs:

2.1 Four key foraging criteria

  1. Predictability – prey occurs in known places/times.

  2. Accessibility – prey is easy to capture (not tightly schooling).

  3. Prevalence – prey encountered frequently.

  4. Abundance – high biomass when encountered.

2.2 “Seabird supermarket” concept

  • Ideal foraging habitat = predictable, abundant, accessible food source at known locations.

  • Especially critical during breeding, when birds must feed themselves and chicks efficiently.

3. Biophysical Coupling

Biophysical coupling = interaction between physical processes (currents, tides, topography) and biological processes (plankton, fish, predators).

3.1 Analogy: A stream with boulders

  • Simple flow becomes complex when obstacles (boulders) are added:

    • Accelerated flow between boulders

    • Slack water behind them

    • Eddies, turbulence, shear zones

  • The ocean behaves the same way, but at much larger scales.

3.2 In the ocean

  • Flows = tides + ocean currents

  • Topography = seabed depth, roughness, islands, headlands, channels

  • Together they create a mosaic of physical habitats that predators exploit.

4. Physical Drivers: Tides and Currents

4.1 Tides

Two main cycles:

(a) Flood–Ebb Cycle (12-hour cycle)

  • Flood tide: water flows in

  • High tide: slack water

  • Ebb tide: water flows out

  • Low tide: slack water

  • Currents change speed and direction predictably.

(b) Spring–Neap Cycle (14-day cycle)

  • Spring tides: strongest currents

  • Neap tides: weakest currents

  • Predictable, repeating pattern.

4.2 Ocean Currents

  • Large‑scale, persistent flows (e.g., Gulf Stream, Kuroshio).

  • Driven by:

    • Coriolis effect

    • Wind patterns

    • Density gradients

  • Northern Hemisphere gyres rotate clockwise; Southern Hemisphere anticlockwise.

5. Topography–Flow Interactions

Topography modifies flow, creating predictable hotspots.

5.1 Shallow channels

  • Currents squeezed between landmasses → accelerate.

  • Also squeezed vertically into shallow water → further acceleration.

5.2 Headlands

  • Currents forced around protruding land → speed up.

  • Eddies form in the lee of the headland.

5.3 Result

  • Complex, predictable patterns of:

    • Fast and slow currents

    • Eddies

    • Shear zones

    • Upwelling/downwelling

  • Animals can learn and exploit these patterns.

6. Predictability of Physical Features

  • Tides = predictable

  • Ocean currents = predictable

  • Topography = fixed
    → Therefore foraging hotspots are predictable in space and time.

Examples

  • Fast currents in:

    • Pentland Firth

    • North Channel

    • Menai Strait

    • Around Anglesey headlands (South Stack, Carmel Head)

  • Slow currents in:

    • Celtic Sea

    • Cardigan Bay

    • Liverpool Bay

7. Linking Physics to Prey: Nutrients + Light

Primary productivity requires:

  1. Sunlight

  2. Nutrients

Problem:

  • Nutrients sink to seabed.

  • Light only at surface.
    → They are usually separated.

Solution:
Topography + flow can mix nutrients upward, combining them with light → local productivity hotspots.

8. Mechanism 1: Seabed Features (Topographic Upwelling)

8.1 Flume experiment (Jaco’s video)

  • Smooth seabed → laminar flow → little mixing.

  • Increasing roughness → turbulence → vertical mixing.

  • Artificial turf (very rough) → strong mixing → nutrients lifted upward.

Concept

  • Rough seabed = turbulence = nutrients forced into photic zone = enhanced primary productivity.

8.2 Major seabed‑driven hotspots

(a) Shelf edges

  • Coriolis forces push currents onto shelf break → upwelling.

  • Very high productivity → attracts fish → attracts seabirds.

(b) Seamounts

  • Large underwater mountains disrupt flow → mixing + upwelling.

  • Biodiversity hotspots.

(c) Submarine canyons

  • Deep incisions funnel currents upward → nutrient injection.

8.3 Example: Black‑legged kittiwakes (Norway)

  • GPS‑tagged birds travelled directly to the shelf edge.

  • Birds in poor condition were more likely to travel to the shelf edge → reliable feeding.

  • Shelf edge = predictable, abundant foraging habitat.

9. Mechanism 2: Tidal Fronts

9.1 Formation

  • Shallow, fast‑flowing water = well‑mixed

  • Deeper, slow‑flowing water = stratified

  • Boundary between them = tidal front

9.2 Why fronts matter

  • Friction between water masses → circular flows

  • Nutrients accumulate on stratified side

  • Enhanced phytoplankton → zooplankton → fish → seabirds

9.3 UK examples

  • Celtic Sea front

  • Irish Sea front

  • Flamborough front (Yorkshire)

  • Ushant front (NW France)

9.4 Example: Gannets (Scales et al. 2014)

  • GPS‑tagged gannets initiated feeding dives more often at persistent tidal fronts.

  • Demonstrates strong predator–front association.

10. Mechanism 3: Prey Manipulation (Making Prey Accessible)

10.1 Why accessibility matters

  • Dense fish schools = hard to catch

  • Loose, dispersed fish = easier to capture

  • Birds prefer:

    • Fish feeding on plankton (looser schools)

    • Fish disrupted by physical forcing (shear, turbulence)

11. Example: Sandbank Systems

11.1 How sandbanks work

  • Currents over banks create:

    • Localised upwelling at front

    • Internal waves behind bank

  • Both processes aggregate plankton.

11.2 Consequences

  • Dense plankton → feeding fish → seabirds and marine mammals.

11.3 Examples of major banks

  • Thames Estuary banks (important for red‑throated divers)

  • Skerries Bank (porpoises)

  • Dogger Bank (seabirds, porpoises)

11.4 Example: Storm petrels (Scott et al. 2013)

  • More storm petrels feeding on the bank than off it.

  • Feeding increased when currents were fastest (more internal waves).

12. Mechanism 4: Headlands and Shear Lines

12.1 How headlands manipulate prey

  • Fast flow around headland + slow flow behind → shear line.

  • Shear breaks up fish schools → makes prey accessible.

12.2 Importance

  • Birds often forage along shear lines where prey is disaggregated.

13. Summary

Biophysical coupling creates predictable, accessible, abundant prey patches by:

  1. Topographic upwelling (shelf edges, seamounts, canyons)

  2. Tidal fronts (mixed–stratified boundaries)

  3. Sandbanks (internal waves + upwelling)

  4. Headlands (shear lines breaking up prey schools)

These features:

  • Enhance primary productivity

  • Aggregate plankton

  • Concentrate fish

  • Provide reliable foraging hotspots for seabirds and other predators