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# Sources and supply of wrack: quantifying vectors of connectivity through a biomechanistic approach

Thibaut de Bettignies, Thomas Wernberg, Paul S. Lavery

► To cite this version:

Thibaut de Bettignies, Thomas Wernberg, Paul S. Lavery. Sources and supply of wrack: quantifying vectors of connectivity through a biomechanistic approach. 46th Annual Conference for the Australian Marine Sciences Association, Jul 2009, Adelaide, SA, Australia, Australia. hal-05345732

HAL Id: hal-05345732

https://hal.science/hal-05345732v1

Submitted on 4 Nov 2025

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## Coastal marine waters have high connectivity

* Movement of plant detritus (especially kelp) is a key connectivity process.

* Kelp forest are highly productive & *Ecklonia radiata* is the most abundant macroalgae on subtidal reefs of temperate Australia.

* Kelps are detached during storms and transported to adjacent habitats.

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## Physical disturbance is common

* The coastline of Western Australia is exposed to wind and waves imposing large hydrodynamic forces on benthic habitats.

* Wave pattern: strong seasonality.

* Hydrodynamic drag generated by waves is the main force acting on *Ecklonia radiata*.

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* Frequent storms in winter generate large swell and waves

* Maximal horizontal water velocity at the substratum is related to:

* wave height and period

* reef depth

* Storm: up to 6m.s⁻¹ at 5m depth

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AIM: Understanding how physical disturbance drive kelp dislodgment and wrack delivery

## Impacts of Physical Disturbance on Kelp

1

**Hydrodynamic conditions:**

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**Drag experienced by the kelp:**

$$F_{drag} = \frac{1}{2} \rho \times U^2 \times C_{drag} \times A_{plan}$$

**Hydrodynamic properties of algae:**

$$C_{drag} = \frac{(2 \times F_{drag})}{(\rho \times U^2 \times A_{plan})}$$

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2

**Frequency distribution of attachment strengths**

Biomechanical breakage test

3

**Dislodgment probabilities as a function of water motion**

Using data from 1 and 2

Break velocity when Fbreak = Fdrag

$$U_{break} = \frac{(2 \times F_{break})}{(C_{drag} \times A_{plan} \times \rho)}$$

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4

**Export to recipient habitats**

Accumulation & Consumption

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Understanding wrack supply (4) requires understanding parts 1, 2 and 3 of this model

## Main questions

* Does algae morphology affect drag ? (1)

* Is there a spatial / temporal pattern of attachment strength in relation to wave energy ? (2)

* What % is lost ? Where ? How ? When ? (3)

* Which hydrodynamic conditions explain the canopy removal process ? (1,2,3)

* Is there a temporal pattern to loss ? (Pulsed vs Continuous delivery) (3)

* Does drag explain the real dislodgment rate ? (1,2,3)

## Study site

<figure><img src="image_16.png" /><figcaption>Marmion Lagoon</figcaption></figure>

**Study site:**

* Marmion Lagoon: 20km N. from Perth, 10m depth

* 3 successive reefs running parallel to the shore (offshore, midshore, inshore reefs)

**Investigations:**

i. The influence of morphology on hydrodynamic drag

ii. Variability in dislodgment force within/among reefs

iii. Spatio-temporal variation of kelp dislodgment and validation of dislodgment predictions