Date of Award

2013

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Natural Sciences

Faculty

Computing, Health and Science

First Advisor

Professor Paul Lavery

Second Advisor

Associate Professor Thomas Wernberg

Third Advisor

Dr Mat Vanderklift

Abstract

In temperate waters, rocky reefs dominated by extremely productive kelp beds export considerable primary production. Despite the extensive body of work on kelp detritus as a trophic subsidy, many questions remain about the production of this detritus. The aim of this thesis was to determine the rate and mechanisms of kelp detritus production, for Ecklonia radiata, the dominant kelp species in temperate Australia. Most of the work was conducted in Marmion Lagoon located 20 km North of Perth, south-western Western Australia, a region strongly influenced by oceanic swell and winter storms. The study comprised of four major components: the impact of kelp morphology on the drag forces acting on kelp thalli; investigation of wound patterns in kelp tissue and the biomechanical implications for kelp detritus production; the relative contribution of erosion of frond material and dislodgement of whole thalli to detritus production; and the relationship between kelp dislodgement and peak water velocities, implemented with a kelp dislodgement model.

The initial work in chapter 1 revealed that only size (total area), not morphology, was important in determining the drag acting on E. radiata at peak velocities. This implied that at storm velocities the only way drag forces acting on a kelp can be reduced is by a reduction of total thallus area (biomass) and not by modification of thallus shape. These results constituted the first step to build a mechanistic model of kelp dislodgement.

In chapter 2, it is shown that wounds were highly abundant on kelps before peaks in winter storms and that simulated wounds caused significant loss of tissue integrity and strength. Collectively, these findings suggest that accumulation of wounds over summer results in kelp pruning (tissue fragmentation) in early winter. Paradoxically, this may increase kelp survival during winter storms because the biomechanical drag is much lower on small, pruned kelps (lower biomass).

Results presented in chapter 3 indicated that erosion accounted for 80% of the annual detrital production with a pulse in autumn, whereas dislodgement accounted for a smaller and more constant proportion throughout the year. Neither erosion nor dislodgement correlated with increasing water velocity. Instead, the pulse of detrital 4 production coincided with sporogenesis, leading to the hypothesis that weakening of structural tissue through the formation and release of spores made E. radiata more susceptible to wound accumulation (Ch. 2) and erosion.

In chapter 4, results are presented that show no increase in kelp dislodgement with increasing water velocity, except during the most severe storms. The dislodgement model indicated that the seasonal variation in individual kelp biomass, resulting from erosion of frond tissue (Ch. 3), resulted in lower susceptibility to dislodgement (lower biomass) at times of peak water velocities. The benefit of erosion in reducing drag acting on the thallus, as proposed in the previous chapters, was therefore demonstrated by the model.

The commonly accepted model of wave-driven mortality of kelp during storms in winter was refined by the results. The experimental, field sampling and modelling studies have been synthesised into an alternative model of kelp dislodgement, in which kelp beds are in dynamic equilibrium with wave disturbance. This equilibrium is mediated through erosion-driven adjustment of individual kelp biomass in autumnwinter which lowers drag on kelp thalli during the period of peak water velocity. This relationship between erosion and the susceptibility of E. radiata to dislodgement suggests an adaptation of the kelp E. radiata to its environment, critical to kelp survival in one of the most hydrodynamically challenging environment.

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