Date of Award

2016

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Science

First Advisor

Professor Paul Lavery

Second Advisor

Mat Vanderklift

Abstract

The impact of increased sediment exposure on coastal marine ecosystems is one of the most important contemporary environmental issues. Sediment exposure is defined as the concentrations of sediment particles suspended in the water column and the amount of sediment depositing on the seabed. In addition to natural events, such as land erosion, rainfall, and tidal currents, anthropogenic activities such as land use, road building, logging, mining, port maintenance and dredging, contribute to the discharge of a great amount of sediment in the water column. As sessile suspension-feeder assemblages play a critical role in marine ecosystems through their active transfers of organic and inorganic between the water column and the seabed (i.e. benthic-pelagic coupling), increased sediment exposure generated by anthropogenic activities may negatively impact these organisms. This study set out to evaluate the influence of increased sediment exposure on suspension-feeder assemblages in a temperate seagrass habitat. Most of the research was conducted within Posidonia sinuosa meadows near Woodman Point (32◦7’S, 115◦44’E), south of Fremantle, Western Australia. At this location, persistent sediment plumes from a cement manufacturer’s wash plant provided an opportunity to study the mechanisms through which increased sediment exposure can affect suspension feeder assemblages. This study was examining: the relationship between sediment exposure and suspension feeder assemblage composition; the influence of increased sediment exposure on the contribution of potential food sources to suspension feeder diets; and the influence of sediment exposure on suspension-feeding mechanisms (filtration and retention rates) and strategies (food particle selection).

The descriptive work, presented in Chapter 1, revealed a strong gradient in sediment exposure with decreasing sediment deposition with distance from the wash plant. There was little dissimilarity, in term of species diversity and biomass, among suspension-feeder assemblages under high sediment exposure and those experiencing natural sedimentation regimes. These findings indicated that the suspension-feeder assemblages at the study site were resistant to high sediment exposure and that some species could potentially display compensatory mechanisms. Thus, the degree to which increased sediment exposure influences suspension feeders was more likely to be species specific and depend on the resilience of their feeding mechanisms and strategies. Those findings underlined the need to test the causal parameters underlying responses to suspension-feeding activity and selectivity due to increased sediment exposure.

Results presented in Chapter 2 indicated that the three most conspicuous suspension feeder in term of biomass and abundances presented distinct isotopic signatures, implying dissimilarities in their diets. Differences in δ13C and δ15N can be explained by consumption of different types of picoplanktonic particles and the degree to which sedimentary organic matter contributed to their diets. Increased sediment exposure had no influence on the natural diets of the ascidian Herdmania momus and the bivalve Pinna bicolor. For the sponge Tethya sp, the contribution of sedimentary organic matter to its diet increased at sites with high sediment exposure, suggesting a potential benefit to its diet. Overall, the influence of sediment exposure on suspension feeder diets was species specific and dependent on the nature (e.g. organically rich versus organically depleted) and concentrations of the sediment.

In Chapter 3 data are presented to show that feeding mechanisms and strategies of three species of suspension feeders best representing the benthic assemblages were influenced by sediment exposure. Under high sediment exposure, the ascidian Herdmania momus had lower pumping rates, but maintained a relatively constant food retention rate, and optimised its food intake by expanding its food sources from mainly cyanobacteria (Synechococcus) to a wider range of food sources. The bivalve Pinna bicolor also had lower filtration activity with high sediment exposure, again, maintaining relatively constant total retention rates and displaying a change in particle selection from bacteria and Synechococcus to larger picoeukaryotic cells of higher carbon content. The sponge Tethya sp. appeared to benefit from elevated sediment concentrations, as filtration and retention rates increased, potentially related to a lack of food selectivity.

In Chapter 4, a short-term laboratory experiment was combined with a field transplant experiment to investigate the response of suspension feeders to increased sediment exposure. Under elevated sediment exposure, Herdmania momus and Pinna bicolor modulated their particle selection to optimise food intake, while the non-selective suspension feeder Tethya sp increased its particle retention rate and efficiency. Both the laboratory and transplant experiment findings corroborated observations made in Chapters 1 and 2; compensatory adaptations associated with the feeding activity, such as the modulation of pumping rates and the optimisation of food intake by particle selection, help to explain the potential resistance of suspension feeder community structure to high sediment exposure. A major outcome of this research is that it informs shallow coastal ecosystems stakeholders of the possible consequences of anthropogenic activities that increase sediment exposure, particularly those in the order of TSS at 40 mg·l-1 and deposition rates in the order of 10 g·cm- 2·month-1. Despite this magnitude of sediment exposure had little influence on suspension feeder assemblage composition, the effects on the suspension-feeding function, including the increase of the filtration activity and transfers of carbon to the benthos, potentially influence benthicpelagic coupling and other ecosystem-scale processes. Given the variation in sensitivity to sediment exposure among suspension feeder species, meaningful criteria to limit the effects of anthropogenic sediment loading on shallow coastal ecosystems should take into account the whole species assemblage present at any given site.

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