Date of Award

2020

Degree Type

Thesis

Degree Name

Master of Science (Environmental Management)

School

School of Science

First Advisor

Associate Professor Kathryn McMahon

Second Advisor

Professor Paul Lavery

Third Advisor

Dr Megan Huggett

Abstract

Seagrasses are highly valued for their productivity and ecosystem services. Despite their importance, seagrasses are declining globally, with losses comparable to tropical rainforests and coral reef ecosystems. Eutrophication and light reduction are two major environmental pressures contributing to seagrass loss. Eutrophication events are a consequence of increased nutrient loads entering the coastal environment. Light reduction can occur from a number of processes including eutrophication, but also increased sediment loads entering the water column through anthropogenic activities on the land and in the sea.

Seagrass managers require a suite of tools to enable monitoring and the assessment of the effects of nutrient enrichment and light reduction on seagrass meadows to assist in the management of these vulnerable ecosystems. Biofilm have displayed some promising attributes as a good bioindicator of nutrient enrichment within benthic environments, however there are limited studies within seagrass habitats. Algal epiphytes within seagrass meadows are frequently studied and are considered good bioindicators of light limitation, however, faunal epiphytes have rarely been considered.

The objective of this research was to determine the effect of two different environmental pressures on seagrass biofilm and epiphytes within temperate seagrass meadows in Western Australia. To examine this, two separate experiments were run, a mensurative experiment assessing seagrass biofilm and the effect of nutrient exposure from drain discharge, and an in situ manipulative experiment assessing the effects of light reduction on seagrass epiphytes. For the mensurative experiment the abundance and taxonomic composition of biofilm was compared at sites near-to and far-from stormwater discharge points, the known sources of nutrient discharge. Within the in situ experiment the algal and faunal epiphyte abundance was examined under different light shading intensities, over different durations and times of year. There were no consistent differences in biofilm abundance near-to and far-from drains, however the drain with the highest nutrient loads had the highest autotrophic abundance and total biofilm biomass. Additionally, biofilm prokaryotic and eukaryotic composition displayed no significant difference between near-to and far-from drain sites. The biofilm composition also seemed to be driven by the nutrient loadings of each respective drain, as sites with higher nutrient loads displayed higher abundances of taxa typically found within nutrient rich or polluted environments. In regard to the in situ experiment, the algal epiphytes displayed a clear response to reduced light intensity and increased duration, however the response was not consistent at different times of the year, as there was a delay in response during the post-winter period. The difference in response was likely due to a difference in the absolute amount of light during the two periods. Furthermore, the algal epiphytes to seagrass biomass (g epiphyte DW to g-1 seagrass DW) displayed a more consistent response compared to the algal epiphyte biomass as (g DW m-2), especially on the seagrass leaves. The faunal epiphytes displayed no consistent response to the shading treatments. This inconsistency is likely due to differing metabolic and substrate requirements compared to algal epiphytes.

This study has displayed that biofilm is a good potential bioindicator of increased nutrient availability within seagrass ecosystems, however, the use of this needs to be interpreted with a good understanding of the hydrodynamics of the system. Additionally, algal epiphytes are a good bioindicator of reduced light availability on seagrass meadows, and the best way to measure the response of the algal epiphytes is to standardise to the seagrass biomass (g epiphyte DW to g-1 seagrass DW). Also, algal epiphytes specifically on seagrass leaves is the indicator variable that responds most consistently. Additionally, it is important to consider the absolute magnitude of the influencing light before predicting responses, and to understand that these responses change with the total amount of light during different seasonal periods. Finally, faunal epiphytes were poor bioindicators of this environmental stress.

Available for download on Tuesday, March 08, 2022

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