Heterozostera resilience

Abstract

Seagrass perform critical provisioning, regulating, cultural and supporting ecological functions and services humans rely on worldwide. Unfortunately, many seagrass ecosystems are vulnerable to disturbance and are being lost at alarming rates. Some seagrass species have been listed by the International Union for the Conservation of Nature as threatened and endangered where population sizes are small or are highly restricted in geographic distribution. For several other species, there is not enough data to determine whether a species is at risk or not.

When ecological resilience thresholds for species survival and reproduction are exceeded, declines in seagrass may occur. Declines over time and space may be episodic or ongoing and occur as a result of impacts from both natural and anthropogenic stressors, such as turbidity, eutrophication, hypersalinity and urbanization. Increasing global change phenomena are expected to exacerbate declines of seagrass in many parts of the world, and vast losses of seagrass have functional implications for other ecosystems and organisms.

Heterozostera are unique seagrass broadly distributed throughout coastal southern latitudes especially in Australia. Only three populations of Heterozostera are found outside of Australia, in a small region of the eastern Pacific in Chile. The populations are made up of 2 nonflowering clones. Therefore, Heterozostera of Australia may be functionally endemic to southern Australia, including Tasmania.

Substantial loss of Heterozostera has been reported from the middle of Heterozostera's range in several locations of Victoria, Australia. Subsequent recovery of diminished populations has been slow in some sites, especially in Western Port, Victoria. Few published accounts related to resilience, including seagrass population monitoring for conservation and restoration of Heterozostera in the southern hemisphere have been completed.

The aim of this thesis is to advance basic botanical research on Heterozostera in support of taxonomic resolution and the development of conservation strategies, especially those focused on modeling and applied seagrass resilience and restoration in Australia. More studies are needed to contribute to the development of restoration and resilience management strategies for lesser studied southern hemisphere seagrass ecosystems, and this work supports the generation of additional discoveries.

Specifically, the objective of this dissertation is to identify and examine the autoecology and resilience Heterozostera, including sexual and clonal reproductive success under variable environmental cues. In all, five comprehensive studies of Heterozostera biogeography and biology are included in this dissertation, along with a theoretical framework and summary exploring the roles of environmental parameters in the autoecology of resilience for temperate Australian Heterozostera. Chapter 1 presents a review of research in support of Australian Zosteraceae, including trends and gaps in our understanding of seagrass resilience mechanisms, including both resistance and recovery pathways in Heterozostera. Chapter 2 details the results of an extensive survey of existing sediment and nutrient conditions for Heterozostera populations across 15 seagrass meadows located in southeastern Australia's Port Phillip Bay. Chapter 3 summarizes a series of experiments aimed at discovering effective Heterozostera seed storage and collection protocols for use in land based aquaculture, and specifically examining the potential roles of seed colour, sterilisation and refrigeration with the goal of improving long term seed viability. Chapter 4 contains a fully factorial germination assay undertaken to identify potential cues to Heterozostera germination, including sediment type and sterilization, nutrient loads, and refrigeration. Chapter 5 details a germination experiment using a 2 ppm copper sulphate (CuSO4) solution to cue germination of Heterozostera seed. Lastly, Chapter 6 explores the rates of asexual growth and survival of three types of clonal reproductive Heterozostera propagules (rhizome, plantlet and shoot). This work provides novel information about several topics of research with regard to the autoecology of Heterozostera including: (1) species identification, (2) species ecology and biogeography, (3) reproductive material collection and storage, (4) seed germination, and (5) clonal transplanting.

This research demonstrates Heterozostera is capable of widespread recovery across a range of sediment and nutrient conditions. Continued work on lesser known Heterozostera species (H. tasmanica and H. polychlamys), and other rarer seagrasses world-wide are critical for understanding the potential loss of resilience and increased vulnerability (or conversely, the potential for recovery, adaptation and survival) of threatened and endangered seagrass populations with limited global dispersal. Collectively these studies details methods to support future Heterozostera research in both laboratory and field settings where culture of reproductive materials are required. In addition, the outcomes from this research provides novel information in support of progressing insitu seeding and transplanting efforts aimed to improve operation of land based seagrass nurseries and research studies. Novel evidence builds support that Heterozostera nigricaulis as a functionally resilient species of seagrass, largely due to its ability to inhabit variable ecological conditions and to utilize multiple sexual and asexual reproductive strategies to recover populations following disturbances.