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1. Seagrass habitat is globally threatened from the cumulative impact of human activities on coastal ecosystems. Successful conservation and restoration of seagrass requires knowledge of the environmental factors that limit seagrass presence; however, quantifying the relative importance of these environmental factors is a significant challenge. 2. To resolve this issue, a species distribution model (SDM) for seagrass was constructed from a range of data collected at different spatial and temporal resolutions in Moreton Bay, Australia. Mean annual benthic light availability, significant wave height, and sediment settling time (calculated from sediment size distribution) were the environmental factors predicting seagrass presence in the model, which was calibrated and validated using maps of seagrass cover from 2004 and 2011, respectively. 3. The SDM correctly classified seagrass presence/absence in 85% of cases for calibration to 2004 data and 88% of cases for validation to 2011 data. 4. Application of the SDM to 12 regions of 10–100 km2 size within the study area demonstrated that the environmental factors limiting seagrass presence (i.e. either threatening seagrass that is present or hindering its colonization if seagrass is absent) varied substantially between these regions. For example, seagrass presence in the regions Waterloo Bay and Central West was predicted to be limited by their high mud content, while in the region Deception Bay South, light availability was the major limiting factor. 5. The results demonstrate that SDMs need to be developed on a sufficiently broad scale to capture significant variability in environmental conditions. These models can then be applied at finer scales to assess management options based on the local environmental conditions.

A feedback between seagrass presence, suspended sediment and benthic light can induce bistability between two ecosystem states: one where the presence of seagrass reduces suspended sediment concentrations to increase benthic light availability thereby favoring growth, and another where seagrass absence increases turbidity thereby reducing growth. This literature review identifies (1) how the environmental and seagrass meadow characteristics influence the strength and direction (stabilizing or destabilizing) of the seagrass‐sediment‐light feedback, and (2) how this feedback has been incorporated in ecosystem models proposed to support environmental decision making. Large, dense seagrass meadows in shallow subtidal, non‐eutrophic systems, growing in sediments of mixed grain size and subject to higher velocity flows, have the greatest potential to generate bistability via the seagrass‐sediment‐light feedback. Conversely, seagrass meadows of low density, area and height can enhance turbulent flows that interact with the seabed, causing water clarity to decline. Using a published field experiment as a case study, we show that the seagrass‐sediment‐light feedback can induce bistability only if the suspended sediment has sufficient light attenuation properties. The seagrass‐sediment‐light feedback has been considered in very few ecosystem models. These models have the potential to identify areas where bistability occurs, which is information that can assist in spatial prioritization of conservation and restoration efforts. In areas where seagrass is present and bistability is predicted, recovery may be difficult once this seagrass is lost. Conversely, bare areas where seagrass presence is predicted (without bistability) may be better targets for seagrass restoration than bare areas where bistability is predicted.

Oxygen flux between aquatic ecosystems and the water column is a measure of ecosystem metabolism. However, the oxygen flux varies during the day in a “hysteretic” pattern: there is higher net oxygen production at a given irradiance in the morning than in the afternoon. In this study, we investigated the mechanism responsible for the hysteresis in oxygen flux by measuring the daily pattern of oxygen flux, light, and temperature in a seagrass ecosystem (Zostera muelleri in Swansea Shoals, Australia) at three depths. We hypothesised that the oxygen flux pattern could be due to diel variations in either gross primary production or respiration in response to light history or temperature. Hysteresis in oxygen flux was clearly observed at all three depths. We compared this data to mathematical models, and found that the modification of ecosystem respiration by light history is the best explanation for the hysteresis in oxygen flux. Light history-dependent respiration might be due to diel variations in seagrass respiration or the dependence of bacterial production on dissolved organic carbon exudates. Our results indicate that the daily variation in respiration rate may be as important as the daily changes of photosynthetic characteristics in determining the metabolic status of aquatic ecosystems.

Seagrass meadows are vital ecosystems in coastal zones worldwide, but are also under global threat. One of the major hurdles restricting the success of seagrass conservation and restoration is our limited understanding of ecological feedback mechanisms. In these ecosystems, multiple, self‐reinforcing feedbacks can undermine conservation efforts by masking environmental impacts until the decline is precipitous, or alternatively they can inhibit seagrass recovery in spite of restoration efforts. However, no clear framework yet exists for identifying or dealing with feedbacks to improve the management of seagrass ecosystems. Here we review the causes and consequences of multiple feedbacks between seagrass and biotic and/or abiotic processes. We demonstrate how feedbacks have the potential to impose or reinforce regimes of either seagrass dominance or unvegetated substrate, and how the strength and importance of these feedbacks vary across environmental gradients. Although a myriad of feedbacks have now been identified, the co‐occurrence and likely interaction among feedbacks has largely been overlooked to date due to difficulties in analysis and detection. Here we take a fundamental step forward by modelling the interactions among two distinct above‐ and belowground feedbacks to demonstrate that interacting feedbacks are likely to be important for ecosystem resilience. On this basis, we propose a five‐step adaptive management plan to address feedback dynamics for effective conservation and restoration strategies. The management plan provides guidance to aid in the identification and prioritisation of likely feedbacks in different seagrass ecosystems.

Recent assessments of marine reserves have emphasized the importance of socio-economic factors in reserve performance. Debates continue, however, about whether we should avoid or promote the placement of reserves near potentially detrimental forces, including coastal cities or rivers. We performed a global meta-analysis to test whether proximity to major coastal influences affected the ability of marine reserves to enhance the abundance of organisms relative to surrounding areas. A strong effect of reserve performance was evident for a range of trophic groups. Positive effects of reserves were undiminished by proximity to coastal cities or river discharges for the majority of taxonomic groups under conservation. We conclude that reserves placed in coastal areas are likely to protect marine populations to a similar extent as reserves in remote or less-developed locations. Marine reserves in coastal settings can be an important tool to protect species and ecosystems in places threatened by human activities.

Full Text, Thesis (PhD Doctorate), Doctor of Philosophy (PhD), Griffith School of Environment, Science, Environment, Engineering and Technology, In response to growing global impact on ecosystems, we design programs for conservation and restoration to maintain and enhance biodiversity, productivity and ecosystem resilience. To ensure the greatest return for these programs, there is an implicit requirement for identifying and understanding the complex non-linear relationships that can exist between impact gradients, ecosystem structure and the processes that mediate the two. Ecological resilience theory has developed as one of the fundamental explanations of this complexity. The application of ecological resilience theory in a local management context, however, is often hampered by a disparity between the theory and what is practical to test empirically. This thesis used seagrass ecosystems in Moreton Bay, Queensland as a model system for the development and testing of a practical framework to examine the potential for incorporating measures of feedback loops into the empirical assessment of resilience. I focussed on the behaviour of feedback processes in relation to changing levels of impact with a view to developing a generic, testable hypothesis that could be used to assess ecological resilience more broadly. A Bayesian network was used to synthesis the known relationships between impact and seagrass response to identify three key feedback processes that stabilise seagrass ecosystems in Moreton Bay......

Summary Severe events such as floods or cyclones can have large ecological effects on the structure and functioning of ecosystems. The capacity of an ecosystem to adapt to, or absorb, the effects of a severe event depends on the severity and longevity of the event and the tolerance of the species present. Seagrasses exhibit phenotypic plasticity at the plant to meadow scale through a variety of physiological and morphological acclimations to light stress to enhance photosynthetic capacity. These acclimations provide early warning of the possible risk of larger scale seagrass loss and can therefore be used in predicting how ecosystems might respond to severe events. The physiological and morphological responses of 12 seagrass (Zostera muelleri) meadows to a severe flood were examined to test two main hypotheses: (i) that the physiological and morphological characteristics of seagrass would differ between meadows along the established chronic water quality gradient, in a pattern consistent with prior acclimations which have been shown to enhance photosynthetic capacity and (ii) that physiological and morphological responses to the flood would differ between meadows in a manner consistent with their position along the water quality gradient. Meadows had different physiological and morphological characteristics across the water quality gradient, with meadows subject to chronically poorer water quality exhibiting characteristics consistent with those that maximize photosynthetic capacity. Despite a large discrepancy in impact among meadows, all meadows sampled responded consistently to the flood, exhibiting only physiological changes with no significant reduction in biomass. This suggests that photoacclimation to chronically poor conditions can enable seagrasses to withstand the effects of severe events, such as floods. Synthesis. Phenotypic plasticity in habitat‐forming species can result in a large variation in their responses to severe events, such as floods or