Paul Maxwell

Unravelling complexity in seagrass systems for management: Australia as a microcosm

Environmental decision-making applies transdisciplinary knowledge to deliver optimal outcomes. Here we synthesise various aspects of seagrass ecology to aid environmental decision-making, management and policy. Managers often mediate conflicting values and opinions held by different stakeholders. Critical to this role is understanding the drivers for change, effects of management actions and societal benefits. We use the diversity of seagrass habitats in Australia to demonstrate that knowledge from numerous fields is required to understand seagrass condition and resilience. Managers are often time poor and need access to synthesised assessments, commonly referred to as narratives. However, there is no single narrative for management of seagrass habitats in Australia, due to the diversity of seagrass meadows and dominant pressures. To assist the manager, we developed a classification structure based on attributes of seagrass life history, habitat and meadow form. Seagrass communities are formed from species whose life history strategies can be described as colonising, opportunistic or persistent. They occupy habitats defined by the range and variability of their abiotic environment. This results in seagrass meadows that are either transitory or enduring. Transitory meadows may come and go and able to re-establish from complete loss through sexual reproduction. Enduring meadows may fluctuate in biomass but maintain a presence by resisting pressures across multiple scales. This contrast reflects the interaction between the spatial and temporal aspects of species life history and habitat variability. Most management and monitoring strategies in place today favour enduring seagrasses. We adopt a functional classification of seagrass habitats based on modes of resilience to inform management for all seagrass communities. These concepts have world-wide relevance as the Australian case-studies have many analogues throughout the world. Additionally, the approach used to classify primary scientific knowledge into synthesised categories to aid management has value for many other disciplines interfacing with environmental decision-making.

An integrated field and remote sensing approach for mapping Seagrass Cover, Moreton Bay, Australia

Creating accurate maps of seagrass cover is a challenging procedure in coastal waters with variable water clarity and depths. This paper presents an approach for mapping seagrass cover from data sources commonly collected by natural resource management agencies responsible for coastal environments. The aim of the study was to develop an approach for mapping classes of seagrass cover from field and/or image data for an area with variable water clarity and depths. The study was carried out in Moreton Bay in eastern Australia. A Landsat 5 Thematic Mapper satellite image was acquired for the same area in August 2004. The image data were used to map seagrass cover in the exposed inter‐tidal and clear shallow water areas to depths of 3 m. Field survey data were collected, in July – August 2004, to map deep (> 3 m) and turbid sub‐tidal areas, using: real time video, snorkeller observations and transect surveys . The resulting maps were combined into a single layer of polygons, with the same seagrass cover classes used as existing mapping programs and with each polygon assigned to one of five cover classes (0 %, 1–25 %, 25–50 %, 50–75 %, 75–100 %). As independent field data were not available for accuracy assessment, a reliability assessment indicated that > 75 percent of the Bay was mapped with high categorical reliability. Most previously published seagrass mapping projects covered areas < 400 km2, were based on single data sets, and lacked assessment of accuracy or reliability. Our approach and methods address this gap and present guidelines for a generally applicable method to integrate image and field data sets over large areas (> 1000 km2) commonly used for monitoring and management.

Phenotypic plasticity promotes persistence following severe events: physiological and morphological responses of seagrass to flooding

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 cyclones. Acclimation to prior poor environmental conditions can promote persistence in habitat-forming species, such as seagrasses, following severe events. The measurement of phenotypic characteristics along an impact gradient can therefore provide an indication of the response of habitat-forming species to severe events. Phenotypic plasticity in habitat-forming species can result in a large variation in their responses to severe events, like floods or cyclones. Acclimation to prior poor environmental conditions can promote persistence in habitat forming species, like seagrasses, following severe events. The measurement of phenotypic characteristics along an impact gradient can therefore provide an indication of the response of habitat forming species to severe events.

The fundamental role of ecological feedback mechanisms for the adaptive management of seagrass ecosystems - a review

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.

Redistribution of sewage-nitrogen in estuarine food webs following sewage treatment upgrades.

Stable nitrogen isotopes were used to assess the effects of wastewater treatment plant (WWTP) upgrades on the utilisation of sewage-N by estuarine biota in Moreton Bay, Australia. We measured delta(15)N of filamentous algae, mangrove leaves and shore crabs at the Brisbane and Logan Rivers before and after scheduled WWTP upgrades, and at two reference rivers where WWTPs had been upgraded >4 years previously. The total N discharged into Brisbane River decreased by >80% after the upgrades had occurred, but N loads remained similar at Logan River despite the upgrade. In Brisbane River, delta(15)N values of algae and crabs decreased and were comparable to the reference rivers within 1-2 years but no changes occurred at Logan River. The delta(15)N of mangrove leaves remained elevated in all rivers, indicating that sewage-N remained a major source to mangroves either from residual WWTP discharges or from N accumulated in the sediments over many years.

Marine reserves help coastal ecosystems cope with extreme weather

Natural ecosystems have experienced widespread degradation due to human activities. Consequently, enhancing resilience has become a primary objective for conservation. Nature reserves are a favored management tool, but we need clearer empirical tests of whether they can impart resilience. Catastrophic flooding in early 2011 impacted coastal ecosystems across eastern Australia. We demonstrate that marine reserves enhanced the capacity of coral reefs to withstand flood impacts. Reserve reefs resisted the impact of perturbation, whilst fished reefs did not. Changes on fished reefs were correlated with the magnitude of flood impact, whereas variation on reserve reefs was related to ecological variables. Herbivory and coral recruitment are critical ecological processes that underpin reef resilience, and were greater in reserves and further enhanced on reserve reefs near mangroves. The capacity of reserves to mitigate external disturbances and promote ecological resilience will be critical to resisting an increased frequency of climate-related disturbance.

Identifying habitats at risk: simple models can reveal complex ecosystem dynamics

The relationship between ecological impact and ecosystem structure is often strongly nonlinear, so that small increases in impact levels can cause a disproportionately large response in ecosystem structure. Nonlinear ecosystem responses can be difficult to predict because locally relevant data sets can be difficult or impossible to obtain. Bayesian networks (BN) are an emerging tool that can help managers to define ecosystem relationships using a range of data types from comprehensive quantitative data sets to expert opinion. We show how a simple BN can reveal nonlinear dynamics in seagrass ecosystems using ecological relationships sourced from the literature. We first developed a conceptual diagram by cataloguing the ecological responses of seagrasses to a range of drivers and impacts. We used the conceptual diagram to develop a BN populated with values sourced from published studies. We then applied the BN to show that the amount of initial seagrass biomass has a mitigating effect on the level of impact a meadow can withstand without loss, and that meadow recovery can often require disproportionately large improvements in impact levels. This mitigating effect resulted in the middle ranges of impact levels having a wide likelihood of seagrass presence, a situation known as bistability. Finally, we applied the model in a case study to identify the risk of loss and the likelihood of recovery for the conservation and management of seagrass meadows in Moreton Bay, Queensland, Australia. We used the model to predict the likelihood of bistability in 23 locations in the Bay. The model predicted bistability in seven locations, most of which have experienced seagrass loss at some stage in the past 25 years providing essential information for potential future restoration efforts. Our results demonstrate the capacity of simple, flexible modeling tools to facilitate collation and synthesis of disparate information. This approach can be adopted in the initial stages of conservation programs as a low-cost and relatively straightforward way to provide preliminary assessments of.nonlinear dynamics in ecosystems.

Incorporating surrogate species and seascape connectivity to improve marine conservation outcomes.

Conservation focuses on maintaining biodiversity and ecosystem functioning, but gaps in our knowledge of species biology and ecological processes often impede progress. For this reason, focal species and habitats are used as surrogates for multispecies conservation, but species-based approaches are not widely adopted in marine ecosystems. Reserves in the Solomon Islands were designed on the basis of local ecological knowledge to conserve bumphead parrotfish (Bolbometopon muricatum) and to protect food security and ecosystem functioning. Bumphead parrotfish are an iconic threatened species and may be a useful surrogate for multispecies conservation. They move across tropical seascapes throughout their life history, in a pattern of habitat use that is shared with many other species. We examined their value as a conservation surrogate and assessed the importance of seascape connectivity (i.e., the physical connectedness of patches in the seascape) among reefs, mangroves, and seagrass to marine reserve performance. Reserves were designed for bumphead parrotfish, but also enhanced the abundance of other species. Integration of local ecological knowledge and seascape connectivity enhanced the abundance of 17 other harvested fish species in local reserves. This result has important implications for ecosystem functioning and local villagers because many of these species perform important ecological processes and provide the foundation for extensive subsistence fisheries. Our findings suggest greater success in maintaining and restoring marine ecosystems may be achieved when they are managed to conserve surrogate species and preserve functional seascape connections.

Moreton Bay and its estuaries: a sub-tropical system under pressure from rapid population growth

Moreton Bay and its associated estuaries are an example of a complex aquatic system that is under increasing pressure from rapid population growth and urbanisation. Although the extent of decline in ecosystem health within Moreton Bay and its associated estuaries is significant and well documented, a range of innovative management responses have been implemented to reverse current declines. An overview of the development of Moreton Bay is provided, highlighting the dynamic and resilient nature of the system over geological time. The ecological responses that occur at decadal timeframes are presented along with a summary of the current state of the Bay’s ecology. The future challenges that are posed by predicted population increases, urbanisation and changes to the region’s climate are also discussed. The highly variable nature of the system over relatively short timeframes (i.e. flood vs non-flood conditions) as well as the ability of the system to adapt to long term changes (i.e. past morphological and ecosystem shifts) suggests that Moreton Bay and its associated estuaries have significant capacity to adapt to change. Whether the current rate of anthropogenically induced change is too rapid for the system to adapt, or whether such adaptions will be undesirable, is unable to be ascertained in any detail at this stage. Notwithstanding the above, the combination of a science-based management framework and the collaborative decision making processes that have been implemented to halt the decline of Moreton Bay have shown remarkable progress in a relatively short period of time.

Seascape context and predators override water quality effects on inshore coral reef fish communities

Understanding the relative influence of factors that influence faunal community structure, such as habitat and landscape arrangement, has been a long-standing goal of ecologists. This is complicated in marine environments by the high importance of physico-chemical water factors in determining species distributions relative to their physiological or behavioural limits. In this study, we rank the relative importance of 17 seascape, habitat and physico-chemical water factors for structuring the composition of fish communities on the inshore coral reefs of Moreton Bay, eastern Australia. Fish were surveyed at 12 reef sites along the ambient estuarine water gradient in the bay during summer and winter using a baited underwater video approach. Multivariate random forest analyses showed that reef fish community composition correlated most strongly with the local abundance of piscivorous fish and the seascape context of individual reefs (especially distance to nearest seagrass and mangroves), while water quality factors ranked much lower in importance. However, fish communities from sites nearer to rivers were more spatiotemporally variable than those from sites nearer to oceanic waters, indicating that water quality can drive variations in community structure along short-term temporal scales. In turn, piscivore abundance was greatest on reefs near large areas of seagrass, and with low sand cover, high coral cover and high water clarity. Our findings demonstrate that a reef’s location within the broader seascape can be more important for fish communities than factors relating to the reef habitat itself and exposure to reduced water quality. To improve the spatial conservation of marine ecosystems, we encourage a more intimate understanding of how these factors contribute to structuring the use of habitats across seascapes by mobile species.