Emerging Solutions to Return Nature to the Urban Ocean

Urban and periurban ocean developments impact 1.5% of the global exclusive economic zones, and the demand for ocean space and resources is increasing. As we strive for a more sustainable future, it is imperative that we better design, manage, and conserve urban ocean spaces for both humans and nature. We identify three key objectives for more sustainable urban oceans: reduction of urban pressures, protection and restoration of ocean ecosystems, and support of critical ecosystem services. We describe an array of emerging evidence-based approaches, including greening grayinfrastructure, restoring habitats, and developing biotechnologies. We then explore new economic instruments and incentives for supporting these new approaches and evaluate their feasibility in delivering these objectives. Several of these tools have the potential to help bring nature back to the urban ocean while also addressing some of the critical needs of urban societies, such as climate adaptation, seafood production, clean water, and recreation, providing both human and environmental benefits in some of our most impacted ocean spaces. </jats:p

Below-ground ecosystem engineers enhance biodiversity and function in a polluted ecosystem

Many important ecosystem functions are underpinned by below ground biodiversity and processes Marine sediments one of the most abundant habitats on earth are essential to the mineralisation of organic matter However they are increasingly polluted by urban activities leading to the loss of biodiversity and the functions they provide While traditional sediment remediation strategies are focussed on microbial and engineering solutions we propose that the reintroduction of below ground ecosystem engineers bioturbators is important to rehabilitate polluted sediments and drive recovery of their functions in urban coastal ecosystems We tested this notion by introducing bioturbators to nutrient polluted sediments to assess their survival as well as their capacity to drive biodiversity and oxygenation and their potential to remediate nutrient pollution Polychaete worms Diopatra aciculata and clams Katelysia sp were added to mesocosms ex situ and the worms also added to experimental plots in situ Potential for remediation was assessed with measures of nutrient content All animals survived when introduced to polluted sediments and showed no evidence of sub lethal effects Worms oxygenated sediments and reduced organic matter content by up to 50 in situ The worms also drove shifts in the receiving communities at all locations and increased the number of taxa at one location On the other hand the effects of clams were variable showing opposite effects in organic matter content at different sites and levels of pollution Synthesis and applications Global seafloor habitats are becoming increasingly degraded and novel strategies that combine biodiversity restoration with remediation are urgently needed to return function Tube building bioturbators can stimulate nutrient processing in sediments proving multiple functional outcomes but these effects are dependent on the receiving environment In areas with medium levels of pollution they can kick start recovery in a feedback loop whereby bioturbation increases oxygenation and nutrient remediation shifting sediment biodiversity and contributing to further recovery This can drive long term changes in sediment communities particularly in urban areas where unvegetated sediments are conspicuous 2022 The Authors Journal of Applied Ecology published by John Wiley Sons Ltd on behalf of British Ecological Societ

Recruitment of a threatened foundation oyster species varies with large and small spatial scales.

Understanding how habitat attributes (e.g., patch area and sizes, connectivity) control recruitment and how this is modified by processes operating at larger spatial scales is fundamental to understanding population sustainability and developing successful long-term restoration strategies for marine foundation species-including for globally threatened reef-forming oysters. In two experiments, we assessed the recruitment and energy reserves of oyster recruits onto remnant reefs of the oyster Saccostrea glomerata in estuaries spanning 550 km of coastline in southeastern Australia. In the first experiment, we determined whether recruitment of oysters to settlement plates in three estuaries was correlated with reef attributes within patches (distances to patch edges and surface elevation), whole-patch attributes (shape and size of patches), and landscape attributes (connectivity). We also determined whether environmental factors (e.g., sedimentation and water temperature) explained the differences among recruitment plates. We also tested whether differences in energy reserves of recruits could explain the differences between two of the estuaries (one high- and one low-sedimentation estuary). In the second experiment, across six estuaries (three with nominally high and three with nominally low sedimentation rates), we tested the hypothesis that, at the estuary scale, recruitment and survival were negatively correlated to sedimentation. Overall, total oyster recruitment varied mostly at the scale of estuaries rather than with reef attributes and was negatively correlated with sedimentation. Percentage recruit survival was, however, similar among estuaries, although energy reserves and condition of recruits were lower at a high- compared to a low-sediment estuary. Within each estuary, total oyster recruitment increased with patch area and decreased with increasing tidal height. Our results showed that differences among estuaries have the largest influence on oyster recruitment and recruit health and this may be explained by environmental processes operating at the same scale. While survival was high across all estuaries, growth and reproduction of oysters on remnant reefs may be affected by sublethal effects on the health of recruits in high-sediment estuaries. Thus, restoration programs should consider lethal and sublethal effects of whole-estuary environmental processes when selecting sites and include environmental mitigation actions to maximize recruitment success

Variation in the density and body size of a threatened foundation species across multiple spatial scales

Population characteristics e g density and body sizes of foundation species can affect their own persistence and provisioning of ecosystem functions Understanding the drivers of population characteristics of foundation species at multiple spatial scales is therefore critical for maximizing ecosystem functions of restored habitats We analyzed variation in population characteristics densities 95th percentile and median lengths of live oysters of the Sydney rock oyster Saccostrea glomerata on remnant oyster reefs at regional scales among three estuaries along an approximately 250 km of coastline in New South Wales Australia We then analyzed how population characteristics were further related to spatial attributes at smaller spatial scales including within patches rugosity distance to patch edge and elevation whole patches size and shape and among patch connectivity within each estuary The densities and body sizes of S glomerata were related to spatial attributes occurring within patch e g elevation whole patch e g shape and landscape i e connectivity scales but these relationships varied among estuaries The greatest variation in oyster density and size occurred at regional scales suggesting that processes acting at larger spatial scales e g water quality and or climate set the context for smaller scale influences on oyster characteristics Our results highlight the potential importance of incorporating site specific spatial attributes in the design of restored oyster reefs to maximize ecosystem services and functions provided by restoration efforts 2022 The Authors Restoration Ecology published by Wiley Periodicals LLC on behalf of Society for Ecological Restoratio

Nitrogen-cycling genes in oyster reefs and surrounding sediments: Relationships with environmental factors and respective nitrogen rates

To investigate nitrogen (N) cycling in oyster reef habitats along the East coast of Australia, we assessed N-cycling gene abundances in oyster shell biofilms and surrounding sediments, and explored their correlation with environmental factors and respective N rates. We found higher abundances of the denitrification gene nosZII in oyster shell biofilms, while there were not significant differences in the denitrification genes nirS and nirK between oyster biofilms and sediments. Additionally, oyster shell biofilms had a lower (nirS + nirK)/nosZII ratio, indicating a greater capacity for N removal and limited nitrous oxide release compared to sediments. Abundance of nirS, nirK, and dissimilatory nitrate reduction to ammonium (nrfA) genes in sediments decreased with increasing content of organic material, suggesting the influence of large-scale environmental conditions. N-cycling gene abundances did not relate to N rates, emphasising the importance of investigating microbial genes to enhance our understanding of the N cycle in oyster reef habitats

Assessing the ecological impacts of invasive species based on their functional responses and abundances

Invasive species management requires allocation of limited resources towards the proactive mitigation of those species that could elicit the highest ecological impacts. However, we lack predictive capacity with respect to the identities and degree of ecological impacts of invasive species. Here, we combine the relative per capita effects and relative field abundances of invader as compared to native species into a new metric, “Relative Impact Potential” (RIP), and test whether this metric can reliably predict high impact invaders. This metric tests the impact of invaders relative to the baseline impacts of natives on the broader ecological community. We first derived the functional responses (i.e. per capita effects) of two ecologically damaging invasive fish species in Europe, the Ponto-Caspian round goby (Neogobius melanostomus) and Asian topmouth gudgeon (Pseudorasbora parva), and their native trophic analogues, the bullhead (Cottus gobio; also C. bairdi) and bitterling (Rhodeus amarus), towards several prey species. This establishes the existence and relative strengths of the predator-prey relationships. Then, we derived ecologically comparable field abundance estimates of the invader and native fish from surveys and literature. This establishes the multipliers for the above per capita effects. Despite both predators having known severe detrimental field impacts, their functional responses alone were of modest predictive power in this regard; however, incorporation of their abundances relative to natives into the RIP metric gave high predictive power. We present invader/native RIP biplots that provide an intuitive visualisation of comparisons among the invasive and native species, reflecting the known broad ecological impacts of the invaders. Thus, we provide a mechanistic understanding of invasive species impacts and a predictive tool for use by practitioners, for example, in risk assessments

Learning from nature to enhance Blue engineering of marine infrastructure

The global sprawl of urban centres is replacing complex natural habitats with relatively flat and featureless infrastructure that supports low biodiversity. In a growing countermovement, artificial microhabitats are increasingly incorporated into designs for “Green” and “Blue” infrastructure. In order to maximise the ecological value of such interventions, we need to inform the designs with observations from natural systems and existing Green and Blue infrastructure. Here, we focussed on water retaining features mimicking intertidal rock pools, as this is a widely used intervention in coastal ecosystems. Using a meta-analysis and a qualitative literature review, we compiled information on diversity and function of rock pools on natural rocky shores and built structures to assess the potential ecological benefits of water retaining microhabitats and the design metrics of rock pools that affect diversity and function. Our meta-analysis showed higher species richness in rock pools compared to emergent surfaces on built structures, but this was variable among locations. The qualitative review revealed that rock pools on both natural and artificial shores generally hosted species that were not present on emergent rock and can also host non-indigenous species, suggesting that the addition of these features can sometimes have unwanted consequences and local ecological knowledge is essential to implement successful interventions. Relationships between species richness and design metrics, such as height on shore, volume, surface area and depth of pool were taxa-specific. For example, results from the meta-analysis suggest that building larger, deeper pools could increase diversity of fish, but not benthic organisms. Finally, this study highlights major gaps in our understanding of how the addition of rock pools and design metrics influence diversity and the variables affecting the ecological functioning of rock pools. Based on the knowledge gathered so far, recommendations for managers are made and the need for future studies to add knowledge to expand these recommendations is discussed

Artificial structures alter kelp functioning across an urbanised estuary

Assessments of human impacts on natural habitats often focus on the abundance of component species, yet physiological and/or sub-lethal effects of stressors on functional attributes may be equally important to consider. Here we evaluated how artificial structures, an integral part of urbanisation in the marine environment, affects key functional properties of the habitat-forming kelp Ecklonia radiata. Given that stressors rarely occur in isolation, we assessed the effects of infrastructure across an urbanised estuary. Estuaries are ideal for studying how multiple anthropogenic and natural stressors influence potential impacts of infrastructure on habitat-forming species because these habitats usually face a wide range and levels of stressors. Here, we compared the abundance of habitat-forming macro-algae as well as the growth, erosion and photosynthetic activity of kelp in artificial and natural habitats across one of the largest urbanised estuaries in the word - Sydney Harbour. We predicted that effects of artificial structures on functional attributes of kelps would be stronger in the inner area of the Harbour, characterised by higher levels of human impacts and low flushing. Contrary to our predictions, we found that effects of infrastructure were consistent across the estuary, regardless of the ecological footprint caused by human activities or natural environmental gradients. When differences were observed between areas of the estuary, they mostly occurred independently of impacts of substrate type. Importantly, we found lower erosion rates of kelp on pilings than on reefs, likely resulting in lower production of detritus in estuaries where natural reefs are degraded or lost and pilings added. Such impacts have important implications for the connectivity among coastal habitats and secondary productivity in adjacent and remote habitats, which are highly dependent on the exportation of kelp detritus. Our study is the first to assess potential functional consequences of urbanisation through physiological and/or biomechanical effects on habitat-formers, an often overlooked mechanism of environmental impact on ecosystem functioning

A novel framework for the use of remote sensing for monitoring catchments at continental scales

Historical ecology can teach us valuable lessons on the processes and drivers of environmental change that can inform future monitoring priorities and management strategies. Environmental data to study environmental history, however, is often absent or of low quality. Even when studying changes occurring during the last few decades, monitoring efforts are scarce due to logistical and cost limitations, leaving large areas unassessed. The aim of this study is to evaluate the use of estuarine water colour as an indicator of historical environmental change in catchments. Water colour change was assessed in estuaries in Australia from 1987 to 2015 using satellite remote sensing. Random points were selected for each estuary and applied to the Australian Geoscience Data Cube (based on Landsat images) to obtain reflectance data through time. We propose a framework where (i) water colour is used to detect historical changes in catchments using generalised additive models, (ii) possible stressors and pressures driving those changes are evaluated using other available historical data, and (iii) lessons learned inform appropriate monitoring and management actions. This framework represents a novel approach to generate historical data for large-scale assessments of environmental change at catchment level, even in poorly studied areas

Building ‘blue’: An eco-engineering framework for foreshore developments

Urbanisation in terrestrial systems has driven architects, planners, ecologists and engineers to collaborate on the design and creation of more sustainable structures. Examples include the development of ‘green infrastructure’ and the introduction of wildlife corridors that mitigate urban stressors and provide positive ecological outcomes. In contrast, efforts to minimise the impacts of urban developments in marine environments have been far more restricted in their extent and scope, and have often overlooked the ecological role of the built environment as potential habitat. Urban foreshore developments, i.e. those built on the interface of intertidal and/or subtidal zones, have the potential to incorporate clear multi-functional outcomes, by supporting novel ecosystems. We present a step-by-step eco-engineering framework for ‘building blue’ that will allow coastal managers to facilitate planning and construction of sustainable foreshore developments. Adopting such an approach will incorporate ecological principles, thereby mitigating some of the environmental impacts, creating more resilient urban infrastructure and environments, and maximising benefits to the multiple stakeholders and users of marine urban waterfronts