Katherine Erickson

Over 85% of oyster reef habitat has been lost or significantly degraded globally due to anthropogenic climate change, urbanization, and pollution. These systems provide critical habitat for fish communities, yet the relationship between oyster reef patch characteristics and fish assemblages remains poorly understood, limiting the effectiveness of restoration efforts. To address this gap, we conducted an extensive 3‐year survey (2019–2021) of fish assemblages on natural remnant oyster reefs across three estuaries in temperate New South Wales, Australia, using unbaited remote underwater video systems. Sampling was conducted seasonally during summer and winter, providing insights into temporal variability and seasonal influences on fish communities and their associated relationships with reef patch characteristics. This study shows that oyster reef patch characteristics are important drivers of fish abundance, richness, and diversity, with relationships varying across spatial and temporal scales. While there were significant estuary‐specific patterns and seasonal variability, some patterns were observed across the systems. Patch shape influenced fish assemblage metrics in contrasting ways, with circular patches supporting greater abundance, while elongate patches supported a greater richness and diversity of fish. Edge habitat was consistently important, with complex edges and higher perimeter‐to‐area ratios positively associated with fish assemblage metrics. Patch connectivity exhibited contrasting effects: greater network isolation supported fish abundance, while greater connectivity promoted diversity. These findings have practical applications for conservation and management. By identifying which patch sizes, shapes, and configurations maximize fish abundance or diversity, managers and restoration practitioners can strategically design restoration projects to achieve greater ecological outcomes per dollar spent. This knowledge helps avoid investing resources in sub‐optimal designs, streamlining project implementation towards configurations more likely to achieve restoration goals. Specifically, restoration practitioners should prioritize creating elongated patches with complex edges and maintain high connectivity among patches to maximize fish habitat value. These recommendations are timely, given the global surge in oyster reef restoration efforts driven by their ecological and economic importance and dramatic decline.

Summary: Belowground microbes are increasingly recognised as mediators of plant responses to stress, but it remains unclear whether the thermal histories of marine plants and their associated belowground microbes influence plant performance under ocean warming. We conducted a common‐garden field experiment at a long‐term warmed site in Lake Macquarie (NSW, Australia, > 30 yr, 1–3°C warming), to test the effects of sediment origin (ambient vs warm‐origin), plant origin (ambient vs warm‐origin) and microbial disruption (intact vs disrupted rhizosphere and bulk sediment microbial communities) on seagrass ( Zostera muelleri ) performance. Both plant origins had lower aboveground biomass in intact warm‐origin sediments, but warm‐origin plants recovered when bulk sediment microbial communities were disrupted, indicating that warming‐altered sediment microbial communities can suppress seagrass performance. Disrupted warm‐origin sediments were enriched in sulphide‐oxidising bacteria, which likely contributed to enhanced performance. In warm‐origin sediments, rhizosphere bacterial communities were similar across plant origins, indicating that sediments shape rhizosphere bacterial assembly. Rhizosphere microbial disruption had no effect on plant performance. We show that ocean warming can shape sediment microbial communities in ways that suppress seagrass performance, suggesting that sediment microbial communities can override plant thermal history and act as hidden constraints on seagrass tolerance to warming.

Background: Assessing fish assemblages in subtidal and intertidal habitats is challenging due to the structural complexity of many of these systems. Trapping and collecting are regarded as optimal ways to sample these assemblages, but this method is costly and destructive, so researchers also use video techniques. Underwater visual census and baited remote underwater video stations are commonly used to characterise fish communities in these systems. More passive techniques such as remote underwater video (RUV) may be more appropriate for behavioural studies, or for comparing proximal habitats where the broad attraction caused by bait plumes could be an issue. However, data processing for RUVs can be time consuming and create processing bottlenecks. Methods: Here, we identified the optimal subsampling method to assess fish assemblages on intertidal oyster reefs using RUV footage and bootstrapping techniques. We quantified how video subsampling effort and method (systematic vs random) affect the accuracy and precision of three different fish assemblage metrics; species richness and two proxies for the total abundance of fish, MaxN_T and MeanCount_T, which have not been evaluated previously for complex intertidal habitats. Results: Results suggest that MaxN_T and species richness should be recorded in real time, whereas optimal sampling for MeanCount_T is every 60 s. Systematic sampling proved to be more accurate and precise than random sampling. This study provides valuable methodology recommendations which are relevant for the use of RUV to assess fish assemblages in a variety of shallow intertidal habitats.

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 wormsDiopatra aciculataand clamsKatelysiasp. 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.