Vulnerability of Temperate Mesophotic Ecosystems (TMEs) to environmental impacts: Rapid ecosystem changes at Lough Hyne Marine Nature Reserve, Ireland

Temporal variability in temperate mesophotic ecosystems revealed with over a decade of monitoring with an autonomous underwater vehicle

Rocky reef temperate mesophotic ecosystems (TMEs) are increasingly recognised for their spatial extent and high biodiversity. Platforms such as autonomous underwater vehicles (AUVs) allow large-scale collection of benthic imagery, facilitating descriptions of TMEs, but these efforts currently remain geographically restricted. Furthermore, descriptions of temporal changes in TMEs are extremely rare and typically limited to a single site with few repeated surveys, leaving critical gaps in our understanding of ecosystem variability. Here, we report on temporal changes in abundance and size structure of sessile biota across TMEs in three Australian Marine Parks (AMPs) across decadal time scales, using AUV-collected benthic imagery, enhanced with AI tools for estimating biota size. Our results challenge the common assumption of TME stability, revealing significant fluctuations in key biota over 2-13-year periods. At the phyla-level, cnidaria exhibited threefold changes and bryozoa fivefold changes at individual sites over ∼5 years. Some individual morphospecies also showed more than twofold change over ∼5 years. We found that higher-level taxonomic/morphological groupings could track changes in dominant taxa, but often masked significant trends at the morphospecies level. Size structure data offer important insights into the population dynamics that abundance or cover data alone could not capture, particularly in terms of recruitment events and size shifts. Our findings highlight that mesophotic ecosystems are dynamic and underscore the need for ongoing monitoring to better understand the temporal changes within TMEs and to inform the development of effective indicators. Coupling image-based surveys with physical data collection such as temperature data should be a priority in future monitoring to better link biotic changes to environmental drivers.

Mesophotic zone as buffer for biodiversity protection: A promising opportunity to enhance MPA effectiveness

Coastal areas conservation strategies often left deeper habitats, such as mesophotic ones, unprotected and exposed to anthropogenic activities. In this context, an approach for including the mesophotic zone inside protection plans is proposed, considering 27 Italian Marine Protected Areas (MPAs) as a model. MPAs were classified considering their bathymetries, exposure to marine heat waves (MHWs), mass mortality events (MMEs) and, using a local ecological knowledge (LEK) approach, the estimated resilience of certain sessile species after MMEs. Only 8 MPAs contained considerable mesophotic areas, with stronger MHWs mainly occurring in shallower MPAs, and MMEs mostly affecting coralligenous assemblages. Even with only a 10% response rate, the LEK approach provided useful information on the resilience of certain species, allowing us to suggest that the presence of nearby mesophotic areas can help shallower habitats facing climate change, thus making the "deep refugia" hypothesis, usually related to tropical habitats, applicable also for the Mediterranean Sea.

Global status, impacts, and management of rocky temperate mesophotic ecosystems

The ecology and function of rocky temperate mesophotic ecosystems (TMEs) remain poorly understood globally despite their widespread distribution. They typically occur at 20-150 m (the limit of photosynthesis), and on rocky substratum they support rich benthic communities and mobile fauna. We determined the distribution of rocky TMEs, their conservation status, and their most characteristic biological groups. Rocky TMEs were dominated by algae, turf-invertebrate matrices (<50 m only), sponges, bryozoans, and cnidarians. The community composition of TMEs differed significantly from shallow (0-15 m) subtidal reefs. Data were geographically biased and variable, available only from the North and South Atlantic, Mediterranean, and Temperate Australasia. Degree of protection of rocky TMEs varied considerably across the world. The biggest threats to rocky TMEs were identified changes in temperature, sedimentation rates, nutrient concentrations, and certain fishing types. We propose a conservation framework to inform future rocky TME management and conservation, highlighting the need to recognize the importance of these biologically diverse and functionally important ecosystems.

Environmental DNA reveals temporal variation in mesophotic reefs of the Humboldt upwelling ecosystems of central Chile: Toward a baseline for biodiversity monitoring of unexplored marine habitats

Temperate mesophotic reef ecosystems (TMREs) are among the least known marine habitats. Information on their diversity and ecology is geographically and temporally scarce, especially in highly productive large upwelling ecosystems. Lack of information remains an obstacle to understanding the importance of TMREs as habitats, biodiversity reservoirs and their connections with better-studied shallow reefs. Here, we use environmental DNA (eDNA) from water samples to characterize the community composition of TMREs on the central Chilean coast, generating the first baseline for monitoring the biodiversity of these habitats. We analyzed samples from two depths (30 and 60 m) over four seasons (spring, summer, autumn, and winter) and at two locations approximately 16 km apart. We used a panel of three metabarcodes, two that target all eukaryotes (18S rRNA and mitochondrial COI) and one specifically targeting fishes (16S rRNA). All panels combined encompassed eDNA assigned to 42 phyla, 90 classes, 237 orders, and 402 families. The highest family richness was found for the phyla Arthropoda, Bacillariophyta, and Chordata. Overall, family richness was similar between depths but decreased during summer, a pattern consistent at both locations. Our results indicate that the structure (composition) of the mesophotic communities varied predominantly with seasons. We analyzed further the better-resolved fish assemblage and compared eDNA with other visual methods at the same locations and depths. We recovered eDNA from 19 genera of fish, six of these have also been observed on towed underwater videos, while 13 were unique to eDNA. We discuss the potential drivers of seasonal differences in community composition and richness. Our results suggest that eDNA can provide valuable insights for monitoring TMRE communities but highlight the necessity of completing reference DNA databases available for this region.

Sponge functional roles in a changing world

Sponges are ecologically important benthic organisms with many important functional roles. However, despite increasing global interest in the functions that sponges perform, there has been limited focus on how such functions will be impacted by different anthropogenic stressors. In this review, we describe the progress that has been made in our understanding of the functional roles of sponges over the last 15 years and consider the impacts of anthropogenic stressors on these roles. We split sponge functional roles into interactions with the water column and associations with other organisms. We found evidence for an increasing focus on functional roles among sponge-focused research articles, with our understanding of sponge-mediated nutrient cycling increasing substantially in recent years. From the information available, many anthropogenic stressors have the potential to negatively impact sponge pumping, and therefore have the potential to cause ecosystem level impacts. While our understanding of the importance of sponges has increased in the last 15 years, much more experimental work is required to fully understand how sponges will contribute to reef ecosystem function in future changing oceans.

Anchor scour from shipping and the defaunation of rocky reefs: A quantitative assessment

Anchor scour from shipping is increasingly recognised as a global threat to benthic marine biodiversity, yet no replicated ecological assessment exists for any seabed community. Without quantification of impacts to biota, there is substantial uncertainty for maritime stakeholders and managers of the marine estate on how these impacts can be managed or minimised. Our study focuses on a region in SE Australia with a high proportion of mesophotic reef (>30 m), where ships anchor while waiting to enter nearby ports. Temperate mesophotic rocky reefs are unique, providing a platform for a diversity of biota, including sponges, ahermatypic corals and other sessile invertebrates. They are rich in biodiversity, provide essential food resources, habitat refugia and ecosystem services for a range of economically, as well as ecologically important taxa. We examined seven representative taxa from four phyla (porifera, cnidaria, bryozoan, hydrozoa) across anchored and 'anchor-free' sites to determine which biota and which of their morphologies were most at risk. Using stereo-imagery, we assessed the richness of animal forest biota, morphology, size, and relative abundance. Our analysis revealed striking impacts to animal forests exposed to anchoring with between three and four-fold declines in morphotype richness and relative abundance. Marked compositional shifts, relative to those reefs that were anchor-free, were also apparent. Six of the seven taxonomic groups, most notably sponge morphotypes, exhibited strong negative responses to anchoring, while one morphotype, soft bryozoans, showed no difference between treatments. Our findings confirm that anchoring on reefs leads to the substantial removal of biota, with marked reductions of biodiversity and requires urgent management. The exclusion of areas of high biological value from anchorages is an important first step towards ameliorating impacts and promoting the recovery of biodiversity.

The Hidden Diversity of Temperate Mesophotic Ecosystems from Central Chile (Southeastern Pacific Ocean) Assessed through Towed Underwater Videos

The largely unexplored diversity in temperate mesophotic ecosystems (TME, ~30–150 m depth) has attracted much attention over the past years. However, the number of studies and knowledge of TME diversity and ecology remains limited and geographically restricted. The absence of information on how assemblages vary across environmental gradients and with depth for most regions also limits our capacity to delimit conservation areas and devise management plans effectively. This study focuses on TME from central Chile and describes the depth distribution of reef fishes and benthic invertebrates and algae for the first time. Through the analysis of towed underwater video surveys between 4.7–95.5 m in multiple sites, we show that total reef fish density and richness decrease with depth but increase with local topographic complexity. The depth-related density varies among fish species and trophic groups, and it reverses in the case of Sebastes oculatus, which increases in density with depth. Sponges and gorgonians dominate benthic assemblages below 20 m depth, and brachiopods and anemones increase below 40 and 60 m, respectively. Some of these species form animal forests which, to some extent, replace the shallow-water kelp forests as structural habitat providers. Nevertheless, the reef fish and benthic community do not show a clear structure with depth or across studied sites. We highlight the urgency to intensify and expand the quantitative characterization of these communities, through this and other methodologies, to better define ecological patterns and advance towards conservation plans for TME, including the Souteastern Pacific region.

Marine heat waves drive bleaching and necrosis of temperate sponges

Marine heat waves (MHWs) are extended periods of excessively warm water¹ that are increasing in frequency, duration, intensity, and impact, and they likely represent a greater threat to marine ecosystems than the more gradual increases in sea surface temperature.^2,3,4 Sponges are major and important components of global benthic marine communities,^5,6,7 with earlier studies identifying tropical sponges as potential climate change "winners."^8,9,10,11 In contrast, cold-water sponges may be less tolerant to predicted ocean warming and concurrent MHWs. Here, we report how a series of unprecedented MHWs in New Zealand have impacted millions of sponges at a spatial scale far greater than previously reported anywhere in the world. We reported sponge tissue necrosis¹² and bleaching (symbiont loss/dysfunction),¹³ which have been previously associated with temperature stress,^6,12,14 for three common sponge species across multiple biogeographical regions, with the severity of impact being correlated with MHW intensity. Given the ecological importance of sponges,¹⁵ their loss from these rocky temperate reefs will likely have important ecosystem-level consequences.

Adaptive strategies of sponges to deoxygenated oceans

Ocean deoxygenation is one of the major consequences of climate change. In coastal waters, this process can be exacerbated by eutrophication, which is contributing to an alarming increase in the so-called 'dead zones' globally. Despite its severity, the effect of reduced dissolved oxygen has only been studied for a very limited number of organisms, compared to other climate change impacts such as ocean acidification and warming. Here, we experimentally assessed the response of sponges to moderate and severe simulated hypoxic events. We ran three laboratory experiments on four species from two different temperate oceans (NE Atlantic and SW Pacific). Sponges were exposed to a total of five hypoxic treatments, with increasing severity (3.3, 1.6, 0.5, 0.4 and 0.13 mg O₂  L^-1 , over 7-12-days). We found that sponges are generally very tolerant of hypoxia. All the sponges survived in the experimental conditions, except Polymastia crocea, which showed significant mortality at the lowest oxygen concentration (0.13 mg O₂  L^-1 , lethal median time: 286 h). In all species except Suberites carnosus, hypoxic conditions do not significantly affect respiration rate down to 0.4 mg O₂  L^-1 , showing that sponges can uptake oxygen at very low concentrations in the surrounding environment. Importantly, sponges displayed species-specific phenotypic modifications in response to the hypoxic treatments, including physiological, morphological and behavioural changes. This phenotypic plasticity likely represents an adaptive strategy to live in reduced or low oxygen water. Our results also show that a single sponge species (i.e., Suberites australiensis) can display different strategies at different oxygen concentrations. Compared to other sessile organisms, sponges generally showed higher tolerance to hypoxia, suggesting that sponges could be favoured and survive in future deoxygenated oceans.