Temperate Reef Research Articles

Optimal fishing effort benefits fisheries and conservation

The ecosystem effects of all commercial fishing methods need to be fully understood in order to manage our marine environments more effectively. The impacts associated with the most damaging mobile fishing methods are well documented leading to such methods being removed from some partially protected areas. In contrast, the impacts on the ecosystem from static fishing methods, such as pot fishing, are less well understood. Despite commercial pot fishing increasing within the UK, there are very few long term studies (> 1 year) that consider the effects of commercial pot fishing on temperate marine ecosystems. Here we present the results from a controlled field experiment where areas of temperate reef were exposed to a pot fishing density gradient over 4 years within a Marine Protected Area (MPA), simulating scenarios both above and below current levels of pot fishing effort. After 4 years we demonstrate for the first time negative effects associated with high levels of pot fishing effort both on reef building epibiota and commercially targeted species, contrary to existing evidence. Based on this new evidence we quantify a threshold for sustainable pot fishing demonstrating a significant step towards developing well-managed pot fisheries within partially protected temperate MPAs.

Genomic and morphological evidence of distinct populations in the endemic common (weedy) seadragon Phyllopteryx taeniolatus (Syngnathidae) along the east coast of Australia.

The common or weedy seadragon, Phyllopteryx taeniolatus, is an iconic and endemic fish found across temperate reefs of southern Australia. Despite its charismatic nature, few studies have been published, and the extent of population sub-structuring remains poorly resolved. Here we used 7462 single nucleotide polymorphisms (SNPs) to identify the extent of population structure in the weedy seadragon along the temperate southeast coast of Australia. We identified four populations, with strong genetic structure (FST = 0.562) between them. Both Discriminant Analysis of Principle Components (DAPC) and Bayesian clustering analyses support four distinct genetic clusters (north to south: central New South Wales, southern NSW, Victoria and Tasmania). In addition to these genetic differences, geographical variation in external morphology was recorded, with individuals from New South Wales shaped differently for a few measurements to those from the Mornington Peninsula (Victoria). We posit that these genetic and morphological differences suggest that the Victorian population of P. taeniolatus was historically isolated by the Bassian Isthmus during the last glacial maximum and should now be considered at least a distinct population. We also recorded high levels of genetic structure among the other locations. Based on the genomic and to a degree morphological evidence presented in this study, we recommend that the Victorian population be managed separately from the eastern populations (New South Wales and Tasmania).

Reproductive Output, Synchrony across Depth and Influence of Source Depth in the Development of Early Life stages of Kelp.

Ecklonia radiata is the main foundation species in Australian temperate reefs, yet little has been published on its reproduction and how this may change across its depth range (1-50+m). In this study, we examined differences in sporophyte morphology and zoospore production during a reproductive season and across four depths (7, 15, 25, and 40m). Additionally, we examined differences in germination rate, survival, and morphological traits of gametophytes obtained from these four depths, cultured under the same light and temperature conditions. Multivariate morphology of sporophytes differed significantly between deep (~40m) and shallow sites (7 and 15m), but individual morphological traits were not significantly different across depths. Total spore production was similar across depths but the peak of zoospore release was observed in February at 15m of depth (6,154zoospores · mm-2 of tissue) and the minimum observed in January at 7, 25, and 40m (1,141, 987, and 214zoospores · mm-2 of tissue, respectively). The source depth of zoospores did not have an influence in the germination rate or the survival of gametophytes, and only gametophytes sourced from 40m sites presented significantly less surface area and number of branches. Overall, these results indicate that E. radiata's reproductive performance does not change across its depth range and that kelp beds reproducing in deeper areas may contribute to the replenishment of their shallow counterparts. We propose that deep kelps may constitute a mechanism of resilience against climate change and anthropogenic disturbances.

The ecology of temperate reefs in a changing world

The world’s temperate coastlines support a rich marine biodiversity and provide billions of dollars of ecosystem services to the global economy annually. Temperate coasts are, however, some of the most densely populated and impacted coastal regions of the world and are increasingly modified by a range of local and global impacts. Understanding the natural functioning of these systems, and the impact of human activities, has been the focus of intense research for more than a century. This focus has led to a community of scientists that gathers at the International Temperate Reefs Symposium (ITRS) to share advances in the field. The papers in this Theme Section (TS) are a selection of the research presented at the 12th ITRS in January 2019. The papers cover the breadth of research at the conference, from algal and animal physiology, to population dynamics and range-shifts, the effects of natural and anthropogenic disturbances on systems, and new insights into efforts to mitigate these impacts. These contributions bring together the rich history of the field and enhance our understanding of how these systems will function into the future.

Ontogenetic shifts in social aggregation and habitat use in a temperate reef fish

AbstractCover, both from physical structure or association with social groups, can reduce predation risk and increase foraging, leading to enhanced growth and survival, and is therefore a critical aspect of the niche for many organisms. However, the need for cover, or the need for a specific type of cover, may change as an individual grows in size, leading to niche shifts throughout development to meet these changing needs. In this study, we examine ontogenetic shifts in cover use by wild populations of a temperate reef fish, the southern hulafish, Trachinops caudimaculatus, a small, abundant planktivorous social aggregator found on temperate reefs in southern Australia. Through repeated in situ surveys, we found clear evidence of ontogenetic shifts in both microhabitat use and aggregation patterns by T. caudimaculatus juveniles in the first three months on the reef. The microhabitat associations of juvenile T. caudimaculatus became more similar to those of adult conspecifics over the study period, and over the same time frame, juveniles increasingly aggregated with adult shoals. Our findings also suggest that trade‐offs between structural and social cover are context‐dependent, with juveniles relying on structural cover longer when adult conspecific density (and thus intra‐specific competition) and/or habitat complexity (and thus the availability of shelter) is high. These findings provide rare and important observations into the complex interplay of social aggregations, habitat use, and ontogeny in wild fish populations.

Maternal age, length, and weight influence larval characteristics and viability in the viviparous temperate reef fish Helicolenus percoides

Mature age structure has been recognized as an important determinant of fecundity in fish populations. More recently it has been demonstrated that in some species, older, larger females can produce faster-growing and more viable larvae than younger, smaller females. We investigated this effect in sea perch Helicolenus percoides and collected information on fecundity as well as notochord length and oil globule volume (OGV) of larvae on the day of parturition, time to 50% mortality, and rate of change in length and OGV of cohorts of larvae in fed and unfed treatments. Absolute fecundity increased proportionally with maternal age and total length. Older, larger females produced cohorts of larvae with larger OGVs. These cohorts also had faster positive rates of change in length in the fed treatment than did cohorts of larvae produced by younger, smaller females. The OGV was positively correlated with rates of change in length and survival in the fed treatment; however, we did not detect influences on growth or survival in the unfed treatment. Further, cohorts of larvae in the unfed treatment had negative rates of change in length and lower survival compared to cohorts in the fed treatment. The results indicated that the OGV, which was affected by maternal age and size, was an important factor influencing larval growth and viability when external energy sources were available. These data provide insight into the reproductive biology of sea perch and highlight the importance of an old-growth age structure for effective larval viability and growth.

Trophic downgrading reduces spatial variability on rocky reefs

Trophic downgrading in coastal waters has occurred globally during recent decades. On temperate rocky reefs, this has resulted in widespread kelp deforestation and the formation of sea urchin barrens. We hypothesize that the intact kelp forest communities are more spatially variable than the downgraded urchin barren communities, and that these differences are greatest at small spatial scales where the influence of competitive and trophic interactions is strongest. To address this, benthic community surveys were done in kelp forests and urchin barrens at nine islands spanning 1230 km of the Aleutian Archipelago where the loss of predatory sea otters has resulted in the trophic downgrading of the region’s kelp forests. We found more species and greater total spatial variation in community composition within the kelp forests than in the urchin barrens. Further, the kelp forest communities were most variable at small spatial scales (within each forest) and least variable at large spatial scales (among forests on different islands), while the urchin barren communities followed the opposite pattern. This trend was consistent for different trophic guilds (primary producers, grazers, filter feeders, predators). Together, this suggests that Aleutian kelp forests create variable habitats within their boundaries, but that the communities within these forests are generally similar across the archipelago. In contrast, urchin barrens exhibit relatively low variability within their boundaries, but these communities vary substantially among different barrens across the archipelago. We propose this represents a shift from small-scale biological control to large-scale oceanographic control of these communities.

Sponges in shallow tropical and temperate reefs are important habitats for marine invertebrate biodiversity

Habitat availability underpins the diversity and distribution of benthic marine communities. Sponges are significant structural components of seabeds; therefore, understanding sponge-community associations are important for the effective management of marine biodiversity. Invertebrate communities were quantified from 11 sponge species having distinct morphologies from Ningaloo Reef (tropical) and Rottnest Island (temperate), Western Australia. Communities from substrate adjacent to sponges were additionally sampled for comparisons to sponge-associated fauna. Gross and fine-scale morphological features of sponge host species were quantified to assess their effects on faunal abundance and diversity. A total of 3966 individuals from 125 taxa were extracted, showing low co-occurrences of taxa from both sponges and the surrounding substrate (Ningaloo 8.9%; Rottnest 11.2%). Four out of the 11 sponges supported higher fauna abundance compared to their surrounding substrate, including Haliclona sp. NTM148 (Ningaloo; 1.21 ± 0.54 N.cm−3, 60 × higher than substrate) and Monanchora clathrata (Rottnest; 2.87 ± 1.7 N.cm−3, 32 × higher than substrate). These communities were dominated by the barnacle Acastinae sp.4 (100%) and sedentary polychaete Spionidae sp. 1 (99%), respectively, highlighting strong host-specific associations. Sponge size (volume), % of internal space, minimum diameter of internal space, and gross morphological complexity were important at explaining variation in faunal assemblage, with larger sponges having more internal space of larger minimum canal diameter supporting higher community abundance. This study highlights the significance of large and long-lived sponges as sources of unique marine biodiversity that are yet to be discovered and the importance of sponge gardens for the conservation of cryptic marine biodiversity.

Integration of close‐range underwater photogrammetry with inspection and mesh processing software: a novel approach for quantifying ecological dynamics of temperate biogenic reefs

AbstractCharacterizing and monitoring changes in biogenic 3‐dimensional (3D) structures at multiple scales over time is challenging within the practical constraints of conventional ecological tools. Therefore, we developed a structure‐from‐motion (SfM)‐based photogrammetry method, coupled with inspection and mesh processing software, to estimate important ecological parameters of underwater worm colonies (hummocks) constructed by the sabellariid polychaete Sabellaria alveolata, using non‐destructive, 3D modeling and mesh analysis. High resolution digital images of bioconstructions (hummocks) were taken in situ under natural conditions to generate digital 3D models over different sampling periods to analyse the morphological evolution of four targeted hummocks. 3D models were analysed in GOM Inspect software, a powerful and freely available mesh processing software to follow growth as well as morphology changes over time of each hummock. Linear regressions showed 3D models only slightly overestimated the real dimensions of the reference objects with an average error < 5% between measured and model‐estimated dimensions for both length and volume. Manual inspection of models and semi‐automated surface‐to‐surface comparison allowed the computation of important metrics linked to the ecology of temperate reefs such as volume, surface area, surface complexity/rugosity, number and size of holes and creeks and the mean density of living worms per colony. Moreover we demonstrated the reliability of 3D surface complexity estimates against two linear rugosity measures: a traditional and a virtual variant of the ‘chain‐and‐tape’ method. Finally, besides 3D models deviation analysis via surface comparison, a Bayesian latent variable model approach was adopted to highlight the significative effects of sea state conditions on S. alveolata hummocks metrics. We demonstrated without damaging the benthic organisms that SfM approach allow continuous study of temperate bioconstruction leading to a fine description of short‐term structural modification mediated by hydrodynamics, making this technique accessible and repeatable to many other areas of ecological research.

Local adaptation of antipredator behaviors in populations of a temperate reef fish.

The temperament of animals can vary among individuals and among populations, but it is often unclear whether spatial variation in temperament is the result of acclimation to local environmental conditions or genetic adaptation to spatial differences in natural selection. This study tested whether populations of a marine fish that experience different levels of mortality and fishing exhibited local adaptation in behaviors related to predator avoidance and evasion. First, we measured variation in reactivity to perceived risk in wild populations of black surfperch (Embiotoca jacksoni). We compared flight initiation distances (FID) between populations with significantly different mortality rates. After finding that FID values were substantially lower in the low-risk locations, we tested for local adaptation by rearing lab-born offspring from both high- and low-risk populations in a common environment before measuring their behavior. Lab-reared offspring from high- and low-risk populations exhibited significant differences in several behaviors related to reactivity. Between 23 and 43% of the total variation in behaviors we measured could be attributed to source population. These results thus suggest that a substantial amount of spatial variation in behaviors related to predator evasion may represent local adaptation. In addition, behaviors we measured had an average, broad-sense heritability of 0.24, suggesting that the behavioral tendencies of these populations have some capacity to evolve further in response to any changes in selection.

Soundscapes of natural and artificial temperate reefs: similar temporal patterns but distinct spectral content

Marine soundscapes often differ among habitats; however, relatively little is known about whether soundscapes on naturally occurring habitats differ from soundscapes on human-made structures. To address this knowledge gap, we investigated whether temporal and spectral characteristics of biological sound production differ between natural and artificial offshore reefs. Specifically, we analyzed recordings from 5 week-long hydrophone deployments on 2 natural rocky reefs and 2 artificial reefs on the North Carolina, USA, continental shelf. Analysis of sound pressure levels (SPLs) on hourly and seasonal scales revealed similar temporal patterns between the reef types. These patterns were largely driven by 4 dominant fish vocalizers with seasonal chorusing patterns, including a toadfishOpsanussp. Despite similar temporal patterns within reef types, soundscape spectral content was more similar within than between reef types, especially during the April deployment, which had the most acoustic activity. Our findings suggest that the soundscapes of shipwreck artificial reefs may differ from the soundscapes of natural rocky reefs, possibly due to differing community composition. As sound plays an important role in the navigation and settlement of many marine species, soundscape differences between natural and artificial habitats could affect ecosystem function through species behavior and interactions.

Downscaling global ocean climate models improves estimates of exposure regimes in coastal environments

Climate change is expected to warm, deoxygenate, and acidify ocean waters. Global climate models (GCMs) predict future conditions at large spatial scales, and these predictions are then often used to parameterize laboratory experiments designed to assess biological and ecological responses to future change. However, nearshore ecosystems are affected by a range of physical processes such as tides, local winds, and surface and internal waves, causing local variability in conditions that often exceeds global climate models. Predictions of future climatic conditions at local scales, the most relevant to ecological responses, are largely lacking. To fill this critical gap, we developed a 2D implementation of the Regional Ocean Modeling System (ROMS) to downscale global climate predictions across all Representative Concentration Pathway (RCP) scenarios to smaller spatial scales, in this case the scale of a temperate reef in the northeastern Pacific. To assess the potential biological impacts of local climate variability, we then used the results from different climate scenarios to estimate how climate change may affect the survival, growth, and fertilization of a representative marine benthic invertebrate, the red abalone Haliotis rufescens, to a highly varying multi-stressor environment. We found that high frequency variability in temperature, dissolved oxygen (DO), and pH increases as pCO2 increases in the atmosphere. Extreme temperature and pH conditions are generally not expected until RCP 4.5 or greater, while frequent exposure to low DO is already occurring. In the nearshore environment simulation, strong RCP scenarios can affect red abalone growth as well as reduce fertilization during extreme conditions when compared to global scale simulations.

Size and stage-dependent vertical migration patterns in reef-associated fish larvae off the eastern coast of Australia

Larval fish have been shown to vertically migrate with ontogeny, a behaviour that influences their dispersal by exposing the larvae to currents with different velocities at different stages of development. Most studies have focused on studying the vertical migration of tropical reef fishes, that have been shown in general to migrate downwards with ontogeny. Here we investigated if similar ontogenetic vertical migration patterns also occur for seven families of temperate reef fish along the eastern coast of Australia, sampling at three depth ranges (0–1 m, 5–50 m, 51–100 m) across sixteen stations along the continental shelf, and eight within cold-core eddies. Larval fish were identified to the lowest taxonomic level possible, their standard length measured, and ontogenetic stage classified to preflexion, flexion or postflexion. Generalised linear models were used to determine if the larval fish exhibited patterns of vertical migration with ontogeny and length. Four families (Labridae, Serranidae, Scorpaenidae, and Synodontidae) exhibited a downwards migration with ontogeny similar to vertical distribution patterns seen in tropical reef fish studies, Mullidae displayed different vertical migration patterns within eddies compared to coastal waters, and two families showed no ontogenetic vertical migration (Pomacentridae and Scaridae). For all families other than Scaridae, the larvae inside an eddy were longer suggesting larval retention. The vertical migration patterns observed, while varying among taxa, show that reef-associated larval fish off the eastern Australian coast do change their depth with ontogeny. The results of this study will be useful in parameterising connectivity modelling studies for these temperate reef-associated families.

The Biophysical Controls of Macroalgal Growth on Subtropical Reefs

The importance of macro-grazers in controlling macroalgal cover has long been recognized on tropical and temperate reefs, with fishes of primary importance on the former and sea urchins on the latter. However, the functional role of herbivorous urchins and fishes on subtropical marginal reefs remains poorly explored. To evaluate the relative importance of fishes and urchins on marginal subtropical reefs, this study used exclusion devices (excluding all grazers, fishes, or urchins) at two depths (1-2, 5-6 m) on Brazilian rocky reefs. Depth influenced responses within cages, with shallow sites changing from patchy barrens (dominated by crustose coralline algae) to epilithic algae-dominated within exclusion treatments, and sea urchins being the primary driver of benthic dynamics. In deeper water, the growth and senescence cycle of Sargassum species was associated with upwelling events during the spring and summer, and dominated benthic dynamics. No clear influence of herbivorous fishes was detected on benthic cover at either depth, despite biomasses similar to comparable tropical reefs where they do control macroalgal populations. Thus, abiotic factors seem to be a strong driver of benthic dynamics in the studied region, and top-down processes act only at shallow depths. Consequently, despite Brazilian subtropical communities being dominated by tropical species, the ecological drivers of these reefs may be more similar to temperate systems.

Relative Effects of Multiple Stressors on Reef Food Webs in the Northern Gulf of Mexico Revealed via Ecosystem Modeling

Since 2010, the northern Gulf of Mexico (NGoM) has experienced two unique environmental stressors. First, the 2010 Deepwater Horizon oil spill (DWH) impacted a broad range of taxa and habitats and resulted in declines of small demersal reef fish over the study area (88.5°-85.5° W, 29°-30.5° N). Then, from 2011-2014 the invasive Indo-Pacific lionfish (Pterois volitans) underwent exponential population growth, leading to some of the highest densities in their invaded range. The primary objective of this study was to evaluate the effect of these stressors on reef ecosystems, and specifically how invasive lionfish and fishing may have impacted recovery following DWH. Site-specific datasets on fish density and diet composition were synthesized into an Ecopath with Ecosim food web model of a NGoM reef ecosystem. The model consisted of 63 biomass groups and was calibrated to time series of abundance from 2009-2016. The model accounted for mortality from the DWH using forcing functions derived from logistic dose-response curves and oil concentrations. Eight stressor scenarios were simulated, representing all combinations of DWH, lionfish, and fishing. Simulated biomass differed across model groups due to singular and cumulative impacts of stressors and direct and indirect effects arising through food web interactions. Species with high exploitation rates were influenced by fishing more than lionfish following DWH. Several small demersal fish groups were predicted to be strongly influenced by either the cumulative effects of lionfish and DWH or by lionfish alone. A second group of small demersal fish benefited in the stressor scenarios due to reduced top-down predation and competition in the combined stressor scenarios. We conclude that lionfish had a major impact on this ecosystem, leading to slower recoveries following DWH and lower fish biomass and diversity. Additionally, the lack of recovery for some groups in the absence of lionfish suggests system reorganization may be preventing return to a pre-DWH state. We intended for this work to improve our understanding of how temperate reef ecosystems, like those in the NGoM, respond to broad scale stressors and advance the state of applied ecosystem modeling for resource damage assessment and restoration planning.

Limited effects of Sargassum horneri, an invasive alga, on temperate reef fish assemblages

Biological invasions can dramatically affect the ecology of invaded regions, and globally have resulted in economic damages that total billions of dollars annually. In recent years, an invasive alga, Sargassum horneri, has become established and spread along the coast of southern California (USA). Using field observations and a field experiment, we explored how this non-native alga influences the structure of fish assemblages on temperate reefs in southern California where S. horneri has become prolific. Fish and algal assemblages were quantified along transects on rocky reefs at depths of 3 and 6 m at 6-8 study sites spanning 5 km on 4 occasions over 1.5 yr. Spatiotemporal variation in the fish assemblage was not strongly correlated with the abundance of invasive S. horneri over this period, although it became less variable as native giant kelp Macrocystis pyrifera disappeared from the study sites due to a warm-water event, during which the invasive S. horneri became more dominant. An experiment removing a total of 4.25 t of S. horneri from 6 × 6 m plots (n = 14) revealed that the invasive alga did not affect fish abundance, species richness, species diversity (H’), or multivariate assemblage structure over a 5 mo period. Overall, we found little evidence of negative effects of S. horneri on fishes even though it drastically changed the underwater landscape. Nevertheless, we advise cautionary management actions to limit the movement of this invasive alga because its effects on other community members, such as other algal species, may be detrimental, and longer-term effects on fishes might develop.

Differential response of forest-forming seaweeds to elevated turbidity may facilitate ecosystem shifts on temperate reefs

Underwater light is essential for fuelling coastal productivity. However, elevated turbidity, resulting from land-based activities and climate change, is often overlooked as a threat to coastal ecosystems. Understanding how low light, and specifically the temporal delivery of light, affects the productivity of forest-forming species is necessary to predict how ecosystems and species will respond to future increases in turbidity. Outdoor mesocosm experiments were used to compare the low-light tolerance of 2 forest-forming macrophytes that vary in their distribution in relation to turbidity, and investigate how the temporal delivery of light, i.e. press vs. pulse low-light disturbance, affects net primary productivity (NPP). We showed that the kelpEcklonia radiata, which dominates reefs with low turbidity, is more productive per unit biomass under high-light conditions than the fucoidCarpophyllum flexuosum, which typifies more turbid waters. Under low light,E. radiatasuffered greater tissue loss and had lower NPP thanC. flexuosum. Under both press and pulse treatments,E. radiatashowed significant losses of lamina biomass and reduced NPP, whileC. flexuosumshowed net growth under press disturbance, and only lost tissue and had reduced NPP under pulse disturbance. The greater tolerance ofC. flexuosumto decreased light, and differential responses ofE. radiataandC. flexuosumto press and pulse low-light conditions, provide mechanistic support forC. flexuosumbeing better suited to turbid low-light environments thanE. radiata. These results suggest future increases in turbidity may facilitate a shift from kelp-dominated forests to alternate states, resulting in reduced primary productivity.

Plasticity in the purple sea urchin (Strongylocentrotus purpuratus): Tube feet regeneration and adhesive performance

Sea urchins are key members of hydrodynamically intense habitats, such as open coast intertidal and shallow reefs. Secure attachment to the substrata, by means of tube feet and spines, is essential for survival. Previous studies suggest that environmental variables (i.e. hydrodynamics, substrate) can influence the morphology and adhesive performance of tube feet. Catastrophic failure of tube feet occurs often due to strong hydrodynamic forces. The time and ability of amputated tube feet to return to pre-amputation adhesive performance, and the interaction with environmental factors are unknown. To assess the dynamics of adhesive performance and regeneration of tube feet, we 1) evaluated phenotypic plasticity of disc surface area of two urchin populations inhabiting sites with different lithologies (field - in-situ conditions) and, 2) assessed regeneration of their functionality (tube foot length, time of disc first appearance, disc surface area, and tenacity) after amputation (laboratory - ex-situ conditions). Results showed that in the field, tube feet are plastic in their phenotype: sea urchins from the two sites differed in their disc surface area. Plasticity was also observed in the laboratory, where urchins regenerate tube feet that are shorter in length and have smaller discs compared to pre-amputation measurements. Thus, sea urchin tube feet recovering from amputation events in the intertidal may differ in their initial morphology and in the recovery period after amputation. Moreover, tube feet that were never amputated also decreased their disc area, suggesting that plasticity occurs on tube feet of urchins maintained in laboratory conditions. In shallow temperate reef systems, urchins are ecosystem engineers that affect the abundance and distribution of many other organisms. The plastic nature of morphological and adhesive abilities of sea urchin tube feet influences their ability to survive and dominate wave-battered habitats.

Tropicalization may invert trophic state and carbon budget of shallow temperate rocky reefs

Abstract Global warming mediates and maintains the tropicalization of temperate marine ecosystems. Recent studies have demonstrated that this process causes shifts from algal forests to denuded non‐canopy states in temperate reefs. It has been suggested that these changes would incur significant consequences to ecosystem functioning. In this study, we tested how tropicalization affects habitat‐provisioning functions and carbon turnover of a shallow reef in the fast‐warming and highly invaded southeastern Mediterranean Sea, in‐situ. On a single shallow reef, we conducted measurements of these functions in three habitats: dwindling native brown algal (Cystoseira) forest, dominant turf (formed by overgrazing of tropical rabbitfish) and expanding tropical shrubs dominated by red calcifying algae (Galaxaura rugosa). Algal forest was an autotrophic net carbon sink and provided habitat for high species diversity and the largest community biomass. The denuded turf was heterotrophic and provided habitat for the lowest species diversity and community biomass. While diversity was as high in tropical shrubs as in the algal forest, it had lower biomass and functioned as a heterotrophic net carbon source. Synthesis. Our study exemplifies possible functional consequences of tropicalization‐driven regime shifts on shallow temperate rocky reefs, and how these can invert the net trophic state and carbon balance.

Snapshot of Macroalgae and Fish Assemblages in Temperate Reefs in the Southern European Atlantic Ecoregion

Most of the biodiversity studies in the South European Atlantic Shelf ecoregion are limited to shallow subtidal or intertidal habitats, while deeper reef habitats, also of relevant ecological importance, are particularly understudied. Macroalgal communities, associated fauna, and sea surface temperature were studied in deep reefs (25–30 m) at two locations in this ecoregion: Parcel, North of Portugal (41° N), and Tarifa, Southern Spain (35° N). Specifically, algal assemblages were assessed using biomass collection and associated ichthyofauna was assessed using visual census techniques using scuba. Seawater surface temperature was higher (>3 °C) in the southern region—Tarifa, compared to the northern region—Parcel. Our survey revealed 18 fish species and 23 algae species. The highest abundance of cold-water species (both macroalgae and fish species) was recorded in Parcel and warm-water species were dominant in Tarifa. In light of climate global trends, both regions might experience biodiversity shifts towards tropicalization. Current knowledge on their biodiversity is imperative to further evaluate potential shifts.