Marine deforestation leads to widespread loss of ecosystem function

Oxygen and pH fluxes in shallow bay habitats: Evaluating the effectiveness of a macroalgal forest restoration

Marine macroalgae are important primary producers in coastal ecosystems. Within sheltered and shallow bays in the Mediterranean, various Fucalean macroalgae and seagrasses coexist, creating habitats of high ecological importance. These habitats have historically suffered from various disturbances, and on this basis, active restoration actions have been proposed as potential solutions for their recovery. Here, we assessed the restoration success of a 10-year restored macroalgal forest by evaluating the recovery in terms of oxygen and pH fluxes and comparing those data with those of a healthy marine forest and a degraded habitat counterpart. We estimated the overall changes in dissolved oxygen and pH using light and dark community in situ incubations. We also determined the biomass and composition of macroalgal and macroinvertebrate compartments of each assemblage. During light incubations, the healthy and restored forest assemblages showed similar average net oxygen production, 5.7 times higher than in the degraded one, and a greater increase in pH. More than 95% of the incubated biomass corresponded to macroalgal and seagrass species. The restored forest showed a six-fold increase in biomass, most likely being responsible for the recovery of primary production. This work provides empirical evidence that the restoration of a single structural species, once successful in the early stages, can yield positive results by recovering processes such as primary production and dark respiration. Moreover, these results showcase differences in ecosystem functions between healthy (either mature or restored) and degraded habitats, highlighting the importance of protecting and preserving coastal marine forests.

Kelp forests as nursery and foundational habitat for reef fishes

Conservation of marine biodiversity requires an understanding of the habitats needed to support and replenish species of interest. It also requires knowledge about the abundance and diversity of multispecies assemblages. Variation in the distribution and composition of kelp forests, one of the most productive marine coastal habitats globally, can have major influences on reef fishes-a group of ecologically and socioeconomically important species. In the face of widespread and escalating loss of kelp forests, quantification of these effects is urgently needed to assess and project cascading impacts on biodiversity. Here, we evaluate relationships between kelp forests and associated reef fish populations using a global meta-analysis of experimental kelp removals and comparative surveys of kelp and adjacent non-kelp habitats. These analyses show that kelp forests increase the abundance of reef fishes, though the significance of this effect varied depending on the structural complexity of kelp forests. In experimental studies, kelp forests have a significant positive effect on fish species richness, revealing that kelp act as true foundation species by supporting the diversity of associated multispecies assemblages. Importantly, regardless of kelp forest morphology and type of study (observational or experimental studies), kelp forests enhance the recruitment of early life history stages suggesting they are nursery habitats for many reef fish taxa. Lastly, kelp forests differentially affected species with different functional traits; small body size fishes from low trophic levels (e.g., herbivore and detritivores, micropredators, and mesopredators) and large body size fish from higher trophic level (e.g., piscivores, general carnivores) were both facilitated by kelp forests. Taken together, these results indicate that the loss of kelp forest, particularly those with more complex morphology, can reduce total abundance and diversity of fish, with possible cascading consequences for coastal ecosystem function.

Submarine groundwater discharge drives both direct and indirect effects on organismal and community metabolism on coral reefs

Coral reefs experience numerous environmental gradients affecting organismal physiology and species biodiversity, which ultimately impact community metabolism. This study shows that submarine groundwater discharge (SGD), a common natural environmental gradient in coastal ecosystems associated with decreasing temperatures, salinity and pH with increasing nutrients, has both direct and indirect effects on coral reef community metabolism by altering individual growth rates and community composition. Our data revealed that SGD exposure hindered the growth of two algae, Halimeda opuntia and Valonia fastigiata, by 67 and 200%, respectively, and one coral, Porites rus, by 20%. Community metabolic rates showed altered community production, respiration and calcification between naturally high and low exposure areas mostly due to differences in community identity (i.e. species composition), rather than a direct effect of SGD on physiology. Production and calcification were 1.5 and 6.5 times lower in assemblages representing high SGD communities regardless of environment. However, the compounding effect of community identity and SGD exposure on respiration resulted in the low SGD community exhibiting the highest respiration rates under higher SGD exposure. By demonstrating SGD's role in altering community composition and metabolism, this research highlights the critical need to consider compounding environmental gradients (i.e. nutrients, salinity and temperature) in the broader context of ecosystem functions.

Kelp forests versus urchin barrens: a comparison of ecosystem functions and services provided by two alternative stable marine habitats

Kelp forests and urchin barrens are two stable states in rocky reef ecosystems, each providing unique ecosystem functions like habitat for marine species and primary production. While studies frequently show that kelp forests support higher levels of some ecosystem functions than urchin barren habitats, no research has yet compared average differences. To address this gap, we first conducted a meta-analysis of studies that directly compared the ecosystem functions, services and general attributes provided by each habitat. We also compiled individual studies on ecosystem properties from both habitats and qualitatively assessed the benefits provided. The meta-analysis included 388 observations from 55 studies across 14 countries. We found that kelp forests consistently delivered higher levels of ecosystem properties such as biodiversity, species richness, abalone abundance and sea urchin roe quality. Urchin barrens supported higher urchin density and crustose coralline algae cover. The qualitative review further supported these findings, showing that kelp forests ranked higher in 11 out of 15 ecosystem properties. These findings can help guide decisions on managing rocky reef habitats and demonstrate the benefits of preserving or expanding kelp forests.

Shrinking suitable habitat of a sub-Arctic foundation kelp under future climate scenarios

Climate change has profound effects on the distribution of kelp forests in the Arctic and sub-Arctic. However, studies on the responses of kelps to climate change, particularly along the sub-Arctic regions of the Alaska coast, are limited. Eualaria fistulosa is a foundational kelp species in the Aleutian Islands, with an east-west distribution that extends from Japan to southern southwest Alaska. In this study, we utilized a species distribution model (SDM) to explore changes in the future habitat suitability of E. fistulosa under contrasting Shared Socioeconomic Pathway (SSP) scenarios. Our model exhibited relatively high predictive performance, validating our SDM predictions. Notably, the SDM results indicate that minimum sea surface temperature, annual range in sea surface temperatures, and annual mean current velocities are the three most important predictor variables determining E. fistulosa's distribution. Furthermore, the projected geographic distribution of Eualaria is generally consistent with its observed occurrence records. However, under high emission scenarios (SSP5-8.5), E. fistulosa is predicted to contract its distribution range by 9.0% by 2100, with widespread disappearance along the southeast Alaskan coast and limited northward migration to Kamchatka Krai in Russia and Bristol Bay in Alaska. These findings contribute valuable insights for conservation strategies via addressing climate-induced alterations in sub-Arctic kelp distribution.

Bimodal spore release heights in the water column enhance local retention and population connectivity of bull kelp, Nereocystis luetkeana

Dispersal of reproductive propagules determines recruitment patterns and connectivity among populations and can influence how populations respond to major disturbance events. Dispersal distributions can depend on propagule release strategies. For instance, the bull kelp, Nereocystis luetkeana, can release propagules (spores) from two heights in the water column ("bimodal release"): at the water surface, directly from the reproductive tissues (sori) on the kelp's blades, and near the seafloor after the sori abscise and sink through the water column. N. luetkeana is a foundation species that occurs from central California to Alaska and is experiencing unprecedented levels of population declines near its southern range limit. We know little of the kelp's dispersal distributions, which could influence population recovery and restoration. Here, we quantify how bimodal spore release heights affect dispersal outcomes based on a numerical model specifically designed for N. luetkeana. The model incorporates oceanographic conditions typical of the species' coastal range and kelp biological traits. With bimodal release heights, 34% of spores are predicted to settle within 10 m of the parental alga and 60% are predicted to disperse beyond 100 m. As an annual species, bimodal release heights can facilitate the local regeneration of adults within a source kelp forest while also supporting connectivity among multiple forests within broader bull kelp metapopulations. To leverage this pattern of bimodal spore dispersal in bull kelp restoration management, directing resources toward strategically located focal populations that can seed other ones could amplify the scale of recovery.

Shortened food chain length in a fished versus unfished coral reef

Direct exploitation through fishing is driving dramatic declines of wildlife populations in ocean environments, particularly for predatory and large-bodied taxa. Despite wide recognition of this pattern and well-established consequences of such trophic downgrading on ecosystem function, there have been few empirical studies examining the effects of fishing on whole system trophic architecture. Understanding these kinds of structural impacts is especially important in coral reef ecosystems-often heavily fished and facing multiple stressors. Given the often high dietary flexibility and numerous functional redundancies in diverse ecosystems such as coral reefs, it is important to establish whether web architecture is strongly impacted by fishing pressure or whether it might be resilient, at least to moderate-intensity pressure. To examine this question, we used a combination of bulk and compound-specific stable isotope analyses measured across a range of predatory and low-trophic-level consumers between two coral reef ecosystems that differed with respect to fishing pressure but otherwise remained largely similar. We found that even in a high-diversity system with relatively modest fishing pressure, there were strong reductions in the trophic position (TP) of the three highest TP consumers examined in the fished system but no effects on the TP of lower-level consumers. We saw no evidence that this shortening of the affected food webs was being driven by changes in basal resource consumption, for example, through changes in the spatial location of foraging by consumers. Instead, this likely reflected internal changes in food web architecture, suggesting that even in diverse systems and with relatively modest pressure, human harvest causes significant compressions in food chain length. This observed shortening of these food webs may have many important emergent ecological consequences for the functioning of ecosystems impacted by fishing or hunting. Such important structural shifts may be widespread but unnoticed by traditional surveys. This insight may also be useful for applied ecosystem managers grappling with choices about the relative importance of protection for remote and pristine areas and the value of strict no-take areas to protect not just the raw constituents of systems affected by fishing and hunting but also the health and functionality of whole systems.

Season-specific impacts of climate change on canopy-forming seaweed communities

Understory assemblages associated with canopy-forming species such as trees, kelps, and rockweeds should respond strongly to climate stressors due to strong canopy-understory interactions. Climate change can directly and indirectly modify these assemblages, particularly during more stressful seasons and climate scenarios. However, fully understanding the seasonal impacts of different climate conditions on canopy-reliant assemblages is difficult due to a continued emphasis on studying single-species responses to a single future climate scenario during a single season. To examine these emergent effects, we used mesocosm experiments to expose seaweed assemblages associated with the canopy-forming golden rockweed, Silvetia compressa, to elevated temperature and pCO2 conditions reflecting two projected greenhouse emission scenarios (RCP 2.6 [low] & RCP 4.5 [moderate]). Assemblages were grown in the presence and absence of Silvetia, and in two seasons. Relative to ambient conditions, predicted climate scenarios generally suppressed Silvetia biomass and photosynthetic efficiency. However, these effects varied seasonally-both future scenarios reduced Silvetia biomass in summer, but only the moderate scenario did so in winter. These reductions shifted the assemblage, with more extreme shifts occurring in summer. Contrarily, future scenarios did not shift assemblages within Silvetia Absent treatments, suggesting that climate primarily affected assemblages indirectly through changes in Silvetia. Mesocosm experiments were coupled with a field Silvetia removal experiment to simulate the effects of climate-mediated Silvetia loss on natural assemblages. Consistent with the mesocosm experiment, Silvetia loss resulted in season-specific assemblage shifts, with weaker effects observed in winter. Together, our study supports the hypotheses that climate-mediated changes to canopy-forming species can indirectly affect the associated assemblage, and that these effects vary seasonally. Such seasonality is important to consider as it may provide periods of recovery when conditions are less stressful, especially if we can reduce the severity of future climate scenarios.

Habitat loss predicts the functional extinction of fish from Amazonian streams during the Anthropocene

The evaluation of extinction risk has typically focused on individual species, although a shift to a focus on ecosystem functioning would appear to be an urgent priority for conservation planning, especially considering that a sixth mass extinction event has already begun. In the present study, we investigated how fish extinction driven by habitat loss may modify the functioning of freshwater Amazonian ecosystems. We sampled the fish and environmental conditions of 63 streams in the eastern Amazon and simulated extinction based on the vulnerability of the species to habitat loss, which is the principal threat to tropical biodiversity. The simulated extinction of vulnerable species led to a decrease in both the mean body size of the community and functional rarity and culminated in abrupt losses of ecosystem functions after 5% and 10% of extinction at local and regional scales. Our functional approach demonstrated the progressive loss of ecological functions in Amazon streams, which may collapse altogether following the extinction of functions related to protection against biological invasions, and associated alterations in nutrient cycling and water quality. We provide robust predictions on the modification of the ecosystem following the extinction of fish species, which is a major step toward the development of effective conservation measures that ensure the avoidance of the predicted processes, and help to prevent the loss of biodiversity and the potentially irreversible modifications to ecosystem functioning.

In-Situ Estimates of Net Ecosystem Metabolisms in the Rocky Habitats of Dokdo Islets in the East Sea of Korea

We measured oxygen (O2) fluxes in two major shallow subtidal benthic habitats (kelp bed (KB) and bare rock (BR) covered with crustose coralline algae) of Dokdo islet in the East Sea by applying noninvasive in-situ aquatic eddy covariance (AEC). The AEC device allows time series measurements (~24 h) of three-dimensional velocity (u, v, and w components) and high-resolution dissolved O2. This allows estimation of O2 exchange flux via benthic habitats. Local flow rates and irradiance levels were found to be major factors controlling O2 exchange flux in the rocky habitats. Gross primary production rates tended to be significantly higher in KB (163 mmol O2 m–2 d–1) than in BR (51 mmol O2 m–2 d–1). The net ecosystem metabolisms were assessed as opposite types, with 8 mmol O2 m–2 d–1 in KB (autotrophy) and –12 mmol O2 m–2 d–1 in BR (heterotrophy). Our results indicate that kelp beds are important for organic carbon cycling in rocky coastal waters and that AEC application to macroalgae habitats is a useful assessment approach.

Rewilding the Sea? A Rapid, Low Cost Model for Valuing the Ecosystem Service Benefits of Kelp Forest Recovery Based on Existing Valuations and Benefit Transfers

Kelp forests and seagrasses are important carbon sinks that are declining globally. Rewilding the sea, through restoring these crucial habitats, their related biodiversity and ecosystem contributions, is a movement and concept, gathering pace in the United Kingdom and globally. Yet understanding of the economic costs and benefits for setting areas of the sea aside—and removing some human impacts from them—is not well understood. The potential benefits and distributional impacts on marine users and wider society is critical to make evidence based decisions. Ensuring that areas of the sea recover, and that the impacts (both positive and negative) are understood, requires targeted research to help guide decisions to optimize the opportunity of recovery, while minimizing any negative impacts on sea users and coastal communities. We approach the problem from an ecosystem services perspective, looking at the opportunity of restoring a kelp bed in Sussex by removing fishing activity from areas historically covered in kelp. Development of an ecosystem services valuation model showed restoring kelp to its highest mapped past extent (96% greater, recorded in 1987) would deliver a range of benefits valued at over £ 3.5 million GBP. The application of an ecosystem services approach enabled the full range of benefits from habitat restoration to be assessed. The results and the gaps identified in site specific data and values for this area, have broader implications in fisheries management and natural resource management tools for restoring marine habitats and ecosystems in the United Kingdom.

Leveraging the blue economy to transform marine forest restoration

The UN Decade of Ecosystem Restoration is a response to the urgent need to substantially accelerate and upscale ecological restoration to secure Earth's sustainable future. Globally, restoration commitments have focused overwhelmingly on terrestrial forests. In contrast, despite a strong value proposition, efforts to restore seaweed forests lag far behind other major ecosystems and continue to be dominated by small-scale, short-term academic experiments. However, seaweed forest restoration can match the scale of damage and threat if moved from academia into the hands of community groups, industry, and restoration practitioners. Connecting two rapidly growing sectors in the Blue Economy-seaweed cultivation and the restoration industry-can transform marine forest restoration into a commercial-scale enterprise that can make a significant contribution to global restoration efforts.

Differential impacts of alternate primary producers on carbon cycling

Disturbance impacts the spatial distribution of primary producers, which can have cascading effects on ecosystem function. The lower-intertidal zone on the rocky shores of the Pacific Northwest is one such place where wave energy creates a mosaic-like distribution between two assemblages: surfgrass (Phyllospadix scouleri) meadows and macroalgal forests dominated by kelp. We simulated wave disturbance by experimentally removing patches of surfgrass monocultures, resulting in a macroalgal assemblage with increased diversity, biomass, and net primary productivity in the following year. Although surfgrass had a higher C:N compared to macroalgal assemblages, macroalgal assemblages achieved a higher biomass, fixed carbon at a faster rate, and released more dissolved organic carbon (DOC) during photosynthesis. Thus, despite similar standing amounts of carbon, macroalgal assemblages have increased carbon turnover-from fixation to DOC release. Comparative photophysiology indicated that surfgrasses have a competitive advantage over other macrophytes at low light levels, allowing them to persist when disturbance is reduced. Unexpectedly, disturbance in this system increased the potential for carbon sequestration when surfgrass monocultures were replaced by diverse macroalgae.

Evaluating bloom potential of the green-tide forming alga Ulva ohnoi under ocean acidification and warming

The occurrence of green-tides, whose bloom potential may be increased by various human activities and biogeochemical process, results in enormous economic losses and ecosystem collapse. In this study, we investigated the ecophysiology of the subtropical green-tide forming alga, Ulva ohnoi complex (hereafter: U. ohnoi), under simulated future ocean conditions in order to predict its bloom potential using photosynthesis and growth measurements, and stable isotope analyses. Our mesocosm system included four experimental conditions that simulated the individual and combined effects of elevated CO₂ and temperature, namely control (450 μatm CO₂ & 20 °C), acidification (900 μatm CO₂ & 20 °C), warming (450 μatm CO₂ & 25 °C), and greenhouse (900 μatm CO₂ & 25 °C). Photosynthetic electron transport rates (rETR) increased significantly under acidification conditions, but net photosynthesis and growth were not affected. In contrast, rETR, net photosynthesis, and growth all decreased significantly under elevated temperature conditions (i.e. both warming and greenhouse). These results represent the imbalance of energy metabolism between electron transport and O₂ production that may be expected under ocean acidification conditions. This imbalance appears to be related to carbon and nitrogen assimilation by U. ohnoi. In particular, ¹³C and ¹⁵N discrimination data suggest U. ohnoi prefers CO₂ and NH₄⁺ over HCO₃^- and NO₃^- as sources of carbon and nitrogen, respectively, and this results in increased N content in the thallus under ocean acidification conditions. Together, our results suggest a trade-off in which the bloom potential of U. ohnoi could increase under ocean acidification due to greater N accumulation and through the saving of energy during carbon and nitrogen metabolism, but that elevated temperatures could decrease U. ohnoi's bloom potential through a decrease in photosynthesis and growth.

Variation in Photosynthetic Performance Relative to Thallus Microhabitat Heterogeneity in Lithothamnion australe (Rhodophyta, Corallinales) Rhodoliths

Rhodoliths are free-living, coralline algae that create heterogeneous structure over sedimentary habitats. These fragile ecosystems are threatened by anthropogenic disturbances that reduce their size and three-dimensional structural complexity. We investigated how physical disturbance from boat moorings affects photosynthetic performance in the rhodolith Lithothamnion australe. Photosynthetic parameters were measured for intact rhodoliths and crushed rhodolith fragments of two sizes (ca. 1 and 2 cm diameter), while chlorophyll fluorescence was measured at the surface of rhodoliths of these two sizes, between the interior branches of the larger rhodoliths, and at the surface of 52 various sized (0.4-3.5 cm diameter) rhodoliths. Gross productivity and net productivity were 15% and 36% higher, respectively, in the smaller L. australe, while respiration was 10% higher in the larger individuals. Thallus crushing reduced gross productivity by 20% and 41%, and net productivity by 9% and 14% in the smaller and larger rhodoliths, respectively. It also reduced respiration by 33% and 60% in the smaller and larger rhodoliths, respectively. Fluorescence parameters were all greater at the surface of the larger L. australe than the smaller individuals, and greater at the surface than in the interior parts of the larger individuals. Across a range of rhodolith sizes, surface fluorescence parameters were at their maxima in 1.54 to 2.32 cm diameter individuals. These results show that L. australe's complex structure creates heterogeneity in photosynthesis and respiration between their surface and interior parts and among rhodolith sizes. This information can help predict how rhodoliths may respond to disturbance and environmental stressors.

Imprint of Climate Change on Pan-Arctic Marine Vegetation

The Arctic climate is changing rapidly. The warming and resultant longer open water periods suggest a potential for expansion of marine vegetation along the vast Arctic coastline. We compiled and reviewed the scattered time series on Arctic marine vegetation and explored trends for macroalgae and eelgrass (Zostera marina). We identified a total of 38 sites, distributed between Arctic coastal regions in Alaska, Canada, Greenland, Iceland, Norway/Svalbard, and Russia, having time series extending into the 21st Century. The majority of these exhibited increase in abundance, productivity or species richness, and/or expansion of geographical distribution limits, several time series showed no significant trend. Only four time series displayed a negative trend, largely due to urchin grazing or increased turbidity. Overall, the observations support with medium confidence (i.e., 5–8 in 10 chance of being correct, adopting the IPCC confidence scale) the prediction that macrophytes are expanding in the Arctic. Species distribution modeling was challenged by limited observations and lack of information on substrate, but suggested a current (2000–2017) potential pan-Arctic brown macroalgal distribution area of 655,111 km2(140,433 km2intertidal, 514,679 km2subtidal), representing an increase of about 45% for subtidal- and 8% for intertidal macroalgae since 1940–1950, and associated polar migration rates averaging 18–23 km decade–1. Adjusting the potential macroalgal distribution area by the fraction of shores represented by cliffs halves the estimate (340,658 km2). Warming and reduced sea ice cover along the Arctic coastlines are expected to stimulate further expansion of marine vegetation from boreal latitudes. The changes likely affect the functioning of coastal Arctic ecosystems because of the vegetation’s roles as habitat, and for carbon and nutrient cycling and storage. We encourage a pan-Arctic science- and management agenda to incorporate marine vegetation into a coherent understanding of Arctic changes by quantifying distribution and status beyond the scattered studies now available to develop sustainable management strategies for these important ecosystems.

Geographic variation in responses of kelp forest communities of the California Current to recent climatic changes

The changing global climate is having profound effects on coastal marine ecosystems around the world. Structure, functioning, and resilience, however, can vary geographically, depending on species composition, local oceanographic forcing, and other pressures from human activities and use. Understanding ecological responses to environmental change and predicting changes in the structure and functioning of whole ecosystems require large-scale, long-term studies, yet most studies trade spatial extent for temporal duration. We address this shortfall by integrating multiple long-term kelp forest monitoring datasets to evaluate biogeographic patterns and rates of change of key functional groups (FG) along the west coast of North America. Analysis of data from 469 sites spanning Alaska, USA, to Baja California, Mexico, and 373 species (assigned to 18 FG) reveals regional variation in responses to both long-term (2006-2016) change and a recent marine heatwave (2014-2016) associated with two atmospheric and oceanographic anomalies, the "Blob" and extreme El Niño Southern Oscillation (ENSO). Canopy-forming kelps appeared most sensitive to warming throughout their range. Other FGs varied in their responses among trophic levels, ecoregions, and in their sensitivity to heatwaves. Changes in community structure were most evident within the southern and northern California ecoregions, while communities in the center of the range were more resilient. We report a poleward shift in abundance of some key FGs. These results reveal major, ongoing region-wide changes in productive coastal marine ecosystems in response to large-scale climate variability, and the potential loss of foundation species. In particular, our results suggest that coastal communities that are dependent on kelp forests will be more impacted in the southern portion of the California Current region, highlighting the urgency of implementing adaptive strategies to sustain livelihoods and ensure food security. The results also highlight the value of multiregional integration and coordination of monitoring programs for improving our understanding of marine ecosystems, with the goal of informing policy and resource management in the future.

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.

Global warming offsets the ecophysiological stress of ocean acidification on temperate crustose coralline algae

Dramatic increases in the release of anthropogenic CO₂ and global temperatures have resulted in alterations to seawater carbonate chemistry and metabolisms of marine organisms. There has been recent interest in the effects of these stressors on crustose coralline algae (CCA) because photosynthesis and calcification are influenced by all components of carbonate chemistry. To examine this, a mesocosm experiment was conducted to evaluate photosynthesis, calcification and growth in the temperate CCA Chamberlainium sp. under acidification (doubled CO₂), warming (+5 °C), and greenhouse (doubled CO₂ and +5 °C) conditions compared to present-day conditions. After 47 days of acclimation to these conditions, productivity was lowest under acidification, although photochemical properties were improved, while respiration was highest under warming. Likewise, growth was lowest under acidification, but this negative response was offset by elevated temperature under greenhouse. Together, these results suggest that warming offsets the negative effects of acidification by creating more suitable conditions for photosynthesis and growth.