Kelp Loss Research Articles

Sea urchin roe quality within urchin barrens and improvement through kelp restoration

AbstractSea urchin overgrazing is a major cause of kelp loss and consequently sea urchin removal is frequently promoted as a tool for restoring kelp in urchin barrens. While many sea urchin species support high‐end roe fisheries, roe condition is generally lower in barrens than in kelp forests due to reduced food availability. Understanding how roe condition varies between kelp and barren habitats, and in response to kelp restoration, is key to developing effective, culturally and socially acceptable approaches to sea urchin removal and for assessing the potential benefits of kelp restoration. We assessed whether sea urchin roe quality within urchin barrens in northeastern Aotearoa New Zealand can be sufficient for harvest, and whether sea urchin removal can ultimately enhance urchin roe condition (by increasing food quality and quantity). We monitored roe condition (roe weight and gonadosomatic index [GSI]) in kelp forest and urchin barrens monthly for one year at two sites, and quarterly over two years at four sites following large‐scale (1.6–2 ha) sea urchin removal from urchin barrens. While roe condition was generally lower in barrens, the magnitude of this difference varied seasonally and among sites. Sea urchin roe quality within barrens was sufficient for commercial harvest (GSI ≥ 6%) during spring–summer at three sites. Following large‐scale sea urchin removal, kelp and fucoid density increased over two years (~9–58 plants m−2), but remained low in barren control areas. Although variable, sea urchins from removal areas (restored kelp) typically showed increased GSI, roe weight, and improved roe color within nine months following removal. These results suggest that, at some locations, harvest may provide a viable method for removing urchins from urchin barrens for kelp restoration purposes. However, to ensure sea urchin densities are reduced and maintained below the critical levels needed to achieve restoration outcomes, strategic approaches with critical evaluation of methods are required. Furthermore, we demonstrate that effectively removing urchins and promoting kelp recovery can ultimately improve urchin roe quality.

Coastal ecological disasters triggered by an extreme rainfall event thousands of kilometers inland

The world is experiencing an increase in the frequency and intensity of extreme weather events, yet the influences of remote inland extreme weather events on the coastal ecosystem thousands of kilometers away remain poorly understood. Here we tracked the chain ecological effects of an extreme rainfall event in North China from terrestrial rivers to coastal aquaculture area of the eastern Shandong Peninsula. Our data suggest the autumn flood resulted from extreme rainfall event leads to abnormally low turbidity in the North Shandong Coastal Currents and coastal red tide blooms by introducing anomalous freshwater with an exceptionally high nitrogen-to-phosphorus ratio into the Bohai Sea. Lower salinity, stronger light conditions caused by limpid coastal currents, and phosphorus limitation resulting from red tide blooms account for huge kelp loss offshore of the eastern Shandong Peninsula. This study underscores the importance of considering multidisciplinary observation for risk management of unexpected extreme weather events.

Local and regional variation in kelp loss and stability across coastal British Columbia

Kelp forests are among the most abundant coastal marine habitats but are vulnerable to climate change. The Northeast Pacific has experienced recent large-scale changes in kelp abundance and distribution, but little is known about changes north of the British Columbia (BC)-Washington border. Here, we assessed whether and how floating canopy kelp (Macrocystis pyrifera, Nereocystis luetkeana) distributions have changed in recent decades along the extensive coast of BC. We assembled and analysed available kelp distributional data, comparing snapshots of kelp linear extent from 1.5-3 decades ago (1994-2007) to recently collected data (2017-2021) across 11 different subregions spanning the province. We then leveraged timeseries, where available (n = 7 data sets), to contextualise patterns of change. In aggregate, the data suggest that kelp forests have declined considerably in some parts of the province, but with variable patterns of change across BC. In the warmest areas (southern BC), kelp persistence was negatively correlated with mean summer sea surface temperatures, which at times exceeded known thermal tolerances. In contrast, in northern subregions, top-down control by sea urchins and otters appeared to modulate kelp dynamics, with declines occurring in 2 subregions despite cool ocean temperatures. Timeseries data suggest that many declines occurred around the 2014-2016 marine heatwave, an event associated with sustained warming and altered trophic dynamics. Our results suggest that the extent of BC’s kelp forests has declined in some places in recent decades, but that regional and local-scale factors influence their responses to environmental change.

Marine protected areas can be useful but are not a silver bullet for kelp conservation.

Kelp forests are among the most valuable ecosystems on Earth, but they are increasingly being degraded and lost due to a range of human-related stressors, leading to recent calls for their improved management and conservation. One of the primary tools to conserve marine species and biodiversity is the establishment of marine protected areas (MPAs). International commitments to protect 30% of the world's ecosystems are gaining momentum, offering a promising avenue to secure kelp forests into the Anthropocene. However, a clear understanding of the efficacy of MPAs for conserving kelp forests in a changing ocean is lacking. In this perspective, we question whether strengthened global protection will create meaningful conservation outcomes for kelp forests. We explore the benefits of MPAs for kelp conservation under a suite of different stressors, focusing on empirical evidence from protected kelp forests. We show that MPAs can be effective against some drivers of kelp loss (e.g., overgrazing, kelp harvesting), particularly when they are maintained in the long-term and enforced as no-take areas. There is also some evidence that MPAs can reduce impacts of climate change through building resilience in multi-stressor situations. However, MPAs also often fail to provide protection against ocean warming, marine heatwaves, coastal darkening, and pollution, which have emerged as dominant drivers of kelp forest loss globally. Although well-enforced MPAs should remain an important tool to protect kelp forests, successful kelp conservation will require implementing an additional suite of management solutions that target these accelerating threats.

Effects of temperature and nutrients on microscopic stages of the bull kelp (Nereocystis luetkeana, Phaeophyceae).

Warming ocean temperatures have been linked to kelp forest declines worldwide, and elevated temperatures can act synergistically with other local stressors to exacerbate kelp loss. The bull kelp Nereocystis luetkeana is the primary canopy-forming kelp species in the Salish Sea, where it is declining in areas with elevated summer water temperatures and low nutrient concentrations. To determine the interactive effects of these two stressors on microscopic stages of N. luetkeana, we cultured gametophytes and microscopic sporophytes from seven different Salish Sea populations across seven different temperatures (10-22°C) and two nitrogen concentrations. The thermal tolerance of microscopic gametophytes and sporophytes was similar across populations, and high temperatures were more stressful than low nitrogen levels. Additional nitrogen did not improve gametophyte or sporophyte survival at high temperatures. Gametophyte densities were highest between 10 and 16°C and declined sharply at 18°C, and temperatures of 20 and 22°C were lethal. The window for successful sporophyte production was narrower, peaking at 10-14°C. Across all populations, the warmest temperature at which sporophytes were produced was 16 or 18°C, but sporophyte densities were 78% lower at 16°C and 95% lower at 18°C compared to cooler temperatures. In the field, bottom temperatures revealed that the thermal limits of gametophyte growth (18°C) and sporophyte production (16-18°C) were reached during the summer at multiple sites. Prolonged exposure of bull kelp gametophytes to temperatures of 16°C and above could limit reproduction, and therefore recruitment, of adult kelp sporophytes.

Mapping warming reefs—An application of multibeam acoustic water column analysis to define threatened abalone habitat

Robust definition of the spatial extent of seafloor habitats and how they may be changing through time is a holy grail for ecosystem management, particularly if an ecosystem is approaching a tipping point beyond which irreversible changes may occur. Here we generate and explore a new data set for the management of warming reefs in eastern Tasmania, Australia that will significantly improve the baseline maps required for fine-scaled spatial modelling and management that is, both robust at regional scales and is highly resolved within the water column. This procedure enabled the relative density of kelp vegetation to be identified in a region that is being overwhelmed by the range extension of a destructive grazer, the Longspined Sea Urchin, Centrostephanus rodgersii. We present a new online tool to visualize multibeam water column acoustic data as surfaces of kelp density at high resolution (50 cm) scale over seafloor terrain maps (spanning a total straight-line distance of 594 km and a total area of 29.14 km2) to reveal the types of reef structure on the East Coast of Tasmania where abalone habitat is threatened by kelp loss.

Diverse stability responses of a sub-Antarctic invertebrate understorey community to experimental giant kelp removal

Ecological stability is crucial for understanding anthropogenic biodiversity loss and its consequences, especially when disturbances affect species that influence numerous others. The giant kelp Macrocystis pyrifera, an iconic foundation species, supports diverse communities on temperate and subpolar coasts. While giant kelp communities in temperate regions have been extensively studied, field-based manipulative research in subpolar latitudes is still limited. We investigated the resistance, resilience, and recovery of a community subjected to experimental giant kelp removal in a sub-Antarctic ecosystem. We simulated pulsed kelp loss caused by destructive storms and monitored the macrobenthic invertebrate understorey for 12 mo. The experimental disturbance elicited varying stability responses of the understorey. Understorey community biomass was strongly resistant to the disturbance but declined gradually (i.e. low resilience), showing incomplete recovery compared to undisturbed communities. Community density (total number of individuals per sample) increased notably following the removal (i.e. weak resistance) but ultimately returned to undisturbed levels (i.e. high resilience and complete recovery). Species composition exhibited low resistance and resilience, and no recovery from the disturbance. Secondary coextinctions of several understorey species accounted for the observed low compositional stability. In contrast to giant kelp communities in temperate latitudes, the slower recovery rates of kelp and the loss of key positive interactions likely contributed to the observed stability responses in this subpolar ecosystem’s invertebrate understorey. By examining the response of a sub-Antarctic understorey community to giant kelp removal, our study enhances our understanding of how foundation species sustain local biodiversity.

Using unoccupied aerial vehicles to map and monitor changes in emergent kelp canopy after an ecological regime shift

AbstractKelp forests are complex underwater habitats that form the foundation of many nearshore marine environments and provide valuable services for coastal communities. Despite their ecological and economic importance, increasingly severe stressors have resulted in declines in kelp abundance in many regions over the past few decades, including the North Coast of California, USA. Given the significant and sustained loss of kelp in this region, management intervention is likely a necessary tool to reset the ecosystem and geospatial data on kelp dynamics are needed to strategically implement restoration projects. Because canopy‐forming kelp forests are distinguishable in aerial imagery, remote sensing is an important tool for documenting changes in canopy area and abundance to meet these data needs. We used small unoccupied aerial vehicles (UAVs) to survey emergent kelp canopy in priority sites along the North Coast in 2019 and 2020 to fill a key data gap for kelp restoration practitioners working at local scales. With over 4,300 hectares surveyed between 2019 and 2020, these surveys represent the two largest marine resource‐focused UAV surveys conducted in California to our knowledge. We present remote sensing methods using UAVs and a repeatable workflow for conducting consistent surveys, creating orthomosaics, georeferencing data, classifying emergent kelp and creating kelp canopy maps that can be used to assess trends in kelp canopy dynamics over space and time. We illustrate the impacts of spatial resolution on emergent kelp canopy classification between different sensors to help practitioners decide which data stream to select when asking restoration and management questions at varying spatial scales. Our results suggest that high spatial resolution data of emergent kelp canopy from UAVs have the potential to advance strategic kelp restoration and adaptive management.

Death of A Star


 Sea star wasting disease (SSWD), originally identified in 2013, is an epidemic impacting 20 species of sea stars along the west coast of North America. Two of the most impacted species, Pisaster ochraceus and Pycnopodia helianthoides are keystone species. The loss of these sea stars impacts oceanic life. Literature reviews were conducted from January to May 2022 to investigate the effect of SSWD on the food web of sea stars and decline of kelp. The apex predators of the ecosystem, P. helianthoides and P. ochraceus, have decreased drastically, while their prey, urchins, have increased immensely. As a result, kelp, a producer, has decreased significantly. The decline of the keystone species is highly concerning and could have drastic ecological impacts. The loss of kelp is extremely damaging for both marine life and Earth itself. If sea stars continue to decline, the amount of carbon dioxide and loss of kelp will become increasingly problematic for the ocean and health of the planet.

Cascading impacts of a climate-driven ecosystem transition intensifies population vulnerabilities and fishery collapse

The recent large-scale intensification of marine heatwaves, and other climate-related stressors, has dramatically impacted biogenic habitats around the globe, including marine ecosystems such as coral reefs, seagrasses, and kelp forests. While the impacts to foundation species may be of particular concern, these ecological catastrophes underscore the need to examine how whole systems respond to a suite of stressors. The recent climate-driven collapse of the bull kelp forest and recreational red abalone fishery in northern California provides an example of unanticipated ripple and lagged effects in the system, intensifying vulnerabilities and accelerating population and fishery collapse. For this case study, we examined 15 years (2003–2018) of biological survey data on the bull kelp forest ecosystem—before, during, and after an extreme climate event. We document the interactions and complexity of impacts over time, as well as the resulting increased vulnerability of red abalone to additional anthropogenic, biological, and environmental stressors. We observed progressively stronger population-level responses of the red abalone to the marine heatwave and the regional loss of kelp, driving the movement of adults and juveniles in search of food. As food remained scarce, we documented the loss of productivity with diminished gonad and body condition, the absence of larval or newly-settled abalone, mass mortalities, and shoreward shifts in depth distributions. With 40% of the population dead or dying, juvenile and trophy-sized abalone abandoning cryptic habitats, the shift in the distribution to shallower depths increased the vulnerability of red abalone to the fishery. Other anthropogenic, biological, and climate-related stressors that disproportionately impact shallow habitats are now a growing concern for the survivors. For red abalone, previously unanticipated cascading risks include increased wave energy, warming air temperatures, freshwater flooding, landslides, as well as possible oil spills and harmful algal blooms. Climate-driven changes in vulnerability to fishing and environmental stressors present significant challenges for sustainable natural resource management in dynamic stressed systems, and underscore the need for continued system-focused monitoring. We present a conceptual framework supporting similar ecosystem investigations of recent and future climate impacts to inform adaptive ecosystem-based management strategies.

Microclimate predicts kelp forest extinction in the face of direct and indirect marine heatwave effects.

Marine heatwaves threaten the persistence of kelp forests globally. However, the observed responses of kelp forests to these events have been highly variable on local scales. Here, we synthesize distribution data from an environmentally diverse region to examine spatial patterns of canopy kelp persistence through an unprecedented marine heatwave. We show that, although often overlooked, temperature variation occurring at fine spatial scales (i.e., a few kilometers or less) can be a critical driver of kelp forest persistence during these events. Specifically, though kelp forests nearly all persisted toward the cool outer coast, inshore areas were >3°C warmer at the surface and experienced extensive kelp loss. Although temperatures remained cool at depths below the thermocline, kelp persistence in these thermal refugia was strongly constrained by biotic interactions, specifically urchin populations that increased during the heatwave and drove transitions to urchin barrens in deeper rocky habitat. Urchins were, however, largely absent from mixed sand and cobble benthos, leading to an unexpected association between bottom substrate and kelp forest persistence at inshore sites with warm surface waters. Our findings demonstrate both that warm microclimates increase the risk of habitat loss during marine heatwaves and that biotic interactions modified by these events will modulate the capacity of cool microclimates to serve as thermal refugia.

Shield wall: Kelps are the last stand against corals in tropicalized reefs

Abstract Communities inhabiting biogeographic transition zones are shifting in composition as a result of progressive warming and heatwaves. In the marine environment, corals are expanding onto higher latitude reefs historically dominated by temperate kelp forests, initiating a shift towards warm affinity coral‐dominated states. Although these coral expansions are a global phenomenon, the mechanisms that are underpinning the expansion process remain poorly understood, which limits the projections of the rate and extent of ecosystem reconfiguration. Here, we investigated the interaction between the kelp Ecklonia radiata and the high latitude scleractinian coral Plesiastrea versipora in several of Western Australia's temperate reefs, where coral colony abundance has increased by 50% in recent years. Combining field surveys with field and laboratory experiments, we test the importance of physical (abrasion and light reduction) and chemical (allelopathic) effects of kelp canopies on coral tissue cover, photosynthetic parameters and calcification rates. In the field, kelp cover had a negative effect on coral density that was overwhelming in comparison to other dominant macroalgal taxa. Abrasion by kelp whiplash was the predominant mechanism by which kelp exerted a negative effect on P. versipora fitness, scraping up to 80% of live coenosarc from experimental colonies. In contrast, canopies had no effects on P. versipora photochemical efficiency and laboratory incubations showed that there were no allelochemical effects from kelp on P. versipora. We conclude that E. radiata inhibits P. versipora establishment and development through abrasion, and the survey data confirmed that recent climate‐driven kelp loss released corals from this effect, facilitating their expansion on high‐latitude reefs in Western Australia. This shows how competitive interactions actively shield against species expansion in biogeographic transition zones and suggests a continued decline of kelp canopies will increase the permeability of temperate reefs to warm affinity species such as scleractinian corals. Read the free Plain Language Summary for this article on the Journal blog.

Extreme flooding and reduced salinity causes mass mortality of nearshore kelp forests

Extreme flooding due to climate-induced increases in storm frequency and intensity may negatively impact marine species and habitats. These impacts may be particularly severe in nearshore marine systems where freshwater influx and reduced salinity will be most apparent. In March 2021, severe storms associated with an intense La Niña occurred along 450 km of the east Australian coast causing widespread flooding and freshwater incursion into estuaries and nearshore marine systems. These floods were linked to widespread kelp mortality (Ecklonia radiata) in nearshore systems, particularly near and within estuaries. We quantified levels of kelp mortality with distance from the entrance and depth in one of the most impacted estuarine embayments, Port Stephens. Experiments were conducted to confirm that the flood-induced reduction in salinity was the primary cause of the observed mass mortality of kelp sporophytes and to determine the likelihood of recovery from surviving gametophytes. Floods caused a ∼50% net loss of kelp within the embayment, with almost 100% mortality at shallow depths (<4 m) and increased loss with increased distance from the embayment mouth. Experiments verified that reduced salinity, at levels comparable to those during the floods, caused severe reductions in kelp health, photosynthetic yield and tissue strength. Contrastingly, experiments on gametophytes revealed that they were resilient to short-term reduced salinity. This likely facilitated the rapid recovery of kelp that was observed seven months after the floods. Overall, we identified that future increases in climate-induced severe weather and storms events are likely to increasingly impact kelp and the ecosystems they underpin in nearshore systems.

Distinct realized physiologies in green sea urchin (Strongylocentrotus droebachiensis) populations from barren and kelp habitats

Overgrazing of habitat-forming kelps by sea urchins is reshaping reef seascapes in many temperate regions. Loss of kelp, in particular as a food source, may alter individual consumer physiology, which in turn may impair their ability to respond to climate warming. Here, we measured the temperature dependence of absolute and mass-independent oxygen consumption ([Formula: see text]) using two different exposure protocols (acute exposure and temperature “ramping”), as proxies of realized physiology, between green sea urchin ( Strongylocentrotus droebachiensis) populations from neighbouring barren and kelp habitats. Sea urchins from kelp habitats consumed 8%–78% more oxygen than sea urchins from barrens (across the range of temperatures tested (4–32 °C)) and had higher maximum [Formula: see text] values (by 26%). This was in part because kelp urchins typically had greater body masses. However, higher mass-independent [Formula: see text] values of kelp urchins suggest metabolic plasticity in response to habitat per se. In addition, the [Formula: see text] of sea urchins from kelp habitats was less sensitive to increases in temperature. We conclude that sea urchins from barren and kelp habitats of comparable body mass represent different energetic units. This highlights that habitat type can drive population-level variation that may shape urchins activities and environmental impact. Such variation should be integrated into energy-based models.

Ecological interactions mediate projected loss of kelp biomass under climate change

AbstractAimClimate change has precipitated loss of kelp in temperate regions globally, with further losses projected in response to ocean warming. Projected climate‐driven changes to kelp are commonly derived from responses to sea surface temperature and neglect the effects of important ecological interactions, such as herbivory. Here, we use a stacked species distribution model to project impacts of future climate scenarios on kelp distribution and biomass incorporating the effects of key ecological interactions.LocationThe eastern Australian coastline (28.0–37.5°S).MethodsA stacked species distribution model, incorporating ecological interactions with urchins, was used to project future effects of climate change on kelp. This model incorporated key drivers of kelp occurrence and cover (habitat availability, light and in situ temperatures) that are rarely considered when projecting the effects of climate change on kelp.ResultsConsistent with previous research, we find range contractions of kelp (~275 km) at lower latitudes (&lt;32°S), due to ocean warming under RCP 8.5 (~3.4°C) by 2100. However, in contrast to previous studies, we find range contractions will not occur under RCP 2.6 (~0.8°C), and only moderate contractions are projected (~55 km) under RCP 6.0 (~2.3°C). Instead, vertical migration of kelp, from shallow areas to cooler deep water refugia, is likely under future change. Interestingly, range contractions at lower latitudes are projected to be offset by increased kelp biomass at higher latitudes (32–37.5°S) resulting from reductions in urchin herbivory. Consequently, a net gain in kelp biomass is projected for mainland eastern Australia by 2100 under RCPs 2.6 and 6.0 (up to 29.4%), while relatively small net losses are projected under RCP 8.5 (−3.5%).Main conclusionsConsidering key ecological interactions when developing species distribution models can significantly change projections of habitat loss and must be integrated into assessments to accurately evaluate future change and enable evidence‐based management.

Loss of Giant Kelp, Macrocystis pyrifera, Driven by Marine Heatwaves and Exacerbated by Poor Water Clarity in New Zealand

Marine heatwaves (MHW) are becoming stronger and more frequent across the globe. MHWs affect the thermal physiology of all biological organisms, but wider ecosystem effects are particularly impactful when large habitat-forming foundation species such as kelps are affected. Many studies on impacts from MHWs on kelps have focused on temperature effects in isolation, except for a few studies that have integrated co-occurring stress from grazers, wave exposure and nutrient limitation. It is likely that many stressors act in concert with MHWs and exacerbate their effects. Here we analyzed satellite images over 60 months to assess temporal changes in abundance of surface canopies of the giant kelp Macrocystis pyrifera in the New Zealand coastal zone. The analysis encompassed the most extreme MHW on record (2017/18), across a 6° latitudinal gradient of four regions southward from the northern distributional limit of Macrocystis along mainland New Zealand. We tested the association of surface canopy cover of Macrocystis with sea surface temperature, temperature anomalies, chlorophyll-a (a proxy for nutrient availability) and water clarity (diffuse attenuation coefficient). We found a reduced cover of Macrocystis across all regions during and after the 2017/18 MHW, with least impact at the most southern region where the maximum temperatures did not exceed 18°C. There was also an important and significant interaction between temperature and water clarity, showing that temperature-induced kelp loss was greater when water clarity was poor. These results show that notable negative effects occurred across the coastal range of this foundation species and highlight the importance of studying MHW effects across latitudinal gradients and in concert with other co-occurring stressors.

Trophic ecology surrounding kelp and wood falls in deep Norwegian fjords

Food availability in fjords is unusual among deep-sea environments due to the increased availability of nutritional sources that are comparatively rare in the open sea, such as influxes of macrophytodetritus and terrestrial organic matter. In open waters, these deep-sea ‘organic falls’ have been shown to increase beta diversity in comparison to surrounding, unenriched sediments, and support a range of specialised fauna. There has been unprecedented loss of coastal kelp forests in Norway and this is likely to greatly reduce the transfer of an important carbon subsidy to deep-sea communities in Norwegian fjords. In contrast, progress in terrestrial land management and reforestation has resulted in a large increase in the area covered by boreal forests. With such an expansion in forests fringing deep Norwegian fjords, an increase in the transport of wood material and forest detritus to these deep-sea habitats seems inevitable. Benthic landers containing experimental substrates (wood blocks and kelp parcels) were deployed for 10 months at a depth of 530 m in Osterfjorden, and stable isotope analysis (13C, 15N) was used to investigate and compare trophic relationships between wood and kelp substrates and the macrofauna that colonised them. Trophic shift analyses showed a dependence on kelp either as a direct dietary source or a primary source of carbon in most of the fauna analysed, whereas there was little evidence of wood providing an importance dietary subsidy. Modelling analyses showed that kelp detritus comprised a large percentage of the diet of all of the taxa collected from the kelp samples, and several from the wood samples. Ontogenetic trophic changes (i.e., differences in δ13C or δ15N depending on animal size) were seen in several of the taxa analysed, revealing changes in dietary preference and increasing trophic level as a function of size/age. This study provides evidence of the importance of kelp in the trophic ecology of communities living at the Norwegian deep-sea floor. The continued loss of kelp from shallow, coastal systems may have profound effects for these communities as they face a reduction in this important subsidy.

Tropicalization and kelp loss shift trophic composition and lead to more winners than losers in fish communities.

Climate-mediated species redistributions are causing novel interactions and leading to profound regime shifts globally. For species that expand their distribution in response to warming, survival depends not only on their physiological capacity, but also on the ability to coexist or be competitive within the established community. In temperate marine reefs from around the world, the range expansion of tropical species, known as 'tropicalization', has been linked to the disappearance of temperate habitat-forming kelps and shifts to dominance by low-biomass turfing algae. The consequences of these range expansions and habitat changes on resident fish communities are, however, unclear. Here, we use data derived from baited remote underwater video (BRUV) surveys to analyse changes in diversity and abundance of marine fishes over a 17-year period in warming reefs that have experienced kelp loss (occurring c. 2009). Despite the loss of kelp, we found that species richness and overall abundance of fishes (measured as probability of occurrence and relative abundance), including both tropical and temperate species, increased through time. We also found dramatic shifts in the trophic composition of fish assemblages. Tropical herbivorous fish increased most markedly through time, and temperate-associated planktivores were the only group that declined, a potential consequence of tropicalization not previously identified. At the species level, we identified 22 tropical and temperate species from four trophic guilds that significantly increased in occurrence, while only three species (all temperate associated) declined. Morphological trait space models suggest increases in fish diversity and overall occurrence are unlikely to be driven by uniqueness of traits among tropical range expanders. Our results show more winners than losers and suggest that pathways of energy flow will change in tropicalized systems, as planktonic inputs become less important and a higher proportion of algal productivity gets consumed locally by increasingly abundant herbivores.

Marine heatwaves and the collapse of marginal North Atlantic kelp forests

Extreme climatic events including marine heatwaves (MHWs) are becoming more frequent and severe in the Anthropocene. However, our understanding of how these events affect population dynamics of ecologically important species is limited, in part because extreme events are rare and difficult to predict. Here, we quantified the occurrence and severity of MHWs over 60 years in warm range edge kelp forests on both sides of the North Atlantic. The cumulative annual intensity of MHWs increased two- to four-fold during this period, coinciding with the disappearance of kelps. We experimentally demonstrated a relationship between strong and severe 2018 heatwaves and high kelp mortality in both regions. Patterns of kelp mortality were strongly linked to maximum temperature anomalies, which crossed lethal thresholds in both regions. Translocation and tagging experiments revealed similar kelp mortality rates on reefs dominated by healthy kelp forests and degraded sediment-laden algal ‘turfs’, indicating equal vulnerability to extreme events. These results suggest a mechanistic link between MHWs and broad-scale kelp loss, and highlight how warming can make ecosystem boundaries unstable, forcing shifts to undesirable ecosystem states under episodically extreme climatic conditions.

Combined mechanistic modelling predicts changes in species distribution and increased co‐occurrence of a tropical urchin herbivore and a habitat‐forming temperate kelp

AbstractAimIdentify climate change impacts on spawning and settlement of a tropical herbivore (Tripneustes), optimal habitat of a temperate kelp (Ecklonia) and implications for these species regions of interaction under future climate.LocationAlong eastern Australia and into the Tasman Sea including Lord Howe Island (LHI).Time periodA contemporary scenario (2006–2015) and future “business as usual” RCP 8.5 climate change scenario (2090–2100).Major taxa studiedThe tropical sea urchin, Tripneustes gratilla, and the temperate kelp, Ecklonia radiata.MethodsWe combined mechanistic models to create a predictive map of Ecklonia and Tripneustes distributions, and their future potential to co‐occur. We use 3D velocity and temperature fields produced with a state‐of‐the‐art configuration of the Ocean Forecasting Model version 3 that simulates the contemporary oceanic environment and projects it under an RCP 8.5 climate change scenario. We map the contemporary and future Ecklonia's realized and fundamental thermal niche; and simulate Tripneustes larval dispersal under both climate scenarios.ResultsBased on the thermal affinity of kelp and increases in projected temperatures, we predict a poleward range contraction of ~530 km by 2100 for kelp on Australia's east coast. Climate‐driven changes in dispersal of Tripneustes lead to its range expansion into Bass Strait and poleward, ~340–650 km further into Ecklonia's habitat range inducing new areas of co‐occurrence in the future. We find warming decreases spawning and settlement of Tripneustes in the tropics by 43%, and causes significant connectivity changes for LHI with future reliance on self‐recruitment.Major conclusionsWe predict novel regions of co‐occurrence between a temperate Ecklonia and tropical Tripneustes species which may lead to greater loss of kelp. Our results provide a new modelling approach for predicting species range shifts that is transferable to other marine ecosystems; it considers species response to abiotic change, predicts spatial spread and anticipates new regions for species interactions.