Kelp Forests Research Articles

Invisible Kelp Forest

Invisible Kelp Forest: From Smell to Sound is a speculative fiction and an 8-channel sonic composition that explores the possibility that sound is an ideal medium for translating senses of smell, or chemosensation underwater. It invites the listener to smell the kelp forest with their ears, a mode of synaesthesia. We explore the ways that sound and smell can both convey intensity, distance, dispersion, texture and elements of memory that may be specific to particular organisms. We imagine that the olfactory memory of the kelp forest is multiple, linked to what is meaningful for different marine fauna. In a literary narrative that explores the sensations of four different marine organisms, we develop sonic impressions of their chemosensory experiences of the kelp forest in a scientifically-informed manner. Invisible Kelp Forest plays with invisibility in several ways: by denying the listener any visual cues, they must use their imagination to conjure a spatial sense of the kelp forest on their own. We invite listeners to pay attention to the way that listening for smell feels in the body, perhaps deterritorialising the sensorium.

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.

Kelp-associated variability in seawater chemistry during a marine heatwave event connects to transgenerational effects in the purple urchin Strongylocentrotus purpuratus

Giant kelp Macrocystis pyrifera provides the foundation for immense biodiversity on the coast of California, USA. Kelp forests can change seawater retention time, altering water chemistry, including pH and dissolved oxygen (DO), as well as the magnitude and predictability of variability in the same properties. Environmental heterogeneity across space and time could drive organismal performance and processes such as transgenerational plasticity (TGP), where parental experience modifies the offspring phenotype, potentially conferring tolerance to future environmental stress. We monitored environmental variability by deploying temperature, pH, and DO sensors inside and outside a temperate kelp forest in the Santa Barbara Channel (SBC) throughout the gametogenesis period of a key herbivore, the purple urchin Strongylocentrotus purpuratus. Over the 6 mo period, pH and temperature were slightly elevated inside the kelp forest, accompanied by more predictable, low-frequency variability relative to outside. Adult S. purpuratus were conditioned inside and outside the kelp spanning gametogenesis. The urchins were spawned and their larvae were raised under high (1053 µatm) and low pCO2 (435 µatm) at 15°C in the laboratory to assess their physiological response to the maternal and developmental environments. Larvae raised under high pCO2 were more susceptible to acute thermal stress; however, within each larval treatment, progeny from outside-conditioned mothers had a 0.4°C higher lethal temperature (LT50). Our results indicate that heterogeneity in abiotic factors associated with kelp can have transgenerational effects in the field, and interactions between factors, including temperature and pH, will impact purple urchins as local variability associated with marine heatwaves and upwelling evolves with climate 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.

Kelp holdfasts in the Great African Seaforest provide habitat for diverse assemblages of macroinvertebrates

Kelp forests along the southwestern and west coasts of South Africa, dominated by the species Ecklonia maxima and Laminaria pallida, are locally termed ‘the Great African Seaforest’. They form 3-dimensional biogenic habitats that provide 4 distinct microhabitats—canopy, fronds, stipe and holdfast—with the latter typically supporting the highest abundance and diversity of associated macroinvertebrates. The macrofauna inhabiting kelp holdfasts in South Africa have rarely been studied, resulting in a near complete lack of baseline data. In this study, macrobenthic assemblages from 40 E. maxima holdfasts were examined over 2 marine ecoregions and 4 locations. Macroinvertebrates were identified and counted for univariate and multivariate analyses using family-level data. A total of 120 families from 9 phyla were identified and were generally dominated by Arthropoda (48 families), Annelida (24 families) and Mollusca (23 families). Marine ecoregion had no significant effect on composition of macroinvertebrate assemblages, whereas location had a significant effect. There was no significant relationship between holdfast volume and macroinvertebrate diversity or abundance, suggesting that other environmental and physicochemical factors are important in determining community structure. This study serves as a baseline for future research aimed at understudied holdfast macroinvertebrate communities in the Great African Seaforest.

A marine heatwave changes the stabilizing effects of biodiversity in kelp forests.

Biodiversity can stabilize ecological communities through biological insurance, but climate and other environmental changes may disrupt this process via simultaneous ecosystem destabilization and biodiversity loss. While changes to diversity-stability relationships (DSRs) and the underlying mechanisms have been extensively explored in terrestrial plant communities, this topic remains largely unexplored in benthic marine ecosystems that comprise diverse assemblages of producers and consumers. By analyzing two decades of kelp forest biodiversity survey data, we discovered changes in diversity, stability, and their relationships at multiple scales (biological organizational levels, spatial scales, and functional groups) that were linked with the most severe marine heatwave ever documented in the North Pacific Ocean. Moreover, changes in the strength of DSRs during/after the heatwave were more apparent among functional groups than both biological organizational levels (population vs. ecosystem levels) and spatial scales (local vs. broad scales). Specifically, the strength of DSRs decreased for fishes, increased for mobile invertebrates and understory algae, and were unchanged for sessile invertebrates during/after the heatwave. Our findings suggest that biodiversity plays a key role in stabilizing marine ecosystems, but the resilience of DSRs to adverse climate impacts primarily depends on the functional identities of ecological communities.

Learning From ‘My Octopus Teacher’

AbstractIn this paper, I present the idea that the documentary film My Octopus Teacher (Ehrlich & Reed) is an evocative allegory for some key threads in the ongoing learning at the heart of psychotherapy. On the one hand, the film is a narrative about a relationship formed between the narrator and documentary‐maker Craig and an octopus that he encounters in daily dives in an underwater kelp forest. On the other hand, it is a story–dream of a man and an octopus who swim together in the proto‐mental seas of the unconscious, a space where fluidity and symmetry rule, and where the boundaries between I and thou dissolve. Alongside the theme of mutual dream work, the documentary presents an evocative allegory of what it takes to practice as a therapist: the maps of our own disintegration that inform our work, and the key dispositions of learning to watch and observe and to fine tune our faith.

Limited effects of management and ecological subsidies on the size-spectra of kelp forest fish communities

Animal body size influences key ecological processes across biological hierarchies. For instance, densities (N) and community biomass (B) are allometric functions of body mass (M). Energetic equivalence predicts that density scales with body size as N ∝ M-0.75 and that biomass scales with body size as B ∝ M0.25. However, the way fish size-spectra are influenced by external processes, such as ecological subsidies (e.g. nutrients from upwelling zones) and fisheries management, is not well understood. We investigated the relationship of body size with density and biomass of reef fishes associated with subtidal kelp forests of Lessonia trabeculata that were influenced by the separate and interactive effects of management (Territorial Use Rights for Fisheries [TURF] or open access) and upwelling regimes (upwelling or non-upwelling zones). Fish densities and lengths were recorded using underwater visual censuses. Within each of 4 locations, paired TURF and open-access sites were surveyed. We surveyed 18 fish species, encompassing 1511 individuals ranging between 2 and 6639 g. We observed that fish size-spectra deviated from energetic equivalence, as N ∝ M-0.32 and B ∝ M2.2, indicating that the contribution of large-sized fishes to community density and biomass was greater than that predicted by energetic equivalence. Multi-model inference suggested that TURF and upwelling scenarios had weak effects on fish size-spectra. Results indicated that fish communities may have access to external food sources beyond local kelp forests. In addition, size-spectra may be a spatially persistent attribute of these fish communities.

From sink to source: Dynamic of greenhouse gases emissions from beach wrack accumulations in a temperate coastal bay

Coastal ecosystems such as salt marshes, seagrass meadows, and kelp forests contribute to climate regulation as carbon sinks. However, coastal ecosystems may act as carbon sources as beach wrack accumulations may release greenhouse gases (GHG) during decomposition. The magnitude of GHG emissions of beach wrack accumulations under natural conditions are poorly understood, hampering accurate blue carbon accountings. In this study, we assessed the spatio-temporal variability and environmental factors driving CO2, CH4 and N2O emissions from beach wrack accumulations on a temperate sandy beach. Beach wrack accumulations, dominated by Zostera marina and opportunistic brown macroalgae, presented variable spatio-temporal dynamics. Annual beach wrack GHG emissions achieved up to 77,915 mg m−2 d−1 CO2e (CO2 equivalents) and varied largely throughout the study period due to interactive effects of temperature, wave exposure, beach wrack biomass moisture, abundance, and species composition. Our findings showed that methane emissions in new, freshly deposited, and in drifting wrack in the water reached up to 100 mg m−2 d−1, representing up to 57 % of annual CO2e emissions occurring throughout the year. Nitrous oxide emissions were highly variable and comprised a minor extent (i.e., up to 4 %) of annual CO2e emissions. Together, wrack CH4 and N2O emissions provided 13.69 g CO2 m−2 per year to the atmosphere. Our findings indicate that excessive opportunistic macroalgae biomass driven by eutrophication may explain increased CO2 and N2O emissions. We conclude that whilst beach wrack depositions are a natural and essential part of coastal ecosystems, they may provide an extra source of GHG to the atmosphere, potentially counteracting the role of vegetated coastal ecosystems as carbon sinks.

Capturing accurate kelp canopy extent: integrating tides, currents, and species-level morphology in kelp remote sensing

Surface-canopy forming kelps (Macrocystis pyrifera and Nereocystis luetkeana) can be monitored along the Northeast Pacific coast using remote sensing. These kelp canopies can be submerged by tides and currents, making it difficult to accurately determine their extent with remote sensing techniques. Further, both species have morphologically distinct canopies, each made up of structures with differing buoyancies, and it is not well understood whether the differing buoyancies between these species’ canopies affects their detectability with remote sensing technologies. Here, we collected in situ above-water spectral signatures for the surface-canopies of Nereocystis and Macrocystis, providing the first direct hyperspectral comparison between the structures that make up the canopies of these species. Additionally, we compare the strength of their red-edge and near-infrared band signals, as well as the normalized difference red-edge (NDRE) and normalized difference vegetation index (NDVI) values. At the bed level, we compare detection of kelp canopy extent using both NDRE and NDVI classified unoccupied aerial vehicle imagery. We also characterized how changing tides and currents submerge the canopies of both species, providing insights that will allow remote sensors to more accurately determine the extent of kelp canopy in remote sensing imagery. Observations of canopy structures paired with in situ hyperspectral data and simulated multispectral data showed that more buoyant kelp structures had higher reflectance in the near-infrared wavelengths, but even slightly submerged canopy structures had a higher reflectance in the red-edge rather than the near-infrared. The higher red-edge signal was also evident at the bed level in the UAV imagery, resulting in 18.0% more canopy classified with NDRE than with NDVI. The area of detected canopy extent decreased by an average of 22.5% per meter of tidal increase at low current speeds (<10 cm/s), regardless of the species present. However, at higher current speeds (up to 19 cm/s), Nereocystis canopy decreased at nearly twice the average rate of kelp beds in low-current conditions. Apart from the strong differences in high-current regions, a robust linear relationship exists between kelp canopy extent and tidal height, which can aid in understanding the errors associated with remote sensing imagery collected at different tidal heights.

The flourishing of Laminaria ochroleuca in the strait of Messina (Sicily, Italy): resilience population between "Scylla and Charybdis"

The kelp forest and bed of Laminaria ochroleuca were found in October 2023 in Porticello (Scylla -Reggio Calabria, Italy) between 48 and 55 m depth. Samples were collected for future genetic and sedimentological analyses and identification of associated fauna and flora. The aim of the upcoming research campaigns is to update the current knowledge of Laminaria forests in the Messina Strait, estimating the action and consequences of climate change at the local level. The project will involve the community of technical divers in long term monitoring (3–5 years), in order to measure ecosystem services. The information will be used to draft a policy document for the proposal of protected areas of the most extensive forests.

Recruitment as a possible indicator of declining resilience in degraded kelp forests

Kelp forests are critical marine ecosystems that provide habitat and ecological services, as well as economic benefits. However, kelp forests worldwide are facing multiple pressures, including climate change and human activities. In this study, we investigated whether recruitment success, an infrequently recorded variable in kelp monitoring studies, was affected in degraded kelp reefs where adult golden kelps (Laminaria ochroleuca) were persistently absent due to fish consumption. We conducted a three-year seasonal monitoring of kelp recruits and juveniles in both healthy and degraded kelp reefs in northwest Spain. Our findings reveal a decline in spring kelp recruitment over time on degraded reefs, while it remained stable on healthy reefs. The results indicate that continued herbivory pressure can decrease kelp recruitment, suggesting the potential exhaustion of kelp stand resilience. Altogether, our study highlights the importance of biotic interactions, such as herbivory, in understanding changes in kelp forest dynamics. Moreover, it emphasizes the significance of including recruitment in kelp forest assessments as an indicator of resilience.

Optimizing the biomass balance of macroalgae and sea urchins in kelp beds by removing the urchins

ABSTRACT In this study, we conducted sea urchin removal (SUR) experiments at two sites along Hokkaido’s southwestern and northwestern coasts in the Sea of Japan and one site along the northeast coast in the Sea of Okhotsk, to elucidate the biomass balance between macroalgae and sea urchins at different urchin barren sites. The experiments resulted in the restoration of macroalgal biomass at all sites and an increase in macroalgal diversity, indicating that it was the impact of high grazing pressure reducing the macroalgae, regardless of the particular experimental site. Notably, the recovery of the macroalgal biomass was characterized by increased kelp biomass, including Saccharina japonica var. religiosa and S. japonica var. ochotensis. In addition, negative correlations were observed between macroalgal biomass and sea urchin density. The lowest sea urchin density inducing an increase of macroalgal biomass was approximately the same in the reproductive growth phase (0.1 kg m−2) and in the vegetative growth phase (0.15 kg m−2) of Saccharina spp., the dominant kelps, regardless of the experimental site. These values can be helpful indicators for managing sea urchin density in order to restore macroalgae. In contrast, the maximum sea urchin density inducing the lowest macroalgal biomass varied markedly depending on the study site, indicating that different levels of kelp productivity are influenced by oceanographic conditions. Moreover, a strong positive correlation was observed between macroalgal biomass and sea urchins’ gonadosomatic index (GSI). Based on this correlation, macroalgal biomass supporting sea urchins with commercial value (GSI ≥ 15%) was estimated to be 1.6 kg m−2. This sea urchin density value is also a helpful indicator for managing macroalgal biomass and commercially valuable sea urchins. Moreover, this value will depend on the influence of the oceanographic environment of the site, particularly the nutrient conditions, on macroalgal productivity. In addition, we have also discussed how macroalgae and sea urchins can best coexist at the experimental sites.

Dispersal synchronizes giant kelp forests.

Spatial synchrony is the tendency for population fluctuations to be correlated among different locations. This phenomenon is a ubiquitous feature of population dynamics and is important for ecosystem stability, but several aspects of synchrony remain unresolved. In particular, the extent to which any particular mechanism, such as dispersal, contributes to observed synchrony in natural populations has been difficult to determine. To address this gap, we leveraged recent methodological improvements to determine how dispersal structures synchrony in giant kelp (Macrocystis pyrifera), a global marine foundation species that has served as a useful system for understanding synchrony. We quantified population synchrony and fecundity with satellite imagery across 11 years and 880 km of coastline in southern California, USA, and estimated propagule dispersal probabilities using a high-resolution ocean circulation model. Using matrix regression models that control for the influence of geographic distance, resources (seawater nitrate), and disturbance (destructive waves), we discovered that dispersal was an important driver of synchrony. Our findings were robust to assumptions about propagule mortality during dispersal and consistent between two metrics of dispersal: (1) the individual probability of dispersal and (2) estimates of demographic connectivity that incorporate fecundity (the number of propagules dispersing). We also found that dispersal and environmental conditions resulted in geographic clusters with distinct patterns of synchrony. This study is among the few to statistically associate synchrony with dispersal in a natural population and the first to do so in a marine organism. The synchronizing effects of dispersal and environmental conditions on foundation species, such as giant kelp, likely have cascading effects on the spatial stability of biodiversity and ecosystem function.

Identifying potential high-risk zones for land-derived plastic litter to marine megafauna and key habitats within the North Atlantic

The pervasive use of plastic in modern society has led to plastic litter becoming ubiquitous within the ocean. Land-based sources of plastic litter are thought to account for the majority of plastic pollution in the marine environment, with plastic bags, bottles, wrappers, food containers and cutlery among the most common items found. In the marine environment, plastic is a transboundary pollutant, with the potential to cause damage far beyond the political borders from where it originated, making the management of this global pollutant particularly complex. In this study, the risks of land-derived plastic litter (LDPL) to major groups of marine megafauna – seabirds, cetaceans, pinnipeds, elasmobranchs, turtles, sirenians, tuna and billfish – and a selection of productive and biodiverse biogenic habitats – coral reefs, mangroves, seagrass, saltmarsh and kelp beds – were analysed using a Spatial Risk Assessment approach. The approach combines metrics for vulnerability (mechanism of harm for megafauna group or habitat), hazard (plastic abundance) and exposure (distribution of group or habitat). Several potential high-risk zones (HRZs) across the North Atlantic were highlighted, including the Azores, the UK, the French and US Atlantic coasts, and the US Gulf of Mexico. Whilst much of the modelled LDPL driving risk in the UK originated from domestic sources, in other HRZs, such as the Azores archipelago and the US Gulf of Mexico, plastic originated almost exclusively from external (non-domestic) sources. LDPL from Caribbean islands - some of the largest generators of marine plastic pollution in the dataset of river plastic emissions used in the study - was noted as a significant input to HRZs across both sides of the Atlantic. These findings highlight the potential of Spatial Risk Assessment analyses to determine the location of HRZs and understand where plastic debris monitoring and management should be prioritised, enabling more efficient deployment of interventions and mitigation measures.

Detecting kelp-forest associated metazoan biodiversity with eDNA metabarcoding

Environmental DNA (eDNA) metabarcoding is a promising tool for monitoring marine biodiversity, but remains underutilised in Africa. In this study, we evaluated the ability of aquatic eDNA metabarcoding as a tool for detecting biodiversity associated with a South African kelp forest, an ecosystem that harbours high diversity of species, many of which are endemic, but are also sensitive to changing environmental conditions and anthropogenic pressures. Using fine-scale spatial (1 m and 8 m) and temporal (every four hours for 24 h) sampling of aquatic environmental DNA and targeting two gene regions (mtDNA COI and 12S rRNA), metabarcoding detected 880 OTUs representing 75 families in the broader metazoan community with 44 OTUs representing 24 fish families. We show extensive variability in the eDNA signal across space and time and did not recover significant spatio-temporal structure in OTU richness and community assemblages. Metabarcoding detected a broad range of taxonomic groups, including arthropods, ascidians, cnidarians, echinoderms, ctenophores, molluscs, polychaetes, ichthyofauna and sponges, as well as Placozoa, previously not reported from South Africa. Fewer than 3% of OTUs could be identified to species level using available databases (COI = 19 OTUs, 12S = 11 OTUs). Our study emphasizes that kelp-forest associated biodiversity in South Africa is understudied, but that with careful consideration for sampling design in combination with increased barcoding efforts and the construction of regional databases, eDNA metabarcoding will become a powerful biomonitoring tool of kelp-forest associated biodiversity.

Herbivore effects increase with latitude across the extent of a foundational seagrass.

Climate change is altering the functioning of foundational ecosystems. While the direct effects of warming are expected to influence individual species, the indirect effects of warming on species interactions remain poorly understood. In marine systems, as tropical herbivores undergo poleward range expansion, they may change food web structure and alter the functioning of key habitats. While this process ('tropicalization') has been documented within declining kelp forests, we have a limited understanding of how this process might unfold across other systems. Here we use a network of sites spanning 23° of latitude to explore the effects of increased herbivory (simulated via leaf clipping) on the structure of a foundational marine plant (turtlegrass). By working across its geographic range, we also show how gradients in light, temperature and nutrients modified plant responses. We found that turtlegrass near its northern boundary was increasingly affected (reduced productivity) by herbivory and that this response was driven by latitudinal gradients in light (low insolation at high latitudes). By contrast, low-latitude meadows tolerated herbivory due to high insolation which enhanced plant carbohydrates. We show that as herbivores undergo range expansion, turtlegrass meadows at their northern limit display reduced resilience and may be under threat of ecological collapse.

Decline of a North American rocky intertidal foundation species linked to extreme dry, downslope Santa Ana winds

Foundation species are essential to ecosystem function, but their role as habitat providers is predicated on their spatial dominance. Worldwide, kelps, seagrasses, corals, and other marine foundation species have declined. This is true also for rockweeds, the canopy-forming analog of subtidal kelp forests in temperate rocky intertidal ecosystems. On the west coast of North America, dense beds of the rockweed Silvetia compressa occur across large biogeographic regions, benefitting numerous species by ameliorating physical stress caused by sun exposure, desiccation, heat, and wave disturbance. Like many rockweed species, Silvetia is long-lived, slow-growing, and short-dispersing – characteristics that reduce its resilience to disturbance. Using a generalized additive mixed-effects model with explicit spatial effects, we analyzed canopy cover data from 30 sites spanning 18 years, and we tested the hypothesis that Silvetia population trends are tightly linked to atmospheric climate conditions, particularly Santa Ana wind events (SAWs): strong, hot, and dry downslope winds that originate inland and move offshore. We found that the rockweed had declined markedly, particularly at sites south of the major biogeographic break, Point Conception (PC), including the California Channel Islands and southern California mainland, and a highly significant negative effect of dewpoint depression, a measure of moisture content in the atmosphere, on Silvetia cover across all three regions in this study. Our results suggest that any increases in the frequency or intensity of SAWs are likely to lead to large declines and possible extirpation of Silvetia, as well as the important ecological services the species provides.

Kelp dynamics and environmental drivers in the southern Salish Sea, British Columbia, Canada

The impacts of local-scale temperatures and winds on bull kelp (Nereocystis luetkeana) vary along a coastal gradient, while also being influenced by corresponding global-scale oceanic conditions. Around Vancouver Island and the Gulf Islands, BC, Canada, bull kelp floating canopies were mapped using high-resolution imagery from 2005 to 2022, whereas the largest kelp bed of the area was mapped with medium-resolution imagery spanning from 1972 to 2022. In order to understand spatial patterns of kelp resilience, the abiotic characteristics were used to organize the coastline into four clusters, ranging from the coldest and most exposed coast to a more sheltered and warmer location. Additionally, local-scale sea surface temperatures, winds, and marine heatwaves were categorized by global-scale temporal conditions defined by the positive/negative oceanic oscillations of the Oceanic Niño Index (ONI) and Pacific Decadal Oscillation (PDO). Comparing spatial and temporal categories, we observed that years with positive ONI and PDO, in particular the 2014–2019 period, concentrated most of the marine heatwaves and the spring temperature peaks. However, there are some indications of an underlying long-term trend. During the period 2020–2022, when ONI and PDO were negative, summer temperatures kept increasing and wind displayed a higher frequency of extreme events. Mapped kelp showed different trends to these stressors: the coldest and most exposed area showed a constant presence of kelp during the entire period, even dating back to 1972. Warmer and semi-sheltered coasts increased in kelp percentage cover after the positive ONI+PDO period of 2014–2019, and the coasts facing the Strait of Georgia displayed a lower kelp percentage cover than the other clusters. In summary, bull kelp was resilient in the study area, but for different reasons: colder and more exposed coasts had the most favorable conditions for kelp, but warmer and more sheltered coastal kelp beds may have benefited from wind-wave forcing.

Bathymetric origin shapes the physiological responses of Pterygophora californica (Laminariales, Phaeophyceae) to deep marine heatwaves.

Kelp communities are experiencing exacerbated heat-related impacts from more intense, frequent, and deeper marine heatwaves (MHWs), imperiling the long-term survival of kelp forests in the climate change scenario. The occurrence of deep thermal anomalies is of critical importance, as elevated temperatures can impact kelp populations across their entire bathymetric range. This study evaluates the impact of MHWs on mature sporophytes of Pterygophora californica (walking kelp) from the bathymetric extremes (8-10 vs. 25-27 m) of a population situated in Baja California (Mexico). The location is near the southernmost point of the species's broad distribution (from Alaska to Mexico). The study investigated the ecophysiological responses (e.g., photobiology, nitrate uptake, oxidative stress) and growth of adult sporophytes through a two-phase experiment: warming simulating a MHW and a post-MHW phase without warming. Generally, the effects of warming differed depending on the bathymetric origin of the sporophytes. The MHW facilitated essential metabolic functions of deep-water sporophytes, including photosynthesis, and promoted their growth. In contrast, shallow-water sporophytes displayed metabolic stress, reduced growth, and oxidative damage. Upon the cessation of warming, certain responses, such as a decline in nitrate uptake and net productivity, became evident in shallow-water sporophytes, implying a delay in heat-stress response. This indicates that variation in temperatures can result in more prominent effects than warming alone. The greater heat tolerance of sporophytes in deeper waters shows convincing evidence that deep portions of P. californica populations have the potential to serve as refuges from the harmful impacts of MHWs on shallow reefs.