Kelp Density Research Articles

Advanced Detection and Classification of Kelp Habitats Using Multibeam Echosounder Water Column Point Cloud Data

Kelps are important habitat-forming species in shallow marine environments, providing critical habitat, structure, and productivity for temperate reef ecosystems worldwide. Many kelp species are currently endangered by myriad pressures, including changing water temperatures, invasive species, and anthropogenic threats. This situation necessitates advanced methods to detect kelp density, which would allow tracking density changes, understanding ecosystem dynamics, and informing evidence-based management strategies. This study introduces an innovative approach to detect kelp density with multibeam echosounder water column data. First, these data are filtered into a point cloud. Then, a range of variables are derived from these point cloud data, including average acoustic energy, volume, and point density. Finally, these variables are used as input to a Random Forest model in combination with bathymetric variables to classify sand, bare rock, sparse kelp, and dense kelp habitats. At 5 m resolution, we achieved an overall accuracy of 72.5% with an overall Area Under the Curve of 0.874. Notably, our method achieved high accuracy across the entire multibeam swath, with only a 1 percent point decrease in model accuracy for data falling within the part of the multibeam water column data impacted by sidelobe artefact noise, which significantly expands the potential of this data type for wide-scale monitoring of threatened kelp ecosystems.

The influence of subtidal Laminaria canopies on local environmental conditions and the structure of understorey communities.

Understanding the role of species interactions (e.g. competition and facilitation) in structuring communities is a fundamental goal of ecology. It is well established that large canopy-forming seaweeds (e.g. kelps and fucoids) exert a strong influence on community structure, by offering biogenic habitat, altering environmental conditions and interacting with other species. While many studies have manipulated the density of seaweeds to causatively examine their effects on the local environment and associated communities, they are biased towards certain regions and canopy-forming species. We conducted a manipulative experiment at two subtidal sites characterised by mixed Laminaria canopies, in southwest England, UK. Three treatments were established in multiple replicate circular plots (area of 7.1m2): 0% kelp removal (i.e. unmanipulated control), 50% kelp removal (i.e. thinning), and 100% removal. Within each plot, temperature, light levels and sedimentation rates were monitored over 3 months, and after 5 months the density of juvenile kelp recruits, the biomass of understorey macroalgae and the abundance of recruiting fauna were quantified. We found kelp canopies had no impact on temperature or sedimentation rates but their removal led to marked increases in light availability, juvenile kelp recruitment and understorey macroalgal biomass. Overall, our study shows that physical disturbance to Laminaria canopies alters resource availability (i.e. light and space), leading to increased abundances of kelp recruits and understory algae. Significant reductions in kelp canopy density, driven by storm disturbance, harvesting or decreased water quality for example, would lead to shifts in community structure and ecological functioning.

Marine Protected Areas That Preserve Trophic Cascades Promote Resilience of Kelp Forests to Marine Heatwaves.

Under accelerating threats from climate-change impacts, marine protected areas (MPAs) have been proposed as climate-adaptation tools to enhance the resilience of marine ecosystems. Yet, debate persists as to whether and how MPAs may promote resilience to climate shocks. Here, we use 38 years of satellite-derived kelp cover to empirically test whether a network of 58 temperate coastal MPAs in Central and Southern California enhances the resistance of kelp forest ecosystems to, and their recovery from, the unprecedented 2014-2016 marine heatwave regime that occurred in the region. We also leverage a 22-year time series of subtidal community surveys to mechanistically understand whether trophic cascades explain emergent patterns in kelp forest resilience within MPAs. We find that fully protected MPAs significantly enhance kelp forests' resistance to and recovery from marine heatwaves in Southern California, but not in Central California. Differences in regional responses to the heatwaves are partly explained by three-level trophic interactions comprising kelp, urchins, and predators of urchins. Urchin densities in Southern California MPAs are lower within fully protected MPAs during and after the heatwave, while the abundances of their main predators-lobster and sheephead-are higher. In Central California, a region without lobster or sheephead, there is no significant difference in urchin or kelp densities within MPAs as the current urchin predator, the sea otter, is protected statewide. Our analyses show that fully protected MPAs can be effective climate-adaptation tools, but their ability to enhance resilience to extreme climate events depends upon region-specific environmental and trophic interactions. As nations progress to protect 30% of the oceans by 2030, scientists and managers should consider whether protection will increase resilience to climate-change impacts given their local ecological contexts, and what additional measures may be needed.

Assessing the role of natural kelp forests in modifying seawater chemistry

Climate change is causing widespread impacts on seawater pH through ocean acidification (OA). Kelp forests, in some locations can buffer the effects of OA through photosynthesis. However, the factors influencing this variation remain poorly understood. To address this gap, we conducted a literature review and field deployments of pH and dissolved oxygen (DO) loggers within four habitats: intact kelp forest, moderate kelp cover, sparse kelp cover and barrens at one site in Port Phillip Bay, a wind-wave dominated coastal embayment in Victoria, Australia. Additionally, a wave logger was placed directly in front of the intact kelp forest and barrens habitats. Most studies reported that kelp increased seawater pH and DO during the day, compared to controls without kelp. This effect was more pronounced in densely populated forests, particularly in shallow, sheltered conditions. Our field study was broadly consistent with these observations, with intact kelp habitat having higher seawater pH than habitats with less kelp or barrens and higher seawater DO compared to barrens, particularly in the afternoon and during calmer wave conditions. Although kelp forests can provide local refuges to biota from OA, the benefits are variable through time and may be reduced by declines in kelp density and increased wave exposure.

Detection of Toxoplasma gondii in wild bivalves from the Kerguelen and Galapagos archipelagos: influence of proximity to cat populations, exposure to marine currents and kelp density

Oocysts of the protozoan Toxoplasma gondii are found in felid feces and can be washed into coastal waters, where they persist for months, attaching to algae and accumulating in invertebrates. We used wild bivalves to assess contamination of coastal waters of the Kerguelen and Galapagos archipelagos by this zoonotic parasite. Additionally, we leveraged the contrasting situations of these archipelagos to identify some potential drivers of contamination. In the Galapagos, with a cat density reaching 142 per km2, 15.38% of the sampled oysters (Saccostrea palmula) tested positive for T. gondii by quantitative real-time PCR (qPCR) (n = 260), and positive samples were found in all eight sampling sites. In Kerguelen, with 1-3 cats per km2, 40.83% of 120 tested mussels (Mytilus edulis platensis) were positive, and positive samples were found in four out of the five sampling sites. These findings provide evidence of T. gondii contamination in the coastal waters of these archipelagos. Furthermore, T. gondii-positive bivalves were found on islands located 20 km away (Galapagos) and 5 km away (Kerguelen) from the nearest cat population, indicating that T. gondii oocysts can disperse through waterborne mechanisms over several kilometers from their initial deposition site. In the Galapagos, where runoff is infrequent and all sites are exposed to currents, the prevalence of qPCR-positive bivalves did not show significant variations between sites (p = 0.107). In Kerguelen where runoff is frequent and site exposure variable, the prevalence varied significantly (p < 0.001). The detection of T. gondii in Kerguelen mussels was significantly correlated with the site exposure to currents (odds ratio (OR) 60.2, p < 0.001) and the on-site density of giant kelp forests (OR 2.624, p < 0.001). This suggests that bivalves can be contaminated not only by oocysts transported by currents but also by consuming marine aggregates containing oocysts that tend to form in kelp forests.

Dynamic structural equation models synthesize ecosystem dynamics constrained by ecological mechanisms

Abstract Ecological analyses typically involve many interacting variables. Ecologists often specify lagged interactions in community dynamics (i.e. vector‐autoregressive models) or simultaneous interactions (e.g. structural equation models), but there is less familiarity with dynamic structural equation models (DSEM) that can include any simultaneous or lagged effect in multivariate time‐series analysis. We propose a novel approach to parameter estimation for DSEM, which involves constructing a Gaussian Markov random field (GMRF) representing simultaneous and lagged path coefficients, and then fitting this as a generalized linear mixed model to missing and/or non‐normal data. We provide a new R‐package dsem, which extends the ‘arrow interface’ from path analysis to represent user‐specified lags when constructing the GMRF. We also outline how the resulting nonseparable precision matrix can generalize existing separable models, for example, for time‐series and species interactions in a vector‐autoregressive model. We first demonstrate dsem by simulating a two‐species vector‐autoregressive model based on wolf–moose interactions on Isle Royale. We show that DSEM has improved precision when data are missing relative to a conventional dynamic linear model. We then demonstrate DSEM via two contrasting case studies. The first identifies a trophic cascade where decreased sunflower starfish has increased urchin and decreased kelp densities, while sea otters have a simultaneous positive effect on kelp in the California Current from 1999 to 2018. The second estimates how declining sea ice has decreased cold‐water habitats, driving a decreased density for fall copepod predation and inhibiting early‐life survival for Alaska pollock from 1963 to 2023. We conclude that DSEM can be fitted efficiently as a GLMM involving missing data, while allowing users to specify both simultaneous and lagged effects in a time‐series structural model. DSEM then allows conceptual models (developed with stakeholder input or from ecological expertise) to be fitted to incomplete time series and provides a simple interface for granular control over the number of estimated time‐series parameters. Finally, computational methods are sufficiently simple that DSEM can be embedded as component within larger (e.g. integrated population) models. We therefore recommend greater exploration and performance testing for DSEM relative to familiar time‐series forecasting methods.

Modeling carbon dioxide removal via sinking of particulate organic carbon from macroalgae cultivation

Macroalgae cultivation is receiving growing attention as a potential carbon dioxide removal (CDR) strategy. Macroalgae biomass harvesting and/or intentional sinking have been the main focus of research efforts. A significant amount of biomass is naturally lost through erosion and breakage of cultivated or naturally growing seaweed, but the contribution of the resulting particulates to carbon sequestration is relatively unexplored. Here, we use a fully coupled kelp-biogeochemistry model forced by idealized parameters in a closed system to estimate the potential of macroalgal-derived particulate organic carbon (POC) sinking as a CDR pathway. Our model indicates that at a kelp density of 1.1 fronds m−3, macroalgal POC sinking can export 7.4 times more carbon to the deep sea (depths &amp;gt; 500m) and remove 5.2 times more carbon from the atmosphere (equivalent to an additional 336.0 gC m−2 yr−1) compared to the natural biological pump without kelp in our idealized closed system. The results suggest that CDR associated with POC sinking should be explored as a possible benefit of seaweed farming and point to the need for further study on organic carbon partitioning and its bioavailability to quantify the effectiveness and impacts of macroalgal cultivation as a CDR strategy.

A model of tidal flow and tracer release in a giant kelp forest

Benthic macroalgae (including brown macroalgae or kelp) constitute one of the largest contributors to coastal primary production, but their ability to store and sequester carbon remains uncertain. Here, we use a numerical model of the flow/kelp interactions to study how tidal currents interact with an idealised numerical model of a giant kelp (Macrocystis pyrifera) forest, intending to better understand the potential for kelp growth in nutrient-limited conditions and the export of important tracers such as dissolved organic carbon. We calibrate and test our model using observations of currents within and surrounding a kelp forest in Southern California. By varying the density of kelp in our model, we find that there is a kelp density that maximises the export of tracer released from the kelp forest. Since the tracer advection/diffusion equation is linear with respect to the tracer concentration, the same kelp density corresponds to the maximum uptake for a tracer with a constant far-field concentration. The density at which this maximum occurs coincides with the density typical of natural kelp forests, where kelp growth may be limited by the uptake of dissolved nutrients from the surrounding water. Additionally, the drag induced on the tidal currents by the kelp forest results in a mean circulation through the kelp forest and a mean displacement of the kelp forest canopy.

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.

Fish functional diversity is modulated by small-scale habitat complexity in a temperate ecosystem

Rocky reefs and kelp forests form conspicuous habitats that promote species diversity and support livelihoods in coastal communities. However, taxonomic approaches often disregard biological identity and differences between species. In this study, we explore the relationship between functional diversity of fish communities and habitat complexity in temperate reefs and test if greater habitat complexity, such as rugosity or kelp three-dimensional structure, would result in higher functional diversity. We conducted fish surveys using SCUBA in four kelp forest sites and rocky reef sites. Although the rocky reef and kelp forest sites showed differences in habitat complexity, no significant differences in fish taxonomic or functional diversity were found between the two habitat types. However, we did find differences at smaller spatial scales for the kelp forest sites, where number of stipes, stipe bundle diameter, and kelp density influenced functional richness, but not species richness, highlighting the importance of functional approaches in certain ecosystems. The differences found among kelp forest sites may be linked with small-scale spatiotemporal oceanographic drivers of productivity such as upwelling exposure or nutrient availability. We recommend considering small-scale spatial drivers when aiming to understand how habitat characteristics link with functional diversity.

Rebirth of a reef: As-built description and rapid returns from the Palos Verdes Reef Restoration Project

Palos Verdes Reef (PVR) is an artificial reef designed to restore rocky-reef associated marine species by directly restoring rocky-reef habitat that has been impacted by scour, sedimentation, and burial in the shallow subtidal portion of the Palos Verdes Peninsula in Los Angeles County, California, USA. The restoration reef provides high-quality habitat that is contiguous with the natural reef and allows for rapid succession. This project is a unique endeavor as restoring lost habitat in situ and has not been attempted in a temperate rocky reef and kelp forest community. While the primary design criteria for PVR is fish production in an area where already-limited hard substrate had been lost, it is also designed to be resilient to ongoing sedimentation and turbidity challenges on the peninsula. Following over a decade of design, planning, outreach, site surveying, and permitting, PVR was built in 2020 as 18 discrete modules using 52,729 tons of quarry rock placed approximately 8–80 m from existing, unburied rocky-reef habitat. There was no significant accumulation or scouring of sediment due to the placement of the reef and temperature data shows that internal tides regularly inundate the reef with cool, nutrient rich water. Rocky-reef associated taxa rapidly recruited to the restoration site, with visible changes occurring within just a few months post-construction. PVR modules showed rapid, significantly positive responses in fish density, fish biomass, kelp density, and biotic benthic cover less than 18 months after reef placement with a general pattern of succession in giant kelp growth from nearshore to offshore resulting in an established, thick canopy, in the nearshore, shallow modules. The newly available, high-quality habitat was quickly colonized and already shows late successional patterns with respect to fish and benthic communities. This restoration reef will produce large amounts of biomass over the long-term, though future surveys of multiple restored, adjacent, and reference sites will determine if high biomass at PVR is a result of new secondary production or attraction from nearby reefs.

Coastal upwelling may strengthen the controls of herbivory and light over the population dynamics of Hedophyllum sessile in the Oregon rocky intertidal

Survival of early life history stages is critical to the successful establishment of benthic populations. Although light availability and herbivory are likely to influence the passage of marine macroalgae through a “recruitment bottleneck” at the sporeling stage, the interactive effect of these factors on subsequent community patterns of macroalgae is not well studied. We experimentally tested the effect of light and grazing on sporelings of the common intertidal kelp Hedophyllum sessile. Studies were conducted at two sites along the Oregon coast: Strawberry Hill (an intermittent upwelling region) and Cape Blanco North (a persistent upwelling region). Herbivory and light availability were manipulated and kelp performance metrics (density and length) were measured monthly from May to November 2019. We found that the effects of herbivory and light availability were pronounced at Cape Blanco North but negligible at Strawberry Hill. At Cape Blanco North, herbivory had strong but opposing effects on the density and length of H. sessile. Kelp density was higher in treatments without herbivores while kelp length was greater in treatments with herbivores. Responses also differed with the life history stage. Herbivory had negative effects on juvenile kelp but positive effects on adult kelp while light availability had opposing effects on the length of juvenile and adult kelps. The length of juvenile kelps was higher in shaded treatments while the length of adult kelps was higher in unshaded treatments. Our study highlights the potential importance of coastal geophysical processes (and subsequently, nutrients) in modifying herbivore and light effects on population dynamics of H. sessile, and how these dynamics may be further influenced by different characteristics of the kelp (i.e., demographic traits and life history stages).

Kelp Patch-Specific Characteristics Limit Detection Capability of Rapid Survey Method for Determining Canopy Biomass Using an Unmanned Aerial Vehicle

Kelp forests dominate autotrophic biomass and primary productivity of approximately 30,000 to 60,000 km of shallow temperate and Arctic rocky reef coastline globally and contribute significantly to carbon cycling in the coastal ocean. Rapid biomass turnover is driven by very high growth rates and seasonal environmental drivers. As a result, kelp biomass varies greatly with time, space, and by species. In the northeast Pacific region, bull kelp (Nereocystis leutkeana) and giant kelp (Macrocystis pyrifera) form extensive floating surface canopies with a distinct spectral signature compared to the surrounding water. Studies have shown that remote sensing is advantageous for deriving large-scale estimates of floating surface canopy biomass, which comprises more than 90% of bull and giant kelp standing stock. However, development and validation of remotely derived kelp canopy biomass is lacking because existing approaches are time intensive and costly. This study attempted to close that gap by developing a rapid survey design utilizing diver and unmanned aerial vehicle (UAV) imagery across six sites in northern and central California. Kelp sporophytes were collected and measured for morphometric characteristics and genera-specific allometry to canopy biomass. Kelp density was measured using in situ diver surveys and coupled with UAV imagery to quantify kelp canopy biomass at a range of ground sampling distances. We successfully estimated kelp canopy biomass from UAV imagery at 33% (2/6) of the survey sites, but consistently determining canopy biomass via this approach was challenged by both survey design and kelp patch-specific spatial characteristics. The morphologies of bull kelp in Monterey were significantly different than other regions measured, but further work should be conducted to fully characterize differences in canopy biomass at the regional and sub-regional scale. We use this opportunity to suggest survey design strategies that will increase the success of future methodological development of UAV biomass retrieval. We also recommend developing long-term, annual genera-specific monitoring programs across the northeast Pacific region and beyond to validate remote sensing derived biomass estimates beyond the small number of existing well-characterized sites.

Influence of Kelp Forest Biomass on Nearshore Currents

AbstractAs part of a project focused on the coastal fisheries of Isla Natividad, an island on the Pacific coast of Baja California, Mexico, we conducted a 2‐1/2 year study of flows at two sites within the island's kelp forests. At one site (Punta Prieta), currents are tidal, whereas at the other site (Morro Prieto), currents are weaker and may be more strongly influenced by wind forcing. Satellite estimates of the biomass of the giant kelp (Macrocystis pyrifera) for this period varied between 0 (no kelp) and 3 kg/m2 (dense kelp forest), including a period in which kelp entirely was absent as a result of the 2014–2015 “Warm Blob” in the Eastern Pacific. During this natural “deforestation experiment”, alongshore velocities at both sites when kelp was present were substantially weaker than when kelp was absent, with low‐frequency alongshore currents attenuated more than higher frequency ones, behavior that was the same at both sites despite differences in forcing. The attenuation of cross‐shore flows by kelp was less than alongshore flows; thus, residence times for water inside the kelp forest, which are primarily determined by cross‐shore velocities, were only weakly affected by the presence or absence of kelp. The flow changes we observed in response to changes in kelp density are important to the biogeochemical functioning of the kelp forest in that slower flows imply longer residence times, and, are also ecologically relevant in that reduced tidal excursions may lead to more localized recruitment of planktonic larvae.

Ecosystem engineering kelp limits recruitment of mussels and microphytobenthic algae

Ecosystem engineers often exert strong effects on the recruitment of other species through modification of the local abiotic and biotic environment. In 2015, artificial reefs in eastern Tasmania (− 42.64693, 148.01481) spanning seven different patch sizes (0.12–7.68 m2) and supporting four densities of transplanted kelp (Ecklonia radiata at 0, 4.1, 8.2 and 16.4 kelp m−2) were used to determine how the patch size and density of this ecosystem engineer influenced the recruitment of microphytobenthic (MPB) algae, and a secondary ecosystem engineer, the mussel Mytilus galloprovincialis. Increasing kelp density and patch size inhibited the establishment of subcanopy MPB algae on settlement slides and reduced the recruitment of mussels in standardised rope fibre habitats (RFHs). The productivity:biomass ratio (P:B) of mussel recruits tended to be lower on small reefs and reefs without kelp, relative to larger reefs with high densities of kelp. Canopy shading and reduced cover of turf algae appeared to negatively impact the recruitment of MPB algae and mussels, whilst reduced sediment accumulation on the reefs due to the kelp was also negatively associated with mussel recruitment. These findings highlight the role of ecosystem engineering by kelp in inhibiting the establishment of other species which may additionally impact community dynamics and primary and secondary productivity. The limited capacity of small kelp patches to inhibit the recruitment of other organisms supports the notion that fragmented patches of ecosystem engineers could be more suspectable to adverse outcomes from species interactions making them less resistant to shifts towards an alternative ecosystem state.

Restored kelp facilitates lobster recruitment but not other mid‐trophic macroinvertebrates

Abstract The patch dynamics of foundation species profoundly affects community assembly and thus has important implications for ecosystem restoration. However, it is unclear how restored kelp patches that vary in size and density will influence the establishment of mid‐trophic level (MTL) macroinvertebrates, a key functional group in coastal ecosystems. Artificial reefs with transplants of the canopy‐forming kelp, Ecklonia radiata, were used to quantify the effect of patch size and kelp density on the densities of MTL macroinvertebrates (primarily decapod crustaceans) and on the recruitment of an ecologically important and commercially valuable lobster species. Densities of MTL macroinvertebrates, which were dominated by hermit crabs, decreased with increasing patch size but responded inconsistently to kelp density. There was, however, an overall positive relationship between MTL macroinvertebrates and the density of small epifaunal grazers (a potential food source), along with a negative association with cover of understorey foliose algae. In contrast, the total abundance and density of lobster recruits was higher on larger reefs, and reefs with kelp had up to double the number of recruits relative to reefs with no kelp. After 12 months, most of the surviving lobster recruits occurred on reefs supporting low and medium densities of kelp. These results show that patchy reef substratum is effective in supporting high densities of some MTL macroinvertebrates, irrespective of kelp presence. Although conversely, larger reefs with restored kelp at natural – or even relatively low – densities appear critical to the recruitment of lobsters, which could motivate and provide positive feedback for kelp restoration projects in some locations. Patch dynamics may be used to support restoration efforts by helping to accelerate the recovery of key species and ecosystem services; however, trade‐offs will exist through different taxa responding to patch characteristics in different ways, some positive and some negative.

Selection of predictor variables for species distribution models: a case study with an invasive marine bryozoan.

Species distribution models (SDMs) are important tools for predicting the occurrence and abundance of organisms in space and time, with numerous applications in ecology. However, the accuracy and utility of SDMs can be compromised when predictor variables are selected without careful consideration of their ecophysiological relevance to the focal organism. We conducted an in-depth examination of the variable selection process by evaluating predictors to be used in SDMs for Membranipora membranacea, an ecologically significant marine invasive species with a complex lifecycle, as a case study. Using an information-theoretic and multi-model inference approach based on generalized linear mixed models, we assessed multiple environmental variables (depth, kelp density, kelp substrate, temperature, and wave exposure) as predictors of the abundance of multiple life stages of M. membranacea, investigating species-environment relationships and relative and absolute variable importance. We found that the relative importance of a predictor, the metric calculated to represent a predictor, and whether a predictor was proximal or distal were important considerations in the variable selection process. Data constraints (e.g. sample size, characteristics of available predictor data) may inhibit accurate assessment of predictor variables during variable selection. Importantly, our results suggest that species-environment relationships derived from small-scale studies can inform variable selection for SDMs at larger spatiotemporal scales. We developed a conceptual framework for variable selection for SDMs which can be applied to most contexts of species distribution modelling, but particularly those with several candidate predictors and a large dataset.

Southeast Alaskan kelp forests: inferences of process from large-scale patterns of variation in space and time.

Humans were considered external drivers in much foundational ecological research. A recognition that humans are embedded in the complex interaction networks we study can provide new insight into our ecological paradigms. Here, we use time-series data spanning three decades to explore the effects of human harvesting on otter–urchin–kelp trophic cascades in southeast Alaska. These effects were inferred from variation in sea urchin and kelp abundance following the post fur trade repatriation of otters and a subsequent localized reduction of otters by human harvest in one location. In an example of a classic trophic cascade, otter repatriation was followed by a 99% reduction in urchin biomass density and a greater than 99% increase in kelp density region wide. Recent spatially concentrated harvesting of otters was associated with a localized 70% decline in otter abundance in one location, with urchins increasing and kelps declining in accordance with the spatial pattern of otter occupancy within that region. While the otter–urchin–kelp trophic cascade has been associated with alternative community states at the regional scale, this research highlights how small-scale variability in otter occupancy, ostensibly due to spatial variability in harvesting or the risk landscape for otters, can result in within-region patchiness in these community states.

Different kelp species support unique macroinvertebrate assemblages, suggesting the potential community‐wide impacts of kelp harvesting along the Humboldt Current System

Abstract Kelp forests provide habitat for myriad species yet remain poorly studied in some ecoregions, including where there is increasing interest in commercial kelp harvesting for the lucrative alginate industry. To identify indicators for monitoring the impacts of kelp harvesting, the structure and composition of macrobenthic invertebrate assemblages associated with large brown macroalgae, Eisenia cokeri, Lessonia trabeculata and Macrocystis pyrifera, from the Humboldt Current System of Peru were determined. Within kelp habitats two distinct habitats were sampled: (a) adult kelp holdfasts; and (b) the benthos adjacent to the kelp sporophyte (hereafter under‐canopy rock). For each sample, organisms were identified, enumerated and weighed. Totals of 108, 102 and 113 different species were found associated with Eisenia cokeri, M. pyrifera and L. trabeculata, respectively comprising distinct assemblages associated with each kelp forest. Both habitat types (holdfasts and under‐canopy rock) supported diverse, but different, macroinvertebrate assemblages, with richness, abundance and functional trophic groups in general higher in holdfasts compared with on under‐canopy rock. Lastly, macroinvertebrate abundance, biomass, and species richness significantly increased with holdfast size. Results confirm that each kelp species is unique in terms of associated assemblage composition and indicate that kelp harvesting for the alginate industry may have large impacts. We suggest that these effects on associated biodiversity should be incorporated within kelp harvesting management plans. We suggest that holdfast diameter is a good ecological indicator as reductions in holdfast size across the population will indicate a decline in the diversity of macroinvertebrates that are important food sources for higher order organisms. We therefore suggest that monitoring studies include morphological measurements as well as measurement of kelp density.

Examining the influence of regional‐scale variability in temperature and light availability on the depth distribution of subtidal kelp forests

AbstractFoundation species play a disproportionate role in maintaining biodiversity and ecosystem functioning. Improved understanding of how environmental factors influence the distribution and population structure of foundation species therefore contributes to management and conservation of entire ecosystems. We surveyed subtidal kelp forests within four regions of the U.K., distributed over 9° of latitude and a mean sea temperature gradient of ~ 2.5°C. Our aims were: (1) to examine relationships between light availability and the structure and depth distribution of Laminaria hyperborea populations and (2) to determine whether depth‐related patterns were consistent across regions with different temperature regimes. We recorded marked depth‐related shifts in structure with decreasing light levels strongly correlated with declines in kelp density, cover, plant biomass, standing biomass, plant length, and age. We also recorded an effect of latitude; populations at our two colder, northernmost regions exhibited greater wet weight and length and higher standing biomass than populations in the warmer southern regions when under similar or even reduced light conditions, indicating an interactive effect of latitude, most likely related to temperature variability. We show that shifts in kelp population structure along depth gradients are strongly driven by light availability, although regional variability in the strength and nature of these relationships may be promoted by other factors such as temperature. Maximum depth penetration, standing biomass, plant density, and plant weight are useful indicators of light availability and, over time, could be monitored to detect changes in the quality of the overlying water column.