Caribbean seagrass beds are facing increasing anthropogenic stress, yet comprehensive ground-level monitoring programs that capture the structure of seagrass communities before the 1980s are rare. We measured the distribution of seagrass beds and species composition and abundance of seagrass and associated macroalgae and macroinvertebrates in 3 years over a 47-year period (1969, 1994, 2016) at Carriacou, Granada, an area not heavily impacted by local human activity. Seagrass cover and physical parameters of fringing beds were measured in transects at high (HWE) and low wave energy (LWE) sites; frequency of occurrence of all species, and biomass and morphology of seagrasses, were measured at 100 m2 stations around the island. Losses in nearshore seagrass cover occurred at HWE but not LWE sites between 1969 and 2016 and were associated with increases in the seagrass-free inshore zone (SFI) and erosional scarps within beds. Total biomass did not vary across years although there were progressive changes in seagrass composition: a decline in the dominant Thalassia testudinum and concomitant increase in Syringodium filiforme, and establishment of invasive Halophila stipulacea in 2016 at LWE sites. Species richness and diversity of the seagrass community were highest in 1994, when 94% of macroalgae (excluding Caulerpa) were most abundant, and sea urchins were least abundant, compared to 1969 and 2016. Multivariate statistical analyses showed differences in community composition across the 3 years that were consistent with trends in urchin abundance. Increases in SFI and scarp number in seagrass beds at HWE sites occurred mainly after 1994 and likely were related to increased wave forcing following degradation of offshore coral reefs between 1994 and 2016. Our observations suggest that landward migration of seagrass beds with rapidly rising sea level in future will not be realized in reef-protected seagrass beds at Carriacou barring reversal in the processes that have caused reef flattening.

Artificial reefs (ARs) have been used on coral reefs for ecological research, conservation, and socio-cultural purposes since the 1980s. We examined spatio-temporal patterns in AR deployment in tropical and subtropical coral reefs (up to 35° latitude) and evaluated their efficacy in meeting conservation objectives, using a systematic review of the scientific literature. Most deployments (136 studies) were in the North Atlantic and Central Indo-Pacific in 1980s – 2000s, with a pronounced shift to the Western Indo-Pacific in 2010s. Use of ARs in reef restoration or stressor mitigation increased markedly in response to accelerating coral decline over the last 2 decades. Studies that evaluated success in meeting conservation objectives (n = 51) commonly reported increasing fish abundance (55%), enhancing habitat quantity (31%) or coral cover (27%), and conserving target species (24%). Other objectives included stressor mitigation (22%), provision of coral nursery habitat (14%) or source populations (2%) and addressing socio-cultural and economic values (16%). Fish (55% of studies) and coral (53%) were the most commonly monitored taxa. Success in achieving conservation objectives was reported in 33 studies. Success rates were highest for provision of nursery habitat and increasing coral cover (each 71%). Increasing fish abundance or habitat quantity, mitigating environmental impacts, and attaining socio-cultural objectives were moderately successful (60–64%); conservation of target species was the least successful (42%). Failure in achieving objectives commonly was attributed to poor AR design or disruption by large-scale bleaching events. The scale of ARs generally was too small (m2 –10s m2) to address regional losses in coral cover, and study duration too short (< 5 years) to adequately assess ecologically relevant trends in coral cover and community composition. ARs are mostly likely to aid in reef conservation and restoration by providing nursery habitat for target species or recruitment substrate for corals and other organisms. Promoting local socio-cultural values also has potential for regional or global impact by increasing awareness of coral reef decline, if prioritized and properly monitored.

Population dynamics and life history traits of the ‘giant’ limpetScutellastra laticostataon intertidal limestone platforms at Rottnest Island, Western Australia, were recorded by interannual (January/February) monitoring of limpet density and size structure, and relocation of marked individuals, at 3 locations over periods of 13-16 yr between 1993 and 2020. Limpet densities ranged from 4 to 9 ind. m-2on wave-swept seaward margins of platforms at 2 locations and on a rocky notch at the landward margin of the platform at a third. Juvenile recruits (25-55 mm shell length) were present each year, usually at low densities (<1 m-2), but localized pulses of recruitment occurred in some years. Annual survival rates of marked limpets varied among sites and cohorts, ranging from 0.42 yr-1at the notch to 0.79 and 0.87 yr-1on the platforms. A mass mortality of limpets on the platforms occurred in 2003, likely mediated by thermal stress during daytime low tides, coincident with high air temperatures and calm seas. Juveniles grew rapidly to adult size within 2 yr. Asymptotic size (L∞, von Bertalanffy growth model) ranged from 89 to 97 mm, and maximum size from 100 to 113 mm, on platforms. Growth rate and maximum size were lower on the notch. Our empirical observations and simulation models suggest that these populations are relatively stable on a decadal time scale. The frequency and magnitude of recruitment pulses and high rate of adult survival provide considerable inertia, enabling persistence of these populations in the face of sporadic climatic extremes.

Amoebae (20-30 pm) are identified in tissues of echinoids (Strongylocentrotus droebachiensis) showing symptoms of disease. The amoebae have a parasome and are morphologically similar to the genus Paramoeba. They are distributed widely but in low density in body wall, water vascular system, nerves and gut in individuals in intermediate or late stages of disease. They are less common in individuals in early stages of disease, and have not been found in healthy echinoids. The amoebae ingest cell debris in degenerating tissue, but it is not known whether they are pathogenic or are secondary invaders in diseased echinoids. Affected echinoid tissues are infiltrated by coelomo-cytes during the course of the disease, but no specific response by coelomocytes to amoebae has been observed.

Strongylocentrotus droebachiensis, the green sea urchin, has a broad Arctic-boreal distribution and is commonly associated with laminarian kelp. As an omnivorous grazer, its feeding capabilities and preferences have profound effects on the structure and dynamics of benthic communities. It exhibits an annual reproductive cycle and has planktrophic larval development. Growth and reproductive rates are largely dependent on the quantity and quality of available food. Larval behavior can influence patterns of dispersal in the plankton and settlement on the seabed, but the importance of predation or other agents of mortality at early life history stages is poorly understood. Fish and decapod crustaceans are major predators of larger juveniles and adults and may play an important role in population regulation. Strongylocentrotus droebachiensis is susceptible to acute and chronic infections by microbial pathogens and parasitic nematodes. In the northwest Atlantic, mass mortality during outbreaks of an amoebic disease can have profound impacts on sea urchin populations and ecosystem state. Green sea urchins have been extensively fished or cultured for roe since the late 1980s. Aquaculture research is contributing new knowledge of the nutritional and reproductive physiology of the species.

Evaluating the efficacy of artificial structures in enhancing or sustaining biodiversity on tropical coral reefs is key to assessing their role in reef conservation or management. Here, we compare spatial and temporal patterns of colonization and succession of the benthic assemblage on settlement collectors (ceramic tiles) in a 13-mo mensurative experiment on a suspended artificial reef, a seafloor artificial reef, and two nearby natural reefs at Eilat, Gulf of Aqaba. We also conducted a concurrent 7-mo manipulative experiment on the suspended reef and one of the natural reefs, and monitored fish feeding behaviour on experimental collectors, to examine effects of large mobile consumers on these patterns. In both experiments, taxonomic composition as percent planar cover for the whole community or biomass for the invertebrate component differed between collector topsides, dominated by a filamentous algal matrix, and shaded undersides with a profuse assemblage of suspension- or filter-feeding invertebrates. In the mensurative experiment, we found differences in final community and invertebrate composition between sites, which clustered according to reef type (artificial vs. natural) for collector undersides. Invertebrate biomass was greater at both artificial reefs than at one (undersides) or both (topsides) natural reefs. In the manipulative experiment, we found similar differences in composition between sites/reef types as well as between treatments (exclusion vs. control), and the invertebrate biomass was greater on the artificial reef. Invertebrate biomass was greater in the exclusion treatment than the control on collector undersides, suggesting mobile consumers can affect community composition and abundance. Predominant fish species observed interacting with collectors differed between artificial and natural reefs, likely contributing to differences in patterns of colonization and succession between sites and reef types. Our findings suggest artificial reefs have the potential to enhance cover and biomass of certain reef-associated assemblages, particularly those occupying sheltered microhabitats.

The pathogenic amoeba Paramoeba invadens causes recurrent mass mortalities of green sea urchins Strongylocentrotus droebachiensis in coastal Nova Scotia, Canada, driving regime shifts from urchin barrens to kelp beds. Outbreaks of the disease (paramoebiasis) are sporadic, and the source population(s) and epizootiology of the amoeba are poorly understood. We developed PCR-based detection of P. invadens in urchin tissue, sediment, and seawater. Primers specific to the P. invadens nuclear SSU rRNA gene were designed and used in PCR and qPCR analyses to better detect and quantify P. invadens during, following, and in the absence of a natural disease outbreak. A comparison of pathogen load in asymptomatic and symptomatic sea urchins indicated a lower threshold of similar to 1 cell mg(-1) tissue for observing overt signs of paramoebiasis in urchins. P. invadens was detected for the first time in sediment during and following an outbreak of disease in 2014. It also was detected in low abundance (<10 cells 1(-1)) in seawater in fall 2015 in the absence of sea urchin mass mortality or a strong storm event, but not under similar conditions in summer/fall 2016 and 2017. The ability to detect and quantify this pathogen in sea urchins and environmental samples sheds new light on mechanisms of introduction, spread, and persistence of P. invadens along the Nova Scotian coast and the role of large-scale meteorological events and ocean warming in these processes.

Declines in kelp abundance over the past 3 decades have resulted in a shift from luxuriant kelp beds to extensive mats of turf-forming algae in Nova Scotia, Canada. With the reduced availability of open rocky substrate, kelps are increasingly recruiting to turf algae. At 3 sites near Halifax, we found that turf-attached kelp Saccharina latissima was generally restricted to smaller size classes (<50 cm length) than rock-attached kelp at 12 m depth. Turf-attached kelp allocated a greater proportion of biomass to the holdfast (anchoring structure), which differed morphologically from that of rock-attached kelp and had lower attachment strength. To assess how these differences affect survival, we monitored kelp in 2 m diameter plots at 11 m depth over 40 wk at 1 site. Smaller kelps were predominantly turf-attached and larger ones rock-attached in late summer and autumn, but there was near-complete loss of both turf-and rock-attached kelp over winter when wave action was greatest. In a concurrent manipulative experiment at 5 m depth at another site, we transplanted small boulders with turf-or rock-attached kelp to a wave-exposed or protected location. Survival was greater for rock-attached transplants at both locations after 12 wk, with a complete loss of turf-attached kelp in the wave-exposed treatment. Classification based on holdfast morphology showed that 76% of drift kelp within a depositional area at this site was once turf-attached. Low survival of kelps that recruit to turf algae, likely due to wave dislodgement, may represent an important feedback that increases resilience of a turf-dominated state and prevents reestablishment of kelp.

Low recruitment due to limitations of propagule supply or post-settlement survival reinforces dominance of turf algal assemblages that replace canopy algae following large-scale losses. However, post-recruitment processes that hinder juvenile growth and survival (epiphytic overgrowth, grazing, physical stress) also could impede recovery. To evaluate the contribution of recruitment, growth, and survival of young sporophytes to recovery of degraded kelp populations and key factors driving post-recruitment tissue loss and mortality, we followed cohorts of juvenile kelp Saccharina latissima at two defoliated sites in Nova Scotia. We also monitored kelp recruitment, abundance, size structure, and macroalgal composition for 5.5 years. Recruit densities were an order of magnitude lower compared to previous studies in the region. Large decreases in blade area of juveniles were related to cover by the invasive bryozoan Membranipora membranacea, grazing by small snails Lacuna vincta, and warm seawater temperatures. Cohort survival was low (time to 50% mortality 2.5–5.5 months) and increased risk of death was directly related to bryozoan encrustation. Modest seasonal or interannual gains in kelp abundance were lost during periods of peak temperature, which showed a warming trend during the study, favouring persistence of widespread turf-forming, opportunistic and invasive algae. We conclude that low recruitment success, high rate of tissue loss relative to growth, and high mortality, inhibited kelp recovery. Impacts of epiphytic overgrowth, grazing, and warm temperatures on these processes highlight the need to protect intact kelp populations, growing in favourable conditions, to maintain positive interactions that increase resilience to undesirable regime shifts.

The invasive seagrass Halophila stipulacea has spread throughout the eastern Caribbean since it was first recorded in Grenada in 2002. We quantified the distribution and abundance of H. stipulacea, and its associated macro invertebrate fauna, in sampling stations and transects around the island of Carriacou (a nearby dependency of Grenada) in early 2016. Halophila stipulacea occurred in extensive monospecific stands (average bottom cover, 62%), or smaller mixed stands with native seagrass (Thalassia testudinum, Syringodium filiforme, Halodule wrightii), at 1-5 m depth in large bays along the leeward (west) coast. It was sparsely distributed on the more wave-exposed east and south coasts, usually in mixed patches with native seagrass. In leeward bays, H. stipulacea has largely replaced the native seagrass H. wrightii, providing a novel biogenic habitat for various filter-feeding invertebrates living within the turf-like leaf canopy (e.g., sponges, ascidians, bivalves, ophiuroids), and sea urchins (mainly Tripneustes ventricousus) and a microphagous sea star (Oreaster reticulatus) that graze upon it. Populations of the sea star consisted mainly of juveniles indicating the seagrass may serve as a nursery habitat for this endangered species. The spread of H. stipulacea along the leeward coast of Carriacou in recent years represents a community-level shift in the shallow subtidal zone, with attendant changes in habitat structure, species composition, and trophic interactions.

Understanding processes that drive sudden shifts in ecosystem structure and function has become an important research focus for coastal management. In kelp bed ecosystems, regime shifts occur when high densities of sea urchins destructively graze kelp and create coralline algal barrens. While the importance of predation and disease in mediating shifts between kelp beds and barrens on shallow rocky reefs has been well documented, little is known about the role of deep-living urchins in these alternative stable-state dynamics. In this study, we test the hypothesis that deep-living urchins along the central Atlantic coast of Nova Scotia move onshore and trigger shifts from kelp beds to barrens on shallow rocky reefs. We documented urchin distribution and abundance using tow-camera surveys down to 140 m depth and spanning 140 km of coast and created a predictive species-distribution model using these observations and spatial data on environmental factors that likely delineate suitable habitat for urchins. We used a random forest model to generate our predictions, which correctly classified 91% of observations into a positive or negative occurrence of urchins. Sea urchins predominantly occurred within 1.5 km of shore, in depressions and flat habitats between 40 and 85 m depth. We found that shallow regions where destructive grazing fronts have been documented over the past four decades were closer to deep-living sea urchin habitats compared to regions that remained in a kelp bed state during the same period. This supports our prediction that deep-living urchins play an important role in driving shallow regime shift dynamics, and indicates that their distribution can help identify areas of coast that are most vulnerable to a collapse to barrens.

The movement of resource subsidies across natural systems can have important effects on recipient communities and has emerged as a key research area in ecology. Detrital subsidies are critical in marine ecosystems where communities are reliant on external sources of primary production, yet few studies have quantified the spatial extent of drift algae at coastal scales. Using observations of the seafloor (up to 140 m depth) from tow-camera surveys along 145 km of Nova Scotia coast, and bathymetric data of this region, we created the first predictive map of drift subsidy in a marine ecosystem. We used a random forest model to generate our predictions, which correctly classified 95 % of observations into a presence or absence of drift. Distance from source, slope, and bathymetric position index (elevation relative to surrounding landscape) was the main predictor variables of the occurrence of drift. Drift algae occurred across a range of benthic habitats within our study area, but most frequently within 1.4 km of the coast on flat bottoms or in regions with zero or negative bathymetric position index. Such areas were coincident with seafloor depressions and flat low-energy habitats. Repeated observations at some locations indicated that areas with steep slopes or large curvature tended to have variable patterns of drift compared to areas with little or no slope or curvature. We predict that deep subtidal environments receiving drift subsidy will be impacted by the declines in kelp biomass projected for this region (and others) due to changes in ocean climate.

Along the Atlantic coast of Nova Scotia sea urchin disease outbreaks are statistically linked to North Atlantic hurricanes and warm sea temperatures. The amoebic pathogen Paramoeba invadens, which causes these disease outbreaks, is unable to withstand typical minimum sea temperatures along this coast, suggesting that it is reintroduced during periods of peak temperatures. Here, we examine hypotheses for mechanisms of introduction or persistence of P. invadens using data on sea urchin disease outbreaks from a 5-yr field experiment (2010-2014), in combination with high-temporal-resolution oceanographic and meteorologic data. Disease outbreaks were observed in 4 yr, with the onset of mass mortality (>50% morbidity or mortality) ranging from mid August to mid October. Physical data suggest that P. invadens originates in warm offshore surface waters that are transported to the coast during strong storms. Disease outbreaks were associated with passing hurricanes in 2 yr (2010 and 2011). In 2012, a disease outbreak occurred in the absence of a storm and following a strong positive anomaly in winter sea temperature, suggesting survival of the pathogen from 2011. In 2014, a disease outbreak occurred in association with a strong storm (nor'easter) that was not a hurricane. Our findings indicate that changing environmental conditions of increasing minimum sea temperatures and increasing intensity of storms may be altering the dynamics of this disease. These inferences remain equivocal, however, indicating the need for high-resolution dynamical modeling of the physical environment and rapid identification of P. invadens in the environment using genetic tools.

Small invertebrate grazers can disproportionately reduce plant fitness by discriminately consuming valuable tissues, but the context and attendant consequences of this activity at higher levels of ecological organization rarely are considered. To assess the impact of a gastropod mesograzer Lacuna vincta on fecundity and potential reproductive output of the habitat-forming kelp Saccharina latissima, we measured the intensity and distribution of grazing damage on kelp blades at five sites of varying kelp density, during the annual reproductive peak (October-November) in Nova Scotia. We found most grazing damage on reproductive individuals consisted of superficial excavations, and was concentrated on the central sorus (region where sporangia develop) compared to the vegetative blade margins. Grazing intensity on sori (percent grazed) averaged 29.6% across sites and sampling periods. The distribution of grazing on non-reproductive individuals was opposite to that of reproductive ones, indicating that snails shift feeding from blade margins to the center as sori develop. Choice and no-choice feeding assays in the laboratory revealed that focused grazing on sori is likely due to an active feeding preference for sporogenous over vegetative tissue. This preference was correlated with the distribution of chemical defense between tissues (phlorotannin content was ~2.5 times higher in vegetative tissue than sori), but not nutritional quality (no difference in C/N ratio). We deduce, with support from histological observations, that consumption of sorus tissue by L. vincta reduces fecundity of S. latissima. Extrapolating our results to estimate potential reproductive output within kelp beds suggests that spore supply and recruitment limitation may be predominantly imposed by the scarcity of reproductive individuals in the most degraded kelp beds. However, loss of reproductive output to grazing could extend recruitment limitations that impede recovery of waning kelp populations in Nova Scotia.

During mass spawning of tropical corals, extremely dense accumulations of eggs develop throughout the water column, the fate of which depends on physical conditions, such as topography, currents, and prevailing winds. We measured egg transport [mainly acroporid corals and the palolo worm, Eunice viridis (Gray, 1847)] during a mass spawning at Ningaloo Reef, Western Australia, in April 2010. We tracked changes in egg and embryo abundance in the lagoon over five consecutive days with plankton tows at the sea surface immediately after spawning and four and 16 hours later, and characterized the flow regime with drifter deployments. Egg dispersal was rapid with a pronounced decrease in concentration within four hours of spawning, from approximately 106 to 101 and 104 m–3 for corals and E. viridis, respectively, on the peak date; few eggs or embryos remained within the lagoon after 16 hrs. Changes in concentration of eggs and embryos along transects inside the lagoon and perpendicular to the reef flat reflected the local hydrodynamics and high flushing rate, providing empirical support for a biophysical model from the same site.

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 543:141-152 (2016) - DOI: https://doi.org/10.3354/meps11554 Large-scale degradation of a kelp ecosystem in an ocean warming hotspot Karen Filbee-Dexter1,*, Colette J. Feehan1,2, Robert E. Scheibling1 1Department of Biology, Dalhousie University, Halifax, NS, B3H4R2, Canada 2Present address: Friday Harbor Laboratories, University of Washington, Friday Harbor, WA 98250, USA *Corresponding author: kfilbeedexter@gmail.com ABSTRACT: Understanding the impacts of climate change on biological systems requires observational data over multi-decadal time spans and broad spatial scales. Extensive research at an ocean warming hotspot off Nova Scotia, Canada, enabled us to evaluate the impact of 3 decades of observed temperature rise on a coastal marine ecosystem. Here, we document changes in the kelp community from sites monitored since 1949, 1968 and 1984, and from coastal surveys in 1982, 2000, 2007 and 2014. We show that mean kelp biomass has declined by 85-99% over the past 4-6 decades, and a catastrophic phase shift has occurred from luxuriant kelp beds to rocky reefs dominated by opportunistic turf-forming and invasive algae. This shift likely represents a persistent change, driven by multiple biotic and abiotic interactions, with positive feedback mechanisms (e.g. sediment accumulation) that stabilize the invasive/turf-algal state. This study is the first to show multi-decadal declines in kelp related to warming temperatures in the Northwest Atlantic. The large-scale degradation of an important coastal ecosystem within a warming hotspot presents a troubling example of the instability of marine systems in a rapidly changing ocean environment. KEY WORDS: Kelp beds · Turf algae · Invasive seaweeds · Climate change · Phase shift · Rocky reefs · Seawater temperature Full text in pdf format Supplementary material PreviousNextCite this article as: Filbee-Dexter K, Feehan CJ, Scheibling RE (2016) Large-scale degradation of a kelp ecosystem in an ocean warming hotspot. Mar Ecol Prog Ser 543:141-152. https://doi.org/10.3354/meps11554 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 543. Online publication date: February 03, 2016 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2016 Inter-Research.

Predicting the effect of climate change on communities requires an understanding of the effects of environmental conditions on species and their interactions. We investigated the potential for warming seawater temperature to modify the interactions of the gastropod mesograzer Lacuna vincta and the invasive bryozoan Membranipora membranacea with kelps in Nova Scotia. The nutritional content (C/N) of the kelps Saccharina latissima, Laminaria digitata and Agarum clathratum were unaffected by temperature (11, 18 and 21 degrees C), and chemical defenses (phlorotannins) were reduced only in A. clathratum after 1 wk exposure to 21 degrees C. C/N and phlorotannin content increased over the season in S. latissima collected monthly in summer 2013 and 2014. The effect of temperature-induced changes in kelp on the grazing of L. vincta was assessed using feeding experiments with S. latissima pretreated at 11 or 21 degrees C. Snails consumed more kelp pretreated at 21 degrees C only when grazing rate was high. The quality of S. latissima as a food source for L. vincta was not affected by temperature, as diets of kelp pretreated at 11 and 21 degrees C supported similar growth, reproduction, and survival of snails. Temperature also did not affect the quality of kelp as a substrate for M. membranacea, since settlement rates were not different between S. latissima pretreated at ambient temperature (9 to 14) and 21 degrees C. The absence of temperature-induced changes in kelp quality suggests that the effects of L. vincta and M. membranacea will act additively with the direct effects of temperature and cause increased biomass loss from kelp beds.

We examined how large-scale distur- bances that defoliate kelp beds (outbreaks of an invasive bryozoan, hurricanes) alter local-scale grazing dynamics of an abundant herbivore, the gastropod Lacuna vincta, on the Atlantic coast of Nova Scotia, Canada. From field observations and a 5 wk kelp-thinning experiment that simulated dis- turbance, we found that snail density and grazing intensity on the kelp Saccharina latissima increased non-linearly with decreasing kelp biomass, as it varied within a site. Grazing intensity on S. latissima also increased non-linearly with decreasing stand- ing kelp biomass across 5 sites spanning 40 km (linear distance) of coast and 2 yr, but we did not find strong support for this relationship for the kelp Laminaria digitata. Intensification of grazing aug- ments the indirect effect of L. vincta on S. latissima (increased blade erosion and fragmentation), and drives it beyond a threshold for further losses of kelp biomass with subsequent storms. This positive feedback between large-scale disturbances and local-scale grazing could reinforce the depletion of kelp and facilitate the establishment of turf-forming algae on Nova Scotian rocky reefs. We conclude that interactions of large external perturbations with local natural perturbations must be considered to under stand how drivers of ecosystem change collec- tively disrupt the balance of top-down and bottom- up forces to cause shifts to unexpected community states.

Recent declines and losses of highly productive and diverse kelp beds have been observed worldwide and linked to increases in ocean temperature. We investigated the impacts of 4 temperature treatments (11, 14, 18 and 21 degrees C) on growth, net length change and mortality of the dominant kelp species in Nova Scotia: Saccharina latissima, Laminaria digitata and Agarum clathratum. Growth rates of A. clathratum were reduced at 18 degrees C over 3 wk of exposure, and all species experienced negative net changes in length at this temperature. Exposure to 21 degrees C led to tissue loss at least twice that observed at 11 degrees C and mortality within the first 2 wk of exposure. Exposure to 21 degrees C for 1 wk reduced blade tissue strength (breaking stress) and extensibility (breaking strain) by 40 to 70% in S. latissima and L. digitata, and all 3 species exhibited reduced strength after 3 wk exposure to 18 degrees C. Histological examination of the blade tissue showed temperature-induced damage to the cellular structure of blades of S. latissima and L. digitata. A. clathratum displayed limited tissue damage and was less susceptible to temperature-induced tissue weakening and loss. Our findings provide a mechanism by which rising temperatures could contribute to observed population declines of kelp species.