Inundations of pelagic Sargassum plague the tropical Atlantic, with size and impacts steadily increasing to surpass 30 million tons in 2025. Understanding the drivers of Sargassum growth in the so-called Great Atlantic Sargassum Belt is fundamental to developing effective mitigation strategies for affected nations. We present a nonlinear regression model that both explains the seasonal and interannual variability observed between 2011 and 2022 and predicts Sargassum concentrations in 2023 and 2024. The growth of Sargassum, initiated by a prolonged negative phase of the North Atlantic Oscillation, is initially enhanced through winter mixed layer deepening in response to stronger winds. An additional overlooked driver is the recycling of nutrients within the mixed layer, carried out by the community of organisms associated with Sargassum and aging Sargassum mats. This contribution increases over time to become dominant in recent years, offsetting the increase in stratification in 2023 and 2024.

Detritivorous sea cucumbers are important processors of reef sediments and have been hypothesized to enhance coral growth by altering local nutrient availability and seawater chemistry, but whether these effects translate into improved coral performance remains untested. We conducted two analogous field experiments in Mo’orea, French Polynesia, to test whether the sea cucumber Holothuria atra enhances coral growth or photosynthetic efficiency. In the first experiment, we enclosed Acropora pulchra with zero versus two sea cucumbers per 0.25 m2, approximating natural densities at our site, and found no detectable effect of sea cucumber presence on growth. In 2025, using densities of four per 0.25 m2 and enclosing three coral species, A. pulchra growth was 41

Sharks play important roles in marine ecosystems but have experienced mass overexploitation during recent decades. Despite their ecological significance, the trophic ecology of certain species remains poorly known in many regions. We conducted bulk carbon stable isotope analysis and compound-specific stable nitrogen isotope analysis of amino acids in shark vertebral tissue to elucidate the ontogenetic trophic and spatial shifts for 2 large oceanic predators, the scalloped hammerhead Sphyrna lewini and the great hammerhead S. mokarran , off the southeastern USA and in the eastern Gulf of Mexico (federally recognized in the USA as the Gulf of America). S. lewini occupies a higher trophic position throughout its life and more offshore pelagic habitats than S. mokarran , which occupies a lower trophic position and generally relies more on benthic and inshore habitats. Both species broadly increase their trophic positions with age, and S. mokarran demonstrates more spatial variability than S. lewini throughout most of its life. S. lewini juveniles spend more time offshore than their adult counterparts, while the opposite is true of S. mokarran . This study elucidates the nuances of the trophic ecology of these predators and finds no evidence of within-species differences in sex or location with regard to trophic position or spatial habitat use.

Biodiversity loss threatens ecosystems worldwide, but we lack a thorough understanding of how genotypic diversity-as opposed to species diversity-impacts threatened foundation species, such as corals on tropical reefs. Corals of the genus Pocillopora have been important to natural reef recovery and human-assisted restoration in the Pacific. They consistently perform better in polycultures with other coral species than in monocultures, but whether genotypic diversity within a species enhances performance remains untested. We assessed whether increased genotypic diversity of the coral Pocillopora verrucosa (ie the number of genetically distinct source colonies per plot) improved growth, reduced tissue mortality, or increased resistance to invasion of competing seaweeds over 3.5 years in experimental plots on reefs in Mo'orea, French Polynesia. We could detect no significant differences in any of these performance metrics among plots comprised of one, three, six, or nine genotypes. Moreover, when we quantified mitochondrial genetic divergence among genotypes within each plot using mean pairwise nucleotide distance, performance was unrelated to the degree of genetic differentiation represented. Thus, we detected no effect of genotypic diversity on Pocillopora performance. Pocillopora acclimates rapidly to variable physical and biological conditions, which may make genotypic diversity effects less critical for these corals.

As human impacts on ecosystems accelerate, understanding how shifts in resource quality alter animal behaviors that structure communities are becoming increasingly important. Nutrient pollution is especially important for oligotrophic ecosystems such as coral reefs, where small changes in nutrient inputs can impact production and biotic interactions. Here, we conducted a before-after control-impact experiment to examine the effects of short-term nutrient addition on the behavior of the damselfish Stegastes nigricans, a common ecosystem engineer on Pacific coral reefs. We added fertilizer to treatment versus control territories of this fish and quantified changes in territory size, aggression, and grazing behavior. After two weeks, fertilizer-enrichment decreased territory size by 18 ± 5

For five decades, Jim A. Estes studied sea otters in the Aleutian archipelago of Alaska, discovering how otters structured entire communities. By consuming sea urchins, otters released kelp from herbivory, kelp beds flourished, and kelps sheltered a diversity of fishes, and invertebrates. When otters were extirpated by paleohumans, modern humans, or killer whales, urchins proliferated, reduced kelp forests to "urchin barrens," and species-rich assemblages of fishes and invertebrates were lost. These losses affected seals, eagles, sea gulls, fishes, and sea stars that depended on these prey as well as adjacent habitats that had been nourished by exported kelp production. Jim's research became the poster child for keystone species, trophic cascades, and the critical role of predators in structuring ecosystems. Similar discoveries followed in freshwaters, forests, and grasslands.

The utility of passive vs. active coral restoration continues to be debated as reefs decline worldwide. Here, we evaluated the efficacy of coral transplantation into the degraded East and Center lagoons of Palmyra Atoll. Corals have failed to recover in these sites over the eight decades since disturbances associated with World War II, despite high coral cover elsewhere around the atoll that could theoretically provide source propagules. We explicitly compared (i) species beginning to recolonize the lagoons with species common elsewhere at Palmyra, (ii) performance between the East and Center lagoons, and (iii) coral growth at sites near vs. far from causeway inlets as a proxy for the benefits of flow. We found that six common coral species were all physiologically capable of growing in the lagoon, but there were: i) large among-species difference in survival, ii) less, but still significant, differences in growth among species, and iii) localized differences in growth and survival across the eight test locations. Many of these differences appeared to be driven by patterns in fish predation on corals. Survival was greatest for Porites and Pavona species, neither of which have substantially colonized the lagoons. Their superior performance relative to Acropora and Pocillopora species that have begun to recolonize the East Lagoon suggests that transplantation of hardier Porites and Pavona species may accelerate recovery. Coral reef restoration efforts often focus on more threatened and fragile corals like Acropora and Pocillopora. Prioritizing the initial planting of hardier corals like Porites and Pavona may help establish foundational reef functions before introducing more fragile species.

Sea cucumbers have been overharvested world-wide, making assessments of their ecological effects challenging, but recent research demonstrated that sea cucumbers increased coral survival via disease suppression and were therefore important for facilitating reef health. The mechanisms underpinning the sea cucumber-coral interaction are not well understood but are likely mediated through sea cucumber grazing of microbes from reef sediments. We explored how sea cucumber grazing alters the sediment microbiome by leveraging a healthy sea cucumber population on a reef in French Polynesia. We used quantitative PCR, 16S rRNA gene sequencing, and shotgun metagenomics to compare the sediment microbiome in cages placed in situ with or without sea cucumbers. We hypothesized that grazing would lower microbial biomass, change sediment microbiome composition, and deplete sediment metagenomes of anaerobic metabolisms, likely due to aeration of the sediments. Sea cucumber grazing resulted in a 75% reduction in 16S rRNA gene abundances and reshaped microbiome composition, causing a significant decrease of cyanobacteria and other phototrophs relative to ungrazed sediments. Grazing also resulted in a depletion of genes associated with cyanotoxin synthesis, suggesting a potential link to coral health. In contrast to expectations, grazed sediment metagenomes were enriched with marker genes of diverse anaerobic or microaerophilic metabolisms, including those encoding high oxygen affinity cytochrome oxidases. This enrichment differs from patterns linked to other bioturbating invertebrates. We hypothesize that grazing enriches anaerobic processes in sediment microbiomes through removal of oxygen-producing autotrophs, fecal deposition of sea cucumber gut-associated anaerobes, or modification of sediment diffusibility. These results suggest that sea cucumber harvesting influences biogeochemical processes in reef sediments, potentially mediating coral survival by altering the sediment microbiome and its production of coral-influencing metabolites.

Seaweed–coral competition is increasingly important as reef communities degrade, with algal turfs being the most common competitor. However, experiments assessing the impacts and mechanisms involved in turf–coral competition under field conditions are rare. We evaluated turf–coral interactions and their impacts relative to those of macroalgae by placing corals (Acropora pulchra and Porites rus) in contact with turf communities from territories of two species of damselfishes, with two common macroalgae, and with inert algal mimics as physical controls. After 13 d, turfs reduced coral photosynthesis by 31–59

As coral reefs degrade worldwide, researchers and managers need to determine whether corals can acclimatize to altered local conditions or whether their fixed phenotypes prevent coral persistence under these new environmental conditions. Fixed phenotypes could produce environmental mismatches that reduce population connectivity and exacerbate decline in the near-term, but a capacity for acclimatization could be harnessed in both passive and proactive coral restoration efforts. Here, we conducted a reciprocal transplant experiment in Mo'orea, French Polynesia, to test how intraspecific performance of 2 common coral species (Acropora hyacinthus and Pocillopora verrucosa) varied between a neighboring forereef and backreef that differed dramatically in trajectories of coral loss, resilience over decadal time scales, and cover of corals versus competing macroalgae. We also tested how corals responded to 2 common stressors-corallivory and macroalgal competition-and how this varied as a function of transplant location and the area of origin. Growth and survival of both coral species were affected by macroalgal competition, corallivory, transplant location, or some combination thereof, but we found limited evidence that the habitat of origin significantly impacted intraspecific performance. These results suggest that acclimatization capacity may outweigh local adaptation for these common reef-building species and could be leveraged to facilitate coral restoration.

Hundreds of studies now document positive relationships between biodiversity and critical ecosystem processes, but as ecological communities worldwide shift toward new species configurations, less is known regarding how the biodiversity of undesirable species will shape the functioning of ecosystems or foundation species. We manipulated macroalgal species richness in experimental field plots to test whether and how the identity and diversity of competing macroalgae affected the growth, survival, and microbiome of a common coral in Mo'orea, French Polynesia. Compared to controls without algal competitors, coral growth was significantly suppressed across three macroalgal monocultures, a polyculture of the same three macroalgae, and plots containing inert seaweed mimics; coral mortality was limited and did not differ significantly among treatments. One macroalga suppressed coral growth significantly less than the other two, but none differed from the inert mimic in terms of coral suppression. The composition, dispersion, and diversity of coral microbiomes in treatments with live macroalgae or inert plastic mimics did not differ from controls experiencing no competition. Microbiome composition differed between two macroalgal monocultures and a monoculture versus plastic mimics, but no other microbiome differences were observed among macroalgal or mimic treatments. Together, these findings suggest that algal diversity does not alter harmful impacts of macroalgae on coral performance, which could be accounted for by physical structure alone in these field experiments. While enhancing biodiversity is a recognized strategy for promoting desirable species, it would be worrisome if biodiversity also enhanced the negative impacts of undesirable species. We documented no such effects in this investigation.

Coral reefs are in global decline with coral diseases playing a significant role. This is especially true for Acroporid corals that represent ~25% of all Pacific coral species and generate much of the topographic complexity supporting reef biodiversity. Coral diseases are commonly sediment-associated and could be exacerbated by overharvest of sea cucumber detritivores that clean reef sediments and may suppress microbial pathogens as they feed. Here we show, via field manipulations in both French Polynesia and Palmyra Atoll, that historically overharvested sea cucumbers strongly suppress disease among corals in contact with benthic sediments. Sea cucumber removal increased tissue mortality of Acropora pulchra by ~370% and colony mortality by ~1500%. Additionally, farmerfish that kill Acropora pulchra bases to culture their algal gardens further suppress disease by separating corals from contact with the disease-causing sediment—functioning as mutualists rather than parasites despite killing coral bases. Historic overharvesting of sea cucumbers increases coral disease and threatens the persistence of tropical reefs. Enhancing sea cucumbers may enhance reef resilience by suppressing disease.

Overfishing is a worldwide occurrence that simplifies marine food webs, changes trophic patterns, and alters community structure, affecting not only the density of harvested species but also their trophic function. The northwestern Atlantic has a history of heavy fishing, and over the past century has also experienced destructive bottom fishing and harmful mobile fishing gear. After confirming that preservation solvent did not alter the nitrogen stable isotopes of preserved samples, we used museum specimens and modern samples to analyze nitrogen stable isotopes in tissues of two common demersal fishes pre-1950 (1850 to 1950) compared to 2021 to assess changes in trophic positions of coastal New England consumers over this time period. Both the mesopredator Centropristis striata (black sea bass) and the benthivore Stenotomus chrysops (scup) experienced significant declines in trophic position during this time. C. striata declined almost a full trophic level, S. chrysops declined half a trophic level, and these species are now occupying almost the same trophic position. Heavy fishing activities potentially shorten food chains, simplify trophic complexity, lessen the separation of trophic niches, and generally flatten food webs. The consequences of these within-species shifts are poorly investigated but could generate underappreciated cascading impacts on community structure and function. Archived natural-history collections are an invaluable resource for investigating ecological changes in natural communities through time. The evaluation of changing trophic positions via stable isotope analysis may allow fisheries managers to quantify large-scale effects of fishing on ecosystems and food webs over time.

A mass sea urchin die-off in the Caribbean Sea in the 1980s resulted from a single-cell protist called a scuticociliate.

Coral reefs are undergoing precipitous decline due to coral bleaching and disease following warming events, with impacted reefs often shifting from coral to macroalgal dominance. We reciprocally transplanted three common coral species between two pairs of coral-dominated marine-protected areas (MPAs) and adjacent macroalgal-dominated fished areas to test for the effects of reef origin and transplant area on coral defense against the common coral pathogen Vibrio coralliilyticus. For the ecologically sensitive species Acropora millepora, both reef origin and transplant area influenced the potency of defense, but for the ecologically hardy coral Porites cylindrica or the weedy coral Pocillopora damicornis, potency was not altered. A. millepora colonies that originated from, or were transplanted into, coral-dominated MPAs exhibited a 46% and 38% increase, respectively, in inhibition of V. coralliilyticus relative to those that originated from, or were transplanted into, macroalgal-dominated fished areas. A. millepora also exhibited reef origin effects on its microbial community composition, notably with persistently higher relative abundances of Vibrionaceae among individuals that originated from macroalgal-dominated fished areas compared to individuals that originated from coral-dominated MPAs. For ecologically important but disease and bleaching susceptible species like acroporids, macroalgal-dominated reefs may suppress coral defense against Vibrio pathogens and facilitate blooms of Vibrio bacteria that may harm corals during periods of thermal stress. However, their defense against Vibrio coralliilyticus recovered when not subjected to degraded reefs dominated by macroalgae.

The negative effects of warming temperatures on coral physiology are well-documented. However, research increasingly suggests that these effects are variable and that the degree to which corals are impacted by higher temperatures are dependent on a range of environmental and physiological variables. In tropical corals, which live near their upper thermal limits, these disparities have led to significant differences in bleaching and other negative health effects even over relatively small spatial scales. However, the response of temperate corals across small spatial scales is less well understood despite their occurrence in areas with larger temperature ranges than their tropical counterpart. To determine whether responses to thermal stress (27.4 versus 29 degrees C) differed in a common temperate coral species across a small geographic area, as has been documented in tropical corals, we evaluated coral growth and photosynthetic efficiency (Fv/Fm) of the coral Oculina arbuscula over the course of eight weeks when grown at differing temperatures. When measured as buoyant mass (calcification), growth of O. arbuscula did not differ as a function of temperature or site of collection. In contrast, when measured as change in tissue mass or total wet mass, elevated temperature suppressed growth. Change in total wet mass also varied significantly by collection site, corals from some sites experiencing little if any growth and those from other sites experiencing negative growth. Photosynthetic efficiency (Fv/Fm) was significantly lower at 29 degrees C than at 27.4 degrees C. Analyses of symbionts showed that sequence variants related to the genus Breviolum made up similar to 98% of all Symbiondiniaceae across all sites and temperature treatments with one variant (100% sequence identity to Breviolum psygmophilum) being 91% of the total community. Only one Symbiodiniaceae, with a 99.28% sequence identity to Breviolum psygmophilum, differed significantly as a function of temperature. It also comprised only 1-3% of the community. Bacterial microbiome variance increased during our experiment, but there were no significant effects of temperature or collection site. For O. arbuscula from these collection sites, location did not impact the coral's ability to withstand the effects of temperature stress.

Detritivore sea cucumbers appear to have been abundant on historic tropical reefs, but (1) have been heavily exploited since at least the mid-1800s, (2) often show minimal recovery post-harvest, and (3) are relatively depleted from modern marine communities. Because they were more abundant, fed on bacteria, microalgae, and other organics, and processed tremendous masses of sediments, removing these detritivores from tropical seas may have suppressed removal of sedimentary pathogens, and impacted co-occurring species in ways that are not documented. We conducted enclosure and exclosure experiments of the sea cucumber Holothuria atra in a back reef lagoon in Moorea, French Polynesia and found that excluding this sea cucumber increased a measure of sediment surface pigmentation by about 10 × but also decreased the potency of extracts from a co-occurring coral ( Acropora cytherea ) against the heat-activated coral pathogen Vibrio coralliilyticus by a significant 52%. This suggests that the large-scale removal of detritovores from shallow tropical seas may make some co-occurring foundation species more susceptible to pathogens during periods of elevated temperatures or other stresses.

Species loss threatens ecosystems worldwide, but the ecological processes and thresholds that underpin positive biodiversity effects among critically important foundation species, such as corals on tropical reefs, remain inadequately understood. In field experiments, we manipulated coral species richness and intraspecific density to test whether, and how, biodiversity affects coral productivity and survival. Corals performed better in mixed species assemblages. Improved performance was unexplained by competition theory alone, suggesting that positive effects exceeded agonistic interactions during our experiments. Peak coral performance occurred at intermediate species richness and declined thereafter. Positive effects of coral diversity suggest that species’ losses on degraded reefs make recovery more difficult and further decline more likely. Harnessing these positive interactions may improve ecosystem conservation and restoration in a changing ocean.