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1. Decadal‐scale time series highlight the role of chronic disturbances in driving ecosystem collapse in the Anthropocene

   https://doi.org/10.1002/ecy.4360  

   Edmunds, Peter J (June 2024, Ecology)

   Aronson, R (Ed.)

   Abstract Biome degradation characterizes the Anthropocene Epoch, and modern ecology is deeply involved with describing the changes underway. Most research has focused on the role of acute disturbances in causing conspicuous changes in ecosystem structure, which leads to an underappreciation of the chronic effects causing large changes through the cumulative effects of small perturbations over decades. Coral reefs epitomize this trend, because the changes in community structure are profound, yet the data to quantify these effects are usually insufficient to evaluate the relative roles of different disturbance types. Here, four decades of surveys from two coral reefs (9 and 14 m depth) off St. John, US Virgin Islands, are used to quantify the associations of acute and chronic events with the changes in benthic community structure. These reefs profoundly changed over 36 years, with coral death altering species assemblages to depress abundances of the ecologically important coralOrbicellaspp. and elevating the coverage of macroalgae and crustose coralline algae/turf/bare space (CTB). Linear mixed models revealed the prominent role of chronic variation in temperature in accounting for changes in coverage of corals, macroalgae, and CTB, with rising temperature associated with increases in coral cover on the deep reef, and declines on the shallow reef. Hurricanes were also associated with declines in coral cover on the shallow reef, and increases on the deep reef. Multivariate analyses revealed strong associations between community structure and temperature, but weaker associations with hurricanes, bleaching, and diseases. These results highlight the overwhelming importance of chronically increasing temperature in altering the benthic community structure of Caribbean reefs. 

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2. Ecosystem engineers on tropical reefs in transition: giant barrel sponges in the Anthropocene

   Pawlik, Joseph (May 2025, Journal of experimental biology)

   Tropical coral reef ecosystems are changing rapidly to an alternative state in which sponges are the dominant living habitat, with giant barrel sponges (GBSs, Xestospongia spp.) representing the largest biomass. Unlike other benthic reef organisms, GBSs are ecosystem engineers that pump large volumes of seawater, disrupting the benthic boundary layer and directing flow away from the reef surface and into the water column. The morphology and size of GBSs have made them particularly good experimental subjects to study the hydraulics of sponge pumping and the transformation that occurs as seawater is processed by the sponge holobiont (sponge cells and microbial symbionts). This Review is part of a series marking the 100th birthday of The Company of Biologists, which was founded by marine biologist George Parker Bidder III, who primarily worked on sponges. The Review provides an integrative assessment of research on GBSs with comparisons with what is known about other marine sponges. Recent discoveries suggest that ancient lineages of morphologically indistinguishable GBSs are responding to environmental changes over sub-decadal time periods to rapidly populate reefs stripped of coral cover by climate change. If GBSs remain robust to rising seawater temperatures, they will become the greatest source of habitat complexity on reefs of the future, so knowledge of their biology and physiology will be important to our understanding of these ecosystems. 

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3. The giant barrel sponge Xestospongia muta takes up dissolved organic matter from benthic cyanobacterial mats

   Olinger, LK; Strangman, WK; McMurray, SE; Mead, R; Pawlik, JR (February 2025, Organic geochemistry)

   With the decline of reef-building corals, other organisms are taking over Caribbean reefs, including sponges and benthic cyanobacterial mats (BCM). Sponges take up dissolved organic matter (DOM), but the sources and chemical characteristics of DOM taken up by sponges are unknown. One likely DOM source is benthic autotrophs, including BCM, which are prolific producers of DOM. We tested the hypothesis that sponges take up BCM-derived DOM using laboratory experiments in which seawater samples were collected before and after sequential incubations of BCM and small individuals of the giant barrel sponge Xestospongia muta. The concentration of DOC and relative abundance of individual features in the high resolution mass spectra using untargeted metabolomics were determined for each sample. There was a significant increase in DOC after BCM incubations, followed by a significant decrease after sponge incubations. These changes were mirrored in single feature relative abundances, with 2101 out of 3667 features significantly enriched during BCM incubations, and 54% of these (1142) depleted during sponge incubations. Among BCM-enriched and sponge-depleted features, many were halogenated, some were known BCM-derived secondary metabolites (e.g., carriebowmide, barbamide), and others matched unidentified sponge-depleted features from seawater samples collected on the reef. To our knowledge, this is the first report that sponges take up BCM exudates, including some that were detectable in reef DOM, revealing a path of molecules from source to sink through their environment. The BCM exudates taken up by sponges may be used as a food source or incorporated into sponge secondary metabolites for holobiont maintenance or chemical defenses. 

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4. Looking for the sponge loop: analyses of detritus on a Caribbean forereef using stable isotope and eDNA metabarcoding techniques

   https://doi.org/10.7717/peerj.16970  

   Olinger, Lauren K; McClenaghan, Beverly; Hajibabaei, Mehrdad; Fahner, Nicole; Berghuis, Lesley; Rajabi, Hoda; Erwin, Patrick; Lane, Chad S; Pawlik, Joseph R (January 2024, PeerJ)

   Hull, M (Ed.)

   Coral reefs are biodiverse ecosystems that rely on trophodynamic transfers from primary producers to consumers through the detrital pathway. The sponge loop hypothesis proposes that sponges consume dissolved organic carbon (DOC) and produce large quantities of detritus on coral reefs, with this turn-over approaching the daily gross primary production of the reef ecosystem. In this study, we collected samples of detritus in the epilithic algal matrix (EAM) and samples from potential sources of detritus over two seasons from the forereef at Carrie Bow Cay, Belize. We chose this location to maximize the likelihood of finding support for the sponge loop hypothesis because Caribbean reefs have higher sponge abundances than other tropical reefs worldwide and the Mesoamerican barrier reef is an archetypal coral reef ecosystem. We used stable isotope analyses and eDNA metabarcoding to determine the composition of the detritus. We determined that the EAM detritus was derived from a variety of benthic and pelagic sources, with primary producers (micro- and macroalgae) as major contributors and metazoans (Arthropoda, Porifera, Cnidaria, Mollusca) as minor contributors. None of the sponge species that reportedly produce detritus were present in EAM detritus. The cnidarian signature in EAM detritus was dominated by octocorals, with a scarcity of hard corals. The composition of detritus also varied seasonally. The negligible contribution of sponges to reef detritus contrasts with the detrital pathway originally proposed in the sponge loop hypothesis. The findings indicate a mix of pelagic and benthic sources in the calmer summer and primarily benthic sources in the more turbulent spring. 

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5. Changing disturbance regimes, material legacies, and stabilizing feedbacks: Dead coral skeletons impair key recovery processes following coral bleaching

   https://doi.org/10.1111/gcb.17504  

   Kopecky, Kai_L; Holbrook, Sally_J; Partlow, Emalia; Cunningham, Madeline; Schmitt, Russell_J (September 2024, Global Change Biology)

   Abstract Ecosystem responses to disturbance depend on the nature of the perturbation and the ecological legacies left behind, making it critical to understand how climate‐driven changes in disturbance regimes modify resilience properties of ecosystems. For coral reefs, recent increases in severe marine heat waves now co‐occur with powerful storms, the historic agent of disturbance. While storms kill coral and remove their skeletons, heat waves bleach and kill corals but leave their skeletons intact. Here, we explored how the material legacy of dead coral skeletons modifies two key ecological processes that underpin coral reef resilience: the ability of herbivores to control macroalgae (spatial competitors of corals), and the replenishment of new coral colonies. Our findings, grounded by a major bleaching event at our long‐term study locale, revealed that the presence of structurally complex dead skeletons reduced grazing on turf algae by ~80%. For macroalgae, browsing was reduced by >40% on less preferred (unpalatable) taxa, but only by ~10% on more preferred taxa. This enabled unpalatable macroalgae to reach ~45% cover in 2 years. By contrast, herbivores prevented macroalgae from becoming established on adjacent reefs that lacked skeletons. Manipulation of unpalatable macroalgae revealed that the cover reached after 1 year (~20%) reduced recruitment of corals by 50%. The effect of skeletons on juvenile coral growth was contingent on the timing of settlement relative to the disturbance. If corals settled directly after bleaching (before macroalgae colonized), dead skeletons enhanced colony growth by 34%, but this benefit was lost if corals colonized dead skeletons a year after the disturbance once macroalgae had proliferated. These findings underscore how a material legacy from a changing disturbance regime can alter ecosystem resilience properties by disrupting key trophic and competitive interactions that shape post‐disturbance community dynamics. 

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