Abstract. Ecologically dominant species often define ecosystem states, but as human disturbances intensify, their subordinate counterparts increasingly displace them. We consider the duality of disturbance by examining how environmental drivers can simultaneously act as a stressor to dominant species and as a resource to subordinates. Using a model ecosystem, we demonstrate that CO 2 -driven interactions between species can account for such reversals in dominance; i.e., the displacement of dominants (kelp forests) by subordinates (turf algae). We established that CO 2 enrichment had a direct positive effect on productivity of turfs, but a negligible effect on kelp. CO 2 enrichment further suppressed the abundance and feeding rate of the primary grazer of turfs (sea urchins), but had an opposite effect on the minor grazer (gastropods). Thus, boosted production of subordinate producers, exacerbated by a net reduction in its consumption by primary grazers, accounts for community change (i.e., turf displacing kelp). Ecosystem collapse, therefore, is more likely when resource enrichment alters competitive dominance of producers, and consumers fail to compensate. By recognizing such duality in the responses of interacting species to disturbance, which may stabilize or exacerbate change, we can begin to understand how intensifying human disturbances determine whether or not ecosystems undergo phase shifts.

Reduction in seawater pH due to rising levels of anthropogenic carbon dioxide (CO) in the world's oceans is a major force set to shape the future of marine ecosystems and the ecological services they provide [1,2]. In particular, ocean acidification is predicted to have a detrimental effect on the physiology of calcifying organisms [3]. Yet, the indirect effects of ocean acidification on calcifying organisms, which may counter or exacerbate direct effects, is uncertain. Using volcanic CO vents, we tested the indirect effects of ocean acidification on a calcifying herbivore (gastropod) within the natural complexity of an ecological system. Contrary to predictions, the abundance of this calcifier was greater at vent sites (with near-future CO levels). Furthermore, translocation experiments demonstrated that ocean acidification did not drive increases in gastropod abundance directly, but indirectly as a function of increased habitat and food (algal biomass). We conclude that the effect of ocean acidification on algae (primary producers) can have a strong, indirect positive influence on the abundance of some calcifying herbivores, which can overwhelm any direct negative effects. This finding points to the need to understand ecological processes that buffer the negative effects of environmental change.

The study of ancient DNA (aDNA) from sediments (sedaDNA) offers great potential for paleoclimate interpretation, and has recently been applied as a tool to characterise past marine life and environments from deep ocean sediments over geological timescales. Using sedaDNA, palaeo-communities have been detected, including prokaryotes and eukaryotes that do not fossilise, thereby revolutionising the scope of marine micropalaeontological research. However, many studies to date have not reported on the measures taken to prove the authenticity of sedaDNA-derived data from which conclusions are drawn. aDNA is highly fragmented and degraded and extremely sensitive to contamination by non-target environmental DNA. Contamination risks are particularly high on research vessels, drilling ships and platforms, where logistics and facilities do not yet allow for sterile sediment coring, and due consideration needs to be given to sample processing and analysis following aDNA guidelines. This review clarifies the use of aDNA terminology, discusses common pitfalls and highlights the urgency behind adopting new standards for marine sedaDNA research, with a focus on sampling optimisation to facilitate the incorporation of routine sedaDNA research into International Ocean Discovery Program (IODP) operations. Currently available installations aboard drilling ships and platforms are reviewed, improvements suggested, analytical approaches detailed, and the controls and documentation necessary to support the authenticity of aDNA retrieved from deep-sea sediment cores is outlined. Beyond practical considerations, concepts relevant to the study of past marine biodiversity based on aDNA, and the applicability of the new guidelines to the study of other contamination-susceptible environments (permafrost and outer space) are discussed.

Metabarcoding of plant DNA recovered from environmental samples, termed environmental DNA (eDNA), has been used to detect invasive species, track biodiversity changes, and reconstruct past ecosystems. The P6 loop of the trnL intron is the most widely utilised gene region for metabarcoding plants due to the short fragment length and subsequent ease of recovery from degraded DNA, which is characteristic of environmental samples. However, the taxonomic resolution for this gene region is limited, often precluding species level identification. Additionally, targeting gene regions using universal primers can bias results as some taxa will amplify more effectively than others. To increase the ability of DNA metabarcoding to better resolve flowering plant species (angiosperms) within environmental samples, and reduce bias in amplification, we developed a multi-gene targeted capture method that simultaneously targets 20 chloroplast gene regions in a single assay across all flowering plant species. Using this approach, we effectively recovered multiple chloroplast gene regions for three species within artificial DNA mixtures down to 0.001 ng/μL of DNA. We tested the detection level of this approach, successfully recovering target genes for 10 flowering plant species. Finally, we applied this approach to sediment samples containing unknown compositions of eDNA and confidently detected plant species that were later verified with observation data. Targeting multiple chloroplast gene regions in environmental samples, enabled species-level information to be recovered from complex DNA mixtures. Thus, the method developed here, confers an improved level of data on community composition, which can be used to better understand flowering plant assemblages in environmental samples.

Seagrass, saltmarsh and mangrove habitats are declining around the world as anthropogenic activity and climate change intensify. To be able to effectively restore and maintain healthy coastal-vegetation communities, we must understand how and why they have changed in the past. Identifying shifts in vegetation communities, and the environmental or human drivers of these, can inform successful management and restoration strategies. Unfortunately, long-term data (i.e. decades to hundreds of years) on coastal vegetated ecosystems that can discern community-level changes are mostly non-existent in the scientific record. We propose implementing DNA extracted from coastal sediments to provide an alternative approach to long-term ecological reconstruction for coastal vegetated ecosystems. This type of DNA is called ‘environmental DNA’ and has previously been used to generate long-term datasets for other vegetated systems but has not yet been applied to vegetation change in coastal settings. In this overview, we explore the idea of using sediment eDNA as a long-term monitoring tool for seagrass, saltmarsh and mangrove communities. We see real potential in this approach for reconstructing long-term ecological histories of coastal vegetated ecosystems, and advocate that further research be undertaken to develop appropriate methods for its use.