First observation of a skate egg case nursery in the Ross Sea

Areas of importance to Southern Ocean skates are poorly defined. Here, we identify a deepwater skate egg case nursery in a discrete location at ~460 m depth off Cape Adare in the Southern Ocean. This is the first confirmed observation of a skate nursery area in the Ross Sea and only the second observation for the Southern Ocean. The morphology and size of the egg cases were consistent with the genus Bathyraja and most likely belong to the Bathyraja sp. (cf. eatonii). The nursery occurs within the "no take" General Protection Zone of the Ross Sea region marine protected area, where commercial fishing is prohibited.

Status, Change, and Futures of Zooplankton in the Southern Ocean

In the Southern Ocean, several zooplankton taxonomic groups, euphausiids, copepods, salps and pteropods, are notable because of their biomass and abundance and their roles in maintaining food webs and ecosystem structure and function, including the provision of globally important ecosystem services. These groups are consumers of microbes, primary and secondary producers, and are prey for fishes, cephalopods, seabirds, and marine mammals. In providing the link between microbes, primary production, and higher trophic levels these taxa influence energy flows, biological production and biomass, biogeochemical cycles, carbon flux and food web interactions thereby modulating the structure and functioning of ecosystems. Additionally, Antarctic krill (Euphausia superba) and various fish species are harvested by international fisheries. Global and local drivers of change are expected to affect the dynamics of key zooplankton species, which may have potentially profound and wide-ranging implications for Southern Ocean ecosystems and the services they provide. Here we assess the current understanding of the dominant metazoan zooplankton within the Southern Ocean, including Antarctic krill and other key euphausiid, copepod, salp and pteropod species. We provide a systematic overview of observed and potential future responses of these taxa to a changing Southern Ocean and the functional relationships by which drivers may impact them. To support future ecosystem assessments and conservation and management strategies, we also identify priorities for Southern Ocean zooplankton research.

Unravelling seasonal trends in coastal marine heatwave metrics across global biogeographical realms

Marine heatwaves (MHWs) can cause dramatic changes to ecologically, culturally, and economically important coastal ecosystems. To date, MHW studies have focused on geographically isolated regions or broad-scale global oceanic analyses, without considering coastal biogeographical regions and seasons. However, to understand impacts from MHWs on diverse coastal communities, a combined biogeographical-seasonal approach is necessary, because (1) bioregions reflect community-wide temperature tolerances and (2) summer or winter heatwaves likely affect communities differently. We therefore carried out season-specific Theil–Sen robust linear regressions and Pettitt change point analyses from 1982 to 2021 on the number of events, number of MHW days, mean intensity, maximum intensity, and cumulative intensity of MHWs, for each of the world’s 12 major coastal biogeographical realms. We found that 70% of 240 trend analyses increased significantly, 5% decreased and 25% were unaffected. There were clear differences between trends in metrics within biogeographical regions, and among seasons. For the significant increases, most change points occurred between 1998 and 2006. Regression slopes were generally positive across MHW metrics, seasons, and biogeographical realms as well as being highest after change point detection. Trends were highest for the Arctic, Northern Pacific, and Northern Atlantic realms in summer, and lowest for the Southern Ocean and several equatorial realms in other seasons. Our analysis highlights that future case studies should incorporate break point changes and seasonality in MHW analysis, to increase our understanding of how future, more frequent, and stronger MHWs will affect coastal ecosystems.

Plankton respiration in the Atacama Trench region: Implications for particulate organic carbon flux into the hadal realm

Respiration is a key process in the cycling of particulate matter and, therefore, an important control mechanism of carbon export to the ocean's interior. Most of the fixed carbon is lost in the upper ocean, and only a minor amount of organic material sustains life in the deep-sea. Conditions are particularly extreme in hadal trenches, and yet they host active biological communities. The source of organic carbon that supports them and the contribution of these communities to the ocean carbon cycle, however, remain uncertain. Here we report on size-fractionated depth profiles of plankton respiration assessed from the activity of the electron transport system in the Atacama Trench region, and provide estimates of the minimum carbon flux (FC) needed to sustain the respiratory requirements from the ocean surface to hadal waters of the trench and shallower nearby sites. Plankton < 100 μm contributed about 90% to total community respiration, whose magnitude was highly correlated with surface productivity. Remineralization rates were highest in the euphotic zone and declined sharply within intermediate oxygen-depleted waters, remaining fairly constant toward the bottom. Integrated respiration in ultra-deep waters (> 1000 m) was comparable to that found in upper layers, with 1.3 ± 0.4 mmol C m^-2 d^-1 being respired in the hadopelagic. The comparison between our FC models and estimates of sinking particle flux revealed a carbon imbalance through the mesopelagic that was paradoxically reduced at greater depths. We argue that large fast-sinking particles originated in the overlying surface ocean may effectively sustain the respiratory carbon demands in this ultra-deep marine environment.

Evidence for the Impact of Climate Change on Primary Producers in the Southern Ocean

Within the framework of the Marine Ecosystem Assessment for the Southern Ocean (MEASO), this paper brings together analyses of recent trends in phytoplankton biomass, primary production and irradiance at the base of the mixed layer in the Southern Ocean and summarises future projections. Satellite observations suggest that phytoplankton biomass in the mixed-layer has increased over the last 20 years in most (but not all) parts of the Southern Ocean, whereas primary production at the base of the mixed-layer has likely decreased over the same period. Different satellite models of primary production (Vertically Generalised versus Carbon Based Production Models) give different patterns and directions of recent change in net primary production (NPP). At present, the satellite record is not long enough to distinguish between trends and climate-related cycles in primary production. Over the next 100 years, Earth system models project increasing NPP in the water column in the MEASO northern and Antarctic zones but decreases in the Subantarctic zone. Low confidence in these projections arises from: (1) the difficulty in mapping supply mechanisms for key nutrients (silicate, iron); and (2) understanding the effects of multiple stressors (including irradiance, nutrients, temperature, pCO2, pH, grazing) on different species of Antarctic phytoplankton. Notwithstanding these uncertainties, there are likely to be changes to the seasonal patterns of production and the microbial community present over the next 50–100 years and these changes will have ecological consequences across Southern Ocean food-webs, especially on key species such as Antarctic krill and silverfish.

Diet of six deep-sea grenadiers (Macrouridae)

Feeding habits of six deep-sea demersal trawl-caught macrourids on Chatham Rise, New Zealand, were examined from stomach contents during the austral summer. Three species were predominantly benthic foragers: smallbanded rattail Coelorinchus parvifasciatus on small epifaunal crustaceans, twosaddle rattail Coelorinchus biclinozonalis on epifaunal decapods and humpback rattail Coryphaenoides dossenus on benthic fishes and epifaunal decapods. Three species were predominantly benthopelagic foragers: banded rattail Coelorinchus fasciatus on hyperiid and gammarid amphipods and calanoid copepods, blackspot rattail Lucigadus nigromaculatus on small epifaunal crustaceans and suprabenthic mysids and Mahia rattail Coelorinchus matamua on epifaunal decapods and calanoid copepods. The most important predictors of diet variability were identified using distance-based linear models and included areal predictors in C. parvifasciatus, L. nigromaculatus and C. dossenus, fish size in C. dossenus, C. biclinozonalis and C. matamua, sample year in C. biclinozonalis and C. fasciatus and depth in C. matamua. Results are compared with previously published data for four other macrourid species from the same study area. The 10 grenadier species comprise benthic, benthopelagic and mesopelagic foraging guilds. This study brings the number of grenadier species for which diet on Chatham Rise has been described in detail to 12.

Diet and trophic niche of Antarctic silverfish Pleuragramma antarcticum in the Ross Sea, Antarctica

The diet of Antarctic silverfish Pleuragramma antarcticum was evaluated by examining stomach contents of specimens collected in the Ross Sea (71°-77° S; 165°-180° E) in January to March 2008. Pleuragramma antarcticum (50-236 mm standard length, L(S)) and prey items were analysed for stable-isotopic composition of carbon and nitrogen. According to index of relative importance (I(RI) ), which incorporates frequency of occurrence, mass and number of prey items, the most important prey items were copepods (81%I(RI) over all specimens), predominantly Metridia gerlachei and Paraeuchaeta sp., with krill and fishes having low I(RI) (2·2 and 5·6%I(RI) overall). According to mass of prey (M) in stomachs, however, fishes (P. antarcticum and myctophids) and krill dominated overall diet (48 and 22%M, respectively), with copepods being a relatively minor constituent of overall diet by mass (9·9%M). Piscivory by P. antarcticum occurred mainly in the extreme south-west of the region and near the continental slope. Krill identified to species level in P. antarcticum stomachs were predominantly Euphausia superba (14·1%M) with some Euphausia crystallophorias (4·8%M). Both DistLM modelling (PRIMER-permanova+) on stomach contents (by I(RI)) and stepwise generalized linear modelling on stable isotopes showed that L(S) and location were significant predictors of P. antarcticum diet. Postlarval P. antarcticum (50-89 mm L(S)) consumed exclusively copepods. Juvenile P. antarcticum (90-151 mm L(S)) consumed predominantly krill and copepods by mass (46 and 30%M, respectively). Small adult P. antarcticum (152-178 mm L(S)) consumed krill, fishes and copepods (37, 36 and 15%M, respectively). Large adult P. antarcticum (179-236 mm L(S)) consumed predominantly fishes and krill (55 and 17%M, respectively), especially in the north (near the Ross Sea slope) and in the SW Ross Sea. Amphipods were occasionally important prey items for P. antarcticum (western Ross Sea, 39%M). General concordance between stomach contents and trophic level of P. antarcticum and prey based on δ(15) N was demonstrated. Pleuragramma antarcticum trophic level was estimated as 3·7 (postlarval fish) and 4·1 (fish aged 3+ years).