Temporal and spatial variability in stable isotope values on seabird islands: what, where and when to sample

Invasive mammal eradications are widely used for managing island ecosystems. However, tracking the outcomes of such large-scale, whole ecosystem projects is challenging and costly, and monitoring all components of an ecosystem is near impossible. Instead, indicators of ecosystem change may provide more practical and integrated measures of ecosystem response to eradications. As high-order marine predators, seabirds subsidise island ecosystems with nutrients isotopically enriched in nitrogen. Invasive mammals have caused a global decline of seabirds on islands, reducing this nutrient subsidisation. Following eradications, nitrogen stable isotope analysis may provide a useful and resource-efficient indicator of ecosystem functional change on eradicated islands. However, isotope ratios are affected by a myriad of factors, with potential sources of variation being introduced by spatial and temporal variation in sampling, and within and between different taxa and ecosystem components. To correctly attribute isotopic change to post-eradication ecosystem function change, these confounding variables need to be understood. To address this need, we analysed stable isotopes of nitrogen in soil, plant, spider, and seabird guano samples collected at different distances from seabird colonies and at different stages of the short-tailed shearwater breeding cycle on six island sites around south-eastern Tasmania, Australia. Across these cool, temperate islands we detected no temporal variability in δ15N throughout the breeding season. However, there was notable spatial variability in δ15N values. The effects of seabird-derived nutrient subsidisation were highly localised with high δ15N values found inside seabird colonies and then rapidly decreasing from the colony boundary. Higher δ15N values also occurred in areas of higher burrow density within a colony. Variability in δ15N values also existed both within and between ecosystem components. Our results highlight the importance of context dependency when using ecological indicators and have important implications for the design, implementation and interpretation of studies employing stable isotopes as indicators for ecosystem change. We provide recommendations for designing future stable isotope studies on seabird islands.</p

The use of swimmers from sediment traps to measure summer community structure of Southern Ocean pteropods

In the Southern Ocean, pteropods play an important role in biogeochemical cycling, and sediment traps are a valuable tool for investigating this role through the collection of passively sinking matter from productive surface waters to deep sea layers. Observations of ‘swimmers’ (e.g. organisms that actively swim into traps) can also prove valuable for studying zooplankton community structure. In this study, we used two separate sediment trap studies during the 2016–2017 summer to study pteropod population structure over time scales of 24 h and 28 days. In both studies, highest densities were measured for veliger-stage Limacina helicina antarctica (0.09–0.3 mm) relative to all species and age classes. Increases in shell diameters of veligers in all traps over time enabled the calculation of an intraseasonal potential growth rate of 0.0068 mm d−1. Swimmer flux rates ranged from 121 to 2674 ind. m−2 d−1 at 53 m depth, and the 24-h vertical flux study measured 960 ind. m−2 d−1 at 57 m depth and 6692 m−2 d−1 at 90 m depth. Among a suite of environmental and biological covariates tested, fluorescence and sinking particulate organic and inorganic carbon (POC and PIC) possessed the most predictive power to explain abundances of near-surface pteropod age class and species composition. Gymnosome abundances were largely influenced by increasing adult L. helicina antarctica counts. Changes to pteropod population and community dynamics in response to climate change will have cascading effects throughout Antarctic epipelagic food webs, and these results provide a regional snapshot of patterns in structure and sedimentation from an under-surveyed region of the Southern Ocean.</p

Two scales of distribution and biomass of Antarctic krill (Euphausia superba) in the eastern sector of the CCAMLR Division 58.4.2 (55°E to 80°E)

Regular monitoring is an important component of the successful management of pelagic animals of interest to commercial fisheries. Here we provide a biomass estimate for Antarctic krill (Euphausia superba) in the eastern sector of the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) Division 58.4.2 (55°E to 80°E; area = 775,732 km2) using data collected during an acoustic-trawl survey carried out in February and March 2021. Using acoustic data collected in day-time and trawl data, areal biomass density was estimated as 8.3 gm-2 giving a total areal krill biomass of 6.48 million tonnes, with a 28.9% coefficient of variation (CV). The inaccessibility of the East Antarctic makes fisheries-independent surveys of Antarctic krill expensive and time consuming, so we also assessed the efficacy of extrapolating smaller surveys to a wider area. During the large-scale survey a smaller scale survey (centre coordinates -66.28°S 63.35°E, area = 4,902 km2) was conducted. We examine how representative krill densities from the small-scale (Mawson box) survey were over a latitudinal range by comparing krill densities from the large-scale survey split into latitudinal bands. We found the small scale survey provided a good representation of the statistical distribution of krill densities within its latitudinal band (KS-test, D = 0.048, p-value = 0.98), as well as mean density (t-test p-value = 0.44), but not outside of the band. We recommend further in situ testing of this approach.</p

Plankton sampling by the training vessel Umitaka-maru in the Indian sector of the Southern Ocean in the austral summer of 2020

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