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Abstract

Recent studies have highlighted the prevalence of microplastic (MP) pollution in the global marine environment and these pollutants have been found to contaminate even remote regions, including the Southern Ocean south of the polar front. Previous studies in this region have mostly focused on MPs larger than 300 μm, potentially underestimating the extent of MP pollution. This study is the first to investigate MPs in marine surface waters south of the polar front, with a focus on small MPs 500–11 μm in size. Seventeen surface water samples were collected in the southern Weddell Sea using an in-house-designed sampling system. The analysis of the entire sample using micro-Fourier transform infrared spectroscopy (μFTIR) with focal plane array (FPA) detection revealed the presence of MPs in all samples, with the vast majority of the MPs detected being smaller than 300 μm (98.3 %). The mean concentration reached 43.5 (± 83.8) MPs m−3, with a wide range from 0.5 to 267.2 MPs m−3. The samples with the highest concentrations differed from the other samples in that they were collected north of the continental slope and the Antarctic Slope Current. Sea ice conditions possibly also influenced these varying concentrations. This study reports high concentrations of MPs compared to other studies in the region. It emphasizes the need to analyze small MPs, down to a size of 11 μm or even smaller, in the Antarctic Treaty Area to gain a more comprehensive understanding of MP pollution and its potential ecological impacts.

Abstract

Large ‘apex’ predators influence ecosystems in profound ways, by limiting the density of their prey and controlling smaller ‘mesopredators’. The loss of apex predators from much of their range has lead to a global outbreak of mesopredators, a process known as ‘mesopredator release’ that increases predation pressure and diminishes biodiversity. While the classifications apex- and meso-predator are fundamental to current ecological thinking, their definition has remained ambiguous. Trophic cascades theory has shown the importance of predation as a limit to population size for a variety of taxa (top–down control). The largest of predators however are unlikely to be limited in this fashion, and their densities are commonly assumed to be determined by the availability of their prey (bottom–up control). However, bottom–up regulation of apex predators is contradicted by many studies, particularly of non-hunted populations. We offer an alternative view that apex predators are distinguishable by a capacity to limit their own population densities (self-regulation). We tested this idea using a set of life-history traits that could contribute to self-regulation in the Carnivora, and found that an upper limit body mass of 34 kg (corresponding with an average mass of 13–16 kg) marks a transition between extrinsically- and self-regulated carnivores. Small carnivores share fast reproductive rates and development and higher densities. Large carnivores share slow reproductive rates and development, extended parental care, sparsely populated territories, and a propensity towards infanticide, reproductive suppression, alloparental care and cooperative hunting. We discuss how the expression of traits that contribute to self-regulation (e.g. reproductive suppression) depends on social stability, and highlight the importance of studying predator–prey dynamics in the absence of predator persecution. Self-regulation in large carnivores may ensure that the largest and the fiercest do not overexploit their resources.

The Atlantic Meridional Overturning Circulation has a major impact on climate, not just in the northern Atlantic but globally. Paleoclimatic data show it has been unstable in the past, leading to some of the most dramatic and abrupt climate shifts known. These instabilities are due to two different types of tipping points, one linked to amplifying feedbacks in the large-scale salt transport and the other in the convective mixing that drives the flow. These tipping points present a major risk of abrupt ocean circulation and climate shifts as we push our planet further out of the stable Holocene climate into uncharted waters.