Sea urchins in a high CO2 world: partitioned effects of body-size, ocean warming and acidification on metabolic rate

Body-size and temperature are the major factors explaining metabolic rate, and the additional factor of pH is a major driver at the biochemical level. These three factors have frequently been found to interact, complicating the formulation of broad models predicting metabolic rates and hence ecological functioning. In this first study of the effects of warming and ocean acidification, and their potential interaction, on metabolic rate across a broad body-size range (two-to-three orders of magnitude difference in body mass) we addressed the impact of climate change on the sea urchin Heliocidaris erythrogramma in context with climate projections for east Australia, an ocean warming hotspot. Urchins were gradually introduced to two temperatures (18 and 23 °C) and two pH (7.5 and 8.0), and maintained for two months. That a new physiological steady-state had been reached, otherwise know as acclimation, was validated through identical experimental trials separated by several weeks. The relationship between body-size, temperature and acidification on the metabolic rate of H. erythrogramma was strikingly stable. Both stressors caused increases in metabolic rate; 20% for temperature and 19% for pH. Combined effects were additive; a 44% increase in metabolism. Body-size had a highly stable relationship with metabolic rate regardless of temperature or pH. None of these diverse drivers of metabolism interacted or modulated the effects of the others, highlighting the partitioned nature of how each influences metabolic rate, and the importance of achieving a full acclimation state. Despite these increases in energetic demand there was very limited capacity for compensatory modulating of feeding rate; food consumption increased only in the very smallest specimens, and only in response to temperature, and not pH. Our data show that warming, acidification and body-size all substantially affect metabolism and are highly consistent and partitioned in their effects, and for H. erythrogramma near-future climate change will incur a substantial energetic cost.

(Tab. XIII.1) TOC content, carbonate content, carbonate mineralogy, percentage of total carbonate, aragonite, low magnesium calcium, and high magnesium calcium, and grain-size contents from samples from the Golfe d'Arguin and from the Western Sahara

Modern carbonate sedimentation takes place on the northern Mauritanian shelf (20°N), where typical tropical components (e.g. hermatypic reefs, calcareous green algae) are absent. Such deposits are reminiscent of extratropical sediment in the geological record. The tropical open shelf of Mauritania is influenced by large siliciclastic dust input and upwelling, highly fertilizing the ocean, as well as strongly limiting the light penetration. In this context, temperature does not appear to be the steering factor of carbonate production. This thesis describes the depositional system of the Golfe d'Arguin off Mauritania and focuses on environmental conditions that control the depositional pattern, in particular carbonate production. The description of this modern analogue provides a tool for paleoenvironmental interpretation of ancient counterparts. The Golfe d'Arguin is a broad shallow shelf comprising extensive shoals (<10 m water depth; i.e. the Banc d'Arguin) on the inner shelf where waters warm up. The sediments collected in water depths between 4 and 600 m are characterized by mixed carbonate and siliciclastic (dust) deposits. They vary from clean coarse-grained, almost pure carbonate loose sediments to siliciclastic-dominated fine-grained sediments. The carbonate content and sediment grain size show a north-south decreasing pattern through the Golfe d'Arguin and are controlled by the hydraulic regime influenced by wind-driven surface currents, swell, and tidal currents. The carbonate grain association is heterozoan. Components include abundant molluscs, foraminifers, and worm tubes, as well as barnacles and echinoderms, elements that are also abundant in extratropical sediments. The spatial distribution of the sedimentary facies of the Golfe d'Arguin does not display a depth zonation but rather a mosaic (i.e. patchy distribution). The depth and climatic signatures of the different sedimentary facies are determined by taxonomic and ecological investigations of the carbonate-secreting biota (molluscs and foraminifers). While certain planktonic foraminifers and molluscs represent upwelling elements, other components (e.g. mollusc and benthic foraminifer taxa) demonstrate the tropical origin of the sediment. The nutrient-rich (and thus also low light-penetration) conditions are reflected in the fact that symbiotic and photosynthetic carbonate-producing organisms (e.g. hermatypic corals) are absent. The Mauritanian deposits represent an environment that is rare in the modern world but might have been more common in the geological past when global temperatures were higher. Taxonomic and ecological studies allow for distinguishing carbonate sediments formed under either tropical high-nutrient or extratropical conditions, thus improving paleoclimate reconstruction.