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Generally speaking, the term benthos refers to all aquatic organisms (marine or freshwater) living in close relationship with the bottom of seas and oceans, lakes and rivers. A distinction is made between the plant benthos or phytobenthos (algae and phanerogams), and the animal benthos or zoobenthos (annelids, molluscs, crustaceans, echinoderms, fish, etc.). Furthermore, the fauna situated on the surface (or epifauna), which may be fixed or free, is different from that which lives inside the sediment (endofauna). Finally, in this section we are interested only in marine macrobenthos (organisms larger than a few mm), from the tidal zone to the abyssal depths. The smaller forms (meiofauna-flora and microfauna-flora), which are practically not documented, are not taken into account in this chapter on the bottoms of the St-Paul and Amsterdam Islands. The area has been explored only sporadically during the passage of a few vessels on scientific expeditions focused on other interests. Only one of them, dedicated to the exploration of the zone from 0 to 3000 m of depth ("Jasus"), took place in 1985, on board the Marion-Dufresne. The purpose of this chapter is to make an assessment of the situation in 2022, in the context of a fishing zone within a marine protected area. {"references": ["F\u00e9ral (J.-P.) 2023. Les composantes biologiques : flore et faune marines subtropicales de l'h\u00e9misph\u00e8re sud \u2013 Le benthos, In: G. Duhamel (coord.) Les \u00eeles Saint-Paul et Amsterdam (Oc\u00e9an Indien sud) : environnement marin et p\u00eacheries. Mus\u00e9um national d'histoire naturelle, Paris, pp. 51-85 (Patrimoines naturels : 84), \u27e8hal-03575176\u27e9, doi : 10.5281/zenodo.7768288"]}

The St-Paul and Amsterdam (SPA) islands are surrounded by giant kelp of which the most abundant is Macrocystis pyrifera, a monospecific genus widely spread in both hemispheres. In SPA, the species is at the northern limit of its southern distribution. M. pyrifera is limited by the winter isotherm 3°C (lethality at 2°C) and the summer isotherm 18°C (lethality between 21 and 25°C). The average temperature of the sea water in SPA fluctuates between 12.5°C in winter and 19.5°C in summer. It is likely that the species may be threatened by the effects of climate change as observed in Tasmania. The problem would be related to the consequences of the El Niño Southern Oscillation (ENSO), notably a decrease in precipitation and an increase in sea surface temperatures as well as mortality due to storms. {"references": ["F\u00e9ral (J.-P.) 2023. Les composantes biologiques : flore et faune marines subtropicales de l'h\u00e9misph\u00e8re sud \u2013 Les ceintures de Phaeophyceae. In: G. Duhamel (coord.) Les \u00eeles Saint-Paul et Amsterdam (Oc\u00e9an Indien sud) : environnement marin et p\u00eacheries. Mus\u00e9um national d'histoire naturelle, Paris, pp. 44-50 (Patrimoines naturels : 84) \u27e8hal-03575185\u27e9, doi : 10.5281/zenodo.7768595"]}