Hills and plains: the influence of topography on deep-sea benthic foraminiferal assemblages
Benthic foraminifera are an important component of soft-bottom meio- and macro-faunal communities on abyssal plains (3500 – 6000 m water depth). Although often perceived as vast flat areas, abyssal plains also encompass numerous abyssal hills. These topographic rises, which reach heights up to 1000 m above the seafloor, are believed to be one of the most pervasive landforms on Earth. Major topographic features, for example, seamounts and submarine canyons, are known to have an effect on food availability and sediment characteristics, which are ultimately linked to community composition and biomass. However, the potential influence of relatively small abyssal hills on associated benthic communities is poorly understood. In the case of benthic foraminifera, previous studies have assessed the effect of small-scale (centimeters to meters) seafloor features (e.g. biogenic structures, organic matter patches) but nothing is known about the effects of larger topographic features, such as abyssal hills, which increase habitat complexity at broader scales (mesoscale, i.e. decimeters to kilometers). In order to address this issue we collected samples from four different sites in the area of the Porcupine Abyssal Plain Sustained Observatory (PAPSO; northeast Atlantic, >4,300 m water depth), two on tops of abyssal hills and two on the adjacent abyssal plain (three replicate samples per site). We test whether (1) benthic foraminiferal abundance and diversity are enhanced on abyssal hills, (2) community composition is different on the hills and the adjacent abyssal plain, and (3) mesoscale environmental heterogeneity is more important in structuring foraminiferal assemblages compared to small-scale patchiness. Preliminary results suggest that benthic foraminiferal density and diversity on the abyssal hills is comparable to that on the neighboring abyssal plain. However, the assemblages on the hills harbor some species not found on the plains, thereby enhancing foraminiferal diversity at regional scales.
Life history traits in deep-sea fishes revealed by otolith microchemistry
Effective fishery management demands an appreciation of spatial and behavioural variations in fish life history but this is particularly challenging in inaccessible deep water species. The chronological properties of otoliths, coupled with stable isotope records of environmental and physiological conditions, offers an approach to reconstruct ontogenetic migrations and metabolic histories in deep-sea fishes. Here we derive ontogenetic records of stable oxygen and carbon isotopes from four contrasting species of deep-sea fishes (Alepocephalus bairdii, Antimora rostrata, Coryphaenoides rupestris and Spectrunculus grandis) and compare these with previous isotopic data in other species.
Oxygen isotope records indicate that ontogenetic increases in depth are a common feature in deep water fishes, but the nature of these migrations varies between species. Larval shallow water phases are found in all investigated species other than the morid, A. rostrata, and early juvenile behaviours split between fishes with relatively extended shallow phases (C. rupestris and A. bairdii) and those with rapid descent (occasionally to ontogenetic depth maxima) during juvenile life periods (Hoplostethus atlanticus, Cataetyx laticeps and S. grandis).
Ontogenetically increasing otolith carbon isotope values reflecting decreasing mass-specific metabolic rates are found for all species, but at different rates. The recovered metabolic history integrates temperature, body mass, growth rates and activity, and the interplay between these factors creates metabolic life-history diversity between species. Life history traits revealed by otolith microchemistry have implications for genetic structure, population connectivity, reproductive rate and vulnerability of these poorly understood deep water faunas.