Spatial and temporal variation in abyssal megabenthic communities, as assessed with seabed photography

Abstract

Large-scale photographic surveys present an opportunity to quantify variation in benthic megafaunal community structure and dynamics on the undersampled abyssal plains, a habitat covering ~70% of the global seabed. The aims were to examine the method, and to use it to improve estimates of megafaunal abundance, biomass and functional groups (e.g. feeding types and rates) at the Porcupine Abyssal Plain (PAP, 4850m depth, NE Atlantic), and also at Station M (4000m, NE Pacific), with the resulting data combined into a food web model.

Method investigations included testing the quality of photo-derived ecological data by comparing three experts’ annotations of a large set of images. Greater consensus was attained in the identification than in detection of megafauna, and variation in the detection of common taxa resulted in significant differences in estimated density and community composition. Randomisation of images should be used to combat annotation bias. Secondly, relationships between measured body dimensions and preserved and fresh wet weights were compiled and applied to dimensions measured in seabed images. Size spectra estimated from photos were similar to those of trawl-caught specimens, but the total fresh wet weight biomass of holothurians and cnidarians in the photos was ~20 times the estimates from trawl catches, suggesting that seabed photography may improve biomass estimates.

Seabed imagery was applied to the ecology of abyssal hills and adjacent plain at the PAP, with substantial impacts to prior estimates of abundance, biomass and community composition. Biomass was 3 times greater on the hills than the plain, and assemblage and trophic compositions (by density and biomass) were significantly different, correlated with environmental conditions. A burrowing anemone, Iosactis vagabunda, was newly found to be the most abundant animal, likely important to carbon cycling with its newly-observed predatory and deposit feeding behaviours. Mobile deposit feeding rates were linked to body size, scaling at a power predicted by the Metabolic Theory of Ecology. This rate appears to increase in response to the deposition of detritus in only one taxon tested, while others remained constant.

These findings all have impacts to carbon cycling. The fate of detritus input on abyssal hills and the plain was examined using a model of carbon flow through the food web, and the improved quantification of biomass and knowledge of feeding modes. Suspension feeding dominated carbon processing on the hill, removing nearly all labile detritus input. On the plain, half the labile detritus was used by deposit feeders. The community on the plain is dependent on a more stable carbon supply than the hill. This represents a substantial improvement to prior assessments of benthic biogeochemistry in the abyss.

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