Ecological controls on density, diversity and community structure of Polar Megabenthos

Abstract

Polar deep water environments are poorly studied. This thesis investigates fundamental questions as to the nature of controlling factors on megabenthic communities as well as the diversity, densities and distributions of organisms present.

The deep (1000-1660m) Faroe-Shetland Channel harbours an Arctic fauna owing to cold deepwater intrusions from the Norwegian Sea. Despite the relatively low species richness of the megabenthos, variation in faunal composition with depth is apparent. Two distinct communities were identified in the north and south of the channel. Epibenthic megafauna in the south are dominantly filter feeders and in the north deposit feeders. Megabenthic diversity and density decreased to the northeast of the channel. Lebensspuren number and area increase northwards in the Channel. The increase in bioturbation and deposit feeder abundance is concurrent with an increase in fine sediment quantity.

The response of a deep, Arctic benthic community to physical disturbance was investigated in the Faroe-Shetland Channel. High levels of physical disturbance, characterised by smothering of the seabed resulted in significant but variable reductions in megafaunal abundance (up to 92.3%). Reductions in diversity, particularly in species richness, were apparent between disturbed (ES(500) = 12.9) and undisturbed areas (ES(500) = 20.6). The implications of selective removal of taxa on ecosystem function and recovery are discussed. Low level disturbance had comparatively little effect on the communities. The effects varied in nature depending on motility and functional group (e.g. motile scavenger abundances were maximal at intermediate distances from disturbance).

Effects of physical factors on the megabenthos of Kangerdlugssuaq Fjord mouth in Arctic Greenland were investigated. Large reductions in faunal density (1881 to 60,132 individuals ha-1) and increases in diversity (H´ = 0.93-2.54), through increases in richness (ES(220) = 7.6-18.8) and reductions in dominance (Berger-Parker Index = 0.77-0.38), were found from 270 to 720m depth. Distinct shallow, intermediate and deep communities were identified. Shallower stations had high levels of iceberg disturbance, directly reducing diversity and creating a complex, patchy environment. Responses to disturbance were taxa specific (e.g. mobile suspension feeders aggregate on disturbed areas), but the shallow area has higher densities of suspension feeding epifauna. Deeper areas experience small scale disturbance from deposition of drop stones but its relatively low frequency and magnitude allows increased diversity. Deposit feeding epifaunal and infaunal taxa increase with depth indicated by increased Lebensspuren. Density decreases result from decreased food supply with depth.

Benthic megafauna were investigated from the NE Weddell Sea (250 to 500m depth), close to the Fimbul Ice Shelf. Faunal density decreased with depth; diversity was variable but not related to depth. Two distinct communities were found, a shallow community with dense patches of suspension feeders in undisturbed areas and a deep community where these were not present. Disturbance from icebergs was very important in controlling faunal distribution. In shallow waters direct effects of
disturbance were observed. In deeper waters habitat changes, caused by past disturbance, controlled faunal distributions. Ice ploughing created a mosaic landscape of fine and coarse sediments. Megafaunal density was highest in coarse sediment and diversity highest in intermediate areas.

Quantitative data on benthic megafaunal abundance and diversity obtained using the same method allowed valid comparisons to be made between these polar areas. Megafaunal abundance was comparable between the Arctic and Antarctic stations, although in both areas abundance decreased with increased depth. Diversity was higher in the Antarctic stations, but species richness and evenness in both areas changed with depth. Iceberg disturbance was found to be very important in structuring megabenthic communities, particularly in the Antarctic.

The results of this study are used to assess the utility of towed camera sleds, remotely operated vehicles and autonomous underwater vehicles as a method for obtaining ecological information in remote environments.

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