Structural aspects of the deep-sea benthos

The availability of food is believed to be the main factor structuring the deep-sea benthos. Results are presented for the quantitative distribution of meiofauna from different oceanic regions. Meiofaunal densities are predictable to a certain extent, and further investigations should be done in conjunction with the other benthic size groups for an evaluation of the total benthos. The size structure of the benthic communities is discussed for the infauna. Standing stock of megafauna seems not to fit into the system, however, data are restricted to continental slope and rise areas and pelagic feeding of fishes, believed to be demersal, result in a biomass too high for benthic food resources. The distribution of detritus and fliter feeders, ontogenetic development, internal structures, content of organic matter per size and weight, low diversity in abyssal regions, and behavior of opportunistic species, all can be viewed under the aspect of severe food limitation, and presumably reduced energy demand, low maintenance costs and high conversion efficiency. Food energy structures the deep sea benthos. The structure of benthic communities is an aspect which was followed from the beginning of biological deep-sea research. While in the early years investigations were addressed to taxonomic questions, the interest later centered around life forms, on the adaptation of species and communities to deep-sea conditions. Ecological factors structuring the deep-sea benthos are described in a number of publications, however, one factor seems to be of overall prime importance: energy in the sense of food availability. The abundance of meiofauna Prior to 1960 no idea was spent on the quantity of meiofauna in the deep sea. Several papers had appeared on meiofaunal taxa, beginning with the Challenger Reports on Ostracoda (1) and Foraminifera (2) and ending with the Galathea Reports on Nematoda (3) dealing with taxono mical problems. It was Wieser (4) who first pointed to quantitative aspects. Without having data, he theorized a decrease in meiofauna abundance in a constant ratio to the already known decrease in macrofauna abundance, postulating for the deep sea the same community structure as established in shallow waters. During the following years some effort has been paid to gather quantitative data. On the abundance of meiofauna not much informa tion is available, however, anticipating one conclusion of this paper, today we do have enough results on meiofauna densities already. Such a statement may sound somewhat premature, but this is elucidated by some results. In figure 1 all those regions are marked, where quantita tive sampling on meiofauna in the deep sea was performed. The distribution of research activities is far from random. Vast ocean areas have not been sampled and mostly the results are based on limited sample numbers and comprise some variation in abundance. However, the data we have, are conclusive under one general aspect and that is the transfer of food energy to the benthic environment. A few examples may demonstrate this: Dinet (5) found higher meiofauna densities south of the Walvis Ridge in the Cape Basin than north of it in the Angola Basin. The Cape Basin exhibits a higher produc tivity in surface waters due to coastal upwelling and this is reflected on the bottom of the deep sea by higher meiofaunal densities, still in the depth of more than 4000 m. Differences of meiofauna abundance on the two slopes of the Iceland-Faroe-Ridge are related to currents. While in the Norwegian Sea currents are neglectable and food sedimentation takes place, the intermittant overflow of water from the Norwegian Sea into the Atlantic Ocean, reaching current speeds up to 50 cm/sec, and the water from the Faroe-Bank-Canal impinging on the Atlantic slope in around 1000 m depth, do not allow settling of food particles or wash them out of the sediment after they may have settled during low current intensities. Accordingly, meiofauna densities in comparable depth are higher on the Norwegian Sea slope than on the Atlantic slope (6). Data from Josephine Seamont, some 300 nautical miles west of southern Portugal, warrant a similar explanation. The production of organic matter in surface waters is the same for the reaches of this seamount. Currents allow for a lesser sedimentation of food on top of the seamont, than on the deeper slope. Accordingly, a slight increase of meiofauna densities was found between top depth with about 200 m and slope depths with 1700 m to 1900 m.