Food-chain relationships in subtidal silty sand marine sediments and the role of meiofauna in stimulating bacterial productivity

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Abstract
From bibliographic data the biomass correlations (organic dry weight) are constructed for the subsurface layer of a hypothetical 30 m deep silty sand station: 200 μg/ml macrofauna (including 120 μg/ml subsurface deposit feeders), 50 μg/ml meiofauna, 20 μg/ml Foraminifera, 1 μg/ml Ciliata and Flagellata, and 100 μg/ml bacteria. ATP-biomass is discussed. Meiofauna and Foraminifera contribute with 30 and 12% to the living biomass in the sediment, and it is assumed that their contribution to the food of deposit-feeding macrofauna is of a similar percentage. This is corroborated by productivity estimations. Bacteria are the main food of deposit feeding macrofauna, meiofauna, and microfauna. From different calculations it becomes evident that the productivity of bacteria in the sediment is far below figures achieved in experimental cultures: the conclusion is that sediment bacteria, in general, do not live under good environmental conditions. A rather large part of the bacterial population in the sediment seems to be in the stationary phase of life, and only a fraction of the total population exhibits high metabolic rates and rapid duplications. Only these active bacteria are of importance for the breakdown of relatively refractive organic matter in the sediment. In soft bottom marine sediments where the input of organic matter is higher than the remineralization rate, benthic animals stimulate by their activities and by nutrient cycling the decomposition of detritus via bacteria. Though meiofauna, in principle, feeds upon the same food resource as macrofauna, there is no real competition for food, because meiofaunal animals by their activities and by excreting metabolic end products induce a bacterial productivity which would not be there without them, and feed on it. There are a few examples where more specialized interactions exist between benthic animals and bacteria; these interactions have been termed “gardening”. They could be highly important in the benthic ecosystem.