Microenvironments in marine sediments

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Abstract
Shallow water and intertidal marine sediments are often heterogeneous. This heterogeneity has led to a definition of microenvironments and their associated biological species. The definition encompasses habitat and species size, species behaviour and physiology, stability and environmental interfaces. We distinguish between open and closed interfaces, and suggest an associated topological structure. Examples are given from the intertidal zone. Three of these are discussed in detail.(i)Sand grains. The occurrence of periphytic micro-organisms on sand grains is considered. The topography of the sand grain surface is closely related to the distribution of microbial colonies some of which contain a range of species. Bacteria, blue-green algae and diatoms have been identified. Species interactions and micro-variation in sediment physico-chemistry are likely to affect the constituents, distribution and abundance of the colonies. The activities of periphytic micro-organisms change the bulk properties of sediments.(ii)Banding in sediments. Characteristic banding patterns are described from a sandy and muddy intertidal shore. Marked discontinuities in the microbial flora and physico-chemical properties occur at interfaces between the bands. For example, Eh, chlorophyll levels, and sulphide can all change dramatically over a few mm. The significance of these alterations for the meiobenthic and interstitial fauna is discussed.(iii)Invertebrate burrow linings.Bioturbation structures including invertebrate burrows change local properties of sediments. The microbial and chemical properties of Nereis diversicolor burrow linings are described in detail. Sediment from the burrow lining closely resembles the sediment surface in many of its attributes. The sediment/water interfacial zone with its associated microbial and chemical properties is therefore extended vertically into sediments by these structures. Attention is drawn to the stabilising function of burrows and to their palaeoecological significance.