The previous chapter identified the predominant physical control on these systems and particularly it showed how the hydrographic regime and the sediments create space and niches for colonisation and how the hydrographic patterns can deliver recruiting organisms and food to an area. It also identified the environmental master factors creating and modifying the conditions against which the biota could colonise these biotope complexes. The present chapter defines the community ecology of the biotope complexes and discusses the interactions both between the different species, and between the animals and their environment. It aims to describe the structure and functioning of the systems whereby structure refers to characteristics at one time, whereas functioning concerns the rate processes in ecosystems.

The biotope complexes discussed can be regarded as a continuum from muds high in the intertidal region, through intertidal sands to subtidal sands. Each of these has their own characteristics and so they are treated separately despite there being common mechanisms responsible for their formation, e.g. hydrographic regime and sediment structure.

The biota of primary importance in sedimentary habitats is:

• the benthos, including the organisms of different sizes (microbenthos, meiobenthos and macrobenthos) and subdivided into the infauna and epifauna;
• the primary producers, including any benthic microalgae (the microphytobenthos) and macroalgal mats;
• the mobile epibenthos, which may be megafauna, and the vertebrate predators, fishes and birds.

The sections below describe and consider each of these biological elements for each of the biotope complexes.

Characteristic and associated species

Ecological functioning and predator-prey relationships

Biological and environmental interactions

References