substrate

noun

The base on which an organism lives; sea bed or lake bed, or other solid surface to which animals or plants attach, or on which they move; the substance forming the bottom of the sea or ocean floor; general term for any benthic habitat. Bottom or bottom materials: large boulders (>1024 mm), small boulders (256-1024 mm), stone (256-600 mm), rubble or large cobble (128-256 mm), cobble or small cobble (64-128 mm), pebble (2-64 mm), coarse gravel (32-64 mm), fine gravel (2-32 mm), sand (0.062-2.0 mm), silt (0.004-0.062) and clay (<0.004).