Marine Biology

Types of Organisms

Oxbow theory

In contrast to sandy beaches, muddy shores often develop a substantial growth of various plants. On the bare mud flats, the most abundant plants are diatoms, which live in the surface layers of mud and often give brownish colour to the surface or low tide. Other plants include large macroalgae, such as species of Gracilaria (red algee), Ulva, and Enteromorpha (green algae). The large algae often undergo seasonal cycles of abundance, becoming common in the warmer months and virtually absent or disappear in cooler months. Other areas, particularly the lowest tidal levels, may be covered with a growth of various sea grasses, such as the genus Zostera. As a result of the occurrence of these primary producers, there is substantial primary productivity in the mud flats.

Mud flats contain large numbers of bacteria, which feed on the abundant organic matter. Bacteria are the only abundant organisms found in the anaerobic layers of the mud shore and constitute a significant biomass. These bacteria are capable of using the potential energy of the various reduced chemical compounds abundant here. These chemolithoautotrophic or sulphur bacteria obtain energy through the oxidation of a number of reduced sulphur compounds, such as various sulphides (e.g. H₂S). These organisms are thus primary producers of organic matter analogous to green plants. They produce organic matter using energy obtained from the oxidation of the reduced sulphur compounds, whereas plants produce organic matter using energy obtained from sunlight.

Since these autotrophic bacteria are located in the RPD layer of mud, mud flats are unique among marine environments in that they have two separate layers in which primary productivity occurs: the surface, where diatoms, algae and marine grasses carry on photosynthesis; and a deep layer, where bacteria conduct chemosynthesis.

The dominant macrofaunal groups on muddy shores are the same as those encountered on sand beaches and sand-flats namely, various polychaete worms, bivalve molluscs, and small and large crustaceans, but of different sorts.