The Sub Thermocline Zone
The sub thermocline zone is the most extensive area of the sound, occupying all depths below 17 m. The extensive mud bottom of this zone exhibits a smooth and barren surface, marked only by scattered burrows, crustacea and the polychaete Spiochaetoperus oculatus and patches of thin purplish algal or bacterial mat. This zone is marked by the common depth of the summer thermocline and is exposed to the oxygen-depleted bottom waters of the hypolimnion. This seasonal lack of oxygen and the presence of hydrogen sulphide limits the biota of the sub thermocline zone to mobile seasonal invaders.
Fine-grain sediment fractions of the sediments increase
in abundance with increasing depth. They are produced by several sources.
The principle sources are: (a) The biochemical degradation of skeletal
material, especially from calcareous algae, (b)The action of boreing sponges
and other boreing or browsing organisms such as rock boring molluscs and
echinoids, and (c) limited physical attrition of friable rock and delicate
skeletal material in the shallow turbulent zone.