Story: Sea floor
Page 2 – Life on the continental shelf
Factors influencing animal distribution
The distribution and abundance of seabed animals is strongly influenced by the nature and topography of the sea floor. Bryozoans, corals and sponges attach themselves to rock and gravel seabeds, whereas mud or sand sea floors are the realm of burrowing worms and shellfish. In addition, the nature of the overlying water (its temperature, salinity, oxygen content, current strength) and the availability of food influence where and how many animals live on the sea floor.
Ocean depth and animal distribution
There is usually a decrease in the abundance and biomass (weight of living matter) of benthic animals with increasing water depth, from some tens of grams per square metre on the continental shelf to only a few grams on the slope. The number of species of any one group of animals, however, may show a unique relationship with depth.
There are regional differences in the distribution of sediment-dwelling animals on the continental shelf. Seventeen different communities have been described around New Zealand, and their distribution is partly related to the type of sediment. Trough shells (Mactridae) are dominant in the shallow sand and gravel off the southern and south-eastern areas of the South Island, and off the west coast of the North Island.
Communities dominated by types of cockles (Pratulum pulchellum and Pleuromeris zelandica) are found in mud at the outer edge of the continental shelf and are dominant off the east coast of the North Island and northern South Island. More restricted is a bristleworm–wedge shell community, which is confined to organic muds off the South Island west coast at depths of 60–70 metres.
Bryozoan hot spots
Bryozoans are animals that form coral-like clumps on the sea floor. Although relatively widespread on the continental shelf, there are exceptionally rich and diverse bryozoan communities at both ends of New Zealand. Three hundred species, or nearly one-third of all bryozoans known in New Zealand, live on the sea floor at the tip of the North Island. Foveaux Strait, at the southern end of the South Island, is another site of high bryozoan diversity.
The sites contain sea floor sediments dominated by broken skeletons and shells of bryozoans, corals and shellfish, and provide a suitable habitat for bryozoans. They are also areas of high productivity and have strong currents sweeping through them. These factors are probably important in ensuring that there is ample food for the bryozoans and that the colonies are kept free of suffocating sediments.