Story: Sea floor
Page 3 – Life on the deep sea floor
The Chatham Rise
The sea floor of the Chatham Rise, to the east of New Zealand, is one of the few areas where deep seabed life has been studied in detail. Animal communities on the muddy northern flanks of the rise are different from those on the muddy southern flanks. These communities are also distinct from the animals on the rise’s sandy crest.
Biomass is highest on the crest and upper southern flanks. The seabed crest is dominated by crustaceans, especially mobile scavengers or carnivores such as the squat lobster Munida gracilis and sabre prawn Campylonotus rathbunae. Sea cucumbers and brittle stars inhabit the muddy flanks.
The composition of the different communities across the Chatham Rise sea floor seems to be related to the productivity of the Subtropical Front. This is an area above the rise where the cold nutrient-rich waters from the subantarctic meet and mix with warm nutrient-poor waters flowing from the subtropics. The movement of these waters brings nutrients into the upper waters, which supports increased plankton growth. This, in turn, feeds the animals in the lower waters of the rise.
Seamounts are prominent and widely distributed in New Zealand’s marine environment, and are often the focus of commercial fisheries. Seamounts typically have a distinctive cone shape and rise hundreds or thousands of metres above the sea floor. They are usually made of hard rocks, often volcanic in origin.
Although relatively few species are found on seamounts made of pumice or volcanic glass, many seamounts sustain very productive ecosystems. Seamounts with great biodiversity can harbour thickets of deep-water coral that provide a habitat for an array of other organisms such as brittle stars, bristleworms and squat lobsters.
Adjacent seamounts, separated by only tens of kilometres, may have entirely different groups of animals living on them. This is not solely due to differences in seamount composition, but is also related to food supply. Food supply, in turn, is influenced by the productivity of the surface waters above seamounts. Currents that disperse eggs and larvae of seabed animals can also determine the composition of seamount communities.
Animals of the deepest reaches
It is particularly difficult to study the sea floor’s deepest reaches (beyond 6,000 metres). Only a handful of samples have been trawled from the Kermadec Trench, which is the deepest seabed in the New Zealand region. There is life down there: worms, sea stars, sea snails, bivalve molluscs, sea cucumbers, shrimps and barnacles have all been discovered.