A habitat is the area where an organism or group of organisms live and breed. One habitat will be distinct from another due to its particular environmental conditions. However, habitats are not discrete, and organisms may interact with different habitats within an ecosystem. For example, organisms may move between habitats on a seasonal basis when looking for food or during different parts of their lifecycle.
Habitats range in size, and their characteristics are determined by a large number of variables. In the marine environment, these variables include light, temperaturesubstrate, wave action and oxygen availability.
The particular combination of variables results in a habitat suitable for particular types of organisms. For example, the stalk-eyed mud crab is adapted to live in mud flats of an estuary, and cockles are adapted to the fluctuating salinity of the subtidal zone.
New Zealand’s marine environment is incredibly diverse and made up of a large number of marine habitats. Scientists estimate that these habitats provide homes for up to 65,000 marine species around New Zealand (although only 15,000 of these species have been named!).
Some of the most important habitats in New Zealand’s marine environment are found in estuaries, fiords and on the continental shelf.
An estuary is a partially enclosed body of water where freshwater (often from a river mouth) mixes with saltwater from the sea. New Zealand has approximately 300 estuaries, made up of a diverse range of subtidal and intertidal habitats that vary from vegetated habitats, like seagrass beds, to mud flats, shellfish beds and tidal channels.
Estuaries are typically nutrient-rich, productive areas because of their proximity to land. The differences in ecology between habitats within an estuary are defined by the physical conditions. The dominant physical condition in an estuary is salinity. Other important factors include topography, tidal flow, the amount of freshwater and sediment entering the estuary, turbidity, and wave action and currents
Each estuarine habitat is home to a diverse range of species. For example, mudflats are full of life – crabs like to burrow through the mud feeding on the microorganisms they come across, many shellfish are partially buried by the mud and help to filter the water by feeding on particulate matter such as tiny planktonic organisms, and bacteria in the mud break down detritus that then provides nutrients for plants that form the base of the food web.
By definition, fiords are estuaries, although they look very different to estuaries you might be more familiar with!
A fiord is a steep-sided valley that has been carved out by glaciers and then flooded by the sea. The freshwater input comes from the surrounding landscapes, via waterfalls and run-off from the mountains and native forests. Fiordland National Park in the south-west of the South Island receives over 7.5 metres of rain every year. Large amounts of this freshwater make their way into the fiords, forming a distinct freshwater layer on top of the more dense seawater.
Fiordland has 14 fiords of varying depths. Where the fiord meets the ocean, the depth is typically about 100 metres. Further into the fiord, depths can reach up to 440 metres, but almost all the marine organisms live within the first 40 metres below the surface. The unique climate, vegetation and topography in this part of the country have created a number of specialised underwater habitats within the fiords. The 3 main habitats are defined by depth – the upper zone, the lower zone and deep basins.
The upper zone includes the surface down to 15 metres. Species that can tolerate the freshwater layer thrive in the top 5 metres – these include green seaweed, mussels, barnacles, shrimps and sea stars. Below 5 metres, the number of species increases, and tubeworms, sponges, soft corals, sea squirts, molluscs, sea stars, urchins, sea snails and sea slugs live in great numbers on the steep rock walls.
The lower zone is 15–40 metres below the surface. The majority of the species live on the rock walls and include large sponges, sea squirts, corals, hydrocorals and lampshells. One unique feature of this habitat is the large colonies of black coral that grow here due to the unusual conditions created by the large freshwater layer and low light levels. In other marine environments, black coral grows only at depths below 45 metres.
Below the lower zone at depths of greater than 40 metres, the habitat is suitable for a few species of heart urchins and tubeworms. Below 200 metres, the dark, muddy habitats resemble those in the open ocean at much greater depths and are inhabited by shellfish, heart urchins and crabs.
Along with all other major landmasses, New Zealand is surrounded by a gently sloping continental shelf. The continental shelf is an underwater extension of the land and is relatively shallow compared to other areas of the ocean that may be thousands of metres deep. Throughout the world, continental shelves form about 7–8% of the total ocean area, but compared to the rest of the ocean, they are extremely biologically active, well understood and commercially exploited for seafood and minerals.
Continental shelves vary widely in width and depth. The continental shelf around New Zealand ranges from a couple of hundred metres to 100 kilometres in width. The greatest depth is usually 100–160 metres, at which point, it drops sharply at the shelf break and becomes the much steeper continental slope.
The continental shelf provides a number of important habitats for New Zealand’s marine species. In the benthic zone, the major influences on habitat type include sediment type (sandy, silty or muddy) and topography (flat, small hills or small valleys).
The numerous habitats on the shelf have been divided into 3 major benthic areas:
- The shallow water zone includes depths of 14–25 metres and is characterised by well-sorted sands.
- The mid-shelf habitat is generally covered in gravel and silt, and reaches to depths of about 90 metres.
- The outer edge of the shelf is from about 90–150 metres deep and typically has a sandy bottom.
For example, off the Otago coastline, species diversity varies across the continental shelf. The mid-shelf habitats have the highest diversity and are dominated by bryozoans and sponges that prefer the rock and gravel seabeds found typically on the mid-shelf.