We often talk about the incredible diversity of species found within the Hauraki Gulf Marine Park/Ko te Pātaka kai o Tīkapa Moana, without mentioning the equally varied marine habitats that support their life. Sponge gardens, seagrass meadows and kelp forests conjure up magical images, while habitats formed by animal life, such as shellfish beds, sound curiously like names for rock bands… horse mussels or dog cockles anyone?
Both types of habitats are called ‘biogenic’ which means ‘formed by life’. For example, a biogenic habitat can be made of a living organisim such as kelp, and from ‘dead’ material created by the activities of organisms. A good example is the shells of molluscs (such as green-lipped mussel/kuku), which, as shellfish beds, provide 3D structures for barnacles to adhere to, or fish larvae to settle amongst.
You could think of biogenic organisms as the architects and construction workers which turn the basic materials of the sea floor into living cities. They do this by engineering the areas around them to support species, by making resources such as oxygen and nutrients available, as well as places to attach or shelter in.
These communities are found throughout the varied underwater landscape of the Hauraki Gulf, ranging from the shallow rocky reefs of our coastline, to the soft sediments of our estuaries and harbours, and the exposed offshore islands and reefs found on the outer edges of the Park.
Within each landscape, three factors determine the type of habitats found like patchwork within them: depth, water current, and type of sea floor sediment (i.e. sand, mud and gravel). For example, depth determines how much sun penetrates to the sea floor for species such as seaweeds to photosynthesise, while the strength of water current dictates which species can live there. Likewise, sea floor sediments affect what kind of infaunal species (organisms that live in sediments) can burrow into it. Each slight adjustment in factors changes the habitat and type of species found there.
Gardens and apartment life
Let’s begin with sponge gardens. Their very name suggests they are plants, and spongy in texture. But sponges are in fact a type of animal with an internal ‘skeleton’ composed of strong and flexible fibres made from ocean sediments, while others make small hard structures. They are filter feeders which means they extract particles and nutrients from the water, cleaning the water as they go. Sponges grow in nearly every environment possible, are every colour imaginable, and come in just as many sizes and shapes: giant barrels, thin tubes, deep bowls, encrusting and branching fans.
Individually, they behave like underwater apartment buildings, providing homes for species such as tiny shrimp. They can become gardens in shaded or deep water where algae and seagrasses can’t grow, providing ideal conditions for species such as anemones, bryozoans, barnacles and soft corals. Sponge gardens are also important feeding, spawning and nursery sites for many fish. You can see wonderful examples of sponge gardens at Cape Rodney-Okakari Point Marine Reserve (Goat Island), near Leigh.
Let the sunshine in
Where sunlight can penetrate to the floor of our rocky reefs, kelp forests grow. Often described as the marine equivalent of trees, kelp forests are considered one of the largest and most productive of biogenic habitats, supporting a wide range of species.
Kelp is not a plant, but a type of brown algae, and can be found in a stunning variety of shapes and sizes, some growing up to 20 metres high. As photosynthesizers, kelp stores carbon and produces oxygen, and as forests, lower the acidity of surrounding waters. Kelp forests support many important fish species including kokiri/leatherjackets, red moki, pakirikiri/spotted wrasse, and kokopara/triplefins, while their blades (leaves) are common hosts for all kinds of larvae. You can explore lush kelp forests on the rocky reefs of Long Bay – Okura Marine Reserve, in north Auckland.
Kelp also has an enemy. Kina/sea urchin is a major predator, chewing their way through kelp forests almost unchecked. This is due to the significant reduction of tamure/snapper and koura/rock lobster who are, in turn, kina/sea urchins natural predators. This produces kina barrens, literally areas that are now bare because of the kina chewing through the kelp.
Meadows make ideal habitats
Found in the sheltered waters of our shallow tidal areas and estuaries, seagrass meadows are one of the most productive ecosystems in the world, providing ideal habitats for juvenile fish including tamure/snapper and invertebrates such as crabs, shrimp, and squid. In a recurring theme of not being quite what they appear, seagrass is not seaweed, but a flowering land plant which evolved to live in the ocean roughly 100 million years ago.
In New Zealand there is only one species of seagrass called rimurēhia which grows thin, ribbon-like olive green leaves. Rimurēhia can be found in both intertidal and subtidal areas, the latter of which is naturally rare and under threat from trawling and the anchors of recreational boaties. Seagrass patches become seagrass meadows when they are one hectare or larger. Meadows were once abundant in the Hauraki Gulf, but have suffered from excessive dredging and trawling of the seafloor in the 1930s. Because of the way they reproduce, existing seagrass meadows can easily expand but are difficult to establish from scratch, making them a priority for protection from further damage.
Getting habitat muscle from mussels
Another important habitat in the Hauraki Gulf that suffered from destructive fishing practices in the earlier half of the 20th century are green-lipped mussels/kuku. In the ocean, kuku grow in dense ‘beds’ in patches more than 100 m2. These kuku beds provide a place for other species to live and grow on vast areas of the seafloor that would otherwise not support much life. In the average kuku bed, you’ll find larvae of fish and shellfish species, seaweed, barnacles, and other invertebrates such as crabs, shrimps, polychaetes, octopus and starfish. Like sponges, mussels are filter feeders – a single kuku can filter an incredible 250 litres of water in a day.
A number of mussel beds have been installed around the Mahurangi Harbour by Revive Our Gulf to create living fish nurseries and help clean the waters of the Gulf. A whopping 70 tonnes of mussels have been laid on the seafloor, and are already supporting species such as baby snapper, koheru, goatfish, spotties and even squid.
Nineteen million reasons to love mangroves
An often-maligned marine habitat is mangroves. Found in the thick, silty mud of sheltered tidal areas, some people blame them for lowering the amenity value of coastal areas. Mangroves have in fact been in Aotearoa for at least 19 million years and are one of the most productive ecosystems on the planet. These amphibious-like trees survive twice-daily submerging in saltwater, and support life such as worms and crabs which feed on their detritus, and barnacles and mussels which grow on the trees’ lower branches and roots. At high tide, aua/yellow eyed mullet, patiki/flounder and other finfish feed and spawn among the mangrove roots. Mangroves also trap sediments in the web of their roots, and help store carbon, making them invaluable in the fight against climate change, sea-level rise and coastal inundation.
One of the best examples of mangroves can be found in Motu Manawa (Pollen Island) Marine Reserve found adjacent to one of our busiest motorways, State Highway 16. Meaning “island of mangroves”, Motu Manawa provides rich feeding grounds for matuku moana/white-faced herons, pūweto/potless crake and the endangered moho-pererū/banded rail.
Tumbleweed to help tiny fish grow
The last habitat we’ll mention is one that’s on the move. Rhodoliths are a type of algae, the same organism that forms seaweed. Rhodoliths produce calcium carbonate, much like the shell of a mussel, but in a beautiful pink colour found in a huge diversity of forms, shapes, and sizes. As they aren’t permanently attached to the seafloor, they’re sometimes called “ocean tumbleweeds”, as currents and waves send them rolling across the seafloor. Clusters form a rhodolith ‘bed’ which are important nurseries for juvenile fish and invertebrates, forming a safe refuge for eggs and larvae to grow. Some have even been found to produce specific chemicals which attract paua larvae to settle and support their early development.
Rhodoliths grow extremely slowly, often just a fingernail thickness per year, which means that they are slow to recover from threats such as sediments which smother and starve them.
Restoration and recovery for a healthy Gulf
Even amongst the few habitats mentioned here, it is easy to see how the Hauraki Gulf’s biogenic habitats are the bedrock of its biodiversity. Sadly, the pressures of marine and land-based activity over the last 150 years has had a profound effect on these habitats, especially those of our coastal urban harbours and estuaries. Habitat loss, sedimentation, pollution, over-fishing and the invasion of non-indigenous marine species have all reduced the abundance of marine biodiversity of the Gulf.
Restoring these habitats will require a range of tactics, the primary of which is creating more marine protected areas (MPA) which will enable these habitats to recover. Active restoration such as the establishment of shellfish beds and seagrass meadows are crucial too. Healthy, intact marine ecosystems benefit all of us whether we look at the Gulf, swim in it, gather from it, sail on it or simply enjoy its place in our lives.
Love It, Restore It, Protect It.