The islands of Rangitāhua lie half way between the eastern North Island and Tonga. There are 18 islands and islets in the Rangitāhua chain extending about 300 km from Nugent Island in the north (29°13'54"S, 177°52'10"W) to L’Esperance in the south (31°21'16"S, 178°49'29"W). Thirteen of these are very small and steeply sided rocky outcrops with very few sheltered areas. The two largest islands are Rangitāhua (also known as Raoul Island) and Macauley Island. Rangitāhua is the indigenous title to Raoul Island as well as to the entire Kermadec Island region.

The islands of Rangitāhua are surrounded by very clear oceanic waters that are generally nutrient-poor (oligotrophic), although nutrient-rich water is brought to the surface by currents and tidal movements. Dense seabird communities on some of the Rangitāhua islands also contribute nutrients to the coastal waters and to the growth of the rimurimu.

Rimurimu can be found in a wide range of habitats – on rocky reefs, boulders, cobbles, shell rubble, pumice, coral, and, in some places, on coarse gravels and sand. They have been found from the intertidal zone through to the mesophotic zone (ca 30–150 m depth). Light available for photosynthesis declines with depth to the point where algae are no longer able to survive.

Some species at Rangitāhua are also found on mainland New Zealand, others are only found on these islands (endemic), while some species are also present in neighbouring areas of the subtropical and tropical Pacific.

Both intertidal and shallow subtidal zones on these rugged islands are exposed to strong wave action and oceanic swells. There are some distinctive features of the growth forms of macroalgae at Rangitāhua – adaptations to the conditions in the region.

Many species grow as crusts or cushions closely attached to substrates and protected from some of the energy of the waves. This prostrate growth habit is seen in species of green, brown and red algae (e.g., Fig. 1A–C). The most conspicuous crusts are non-geniculate or crustose coralline algae (Fig. 1A, C).

In both the intertidal and subtidal zones there are dense patches of turf (Fig. 1A–D) made up of many different types of rimurimu. Red algae make up more than half the Rangitāhua flora and the vast majority of these are less than 2 cm in height, and many of these species grow as turfs or partially prostrate in intertwined, multispecies assemblages. Some of the turfs include articulated or geniculate coralline algae. Even in a small clump a few centimetres in diameter there can be more than 20 species.

A number of species have stolons (e.g., Figs 1D, 2A–C) – stems that attach to the substrate and produce new upright branches, with the stolons weaving in and out of the turf, providing some protection from herbivory. Stolons enable both firm attachment and also the potential for recovery and regrowth if there is any damage from storms or grazing. They can be found in species growing on gravels and sand, often resulting in firmly consolidated patches with the intertwined stolons. Stolons are also found in many of the turfing species providing multiple attachment points to secure the rimurimu in the wave exposed habitats.

A feature of Rangitāhua macroalgae is that most species are quite small – with the majority less than 5 or 10 cm in height. There are a few species that are taller and some of these, such as Asparagopsis taxiformis and Dictyota intermedia, can form dense beds. Some other species such as Delisea aweawetai and some species of Liagora can also reach more than 25 cm in height. The largest brown algae are species of Sargassum which appear to have an annual cycle of growth and are largest in spring and by autumn are small and inconspicuous.

In the coastal waters of the islands of Rangitāhua macro­algae are important primary producers, fixing sunlight energy and providing food for a range of invertebrates and fishes. The nutrient status and growth of macroalgae in the region are enhanced by nutrients contributed by seabirds nesting on the islands.

Although macroalgae at Rangitāhua are much less conspicuous than kelps in colder waters, they play a key role in providing three-dimensional habitat space for a range of fish and invertebrate species. They provide physical shelter and camouflage as well as surfaces for other species to settle on – ranging from bacteria to other algae and invertebrates.

Rimurimu, particularly species of coralline algae, can play key roles in life histories of other species.

Some invertebrates such as corals, settle onto coralline crusts changing from larval stages and developing into their adult forms, with their metamorphosis triggered by chemicals the coralline crusts release. Other species settle preferentially onto branched corallines – getting protection from grazers.

The green seaweeds of Rangitāhua are visually dominated by two genera (Caulerpa and Codium).

There are three species of Caulerpa at Rangitāhua (Fig. 2A–C), ranging from bright to dusky green. This group features stolons that help them to remain attached to the rocks while growing outwards.

Caulerpa chemnitzia (previously known as C. race­mosa in this region) has trumpet shaped ‘leaves’ (Fig. 2A), C. nummularia has flattened, disc-like leaves, often with teeth around the edges (Fig. 2B), and C. webbiana has very small leaves, tightly clustered along stems, giving a pipe-cleaner appearance (Fig. 2C).

There are a variety of other green algae found at Rangitāhua including bushy Bryopsis, bladed species of Ulva and Ryuguphycus, and very fine, filamentous algae such as Cladophora (Fig. 3A) and Lychaete. Species of Codium (Fig. 3B) are usually velvety crusts or upright fingers and are typically bright to dark green. There are six species of Codium at Rangitāhua, including a new species named by Ngāti Kuri, Codium tohuoratai, as well as one that forms a large thin-walled sac. The thin lace-like sheets of Microdictyon (Fig. 3C) can be found on the reefs and mixed in the turfs. Bright green crusts of Palmophyllum umbracola (Fig. 3D), can be found both at Rangitāhua and in the far north of the North Island. This species was recently discovered to belong to a different lineage than other green algae (the Prasinodermatophyta) – most other species in this group are single celled plankton.

The brown rimurimu of Rangitāhua are dominated by members of the order Dictyotales (Fig. 4A–D) but also include some diminutive species and crusts. Dictyota intermedia is conspicuous subtidally and often abundant, in places forming thickets and providing shelter and camouflage for small fish and invertebrates and is one of the taller species on the reefs growing to heights of ca. 25 cm (Fig. 4A). Other species of Dictyota are very small, with narrow dichotomously branched axes and are often iridescent (Fig. 4B). Fan shaped species of Dictyotales include Padina, which is partially calcified (Fig. 4C) and Distromium species (Fig. 4D).

The largest brown algae in the coastal zone belong to the genus Sargassum (Fig. 5A). There are at least 3 species of Sargassum at Rangitāhua. They appear to be seasonal, and they are also heavily browsed by herbivores. Some escape herbivores by living on limpet shells or on cobbles or boulders surrounded by sand or gravel making it harder for particularly urchin herbivores to reach them.

The kelp that grows in northern New Zealand – Ecklonia radiata – has only been found in deep colder waters of the Rangitāhua region (at 70 m depth).

Other brown algae include sacs such as Colpomenia (Fig. 5B) and Hydroclathrus and a range of small filamentous species as well as brown crusts (Fig. 5C).

There are more species of red rimurimu than the greens and browns together. Just like the greens and browns, most of the red rimurimu are small, with most less than 5 cm in height.

Asparagopsis taxiformis (Fig. 6A) is very abundant, found growing on rock faces and on compacted sand and gravel, extending along the substrate with stoloniferous growth. This species is widely distributed worldwide in warm waters. The variety present at Rangitāhua is also found in Hawaii.

Delisea aweawetai found from the shallow subtidal to depths of ca. 30 m, is the largest species of red algae found at Rangitāhua, reaching a height of 25 cm or more, and common in the coastal reef areas. Delisea (Fig. 6B) is dark red in deeper areas but in sites that are brightly lit it bleaches to a yellow/khaki colour in the upper branches. This species has been known for a long time and was thought to be the same as a species in Australia. However recent work has shown it to be distinct and it has been given the name Delisea aweawetai by Ngāti Kuri.

Martensia (Fig. 7A) is very beautiful with a lace-like network around the edges of the blades. There are two species at Rangitāhua currently without scientific names. Sometimes the blades are a milky, pink and blue colour underwater.

The coralline algae (Fig. 8) at Rangitāhua are particularly diverse – both the branched and unbranched growth forms. They form a conspicuous part of the shallow subtidal but also extend to great depths with some recorded as deep as 145 m. There are a number of species of branched corallines at Rangitāhua, most of which are less than 5 cm in height.

The crustose corallines vary a great deal in appearance in colour – from pale pink through to bright purple/pink – as well as texture – from smooth patches to convoluted lumpy crusts, to delicate layers and disc like growths (Fig. 8). These encrusting corallines can be found in the intertidal zone through to deep waters growing on rocks and cobbles, as well as growing as epiphytes on other rimurimu, or on some invertebrates. There are also free-living coralline algae or rhodoliths.

There are many species belonging to a group of red algae called the Nemaliales (Fig. 9). These vary a lot in appearance and belong to a number of different genera. Some are calcified, others have a woolly appearance, and some have a very soft and mucilaginous texture. Most of these species do not have current scientific names.

Global climate change and biodiversity loss are recognised as two of the most important challenges and risks facing the world. Climate and biodiversity are strongly linked with each other, and with the future and well-being of human communities. Rangitāhua is not immune from the global stressors linked to climate change, and rimurimu are affected by a range of climate change impacts – increased temperatures in the sea, altered patterns of rainfall, sea-level rise, increased frequency and intensity of weather events, and oxygen depletion and ocean acidification. Different species vary greatly in their responses and tolerances to these changing environmental conditions. The calcified species of rimurimu are particularly vulnerable to the impacts of ocean acidification: ocean acidification reduces coralline algal habitat complexity and is projected to adversely affect ecosystem services.

Changing conditions also influence the other marine life at Rangitāhua potentially resulting changes in the ranges and abundance of predators and herbivores which in turn are likely to influence the rimurimu of the region.

The oceanic volcanic islands of Rangitāhua occupy a unique position alongside the adjacent trench, and as such can serve as a sentinel and a bellwether of change, offering the potential to guide understanding of future trends and pathways.