Marine habitats, principles and factors

What physical and biotic factors are found in marine habitats?

By Dr J Floor Anthoni (1997)
The sea is a weird environment. The marine environment is stranger than most of us can imagine, and its organisms have adapted in equally strange and surprising ways. But what are the main principles for understanding marine habitats?  Learn about habitats, life-determining factors, limiting factors, habitat types, depth zoning and the sea bed. Understand what it means to live in the sea.
(See also Oceanography and ecology)


Related chapters:
Rocky shore ecology of the intertidal rocky shore, important principles and shore organisms from New Zealand.
Marine conservation and how the sea differs from the land.
Marine reserves: Goat Island, Poor Knights, Kermadecs, Niue,
Soil ecology and biomes of Earth, including a classification of rocks, soils and biomes.
How the sea suffers from land-based pollution, causing massive degradation, and how this works.
Our most recent discovery about how the sea really works, which was overlooked by science.
-- home -- habitat index -- Revised:19971014,1201,20010713,20070710,

What is a habitat?
Literally translated, a habitat is 'a place where an organism lives' but that is not a very helpful definition. A habitat is a place where communities of organisms live, in such a way that one habitat is recognisably the same for that type but equally recognisably different from another type, like forest differs from pasture, which differs from savannah, which differs from tundra.

The easiest way to understand the concept of a habitat is that it is first and foremost caused by physical factors such as (on land) substrate (rock, sand, soil), temperature and rainfall. A certain combination of the three allows only a particular community of plants to live there or that these out-compete all others. Since grazing and browsing animals depend on plants for a living, only a certain community of grazing animals can live from the established community of plants. Carnivores that depend on the grazing animals, complete the picture. To make matters more complicated, animals influence the plant community (like possums killing our native trees, sheep eating grass and tree seedlings) and plants influence the substrate by producing humus and litter. Furthermore, organisms compete for a place, whether they are grazers or predators. Add to this millions of years of co-habitation and the habitat and its community evolve into an interdependent unit where one organism can no longer be separated from the whole. That is a habitat.

We talk about habitats, we see and measure communities but we know little about how it functions.

The three main habitats of the ocean are:

Obviously the transition from open water to soft bottom is quite sudden, and likewise that from water to rock. But there is a transition area. For instance, the hard rock is covered in soft organisms and seaweeds and there are many fish that feed in the open water but sleep on/in the rock. Likewise the sandy bottom may be burrowed into for 30cm and above it one finds fish in the open water, yet always living close to or on the bottom. A cobble beach may have a community employed from both rocky shore and the sea bottom.

In recent years quite a lot of confusion has been propagated about biodiversity and habitats, to the extent that we now firmly believe that we must protect every possible habitat in order to protect biodiversity. But this is not true. Biodiversity and habitats live purely in human minds, and not out there in nature where one encounters only communities. Every community (or habitat as we believe) is made up of species that are also members of other communities (habitats). For instance, a tube worm normally found in the sea bottom, can also be found lodged in a crack on the rocky shore. A rocky shore dweller (sponge) can also be found stuck on a shell on the sandy bottom.
Of the above three basic marine habitats, the rocky shore is the smallest and has also the highest biodiversity for its small size. It follows that it needs the most protection. Yet commercial fishing focuses on the other two habitats, which leaves the rocky shore relatively unexploited (with some exceptions). But the rocky shore is most threatened by runoff from the land, a threat that is difficult to mitigate.

Another important property of the sea one should always keep in the back of one's mind is:

There are many lands but there is only one sea

The terrestrial world is broken into compartments by islands, mountain ridges and even by rivers. In each compartment, over time, a different community evolves, as long as that community has weak or no connections with neighbouring compartments. This has resulted in a high degree of biodiversity on land. However, in the sea this is not the case since all seas are interconnected, such that animal eggs and plant spores can travel unimpededly to any other place in any other sea. The sea should thus have much less biodiversity than the land, but this is not so - a vexing paradox that is answered in part by this introduction to habitats. For more about this, read biodiversity, marine conservation, frequently asked questions about marine conservation, marine degradation and our extraordinary discoveries: the plankton balance and DDA for dummies.

Too many people are unaware that they are insufficiently informed about the sea and why it is so different from the land and what it really needs by way of protection. This introduction to habitats hopes to make the first step.

What physical factors determine marine habitats?
This part is very important for knowing how sea creatures live. On the land, rainfall and temperature are the most important influences for living creatures. In the sea, this is very different.

In the sea the same reasoning for why one habitat differs from another, applies as on the land but the physical factors are different. Let's have a look at all the possible physical factors and it will soon be obvious that a large combination of these creates an equally large variety of habitats. (factor=what makes, circumstance, influence, from Latin facere=to do; to make).

sea temperature New ZealandTemperature:  The machinery of life is provided by complex molecules called enzymes. These make chemical reactions possible at the minimum amount of energy, but their active temperature range is limited. That is why temperature has a major effect on living organisms. For instance, a difference of only 3 degrees can double or halve an organism's metabolic rate. That's why lizards need to warm up in the morning and it is also the reason why warm-blooded animals perform better than cold-blooded ones. On the land, temperature differences are already decisive but the water being 800 times denser than air, has a profoundly cooling effect. For water animals to maintain a temperature only slightly higher than the surrounding water is a major feat which requires much energy and thus also food (whales and dolphins, tuna and swordfish). Temperature appears to affect grazing fish more so than predators. This explains in a way why the sea has so many climatic regions from north to south, compared with the land. In NZ the difference between the North and the South is about 6 degrees, which is about the same as between summer and winter in one place.

Salinity: Aquatic animals live in the water. They 'drink' the water and they must have inbuilt mechanisms to maintain their body salts at a constant level which always differs from that outside. Land animals by contrast, only need to protect against dehydration (losing water) and losing body salts. Humans for instance are not capable of surviving with salt water as our sole drinking water. We are not able to shed excess salt while maintaining our body salts.

Salinity changes gradually in river mouths and estuaries. The difference between seawater (3.5% by weight) and fresh water (0%) is enormous. Only few plants (eelgrass, mangrove) and animals (eel, whitebait, salmon) can make the transition. It stands to reason then that various differing habitats can be found inside estuaries and saline rivermouths, but these have very few species. Out in the open sea, salinity changes very little but some inland seas exist with remarkably different consistencies (Mediterranean Sea, Red Sea, Black Sea, Caspian Sea, Dead Sea)

Salinity also affects flotation. Salt water being heavier than fresh water, has more lift than fresh water (an adult weighs 2-3kg less in salt water as in fresh water, e.g. 3.5% of 80 kg). Objects that just float in salt water may sink in fresh water (like mangrove propagules). Scientists use the specific gravity of seawater or 'density' which is related both to dissolved salts and temperature. (See Oceanography/sea water)

Tides: The forces of moon and sun on the sea level produce a twice daily (or once daily in some places) rise and fall, accompanied by tidal currents (tidal stream). The effect of this on the top two to six metres of coastline is profound. Animals and plants can live here only when capable of resisting heat, cold,dehydration and being capable of living in tide pools with exaggerated salinity. This littoral zone (intertidal zone) leads to a high variety of specialised organisms. (See oceanography/tides and rocky shore)

Currents: Three kinds of current exist: those produced by wind, the tides and global circulation. Winds blowing along the shore can whip up substantial long-shore currents. When blowing across the shore, they can transport the warm surface layer out to sea or towards the land. Persistent wind-currents can affect the underwater communities.
Tidal currents (0.5-5km/hr) are reversing four times a day and cause the mixing of waters (salinity, temperature and nutrients). They also cause eddies that separate bodies of water, and they distribute phyto- and zooplankton. As a result, tidal currents are very important for sessile filter-feeding animals.
Currents from global circulation are slow (0.5-1km/hr) and play a very important role in distributing the heat from the tropics to the poles and the cold from the poles to the tropics. At places they can cause upwellings which are often rich in nutrients. Global currents affect our climate. (see oceanography/currents)

Wind: Wind is a land-phenomenon that won't affect most aquatic organisms. But when living in the intertidal zone, desiccation (drying out) wind can become more decisive than sunshine and heat. Wind causes waves and these can change the substrate in shallow harbours from mud into coarse sand. Wind causes waves that pile up the sand in long, sandy beaches where only a very few specialised organisms can live.

Wave action: Waves are caused by the prolonged action of wind on the water's surface. First turning it into ripples, the surface becomes more and more disturbed, eventually resulting in tall breaking waves. The taller the waves, the deeper their effect but their force decreases rapidly with depth. Two metre high waves, for instance have lost nearly all their power at six metres depth. But the force of waves develops over the distance the wind has been able to build them up. This is called their fetch. Short waves or 'chop' (short fetch) lose their power rapidly with depth but long waves (long fetch) don't. Long waves (swell) arise from distant storms. With inter-crest distances (wave length) of hundreds of metres, these waves can travel 60-100km/hr, fast enough to cover thousands of kilometres in a day. One can experience a beach with dangerous and powerful breaking waves during an otherwise beautiful and calm day! These long waves cause massive movements of water along the sea bottom (ground swell) where the depth is much less than their wave length. Eventually these slow waves increase dramatically in height (a bit like tsunamis do) before breaking in spectacular ways on the beach. This is the kind of surf that surfies like. For the underwater environment, however, the slow waves can be catastrophic, something proved by the dead animals and plants washed up after a big storm. Ironically, as the effect of wave action decreases with depth, it increases again near the bottom. (see oceanography/waves)

Light: On the land, the intensity of the light does not change noticeably with height, although mountainous areas receive more ultraviolet light than lowlands. In the water the light diminishes with depth and in doing so, also its quality changes. First the infrareds disappear, then the ultraviolets, then the reds and finally the yellows. The deep sea turns from dark purplish to black and is completely dark at 600-800m even in the clearest of waters! The light-transmissive qualities of the water (visibility) changes considerably from muddy estuaries (0.2m) to coastal (6m) to barrier islands (15m) to open ocean (40m) to antarctic (100m). Visibility is affected by leachates ('tea'), mud particles ('coffee') and phyto plankton ('soup'). Each absorbs the light in its specific way, resulting in different colours. (brown, milky, green, red, blue). Underwater photographers have to cope with these conditions too! As soon as the camera goes under water, only half to one quarter of the light remains. If the sea has waves or ripples, another half goes. If there's a foam blanket, another half of the light disappears. At 5m the light has halved again and at 10m again, then again at 20m, 30m, depending on the quality of the water. In good visibility one takes photos at 20m depth in 1/20th-1/50th the amount of light (400 ASA f5.6 1/60s) compared to the surface situation (f16 1/500s). Put it another way, with only 2m visibility it will be darker than a dark (no moon) night at 20m depth while the sun is shining brightly at the surface! This is to illustrate that light, or rather the lack of it, affects the underwater world in a most decisive way. (see underwater photography/water)

Plants need light and they can grow only where light is adequate. Each plant species has its own light requirements. Some have pigments to prevent sun-burn, others iridesce to improve efficiency. Three seaweed groups exist, having different kinds of chlorophyll and different light requirements. The green seaweeds need more light, the brown seaweeds less and the red seaweeds less still (this rule is not always clear, however). (see oceanography/radiation and rocky shore)

In the intertidal the sunlight can cause intense heat in the short period of low tide, resulting in drastic variations in the organism's body temperatures. Some molluscs are particularly resistant to being 'cooked'.

Substrate: Substrate is the sea bottom. It can be hard, soft, coarse, smooth, muddy, sandy, shelly, grainy, pebbly, bouldery. Even organisms can act as substrate for other organisms: leaves, roots. The substrate can be broken, fissured, shelved, tunnelled, caved, arched. This all matters to the underwater communities.

Aspect: The slope of the shore is important. Steep shores bounce waves back without dissipating their energy whereas mildly sloping shores dissipate all the waves' energy without bouncing the waves back. A flat or gently sloping sea bottom faces the sunlight better and gets an even amount of light from morning to evening. But steep walls may never face the sunlight or receive it for only short periods of the day. It makes a difference whether the shore faces north or south.

This table shows how strange it is to live in the sea.
Factor Living on the land Living in the sea
Temperature Fluctuates enormously from night to day, winter to summer and from sea level to mountain top. It is relatively easy to protect against the cooling or heating effect of air as air is 800 times less dense than water. Animals can be warm-blooded, which allows them to live in hostile places. Fluctuates little from night to day and winter to summer. The water is 800 times denser than air and its cooling effect is tremendous. Creatures cannot spend the energy to stay warmer than their surroundings and are mainly cold-blooded. Warm-blooded whales and dolphins live in the sea but they don't breathe water! Cold blooded animals spend very little energy when sitting still. They go on 'stand-by'.
Salinity Does not apply. Living IN the water can cause water and salts to enter or leave the body through the skin (osmosis), which creates a real problem for many creatures.
Tides Do not apply. Where the sea begins, all land life ends. While perfectly adapted to living on land, the land creatures have not ventured back into the sea, although some exceptions exist (mangrove, sea grass, dolphins, etc.) The intertidal zone is poor in land creatures.  While perfectly adapted to the water world, creatures find the dry world impossible to live on. Yet some have found ways to survive out of the water for short or long periods. The intertidal zone is rich in sea creatures. Tides cause currents near the coast. These are important for sessile (attached) sea life.
Currents Do not apply. Air is light and although some creatures can fly, they must eventually rest. The air is not filled with food, like the sea. The water makes creatures float. It fills the water with food. Currents push this food past filter-feeding organisms that sit on rocks. Currents are very important for distributing food and nutrients and for transporting mud from the coast. Eggs and larvae are transported easily and far.
Wind Wind is not quite like sea currents or waves but may cause similar effects. Storms can cause major damage and continuous winds can influence the communities decisively. Wind can blow salt spray into the coastal vegetation, thereby selecting those species that can cope with salt on their leaves. Wind is felt only in the intertidal zone where it can accelerate cooling and drying out.
Wave action Not applicable but wind takes its place. Wave action is enormously destructive because water is 800 times heavier than air and waves move to and fro, rather than in just one direction. Long waves can travel very long distances and may be felt for many days. Waves are very important in determining what grows where. Waves destroy but they also cleanse organisms.
Light Light is very important to plants. Light quantity is almost constant over large areas. The mountains have brighter light and more ultraviolet. There is more light above water than below. Light is very important to seaweeds. The quantity and quality of light are very variable and change with depth, amount of plankton and particles, waves, foam. It gives rise to many different habitats. Deeper than 70m in clear oceans, plants can no longer photosynthesise enough to stay alive. Along the coast, this is 15-25m. Runoff from the land makes this worse.
Substrate Plants need moisture and nutrients which are present in loose soil. Rocks are difficult places to live although some organisms are able to do just that (lichens). Plants need to transport moisture, nutrients, oxygen and carbondioxide through vessels from their roots to their leaves and back.  The water is found all around. Seaweeds do not need roots for finding water and nutrients. They need solid rock to hold on to, which they do by means of root-like structures. Seaweeds cannot grow in sand in the open sea, although there are some exceptions (seagrass). Seaweeds do not need to transport liquids very far. The water movement helps to pump liquids around in their tissues.
Aspect Steep hillsides funnel the winds and drain moisture quickly so that they are hard to live on. Soil won't hold on steep slopes. Moderately steep cliffs absorb waves and make currents move faster near them. Because the sun appears to shine from straight overhead, cliffs are not suitable for plants. So they are often rich in animals that filter the water for food. Ironically, vertical cliffs do not absorb wave energy, but bounce it back, thus providing shelter.
Body weight On the land, body weight can be decisive. Elephants can't grow bigger because their bones would not be strong enough (counts for humans too). Most animals can't fly. Birds can, but most waste much energy doing so.  In the sea, all organisms are nearly weightless. That is why whales can become so enormous. Many organisms 'fly', often only as larvae. Larvae can easily be spread over vast distances.
Body shape Most animals (except birds) are not streamlined. Wind resistance does not affect their speed very much. Since water is 800 times denser than air, water resistance affects speed very much. Fast fish are streamlined. Fins propel efficiently, legs don't.
Feeding Most animals are born relatively large, feeding from two or three different kinds of food as they grow up. From baby to adult they grow 10-50 times. Most animals produce large quantities of very small eggs, which are released in the water to fend for themselves. A fish may grow from 0.1mm to 500mm, several million times in weight. In the process, it changes food many times.
Breathing Land animals have lungs for breathing. They breathe in, then out. In the lungs, air passes through ever smaller passages. Lung tissue is thin so that gases can exchange between air and blood. It is relatively difficult for gases to pass from air to liquid and back. Infection through the air is difficult because few organisms can live in air. Sea animals have gills. Water passes through in one direction since reversing the current each time would cost too much energy. The gills are like lungs: the blood passing through ever smaller passages. Like lung tissue, gill tissue is very thin to allow gases to exchange easily. Gases pass easily from liquid to liquid. Infection through the water is easy. It is also teeming with parasites and viruses and decomposing (infectious) bacteria.
Smelling In order to 'smell', substances must be able to dissolve in both water and oil. They must also be able to evaporate in the air where they are carried in very low concentrations that are hard to detect. Yet some animals' sense of smell is amazing. In the sea, concentrations of smelly substances are much higher and thus easier to detect. The sea is full of smells. Smells pass easily from liquid to liquid. Some animals can smell a target at many kilometres distance.
Hearing Sound is produced by vibrations of hard objects. These vibrations are transferred to a soft, compressible medium (air) and in order to be heard, transferred to a hard medium (the ear mechanism). In the process, much energy is lost. Sound or vibrations are transferred from a hard object to an incompressible (hard) medium (the water) to arrive at a hard receptor. Very little energy is wasted this way. Sound travels four times faster in the sea. Waves can easily travel thousands of km. The sea is full of loud vibrations and noises but the human ear is not designed to hear these well.
Electricity Air is a perfect insulator (bad conductor of electricity). Static electricity loses its energy very quickly outside the body. It is impossible for animal senses to sense electric fields in air. Sea water conducts electricity about equally well as the salts inside the body. Transfer of electrical energy to the water is almost ideally matched. Some eels and electric rays can produce electric shocks that kill or stun prey. Others can sense the electric fields produced by working muscles, when nearby (sharks).

What biotic factors play a role?
Have you ever wondered why it is that you sit in the class where you are? Where you sit in the office? How you got your job? Could any of the factors below have been of influence?

The biotic (living) factors discussed here are very similar to those found on the land.

To be first: Often who lives where, is determined not by suitability alone but simply by who was there first. Certain organisms such as barnacles spawn so profusely that they always seem to be first. Once settled, they are hard to remove or to displace.

Competition: Particularly on the shallow shore or seabed where space is in short supply, competition for that space influences who will be able to grow where, to a considerable extent. Even small differences in suitability for a particular habitat, can over time, give preference to just the right species. Organisms try to crowd one another out by smothering or by biochemical warfare.

Co-operation: Unwittingly, species can co-operate to make life easier for some but worse for others. Parore (Girella tricuspidata) graze seaweeds and in doing so remove fast growing algae that could otherwise have smothered these seaweeds. Over time, non-cooperative creatures make room for the more successful co-operative ones.

Altering the environment: Animals everywhere in the world have changed their habitats to suit themselves and (strangely enough) the species they graze. Likewise in the sea. The sea urchin grazes a shallow band below the worst wave action and so displaces both the shallow bladder weeds above it and the stalked kelp below its habitat. In doing so, it also clears the way for other grazers such as snails and limpets. The stalked kelp, in turn, pens up the urchins in their zone, preventing them from straying away, which could leave their habitat insufficiently grazed.

Predation: Predators can influence their habitats decisively, usually by reducing the numbers of grazing animals. They also co-operate by removing dead and dying or sick organisms and by preventing grazing animals to reach levels of overpopulation.

Disease: Disease can considerably upset the balance in a community by removing one or more species totally. Others will then have a chance to take their places.

Extremes: Extremes of weather (storms, droughts, winters), climate (ice ages) and tectonic upheaval (island-forming, volcanic eruptions) have always had a decisive influence on evolution, by killing a large proportion of the population, and thus selecting out those individuals that were able to survive. After an extreme event, the composition of communities can change considerably.

Climate regions
The sea water becomes colder towards the poles. New Zealand runs north to south. The sea water up north is six degrees warmer than down south. It is important to grazing animals and plants.

New Zealand is located in the southern temperate seas of the world that span the globe like a thin ribbon (Ayling 1982). Some of our marine species have managed to migrate across the wide gaps between continents but most have not. Including our off-shore islands, the New Zealand region is characterised by a number of distinct 'climatic' areas ranging from almost tropical to almost antarctic.

Marine provinces around New ZealandThis map shows the temperate zone, encircling the globe and the tropical zone straddling the equator. Of the tropical zone, only the Tropical Indo-Pacific zone is shown. Between the tropics and the temperate zone, lies a subtropical zone. In 1982 Ayling recognised three distinct areas, based on the similarity and dissimilarity in fish species. These areas are the Broad Subtropical area which includes the north of New Zealand, the Specialised Subtropical Region which includes the Poor Knights Islands and White Island in with the Kermadec Islands, Norfolk Island, Lord Howe island and a small section of Australia's East Coast. It is relevant in this context that a difference is found between both countries' east and west coasts. Our subantarctic islands form a separate region, the Subantarctic Region.

Provinces and currents of NZThis map shows the most distinctive oceanic features of the oceans around New Zealand. The dotted lines demarcate boundaries in the sea, running along fronts and convergences. Fronts and convergences are formed when cold and warm water meet, unable to mix because the warmer water lays on top. But along the boundary eddies are split off, enabling nutrient-rich bottom water to surface, resulting in plankton blooms and rich fish life. Ocean currents run along these boundaries, rather than across them. On the map are shown from south to north, the Subantarctic Front, Subtropical Convergence, Tasman Front, and Tropical Convergence, with bodies of water in between them, differing in temperature, from 4ºC to 23ºC in summer. Each has its own assemblages of plankton organisms.

Two current systems are of major influence to the underwater environment. From the north, moving along the Tasman Front, arrives the warm East Auckland Current, which moves offshore and away from New Zealand. But warm water eddies mix with the coastal waters, increasing their temperatures, while introducing warm water species.
From the south, hugging the Subtropical Convergence, arrives a temperate water current, which flows west and east around the South Island in the Westland Current and the Southland Current. The Durville Current flowing in between the North and South Islands, feeds most of this water eastward, to flow out of New Zealand waters past the Chatham Islands.
Some of the temperate water flows north to meet the Tasman Front, veering east around North Cape.

Based on the similarity and dissimilarity of intertidal organisms, the coast of New Zealand has been recognised to have six distinct areas, which brings the total number of climatic regions to eight (see map):

Wave regimes

Waves are very much stronger than wind. They decide very much what grows where.

Our shores can be classified in broad terms by the amount of wave action they receive:

Exposed shores receive the full brunt of the ocean (long waves arriving from a fetch of over 200km) for most or at least some of the time. Very often the regular occurrence of a disastrous event is more of influence to a community than average conditions.

Semi-exposed shores are sheltered by barrier islands but still have to cope with waves that have developed over a fetch of 10-50km. These are a specialised case of exposed shore (Hauraki Gulf) and in the classification of habitats, will be lumped in with them.

Sheltered shores are found in the shelter of peninsulas and inshore islands.

Enclosed shores are found in river mouths and estuaries. These are completely sheltered from the brunt of the sea by either a protective promontory and sandspit or a sand bar.

Depth zoning
When the tide is low, it's just as if the sea has pulled its trousers down. Surprising things can then be found. You see barnacles high up and big snails low down on the shore. You can see that the shore has zones. Under water more zones are to be found. Here are the names for all these habitat zones.
littoral zonesThe effects of tides, waves and depth cause organisms to live in zones, running parallel with the coast. In this drawing a number of important terms are defined. The intertidal zone (eulittoral or midlittoral) extends between low and high tide but during low and high water spring tides, it extends further, allowing intertidal organisms to either live higher up the shore in the upperlittoral zone or lower down in the lowerlittoral zone. Sometimes the transitions between these zones are gradual but often they are surprisingly sudden. Under water two more zones are distinguished: the infralittoral plant zone where sunlight is enough for photosynthesis and below it the circalittoral zone which is dominated by sessile filterfeeders such as sponges.

On many coasts the infralittoral (plant) zone is further divided in a shallow bladderweed zone (stringy seaweeds), a barren urchin zone and the kelp forest. Bladderweeds are stringy seaweeds, strong but flexible. They yield to the water movement, instead of resisting it. Their leaves are small so that they are not torn off easily. In calm conditions, bladderweeds develop float bladders that pull the plants upright towards the sunlight.

Maritime zones and exposureOne would like to have an exact measure of where the agreed zones start and end but this is complicated by a number of factors, one of which is wave action. This drawing shows the periwinkle and barnacle zones in relation to wave exposure. To the left of the image is high exposure (high waves) and to the right is low exposure (low waves). It shows that waves enable the organisms to live higher up the shore in the 'splash zone'. In sheltered areas, marine organisms won't be able to live far above high tide, and both periwinkle and barnacle stay well under EHWS (Extra High Water Spring). But where tall waves are common, they can live high above EHWS. At the same time, the width of the periwinkle zone widens, whereas that of the barnacles stays roughly the same.
It should be noted here that the notions of high and low exposure are by themselves difficult to quantify. Is it related to the average height of the waves or to the occasional destructive storm?

Underwater zoning in N New ZealandWhen observing the plant zones under water in relation to wave exposure, they similarly curve away from the surface, as shown in this diagram (Anthoni, 1993).  This diagram was obtained by measuring north-facing slopes and sorting the many transects by their sand bottom depths. Because this depth is related to wave exposure (with exceptions), this diagram emerged. As it happened, the sheltered water (on right) was also much less clear (more murky), typically only 2-4m visibility, than the deeper, more exposed shores, which had visibility exceeding 15m. Murky water affects the presence of filter feeders, since these organisms are very sensitive to dirty water. It also affects the lower boundary of the stalked kelp because of lack of light. The upper boundary of the stalked kelp Ecklonia radiata, however, is due to the shearing effect of wave action. Above the kelp but under the stringy seaweeds, sea urchins can hold their own, but their upper limit is limited by wave action too. Not shown in the diagram is the disappearance of sea urchins as wave exposure increases. The bare zone on very exposed shores is then taken by short calcareous algae. The three dominant bladder weeds are Carpophyllum plumosum, C. maschalocarpum and C. angustifolium, in order of increasing strength. Wave action sorts them out in zones. Not on this scale, but occurring in very sheltered waters, one finds the weak C. flexuosum, which tolerates murky waters well.

It would be ideal if a place could be found where the amount of wave exposure varied dramatically over a short distance, for such places would show us this diagram in real life. But such places do not exist.

exposure diagram Poor Knights
Wave exposure zoning diagram for the Poor Knights Islands, northern New Zealand. Click on the image for a larger version. Note that these diagrams were drawn 'from memory' by hand and have not been measured like Anthoni's diagram. Source: Ayling, 1975.
exposure diagram South Island
Wave exposure zoning diagram for the South Island. click on the image for a larger version.
Source: Ayling, 1975.

The substrate
The substrate (sub=under, stratum=layer) is very important, especially for plants. Let's first look at what plants need and in doing so, let's also compare seaweeds with land plants.

What plants need in order to live Land plants Sea weeds
Water With their roots, plants find water in loose soil. They can't live on rock. Some water is absorbed by leaves directly (rain, dew). Water is all around. Seaweeds don't need roots for finding water. They just absorb it through their leaves (fronds).
Sunlight Is bright and it comes from one side in the morning, and another in the evening. Is weak and always comes from above. Its colour varies from white to blue.
How they grow Plants grow tall to reach out for the light. Their rigid stems keep them upright while their roots prevent them from falling over. Seaweeds cannot be rigid but must be flexible to survive. They cannot root in sand because the waves would immediately wash them out. But since they don't need the sand for water, they live on the rocks. They hold on to the rock with a root-like holdfast. Float bladders pull them up towards the light.
How they disperse Plants have evolved many different ways for dispersing their seeds. Heavy seeds are carried by birds; some seeds can be dispersed by wind; others attach themselves to animals. Birds and insects are important for plants. Seaweeds are primitive plants. They reproduce like ferns and mosses, with one asexual and one sexual cycle. Their spores are very small and numerous and are dispersed by water, because they are suspended by it. Seaweeds do not depend on animals for dispersal.
Carbon dioxide Is filtered from the air where it exists in very low concentrations. With carbon dioxide and sunlight, plants make carbohydrates using green chlorophyll. Is abundant in the water and filtered from the water. With carbon dioxide and sunlight, seaweeds also make carbo-hydrates but green, brown and red seaweeds use different kinds of 'chlorophyll'.
Oxygen Is taken from the air when there's no sunlight. Oxygen is available everywhere in the same concentration, except in enclosed spaces such as in soil and swamp. Is taken from the water when there's not enough light. Because water can assume different densities depending on salinity and temperature, it can become layered, hindering the exchange of gases. In such cases the deeper layer can become anoxic (lacking oxygen).
Wind/waves Wind effectively transports and mixes atmospheric gases on which plants depend. Currents transport gases to the surface, where they exchange with the atmosphere. Waves allow seaweeds to grow denser than land plants. It moves the dense leaves around so that all get some sunlight. It also helps to keep them clean. Leaves are self-lubricating to prevent damage.
Acidity Only the roots of plants are submerged in moisture. Acidity (pH) of the soil is very important. Seaweeds are submerged entirely in sea water with high alkalinity (high pH) which is not conducive to efficient growth.

Rock. The hard rocks found along our seashores are from hardest to softest: granite, basalt, greywacke, mudstone, sandstone. The hardest rocks have smooth surfaces which are difficult to attach to. The softest rocks fall apart too easily and cannot support large seaweeds. The ones in between are ideal for attachment

Boulders. Boulders are found where hard rocky shores erode. The force of the sea is capable of dislodging huge boulders, the size of a house but more often smaller. The larger boulders take many years before being shifted by enormous storms but the smaller boulders get turned more often. Small boulders may get turned so often that no seaweed can grow on them. As they grind against each other, they wear into round shapes. Boulder beaches above and under water are difficult places to live. Only the most mobile of organisms (skinks, crabs) can manage to live there.

Pebbles. Pebbles are so easily moved by the water that practically nothing can live in them. Big storms throw pebbles around with great force, thereby eroding surrounding rocks and caves more quickly.

Sand. Sand comes in all gradations from coarse to fine and consisting of all kinds of rock particles, quartz and shell fragments. Most of our marine habitats consist of sand (or mud). Many creatures have learned to live in and on the sand. Those living on top of the sand such as flounder and stingray, are able to hide by quickly burrowing in a shallow manner. Sand is for them ideal since it does not create a big cloud of dust, as mud would. Burrowing creatures prevent their burrows from caving in by cementing the insides with slime. This method is not suitable for coarse sand. The finer the sand, the easier it is to burrow into. Many snails live on the sand. They burrow during the day, only to come out at night when their predators sleep.
A very important property of sand, not found in either mud or pebbles, is that it becomes 'liquid' when stirred or shaken. Burrowing animals extensively make use of this fact. The serpent eel is a slim, strong and long eel which can 'swim' in the sandy bottom. The stargazer can sink into the sand by blowing water around its body, and wriggling from side to side. Many clams (cockle, pipi, tuatua) use a similar strategy for burrowing.

Mud. The finest sand particles we call mud. It gathers where the water is calm, such as in sheltered parts of an estuary or in the deep sea. Minuscule organisms such as diatoms and foraminifera can live in and on the mud. For them the mud particles are big enough to hold on to. They can be so numerous that the mud becomes nutricious for other organisms such as worms and mud crabs. Mud is a convenient building material for burrowing but where the mud is too soft, it will smother every bottom dweller.
Nephos. (Gk: nephos= cloud) We'll mention a recent phenomenon here, as it covers larger and larger areas of the sea bottom near large rivers or human populations, in calm waters without currents. The name nephos has been used to describe a kind of fuzzy sea bottom which goes vertically from solid mud to soft mud to ooze to muddy water, without a clear boundary between these. It usually also lacks oxygen so that no recognisable animals or plants can live here. Nephos can also be found in fresh water lakes.

Dead zone: a dead zone is found where nephos occurs, characterised by a severe shortage of oxygen, and often black stinky decomposition. Dead zones are increasing in area and in number all over the world as the oceans degrade due to eutrophication from land-based pollution.

The size of a habitat or community can range from a fist to half the world. The bottom of the deep sea at 3000m may extend for millions of square km without change. Habitat zones (e.g. kelp forest) may extend unaltered over hundreds of km of seashore, only here and there interrupted by others (say, a beach). The holdfast of a stalked kelp plant may predictably house a number of small species, which somehow live together. A rockpool in the intertidal zone may predictably lodge a number of sensitive species, which could make a rocky shore with many rockpools distinctly different from those without.

On a small scale, the coast may provide exceptions to the average which, somehow, could influence the makeup of the habitat's communities. A cave for instance, may have its own community of species but it will almost certainly also play an important role as a refuge for reef fish who graze in other habitats. Cracks and crevices in the rock face provide shelter to crayfish who would otherwise not have been able to live there safely, but they are also inhabited by their own communities. Likewise, promontories and archways will have their own permanent inhabitants and regular visitors.

The open ocean
The open ocean has blue, clear water with very little food. But on the seabottom, things get better even though oxygen levels are low. Here are the names of open ocean and sea bottom habitats.

(See also the chapters on Oceanography and Plankton.) Here is a diagram of the sea bottom far away from the coast. First it slopes gently on the continental shelf, then more steeply on the continental slope, to bottom out in the abyssal planes. The depth contours of 200, 1000 and 2000m have biological significance.

Oceanic zoningThe photic zone in the clear open ocean extends to about 100m but it feeds organisms down to about 200m. This zone is called the epipelagic zone (epi=above, pelagicus= of the open sea). It is where most fish are found. The surface waters of the ocean where the sunlight is brightest, are home to phytoplankton, the meadows of the sea. The single celled plant plankton organisms combine nutrients and sunlight with carbon dioxide while growing and multiplying. They are eaten by plankton animals such as krill. Because both krill and plankton (particularly the bigger organisms) sink, the nutrients are gradually depleted from the surface. It results in very clear water with hardly any sealife at all. (see plankton) Please note that blue seas, like coral reefs may well be far more productive than thought due to recently discovered mixotrophic zooplankton with symbiotic plant cells.

The fishes of the mesopelagic (mesos=middle; pelagos=ocean) are small and strange, with unusually large eyes that allow them to see in the very dim light. (below 800m the sea is pitch dark). Many have lanterns (photophores) in their skins. They can turn the light on and off. Many fish here migrate towards the surface at night in order to feed from the richer epipelagic zone.

The fishes of the bathypelagic (bathos=depth) zone live in complete darkness and in an environment not unlike a desert. Their eyes have disappeared almost completely. They have to live alone because there's not enough food for company.

Some fishes, swimming in the open water, never venture very far from the shore (piper, blue maomao, demoiselle). These we call semi-pelagic.

The seabed
Are you wondering where most of our fish comes from? The seabed is a huge area. The shelf of the continent extends down to 200m. This is a productive area. The continental slope from 200-500m is very dark. The fishes there have large eyes. They come to the surface by night, to feed.

Our country is endowed with an enormous amount of fishable seabed, protected by our Extended Economic Zone (EEZ). Although productivity decreases sharply with depth, the sheer size of these deep sea plateaus provides for rich sustainable fishing (if managed wisely).

NZ continental shelf and slopeThe seabed extends from the rocky shore across the continental shelf and the continental slope to the abyssal plains.

The continental shelf which ends at about 100-200m, is the most productive of these. Here our popular fish species are trawled. The species resting on the bottom are called benthic (flounder, gurnard); those swimming immediately above it, demersal or benthopelagic (tarakihi, red cod, snapper). The slope of the continental shelf is very gradual (about 10m per km), making the sea bottom there look horizontal.

The continental slope is about 70m per km on average but can be much steeper close to subduction zones or ocean troughs. New Zealand is positioned over such a trough, with very deep water close inshore on its east coast and along the Kermadec Islands. Sometimes global currents are pushed up the continental slope, bringing nutrients from the deep to the surface. Such upwellings can cause rich plankton blooms, resulting in an abundance of fish life on the edge of the continental shelf (rattails, hakes, morid cods). Our continental slope habitat is much larger than our continental shelves.

The abyssal plains are completely dark and many fishes bring their own lanterns. These fish eke out an existence in this barren environment where food is scarce. Their flesh is watery and their bones are soft.

See also Oceanography, Commercial Fish,