By Dr J Floor Anthoni (2001)
One of the biggest mistakes is to treat the
sea as if it were an extension of the land. We tend to apply the same scientific
principles and the same ecological laws, without realising that the marine
environment is too strange for these laws to apply. Please read this chapter
with care, and also the related pages, because it gives you a new understanding
of how the sea works and why a century of ocean management has not delivered
the expected results. Also take the challenge to read about our most recent
marine discoveries with the DDA method.
The sea is a totally different place. How does
it differ from the land? How many phyla and species? How does it affect
marine reserves? How can marine reserves fail? Why has ocean management
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Marine biodiversity and differences between land and
The sea is such a different place to live, that it is perhaps not possible
to imagine how different, not even after years of diving and observing
underwater life. Living in water is profoundly different from that on land.
Take some time to read carefully what follows. Since this web site specialises
on the marine world, good information can be found in other sections. Also
look at the photographic library and classifications for images of marine
there are many lands but only one sea
The following summary of characteristics of the seas, may give an idea
how different it is from what we experience ourselves on land.
general: some general characteristics.
only one sea: whereas there are many unconnected continents and
islands, there exists only one sea. People have given different names to
parts of it, while also naming the large parts oceans.
lowest point on land: the sea lies at the bottom of all land and
civilisation. All the water and pollution ends up in the sea.
life began in the sea: the sea is where life began, although the
primordial seas were certainly quite different from those of today. Only
a few creatures crawled onto land, to adapt to a much harsher environment.
Yet today, all organisms carry inside them a body liquid which still resembles
that of seawater.
controls the planet, climate, water circulation: the sea is extraordinarily
large and massive. As a result, it has a stabilising influence on the planet's
temperature. It has a major influence on the climate everywhere and also
on rainfall. Most of the rain falling on land, has originated from the
seabed rocks and sediments are young: the constant forming of new
tectonic plate material at the spreading mid-ocean ridges, and their continual
subduction into ocean trenches, acts like a gigantic conveyor belt. As
a result, the oldest rocks (basalt) found on the seafloor is no more than
200 million years old. By comparison, 3000 million year old rocks can still
be found on land. Erosion on land has carved away old sediments and rocks
in most places, but has exposed old rocks elsewhere.
bulk of the planet's biomass: 90% of the planet's biomass is found
in the oceans, because water is the most important need for life.
life everywhere: life is found everywhere in the oceans because
light shines into the oceans, and some nutrients are found everywhere.
Also, the sea mixes its waters quite well. Everywhere in the ocean, at
least 100 - 1 million cells can be found per ml.
more natural: because humans do not inhabit the sea, plough its
soils, build cities and roads there, the sea is far more natural than the
land, even more natural than national parks where introduced predators
and pests still roam freely, threatening native wildlife.
accessibility: because the sea surface is a continuous flat, encircling
the globe, it is easy to get anywhere by boat. Compare this to unpassable
mountain ranges, river gorges and waterways on land. Even so, the sea is
also difficult to study under water since this is a hostile environment
to humans, as divers know.
largest mammals: the largest mammals are found in the oceans, because
they don't need to carry their body weight, and size gives an energy advantage
with regards to cooling, migrating and hunting.
knowledge: not being able to spend long times in and under the water,
the sea has retained its mysteries far longer than the land which could
be studied much more easily. So the knowledge of the sea is proportionally
far less. Unfortunately, many people are unaware of the enormous differences
between land and sea, and unknowingly make big mistakes.
size of the oceans: seen from space, the oceans are Earth's major
sea is very large compared to land: The surface area of the sea
is 71% versus land 29%, and it is on average about 4000m deep. The oceans
consist of vast, uniform areas. However, when thinking of the Pacific Ocean
as wide as a large living room (10m vs 10,000km), then the pool of water
is only 4mm deep (4/1000m vs 4 km). Within these 4mm, everything happens
- surface currents, deep currents, upwellings and so on. The deepest trenches
are only 10mm deep!
3-Dimensional: unlike forests, which extend in three dimensions
by the strength of their trees, the sea stretches in three dimensions on
account of the density of the water, which carries the organisms. Unlike
forests, which may reach 40-60m height, the sea reaches down to 10,000m,
and contains life at all depths, with a 'canopy' of microscopic plants
close to the surface.
connectivity: connectivity creates stability. Oceans have very stable
and often ancient communities, particularly in the sunless depths, where
major global disasters do not penetrate (meteorite impact, ice ages).
less isolated: all seas are interconnected, and in theory, all organisms
could migrate to every other place in the sea. However, in practice, seas
do have regions and many marine organisms are endemic, although less so
than in the terrestrial world, which is highly fragmented by comparison.
homogenous: marine environments are more homogenous and stable,
and are less patchy.
food from elsewhere: sea water transports organisms (plankton, larvae),
unlike air. Many marine habitats depend on adjacent areas for food and
larvae. Organisms can survive by being stationary, while catching food
that passes by.
less endangered: because oceans are so well connected and large,
they are thought to be less endangered than the land ecosystems. However,
near the coast they are threatened by disturbance from trawl nets, and
by pollution from land and industry.
sea water: sea water is almost constant in its composition, but
nutrients important for life, vary. Variations in temperature and salinity
have profound effects.
heat conduction: water has a high heat capacity and conducts heat
quite well, also by convection (movement). It cools formidably. It is therefore
impossible to be warm-blooded and breathe water as well. So all marine
organisms are poikilotherm (assume the surrounding temperature). Note that
whales and dolphins are warm blooded but they breathe air.
temperature: the temperature of seawater stays quite equitable,
unlike that of the air. Seawater at the poles is much colder than that
at the equator, resulting in ocean currents to equalise the difference.
Warm water is lighter than cold water. The coldest water is found at the
bottom of the deep oceans.
salinity: miraculously, the salinity of sea water is almost constant.
Judging by the amount of salts carried by rivers over eons of time, the
oceans should have been very much saltier, but somehow they shed salt continuously.
Salty water is heavier than fresh water.
density: water is about 800 times denser than air, reason that waves
and tides can exist. The density of seawater also depends much on salinity
and temperature, the combined effect of which leads to surprising phenomena
such as sinking waters, upwellings and stratification.
stratification: ocean water easily forms layers, with lighter (less
dense) water on top. Boundaries between such water masses are called thermoclines
(equal temperature) or haloclines (equal saltiness).
limiting nutrients: the most limiting need on land is water, which
is of course plentifully available in the oceans. Nutrients limited on
land are soluble nitrogen, phosphorus, potassium (NPK), followed by calcium,
sulphur, magnesium (Ca, S, Mg) (see soil/fertility).
Bacteria are capable of making nitrogen compounds from atmospheric nitrogen.
In the ocean, the main limiting nutrients are phosphorus, sulphur, silicon
and iron (P, S, Si, Fe).
good mixing: although being stratified in many places, the oceans
are mixed quite well by their currents, returning nutrients from the deep
back to the surface by means of the large ocean
conveyor belt and local upwellings.
microscopic plants: the plankton ecosystems are a major feature
of the oceans, with single celled microscopic plants ranging from the size
of bacteria to almost 1mm. Larger planktonic organisms such as jellyfish
can reach one metre across.
filterfeeding: because of the availability of plankton, many organisms
have adapted to filtering these and feeding from them. Many ingenious ways
have evolved. This kind of feeding is not known on the land.
even though all nutrients from the land end up in the sea, the oceans are
only moderately productive. Dead organisms and even live diatoms, gradually
sink. Decomposing bacteria are found at all levels, but in the end, nutrients
sink to the deepest parts of the oceans, from where they are returned slowly
by upwellings. Some places are very productive, like rain forests, yet
others compare with deserts.
tiny plants: unlike on land, the seas' main productivity comes from
tiny plants called phytoplankton. Due to their small size, they metabolise
and grow fast while using all available nutrients and light. These diatoms
and dinoflagellates must be small in order not to sink to sunless depths.
food chain: unlike on land, large fish cannot find plants large
enough to feed on. So they depend on the lower hierarchies of the food
chain which convert the sun's energy to ever larger food particles, from
zooplankton to fish larvae to bait fish. Food pyramids in the sea are therefore
necessarily much higher than on land.
economy: marine organisms are very energy-efficient, since they don't
need to keep warm, and often don't need to move either. They do not need
to support their bodies and can swim easily by all kinds of means. It makes
long food chains possible, and large food webs.
highly productive areas: highly productive areas are found in estuaries
(and salt marshes, mangroves), over the continental shelves and over oceanic
upwellings. They are usually poor in species.
continental shelves: these are the shallow margins around continents.
Dead organisms sink only 40-100m down, and their nutrients are recycled
seasonally (much like lakes do), causing one or two annual plankton blooms.
ocean deserts: the open oceans outside continental shelves and slopes,
outside upwellings, behave like a deserts due to their low productivity.
Their area is huge.
east-west differences: The east coasts of oceans (west of land)
are productive but less diverse, due to upwellings, rich plankton blooms
and cold water. The west coasts however, are less productive while being
more diverse, due to clear and warm waters.
long food chains: marine food chains are unusually long, having
5-7 trophic levels.
predator fishing: humans are not harvesting the primary production
(algae, like food crops), not even catching grazers (plankton feeders,
like sheep) but top predators (like lions). This makes it seem that oceanic
productivity is rather low. It also causes fisheries to collapse easily.
reproduction: because of the water's carrying capacity, eggs and
larvae are transported easily and over vast distances. Most marine species
reproduce by eggs. However, some species fertilise internally, like sharks
high fecundity: fish produce very large numbers of eggs. This apparent
waste is necessary to produce food in the food chain from invisible phytoplankton
to bite-sized parcels. One can say that fish reproduce primarily to make
food (99.999%) rather than babies (0.001%).
larval dispersion easy: because of the sea's ease of dispersion,
it provides spawning at a low cost, producing high numbers, which then
become a major food source, unlike seeds. By contrast the most common terrestrial
reproductive strategy places adult and juvenile life stages in close proximity
synchronised mass spawning: to increase the chance of sperm fertilising
eggs, many marine organisms (corals, barnacles, limpets, etc.) synchronise
their spawning. In this manner their juveniles swamp all other planktonic
predators, becoming the main zooplankton for long enough to become successful.
variable recruitment: because of the many vagaries of their juvenile
planktonic stages, recruitment (becoming one year old), is highly variable
from year to year. This also adds a complication to fisheries management.
extraordinary growth: individual fish may grow from a 1mm egg to
a 1m fish, over a billion times in volume or weight. During their fantastic
growth, juveniles have to adapt to many different types and sizes of food.
They do so without parental guidance. Terrestrial species have more prudent
many species reproduce asexually: the sea has some ancient but successful
organisms. Because many are able to reproduce asexually, a mating partner
is not critical, and they can live far apart as rare organisms (anemones,
corals). Some species grow their colonies by asexual reproduction, which
guarantees that their offspring benefit from the favourable conditions
of their parents (anemones). Some organisms grow this way (sponges).
many species can self-fertilise: many species can fertilise themselves
(hermaphrodites), which produces offspring slightly different, but very
much alike (molluscs). It gives them enough adaptability.
biodiversity: because all parts of the sea are interconnected, with
ocean currents as highways between them, marine diversity is not as high
as terrestrial diversity. For instance, the ocean-wandering jellyfish may
comprise only 200 species. However, exceptions exist.
old phyla: the sea has 18 phyla (the top of the species pyramid,
where species are at the bottom) which are not represented on land. Thus
80% of the world's phyla are found in the sea, but only 20% of the world's
species. Variety in phyla is often referred to as variety in body plans,
because they comprise enormous differences in body form and function.
seawater cools: seawater cools its organisms for 100%. They can't
warm up in the sunshine (lizards) or exercise to warm up (insects, birds).
As a result, biological productivity is low. As the temperature changes,
the biochemistry (enzyme optimum) changes enough to introduce new species.
Thus marine species variety is susceptible to latitude (north-south).
sea diversity: 100 new species per 100km transect, or 1 new species/km2.
sea bottom diversity: as scientists are expanding their knowledge
of the sea bottom, they are finding species variety as high as 900 species/m2,
which compares with that of tropical rain forest. The benthic deepwater
species variety increases with depth to 1800-3500m in the Atlantic Ocean,
but not in the Pacific Ocean.
intertidal: intertidal environments are rich in species, all water
pelagic: pelagic environments are poor in species.
rare species: rarity is more likely, due to the stability of the
sea, its many ancient phyla and species, and their ability to live far
apart, while living long lives.
endemic species: marine species are less endemic (belonging to one
place), thus are less prone to extinction.
endangered species: very few on the endangered species list CITES.
But many marine mammals are threatened. See conservation/marine
oxygen/ CO2: oxygen is exchanged only in one place, through the
ocean's surface, and CO2 goes this way too. In the shallow zone, where
sunlight reaches deep enough, and only where there are enough nutrients,
will oxygen be produced and carbondioxide used. At night, and in the deep
ocean, the process reverses, leading to a shortage in oxygen at the deep
stratification: due to the combined effects of salinity and temperature,
many places in the sea are stratified (layered), blocking the convection
of water (up/down movement) and gases. It may lead to severe shortage of
salinity: the sea water is saltier than the body tissues of many
marine organisms, and they lose moisture continually by osmosis. A fish
must drink seawater in order to maintain a balance of body salts.
currents: winds on land are not exactly like currents, but both
transport heat. The heat transport of currents is often underestimated
because they flow slowly (current=0.5km/h; wind=35km/h). However, the heat
capacity of 100m of water column (depth), is enormous. At the surface,
winds and currents exchange heat, resulting in a major influence on climate
and season. (See also oceanography/currents)
wave action: whereas winds blow mainly in one direction, waves swing
to and fro. In the shallows, they are very destructive, also because water
is 800 times heavier than air. This has had a major effect on the shape
and development of seaweeds. Wave action is also important fore mixing
nutrients and exchanging gases.
tides: the moon and sun tides are felt in the sea as a predictable
rise and fall in sea level, accompanied by currents. Such tidal currents
cause extensive mixing and distribution of pollutants and plankton in coastal
waters. In the open ocean, tides are not noticeable.
substrate on the move: everything on land has a fixed place. Plants
are well rooted. In the sea only the rock surface provides reliable holdfast.
Planktonic organisms are constantly on the move. The sandy and muddy seabeds
are stirred regularly, forcing organisms living there, to rebuild their
warrens and to dig deeper.
substrate as attachment: because plants are surrounded by nutrients
and water, they do not need roots and soil. They attach to the rocks and
absorb moisture and nutrients direct through their entire surfaces. Other
organisms can attach themselves likewise, because the food comes towards
them by movement of the currents.
decreasing light intensity: unlike on land, light intensity decreases
rapidly with depth. In the coastal zone, plant life stops at 20-30m depth.
The quality of the light changes too, becoming more blue. The light intensity
and quality gradient gives rise to biodiversity and horizontally zoned
eco3.gif, and eco8.gif).
habitats: due to its unique nature, the sea offers new kinds of
habitat, some of which are mentioned here.
saltmarsh: the salt marsh is a stable brackish water habitat, densely
vegetated and intertidal. It traps mud and thrives from its nutrients.
It is highly productive.
estuarine sand & mud: the sand and mud inside harbours is exposed
to small waves, but protected from large ones that could rip it up. It
thrives on the nutrients from the land and on the warmth of the water,
leading to high productivity of plankton, algae and bacteria.
rocky shore: the coastal rocky shore has an intertidal habitat,
sometimes with rock pools. Below low tide it exhibits zoned seaweed habitats,
depending on its exposure to waves. It is moderately productive.
open ocean pelagic: lacking nutrients, this is a clear water, low
plankton: plankton with all its life forms, is found in all oceans.
Water is its substrate. In the presence of high nutrient concentrations,
it can bloom vigorously, darkening the water and giving rise to harmful
deep sea floor: a very stable habitat, able to live in low oxygen
concentrations but rich in slow growing and rare species. It has a low
productivity, except in the presence of hot water vents.
coral reef: comparable to rain forests, coral reefs have very high
diversity and nutrient/energy recycling, but low overall productivity.
Corals provide the substrate and shelter for other species.
polar seas: nutrients surface in certain places, but once a year
causing blooms of plankton, with krill, and high density of life, but with
low diversity. Because in summer the sun shines all day, the overall amount
of light is high. Due to melting ice, there is diversity in salinity, causing
sea is still more natural: because people do not live there, no
habitat conversions have occurred, nor roads or housing developments. However,
along the coast where people like to live, saltmarshes have been
reclaimed and parts of estuaries. Causeways have changed the hydrology
trawling damage: trawling for finfish and particularly for clams
and shrimps, has caused damage to the sea bed. However, in exposed shallow
seas, waves do more damage naturally.
sea is at the bottom of society: all pollution ends up in the sea.
although the oceans are large, the coastal seas above the continental shelves
are shallow and are not rinsed well because they are too deep for wave
mud: mud from land erosion, agriculture, cropping, is easily stirred
by waves and keeps polluting the waters after every storm or deep ocean
swell. Many organisms are sensitive to mud, which suffocates them.
nutrients: nutrients from farm fertiliser arrive with the mud, to
fertilise the plankton. Heavy plankton blooms result, often with poisonous
species, killing marine organisms and rendering beaches unsafe for swimming.
sewage: sewage and treated sewage end up in the sea, sometimes making
beaches unsafe from swimming. Its nutrients add to those coming from mud
and farm fertiliser.
biocides: biocides from farming, industrial chemicals and heavy
metals cause the disappearance of sensitive marine creatures or stress
access and knowledge: in general, the sea is difficult to access,
but once there, one can travel everywhere, not needing roads.
access to the sea difficult: access is very difficult, particularly
for going down. One can visit the shore at low tide, or use SCUBA gear
for diving. Almost always is a boat needed. A small part of the sea can
be kept in aquaria and studied there. Lately, the seas can be observed
with remotely controlled instruments (ROVs), or by remote sensing such
as aerial photography and satellites. Submersibles have also become tools
man is more mobile at sea than on land: once in a boat, there is
no need for roads. One can go anywhere.
water is mortal to humans: deprived of air, humans die quickly when
submerged. Water can also kill rapidly by cooling. SCUBA diving has its
own hazards. The amount of time we can be in the sea is very limited.
dangerous: the sea is dangerous and unpredictable. The force and
fury of waves counter-intuitive. (See oceanography/waves)
difficult to look through the surface: the surface of the sea is
reflective, making it difficult to look down. Sea life is counter-shaded
and camouflaged to avoid detection. Low level visibility complicates observation
further. The cleanest ocean water has 50-100m visibility (viz), but clean
air has 50-100 km! Most coastal waters have a viz of 2-10m.
low level of knowledge of the sea: our knowledge of the sea has
been obtained by perseverance, by working with the sea, from disasters
and from fishing. Compared with what we know of the land, we know very
little. The sea is also very strange.
data is hard to obtain: because of poor access, and because the
sea is deep and on the move, data is hard to obtain. Plankton organisms
reproduce rapidly, while the water also moves them around.
fewer people are studying the sea: only very few people are studying
the sea, because of unfamiliarity, because we do not use it as intricately
as the land, and because it is expensive and dangerous.
use of the seas: the seas separate continents, and have because
of this been regarded as a nuisance.
fishing: first the coastal seas provided for enough food, but boats
gradually went further afield, complete with factory ships to process fish
at sea. They also progressively fished deeper and for smaller species.
transport: as part of the migrations of people over the world, and
because of trade, the seas have always provided the pathways for people
and cargo. Free access to the oceans is therefore important.
warfare: those countries with navies were able to project their
power across the oceans, waging war and winning the spoils of war.
mining: many minerals, particularly oil, are found under the sea
recreation: the sea has become a place of recreation, ranging from
fun at the beach to ocean yachting.
ownership: not long ago, nobody wanted the oceans. The only interest
was for a strip along the shore. However, that has changed.
vastness: the sheer size of the oceans means that it will always
have places where people won't go; even places that have never been visited.
Even today, little interest exist for those places, although the sea bed
there may contain minerals. The law of the sea infers that these can be
on a first come, first served basis.
fluid environment: the oceans cannot be compartmentalised. They
run across political boundaries. Coastal waters have all been nationalised,
ending tribal ownership. Some fish (tuna, shark, billfish) migrate long
distances, and across national boundaries.
free passage: free passage (beyond 13 nautical miles) was essential
for transport, trade and warfare.
facilities: in order for transport to work, extensive harbours are
needed for shipping (ports), complete with beacons, and surveying for charts.
It has been agreed internationally who surveys which ocean areas. Ocean
rescue has also been agreed on internationally.
open access: until recently, the main ocean strategy was that of
open access to its resources such as fisheries. However, this has led to
uncontrollable overfishing and large fishing nations plundering other coastal
EEZ: the Exclusive Economic Zone has now been ratified as 200 nautical
miles (370km) around sea-bordering nations, with an extension to 500m deep
over the continental slope. Within this area, a nation has exclusive right
to its resources, under the proviso that it exploits them maximally. For
failing to do so, it empowers other nations to move in and to do it instead.
f019624: yellow zoanthid anemones on a red carpet sponge,
in New Zealand. The ocean has life forms that have not become terrestrial.
f020710: A Diadema palmeri sea urchin at the Poor
Knights Islands, New Zealand. Sea urchins, starfish and other echinodermata
are not represented on land or in fresh water.
Triton's trumpet (giant
triton) Charonia tritonis
A very large tropical shell,
reaching 40cm length. It is collected for the ornamental shell trade. Living
in shallow water, these shells are easy to collect. In Indonesia, a survey
of 133 sites on 92 reefs, found only 2 individuals.
Protected in Australian
waters, Seychelles and Fiji.
Giant clam Tridacna gigas
The largest living shelled
mollusc, measuring up to 1.3m and weighing over 200 kg, of which 60 kg
is living tissue. Giant clams may live for 100 years or more. Giant clams
live in shallow water and are easy to harvest for food. They are also collected
for their ornamental shells.
Fully protected in Australia,
Papua New Guinea. Proposed for Appendix 2 of CITES.
Coconut crab Birgus latro
The largest terrestrial
arthropod in the world, measuring up to 1m between stretched legs, weighing
up to 3 kg. A tropical (hermit) crab, which has abandoned the sea, except
that females have to return to the sea to lay their eggs. The crab is regarded
as both an aphrodisiac and delicacy by islanders, who hunt it persistently.
Also predated upon by introduced species like rats, pigs and monkeys.
Fully protected on Aldabra
(Indian Ocean). Further proposals for protection elsewhere.
Threatened species: some tuna,
some sharks, some salmon, all turtles, many sea mammals.
Also read the section on oceanography
which gives further background information on the physics and productivity
of the oceans.
For more details on marine habitats and living in the
sea, read marine environment/habitats/introduction.
Thorne-Miller, Boyce & John Catena (1991): The
living ocean: understanding and protecting marine biodiversity. Island