MANGROVES CRADLE OF THE SEA MY EBOOK
Prologue: Mangroves - Richest of Coastlines
We New Zealanders share a coastline almost equal in length to that of the United States.
From childhood we are accustomed to make use of the sea for sport and recreation. So it
is with interest and concern that most people learn of the dangers the seas of the world
now face as humans come to the limits of their environment.
We shudder to hear that each year industry puts millions of tons of oil into the ocean.
The effect of an oil slick on beaches and boat hulls is immediately apparent.
Unfortunately the marine environment is suffering from human exploitation in many less
obvious ways. It is only by listening carefully to the advice of marine scientists that we
can avoid irretrievable damage to our New Zealand coast.
But who would think that mangroves were a cause for concern? An early New Zealand
novelist wrote of mangroves with loathing: “I never saw one that looked as if it
possessed a decent conscience. Growing always in shallow, stagnant water, filthy black
mud or rank grass, gnarled, twisted and stunted and half bare of foliage, they seem like
crowds of withered, trodden down old criminals, condemned to the punishment of
everlasting life”.
Such an attitude to mangroves has been shared by generations of New Zealanders who
live in the mangrove zone. Northland and the Hauraki Gulf, including harbours like the
Kaipara, Waitemata and Manukau have a vast extent of deeply indented coastline.
Almost every bay, estuary and harbour backwater in this area is lined with mangrove
trees. They reach their greatest sizes around Whangarei and further north, where they
line tidal rivers in a dense green canopy some twenty to thirty feet in height. Further
south they are much smaller and stunted, diminishing to low bushes around Tauranga and
Opotiki on the east coast and as far as Raglan in the west.
Mangroves are scarcely more gnarled and twisted than the beautiful pohutukawa. The
fact that their environment may become foul and pestiferous is largely the fault of
humans who restrict the tidal flow around them.
Instead of looking on mangroves as “trodden down criminals” let us look at them as a
vital element in our coastal ecology. For too long it has been the custom to regard
mangroves as wasteland, only fit for the dumping of refuse or for reclamation. As
monuments to our ignorance Auckland and Northland are dotted with mangrove-killing
causeways which sweep across harbour back waters and estuaries, carrying traffic a little
but faster to their destination often paradoxically to a seaside holiday. Yet in turning our
mangroves into soggy paddocks we are destroying a marine asset perhaps ten times more
valuable.
For the sake of a few acres of grazing land we lose a huge interrelated network of living
systems which scientists as yet do not fully understand. Like the radiator of a car it offers
a maximum of exposed surfaces to the sea. You will have noticed how swiftly any new
jetty pile, boat hull or drifting object is colonised by marine animals and plants. Settling
surfaces are at a premium in the sea. An artificial reef can transform an area of barren
sea bed into a thriving community. In Asian waters fish farms are established around
floating rafts of bamboo. A New Zealand oyster farm or mussel barge is a more
immediate example of man providing a settling surface to enable shelfish to establish
themselves in the self-fertilising sea.
In this light it may be appreciated how serious it is for our coastal environment if we
persist in “tidying up” our scruffy harbours and estuaries with neat paddocks, highways
and rubbish dumps.
Only one third of this planet’s surface is earth. The continental shelves about which we
know so little are equal in area to the entire surface of the moon. Beyond these shelves,
the area representing ninety per cent of the world’s ocean, is a biological desert.
The zone where life is most prolific is the shelves of the continents, stretching from the
seashore down to 600 feet. This is the greatest domain of life on Earth. Unfortunately it
is also the area of the sea most subject to pollution and interference from man, who little
suspects how far reaching his actions may be.
We have to break the habit of treating the open ocean as an infinite resource for a hungry
world. For that we need to keep intact every tide pool, every mangrove, every piece of
shoreline.
We have now entered the solar age. Fossil fuels will decline in importance as finite
deposits are exhausted. The challenge we now face is to recognise the sun as our safest
nuclear reactor, and to grasp the fact that, in a solar age, a country’s GNP is the measure
of the plant material or biomass which its land surface and territorial waters can
photosynthesise each year from the energy of our nearest star. (gross natural product)
The coastal environment is New Zealand’s richest province, in this respect, but also the
most sensitive to abuse.
In terms of solar age accounting, the daily energy transactions on our coastline make it
Wall Street, and the richest areas - sheltered harbours and inlets, estuaries and swamps -
are almost twice as valuable, in terms of biomass production, as our best agricultural
land, tilled and topdressed by machinery.
Ecologists now see the coastline as a series of habitats which fall into two major
categories: those that are sheltered from the wind energy of the open ocean and those
exposed to its full force. Modern man threatens each habitat with a special set of
problems. We must learn to appreciate these marine worlds and to live within them
without causing harm.
As so many life forms rapidly become extinct, we are realising that it is no longer
individual species we must protect, but their entire habitats.
From an ecological viewpoint, the richness or biomass of coastal habitats depends on
their stability.
Sand and gravel beaches provide an insecure and shifting environment for a reduced
range of life forms. Those adapted to a burrowing existence form very dense populations
in such areas - pipis and other bivalves, tube worms and swimming crabs. Further up the
shore the sand dunes provide a very fragile habitat in which the least disturbance of plant
cover can cause erosion. (In Holland access to beaches across sand dunes is now
restricted to foot bridges.)
The boulder beach is the harshest life zone of them all. Every storm tosses the big stones
around and rubs them together, giving them their clean, close-grained surfaces. Despite
the diminished range of organisms in such areas, life is tenacious, and small, fast-moving
creatures such as crustaceans manage to find refuge from predators among the boulders
and feed on the life-giving currents.
The richest marine worlds are the most sheltered parts of our coast. Mud is the basis of it
all: the messy places we insist on “tidying up”, the mud flats we reclaim, the inlets we
straighten with causeways, the swamps and wetlands we drain for pastures, all the
gentlest meeting places of land and sea, are the “black gold” of tomorrow. In thick, black
mud thrive millions of bacteria which readily convert decaying matter into nutrients, a
process on land well known to soil technologists. The difference between clay and fertile
soil is the presence of countless useful bacteria which convert animal manure and leaf
mould into plant food.
In a forest when a leaf falls the solar energy store it represents usually stays within the
forest as leaf litter, but in mangroves each fallen leaf releases its nutrients to the tides and
beyond. In all the world’s oceans, mud and ooze swarm with bacteria. From the upper
waters a rain of dead animal and plant life is attacked by them and broken down into
nutrient salts and minerals. Over the continental shelves upwelling currents bring these
recycled organic chemicals back into contact with sunlight in the upper layers of the sea.
Here microscopic plant plankton feed on them; generating a rich algal soup which
nourishes the whole marine food chain from animal plankton to sardines and great
whales. Such is the importance of algal plankton that it is now believed they are the
main source of oxygen in our planet’s atmosphere and chiefly responsible for its renewal.
For algal plankton the shallow warm expanses of harbour and estuary are a hidden
pasture of the richest resources for here nutrients flow from the land and from the sea.
Twice a day, into every sheltered place on the New Zealand coast, a ‘tidal topdressing
plane’ glides in under moon power. Along estuarine shores where wave violence is
minimal, over harbour flats where exposure to the sun is seldom fierce, amid grasses,
reeds and leafy stalks it flows, bringing fresh food and oxygen to all the low, sly lives
that find refuge in that vast surface area. So the crabs and snails, worms and barnacles,
shrimps and oysters grow fatter, multiply and release their larvae to the sea. As each tide
recedes from all these sheltered havens, fishes gorge themselves on the rich outpourings,
wading birds begin to stalk over the refurbished mud flats and on sandbanks cockles and
tuatuas close their filter intakes until the next tide.
Ecological Role
Mangrove glades play a very important role in the ecology of our coastal waters.
On every fine filament, twig and rootlet, silt is deposited. All rivers carry silt from the
surrounding hill-sides. Usually it flows out to sea. If a river is slowed down with
serpentine windings and increased vegetation, the silt will be retained. This the
mangrove does extremely well, being the only plant able to withstand the harsh
conditions at the meeting place of land and sea. Its roots and branches stabilise mud
banks. Over what would otherwise be an intertidal expanse of smelly mud they extend
very gradually to reclaim the land.
If we alter the mouth of a river, removing mangroves and straightening things up, we
must realise that in so doing we increase the flow of the river right back to its headwaters.
Without the network of mangroves at its mouth, and the meandering turns, the swift
flowing waters bear an increasing volume of silt out to sea, causing erosion in the
hinterland and affecting our fishing grounds. Nature loves the wild disorder of the
mangrove forest. And so do an increasing number of those who enjoy outdoor pursuits.
The vast settling surface offered by the mangroves’ contorted roots and trunks bathed
twice daily in a nutrient tide of salt water, provides a nursery for the pelagic larvae of
many creatures from the adjacent sea. Each outgoing tide releases rich swarms of tiny
crustacean larvae and other living material back into the sea where it helps support our
inshore fish populations. Mussels and oyster spat have a tremendous mortality rate in the
sea.
It is the huge settling surface of our mangrove forests that enables sufficient to be
released for an adequate survival ratio. No area of seashore can sustain the density of life
found in a mangrove forest. Sea plants and animals find a shoreline habitat very difficult
to colonise because of the sharp contrasts in temperature, violent water movements and
the drying out effect of tidal changes. Mangroves help them in that their roots stabilise
these fluctuations. At low tide, water is held around them and there is shade and deep
mud to reduce the extremes. With the rising tide the roots diminish the pressure of the
current and quieten the underwater world during its main feeding period.
The mangroves belong to the highest group of plants, the seed bearers, whose earliest
forms probably developed on land but adapted themselves to a marine environment to
escape competition with other plants. Certainly no plant threatens their dominance of the
difficult world where shore and sea meet. To survive on anaerobic (oxygenless) marine
mud the mangrove puts down a grid of roots forming a dense network beneath the mud.
From these roots arise breathing roots or pneumatophores. Anyone who has been
floundering in a mangrove forest will remember the thickets of pencil-like roots sticking
about four inches out of the mud. Nets get caught in them, fingers are cut by the oysters
which cling to them and they are almost as uncomfortable to walk on as a bed of nails.
But these breathing roots are the secret of the tree’s survival in salt water and mud. If
they are destroyed or smothered the trees will die.
At the height of the northern summer, the story of an individual mangrove tree begins. If
you row quietly through the glades of grey green and bronze colour leaves, soft plopping
sounds come to the ear. These are the mangrove seedlings. Like fat green beans, they
fall from the tree already germinated, a tiny four-pronged root tucked in the thick folds of
the outer skin. When the tide ebbs, thousands of these buoyant seedlings drift out to sea,
eventually to become stranded on another shore. If conditions are right the tiny root is
quickly established in the mud and a leafy stalk is sent up. As it grows in a tangle of
roots, all sorts of debris collect around it: driftwood, leaves, shells and sticks.
Within this shelter crabs and worms take refuge. Molluscs and barnacles settle on the
roots and branches. In a short time there is established a complete community of plants
and animals all tied biologically to the mangrove.
Chapter One: Trees in the Sea: Exploring Life in Mangroves
After several years in the seventies exploring coral reefs aboard Dr Walter Starck’s underwater research vessel El Torito, an intensive period of my life had been completed. I returned to live beside a mangrove river in Northland, New Zealand. I recalled Walt’s words about mangroves: “I’ve always had a fondness for mangroves”, said the scientist-philosopher during cabin talk. “I grew up around them in the Florida Keys. They’re one of the places where life has reached its fullest expression; amongst the most productive communities on the planet. The ultimate evergreens.
“All the world over,” Walt continued, “as water runs off the land it takes silt and nutrients with it. At tidal level it spreads, forming mud banks. Such places are very rich in nutrients and there’s no shortage of water - although it’s salt. A great niche for any plant that can survive under such extraordinary conditions. And there is no competition. It’s too wet and muddy for most land plants and too harsh for marine plants.”
Aroused by coral reef communities and tropical rain forest, my passion for diversity spilled over into the mangroves at my door. Walt’s words kept coming to mind. Why did he value them so highly? Why were they so incredibly productive? To my delight I began to find a complexity of life in a world of mud and flooded forest that matched the tropical world and coral reefs I still yearned for.
Gradually I learnt there were myriad secrets to penetrate amidst those low, sly lives. The greater my effort the more I was rewarded with insights. Soon I was developing new angles of approach: I built an observation platform above a winding river glade; a crab hide let me pry at mud level into a busy society of ecofarmers; I suspended myself face down above crab city in a rectangular net; I crawled beneath overhanging mud banks with a torch and high power lens; by day and night I dived with scuba; on a special surfboard with a window-panel I drifted upstream silently with the tide - the more avenues I tested, the further I was allowed to enter a hidden world.
In isolation near the mouth of the Matapouri River stand the first of its mangroves: two trees surrounded by a broad expanse of tide-rippled sand. Around these sentinels at times of unusually heavy rainfall all the sand is swept away leaving yellow clay pug. The entire root system is exposed - an elaborate radial web of cable roots and risers that extends beyond the height of each tree. In more typical situations the interlaced root mass may be up to five times the diameter of the tree’s canopy. There is no taproot. This massive raft of roots anchors the tree against the most extreme erosive forces of storm energy, wind and water pressure, effectively consolidating the surrounding area.
Upstream from these trees, on the first bend in the mangrove system, I discovered my favourite diving area. At this point the river is still a broad estuary, a wide sandy plain crowded with large, spreading mangroves. The streambed is a firm expanse of cockles and empty shells. At high tide the water here can be very clear. During spring time coastal waters often become green with plant plankton. So effective is the filtration effect of all the plankton sieving life forms in the mangroves that the water is often much clearer when the tide recedes than when it came in.
Like a miniature glass helicopter a translucent shrimp hovers, backing, advancing, rising and falling around the pencil column of a densely encrusted breathing root that quivers in the current. The shrimp tries to avoid me. Rows of swimmerets under its tail pulse in waves that reverse or arrest as it manoeuvres deftly.
Barnacles, dwarf black mussels and oysters cling to this highrise apartment building, feeding on the rich flow of plant plankton borne in by the tide. A fat cats eye sea snail glides over them, its radula rasping at the thin fuzz of algal film that coats every surface in the mangroves, like a child licking at an icecream.
On this bend of the river I can find most of the life forms in the mangrove community and in ideal conditions for photography. To get close to some of the shyest I had to restore to a rare piece of diving apparatus: the bubble-free, silent oxygen rebreather. My lungs expand and contract in rhythm with a rubber bladder strapped to my chest. Replenished from a small steel cylinder warm, friendly oxygen is quietly cycled to and fro and not a single bubble escapes to alarm the tiny prawns and blennies [triplefins] close to my lens. I am weightless and feel very like a fish myself.
An estuarine triplefin hops across the stream-bed towards me. Its foremost dorsal fin is erect, a red splotched signalling flag. This is a male making a bold territorial claim to this huge monocular rubbery monster. Nearby I witness a tragic battle: a blenny has seized the leg of a crab in its jaws. Just like a shark it rotates to tear the limb loose. Other blennies seize the crab’s flailing appendages and provide leverage for each other until their twisting action snaps the legs off, leaving the crab helpless to defend their attacks.
Beyond this first bend the river enters a more typical mangrove world, flowing between steep, deeply undercut banks of the thickest mud.
Along this stretch I set my observation platform among overhanging branches, I constructed my crab hide and I scuba dived intensively by day and night. I found that some fish from the sea rest up here beneath the banks overnight while eels by the dozen prowl for crabs, and a snake eel as thick as my thumb but eight feet in length, lurks in the mud, just its glossy teeth protruding to trap prey.
One day it occurred to me - the riverbank mud does not behave as mud should. I had watched strong currents swirl up and down the river at times of spring tide and storm flood. Once I even feared that our little cottage would be swept out to sea. But the banks remained unchanged. No tree was even displaced. How could normally colloidal mud resist such immense hydraulic pressure? To satisfy my curiosity I had to steel myself for a mucky ordeal. At low tide with a powerful torch and a hand lens I crawled beneath an overhanging mud bank until I could scrutinise the walls of the deepest recesses. To my amazement I found that what I had considered mud was really an array of tiny stones and boulders of varying colours and composition, held in place by a filigree network of fine white threads like cotton. It was just like the system engineers develop: wire netting groynes packed with boulders, to protect bridge abutments from erosion. The delicate hair roots that nourish the mangroves, entrap and retain sediment so firmly that riverbanks of soft mud maintain themselves for years with apparent permanence. For this reason mangrove-lined channels remain deep and navigable. Because of their elaborate root systems, mud no longer oozes as mud should.
After serpenting around four major bends the river narrows with dense mangroves along either bank almost touching overhead. Eventually a leafy tunnel develops that narrows beyond boat width. Even at this stage the riverbed remains half full at low tide and the fringe of breathing roots forms a dense thicket of spears along both sides.
On a golden autumn afternoon a brimming tide curves the river bend with an array of white bubbles that rearrange themselves in geometrical patterns as they conform to the bend (twist). Mangrove boughs are reflected in the water in sharp silhouette against the late sun and the shimmering river. On its face the clouds overhead are received but the sun is too low to strike the river. Its beams just glance over the mangrove canopy in a golden haze. Below the surface breathing roots quiver rhythmically with the tidal flow and silver ellipses spurt past in pairs, groups or alone. Weed-eating parore from the open coast, streaking beneath my tree platform on their journey upstream to graze the algal fuzz sprouting anew on the tree trunks after a day in the sun. A tui explodes with song. Snapping shrimps pop their claws. Wavelets lap the banks. All the sounds resound along the leafy hallways with a special ambiance as within a concert chamber. Below my feet tidal turbulence stirs vortices in the mud and mullet gulp it down.
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