"No Visible Means Of Support-"

Auckland Star, Volume LXX, Issue 99, 29 April 1939, Page 3 (Supplement)

"No Visible Means Of Support-"

THE richness and variety of life in the sea is to the' layman a constant source of wonder. Dur«.L_ C..i .1 i-i

ing the haster holidays the placid waters were teeming with jellyfish, and every little while one noticed that the surface waters were lashed to foam by the working of large schools of kahawai. These were the obvious signs of life—but what of the delicately- f balanced structure that supports c all this life? s The fisherman profits by the evidence lie sees—he knows that both the * kahawai and the terns feed on small c

fishes aiid that the presence of flocks < of working birds invariably denotes a school of kahawai beneath. Actually it i is the prior efforts of the kahawai in 1 attacking the schools of small fish that ( attract the birds to the scene of the 1 feast. We see now that it is the presence t of these tinv school fish that account t for the presence or alisenc; of both the < kahawai and terns. This brings us to t t lie fowl of the tiny fish, which is c plankton, the microscopic drifting and swimming life of the sea. ' c Plankton, mostly invisible to the eye, < is the great food source Of most of the 1 larger sea creatures—but even plankton must feed—and that takes us down to c smaller details. Now plankton is a col- j lective term applied to a variety of \ small creatures both animal and plant, j the lower being the minute diatoms r which are really simple plant forms consisting of a glassy disc of elaborate structure and pattern covered with chlorophyll, the identical green substance found in all plants. To get really down to basic details we find that the whole structure of life, both on land and sea, is directly dependent U]>on the life-giving rays of the sun. Sunlight acting upon chlorophyll, the green substance in plants, enables them to utilise the carbon dioxide and water taken up, and to reform it into the food substances, starch and sugar. Diatoms are thus drifting plants, and seaweeds fixed plants, both deriving their food by chemical processes under the influence of sunlight. Diatoms form the food of slightly larger but still microseqpic life in the form of tiny animals such as foraminifera, and these, in company with diatoms and the larval stages of innumerable animals, make up the plankton which is sifted from the seawater by such creatures as mussels and oysters. Another set of organisms browse on fresh seaweeds, just as land mammals eat grass and herbage. Nothing is wasted in Nature, however, for there is still a large percentage of marine animals that eat neither plankton nor fresh seaweeds. These are the detritus feeders that exist by simply ploughing up soft mud and extracting from it the particles of decaying animal and vegetable matter that becomes impregnated in the mud. Cockles and mud oysters are to a large extent detritus 1

By--A. W. B. Powell

feeder?. Fixed seaweeds are local in occurrence, for they merely fringe the shore and do not extend into the ocean depths below the range of light penetration, this being decided by the inability of seaweeds to utilise their chemical food unless under the influence

UJIITO3 UUUTTI Llie UIUUCIICe of sunlight. ' Animal life in the ocean depths could not exist if it was not for the "crumbs from the rich man's table" in the form of a constant rain of decaying plant and ' animal matter which slowly descends to the depths and provides sustenance to a specialised group of creatures. Those . occurring in depths far from land obtain their food rain from the decay and fall ' of oceanic surface plankton. Every animal community harbours carnivorous species which take their toll of life, and so the structure is completed. To return to the plankton—winds, currents and tides play an important part in its concentration. In Auckland . waters we all know that marked phosphorescence of the sea occurs durin® northerly or easterly weather. This I

m a phosphorescence is occasioned by a heavy concentration of plankton in the water, concentrated in the Gulf by favourable winds. On the West Coast the prevailing wind is westerly to south-westerly, and on the rare occasions when there is a lengthy spell of easterly to northerly off-shore winds the toheroa population invariably suffers. Now the tolieroa is a plankton feeder, and normally its food, in the form of diatoms, is kept concentrated in shore. Off-shore winds disperse this food, and if such conditions are prolonged, the toheroas are virtually

starved. Some years ago a Danish fisheries expert, the late Dr. J. Petersen, found that in the Baltic the ordinary seagrass was the basic food of the fish fauna of the area—not in its living state, however, but as detritus in the mud due to normal seasonal decay of the weed. Almost the same weed, Zostera, occurs extensively on most tidal flats in New Zealand", and in all probability is equally vital to the welfare of certain species of fish and other marine life. Owing to reclamation and other harbour improvements this seagrass is not nearly so abundant in Auckland Harbour as it was in the past, <uid one cannot help associating this fart with the noticeable decrease in fish in local waters as compared with conditions in the past.