Tuatara: Volume 4, Issue 2, December 1951
The least observant, least interested of persons cannot have failed to notice that the sea is not always blue. In fact, in coastal waters, it is almost exceptional that the typical blueness associated with the sea is to be met with, and especially does this apply in enclosed waters such as harbours, sounds and deep bays. To be ‘blue’, sea-water must be free of sand, mud, volcanic dust etc., and be sparse of animal and plant life — in short, it must be clear and without the larger particles of matter, as found in the open ocean. It is only under these conditions that the shorter wave lengths of light (the ‘blue’ end of the spectrum) can penetrate deeply into the water and be scattered by very minute particles to give a visible blueness, in much the same way as light penetrating the atmosphere is scattered to give the blue of the sky. On days of heavy overcast, reflection from the sky will help produce the steel-grey appearance, in alliance with the condition that much of the blue end of the spectrum is absorbed in the cloud mass and does not reach the water to be scattered therein.
Plant or animal life, organic detritus, or sedimentary particles present in any degree of concentration at all immediately affect the colour of a water mass. This is obvious in such cases as flooded muddy rivers entering the sea or a lake. The degree of colour-change is more subtle, however, when concentrations of particles, whether animal, vegetable or mineral are not so great, and especially when the inherent colour of the particles is not strong, as is so with many of the marine organisms concerned with discolouration. Depths at which the organisms occur, may modify the colour of the water. When scattered through some depth, water usually is darker coloured, often being almost black when seen from above, but with the same organisms concentrated close to the surface the colour might be, say, a milky-green. And again, wind-ripples on the surface, the state of the sky, and even the height of the sun, all have their effects on whatever is the colour at the time.
There are chiefly two ways by which animals or plants can reach the concentrations necessary to become visible as a distinctly coloured area, either by ‘blooming’, or by actively aggregating as is found with the strongly-swimming Krill (or whale-feed) of Antarctica or the mana of New Zealand waters. Blooming is a very rapid multiplication of small, single-celled animals or plants, in a large or a small area, when such vast numbers are reached that the affected water is discoloured by them. In so far as wind action on the sea may concentrate these organisms, or others that are relatively non-motile, into definite patterns, then page 42 aggregation can be said to occur in blooms as well. In general, blooms follow similar courses: a development-period, during which rapid multiplication of organisms occurs; a peak-period, lasting days, weeks and occasionally months, during which the organisms are in very high numbers (often several millions per litre); and a dispersion period, usually of only a few days in any one area, when the bloom is dispersed. Dispersion may possibly be due to wholesale deaths because of a change of conditions — a sudden rise (or fall) of temperature, an influx of fresh-water or some other such factor; or it may be due to a rapid rise in numbers of some other organism; or to unhealthy conditions developed by the bloom itself which in turn destroys the bloom. The true conditions necessary for development, maintenance and the final dispersion or destruction of a bloom of organisms are not understood; even though the study of these conditions has now been under way for many years, associated problems are so numerous and so diverse that, until these are solved, little or no progress with the conditions connected with blooming itself can be made.
The marked discolouration sometimes associated with blooms arouses a considerable amount of speculation in the press and in popular accounts, due primarily to the depths of the colours, but due in part too, to the difficulty of finding out anything about the causes. Various shades of red — brick-red, brown-red, rusty-pink and so on — are the most usual, and certainly the most noticeable colours. Less obvious, but none-the-less real, are yellows through various shades of yellow-green to pea-green, olive-green or greenish-khaki; and even less noticeable is the dark, brown-blackish hue of harbour or sound water, seen usually in Spring. These colours, and others too, can all be traced to blooms of the tiny, single-celled organisms collectively known as Protozoa (animals) or Phytoplankton (plants — see also Tuatara Vol. I, No. 1). As coastal waters are those most richly supplied with the necessary phosphates and nitrates on which plants depend, and as water temperatures are frequently higher in the shallower depths so that development and multiplication are more rapid than in the open ocean, it follows that the numbers of organisms per unit volume of coastal water are usually far higher. And this, together with the fact that yellowish, water-soluble pigments dissolve out of many of these organisms, is very largely the reason why such coastal waters are discoloured more often with shades of green than they are blue.
Organisms Causing Discolouration
Besides being of interest to the biologist, the organisms of discoloured waters, on more than one occasion, have been of direct concern in commercial ventures. Ordinarily these animals and plants are harmless enough to other creatures, and in actual fact many of them are essential constituents of food-chains, especially of smaller animals. But on occasion, blooms of particular species may be disastrous. During eight months of 1946-7, a species of dinoflagellate (a group within the phytoplankton) page 43 Gymnodinium brevis, — which bloomed along the coast of Florida and produced vast areas of yellow-water (the so-called Red-Tide), — is believed to have been the direct cause of the deaths of an estimated 60 to 500 million fish. The actual cause of death is regarded as being possibly a toxic substance derived from, but harmless to the organism, or alternatively, a waste product of the metabolism of the organism, and fatal to the fish only in the high concentrations likely to be found when many of the organisms are present. Less highly developed animals did not seem to be affected. In Japan, fairly regular outbursts of various other dinoflagellates are most destructive of the artificially cultured pearl-oysters, and again the true cause of death is not known. Along the California Coast, Gonyaulax, another dinoflagellate, blooms regularly each year between May and November, very often causing red discolouration. Mussels feed on the organisms and are not affected, but should man eat the mussel, he is poisoned, often fatally. By regular laboratory testing this poisonous state is determined each year and notices posted warning people of the consequences. Coming nearer to New Zealand, a bloom of a species of Glenodinium (dinoflagellate), in Sydney harbour in 1891, killed a wide range of shell-fish, crabs, polychaete worms and almost all other invertebrate animals unfortunate enough to come in contact with it. Many other instances of poisoning associated with blooms (many of them of dinoflagellates giving rise to red or yellow water), some selective as with the fish in the Florida ‘Red-Tide’, others geenral, as at Sydney, have been recorded from various parts of the world and in some the economic losses caused have been severe. In New Zealand waters however, and this in spite of a number of most spectacular blooms, no animal deaths have been reported or recorded. Because such destruction of marine organisms has not happened to date does not mean that it will not occur; there is no reason to suppose that one or more of our species, closely related to destructive species elsewhere, may not bloom, given satisfactory conditions, and be just as fatal to life in the sea.
As well as these three organisms, two other, comparatively large animals are known to be causes of conspicuous discolouration. Except for four years during the period 1898-1924, red discolouration was recorded annually in Port Chalmers and often outside the harbour as well. These regular occurrences were all of Munida gregaria, a shrimp-like organism (mana in page 45 New Zealand waters) one to two inches long, which aggregates actively into large shoals, and is an important constituent of the food of some whales migrating along our coasts. The observer, Mr. M. Young, likens the behaviour of these numerous darting, clustering crustaceans to a swarm of bees, and this description is most apt. Munida was collected again in 1951 from large areas of red-water both outside and inside Port Chalmers; it has also been collected and reported from Cook Strait on more than one occasion in the past. A fawn to buff colour, visible over distances of at least two miles, has twice been produced off Dunedin, and also a number of times in Cook Strait. Species of Pyrosoma, slender, more or less cylindrical, pale-coloured colonial ascidians (related to sea-squirts and salps), massed in enormous numbers into lanes and patches were the causes of these discoloured areas.
Although the above organisms are known to be associated with conspicuously discoloured sea-water, there are many records from widely scattered localities about New Zealand, for which no organisms are known. In some of these places discoloured water appears so regularly that it has a local name attached to it — ‘swamp-water’ in the Firth of Thames, ‘fern-dust’ in the Marlborough Sounds; in the former instance at least the organism concerned is unknown and the same is true for many more of the records. This is unfortunate. The organisms concerned with discolouration need to be known from as many areas as possible in order to arrive at a knowledge of their distributions. Also it is desirable, in fact necessary, to know when, and how often, outbursts of a particular organism occur. With these facts, it is possible to determine whether particular organisms are associated with certain conditions of climate or weather, for example, rainfall, amount of sunshine and so on; or whether they are to be found only in limited areas or in bodies of water with special qualities of salinity and temperature. If these, and other, facts can be elucidated, some light may be shed on the great scientific mystery of the causes of blooming among the protozoa and phytoplankton. If the cause for the blooming of but one organism could be determined, it would be invaluable for application in studies of the productivity of the sea.
Notes on Collection and Preservation:
Many of the organisms concerned with discolouration are very small, in the region of 1/500th of an inch long. To collect samples of these from red, or other strikingly discoloured areas, it is usually necessary to fill a bucket or large container from an area of dense concentration of organisms, give the water a light swirl, and leave to stand for a few minutes, preferably out of the sun. The organisms will usually concentrate into patches. Dip from a dense patch with a small container, and add the water (about 1:1) to one of the following preservatives:page 46
|Mercuric chloride||66||(This is very poisonous)|
|Glacial acetic acid||1|
|Saturated aqueous picric acid||75|
|Glacial acetic acid||5|
It is desirable to add the acetic acid just prior to using a quantity of solution.
Failing these preservatives, which are the most satisfactory, 40% commercial formalin added to the sample in the proportions of between one part of formalin to 15-20 of the water containing specimens can be used, although this does not adequately preserve all characters of such small organisms.
The following information should be noted along with the samples:
Approximate area of bloom.
Temperature of water.
Allen, W. E., 1949 — ‘Significances of “red-water” in the Sea.’ Turtox News No. 24, 2pp.
Galtsoff, Paul S., 1949 — ‘The Mystery of the Red Tide.’ The Scientific Monthly Vol. 68 (2): Pp. 109-117.
Gunter, G., F. G. W. Smith, and R. H. Williams, 1947 — ‘Mass Mortality of Marine Animals on the Lower West Coast of Florida, November 1946 to January 1947.’ Science, Vol. 105 (2723): Pp. 256-257.
Gunter, G., et al., 1948 — ‘Catastrophic Mass Mortality of Marine Animals and Coincident Phytoplankton Bloom on the West Coast of Florida, November 1946 to August 1947.’ Ecological Monographs, Vol. 18, Pp. 309-324.
Whitelegge, T., 1891 — ‘On the Recent Discolouration of the Waters of Port Jackson.’ Records of the Australian Museum, Vol. 1, Pp. 179-192.
Young, M. W., 1925 — ‘Notes on the habits of the Whale-feed (“Munida gregaria”).’ N.Z. Journal of Science and Technology, Vol 7, Pp. 317-319.