Phytoplankton

Miss D. A. Crawford, Botany Department, V.U.C.

To most people the vegetation of the sea is limited to the more conspicuous seaweeds found in the littoral zone (interdial regions). But is this all? Does the large expanse of ocean support plant life? It actually sustains the major portion of marine plant life which consists of minute, floating unicellular plants, collectively known as phytoplankton. These plants are found in the upper layers of the sea where the penetration of the light is sufficient for photosynthesis to be carried out. The euphotic zone, as it is called, extends down to approximately 80 metres (266 feet) the depth of course depending on the time of the year and the latitude. In spite of their microscopical size the total bulk of these plants is greater than all the rest of the life of the sea. Sometimes they are found in such quantities that the sea is quite turbid. Their production is greater in enclosed seas and fjords, rather than in more exposed unstable waters, in coastal waters over the continental shelf rather than in mid ocean, in colder waters rather than in the warmer tropical waters. This is illustrated by the huge deposits of plant remains (diatomaceous ooze) which occurs as a more or less continuous belt around the Antarctic and as a band across the North Pacific ocean.

There are two chief components of the phytoplankton, diatoms and dinoflagellates. Besides these, there are several other simple algae which rarely occur in sufficient numbers to be of great significance. Diatoms are all microscopic in size, varying greatly in form and structure. Typically they are yellow-green in colour, due to the presence of both yellow and green pigments. They are unicellular though often joined together in various ways to form chains and other aggregates. The characteristic feature of diatoms is their cell wall. It is composed of translucent silica and the variety of sculpturing shown by the striae (parallel lines) and pits its truly remarkable. These shells are of considerable importance in the formation of siliceous sediments and have formed great fossil deposits known as diatomaceous earth. These deposits have been known from the earliest of times and were used for making bricks for the temples of the ancients, and later for making glass and generally as a source of silica. The variation in form of these plants is largely a matter of structural adaptations to counteract the tendency to sink. Four main type of adaptions are to be found. The bladder type has cells which are relatively large, with a thin peripheral layer of cytoplasm and a large central cavity filled with light cell sap. Examples of this are Coscinodiscus species. In the needle type the cells are long and slender, for example Rhizosolenia species. Fragillaria or Eucampia illustrate the ribbon type with broad flat cells attached to one another to form long chains. The branched type is illustrated by Corethron and Chaetoceros species. These have spines page 16

Plate 1

page 17 produced as projections to resist sinking. Increased buoyancy is achieved in many cases by the formation of chains, and the presence of oil within the cell. Many diatoms grow normally on the bottom in the littoral zone in rock pools, where they may be attached by stalks to rocks or they may glide freely over the bottom. These bottom or benthic forms produce heavily walled cells with the most exquisite designs, while the walls of planktonic forms are much thinner. Diatoms may also grow in profusion on other plants and animals. The littoral genus Licmophora frequently grows on other seaweeds, whilst the massed growth of Cocconeis ceticola flourishing on the skins of whales that have spent considerable time in the cold Antarctic waters, have by their yellow colour, given rise to the name “sulphur bottom” to the blue whale.

Dinoflagellates are an extremely diverse group about which it is hard to generalise. Some possess no cell wall and are more animal in character, while others are truly plant-like having a definite cell wall made of cellulose plates. All possess two whip-like extensions—flagella—which help to propel them through the water to a limited degree. Like diatoms, dinoflaellates also have various structural adaptations to their floating existence. They may have long arm-like extensions or conspicuous wing-like membranes or parachute-like structures, especially in tropical waters where the water is lighter. Many dinoflagellates are luminescent being responsible for much of the brilliant phosphorescence of the sea.

The study of diatoms and dinoflagellates dates back to O. F. Muller (1730-1785) who was one of the first to use the compound miscroscope for the study of the floating life of the sea. Another tremendous step was made with the introduction of the plankton tow-net of fine mesh by Johannes Muller in 1846. The end of the last century and the beginning of this, was the descriptive period when great volumes were written recording new species, new forms of life. This period also saw the founding of the famous marine stations, the first at Naples, then at Plymouth and then Woods Hole in America; in these laboratories researches into the structure, development, life and habits of marine organisms of all kinds has been continued up to the present time.

Next came the investigations of the International Council for the Exploration of the Sea. Scientists of the different nations of Europe together carried out a series of investigations to form part of one great plan. Not only are they enquiring into the lives of fishes, their lifehistories, food and feeding habits, migrations, growth, birth rates, etc., but with continually improved equipment they are studying the distribution of the different plankton forms, the conditions under which they live, the flow of ocean curreent, the chemistry of the sea, and the varying nature of the sea bottom and its life. Since 1918 much work has been done in this direction especially by the “Discovery” Expeditions, page 18 carried out by the Colonial Office on behalf of the Falkland Islands Government. They were concerned about the ecological factors underlying the great whale fisheris in the Antarctic waters. Recent work has become quantitative as well as qualitative. Chemical, physical and biological factors have been measured and correlated.

There are various methods of collecting, concentrating and computing phytoplankton samples, depending on the accuracy required and the number of plants present. The plankton tow-net, consisting of a filtering cone, made of very fine bolting silk (200 meshes to the linear inch being the finest) with a metal ring attached to the wide end and a detachable collecting jar at the tail end, is widely used for the collection of phytoplankton samples. For more accurate quantitative work however, the tow-net has largely been superseded by various other collecting devices such as bottles, buckets and pumps, whereby known volumes of water are collected and out of which the plant population is later concentrated. The method used for concentration depends largely on the quantity of plants present. If large quantities are present they can be concentrated by settling and if free from zooplankton or if estimates of the total plankton are required, the concentrate can be expressed in terms of volume. Concentrating can also be done by filtering through bolting silk. This is commonly employed when large volumes of water are involved as in the use of the pump. The most accurate method is the use of the centrifuge. By this means even the tiniest of plants are retained. The results can be expressed in terms of volume, numbers of plant cells per unit volume, or they can be computed by chemical analysis. In the latter case the plant pigments are extracted with acetone and reported as numbers of plant pigment units. (The tinted acetone is compared colourmetrically with an arbitrary standard prepared by dissolving 25 mg. of potassium chromate and 430 mg. of nickel sulphate in one litre of water. One ml. of standard equals 1 pigment unit.).

By the above methods the seasonal distribution of the phytoplankton can be ascertained for any given locality. ‘Overseas workers correlating the seasonal distribution of the phytoplankton with such factors as sunshine and phosphate content of the water, have shown how interrelated one is with the other. In spring when the temperature and hours of sunshine are increasing, conditions are favourable for the production of phytoplankton which increases to such an extent that soon the phosphate content is almost depleted, limiting the further production of plants. Their numbers then decrease markedly. This makes possible the accumulation of phosphates and other nutritive salts by late summer, the amount being supplemented by a mixing from deeper waters due to an overturning of the water strata as their relative temperatures change. Conditions of light and temperature are still adequate at this time, allowing a further production of phyto- page 19 plankton, which decreases again as winter approaches. Thus the phyto-plankton usually shows two periods when maximum numbers are obtained, one in spring, and another in late summer. The only work that has been done in New Zealand in this direction has yielded similar results, although along with results from off the coast of New South Wales, the spring maximum appears to be one month ahead of the northern hemisphere. Also the waters on the whole not being enclosed, are subject to the influence of incoming currents with the consequence that even in winter the waters are rarely if ever so depleted of phosphate that they can support no plant life.

Thus the cycle of life in the sea continues. The sun shining down on the water provides the energy for plant growth. From the atmosphere, carbon dioxide and oxygen are dissolved in the water and the mineral salts such as phosphates, nitrates are derived from land and spread throughout the sea. These form the raw materials from which plant cells synthesis complex organic material. They alone can utilise these simple substances in solution. On them, directly or indirectly,’ depend all the animal life of the sea.

The phytoplankton forms the food of the zooplankton which consists of very small animals such as copepods, other crustacea, and the larval stages of fish, all of which may occur in immense numbers through the upper layers of the sea. Great quantities of the zooplankton are captured and utilised by carnivorous animals such as many actively swimming fish, certain sea birds and whales. There is also a continuous rain of dead and dying material from above which forms the food of many detritus feeders and scavengers on the sea bottom. Bottom feeding fish prey on many of these forms although the larval stages of the fish are usually planktonic. From the human standpoint, man is regarded as the culmination of the food cycle. In practice this is frequently the case with his inroads on fish populations by net, line and trawl, and depletion of whale populations with the use of the explosive harpoon.

The economic importance of these animals is very great both as a supply of a large percentage of human food, and of very valuable oils. For this reason it is important to understand the factors regulating the production of the marine animals utilised by man.

Results from the study of fundamental factors in the marine food chain are being used in at attempt to understand the fluctuation of fish populations, and to forecast the future numbers and their movements, and to decide whether it is economic to supply nutrient materials to enclosed water masses in order to increase the economic populations they can support.

Already some economic applications of plankton study are being used. At Hull much work has been carried out in an attempt to correlate changes in plankton with those of the fisheries in the north. It page 20 has been found that future stocks of haddock are dependent on the mortality rate of young fish. This is directly affected by the varying quantities of suitable planktonic food. It has been found possible by studying the plankton, to indicate probable variations in fish populations from, year to year. Similarly the distribution and movements of the very important herring populations are closely related to the presence of suitable planktonic food. Knowledge of the distribution of the latter is providing a basis for forecasting probable movements of herring and the best regions for future fishing. Movements of whales is largely determined by the numbers of the shrimps which form whalefeed. The phytoplankton which is so dependent on nutrient salts for their profusion, form the food supply of, the whalefeed. From a knowledge of the interactions of this cycle, it has been stated that a phosphate analysis of the sea water in the southern ocean can supply a very valuable indication as to the likely presence of the whales.

It is apparent then that a study of the changes in plankton can have a direct and important relation to economic problems. This relationship has yet to be elucidated, and utilised in New Zealand waters.

Suggested reading: The Seas, by F. S. Russell and C. M. Yonge, 1928. Explanation of Plate I—Diatoms A-E, Dinoflagellates F-H A. Coscinodiscus sp. B. Corethron criophilum. C. Chaetoceros sp., D. Eucampia sp. E. Rhizosolenia sp., F. Ceratium furca, G. Dinophysis tripos, H. Ceratium tripos type.