THE ZUIDER ZEE

Te Awamutu Courier, Volume 56, Issue 4043, 4 May 1938, Page 10

THE ZUIDER ZEE

HOLLAND’S GIGANTIC UNDERTAKING. Last year I had the unforgettable experience cf crossing the enclosing barrage which runs across the mouth of the Zuider Zee from the village of Zarig in Friesland to the village of Van Ewijksluis in North Holland, a distance of twenty-six miles, writes Dr J. S. Wood in the Melbourne Age. The journey was unforgettable because of two very vivid impressions—first, the sensation of motoring over a narrow road running into the distance, but with great expanses of water on either side as far as one could see; second, the sight of luxuriant crops growing among seashell? on sandy-silty soil which a year oi two previously was under the water. These were seen when a descent was made from the dyke into the newly reclaimed land. Every schoolboy knows that a large part of the Netherlands lies below sea-level. In fact, 38 per cent, of the total area of the country is kept above water by artificial means. The project of damming the Zuider Zee and its ultimate conversion into arable land will increase the area of Holland by 7 per cent.—by an area of about 600,000 acres. At the present time only a portion of the scheme has been completed. The area of land so far reclaimed—the Wieringermeer polder—is 48,009 acres in extent. It is the largest polder (reclaimed area) in the world, but will be by far the smallest of 4 when the three other projected polders are completed. Karel Capek, the famous Czechoslovakian author, has described the process rather humorously, “You take a bit of sea,” he says, “fence it in and pump it out, and at the bottom is left a deposit to which a respectable slice of Europe, by means of its rivers supplies its best swampy soil, and the sea its finest sand; the Dutchman drains it and sows grass there, the cows feed on it. the Dutchman milks them, and thus makes cheese which is sold to England, and this, incidentally is a suggestive example of metabolism.” This is a fine test on which to hang a sermon on the Zuider Zee, whether from the point of view of the engineer, geographer, agriculturalist, economist, or biologist. It is, of course, from the point of view of a great engineering feat that the whole project commands its greatest respect, but in this article I simply intend to say something of the history of the Zuider Zee and of the biological changes which have occurred since its enclosure. This subject is one of peculiar interest to South Australians, since they are faced with a somewhat similar great biological experiment in the building of barrages across the Murray mouth at Goolwa. It is to be regretted deeply that neither funds nor investigators have been provided to study the gradual changes, both environmental and biological, which follow the completion of the barrages. The reclaiming of the Zuider Zee is largely a repossession of land filched by the sea. The last glaciation of Europe (usually dated about 5000 8.C.) was followed by a retreat of the North Sea, and on the sand bars so formed a sand dune system developed, fringing the coast of Holland. Behind these dunes were brackish lagoons which gradually became transformed into fens—that is, areas with wet, peaty soils. This was the state of affairs at the time of the Roman invasion of Holland, as w e know from the historians of the Gallic wars. Beginning in the sth century, A.D., and culminating in the 13th century, the dune system was largely destroyed. So was formed the Zuider Zee, and also, incidentally, began the long fight for the

Dutch to reclaim and retain their lost land—the earliest dykes are sea dykes of Friesland, built about 1000 A.D.

Before the building of the great dyke the Zuider Zee was a large, shallow basin of brackish water fed by the river Yssel in the south, which varied in salinity from about 17.5 grams of chlorine per litre near where the present dyke is built, t 0 about 0.2 grams of chlorine per litre near the estuary of the River Yssel. It is unusual to express salinity in terms of grams of chlorine; one gram of chlorine is equivalent to about 1.8 grams of common salt (sodium chloride) so that the variation in the Zuider Zee was from that of sea water to that of a moderately fresh water. Since the completion of the great barrier dyke the Zuider Zee—or as it is now called, Ysselmeer, that is the Yssel lake—has been gradually decreasing in salinity. It is hoped that eventually the Ysselmeer will be transformed into a large reservoir of fresh water.

As a result of the freshening, important environmental changes have taken place and have produced a profound effect upon the vegetation of the Zuider Zee. Most of the vegetation consists of microscopic free-floating organisms which collectively are termed the plankton. The yield (or number) of these organisms is dependant on the presence of various chemical substances in the water, notably nitrates and phosphates, plentiful aeration, and on the penetration of light radiation.

None of these factors appears to be limiting in the Ysselmeer compared with the Zuider Zee; nitrates and phosphates are present in quantity, and adequate aeration is shown by the absence of black mud on the bottom of the lake—black mud is formed by sulphate reducing bacteria in the absence of oxygen. The greatest change since the erection of the barrage has been the decrease in the salt content, with its consequent changes in the osmotic pressure of the water.

Two-thirds of the total number o( species comprising the plankton before the completion of the barrage consisted chiefly of marine diatoms. These are small, unicellular plants with hard siliceous “shells.” The original plankton consisted of seventytwo species of diatoms, chiefly marine; fifteen Green Algae, seven Blue-green Algae and fourteen Peridinians.

In 1931 a staff of workers began investigations on the changes in the plankton, and at intervals throughout the succeeding years have collected samples of the plankton. Eleven different localities in the lake were sampled on each sampling day. Of the fortyfour species of marine and brackish water diatoms present in the Zuidei Zee, only two have maintained themselves in the Ysselmeer. The invasion of fresh-water forms was very rapid, as the following shows: In 1931 twenty-one species of freshwater diatoms were observed; in 1932, fortytwo; in 1933. fifty-three; in 1934, sixtynine. These forms comprised 31 pei cent., 56 per cent., 65 per cent., and 89 per cent, of the total number of species.

The plankton at the end of 1934 consisted of forty-nine species of diatoms, twenty-one Green Algae. The floia of the Ysselmeer is again a diatom flora, but entirely different in composition from the previous one of the Zuider Zee.

The number of species in the plankton of the Ysselmeer is approximately the same as that of the old Zuider Zee, and the plankton is apparently wholly sufficient to perform its functions in the life-cycle of the lake. It forms the chief food, a peculiar Zuider Zee race of the smelt —the only brackish-water fish which survived the enclosure. Of the fishes originally present, herring, anchovies and plaice have all disappeared, but freshwater fish like bass and carp are tak-

ing their place and are beginning to settle in quantity. Mussels, clams and crabs have all disappeared and are not present in the Ysselmeer. Of the higher algae—seaweeds in the usual sense—the Brown and Rea seaweeds have disappeared, and only one large member of the Green Algae remains. The change in the higher aquatic plants has also been important.

South Australians are familiar with the vast sea meadows in the shallow waters of the gulfs. These meadows are composed almost entirely of flowering plants, which have adopted an aquatic habitat. Their long, strap-like leaves are thrown up in great banks on our beaches aftei storms.

They are species of Zostera, Posdonia and Cymodocea. In the Zuider Zee two species of Zostera (one of the the same as occurs in the gulfs) formed the chief spawning and nursery places for fish; Ihe plants perform

.he same function around our own •oasts. Neither of these plants has survived in the Ysselmeer.

Fortunately, their place is beingtaken by species of Potomogeton, another fresh-water flowering plant This plant performs a similar ecological function to Zostera and is providing veritable hatcheries for freshwater fishes.

The spread of these important freshwater aquatics has been slow, however. The factor hindering the spread has been mentioned before; it is the tenacity with which the bottom of the lake holds its salt. This does not affect to a great extent the plankton which lives in practically fresh water, but it does affect these higher plants whose roots grow in the muddy lake bottom.

From a biological point of view, therefore, the enclosure of the Zuider Zee has resulted in a rapid change from a brackish water community to a fresh-water community, each ecological group of the Zuider Zee being replaced by one closely allied to it in the Ysselmeer. It has been an experiment of extreme interest from this point of view.

' One cannot but deplore again the fact that no such biological .record—which may have important economic results-will be available for the enclosure at the Murray mouth.