the disappearance of zostera - PlyMSEA

the disappearance of zostera - PlyMSEA

2°7 THE DISAPPEARANCE OF ZOSTERA MARINA By W. R. G. Atkins, Sc.D., F.R.S. Head of the Department of General Physiology at the Plymouth Laboratory...

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2°7

THE DISAPPEARANCE

OF ZOSTERA

MARINA

By W. R. G. Atkins, Sc.D., F.R.S. Head of the Department

of General Physiology at the Plymouth Laboratory

In his paper on "The autecology of Zostera marina in relation to its wasting disease", Tutin (1938) states that" In the British Isles the year 1931-2 showed a sunshine deficiency of about 20 % below normal, and no other year in the past ten showed a deficiency approaching this. The scanty figures available for other countries suggest "that this unusual lack of sunshine was a general phenomenon, though in some countries it was less pronounced but oflonger duration." His enquiries showed that about 1931 there was extensive mortality on the Atlantic coast of the U.S.A., though there was some evidence of a local decline in 193°. A year later, in 1932, Zostera had practically disappeared at Beaufort, North Carolina and as far north as Nova Scotia. In 1933 scarcity spread still farther northward to all localities in Canada. At Plymouth, England, the plant is believed to have begun to decrease towards the end of 1931. In § 6 of his summary Tutin writes: "It is suggested that the enfeeblement of the plant due to lack of sunshine in 1931-2 is the fundamental cause of the epidemic, and that recovery depends on the regeneration of the plant from seed and is therefore likely to proceed slowly." It appeared desirable that this interesting suggestion should be tested by an extension of the examination of the meteorological records beyond ten years,

as far as they go, namely

back to 1897, and that the alleged

20

%

deficiency should be checked. Accordingly, taking the sunshine normals as revised up to 1928, the amounts of sunshine received each year in each of the twelve districts of the British Isles were tabulated as percentages of the normal. As is customary mean values were obtained for the districts I-1O, namely excluding the north of Scotland (0) with adjacent islands, and the English Channel Islands (rr). It appears unnecessary to give all the figures, especially as since 1928 such percentages have been published by the Meteorological Office on the 1928 basis, but the following comments may be made upon the records from 1897-1937 inclusive. The most striking thing in the series is the uniformity of the results. As regards low values, in 1898 Ireland S. had 81 % of its normal sunshine. In 19°° and 19°2 England N.E. had 84 %, as had also Scotland E. in 19°2. The year 1912 gave conspicuously low values, and averaged 83 % for the ten districts, Scotland E. and England N.E. showing 76 and 78 % respectively. In 1916 districts 0-4 inclusive showed 83-86 %. In 1920 England Midland, Scotland W. and Ireland S. had

84-86 %, and in 1924Ireland S. had 84 %. In 1927England M. had 82 %. In 1931,the year supposed to have been destructive for Zostera,England S.W. had 83 and England M. 85 %, both districts had 85 % in 1932 and England E.

W. R. G. ATKINS

208

had 85 %. In 1936 England S.W. had 85 % and in 1937 England E. had 84 %. There is obviously nothing very unusual in the low values which occurred in some districts in 1931and 1932. Turning

to the high values, in 1899 England

S.W. had 122

% of

the normal

sunshine with an averageof II4, and in 1901England N.W. had II9. England E. had IIS

% in

1906 and England

N.W. had 120

% in

19II, which averaged

liS %, the maximum average,followedby the minimum in 1912. England M. had IIS % in 1921 and England S.E. had II6 % in 1929 and IIS % in 1933. Table I puts on record the values for the district including Plymouth, namely England S.W. and S. Wales, and the mean values for the ten British Isles districts as usually taken by the Meteorological Office. For neither are the low values of 1931-2 outstandingly low. They have been equalled or surpassed in earlier years. TABLE I. PERCENTAGE OF NORMAL SUNSHINE a, England S.W. and S. Wales (district No.8) calculated on the normal value to 1928, namely 4'28 hr. a day. b, The ten districts of the British Isles, calculated on the normal, 3'95 hr. a day.

a b

1897 106 1°4 19°4 99 99 I9II II8 II5 1918 1°4 100

a b

1925 1°5 101

a b

1932 85 89

a b a b a b

1898 109 97 19°5 101 1°4

1899 122 II4 1906 IIO III

19°° III 100

1901 III

19°7 98 98

1908 102 99

1912

1913 91 91 1920 85 9°

1914 1°4 1°4 1921 III III

1915 101 102

1927 91 93

1928 99 101

97 98 1929 II3 1°9

1934 100 102

1935 97 1°4

1936 85 93

81 83 1919 1°5 1°3 1926 93 94 1933 1°7 106

III

1922

1902 97 93 19°9 1°9 1°4

19°3 98 94 1910

98 97

1916 96 89 1923 95 96

1917 99 98 1924 89 91

193° 9° 95 1937 88

1931 83 89



Table II shows the normal values for the hours of sunshine for the ten districts and the corresponding percentages calculated with respect to the maximum, England S.E., and to the mean. The value for the north of Ireland 3'SI hr. is only 78 % of the maximum, and only 89 % of the mean. In spite of this Zostera formerly grew well in the north of Ireland, though a reduction of sunshine to 89 % of the mean is suggested as having been the cause of the fatal nature of the Zostera disease. Furthermore, the illumination due to sunshine in the north of Ireland or of Scotland is on the averageless than that in the south of England in which with a midsummer maximum solar altitude of 63°, for latitude soo, the vertical

THE

OF ZOSTERA

DISAPPEARANCE

MARINA

209

component of the light from sun and sky is about 122 kilolux (Atkins, Ball & Poole, 1937); for the north of Ireland, around latitude 55°, the maximum altitude is 58°, so the illumination is about II7 kl., similarly with altitude 55° in the north of Scotland the maximum illumination is around 110 kl. This is to some extent offset by the longer duration of daylight during the summer, though the vertical illumination from a low angle sun is small. Zostera has been reported as growing (Borgesen, 1903) as far north as 61° 28' N. in 2-4 m. of water in Vaagfjord in Sydero, the most southerly of the Faeroes. The mean sea temperature at the adjacent Thorshaven is 7.8° C. with minimum 5'4° and maximum 10'6°. No sunshine records are available for the Faeroes. TABLE II.

NORMAL SUNSHINE (1928) AND PERCENTAGES CALCULATED ON THE MAXIMUM AND MEAN VALUES

District Scotland E. England N.E. England E. England M. England S.E. Scotland W. England NoW. and N. Wales England SoW. and S. Wales Ireland N. Ireland So Mean

Hours

%

%

3.63 3'93 4'32 3'82 4'49 3'70 3.89

81 87 96 85 100 82 87

92 100 109 97 II4 94 99

4'28

95

108

J'SI 3'96 3'95

78 88 88

89 100 100

But the assumption that a greater duration of sunlight necessarily means a greater amount of light is not correct. It has been shown (Atkins, 1938) that though the relation usually holds over a year, it does not always do so, for the altitude of the sun when clear is of great importance. According to photo-electric measurements made at Plymouth the year 1931 stood third on the list of eight years 1930-7, as regards the amount of light received, and 1932 was almost identical with four of the other years. Furthermore, the radiation records at London show that in 193I the amount received was not unusually low but rather above normal. Finally, with a water plant, one has to consider the extinction coefficient of the water. In estuarine and coastal waters this is subject to large variations, from about 0'2 to 2'5 even in June, thus introducing more than a tenfold variation in the amount of light reaching the plants (Poole & Atkins, 1937; Cooper & Milne, 1938). At Plymouth Zostera has been under observation since the opening of the laboratory, as it was always exhibited in one of the aquarium tanks. I am indebted to Dr E. J. Allen for the information that never within his 42 years' experience was there a scarcity of Zostera in any way comparable to the shortage of the last few years. There is certainly no ground for attributing its disappearance to any decrease in illumination leaving the plant, thus weakened, an easier prey to disease. . ]OURN. MAR. BIOL. ASSOC.vol. XXIII, 1938

14

210

W. R. G. ATKINS SUMMARY

The suggestion that the enfeeblement of Zostera marina due to lack of sunshine in 1931-2 is the fundamental cause of the epidemic is not supported by the meteorological data available from 1897 onwards, or by the known extinction coefficients of the water. REFERENCES ATKINS, W. R. G., 1938. Photo-electric measurements of the seasonal variations in daylight around 0'41 mfL' from 193° to 1937. Proc. Roy. Soc. A, Vol. 165, pp. 453-65. ATKINS,W. R. G., BALL,N. G. & POOLE,H. H., 1937. The photo-electric measurement of the diurnal variations in daylight in temperate and tropical regions. Proc. Roy. Soc. A, Vol. 160, pp. 526-39. BORGESEN, F., 19°3. Botany of the Faeriies, Vol. 2, p. 706. COOPER,L. H. N. & MILNE, A., 1938. The ecology of the Tamar estuary. II. Underwater illumination. Journ. Mar. Biol. Assoc., Vol. XXII,pp. 5°9-28. POOLE,H. H. & ATKINS,W. R. G., 1937. The penetration into the sea of light of various wave-lengths as measured by emission or rectifier photo-electric cells. Proc. Roy. Soc. B, Vol. 123, pp. 151-65. TUTIN, T. G., 1938. The autecology of Zostera marina in relation to its wasting disease. New Phytologist, Vol. XXXVII,pp. 5°-71.