Náttúrufræðingurinn - 01.06.1968, Blaðsíða 123

NÁTTÚRUFRÆÐINGURINN 235
Syrtlingur eruption began and throughout the summer this crater island
produced ash and pumice at an average rate of 4—5 cubic yards per second.
The case of the pumice that was produced during the first months of
the Surtsey eruption that began in November 1963 was quite different.
This eruption ocurred in the winter when the sea off the South coast of
Iceland was not warm enough to enable Lepas larvae to survive and make
use of the pumice that was available then. And at the beginning of April, 1964,
or before the temperature of the sea off the Icelandic coasts had begun to
rise again to any appreciable degree, the Surtsey eruption ceased to produce
pumice. And as the summer of 1964 advanced the greater part of this pumice
had either been washed upon the mainland shores or drifted away from the
land to the open sea. Besides, the sea temperature was never as high in the
summer of 1964 as in the summer of 1965. This must be the reason why no
traces of goose barnacles were found in the stomachs of 87 gulls that were
shot at Sandvik in the autumn of 1964 and the winter of 1964—1965.
The same is most likely true of the pumice that was produced by the Jolnir
eruption that began at Christmas of 1965. This eruption, which produced
a similar quantity of material as the Syrtlingur eruption, admittedly lasted
until August 10, 1966, and therefore growth of goose barnacles on the pumice
might have taken place that summer, too. Unfortunately, however, other work
prevented us from making a study of this question then.
We can assume that every year larvae of the three Lepas species that were
found on the Surtsey pumice in 1965 are carried to the South coast of Iceland,
but their quantities most likely vary a great deal from one year to another.
Normally, the greater part of this larva mass probably perishes owing to the
absence of requisite living conditions. This is a good example of the enormous
wastage of life which is characteristic of many invertebrates, their reproductive
rate therefore being high to the same degree. Such species can with impunity
stand enormous setbacks and sacrifice countless millions of individuals in their
search of favourable living conditions. It is because of these relatively aimless
volleys of organisms that the Lepas larvae were able to make use of the Syrtling-
ur pumice in 1965 to the large extent indicated above. Then a volley of organ-
isms scored a direct hit, a relatively rare occurrence. But in connection with
this fact the question arises if and to what extent volcanic eruptions and
pumice production in past geological epochs may have contributed to the
cvolution of marine organisms depending on Qoating objects for their survival.