18
THE ESSEX NATURALIST.
Another category of aquatic phenomena is that in which
various microscopic organisms accumulate in such numbers on
or near the surface of the water as to produce a noticeable scum
or layer of floating material. The most important type of this
class, and the one occurring on the largest scale, is what is known
in some parts of England as the "breaking of the meres." In
Germany the phenomenon is known as "Wasser-blute," and it
is now often referred to in English as "water-bloom." The
appearance can be produced by a considerable number of different
organisms, and is due to an exceptionally abundant development
of, usually, one particular species combined with some special
increase in its buoyancy (by production of gas-vacuoles, oil-
globules, etc.). In this way the whole surface of a pond or lake
may be covered, in some cases to a depth of several inches, with
a mass of material consisting entirely of microscopic organisms.
Perhaps the chief of these are certain species of blue-green algae
such as Clathrocystis (Microcystis) aeruginosa, Anabaena spp., but
many other organisms have been reported from time to time
as producing a similar effect.
A somewhat similar phenomenon, though occurring only in
limited patches, is caused by masses of filamentous algae of
various kinds brought to the surface by the entangled gas bubbles
which the algae produce under the influence of sunlight. The
most usual forms involved in this kind of appearance are species
of Spirogyra, Mougeotia, Microspora, etc. In the same way,
especially in shallow waters, pieces of the bottom, when covered
with a felt-like growth of such blue-green algae as Oscillatoria,
will rise to the surface and remain floating for some time. One
very peculiar example of this kind of action, which occurs
pretty regularly in the spring and early summer in parts
of the Norfolk Broads, is the appearance at the surface
in enormous numbers of dark green balls of what is
locally called "sponge-weed." The balls are formed by
masses of radiating threads of the branched filamentous alga
Cladophora (AEgagropila) Sauteri. It appears that, under
certain conditions, some of the little pieces of felt or cushion-like
masses which this alga normally forms on the bottom or on
submerged posts, stems of water plants, etc., become free, and
owing possibly to being rolled about the bottom by the action of
currents, they develop equally in all directions, and thus eventually