Ten years ago, I had a holiday in Denmark with my wife and
daughter and we spent a few days in the wonderful city of Copenhagen. As we
wandered along the waterfront, I noticed that there were large amounts of foam
accumulated at one end of a dock and I started explaining (how typical of an academic)
that this was the result of natural processes and not pollution. We were surprised
to then turn round and find a TV crew just behind us. The reporter had been
sent to cover the appearance of the foam and she had overheard my comments and
asked me to talk about the origins of the foam to camera. I explained that, although
the accumulations look unpleasant, they disperse and do not usually create a
bad odour.
In contrast to this small accumulation in Copenhagen, sea
foams can be of much larger dimension, creating banks of foam over a metre deep
on shore, with winds carrying the flocs inland. One genus of algae is
well-known as the origin of foams - Phaeocystis.
These algae are found as single cells or, commonly, in large groups embedded in
a globe of transparent mucus (see below). When nutrients are plentiful, and when day length
increases, the excellent conditions for algal growth result in blooms. Excess carbohydrate,
resulting from photosynthesis, is exuded from the cells to form a field of
exopolymers 1 around each cell and, in Phaeocystis, the exudates from cells combine and a globe of mucus
results, in which individual algae become embedded. The globe protects
individual cells from capture by herbivores and acts as a means of retaining
nutrients close to each cell. As the mucus consists largely of water bound
within a matrix of carbohydrate to give structure, it is energetically cheap to
produce. 1
So, how does Phaeocystis
contribute to masses of sea foam? Let’s begin with an analogy. When we make
meringue, we whip transparent egg whites (mainly protein in water) to include
air, and the masses of tiny bubbles that we create are trapped within the developing
meringue, each coated by some of the protein. The trapped bubbles then give the
whole its white appearance. Now back to Phaeocystis.
When huge numbers of globes at, or near, the surface of the sea are whipped up
by waves, the mucilaginous colonies are broken up and their organic matter covers
bubbles, so the whole becomes whisked into foam. The more algae, the more foam,
and it is easy to see how this light, floating mass can then be blown ashore. Like
all foams, it consists largely of gas and is white because of all the bubbles
that are included, although green or brown colouration sometimes occurs. This
is because algae or brown organic compounds are also bound into the mass, or
become stuck to its surface.
All surf creates foams because bubbles are covered with the
organic matter that accumulates at the water surface and this means bubbles do
not collapse instantly, like those produced by shaking tap water vigorously, for example.
We’ve all seen the white masses of coated bubbles that form when waves break,
but the huge accumulations that can occur after Phaeocystis blooms are in a different league.
1 Roger S Wotton (2005) The essential role of
exopolymers (EPS) in aquatic systems. Oceanography
and Marine Biology: An Annual Review 42: 57-94.
No comments:
Post a Comment