So in this chapter we will talk about marine
food web structures. So how do marine ecosystems
work? You will learn things about marine food
webs that are very different from land. For
example, you will learn that if you eat a
mackerel, that’s equivalent to eating a
wolf that previously has eaten a fox. So this
is how marine systems work very differently
from land.If you look at bucket or a bottle of ocean
water, usually that’s very transparent and
you ask yourself, so what actually.... Where
does life come from that eventually will lead
to populations of fish that can be harvested
and actually people were asking that for quite
a long time, way into past the Medieval times.
And it was only in 1887 that Victor Hensen
took a plankton net, that’s a net with very
fine meshes, went out, sifted through ocean
waters and with the aid of a microscope found
tiny organisms, tiny plants that are actually
at the base of the food webs.
And through the process of photosynthesis,
as we know it from land, they build up organic
matter that can then be consumed by secondary
consumers, which are if you will, the cows
of the sea, tiny crustaceans called copepods.
So since the plants with which the food web
starts are very small, also you, the first
consumers have to be quite small.
And then those tiny crustaceans and also larvae
of worms and mussels that also have their
larvae in the plankton feeding on phytoplankton,
they are then consumed by still relatively
tiny fish. So either juvenile fish or small
anchovies, small pelagic fish.
So we have already now three steps in a tropical
open ocean marine food chain that would lead
to still a small fish that we didn’t yet
want to put on our plate. Meaning that open
ocean food webs usually take several steps,
up to five or six steps to end then in a big
tuna or in a large shark.
So why is this important? So why is that very
significant for understanding marine productivity
in general? So now in each conversion when
one organism eats another one, about 90% of
the biomass gets lost simply by respiration,
by the maintenance metabolism of the organisms
and only 10% of the matter get...gets transferred
to the next trophic level. And that means
if we eat a tuna steak of 100 grams, if it
trickle...if we, if we go down all the five
trophic levels back to the base of the food
web of phytoplankton, we need to build up
one ton of phytoplankton biomass to sustain
this 100 gram steak of tuna.
And now you can hopefully see the significance.
It means that although oceans cover very vast
areas and you may think that fish populations
cannot really be overexploited, this is very
much the case, because the amount of biomass
that goes into a fish population that’s
very much reduced by the very long way through
a relatively long food chain. And that’s
again, very different to land where we can
directly eat cattle that’s a first consumer
directly eating on the primary producers.
So have four trophic steps less than our tuna.
So to give you another example, if you put
a mackerel on your plate, this would correspond
to, on land, on eating a wolf who has entirely
fed on foxes, because a mackerel is already
the fourth trophic level. And so we would
be actually the eaters of wolf eaters, and
so would be the tuna.
And now you also understand why the largest
animals among the fishes and among the...the
mammals are filter feeders that by means of
their filtering organs, either the baleen
with the whales or gill arches that are modified
with the very large basket shark go directly
for the zooplankton for the first consumers.
They themselves then being the second consumer.
This is because they, they spare all the upper
steps of the food chain of the trophic levels
and this way they can feed on much more biomass
that’s available and this is the only way
how they can maintain their very large biomasses.
We now come from the open ocean to more coastal
systems. Coastal marine systems are generally
much more productive than open ocean systems.
One reason is that we have a threefold input
into the food chain. It’s not just the plankton
that is also there, but we also have in addition,
plants that grow either on rock-like algae
or on sediments like sea grasses and then
we have tiny unicellular algae that themselves
grow on the sediment surface or on the larger
plants themselves. And also we can have very
short food chains. The plant material gets
degraded in to detritus and like earthworms,
worms in coastal sediments, they directly
eat on the primary productivity. And also
bivalves filter directly the plankton, meaning
that they as first consumers are the food
for the place of flatfish or a cod. Then we
have only three partite food web that already
leads to something that humankind can eat.
Much shorter than a typical pelagic food web.
Most of the ocean is much deeper than the
sunlight can enter. Even in the clearest water
like the Sargasso Sea, sunlight can only enter
150 to 200 meters to sustain photosynthesis.
That means that most of the ocean that’s
several kilometers deep relies on the production
of the upper sunned layer. And so dead particles
that are sinking down from the upper sun that
layer be it dead phytoplankton or the feces
from other organisms that have fed each other,
things, the long way down to the deep sea
bottom and on that way there are specialized
organisms like the vampire squid. They are
specialized on...on going after that sinking
menu that has still some calories to digest.
And once this rain is down at the deep sea
bottom, it has gone down to 1% or less of
the digestible biomass that has been there
in the sunlit upper surface. This then again
means that many of the deep sea organisms
have to adapt to extreme food shortage. They
are very low population densities and many
other adaptations like longevity and very
slow growth that, that these deep sea organisms
need to survive.
So I hope in this chapter you have learned
some basic structural features of marine food
webs that are very different to those on land,
generally marine food chains that constitutes
those webs of who it’s...whom they are longer
than on land, that means that sustainable
harvest of many populations is actually much
lower as what you would think at the first
glance. We have many examples of productive
marine ecosystems, mainly in coastal areas
where several nutritional sources come together
and also nutrient levels are higher as in
the open ocean.
