Chapter 9
Pelagic food webs
Food chains were given an equivalent name by an Arab scholar as early as the eighth
century CE (Egerton 2002), and the idea is obvious from sequences like grass eaten
by goats that are eaten by people. Simple, direct food chains of that sort do exist in the
sea, and they support some of the charismatic megafauna. For example, massive and
persistent diatom production during summer in parts of the Southern Ocean is
consumed by Euphausia superba, the antarctic krill. Juvenile and adult krill live in
voluminous, relatively dense schools, making them suitable prey for rorquals: blue,
fin, sei, and minke whales. These whales ingest great gulps of water and schools of
krill, squeeze the water out through baleen, then swallow. They are well defended
from predators (other than explosive harpoons) by size and have great longevity, but
they can be attacked and eaten by killer whales. That is a food chain. However, even
this system, with its obvious direct links, is more complex. Diatoms are also eaten by
dinoflagellates, salps, copepods, and pteropods, and part of the diatom stock mixes or
sinks to the seafloor and is filtered out by clams and worms. Krill are also the main
diet of penguins, crab-eater seals (the teeth of which form a filtering lacework) and
the cod-like notothenid fishes of the antarctic shelves and slopes. As commercial
harvests devastated the rorqual stocks of the Southern Oceans in the decades before
and after the 1985 moratorium on commercial whaling, populations of crab-eater seals
(Lobodon carcinophagus) and penguins increased dramatically (Laws 1984), as did
the leopard seals (Hydrurga leptonyx) that feed on both crab-eater seals and krill.
Thus, even the diatom–krill–whale food chain is just one sequence of transfers in a
complex food web. Marine food webs reside out of sight and underwater, so there are
powerful challenges to characterizing them and quantifying the rates of trophic
transfers. To a degree, those challenges have been met, and we have many insights.
(^) An early illustration (Fig. 9.1) of the complexity of pelagic food webs is Hardy’s
1924 diagram of North Sea trophic transfers from microplanktonic algae (diatoms,
dinoflagellates, small flagellates) to herring. Hardy drew the arrows from predator to
prey, implying the effects of predatory population control. More recent diagrams often
run the arrows in the other direction, that of organic-matter transfer. There are so
many links that not all of them could be drawn, even though Hardy knew that they
exist. The euphausiid Nyctiphanes, for example, has no arrows, but it feeds on smaller
zooplankton, mostly copepods, and phytoplankton. Tomopteris, a predatory