Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 171

nutrients. The large phytoplankton cells, and the faecal pellets released by the
mesozooplankton that graze them, form particulate organic carbon that sinks out
of the photic zone, so providing a vital export of carbon away from the air-sea
boundary and contributing significantly to the flux of CO 2 from the atmosphere to
the sea.
Big phytoplankton cells play two key roles in the ocean: exporting carbon and
providing food (directly, or indirectly through the mesozooplankton) for higher
trophic levels. Both the biogeochemists and the fisheries scientists use a key
particle size of 5

m. Particles above this size are needed for first-feeding fish
larvae (Cushing, 1995 ) , and can sink out of the surface layers to export carbon
(Legendre and Rivkin, 2002 ). Particles smaller than 5

m

m will take part in the
microbial loop; ultimately this can result in their organic material being available
to higher trophic levels or to sinking into deeper water, but the additional steps
required mean that efficiencies of these transfers will be reduced. Figure 5.19
provides a final summary of the generation and fate of organic carbon in the
shelf seas.

m

FURTHER READING

A Mechanistic Approach to Plankton Ecology, by Thomas Kiørboe. Princeton University Press,
2008.
Aquatic Photosynthesis, by Paul Falkowski and John Raven. Princeton University Press,
2007.
Ocean Dynamics and the Carbon Cycle, by Richard Williams and Michael Follows. Cambridge
University Press, 2011.
Population Production and Regulation in the Sea: a Fisheries Perspective, by David Cushing.
Cambridge University Press, 1995.
Influence of nutrient biogeochemistry on the ecology of North-West European shelf seas,
by P. Tett, R. Sanders, and D. Hydes, in Biogeochemistry of Marine Systems,
ed. K. D. Black and G. B. Shimmield. Blackwell, 2003, 293-363.

Problems

....................................................................................................................
5.1. Produce a plot that shows how the depth of the photic zone changes with the
PAR attenuation coefficient, over the range 0.1 0.4 m 1 . Take the < K PAR <

base of the photic zone to be at 1% of the surface PAR.

5.2. Data shown in Lee et al., 2002 suggest that the grazing rate in the North
Seainwinteris0.08d 1 , and in summer, 0.35 d 1 . If the phytoplankton
respiration rate is 0.1 d 1 and the maximum light utilisation coefficient is
0.12 d 1 (W m 2 ) 1 , calculate the compensation depths in winter and
summer. Take the winter surface PAR to be 75 W m 2 and the summer

surface PAR to be 300 W m 2 ,andassumeK PAR ¼ 0.15 m 1 .