Future challenges in shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 368

the shelf edge. Changes in stratification will alter the flux of internal tidal energy, and
hence influence the vertical turbulent mixing of nutrients by internal solitons. Recent
regional modelling of the northwest European shelf seas has suggested significant
strengthening of stratification in a warmer world, as a result of changes in precipita-
tion and in the timing of stratification (Holt, Wakelin, et al., 2010 ).
The temperate shelf seas are currently warming at a greater rate than the open
ocean, with the North Sea, for instance warming at about 0.4 C decade 1 (Sharples
et al., 2006 ) compared to a global average for the ocean surface temperature of
0.16 C decade 1 (Solomon et al., 2007 ). This is largely because the shallower water
column provides much less thermal inertia or scope for vertical mixing of surface
heat. We are already seeing large shifts in the distribution of animals associated with
recent warming (e.g. changes in zooplankton (Beaugrand, 2009 ) and stocks of
commercial fish (O'Brien et al., 2000 ; Hunt et al., 2002 )) which are mainly ascribed
to temperature increases and its effects on growth rates (e.g. Stegert et al., 2010 ;
Brander, 1995 )). There is also a host of physical changes expected as our climate
warms that may affect the ecological environment. Greater heat exchange through
the ocean surface could alter the positions of the tidal mixing fronts, or the timing of
the onset of stratification in the spring. Shifts in rainfall, either total amounts or the
seasonal distribution, could alter the ROFI environment, for instance by changing
the density-driven exchange between the estuaries and ROFIs and the adjacent shelf
sea, or by modifying the strength of coastal buoyancy currents. Changes in the
strength of winds (Kjellstrom et al., 2011 ) will represent an important alteration of
a source of mixing to the shallow seas. Up in the Arctic, the reduction of seasonal ice
cover will have enormous impacts on the biogeochemistry of the high latitude shelf
regions (Kahru et al., 2011 ). In deciding how these changes might affect the higher
trophic levels of the shelf sea ecosystems, the rate at which the changes become
manifest will be a key determinant of whether or not existing species or ecosystems
can adapt, or if we should expect wholesale shifts in shelf sea biology.

Conclusion

....................................................................................................................
The shelf seas are intrinsically interesting, complex systems with often remarkable
gradients in physical and biological dynamics over short horizontal distances. We
have made considerable progress in understanding the fundamental physics of the
shallow seas, including recognising the important contrasts with the open oceans,
and linking the physics to the biology from microbes up to fish. In this chapter we
have suggested several areas where we think further research effort is desirable and
necessary, particularly in improving understanding basic processes, combining
knowledge from different approaches and in transferring our existing knowledge to
try it out on less studied regions.
Interdisciplinary research is now a rapidly developing field. The basic link between
the vertical structure of turbulent mixing and the growth of the phytoplankton is well
established, but there is work required to understand how the diverse shelf sea