Future challenges in shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 354

basic features of ocean circulation. 1 In contrast to the physics, the equations
commonly used to describe the fundamentals of primary production, such as the
shape of the growth-radiation response of the phytoplankton or the uptake of
nutrients, are empirical. While we understand the fundamentals of the biochemistry
of photosynthesis and the molecular diffusion of nutrients to a cell that
underpin these descriptions, there is a real challenge in formulating this basic
biochemistry on a more fundamental level (Falkowski et al., 2008 ). Progress is being
made here, for instance by quantifying the interplay between the costs and benefits
of different acclimation strategies in phytoplankton (Geider et al., 2009 ), so that
there appears to be a real prospect of significant advances in our models of primary
production on the horizon. This is clearly of importance to the modelling of
phytoplankton globally, not just in the shelf seas. However, the marked horizontal
gradients in phytoplankton ecosystem structures that we see in the shelf seas, driven
by the contrasts in physical forcing (e.g. Fig. 10.25 ), provide a particular challenge at
a location which can be reached without several days steaming in a research vessel.
Such a change in the approach to describing phytoplankton biochemistry could
revolutionise our models of phytoplankton communities. Most phytoplankton eco-
system models in regular use are based on a formulation of phytoplankton growth
that can be recognised in the pioneering work of Gordon Riley (Visser and Kiørboe,
2006 ). In a development of the earlier single species models, the phytoplankton
ecosystem is now often split into a number of phytoplankton functional types (PFTs),
e.g. diatoms, dinoflagellates, cyanobacteria, flagellates. Each PFT has a single repre-
sentative 'species' in the model with driving parameters taken from laboratory
cultures or observations. Additional complexity is added in attempts to describe,
for instance, different grazer types, recycling of organic material, and coupling
between the biochemistry of the water column and the benthos. While such models
have been successful in replicating the typical structures of present-day phytoplank-
ton communities (e.g. Kiørboe, 1993 ; Denman and Gargett, 1995 ), there are import-
ant limitations. The phytoplankton species are 'hard-wired' to their PFT
representatives, and so the modelled ecosystem lacks the capacity to adapt to shifts
in its environment in ways that we expect the real ecosystem to do. Also, the complex
biochemical interactions and the parameter values that control them are often
lacking a sound observational basis. We would argue that such models cannot
address the subtleties of the diversity of the phytoplankton community, and yet it
is likely that shifts in phytoplankton diversity in response to physical processes are a
more powerful driver of whole ecosystem biodiversity than simply assessing the role
of physics in determining phytoplankton carbon fixation. This is an area of overlap
between the biological oceanography and the fisheries oceanography that has high
potential reward: in regions where gradients in the physics appear to correlate with
the distribution of higher trophic levels, such as commercially important fish, we need
to move on from simply linking physics to the production of organic carbon and
1 Of course, this fundamental rigour in the physics still has the problem of how to deal with turbulence
and friction, which is an area in which we are still reliant on parameterisations.