Geoscience Reference
In-Depth Information
11.3
Managing shelf sea resources
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There is an increasing demand for sufficient understanding of the biology of the shelf
seas to allow robust predictions of how ecosystems might respond to different
strategies for managing fisheries, against the back-drop of a changing climate.
Oceanographers have responded to this by attempting to predict how the biogeo-
chemistry of the shelf seas might respond to a warming climate, on the basis that the
carbon fixed by the primary producers represents a fundamental control on the
biomass of fish (e.g. Holt et al., 2009 ). We might expect large shifts in
carbon fixation to have effects through the shelf ecosystem, and so some predictive
capability is required. However, we have also seen throughout this topic that
phytoplankton biomass rarely appears as the principal limiter on the survival of
larger organisms. Instead we have seen how physical processes such as frontal jets
and convergences, internal tides and solitons, and density-driven flows play more
direct roles in the life cycles of important fish species. Developing our understanding,
and modelling, of the mechanics of these physical drivers on, for instance, phyto-
plankton community structure (rather than bulk biomass), larval retention and
transport, and prey availability is vital to any meaningful predictions of an ecosys-
tem's response to climate and fishing pressures. Correctly linking the spatial and
temporal components of the physics through to the requirements of fish is also
needed if we are to design marine protected areas on a scale that both supports the
continued existence of fish stocks and provides for sustainable harvesting by fishers
(Botsford et al., 2003 ).
11.4
Shelf seas in the Earth system
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The high level of primary production in the shelf seas relative to that of the
abyssal ocean is now well documented (e.g. Liu et al., 2010 ). Important questions
remain, however, about the extent to which the resultant particulate carbon is
sequestered into shelf sediments, or exported off the shelf and into deep water to
be buried in abyssal sediments, or re-mineralised in the water column. There are also
interesting questions about which parts of the shelf seas act as sinks for atmospheric
CO 2 from the atmosphere while others are net sources (Jahnke, 2010 ).
11.4.1
Shelf sources and sinks of CO 2
We saw in Chapter 10 how recent studies have suggested that, in mid-latitudes,
seasonal stratification exerts a key control on net air-sea fluxes of CO 2 . We saw in
the North Sea (see Section 10.9.2 ) that the net balance appears to be a significant
net drawdown of carbon for the atmosphere. A recent worldwide synthesis of
seasonally averaged fluxes based on measurements of
D
pCO 2 (Chen and Borges,
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