Knowledge and implications of global change in the oceans for biology, ecology, and ecosystem services - Ocean Sustainability in the 21st Century - page 128

4.2.2 Food supply
Climate variation is altering the production of organic carbon (OC) in the surface ocean,
which ultimately comprises the primary food supply to deep-sea ecosystems (Smith et al.,
2008 ; Smith et al., 2009 ) . Changes in upper ocean temperature influence stratification, re-
duce vertical mixing, and can affect the availability of nutrients for phytoplankton produc-
tion with the direct consequence of enhanced variability in primary production and carbon
export flux to the deep-sea sediments (Smith et al., 2009 ). The magnitude of such climate
change effects is likely to be substantial because ocean warming to date appears to have
caused a 6% decline in global ocean primary production (Gregg et al. , 2003 ), and climate
models predict major reductions in ocean productivity over large regions within this century,
especially in the tropical ocean (Bopp et al., 2001 ) . Time-series studies conducted over the
past two decades in the North Pacific and the North Atlantic (at > 4000 m depth) have re-
vealed, unexpectedly, climate-driven changes in deep-ocean ecosystems (in term of faunal
abundance and composition) correlating with changes in surface ocean productivity (Smith
et al., 2009 ). Also, in the equatorial Pacific it has been documented that moderate changes in
net primary production and sea surface temperature have provoked a threefold reduction in
the flux of particulate organic carbon (POC) (e.g. from 1.5 to 0.5 g C m −2 y −1 ; Laws, 2004 ) .
The benthic response to the reduction of food sources available in the sea bottom is a halv-
ing of microbial, nematode, and megafaunal standing stocks, a fivefold reduction in mac-
rofaunal biomass, and two- to fourfold reductions in sediment mixed layer depth, sediment
community oxygen consumption, and bioturbation intensity (Smith et al., 2008 ). Long-term
declines in POC flux, such as are expected in the equatorial abyss, are also likely to yield
reductions in species diversity and body size (McClain et al., 2005 ) , as well as basic shifts in
the taxonomic composition ofabyssal assemblages (Smith et al., 2008 ).All ofthese changes
have a dramatic effect on the functioning of deep-sea ecosystems since an exponential re-
lationship between biodiversity and ecosystem functioning has been observed in these eco-
systems (Danovaro et al., 2008a ). Global warming may also have important implications
for the fish stock as a consequence of the reduction in food sources (zooplankton for ju-