Physical and chemical changes in the ocean over basin-wide zones and decadal or longer time-scales: perspectives on current and future conditions - Ocean Sustainability in the 21st Century - page 98

There are variable signals with regard to changes in nutrient concentrations, with few
studies reportingdecadal changes.Changes havebeenobservedinalloceanbasindeepwa-
ters, but with no clear pattern. An inferred decrease in primary production of 6% from the
early 1980s to the late 1990s is plausible, with a decrease in nutrient inputs from intermedi-
ate waters. This is consistent with decreased ventilation of these water layers, suggested by
their observed decrease in oxygen content, which may also be linked to an increased rate of
oxygen consumption through faster oxidation in warmer water. The uncertainties in these
inferences are underlined by the indications from analysis of model simulations presented
by Sarmiento et al . ( 2004 ) . These suggest a global increase of ocean primary production in
the range 0.7 to 8.1%, with large regional differences.
Shifts and trends in plankton biomass have been observed in the North Atlantic, the
North Pacific, and the Southern Indian Ocean. Trends of decreasing chlorophyll a and
primary production since 1978 are observed in LMEs (Sherman, Aquarone, and Adams,
2009 ) . It should be noted that changes in ocean productivity have been a key to recon-
structing the conditions during other climate periods, including winds and ocean currents,
by means of isotope palaeontology.
While the data are insufficient to conclude whether observed changes are caused by
natural variability or are due to trends, they do indicate large-scale changes in ocean con-
ditions. According to reports from the IPCC, the ongoing changes in the ocean have very
strong repercussions on basic human needs. Fish perish as climate change disrupts ocean-
ic systems. Through the combined impacts of climate change, pollution, over-harvesting,
damaging fishing practices, in particular bottom trawling, migration of species, infestations
of exotic species, and changes in biodiversity, the major fishing grounds are all affected.
A decline of 13% of cumulative catches in the 64 LMEs since the maximum in 1994 has
been confirmed, with 80% of mean annual marine fisheries occurring within these LMEs.
Decreasing trends have been established for chlorophyll a and primary production since
1998. Up to 75% of fishing grounds may be affected by changes in ocean circulation due to
climate change interfering with established circulation, ventilation, over-turning, and mix-
ing. Up to 80% of the primary fish catches are exploited beyond or close to their harvesting