Geoscience Reference
In-Depth Information
CHAPTER 9
on sediment fauna
Stephen Widdicombe, John I. Spicer, and Vassilis Kitidis
9.1 Introduction
able impact on human health and well-being. It is
therefore essential that the impacts of ocean acidii -
cation on sediment fauna, and the ecosystem func-
tions they support, are adequately considered. This
chapter will i rst describe the geochemical environ-
ment within which sediment organisms live. It will
then explore the role that sediment organisms play
as ecosystem engineers and how they alter the envi-
ronment in which they live and the overall biodi-
versity of sediment communities. It will identify
how the impacts of ocean acidii cation could act to
reduce the inl uence of these ecosystem engineers.
Finally, the chapter will identify the physiological
and behavioural mechanisms that currently allow
infaunal animals to live in a 'high-CO
2
' environ-
ment and discuss whether these mechanisms will
make them less vulnerable to ocean acidii cation in
the future.
The vast majority of the seal oor is covered not in
rocky or biogenic reefs but in unconsolidated sedi-
ments and, consequently, the majority of marine
biodiversity consists of invertebrates either residing
in (infauna) or on (epifauna) sediments (Snelgrove
1999). The biodiversity within these sediments is a
result of complex interactions between the underly-
ing environmental conditions (e.g. depth, tempera-
ture, organic supply, and granulometry) and the
biological interactions operating between organ-
isms (e.g. predation and competition). Not only are
sediments important depositories of biodiversity
but they are also critical components in many key
ecosystem functions. Nowhere is this more appar-
ent than in shallow coastal seas and oceans which,
despite covering less than 10% of the earth's sur-
face, deliver up to 30% of marine production and
90% of marine i sheries (Gattuso
et al.
1998 ). These
areas are also the site for 80% of organic matter bur-
ial and 90% of sedimentary mineralization and
nutrient-sediment biogeochemical processes. They
also act as the sink for up to 90% of the suspended
load in the world's rivers and the many associated
contaminants this material contains (Gattuso
et al.
1998). Human beings depend heavily on the goods
and services provided, for free, by the marine realm
( Hassan
et al.
2005) and it is no coincidence that
nearly 70% of all humans live within 60 km of the
sea or that 75% of all cities with more than 10 mil-
lion inhabitants are in the coastal zone (Small and
Nicholls 2003 ; McGranahan
et al.
2007 ) Given these
facts, it is clear that any broad-scale environmental
impact that affects the diversity, structure, and func-
tion of sediment ecosystems could have a consider-
9.2 Distribution of carbon dioxide (CO
2
)
and pH within sediments
The distribution of CO
2
and pH within marine sedi-
ments is largely controlled by microbially mediated
redox reactions that are linked to the mineralization
of organic matter as well as abiotic processes (e.g.
mineral formation and dissolution). However, by
mixing and ventilating the sediment, the activities
of large infaunal organisms can affect the microbial
community, mineral type, and the redox state of
sediments through multiple feedback mechanisms.
Consequently, sediment-dwelling organisms, both
multicellular and microbial, are instrumental in set-
ting the geochemical environment that surrounds
them. However, this relationship between organ-
isms and their environment acts in both directions,
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