Geoscience Reference
In-Depth Information
BIOLOGICAL PROCESSES INVOLVED IN MODERATING SEDIMENT FUNCTION
Intensity of individual
impact on sediment
mixing and irrigation
Provision or depletion
of microbial nutrients
(e.g. NH
4
, O
2
, CO
2
)
Density of bioturbators
Gamete production
Respiration
Metabolic
activity &
physiological
maintenance
Fertilization
Feeding
Larval and juvenile
development
Growth
(Body size)
Abundance
PHYSIOLOGICAL & BEHAVIOURAL PROCESSES POTENTIALLY AFFECTED BY OCEAN ACIDIFICATION
Figure 9.4
The relevance of CO
2
-induced changes in organism physiology and behaviour to the biological control of sediment function.
knowledge, can we inform any generalizations
about the response of sediment assemblages to ele-
vated CO
2
and reduced pH? Studies of the effects of
ocean acidii cation on both present-day and fossil-
ized organisms suggest that there may be specii c
characteristics or traits that could indicate whether
animals are tolerant of elevated CO
2
(hypercapnia)
or low pH. For example, Bambach
et al.
( 2002 ) sug-
gested that organisms will be least affected by hyper-
capnia if they: (1) are 'buffered' against a range of
chemically related physiological stresses, (2) have
high rates of metabolism, (3) have well-developed
gills and circulatory systems, and (4) possess skele-
tons limited in mass or made of materials other than
CaCO
3
(see also Chapter 4 ). Melzner
et al.
( 2009 )
compiled physiological data from taxonomic levels
higher than species. He highlighted that compensa-
tion for extracellular acid-base disturbance together
with high (specii c) metabolic rates (and high levels
of mobility/activity), may characterize CO
2
-resistant
taxa. But how robust are such generalizations? To
i nd out, this chapter reviews current understanding
and assumptions about large infauna before testing
the currently limited knowledge against some of the
key features identii ed above. In doing so, an attempt
will be made to identify differences in the responses
of infaunal and epifaunal species to hypercapnia or
low pH.
A reasonable body of literature exists on the gen-
eral biology of large infauna, but most focuses on
adaptations intended to help the organism maintain
contact with the surface via specialized structures
or by inhabiting permanent or semi-permanent bur-
rows, the deepest of which are actively ventilated
( Eltringham 1971 ; Meadows and Meadows 1991 ;
Little 2000). However, two seemingly opposing
assumptions underpin much of the literature.
Firstly, it is often assumed that these adaptations
reduce the possibility of exposure to low O
2
tension
(hypoxia) and high CO
2
tension (hypercapnia).
While this is partially true, measurements made at
different depths within sediments, or in burrow
water, indicate that environmental hypoxia and
hypercapnia are still issues for burrowing animals.
The second common assumption is that large infau-
nal organisms are physiologically better equipped
to tolerate and respond to hypoxia and hypercapnia
than their epifaunal or open-water counterparts
(e.g. Royal Society 2005 ; Knoll
et al.
2007 ). While
there is reasonable evidence that physiological
adaptation to hypoxia is found in a wide range of
infauna, including annelids, crustaceans, bivalves,
and teleost i sh (Atkinson and Taylor 1991; Mill
1997), empirical evidence of the degree of adapta-
tion within physiological responses to hypercapnia
is scarce. The responses of the oxygen exchange and
transport systems to hypoxia were thought to
greatly exceed any requirement for CO
2
excretion
( Cameron 1986 , 1989 ; Sundin
et al.
2007 ). What is
striking when collating data on the effects of ocean
