Effects of ocean acidification on sediment fauna - Ocean Acidification - page 182

2004). For example, the nitrii cation potential of
Nereis virens burrow walls is estimated to be 1.7 to
4.1 times greater than that of surface sediment
( Kristensen et al. 1985 ), whilst Pelegr

the health and activity of fauna that burrow in, irri-
gate, and mix the sediment. This consideration
should start, but certainly not end, with an under-
standing of the direct impacts on an organism's
physiology (see Chapter 8). From there the challenge
is to predict the biological and ecological responses
that will ultimately shape the structure and function
of bioturbating communities. At the population
level, these responses (summarized in Fig. 9.4)
include bioturbator size (growth), abundance (repro-
ductive success), activity (respiration, feeding), and
supply of nutrients (metabolic activity). From this
point, additional ecological processes, such as com-
petition for space and resources, further dei ne the
structure of bioturbating communities and the over-
all function of sediment ecosystems.

et al. ( 1994 )
showed that the burrows of the amphipod Corophium
volutator stimulated denitrii cation fuelled from
nitrii cation threefold and denitrii cation fuelled by
nitrate from the overlying water i vefold. These
processes can have a signii cant impact on the
exchange of dissolved nutrients across the sedi-
ment-water interface (e.g. Webb and Eyre 2004;
Widdicombe and Needham 2007 ).
Whilst it is important to think of burrows as a dif-
ferent environment from the sediment surface, it
should also be remembered that the nature of the
burrow environment is primarily controlled by the
activity of the burrow builder. This leads to the sec-
ond issue, which is that sediment communities are
taxonomically and functionally diverse. This is
supported by molecular studies of the bacterial com-
munities inhabiting the burrow walls which also
demonstrated that there are large differences between
the communities inhabiting the burrows of different
species. Whilst the bacterial communities inhabiting
the burrows of some macrofauna more closely resem-
ble those in the surrounding subsurface sediment
( Lucas et al. 2003 ; Papaspyrou et al. 2005 ), others are
more similar to the communities found at the sedi-
ment surface (Steward et al. 1996 ; Bertics and Ziebis
2009). Recent methodological advances have allowed
the incorporation of individual species function into
community-level estimates of bioturbation (e.g.
Solan et al. 2004 ; Teal et al. 2009 ). Unfortunately, many
studies and models still generalize the effects of
'deposit feeders', despite the fact that this group
encompasses a wide range of species with different
feeding behaviours to extract the organic material
from the sediment. These differences, together with
those in body size, life-history characteristics, and
mobility, make it impossible to consider bioturbation
as a single uniform process.
Bioturbation is now recognized as a globally
important process (Teal et al. 2008) and changes in its
precise nature and intensity will have enormous
implications for the biodiversity and function of
coastal ecosystems. Therefore, in the light of ocean
acidii cation and warming, it is essential that we
consider the potential impacts of elevated CO 2 on

ί

9.5 Assessing the potential impacts of
ocean acidii cation on infaunal organisms

It is evident that the differing activities of infaunal
organisms can create huge variability between the
structure and function of the burrows in which they
live. These differences create a variety of different
burrow environments and these differences may
therefore also be rel ected in the physiological or
behavioural mechanisms used by different species
to cope with life in low-pH sediments. For example,
Zhu et al. (2006b) found that values of extracellular
l uid (i.e. haemolymph) pH T differed substantially
between two species of polychaete worm, Nereis suc-
cinea and Nephthys incise (~8.0 and ~6.8 respectively),
and that these values also rel ected pH T conditions
within the burrow (~8.2 and ~7.4 respectively). It
may be concluded that since infaunal organisms live
in an environment that is often high in CO 2 , they
will be inherently more immune to ocean acidii ca-
tion than organisms that live on the sediment sur-
face (epifauna). However, before such a conclusion
is reached, there should be an examination of the
physiological mechanisms used by infaunal species
to cope with a high-CO 2 environment.
There is growing evidence that marine organisms
do not respond uniformly when exposed to CO 2 -
acidii ed seawater (Fabry et al. 2008 ; Widdicombe
and Spicer 2008 ; Melzner et al. 2009 ; Ries et al. 2009 ;
Hendriks et al. 2010; see also Chapters 6 and 7). The
question is, given the current state of physiological