Geoscience Reference
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transport of organic matter and the modulation of
chemical reactions within sediments. In addition, by
modii cation of the chemistry and physical structure
of the sediment, together with the disturbance and
displacement of other species, bioturbation may reg-
ulate the species composition and diversity of entire
sediment assemblages (see review by Widdicombe
and Austen 2005). As such, large bioturbating infau-
nal organisms must be considered as ecosystem engi-
neers as dei ned by Lawton ( 1994 ):
9.0
8.5
8.0
7.5
7.0
6.5
pH T
Ecosystem engineers are organisms that
directly or indirectly modulate the availability of
resources (other than themselves) to other spe-
cies, by causing physical state changes in biotic
or abiotic materials. In so doing they modify,
maintain and/or create habitats.
Figure 9.3 pH planar optode image showing the pH T distribution
around a burrow of the polychaete Hediste diversicolor. The dotted line
represents the sediment surface. Image by courtesy of Morten Larsen and
Ronnie N. Glud.
The importance of ecosystem engineering in con-
trolling the biogeochemical and ecological proc-
esses occurring within marine sediments has
highlighted the need to describe and quantify bio-
turbation in greater detail. In doing so this research
has highlighted two key issues.
Firstly, the impact of large burrow-building spe-
cies on biogeochemical cycles is not simply the
result of extending the sediment-water interface
across which nutrients may pass. It is becoming
clear that the burrow is a specialized environment
that often contributes more to nutrient cycling than
would be expected solely based on the additional
area of sediment-water interface provided. As seen
above, and in contrast to surface sediments, the
burrow environment is physically but not chemi-
cally stable (Kristensen et al. 1985 and references
therein) and contains strong chemical gradients
which exert signii cant control over biogeochemical
processes. It has also been shown that the burrow
environment supports highly diverse microbial
communities that are different and more active than
their sediment-surface counterparts (e.g. Laverock
et al. 2010). This has most readily been seen in the
case of nitrogen cycling where studies have shown
that the presence of burrow builders increase the
rates of nitrii cation and denitrii cation above the
rates that would be expected purely from an increase
in exchange surface area (e.g. Kristensen et al. 1985 ;
Pelegrí et al. 1994; Howe et al. 2004 ; Webb and Eyre
observed a CO 2 plume rising from a burrow into
the overlying water, thereby demonstrating clearly
the effect of fauna on solute exchange between the
sediment and overlying water. Active ventilation
removes potentially toxic solutes (e.g. CO 2 , NH 3 ).
On the other hand, mucus used to cement the bur-
row walls may be inhibiting the diffusion of such
solutes (Hannides et al. 2005 ) thereby allowing
macrofauna to conserve energy which would oth-
erwise be spent ventilating the burrow. Thus, dif-
ferent strategies of dealing with potentially toxic
solutes emerge: burrow ventilation, insulation with
cementing mucus, physiological adaptation, etc.
Each one of these strategies carries an energetic
cost and is discussed further below.
9.4 Sediment fauna as 'ecosystem
engineers'
It is clear from the processes described in the previous
section that the activities of infaunal organisms such
as building and irrigating burrows as well as trans-
porting and mixing sediment (collectively termed
'bioturbation') can have a signii cant effect on the
sediment environment. Levinton (1995) reviewed the
role of deposit-feeding marine invertebrates in modi-
fying their sediment habitat and concluded that
bioturbation is the main driving force behind the
 
 
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