Benthic Suspension Feeders in Shallow Coastal Water
27
natural systems, some species of oysters form reefs, while mussels typically
aggregate in beds. These structures interact with the over-flowing waters to
increase turbulent mixing, resuspend feces and pseudofeces, and increase the
surface area of the reef or bed by occupying space in three dimensions.
Vermetid and sabellid aggregations are common to wave-swept, highenergy environments, that appear to dictate their geomorphology. Vermetids
may have an advantage in turbulent environments because of their ability to
change the construction and shape of their shell in response to water-flow
dynamics. Sabellids are constantly building and repairing their tubes using
resuspended sand particles. Because their environment is so energetic and
often facing the open ocean, it is doubtful that these groups have much of an
ecological impact on the transformation of suspended particles. However, the
structures they build probably do influence local currents and wave patterns.
In contrast, serpulid reefs are found in low-energy, poorly flushed lagoons,
bays and fjords. The density of serpulids in these environments implies that
these worm reefs are translocating and transforming large quantities of
suspended materials in order to support their structure. It also suggests that
fjord habitats currently dominated by serpulid reefs may be adaptable to the
raft culture of suspension-feeding bivalves similar to that now taking place at
Carlingford Lough (Ball et al. 1997).
In all but a few cases, encrustations of ascidians, barnacles, bryozoans and
sponges, and populations of solitary clams, echinoderms and polychaetes,
only appear to be important at local scales. In ecosystems, epifaunal reefs or
beds and/or combinations of suspension-feeding types may have the potential to transform shallow coastal environments (Crisp 1979; Buss and Jackson
1981; Hily 1991; Petersen and Riisgard 1992; Vedel and Riisgard 1993;
Lemmens et al. 1996). In addition to encrusting rocks and other firm substrates including seagrass blades, many of these organisms are also found on
the hard surfaces constructed by reef-building organisms or in the habitats
generated by the reefs. In a sense, their functional similarity enhances the
suspension-feeding mode of the reef or bed.
Heip et al. (1995) argued that benthic suspension-feeder biomass is determined by primary production at the scale of entire systems. However, the
distribution of animals on smaller spatial scales is not determined by overall
productivity but by numerous other factors, including predation, competition
and physical environment (Dame 1996). Local distribution patterns, with
suspension-feeder communities occurring at specific sites like slopes of tidal
channels or across narrow inlets, illustrate the importance of localized
hydrodynamic conditions in controlling seston flux (Verhagen 1985; Smaal et
al. 1986; Wildish and Kristmanson 1997). Food depletion is a function of flow,
and Herman et al. (1999) have argued that the maximum limit to the filtration
capacity of the animals is a linear function of current velocity. It can be
expected that animals with a high filtration capacity will be limited to a lower
27
natural systems, some species of oysters form reefs, while mussels typically
aggregate in beds. These structures interact with the over-flowing waters to
increase turbulent mixing, resuspend feces and pseudofeces, and increase the
surface area of the reef or bed by occupying space in three dimensions.
Vermetid and sabellid aggregations are common to wave-swept, highenergy environments, that appear to dictate their geomorphology. Vermetids
may have an advantage in turbulent environments because of their ability to
change the construction and shape of their shell in response to water-flow
dynamics. Sabellids are constantly building and repairing their tubes using
resuspended sand particles. Because their environment is so energetic and
often facing the open ocean, it is doubtful that these groups have much of an
ecological impact on the transformation of suspended particles. However, the
structures they build probably do influence local currents and wave patterns.
In contrast, serpulid reefs are found in low-energy, poorly flushed lagoons,
bays and fjords. The density of serpulids in these environments implies that
these worm reefs are translocating and transforming large quantities of
suspended materials in order to support their structure. It also suggests that
fjord habitats currently dominated by serpulid reefs may be adaptable to the
raft culture of suspension-feeding bivalves similar to that now taking place at
Carlingford Lough (Ball et al. 1997).
In all but a few cases, encrustations of ascidians, barnacles, bryozoans and
sponges, and populations of solitary clams, echinoderms and polychaetes,
only appear to be important at local scales. In ecosystems, epifaunal reefs or
beds and/or combinations of suspension-feeding types may have the potential to transform shallow coastal environments (Crisp 1979; Buss and Jackson
1981; Hily 1991; Petersen and Riisgard 1992; Vedel and Riisgard 1993;
Lemmens et al. 1996). In addition to encrusting rocks and other firm substrates including seagrass blades, many of these organisms are also found on
the hard surfaces constructed by reef-building organisms or in the habitats
generated by the reefs. In a sense, their functional similarity enhances the
suspension-feeding mode of the reef or bed.
Heip et al. (1995) argued that benthic suspension-feeder biomass is determined by primary production at the scale of entire systems. However, the
distribution of animals on smaller spatial scales is not determined by overall
productivity but by numerous other factors, including predation, competition
and physical environment (Dame 1996). Local distribution patterns, with
suspension-feeder communities occurring at specific sites like slopes of tidal
channels or across narrow inlets, illustrate the importance of localized
hydrodynamic conditions in controlling seston flux (Verhagen 1985; Smaal et
al. 1986; Wildish and Kristmanson 1997). Food depletion is a function of flow,
and Herman et al. (1999) have argued that the maximum limit to the filtration
capacity of the animals is a linear function of current velocity. It can be
expected that animals with a high filtration capacity will be limited to a lower