Dynamics in European and North American Soft-Bottom Mussel Beds
51
mussels and natural or mimic mats caused an increase in the silt-clay fraction
and percent organic material of bottom sediment. These results suggest that
active biodeposition by mussels is not the only, or even the primary, reason
why sediment composition is usually finer and more organically enriched in
mussel mats and beds than adjacent bare sediment. They are also consistent
with results demonstrating that physical structure, in this case algae attached
to mussels, causes sediment to accumulate rapidly on Mytilus edulis beds in
the Wadden Sea (Albrecht and Reise 1994; Albrecht 1998).
In addition, the experiments showed that structure of any type had effects
on species abundances. Mats generally had stronger effects than did the
mussels themselves. Compared to bare areas, mats caused a higher abundance
and proportion of infauna with direct development, including oligochaetes
and tan aid crustaceans, and a lower abundance and proportion of planktonic
developers. These results are consistent with results from the studies
described earlier for Mytilus edulis. Compared to mussel mimics, the live
mussels had only a slight enhancement effect on the direct developers. This
result suggests that active in situ biodeposition may not be as important as
passive sediment trapping in creating conditions favorable to direct developers. Compared to mussel mimics, the live mussels had only a slight reduction effect on the planktonic developers. This result suggests that direct ingestion oflarvae by mussels may not be as important as some surface roughness factor that inhibits successful larval settlement. It also suggests that feces
and pseudofeces may not be as important as transported bedload sediment in
smothering recently settled larvae.
These experimental results suggest that mussels of any species probably
have important passive effects that alter environmental conditions for residents of soft-bottom communities. This result should not be surprising, especially in light of recent advances in our understanding of near-bottom flow.
It is possible that flow is the master parameter that affects all the possible
mechanisms listed above, as well as current-induced storm effects that rip
beds apart and transport mussel clumps to new sites. If so, then the structure
of mussel beds in the horizontal and vertical planes deserves more attention
than it has thus far received. The calculations used to estimate flow regimes
are based on the height of roughness elements above the bottom (Eckman
1990; Ke et al. 1994; Abelson and Denny 1997). Mussel beds have complex
surface topographies at many spatial scales, and we know little about water
flow over mussel beds. A field study of the pinnid Atrina zelandica in New
Zealand by Green et al. (1998) showed that seabed drag coefficients were
higher over three horse mussel-bed sites than over a site where the bottom
had no live mussels but consisted of shells, seaweeds, and crab burrows. The
four sites had seabed drag coefficient values greater than the value generally
applied to an abiotic, flat, cohesionless bed. Despite precise site maps with
each mussel's position, orientation, shell height, and shell width, the in-
51
mussels and natural or mimic mats caused an increase in the silt-clay fraction
and percent organic material of bottom sediment. These results suggest that
active biodeposition by mussels is not the only, or even the primary, reason
why sediment composition is usually finer and more organically enriched in
mussel mats and beds than adjacent bare sediment. They are also consistent
with results demonstrating that physical structure, in this case algae attached
to mussels, causes sediment to accumulate rapidly on Mytilus edulis beds in
the Wadden Sea (Albrecht and Reise 1994; Albrecht 1998).
In addition, the experiments showed that structure of any type had effects
on species abundances. Mats generally had stronger effects than did the
mussels themselves. Compared to bare areas, mats caused a higher abundance
and proportion of infauna with direct development, including oligochaetes
and tan aid crustaceans, and a lower abundance and proportion of planktonic
developers. These results are consistent with results from the studies
described earlier for Mytilus edulis. Compared to mussel mimics, the live
mussels had only a slight enhancement effect on the direct developers. This
result suggests that active in situ biodeposition may not be as important as
passive sediment trapping in creating conditions favorable to direct developers. Compared to mussel mimics, the live mussels had only a slight reduction effect on the planktonic developers. This result suggests that direct ingestion oflarvae by mussels may not be as important as some surface roughness factor that inhibits successful larval settlement. It also suggests that feces
and pseudofeces may not be as important as transported bedload sediment in
smothering recently settled larvae.
These experimental results suggest that mussels of any species probably
have important passive effects that alter environmental conditions for residents of soft-bottom communities. This result should not be surprising, especially in light of recent advances in our understanding of near-bottom flow.
It is possible that flow is the master parameter that affects all the possible
mechanisms listed above, as well as current-induced storm effects that rip
beds apart and transport mussel clumps to new sites. If so, then the structure
of mussel beds in the horizontal and vertical planes deserves more attention
than it has thus far received. The calculations used to estimate flow regimes
are based on the height of roughness elements above the bottom (Eckman
1990; Ke et al. 1994; Abelson and Denny 1997). Mussel beds have complex
surface topographies at many spatial scales, and we know little about water
flow over mussel beds. A field study of the pinnid Atrina zelandica in New
Zealand by Green et al. (1998) showed that seabed drag coefficients were
higher over three horse mussel-bed sites than over a site where the bottom
had no live mussels but consisted of shells, seaweeds, and crab burrows. The
four sites had seabed drag coefficient values greater than the value generally
applied to an abiotic, flat, cohesionless bed. Despite precise site maps with
each mussel's position, orientation, shell height, and shell width, the in-