Dynamics in European and North American Soft-Bottom Mussel Beds
49
cavage (1989) performed a mussel density manipulation at the same site in
Maine studied by Commito (1987). They hypothesized that mussels enhance
the abundance of oligochaetes. All mussels were removed from 1-m 2 plots and
put back to create 0 x, 1 x, and 2 x ambient mussel density treatments and
unmanipulated controls. After 3 months, there were significant positive
Spearman rank correlations between oligochaete and Mytilus edulis abundances. Moreover, the 0 x treatment plots had half as many oligochaetes per
core as did the other three treatments, which did not differ among themselves.
The results indicate that mussels enhance oligochaete abundance, and that
this relationship is nonlinear. Above a certain density of mussels, further
increases do not result in greater oligochaete abundance.
Ragnarsson and Raffaelli (1999) recently performed a manipulation of
Mytilus edulis density at an intertidal site in the Ythan estuary, Aberdeenshire,
Scotland. They removed mussels from 1-m 2 plots and also created 0.6-m 2
patches of mussels transplanted to bare sediment. Overall density and
number of taxa declined in the mussel removal patches, particularly the
oligochaetes and amphipods. These two groups also increased in the mussel
transplant (addition) patches. These results were consistent with the observations and predictions from the Maine studies (Commito 1987; Commito
and Boncavage 1989). The silt content in the transplanted patches skyrocketed
within 2 weeks, leading to an increase in deposit-feeding polychaetes and a
decline in suspension-feeding bivalves, as observed by Dittmann (1990) in the
Wadden Sea.
The studies described above were all at intertidal sites and examined the
effects of mussels that form dense beds or mats. What about mussels that live
in other habitat types or, more importantly, do not form densely packed
aggregations? Jaramillo et al. (1992) investigated subtidal, soft-bottom areas
inhabited by Choromytilus chorus and Mytilus chilensis in the Quele River
estuary in southern Chile. They showed that macrofaunal density (primarily
polychaetes), species richness, and the Shannon index of diversity were lower
inside mussel areas than outside. Cummings et al. (1998) sampled patches of
the pinnid horse mussel, Atrina zelandica, at a subtidal location in northern
New Zealand and found no significant differences in macrofauna between
samples taken inside and outside the patches at a muddy site. However, at
their sandy site, they found effects similar to those from Chile. Density
(primarily amphipods and polychaetes), species richness, and the Shannon
index of diversity were lower in pinnid patches than in bare sediment.
The two Chilean mytilids and the pinnid were subtidal and had lower
mussel densities than are found in intertidal Mytilus edulis beds, yet they still
produced an impact on soft-bottom community structure. In both cases, they
generally lowered the abundance of associated in fauna. This result is expected
for polychaetes, but amphipods do not produce free-swimming larvae and
ought to be able to avoid ingestion by mussels. However, unlike cocoon-
49
cavage (1989) performed a mussel density manipulation at the same site in
Maine studied by Commito (1987). They hypothesized that mussels enhance
the abundance of oligochaetes. All mussels were removed from 1-m 2 plots and
put back to create 0 x, 1 x, and 2 x ambient mussel density treatments and
unmanipulated controls. After 3 months, there were significant positive
Spearman rank correlations between oligochaete and Mytilus edulis abundances. Moreover, the 0 x treatment plots had half as many oligochaetes per
core as did the other three treatments, which did not differ among themselves.
The results indicate that mussels enhance oligochaete abundance, and that
this relationship is nonlinear. Above a certain density of mussels, further
increases do not result in greater oligochaete abundance.
Ragnarsson and Raffaelli (1999) recently performed a manipulation of
Mytilus edulis density at an intertidal site in the Ythan estuary, Aberdeenshire,
Scotland. They removed mussels from 1-m 2 plots and also created 0.6-m 2
patches of mussels transplanted to bare sediment. Overall density and
number of taxa declined in the mussel removal patches, particularly the
oligochaetes and amphipods. These two groups also increased in the mussel
transplant (addition) patches. These results were consistent with the observations and predictions from the Maine studies (Commito 1987; Commito
and Boncavage 1989). The silt content in the transplanted patches skyrocketed
within 2 weeks, leading to an increase in deposit-feeding polychaetes and a
decline in suspension-feeding bivalves, as observed by Dittmann (1990) in the
Wadden Sea.
The studies described above were all at intertidal sites and examined the
effects of mussels that form dense beds or mats. What about mussels that live
in other habitat types or, more importantly, do not form densely packed
aggregations? Jaramillo et al. (1992) investigated subtidal, soft-bottom areas
inhabited by Choromytilus chorus and Mytilus chilensis in the Quele River
estuary in southern Chile. They showed that macrofaunal density (primarily
polychaetes), species richness, and the Shannon index of diversity were lower
inside mussel areas than outside. Cummings et al. (1998) sampled patches of
the pinnid horse mussel, Atrina zelandica, at a subtidal location in northern
New Zealand and found no significant differences in macrofauna between
samples taken inside and outside the patches at a muddy site. However, at
their sandy site, they found effects similar to those from Chile. Density
(primarily amphipods and polychaetes), species richness, and the Shannon
index of diversity were lower in pinnid patches than in bare sediment.
The two Chilean mytilids and the pinnid were subtidal and had lower
mussel densities than are found in intertidal Mytilus edulis beds, yet they still
produced an impact on soft-bottom community structure. In both cases, they
generally lowered the abundance of associated in fauna. This result is expected
for polychaetes, but amphipods do not produce free-swimming larvae and
ought to be able to avoid ingestion by mussels. However, unlike cocoon-