140
G.c. Cadee
combined effect of biological and physical mixing. The same holds for mixing
depth estimated from the vertical distribution of isotopes (e. g. 21OPb). Only if
physical mixing is low will these methods estimate bioturbation, as for
Discovery Bay, Jamaica, in the calm summer period, but not in the stormy
winter (Aller and Dodge 1974). For more exposed coastal areas the best
method seems to estimate bioturbation of the (main) bioturbators separately
and than add these estimates for community bioturbation data. Only few
studies give such information. Moreover, comparisons of literature data on
bioturbation face the problem of differences in methods used and data
provided.
The greatest diversity of coastal life occurs in the tropics (Ekman 1953;
Briggs 1995), and this increase in species diversity from high to low latitudes
occurs particularly in epifaunal species such as crabs and nudibranchs
(Thorson 1957). Some of the infaunal groups that Thorson studied did not
show such an increase. This might imply that the diversity of deposit-feeding
polychaetes and bivalves does not show a strong latitudinal variation, but
their density and bioturbation activity might still vary with latitude. As
already mentioned, arctic and temperate coasts lack the rich diversity of holedigging callianassid shrimps and crabs such as ghost crabs Ocypode and
fiddler crabs Uca of (sub)tropical beaches and intertidal flats.
The best -studied coastal faunas are usually in temperate areas, which
makes comparisons from (ant)arctic to tropical areas difficult. Coastal areas
of high latitudes are too hostile for bioturbators due to low temperatures and
the high amount of sediment disturbance by drift ice. As an example of
subarctic intertidal flats I will use the study of Aitken et al. (1988) at Baffin
Island, Canada, near the Arctic Circle. They found a boreal Macoma balthica
community with bioturbators such as the bivalve Macoma balthica and the
polychaete Arenicola marina, which are species that also occur on temperate
tidal flats of i.a. the Wadden Sea. The authors state their sediment-reworking
capacity to equal that of their counterparts in the Wadden Sea, but their
density was lower as well as the period of the year they could bioturbate. For
tidal flats in the temperate Dutch Wadden Sea, I arrived at an annual
bioturbation activity equalling a sediment layer of almost 40 cm by adding
sediment-reworking rates of the most important bioturbators (Cadee 1976,
1979,1990); this is probably four times higher than on Baffin Island.
Myrick and Flessa (1996) studied bioturbation on a tropical tidal flat area
in Bahia la Choya, northern Gulf of California, Mexico. They measured an
annual sediment-reworking rate of 0.5 m by callianassid shrimps on high
tidal flats and of 1 m by rays· on low tidal flats. The study of Aller and Dodge
(1974) in Discovery Bay, Jamaica, provides data for bioturbation in a tropical
lagoon. They confined their measurements to the warm and calm summer
period (April-October), which makes extrapolation to include the cooler
stormy winter period difficult. Deposit-feeding callianassid crustaceans were
G.c. Cadee
combined effect of biological and physical mixing. The same holds for mixing
depth estimated from the vertical distribution of isotopes (e. g. 21OPb). Only if
physical mixing is low will these methods estimate bioturbation, as for
Discovery Bay, Jamaica, in the calm summer period, but not in the stormy
winter (Aller and Dodge 1974). For more exposed coastal areas the best
method seems to estimate bioturbation of the (main) bioturbators separately
and than add these estimates for community bioturbation data. Only few
studies give such information. Moreover, comparisons of literature data on
bioturbation face the problem of differences in methods used and data
provided.
The greatest diversity of coastal life occurs in the tropics (Ekman 1953;
Briggs 1995), and this increase in species diversity from high to low latitudes
occurs particularly in epifaunal species such as crabs and nudibranchs
(Thorson 1957). Some of the infaunal groups that Thorson studied did not
show such an increase. This might imply that the diversity of deposit-feeding
polychaetes and bivalves does not show a strong latitudinal variation, but
their density and bioturbation activity might still vary with latitude. As
already mentioned, arctic and temperate coasts lack the rich diversity of holedigging callianassid shrimps and crabs such as ghost crabs Ocypode and
fiddler crabs Uca of (sub)tropical beaches and intertidal flats.
The best -studied coastal faunas are usually in temperate areas, which
makes comparisons from (ant)arctic to tropical areas difficult. Coastal areas
of high latitudes are too hostile for bioturbators due to low temperatures and
the high amount of sediment disturbance by drift ice. As an example of
subarctic intertidal flats I will use the study of Aitken et al. (1988) at Baffin
Island, Canada, near the Arctic Circle. They found a boreal Macoma balthica
community with bioturbators such as the bivalve Macoma balthica and the
polychaete Arenicola marina, which are species that also occur on temperate
tidal flats of i.a. the Wadden Sea. The authors state their sediment-reworking
capacity to equal that of their counterparts in the Wadden Sea, but their
density was lower as well as the period of the year they could bioturbate. For
tidal flats in the temperate Dutch Wadden Sea, I arrived at an annual
bioturbation activity equalling a sediment layer of almost 40 cm by adding
sediment-reworking rates of the most important bioturbators (Cadee 1976,
1979,1990); this is probably four times higher than on Baffin Island.
Myrick and Flessa (1996) studied bioturbation on a tropical tidal flat area
in Bahia la Choya, northern Gulf of California, Mexico. They measured an
annual sediment-reworking rate of 0.5 m by callianassid shrimps on high
tidal flats and of 1 m by rays· on low tidal flats. The study of Aller and Dodge
(1974) in Discovery Bay, Jamaica, provides data for bioturbation in a tropical
lagoon. They confined their measurements to the warm and calm summer
period (April-October), which makes extrapolation to include the cooler
stormy winter period difficult. Deposit-feeding callianassid crustaceans were