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E. Flach and A. Tamaki
densities (-45 per m 2 ) an average reduction of -50 % in the number of recruits was found. As densities of >30 adult lugworms per m 2 are commonly
observed in the Wadden Sea (Cadee 1976; Beukema and De VIas 1979; Reise
1985), recruitment can therefore be strongly affected. In the middle part of the
transect at Balgzand (Fig. 7.3), high numbers of recruits of different species
(e.g. Macoma, Nereis, Heteromastus) were found during the summer of 1991
(when few adult lugworms were present there), but not in 1990 and 1992
(when adult lugworms were present in their normal density). Significant
negative correlations (p<0.05) were found between the densities of adult lugworms and juvenile lugworms, as well as Macoma, Mya, Nereis, Eteone, Capitella and Pygospio, along various transects sampled during the summer of
1991 in the Dutch Wadden Sea (E. Flach, unpublished data; position of transects given in Flach 1993). The negative effects on the recruits were strongest
later in summer, which implies that it was not the initial settlement of the
juveniles that was prohibited by the presence of lugworms, but that, in the
presence of high lugworm densities, either the survival rate of the juveniles
was lower or the migration rate higher (Flach 1992a).
A strong negative impact of lugworms on the whole population of two
Corophium species (c. volutator and C. arenarium) was also found (Flach
1992a,b, 1993). Within the experimental plots lugworm densities of only 18 per
m 2 already caused a reduction in numbers in both Corophium species of
-50 %, whereas higher lugworm densities (-40-55 per m 2 ) caused reductions
of between 80 and 95 % (Flach 1992a,b, 1993). Removal of adult lugworms in
otherwise undisturbed 'natural' plots within the lugworm zone resulted in a
significant increase in Corophium, whereas addition of lugworms in 'natural'
plots within the Corophium zone caused a significant decrease in Corophium
numbers (Flach 1992b, 1993). It was concluded that the commonly observed
zonation pattern of Corophium dominating the upper tidal zone, and Arenicola
the middle and lower zone, resulted from the strong negative impact of
Arenicola on Corophium, restricting Corophium to the upper tidal zone, which
for Arenicola is physically unfavourable (Beukema and Flach 1995).
This strong negative impact of Arenicola raises the question as to the mechanism by which Arenicola influences other species. To answer this question
aquarium experiments have been carried out with Corophium. In the first
experiment, settlement of Corophium was studied in the presence and absence
of lugworms (Flach 1992a). In the absence of lugworms, Corophium settled
about equally on both sides, but when lugworms were present on one side the
numbers of Corophium on the lugworm-side were significantly lower
(Fig. 7.4a). In a second experiment, migration of Corophium was studied in
the presence and absence of lugworms (Flach 1993). About twice as many
Corophiums had migrated to the empty side within 2 weeks when lugworms
were present compared to the control treatment without lugworms (Flach
1993).
E. Flach and A. Tamaki
densities (-45 per m 2 ) an average reduction of -50 % in the number of recruits was found. As densities of >30 adult lugworms per m 2 are commonly
observed in the Wadden Sea (Cadee 1976; Beukema and De VIas 1979; Reise
1985), recruitment can therefore be strongly affected. In the middle part of the
transect at Balgzand (Fig. 7.3), high numbers of recruits of different species
(e.g. Macoma, Nereis, Heteromastus) were found during the summer of 1991
(when few adult lugworms were present there), but not in 1990 and 1992
(when adult lugworms were present in their normal density). Significant
negative correlations (p<0.05) were found between the densities of adult lugworms and juvenile lugworms, as well as Macoma, Mya, Nereis, Eteone, Capitella and Pygospio, along various transects sampled during the summer of
1991 in the Dutch Wadden Sea (E. Flach, unpublished data; position of transects given in Flach 1993). The negative effects on the recruits were strongest
later in summer, which implies that it was not the initial settlement of the
juveniles that was prohibited by the presence of lugworms, but that, in the
presence of high lugworm densities, either the survival rate of the juveniles
was lower or the migration rate higher (Flach 1992a).
A strong negative impact of lugworms on the whole population of two
Corophium species (c. volutator and C. arenarium) was also found (Flach
1992a,b, 1993). Within the experimental plots lugworm densities of only 18 per
m 2 already caused a reduction in numbers in both Corophium species of
-50 %, whereas higher lugworm densities (-40-55 per m 2 ) caused reductions
of between 80 and 95 % (Flach 1992a,b, 1993). Removal of adult lugworms in
otherwise undisturbed 'natural' plots within the lugworm zone resulted in a
significant increase in Corophium, whereas addition of lugworms in 'natural'
plots within the Corophium zone caused a significant decrease in Corophium
numbers (Flach 1992b, 1993). It was concluded that the commonly observed
zonation pattern of Corophium dominating the upper tidal zone, and Arenicola
the middle and lower zone, resulted from the strong negative impact of
Arenicola on Corophium, restricting Corophium to the upper tidal zone, which
for Arenicola is physically unfavourable (Beukema and Flach 1995).
This strong negative impact of Arenicola raises the question as to the mechanism by which Arenicola influences other species. To answer this question
aquarium experiments have been carried out with Corophium. In the first
experiment, settlement of Corophium was studied in the presence and absence
of lugworms (Flach 1992a). In the absence of lugworms, Corophium settled
about equally on both sides, but when lugworms were present on one side the
numbers of Corophium on the lugworm-side were significantly lower
(Fig. 7.4a). In a second experiment, migration of Corophium was studied in
the presence and absence of lugworms (Flach 1993). About twice as many
Corophiums had migrated to the empty side within 2 weeks when lugworms
were present compared to the control treatment without lugworms (Flach
1993).