Competitive Bioturbators in the Wadden Sea and Ariake Sound
153
In similar experimental plots the settlement of juvenile lugworms was
studied. A significant (r=-0.84, n=12, p
addition, a significant (r=-0.79, n=9, p<0.05) negative relationship between
the mean individual weight of the juveniles and the densities of the juveniles
was also found (Flach and Beukema 1994). Thus, although the numbers of
juveniles in plots with high adult densities were lower, they were larger
individuals. This could be obtained either by different growth rates, or by
migration of small individuals. in the presence of adults, and/or by successful
settlement of large juveniles between adult individuals.
Detailed observations of the lugworm population along the transect
perpendicular to the coast showed the usual pattern of low densities close to
the coast and high densities (-30-40 per m 2 ) in a broad zone nearly towards
the tidal channel in May 1990. In October 1990, adult densities had declined
dramatically (to -10 per m 2 ) in the central part of the formerly high-density
zone. During the summer of 1991 very high numbers of juveniles (up to -400
per m 2 ) were found just in this area of low adult densities (Fig. 7.3), whereas
they were nearly absent in the areas with high adult densities. Fishing in a
nearby tidal channel during the subsequent winter showed that small
juveniles were migrating, whereas the larger juveniles stayed on the tidal flats
(Flach and Beukema 1994). This resulted in lugworm densities in this central
part of the transect of -30-40 per m 2 in the summer of 1992, thus restoring
the situation as found in May 1990.
7.4 Effects of Lugworms on the Benthic Community
Field experiments were carried out at Balgzand (Fig. 7.2) and near the island
of Schiermonnikoog in the Dutch Wadden Sea. Large experimental squares
of 144m2 were defaunated during winter by placing a mat on top of the sediment, which was removed in early spring. Within these squares small plots
of 2.25 m 2 were either restocked with different lugworm densities or left
empty. Initial densities within the small plots were 0,25,50,75 and 100 adult
lugworms per m 2 , all densities at least in duplicate per experimental plot.
The repopulation of these plots was followed during the summer by taking
three 86.6-cm 2 sediment samples to a depth of 15 cm per plot once a month.
These samples were sieved through a 0.5-mm mesh sieve and all macrofaunal species counted. At the same time the numbers of lugworm casts were
counted. In addition to these defaunated plots, lugworm densities were
manipulated in 'natural plots' by either removing or adding lugworms in
small 2.S-m 2 plots at Balgzand and near Schiermonnikoog. Moreover, the
natural distribution patterns of lugworms and all other macrobenthic spe-
153
In similar experimental plots the settlement of juvenile lugworms was
studied. A significant (r=-0.84, n=12, p
addition, a significant (r=-0.79, n=9, p<0.05) negative relationship between
the mean individual weight of the juveniles and the densities of the juveniles
was also found (Flach and Beukema 1994). Thus, although the numbers of
juveniles in plots with high adult densities were lower, they were larger
individuals. This could be obtained either by different growth rates, or by
migration of small individuals. in the presence of adults, and/or by successful
settlement of large juveniles between adult individuals.
Detailed observations of the lugworm population along the transect
perpendicular to the coast showed the usual pattern of low densities close to
the coast and high densities (-30-40 per m 2 ) in a broad zone nearly towards
the tidal channel in May 1990. In October 1990, adult densities had declined
dramatically (to -10 per m 2 ) in the central part of the formerly high-density
zone. During the summer of 1991 very high numbers of juveniles (up to -400
per m 2 ) were found just in this area of low adult densities (Fig. 7.3), whereas
they were nearly absent in the areas with high adult densities. Fishing in a
nearby tidal channel during the subsequent winter showed that small
juveniles were migrating, whereas the larger juveniles stayed on the tidal flats
(Flach and Beukema 1994). This resulted in lugworm densities in this central
part of the transect of -30-40 per m 2 in the summer of 1992, thus restoring
the situation as found in May 1990.
7.4 Effects of Lugworms on the Benthic Community
Field experiments were carried out at Balgzand (Fig. 7.2) and near the island
of Schiermonnikoog in the Dutch Wadden Sea. Large experimental squares
of 144m2 were defaunated during winter by placing a mat on top of the sediment, which was removed in early spring. Within these squares small plots
of 2.25 m 2 were either restocked with different lugworm densities or left
empty. Initial densities within the small plots were 0,25,50,75 and 100 adult
lugworms per m 2 , all densities at least in duplicate per experimental plot.
The repopulation of these plots was followed during the summer by taking
three 86.6-cm 2 sediment samples to a depth of 15 cm per plot once a month.
These samples were sieved through a 0.5-mm mesh sieve and all macrofaunal species counted. At the same time the numbers of lugworm casts were
counted. In addition to these defaunated plots, lugworm densities were
manipulated in 'natural plots' by either removing or adding lugworms in
small 2.S-m 2 plots at Balgzand and near Schiermonnikoog. Moreover, the
natural distribution patterns of lugworms and all other macrobenthic spe-