6.3 RecolonizatIOn After Experimental Disturbances
183
Hardly any species settled in the autumn experiment, as the abundances in the
adjacent sandtlat were low and almost no larvae or postlarvae were available
(Fig. 6.3.3). High abundances of individuals were not recorded until the following
year. In spring and summer 1995, benthic colonization of the spring and autumn
experiment were comparable to each other, only capitellid polychaetes were more
abundant in the autumn experiment and molluscs were absent. As in the colonization of the spring experiment in the preceding year, seasonal events influenced the
course of development. Juvenile polychaetes occurred in higher individual numbers in the experimental than in the control plots.
The experiments with varying intensities of disturbance carried out in 1995
showed that the weaker the disturbance, the faster the recovery of the plots
(Fig. 6.3.4). The mechanical disturbance caused a temporary decline in abundances only. But no recruitment of juvenile spionid polychaetes, as recorded in all
other experimental plots, was observed in this treatment. In the plots that were
disturbed for I and 4 weeks, the recruitment of polychaetes was twice as high as in
the control plots. Dominant species in these experimental plots were P. elegans,
ostracods, Polydora sp., H. flliformis and C. capitata.
The mean species density reached background values fastest after the mechanical disturbance (4 days). This took 5-7 weeks after the spring experiment of 1994
and the 1- and 4-week disturbances in 1995. The autumn experiment still had a
lower species density than control plots after half a year.
The abundance of the small macrofauna species were reduced depending on the
type of disturbance and reached background values in dependence of the timing
and intensity of disturbance. The recovery time ranged from a few days (the mechanical disturbance), over 2-3 months (spring experiment, 1- and 4-week disturbances) to more than 6 months (autumn experiment).
6.3.2
Mode of Recolonization
To assess the mode of transport and the age of the organisms colonizing the experimental plots, the macrofauna was divided into adults and juveniles and samples were grouped according to their sample location at the periphery or centre of
the plots. A comparison of the colonization in the central and peripheral areas
should allow to distinguish between a dispersal through the water column (mainly
larvae and highly mobile postlarvae) or a dispersal with bedload transport (mainly
less mobile postlarvae/adults). With a prevailing water column transport, larvae/postlarvae should show similar abundances all over the plots, with a prevailing
bedload transport it was expected that abundances of adults were higher in the
periphery. However, this analysis is biased, as four samples had been taken from
the periphery and one from the centre of each site. An exemplary data analysis of
the spring experiment 1994 and of the control plots gave no significant differences
for the individual densities between central and peripheral areas. Juveniles were
recorded in the experimental plots after one week, whereas the first adults occurred only after 3 weeks. Thereby it was not possible to separate whether these
adults were derived from recruitment in the plots or from colonization ti·om ambient sandtlat sediments.
183
Hardly any species settled in the autumn experiment, as the abundances in the
adjacent sandtlat were low and almost no larvae or postlarvae were available
(Fig. 6.3.3). High abundances of individuals were not recorded until the following
year. In spring and summer 1995, benthic colonization of the spring and autumn
experiment were comparable to each other, only capitellid polychaetes were more
abundant in the autumn experiment and molluscs were absent. As in the colonization of the spring experiment in the preceding year, seasonal events influenced the
course of development. Juvenile polychaetes occurred in higher individual numbers in the experimental than in the control plots.
The experiments with varying intensities of disturbance carried out in 1995
showed that the weaker the disturbance, the faster the recovery of the plots
(Fig. 6.3.4). The mechanical disturbance caused a temporary decline in abundances only. But no recruitment of juvenile spionid polychaetes, as recorded in all
other experimental plots, was observed in this treatment. In the plots that were
disturbed for I and 4 weeks, the recruitment of polychaetes was twice as high as in
the control plots. Dominant species in these experimental plots were P. elegans,
ostracods, Polydora sp., H. flliformis and C. capitata.
The mean species density reached background values fastest after the mechanical disturbance (4 days). This took 5-7 weeks after the spring experiment of 1994
and the 1- and 4-week disturbances in 1995. The autumn experiment still had a
lower species density than control plots after half a year.
The abundance of the small macrofauna species were reduced depending on the
type of disturbance and reached background values in dependence of the timing
and intensity of disturbance. The recovery time ranged from a few days (the mechanical disturbance), over 2-3 months (spring experiment, 1- and 4-week disturbances) to more than 6 months (autumn experiment).
6.3.2
Mode of Recolonization
To assess the mode of transport and the age of the organisms colonizing the experimental plots, the macrofauna was divided into adults and juveniles and samples were grouped according to their sample location at the periphery or centre of
the plots. A comparison of the colonization in the central and peripheral areas
should allow to distinguish between a dispersal through the water column (mainly
larvae and highly mobile postlarvae) or a dispersal with bedload transport (mainly
less mobile postlarvae/adults). With a prevailing water column transport, larvae/postlarvae should show similar abundances all over the plots, with a prevailing
bedload transport it was expected that abundances of adults were higher in the
periphery. However, this analysis is biased, as four samples had been taken from
the periphery and one from the centre of each site. An exemplary data analysis of
the spring experiment 1994 and of the control plots gave no significant differences
for the individual densities between central and peripheral areas. Juveniles were
recorded in the experimental plots after one week, whereas the first adults occurred only after 3 weeks. Thereby it was not possible to separate whether these
adults were derived from recruitment in the plots or from colonization ti·om ambient sandtlat sediments.