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METHODS
Study site - The exact position of the field experiment within the study area is shown
in Figure 4.1. The sediments there were moderately sorted with median particle size
of 2.64) (Chapter 2) and with no obvious environmental gradient (pers. obs.). The
sediment surface in the selected area was well oxidised and had no macroalgal cover
during the summer of 1996. This was an important consideration for the maintenance
of controls. For example, weed mat establishment within this area during the
experiment would have resulted in a lack of proper controls.
Experimental design - Six experimental blocks were set out in a line perpendicular to
the direction of the flood tide (Figure 4.2). The use of 6 replicates allowed non-
parametric statistical analyses, if necessary, while greater replication was limited by
the practical work involved. Within each block, three plots (each of 1m 2) were
marked out (Figure 4.2). These plots were 0.5m apart from each other and at least
1.5m away from those of neighbouring blocks. Since P. elegans patches were just
above 1m2 in area, this design ensured that replicates would be independent since their
separation was larger than the average P. elegans patch size. The plots were randomly
assigned as unmanipulated control, net plot or weed treatment plot (see later) within
each block. The size of the plots were as large as practically possible: maximising the
number of replicates was considered to be more important than increasing the size of
the plots. The spatial arrangement of experimental plots, therefore, corresponded to a
randomised block design. This design is excellent for field experiments in which the
distributions of the organisms are unknown since it automatically produces an
interspersion of treatments and reduces the effect of chance events on the results of the
experiment (Krebs, 1989). Furthermore, Zar (1984) suggested that the blocking
enables more of the variability among the data to be accounted for, with the desirable
outcome of a smaller mean square and greater statistical power.
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