9780521861724htl 1..2

Pelagic assemblages

For comparison with benthic species size distributions, Warwicket al.(1986)

made a comprehensive collection of pelagic animals at station CS2 in the Celtic

Sea, using a range of sampling devices with mesh sizes of 10 mm, 1mm, 280mm

and 53mm, and determined the adult body weights of all species in the samples.

The centre of the size distribution of pelagic species closely corresponded

with the trough in the bimodal benthic curve, with a few species extending

as a tail across the macrobenthic size range (Fig.11.9). Since the larvae of

the macrobenthos (the meroplankton) are in the plankton when they occupy

the meiobenthic size range, resource partitioning on a size basis between the

meroplankton and holoplankton may have been an important evolutionary

driving force in determining the pelagic size-distribution pattern (Warwick

et al., 1986; Warwick & Joint, 1987 ).

Effects of pollution and disturbance

Disturbed soft-bottom assemblages, and particularly those subjected to organic

enrichment, become dominated by a group of species that have become known

as ‘pollution indicators’ (Pearson & Rosenberg,1978). These species are the

smallest representatives of the macrofauna, mainly small annelids, and the

largest species of the meiofauna, mainly oncholaimid nematodes and harpacti-

coid copepods of the generaTisbeandBulbamphiascus(Table11.3). The body size

of these species falls within the trough between the meiofauna and macrofauna

0

0.05

0.1

(a) (b)

0

10

Number of species

Proportion of species

Number of species

Proportion of species

0

4

0

3

0

0.05

0.1

0102030

X2 geometric weight class

0102030

X2 geometric weight class

benthic adults

eggs

benthic adults

Thorson’s polychaete larvae

Northumberland larvae

Figure 11.5(a) Histogram of the size distribution of eggs of macrobenthic species that have

planktotrophic larvae, compared with a generalized benthic species body-size

distribution curve (from Warwicket al., 1986). (b) Histogram of the sizes of the polychaete

larvae in Figure11.6 and for a variety of newly settled macrobenthic larvae off the coast

of Northumberland, compared with the adult species body-size distribution curve for

the Northumberland station (from Warwick, 1984 ).

218 R.M. WARWICK