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The sole reliance on a planktonic mode of recruitment has implications for not only
the smaller scale (metres) heterogeneity of this species on Drum Sands (Chapter 2) but
also for the larger-scale maintenance of high population densities on Drum Sands in
view of very high losses of planktonic larvae (Thorson, 1950; Mileikovsky, 1971;
Vance, 1973; Bachelet, 1990). Morgan (1997) suggested that for the P. elegans
population in the Baie De Somme, population maintenance was facilitated by the
larval retentative properties of the bay. Active migration in the water column, which
could enable a species to take advantage of directional water flows, has not been
shown for polychaete larvae and therefore passive retentive mechanisms must be
responsible for population maintenance in some areas (Morgan, 1997). The
differences in passive larval retention between bays and the open coast was
investigated by Gaines and Bertness (1992). They found that during periods of long
flushing times, recruitment of barnacles within Naragansett Bay, Rhode Island, was
significantly higher than recruitment to nearby coastal sites, while during periods of
short flushing times, this difference was not observed. Tidally-induced retainment of
spionid larvae have been shown in many bays and estuaries (see Morgan, 1997).
Therefore, it is possible for passively dispersing larvae to be retained within their bay
or estuary of origin by the hydrodynamic characteristics of the area and thus the
possibility of population maintenance by a dispersive mode of reproduction is
increased.
There is evidence to suggest that the planktonic larvae of the P. elegans population on
Drum Sands may be retained within the area to a certain extent. Dyke (1987)
proposed that within the Firth of Forth, tidal residuals are small and that total residuals
are more due to wind than the tide. Craig (1972) indicated that the residence time for
seawater within the area was about 8 months. This suggests that the flushing time
within the Firth of Forth is relatively long and, therefore, larval retention is likely to be
high. Furthermore, Covill (1972, 1975) found that bays or indented coastlines along
the southern Firth of Forth produced eddy currents which tended to distort the general
tidal flow pattern of the Firth of Forth and that this effect was exacerbated by
freshwater flows from rivers (Covill, 1972). The indented shape of Drum Sands,
together with the added effect of the River Almond, suggests that eddy currents
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