164 
Mar Ecol Prog Ser 338: 159-168, 2007 
Fig. 6. Time series for year 2005. (a) Colonisation of traps by Carcinus mae- 
nas megalopae; (b) wind direction; (c) predicted residual currents. Further 
details as in Fig. 3 
val supply has been related to tidal and wind-driven 
currents (Queiroga et. al. 2006). We used a model for 
predicting surface (0 to 8 m depth) currents to assess 
the importance of water circulation; however, distribu 
tion of megalopae may also be affected by circulation 
in deeper layers. If a pycnocline were present in our 
sttidy area we would need to consider movements 
below the pycnocline. However, in the German Bight 
the summer thermocline forms mostly at depths > 40 m, 
well offshore from Helgoland, and haloclines are 
formed near the mouths of the estuaries. During most 
of the year, Helgoland lies outside these areas (Krause 
et al. 1986, Luyten et al, 2003, Loewe et al. 2005), 
The fact that >90% of individuals colonising the 
traps metamorphosed to juveniles after <6 d (L. Gimé 
nez unpubl. data) suggests that we captured only lar 
vae at an advanced stage of development. Although 
this could have increased the noise in our results 
related to supply, these larvae are those most likely to 
remain in the intertidal. Larval movement during set- 
tlemenl was studied only on the Swedish 
coast, where emigration of megalopae 
from the benthos occurs mainly at dusk 
and at night; emigration is higher if mega 
lopae need >2 d to reach metamorphosis 
(Moksnes et al. 2003). Juvenile blue crabs 
CaWnectes sapidus respond positively to 
currents and turbulence by planktonic dis 
persal (Blackmon & Eggleston 2001). If 
megalopae settling on Helgoland also 
respond to these factors, then the patterns 
in the present study may have resulted 
from movement within the nursery habitat. 
However, such movement cannot explain 
all the patterns observed, since: (1) while 
the most important colonisation peaks 
were separated by periods >10 d, most 
megalopae (>90%) transferred to the labo 
ratory took <6 d to reach the first juvenile 
stage. Therefore, each colonisation peak 
should represent a new cohort of mega 
lopae. (2) We recovered and deployed col 
lectors during daylight hours to avoid vari 
ability due to twilight or nocturnal 
emigration. (3) Strong eastward currents 
produced as much turbulence in the inter 
tidal westward currents (L. Giménez pers. 
obs.); however, colonisation of traps was 
consistently low during periods of east 
ward currents. Movements within the set 
tlement habitat could explain a consider 
able proportion of variability at a scale of 
<5 d: variation at this scale was recorded in 
2003 and 2005 during periods of high 
colonisation. 
The present data suggest that colonisation rates 
were indeed affected by transport processes through 
changes in larval supply. In 2003 and 2004, colonisa 
tion rate was negatively correlated with SW winds 
and eastward currents, suggesting that megalopae 
were transported from the east of Helgoland, per 
haps from near the Wadden Sea or from the mouth 
of the Elbe River, where plume fronts are present 
(Krause et al. 1986, Otto et al. 1990). In agreement 
with our hypothesis, larvae of Carcinus maenas con 
centrate in coastal areas of the North Sea (Lindley 
1987). Steiff (1989) found that C. maenas was com 
mon in the inner German Bight, with a distribution 
limit near Helgoland. In the German Bight, SW 
winds lead to cyclonic water transport (Dippner 1998, 
Loewe et al. 2005, and Fig. 7a) and to a northwards 
and onshore displacement of the plumes from rivers 
(Dippner 1993, Luyten et al. 2003). Other wind con 
ditions could move coastal waters towards Helgoland 
(Fig. 7b).