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Mar Ecol Prog Ser 338: 159-168, 2007
Fig. 6. Time series for year 2005. (a) Colonisation of traps by Carcinus maenas
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, distribution
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. Giménez
unpubl. data) suggests that we captured only larvae
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 settlemenl
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 megalopae
need >2 d to reach metamorphosis
(Moksnes et al. 2003). Juvenile blue crabs
CaWnectes sapidus respond positively to
currents and turbulence by planktonic dispersal
(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 laboratory
took <6 d to reach the first juvenile
stage. Therefore, each colonisation peak
should represent a new cohort of megalopae.
(2) We recovered and deployed collectors
during daylight hours to avoid variability
due to twilight or nocturnal
emigration. (3) Strong eastward currents
produced as much turbulence in the intertidal
westward currents (L. Giménez pers.
obs.); however, colonisation of traps was
consistently low during periods of eastward
currents. Movements within the settlement
habitat could explain a considerable
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, colonisation
rate was negatively correlated with SW winds
and eastward currents, suggesting that megalopae
were transported from the east of Helgoland, perhaps
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 concentrate
in coastal areas of the North Sea (Lindley
1987). Steiff (1989) found that C. maenas was common
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 conditions
could move coastal waters towards Helgoland
(Fig. 7b).
