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Mar Ecol Prog Ser 338: 159-168, 2007 
2004, the year with higher variations in winds and pre 
dicted current, we found a significant negative correla 
tion with northward currents and a positive correlation 
with the easterly winds. Therefore, correlations de 
pend on the range in variation and strong SW winds, 
and an anticlockwise circulation pattern reduces 
colonisation. (4) Spectral analysis of the northerly wind 
and eastward current of 2005 revealed a peak at a 
period of nearly 15 d (data not shown). Thus, both var 
ied at approximately the same frequency as the spring- 
neap cycle. Colonisation peaks occurred after rela 
xation of the predicted eastward residual currents 
(Days 200 to 210, Fig. 6). Either these correlations 
arose from coincidental variation between tidal cycles 
and winds, or they indicate that colonisation increases 
after current relaxation. It is thus clear that strong and 
persistent (>5 d long) periods of SW winds and east 
ward currents lead to low colonisation rates, while 
under other wind or current conditions, e,g. current 
relaxation or westward currents, colonisation appar 
ently varies as a function of tide. 
Our hypothesis is consistent with a 2-step model of 
larval supply formulated for coastal marine species in 
general (see Miller & Shanks 2004 for discussion) and 
supported by Carcinus maenas data from Portugal 
(Queiroga et al. 2006). In addition, the influence of 
local movements of megalopae on colonisation pat 
terns may be contingent on the overall number of 
larvae reaching the benthic habitats. 
As an initial step in larval transport, wind-driven 
currents are thought to transport larvae across the 
continental shelf towards the coast (Shanks 1995, Wing 
et al. 2003); in Portugal, the supply of Carcinus maenas 
megalopae to estuaries may be related to upwelling 
fronts reaching the coast after wind relaxation (Al 
meida & Queiroga 2003, Queiroga et. al. 2006). In the 
North Sea, variations in westward wind-driven trans 
port may move larvae away or towards Helgoland. 
Near the coast in mesotidal systems, selective tidal 
stream transport (STST, Forward & Tankersley 2001) 
or some form of tidally mediated water flow may trans 
port larvae shorewards in pulses of about 2 wk, at or a 
few days after spring tides; this may be the second step 
in transport, as observed for Carcinus maenas in Portu 
gal (Queiroga et al, 2006). This second step is contin 
gent on the first step, since 2 wk pulses were observed 
in the absence of strong SW winds or eastward cur 
rents. In the estuaries along the Portuguese coasts, 
zero time lags suggest STST, although the influence of 
internal waves cannot be disregarded (Queiroga et al. 
2006). Following Miller & Shanks (2004) and Queiroga 
et al. (2006), the time lags of 1 to 5 d for Helgoland sug 
gest a combined effect of STST and internal waves on 
larval colonisation. The STST has been invoked for 
megalopae invading estuaries: here cues associated 
with estuarine water (e.g. low salinity) may entrain the 
appropriate swimming behaviour of megalopae. Data 
for coastal zones are sparse (Necora puber. Lee et al. 
2005; C. maenas in mesotidal coastal zone of Wales: 
Zeng & Naylor 1996, Queiroga 1998), but those that 
are available for C. maenas show that larvae are more 
abundant near the surface water at flood tide, espe 
cially at night. This pattern would predict maximum 
larval supply to the intertidal of Helgoland during 
spring tides, since at this time at least 1 high tide 
occurs al night. According to Zeng & Naylor (1996) this 
pattern is not endogenous, but due to some exogenous 
factor, perhaps related to the intertidal proximity (e.g. 
turbulence), 
Tidally mediated flow may be related to internal 
waves through at least 3 different transport mecha 
nisms, involving shoreward movements of (1) conver 
gence zones, (2) upwelling fronts, and (3) cold bores 
(Pineda 1999, Shanks 2006). River plumes, such as 
those of the Elbe and Wesser, may be a source of large- 
amplitude internal waves (Nash & Mourn 2005); 
however, according to W. Alpers (www.internal- 
waveatlas.com) internal waves are present only spo 
radically in the North Sea, mainly in the summer 
months. An analysis of density fronts and residual cir 
culation of the German Bight, under minimal wind 
effects (Luyten et al. 2003) suggest that a southward 
flow can be expected at neap tide, while a northward 
flow can be expected at spring tide. This pattern 
should result in peaks of Carcinus maenas settlement 
on Helgoland around spring tides. In summary, several 
mechanisms may explain the abundance peaks around 
spring tides; however, the fact that spectral analysis 
shows a periodicity of 10 d, instead of 14 to 15 d, sug 
gests that further studies are necessary to elucidate the 
tidal mechanisms operating in the German Bight. 
Finally, the influence of emigration-immigration 
cycles of megalopae on colonisation rates should also 
be contingent on the absence of strong SW winds. This 
was the case in the 2005 and part of the 2003 
settlement period, with marked short-period (ca. 5 d) 
variability in colonisation rate. SW winds or eastward 
currents should lead to low settlement rates and a low 
number of megalopae colonising the sediment. 
In summary, variability in intertidal colonisation by 
megalopae of the shore crab Carcinus maenas in the 
North Sea may be related to wind-driven current and 
tidal conditions. Colonisation is low under strong and 
persistent eastward currents, whereas under other 
conditions (westward currents or during relaxation 
events), colonisation may be high and related to sping- 
neap tidal cycles. Future work should elucidate the 
mechanisms of larval transport in the North Sea and 
relationships between larval supply, colonisation and 
juvenile recruitment in the intertidal.