Appendix A. Sampling and laboratory methods, biology of Carcinus maenas, and characteristics of the study site.
Sampling and laboratory
The collection of individuals was made in the Island of Helgoland (Fig. A1) using traps deployed at low tide, placed randomly, between MHW and MLW, changing the position each day; traps were spaced about 10 m, but the distance varied with day . The traps were made of a PVC ring of 4 cm width and 314 cm2 surface area with a nylon mesh of 500 μm at the bottom. They were filled with natural defaunated coarse gravel and cobble. Each day, traps were carried out to the laboratory, megalopae were separated from the sediment, identified and used for experiments. Data on patterns of colonization and its relationship with transport processes were reported in Giménez and Dick (2007).
Experiments to evaluate juvenile growth were done under controlled conditions of temperature (18°C), salinity (32‰) and photoperiod (12:12), using filtered (1μm) seawater. Larvae and juveniles were fed with freshly hatched Artemia sp. nauplii; water and food was changed every other day. Megalopae were reared in glasses of 300 mL with plastic gauze at the bottom, under ad libitum food conditions or without food, depending on the experiment (see below), until metamorphosis to the first juvenile crab. Juveniles were reared in plastic compartments; most juveniles were reared until they moulted to the second stage (J-II), but in 2003 and 2005 a random sample of the collected individuals was reared until the stage J-V.
The study site and its biologicaloceanographic context
The study was carried out in the island of Helgoland, located in the German bight, North Sea, at approximately 64 km from the shore (Fig. A1a). The German Bight is characterized by complex hydrodynamics and by a diverse number of frontal structures (Otto et al. 1990, Dippner 1993a, 1998). There are three main types of fronts: (a) the seasonal thermal fronts separating the well mixed coastal waters from the thermally stratified waters of the central North Sea; these fronts are located away from the study site. (b) Upwelling fronts are caused by easterly winds but they are local and intermittent. (c) Plumes fronts associated to the Elbe River are permanent and the most important features around the island of Helgoland (Figs. A1b, c) and the Eastern sector of the Wadden Sea. Circulation patterns affect the dynamics of these fronts and associated eddies. Tidal variations contribute to front displacement (Krause et al. 1986). Most wind conditions lead to the formation of a clear outer front; the front may change its position according to the wind, but it is located more or less parallel to the German and Danish coasts (Fig. A1b). However, under SW or S winds, water form the central North Sea are transported towards NW and the front is destroyed in most areas or advected onshore towards the mouth of the Elbe River. In this situation no eddies are present and the circulation is characterized by an anticlockwise pattern (Fig. 1c). Skov and Prins (2001) also mention the presence of a not-well studied inner front. These estuarine fronts have a positive effect on the recruitment of fish larvae (Dippner 1993b) and the accumulation of food for piscivorous birds (Skov and Prins 2001).
|FIG. A1. Location of Helgoland and predicted plume frontal dynamics in the German Bight (after Dippner 1993, 1998): (a) Location of sampling site (arrow) off Helgoland (54° 10' 67.0'' N, 7° 53' 54.3'' E); (b) (c) predictions of approximate frontal position (lines) under different wind conditions. In (b) circulation is complex due the presence of several eddies (not shown); in (c) circulation (line arrows) is anticlockwise in the German bight and most fronts are destroyed.|
Early life history of Carcinus maenas
The life history, larval, and juvenile development of this crab is well known, (Anger 2001, Moksnes 2004, Queiroga and Blanton 2004, Giménez and Dick 2007). The larval development of C. maenas occurs in the coastal ocean waters and takes about 46 weeks depending on temperature (Dawirs 1985). Larvae are planktotrophic, i.e., they relay on external food sources to complete their development, being therefore vulnerable to starvation periods or changes in food quality (Dawirs, 1984, Harms and Seeger, 1989). Larvae are transported by wind-driven and tidal currents. From late spring to early autumn, the megalopa stage colonizes the intertidal zones of estuaries and open sheltered shores. In Helgoland, high settlement rates occur around spring tide periods when winds are not from SW; if strong SW winds prevail for long periods (> 4 d) settlement in consistently low (Giménez and Dick 2007).
The juveniles remain in the intertidal and grow very fast during the summer, but growth rate is density-dependent, with smaller individuals growing slower at high densities (Moksnes et al. 2003, Moksnes 2004). Juvenile survival is affected by cannibalism until individuals reach a refuge in size. In the study area, juveniles occur in complex substratum, mainly in gravel shores, but also in beds of turf algae and mussel patches.
Dawirs, R. 1984. Influence of starvation on larval development of Carcinus maenas L. (Decapoda: Portunidae). Journal of Experimental Marine Biology and Ecology 80:4766.
Dawirs, R. 1985. Temperature and larval development of Carcinus maenas (Decapoda) in the laboratory: predictions of larval dynamics. Marine Ecology Progress Series 24:297302.
Dippner, J. 1993a. A frontal-resolving model for the German Bight. Continental Shelf Research 13:4966.
Dippner, J. 1993b. Larvae survival due to eddy activity and related phenomena in the German Bight. Journal of Marine Systems 4:303313.
Harms, J., and B. Seeger, 1989. Larval development and survival in seven decapod species (Crustacea) in relation to laboratory diet. Journal of Experimental Marine Biology and Ecology 133:129139.
Krause, G., G. Budeus, D. Gerdes, K. Schaumann, and K. Hesse.1986. Frontal systems in the German Bight and their physical and biological effects. In: Nihoul, J. (ed.) Marine interphases ecohydrodynamics. Elsevier, Amsterdam, p 119–140.
Otto L, J. Zimmerman, G. Furnes, M. Mork, R. Saetre, and G. Becker. 1990. Review of the physical oceanography of the North Sea. Netherlands Journal of Sea Research 26:161238.
Queiroga, H., and J. Blanton. 2004. Interactions Between Behavior and Physical Forcing in the Control of Horizontal Transport of Decapod Crustacean Larvae. Advances in Marine Biology 47:107214.
Skov, H., and E. Prins. 2001. Impact of estuarine fronts on the dispersal of piscivorous birds in the German Bight. Marine Ecology Progress Series 214:279287.