Diel vertical migration of decapod larvae in the Portuguese coastal upwelling ecosystem: implications for offshore transport

Abstract

The vertical distribution of decapod larvae off the northwest Portuguese coast was analysed in relation to associated environmental conditions from sampling during a 69 h period around a current meter mooring located on the shelf, approximately 21 km off the coast. Plankton samples were collected every 2 h at the surface with a neuston net and through the water column with a Longhurst Hardy Plankton Recorder (Pro-LHPR), allowing a very detailed resolution of larval vertical distribution. Environmental data (temperature, salinity, and chlorophyll a) were obtained every hour. To investigate the horizontal distribution of decapod larvae in relation to the coast, a plankton-sampling grid was carried out before the 69 h fixed station. Larvae of shelf decapod species were widely distributed over the shelf, while those of inshore species were found much closer to the coast. Decapod larvae (zoeae and megalopae) showed clear diel vertical migrations, only appearing in the upper 20 m at night, a migration that did not appear to be affected by physical conditions in the water column. Larval densities were highly variable, 0.01 to 215 ind. m -3 for zoeae and 0 to 93 ind. m -3 for megalopae, the zoeae being generally more abundant. The results indicated that during the day larvae accumulate very close to the bottom. The diel vertical migration behaviour is discussed as one of the contributing mechanisms for larval retention over the shelf, even with offshore transport conditions promoted by coastal upwelling, and is hence of major relevance for the recruitment success of decapod species that inhabit inshore and shelf zones of coastal upwelling systems.

Transport pathways of decapod larvae under intense mesoscale activity in the Canary-African coastal transition zone: implications for population connectivity

“…Studies of larval distributions conducted in the upwelling systems off California (Morgan et al 2009), Humboldt (Yannicelli et al 2006) and Portugal (dos Santos et al 2008) were consistent with the present results, describing how the larvae of shelf crustacean species were widely distributed over the continental shelf, but never far from the upwelling front, while those of inshore species were found much closer to the coast. However, these studies are not completely comparable with ours because in our study the lack of sampling in areas closer than 5 km to shore prevented the complete observation of the African larvae distributions.…”

“…Differences in larval advection patterns among nearshore species cannot be explained by environmental forcing because they are all exposed to the same physical processes. Diel and/or ontogenetic vertical migration behaviours have been stated as key factors for larval retention in upwelling areas (Queiroga and Blanton 2005, dos Santos et al 2008, Morgan et al 2009). We observed a vertical distribution pattern in which most of the larvae were concentrated in the upper 60 m, with little evidence of DVM.…”

Supply of crab larvae to an estuary in the eastern Atlantic upwelling system exhibits predictable and haphazard variation at different temporal scales

“…the internal wave hypothesis, which is invoked to explain shoreward transport of neustonic larvae over the shelf (Shanks & Wright 1987, Pineda 1999, or the flood tide hypothesis, which pertains to the immediate behavioural response of crab megalopae to changes in salinity or hydrostatic pressure during flood (DeVries et al 1994, Tankersley et al 1995, Welch & Forward 2001. However, because of the very short delays between maximum tidal range and maximum supply, and, because Carcinus maenas megalopae are not fully neustonic organisms (dos Santos et al 2008), we would favour the second hypothesis, which implies that the megalopae are already close to shore and are influenced by the tidal current.…”

Temporal genetic homogeneity among shore crab (Carcinus maenas) larval events supplied to an estuarine system on the Portuguese northwest coast

“…Spatial and temporal heterogeneity in oceanographic conditions affect larvae during development and promote episodic and variable supply back to adult populations (Almeida and Queiroga, 2003;Queiroga et al, 2006), thereby influencing which larvae actually recruit to the next generation. The current understanding of the oceanographic mechanisms coupled with larval behaviour shows that diel vertical migration (dos Santos et al, 2008) promotes retention of crab larvae in the inner shelf during spring and early summer, through interaction with the two-layered flow typical of upwelling circulation (Marta- . Relaxation of upwelling causes translocation of the larvae to the near-shore environment; supply into estuaries then occurs by selective tidal stream transport (for example, Queiroga et al, 2006).…”