RESUMEN
The ability of marine invertebrate larvae to control their vertical position shapes their dispersal pattern. In species characterized by large variations in population density, like many echinoderm species, larval dispersal may contribute to outbreak and die-off phenomena. A proliferation of the ophiuroid Ophiocomina nigra was observed for several years in western Brittany (France), inducing drastic changes on the benthic communities. We here studied the larval vertical distribution in this species and two co-occurring ophiuroid species, Ophiothrix fragilis and Amphiura filiformis, in two contrasting hydrodynamic environments: stratified in the bay of Douarnenez and well-mixed in the bay of Brest. Larvae were collected at 3 depths during 25 h within each bay. In the bay of Brest, all larvae were evenly distributed in the water column due to the intense vertical mixing. Conversely, in the bay of Douarnenez, a diel vertical migration was observed for O. nigra, with a night ascent of young larvae, and ontogenetic differences. These different patterns in the two bays mediate the effects of tidal currents on larval fluxes. O. fragilis larvae were mainly distributed above the thermocline which may favour larval retention within the bay, while A. filiformis larvae, mostly concentrated near the bottom, were preferentially exported. This study highlighted the complex interactions between coastal hydrodynamics and specific larval traits, e.g. larval morphology, in the control of larval vertical distribution and larval dispersal.
Asunto(s)
Organismos Acuáticos , Equinodermos , Ambiente , Animales , Bahías , Larva , Densidad de Población , Dinámica Poblacional , Movimientos del AguaRESUMEN
The implementation of the European Water Framework Directive requires new tools for predicting the effect of expected measures taken in the watershed on water quality at the scale of large regional river systems. In the Seine basin, four models, developed in a research context, have been chained to each other to simulate water quality and biogeochemical functioning of the hydrosystem from headwater streams to the coastal marine area. All four models are based on a similar deterministic approach and share a common description of the biogeochemical processes, allowing them to exchange information. Each model differently represents the hydro-sedimentological processes, and uses different time and space resolution, in order to tackle with the specific problematic of each sub-system. This cascade of models has been used for testing a prospective scenario of water resources management at the horizon of 2015, established by Water Authorities of the Seine-Normandy district. The simulation predicts a general improvement of water quality concerning those variables linked to point sources of pollution (ammonium, oxygen, phosphate), even if, locally, this improvement can be insufficient for meeting the expected quality standards. The predicted improvement of the quality of the Seine River downstream from Paris and its estuary is large. However, the predicted very significant drop of phosphate contamination, although beneficial for limiting the problems of coastal marine eutrophication, does not lead to a significant control of phytoplankton development in the rivers upstream from Paris. The simulation also predicts a general increase in nitrate contamination mainly linked to diffuse sources from agricultural areas.