Onshore advection of warm water and larval invertebrate settlement during relaxation of upwelling off central Chile
1 Estación Costera de Investigaciones Marinas & Center for Advanced Studies in Ecology and Biodiversity, Pontificia Universidad Católica de Chile, Casilla 114-D, Santiago, Chile.
2 Bodega Marine Laboratory, University of California Davis, PO Box 247, Bodega Bay, CA 94923, USA.
3 Current address: Departmento de Oceanografía y Estación de Biología Marina Dichato, Casilla 160-C, Concepción, Chile.
ABSTRACT
Observations at a several places around the world suggest that periods of relaxation of upwelling winds favor the onshore transport of larvae entrained in shoreward movement of warm surface waters. However, the generality of this process has not yet been appropriately evaluated. Here we examine the frequency, intensity, spatial extent and inter-annual variability of sea surface temperature fluctuations associated with the relaxation of equatorward winds and their influence on invertebrate settlement in central Chile (33° 30’ S; 71° 40’ W). Our results show that there are marked differences in the intensity and structure of temperature increases following the relaxation of upwelling favorable winds. While most temperature increases were small (0.5-1 °C) and preserved the stratification of the water column, large “warming events” (> 3 °C) led to a breakdown of stratification for periods of three to nine days at least twice during each spring-summer season. These large warming events occur in association with downwelling-favorable (northerly) winds. However, not all northerly winds resulted in similar large temperature increases. Satellite images show onshore advection of warm waters between 30 and 35°S, but not further to the north. Settlement of invertebrates occurred during these events and at other times and this is not quantitatively correlated with relaxation or downwelling conditions. However, during the specific large warming events, we observed significant synchrony in settlement of several marine invertebrate taxa (e.g. crustaceans, gastropods, polychaetes, bivalves and sea urchins). Thus, at this site in central Chile, relaxation events do not dominate settlement and it appears that the upwelling-relaxation model does not adequately represent the larval transport mechanism for any of the species examined. However, the large warming events, which produce synchrony in settlement of several different species, are important in that they result in uniform settlement along the shore and they deliver larvae from distant origins, increasing long-distance demographic and genetic connectivity. Thus, these infrequent events could be a critical factor in homogenizing settlement along the shore.
1 Estación Costera de Investigaciones Marinas y Centro de Estudios Avanzados en Ecología y Biodiversidad, Pontificia Universidad Católica de Chile, Casilla 114-D, Santiago, Chile
2 Center for Coastal Physical Oceanography, Department of Ocean, Earth, and Atmospheric Sciences, Old Dominion University, Crittenton Hall, 768 West 52nd Street, Norfolk, VA 23529, USA.
3 Current address: Departmento de Oceanografía y Estación de Biología Marina Dichato, Casilla 160-C, Concepción, Chile.
4 Programa Regional de Oceanografía Física y Clima, University of Concepción. Cabina 7. Barrio Universitario s/n. Concepción, Chile.
ABSTRACT
Time series and shipboard observations of hydrography and current velocity are used to describe the motion of the Maipo river plume and its influence on inner shelf hydrographic conditions. Observations show fundamental seasonal changes in the forcing of the plume. During austral fall-winter, rapid and irregular changes in salinity were closely associated to rainfall patterns and the subsequent changes in river discharge, which produced drops in onshore-measured salinity that usually lasted several days. In contrast, during spring-summer salinity exhibited a clear diurnal cycle, not associated to river discharge. With daily regularity, the lowest salinity at a location 12 km north (upstream) of the river mouth was observed in late evening, between 17:00 and 22:00 hrs. . Salinity was high again in the morning and early afternoon, between about 2:00 and 14:00 hours. Spatial variability determined from shipboard surveys indicated that during spring-summer, Maipo river waters (~32.2 PSU) strongly influence the surface density field of the study area, but the salinity effect was always shallower than 10 meters depth, characterizing this plume as ‘surface advected’ and allowing a relatively rapid response to daily variation in wind stress. Local wind, current and tides data showed that the primary forcing of the plume is the sea breeze. Breezes that exceed 5 m s-1, which occur regularly in central Chile, produce frictional forces that drive the onshore salinity changes with a 7 hour delay. Earth’s rotation effects modulate the river plume and can drive the plume spatial distribution in the absence of strong onshore wind, as observed from shipboard measurements during winter months. Seasonal variation in river plume spatial distribution was also apparent in satellite images. This constitutes one of the few studies that document diurnal switching of a river plume caused by wind forcing and we discuss the consequences for the inner shore environment.
Estación Costera de Investigaciones Marinas
Undergraduate-degree thesis:
Actual Researchs