Onshore advection of warm water, larval invertebrate settlement, and relaxation of upwelling off central Chile

Abstract

Observations from several places around the world suggest that periods of relaxation of winds tha drive upwelling 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 evalu- ated. We examined the frequency, intensity, spatial extent, and inter-annual variability of sea surface temperature fluctuations associated with the relaxation of equator-ward winds and their influence on invertebrate settlement in central Chile (33° 30' S, 71° 40' W). Our results showed that there were 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 to 1°C) and preserved the stratification of the water column, large 'warming events' (>3°C) led to a breakdown of stratifica- tion for periods of 3 to 9 d at least twice during each spring-summer season. These large warming events occurred in association with downwelling-favorable (northerly) winds, and their formation might have required certain conditions in terms of mesoscale eddy features and local topography. Settlement of invertebrates occurred during these events as well at other times, and was not quanti- tatively correlated with relaxation or downwelling conditions. However, during specific large warm- ing events, we observed significant synchrony in the settlement of several marine invertebrate taxa (e.g. decapods, gastropods, polychaetes, mussels, and sea urchins). Thus, at this site in central Chile and during our period of study, relaxation events did not dominate settlement, and it appeared that the upwelling-relaxation model did not adequately represent the larval transport mechanism for any taxa examined. However, large warming events (which produced synchrony in the settlement of sev- eral different taxa) could be important in that they may result in uniform settlement along the shore and deliver larvae from distant origins, increasing long-distance demographic and genetic connectivity. Thus, these infrequent events are likely to be a critical factor in settlement distribution along the shore.