Bathymetric distribution of chaetognaths and their association with water masses off the coast of Valparaı́so, Chile

Abstract

The bathymetric distribution of chaetognaths was studied at an oceanic station located 14 nautical miles off Valparaı́so. During an annual cycle between July 1994 and September 1995, vertical samples of plankton were taken between 0 and 900 m depth. Temperature, salinity and dissolved oxygen showed temporal and spatial fluctuations within the surface layer (0–100 m), mainly affected by Subantarctic Water and in certain periods by Subtropical Water. Between 150 and 400 m low dissolved oxygen content and higher salinity were found, characteristic of Equatorial Subsurface Waters, and between 400 and 800 m Antarctic Intermediate Water, characterized by its higher content of dissolved oxygen and lower temperature and salinity was present. Fifteen species of chaetognaths were identified: Sagitta bierii, S. enflata, S. minima, S. pacifica, S. lyra, S. planctonis, S. marri, S. macrocephala, S. maxima, S. decipiens, S. tasmanica, S. gazellae, Krohnitta subtilis, Eukrohnia hamata and E. fowleri. The most abundant and frequent species were Sagitta enflata, S. bierii and Eukrohnia hamata. The greatest density of chaetognaths was confined to the 0–200 m water column and the species diversity gradually decreased from the epiplanktonic to the mesoplanktonic domain. The vertical distribution of the chaetognaths showed a strong association with the water masses present. The epipelagic species (0–200 m) S. enflata, S. bierii, S. minima and Krohnitta subtilis are associated mainly with Subantarctic Water and occasionally with Subtropical Water; the mesopelagic species (200–1000 m) S. lyra, S. planctonis, S. macrocephala, S. marri and E. fowleri are associated with Equatorial Subsurface Water and Antarctic Intermediate Water. There is another species assemblage of broad bathymetric distribution (0–900 m) consisting of E. hamata, S. decipiens and S. maxima, which does not show a distinctive association with a given water mass. E. hamata is an indicator species of upwelling events off the Chilean coast, showing an association with waters of low temperatures, greater salinity, and low contents of dissolved oxygen, which are characteristic of Equatorial Subsurface Water.