Egg production, growth and development rates of Oithona similis were measured in the laboratory as a function of food concentration and composition. On an optimum diet, development is isochronal and growth is near exponential. The maximum juvenile growth rate at 15°C (0.2 day" 1) is similar to juvenile growth in calanoid copepods. The maximum weight-specific egg production rate (0.1 day" 1), on the other hand, is substantially less than in free-spawning calanoids, but similar to that in egg-carrying calanoids. In the Kattegat, Oithona spp. egg production is strongly limited by food during summer and controlled by temperature during winter. The seasonal signal in fecundity and population biomass is much weaker than in the co-occurring free-spawning calanoid genera, where fecundity and population biomass undergo dramatic seasonal viaration.

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Hydrography and spatial patterns of mesozooplankton communities in a frontal area off northern Patagonian were investigated in spring 1995 and 1998. Differential wind/tidal relative forcing appeared to drive along-shelf variations in the physical structure of the front and hence, planktonic patterns. Approaching the northern border of the system, the conspicuous presence of Oithona helgolandica, cladocerans and appendicularians, and the low fecundities of calanoid copepods suggest that a high proportion of the primary production may be channelled through microheterotrophic processes. Towards the southern area, calanoids dominated over cyclopoids, 'large' species (Drepanopus forcipatus, Calanoides cf. carinatus) were abundant, and copepod fecundities were high and steady, suggesting that there may be a more classical-like herbivorous food web. Paracalanus parvus and Ctenocalanus vanus occurred over the whole system but seemed to be key species in mixed and weakly stratified waters. Copepods peaked in stratified waters away from the front itself, while the abundance of eggs and nauplii was highest in weakly stratified waters next to the front. Apparently, the general heating occurring over the region in spring 1998 strongly affected both the physical and biological structure of the system. Unlike in 1995, a massive bloom of gelatinous zooplankton took place in 1998 and a bloom of the phagotrophic dinoflagellate Noctiluca sp. occurred in the northern area. Co-occurring copepod populations were depressed and their reproductive patterns seemed to be highly disrupted. Present observations stress the significance of variable external forcing in driving satisfactory habitat conditions for juvenile fish in the area.Key words: zooplankton, tidal front, food webs. RESUMEN: CARACTERÍSTICAS DEL MESOZOOPLANCTON EN UN ÁREA FRONTAL DE PATAGONIA NORTE (ARGENTINA) DURANTE LA PRIMAVERA DE 1995 Y 1998. -Los patrones de distribución del mesozooplancton fueron estudiados en un área frontal ubicada en la plataforma continental norpatagónica (Argentina) durante las primaveras de 1995 y 1998. Las variaciones latitudinales en la intensidad relativa de los forzantes viento y marea serían responsables de las diferencias observadas a lo largo de la estructura física del sistema. Estas diferencias llevan a patrones de composición y distribución del plancton también diferentes. Hacia el norte, la conspicua presencia de Oithona helgolandica, cladóceros y apendicularios, y las bajas fecundidades de copépodos calanoideos que se registraron, sugieren que allí una alta proporción de la producción primaria podría estar siendo canalizada a través de procesos microheterotróficos. Hacia el extremo sur, en cambio, los copépodos calanoideos dominaron sobre los ciclopoideos, fueron abundantes especies relativamente 'grandes' (Drepanopus forcipatus, Calanoides cf. carinatus) y las fecundidades de copépodos fueron altas, sugiriendo allí la existencia de una trama trófica clásica, básicamente herbívora. Los copépodos Paracalanus parvus y Ctenocalanus vanus e...

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