Abstract
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.
Highlights
Hydrographic fronts are physically driven structures that highly influence ecosystem dynamics (e.g.Pingree et al, 1978; Le Fèbre, 1986)
In this study we explore the zooplankton patterns resulting from the changing biophysical conditions across the system, and the characteristics of the communities linked to along-shelf differences in the frontal features off northern Patagonia
The distribution patterns of jellyfish biomass we found are in good agreement with previous findings that indicate large aggregations of mainly Mnemiopsis leidyi in relation to transitional and stratified waters off Península Valdés (Mianzan and Guerrero, 2000), much higher concentrations of this species are recorded in the southern area (Alheit et al, 1991)
Summary
They are at present largely recognized as areas of enhanced primary and secondary production, and often related to major fishing grounds on continental shelves of certain regions (e.g. North Sea -Daan et al, 1990; Georges Bank -Sherman et al, 1988). The Patagonian shelf off Argentina is characterized by high tidal dissipation rates, which in nearshore waters generate a strong vertical mixing that may homogenize the whole water column. This feature, combined with the stratification of shelf waters induced by seasonal surface warming, leads to the occurrence during spring and summer of shelf sea fronts separating highly mixed coastal areas from stratified waters offshore. A major frontal zone is located near Peninsula Valdés (Fig. 1) extending southward along the Patagonian coast from ca. 42°
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