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The aim of this study was to investigate the biotic and abiotic factors controlling the spring phytoplankton blooms at the Patagonian shelf-break front (PSBF). Using a CHEMTAX analysis of HPLC pigment data and other methods, the biomass and spatial variability of plankton communities were studied in four sections (39-48°S) across the PSBF during October 2005. Environmental factors and the biomass and composition of plankton communities exhibited a marked spatial heterogeneity. The latitudinal and cross-shelf progression in the timing of the spring bloom initiation and the nutritive properties of the water masses (Subantarctic Shelf Waters and Malvinas Current Waters) seemed to be the key factors. Three plankton regions were distinguished: a) Outer shelf (OS) b) Shelf-break front (SBF) and c) Malvinas Current (MC). At the highly stratified OS region, the postbloom community showed low-biomasshigh-phytoplankton diversity formed mainly by small cells (haptophytes 30-62%, diatoms 17-49%, chlorophytes 0-34%, and prasinophytes 0-21% of total Chl a). High amounts of degraded fucoxanthin were found associated with the heterotrophic dinoflagellate, Protoperidinium capurroi. Grazing by this microheterotroph on the diatom population seemed to be the most important factor for the spring bloom decay at the OS. A remarkable quasi monospecific bloom (~90%) of a nanodiatom (Thalassiosira bioculata var.raripora) associated with high Chl a (up to 20 mg m-3) occurred along (~1000 km) the SBF and in the most northern extension of the MC. In the southern region, the bloom was developed under absent or incipient density stratification, increasing solar irradiance, high nitrate and phosphate availability, and low numbers of phytoplankton grazers. The average mixedlayer PAR irradiance (< 2.0 mol quanta PAR m-2 d-1) and Si:N ratios (< 0.2) were low, suggesting a diatom population limited by light and under progressive silicate limitation. The more stratified northern region of the SBF showed a later stage of the bloom development, but the large population of diatoms under Si limitation was not in senescence and losses from microzooplankton grazing were minor. The observed high proportion of Chl a below a shallow upper mixed layer (up to 85 %) could directly reach the bed, favoring the development of epibenthic communities and the formation of seed diatom banks and organic iron-rich sediments. The upwelling along the SBF provides a large source of macronutrients and probably the dissolved iron needed to sustain the intense diatom bloom, but also diatom resting stages that could act as seeds for the next spring bloom. The macronutrient-rich MC region showed low chlorophyll (Chl a< 0.8 mg m-3) and a highly diverse phytoplankton community, mainly composed of small cells (diatoms 20-70%, haptophytes 20-40%, chlorophytes 2-25%, prasinophytes 2-18%, and cryptophytes 3-12% of total Chl a).

It has been suggested that the Subtropical Shelf Front (STSF) could be a preferential site for the detrainment of Subantarctic Shelf Water (SASW) and related planktonic shelf species onto the open SW Atlantic Ocean. The offshore detrainment of SASW and planktonic shelf species might be an exportation mechanism, affecting the population abundances of fishing resources in Argentina, Uruguay, and Southern Brazil. In this study, we characterize for the first time the 3-D structure of the STSF and the main routes of offshore export of SASW from the Patagonian shelf during austral summer (summer and early fall) and winter (winter and early spring) by using numerical hydrodynamical model results and Lagrangian tracking simulations of neutrally buoyant floats. The transport of SASW toward the open ocean is~1 Sv (1 Sv = 10 6 m 3 /s) during summer and~0.8 Sv during winter. SASW are exported offshore mainly near the Brazil-Malvinas Confluence region during both seasons. The STSF appears to act as an important retention mechanism for the plankton over the inner and middle shelf mainly during late summer and early fall. Our findings could explain the life cycle of distinct fish species that are distributed in the region, as well as the population abundance variability of such species.Plain Language Summary The Subtropical Shelf Front (STSF) has been suggested to be a preferential site for the detrainment of Subantarctic Shelf Water (SASW) from the Patagonian continental shelf toward the open ocean. This offshore detrainment of SASW might be an exportation mechanism to the open ocean of early larval stages of life of shelf fish species, affecting the population abundances of fishing resources in Argentina, Uruguay, and Southern Brazil. In this work we characterize the 3-D structure of the STSF, its dynamics and variability, and the main routes of offshore export of SASW during austral summer and winter for the first time. We discover that the main offshore detrainment of SASW occurs near the Brazil-Malvinas Confluence, while the STSF appears to act as an important retention mechanism for early larval stages of life of shelf fish species mainly during late summer and early fall. Our findings could explain the life cycle of distinct fish species that are distributed in the region, as well as the population variability of such species.

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Blooms and strandings of Chrysaora plocamia are reported to occur along both Atlantic and Pacifi c South American coasts. First described in Peruvian waters by Lesson (1830) almost two centuries ago as Cyanea plocamia , there is surprisingly little ecological information about this conspicuous animal. This chapter reviews current knowledge about C. plocamia biology and ecology, its relationship with pelagic fi sheries and climate and the problems blooms cause in the Humboldt Current and Patagonian shelf ecosystems. Chrysaora plocamia has important ecological roles, including trophic and symbiotic interactions with fi sh and sea turtles. Population variability has a clear relationship with climate where phases of high C. plocamia biomass were associated with El Niño events occurring during warm "El Viejo" regimes. Interestingly, their estimated biomass occasionally approached those of sardines or anchovies. This large jellyfi sh negatively affects human industries in the region when abundant, including fi sheries, aquaculture, desalination plants and tourism. Understanding relationships between jellyfi sh blooms and environmental drivers (e.g. ENSO, regime shifts) should allow forecasting of the jellyfi sh abundance and potential vulnerabilities such that resource managers and industrial fi sheries owners may prepare for costly outbreaks.

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