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) indicated the establishment of a herbivorous food web structure. Particulate organic carbon (POC) production was mainly due to phytoplankton (98%) and did not differ between periods. However, the observed variability in plankton trophic interactions should affect the magnitude of POC export from this dynamic system. KEY WORDS: Food webs · Carbon fluxes · Bacterioplankton · Phytoplankton · Zooplankton · Cabo Frio · South Brazil Bight Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 363: [109][110][111][112][113][114][115][116][117][118][119] 2008 small phytoplankton and bacteria to larger zooplankton and then higher pelagic trophic levels is mediated by heterotrophic nano-and micro-sized protists (Sherr et al. 1986). Therefore, in these microbial food webs, most of the biogenic carbon produced is recycled within the system through heterotrophic respiration (Legendre & Le Fevre 1995).In contrast, in more eutrophic systems with low vertical stability, larger phytoplankton cells are more representative and usually dominant, where turbulence simultaneously provides nutrient and prevents sinking below the euphotic zone (Kiørboe 1993). In those conditions an herbivorous food web prevails and most of the POC produced is exported via grazing, as the mesozooplankton are the dominant primary consumers (Legendre & Rassoulzadegan 1996). Moreover, carbon export is directly related to the input and uptake of 'new' nutrients, mainly nitrate (N-NO 3 ) to the euphotic zone (Dugdale & Goering 1967), and this nitrogen form is primarily utilized by larger cells, such as diatoms (Price et al. 1985).Coastal waters influenced by upwelling are among the most productive aquatic systems. The periodical wind-driven transport of surface waters offshore and the consequent upwelling of deeper water masses provide enrichment to previously oligotrophic waters. The high nutrient input to the surface waters, especially N-NO 3 , stimulates phytoplankton production, mainly for larger species, resulting in higher mesozooplankton and fisheries productivity. This pattern has been described for several upwelling zones around the world, such as those in the SE Pacific Ocean , Iriarte & Gonzalez 2004, Fernandez-Alamo & Farber-Lorda 2006, NE Pacific Ocean (Peterson et al. 1979, Collins et al. 2003, NE Atlantic Ocean (Bode et al. 2003), and Indian Ocean (Brown et al. 2002).Upwelling zones usually occur along eastern ocean boundaries, between 30°N and 30°S, due to the dominance of the trade winds. In the SW Atlantic, however, the change in the coastal direction at 23°S (Cabo Frio, SE Brazil) from N-S to E-W, along with the proximity of the 100 m isobath to the coast, allow the NE winds to move surface waters offshore and the consequent upwelling of the South Atlantic Central Water (SACW). This results in a high productivity core in the midst of the otherwise oligotrophic Brazilian Current waters (Valentin 1984a). The upwelling of deeper water masses is reversed when shifts in wind direction bring surface w...

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A B S T R A C TThe dynamics of the plankton compartments at the entrance of Guanabara Bay (SE Brazil) were assessed during a short-term temporal survey to estimate their trophic correlations. Size-fractioned phytoplankton (picoplankton: < 2µm, nanoplankton: 2-20µm and microplankton: > 20µm) biomass and photosynthetic efficiency, composition and abundance of the auto-and heterotrophic nano-and microplankton, and mesozooplankton were evaluated at a fixed station for 3 consecutive days at 3-h intervals, in the surface and bottom (20m) layers. The variability of almost all plankton compartments in the surface layer was directly dependent on temperature, indicating the great influence of the circulation at the entrance of the bay on plankton structure. In the surface layer, the mesozooplankton seems to be sustained by both autotrophic nano-and picoplankton, this last being channeled through the microzooplankton. Near the bottom, both auto-and heterotrophic microplankton are probably supporting the mesozooplankton biomass. Our findings thus suggest that the entrance of Guanabara bay presents a multivorous food web, i.e., a combination of both grazing and microbial trophic pathways. R E S U M OA dinâmica dos vários compartimentos do plâncton foi avaliada durante uma série de curta duração na entrada da baía de Guanabara (SE do Brasil), com o objetivo de estimar suas correlações tróficas. A biomassa e eficiência fotossíntética das três frações do fitoplâncton (picoplâncton: < 2µm, nanoplâncton: 2-20µm e microplâncton: > 20µm), juntamente com a composição e abundância do nano-e microplâncton auto-e heterótrofos e do mesozooplâncton, foram determinadas em uma estação fixa durante 3 dias consecutivos, a intervalos de 3h, nas camadas de superfície e de fundo (20m). A variabilidade de quase todos os compartimentos do plâncton na superfície foi diretamente relacionada à temperatura, indicando forte influência da circulação da entrada da baía na estrutura planctônica. Na camada superficial, o mesozooplâncton parece ser alimentado pelo nano-e picoplâncton autótrofos, esse último sendo sustentado pelo microzooplâncton. Próximo ao fundo, o microplâncton auto-e heterótrofo estão possivelmente sustentando a biomassa mesozooplanctônica. Nossos resultados sugerem, portanto, que na entrada da baía de Guanabara esteja estabelecida uma rede trófica multívora, i.e., uma combinação entre as cadeias microbiana e de pastagem.

Bacterial metabolic rates were assessed at the entrance to Guanabara Bay, SE Brazil, during the summer (i.e. wet) season, in order to evaluate short-term effects of tidal oscillation on the magnitude of carbon flow through the bacterioplankton. Bacterial production (BP), respiration (BR) and abundance, phytoplankton production and biomass, particulate and dissolved organic matter (POM and DOM) concentrations and elemental composition, nutrient concentrations and hydrological profiles were estimated at a fixed station during 3 consecutive days with intervals of 3 h, in surface and bottom (20 m) layers. The study period covered the end of the spring tide and the beginning of the neap tide. Bacterial metabolic rates were highly variable, and a small fraction of the dissolved organic carbon (DOC) pool was used by bacterioplankton (DOC turnover: 23 to 71 d). The input of allochthonous DOC, inferred from DOM elemental composition (C:N:P surface~1 50:9:1; C:N:P bottom 245:6:1), and temperature were the main controlling factors of bacterial carbon metabolism. Although the magnitude of carbon flow through the bacterioplankton was highly variable, there were nevertheless consistent differences between layers. At the surface, the particulate organic carbon (POC) production (~14.3 µM C h -1 ), mainly due to phytoplankton, was higher than BR, and bacterioplankton acted equally as POC producers and DOM remineralizers (bacterial growth efficiency ~52%). Near the bottom, BR was equivalent to total POC production (~0.3 µM C h -1 ) and higher than BP (~0.05 µM C h -1 ), and thus bacteria acted mostly as a DOM sink. This study highlights the importance of short-term variations in carbon flow through bacteria for understanding the carbon cycle of estuarine systems. KEY WORDS: Bacterial production · Bacterial respiration · Bacterial growth efficiency · Carbon flux · Guanabara Bay · EstuaryResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 50: 123-133, 2008 ration (BR), which represent, respectively, the amount of carbon incorporated into bacterial biomass and that used for ATP generation and thus lost as CO 2 (Jahnke & Craven 1995, del Giorgio et al. 1997.The sum of BP and BR provides an estimate of the total amount of organic carbon assimilated by bacteria, the bacterial carbon demand (BCD), as well as the proportion of carbon actually converted into bacterial biomass, the bacterial growth efficiency (BGE). The latter is a measure of the physiological state of bacterial cells, and can also be used as an indicator of the bacterioplankton role in a system: a high BGE indicates that bacteria are more important as a source of particulate organic matter, while a low BGE indicates that they act mainly as a sink of organic matter and a source of CO 2 and other inorganic nutrients (del Giorgio & Cole 1998).Estuaries and coastal bays are complex hydrodynamic systems, under both oceanic and terrestrial influence, driven by riverine input on the one hand and tidal oscillation on the ...

Light saturation curves of natural assemblages of phytoplankton at 4 stations in Cabo Frio coastal waters were examined and related to changes in environmental conditions, based on 263 experiments carried out weekly during two years . No differences in specific productivity at saturating light (Pm B ) between stations were detected . Global mean value of Pm B was 4.7 mgC mgChla -1 h -1 with a range from 0.50 to 15 .29. Significant seasonal variations were not observed . Correlation and regression analysis attributed most of PmB variation to temperature, salinity, nutrients and the phaeopigment : chlorophyll-a ratio . 145

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