| Resumo: |
Ultraphytoplankton is composed by a group of organisms phylogenetically diverse, which contribute significantly to the total biomass and primary production in different oceanic and coastal pelagic systems. The success of ultraphytoplankton is attributed to its high efficiency in capturing light and nutrients, due to their small cell size and low sinking rate. The community structure is driven by the interaction among biotic and abiotic variables, such as temperature, nutrients, light and predators. The objective of this work was to assess the ecological importance of ultraphytoplankton organisms at the EPEA station, situated in the Buenos Aires coastal pelagic system (Argentina, SW Atlantic), during the period 2000-2009. In particular, ultraphytoplankton dynamics was analyzed throughout annual and interannual cycles and their strategies to adjust to the environment, as well as their relationships with other components of the microbial food web. Ultraphytoplankton taxonomic identification is usually difficult due to their small size. This study focused, for the first time in this region, on the characterization of the major groups: Synechococcus, picophytoeukariotes, Chrysochromulina like, and cryptophytes. The succession of these groups was related to the seasonal cycle of mixing-stratification in the water column. Cryptophytes developed successfully during winter and were progressively replaced by the Chrysochromulina like with the increase of stabilization in the water column. Finally, Synechococcus dominated the ultraphytoplankton community during the summer period of maximum stabilization. During autumn, when the vertical mixing began, the community was represented by a mixture of organisms belonging to the picophytoeukariotes, small diatoms and the chrysophyte Ollicola. Ultraphytoplankton biomass contributed between 50 and 90 % to total phototrophic biomass during the warm period. In order to understand more objectively the relationships between the main ultraphytoplankton populations and the environment, generalized additive models (known as GAM) were used. According to the first step of descriptive study of this community and the less detailed but methodical analysis of GAM, the following conclusions could be established: Synechococcus dominated the ultraphytoplankton biomass and its habitat was related to low nitrate concentrations, high stability and irradiance in the water column; its potential trophic link would be associated to the heterotrophic flagellates. The optimum habitat for the Chrysochromulina like was during early summer, when the water column was beginning to stabilize, and nutrients and light were not limiting their growth. These organisms have different strategies such as mobility, mixotrophy and production of allelopathic substances, which can favor their growth. Cryptophytes showed preference for cold waters and low light conditions. Picophytoeukariotes was a group taxonomically diverse, mainly composed by prasinophytes, prevailing in conditions of transition between stratified and mixed periods in the water column. This strategy would be associated with the high competitivty of these organisms for nutrients availability, and their adaptation to fluctuating irradiance levels. From a macroecological point of view, the annual trend of the main ultraphytoplankton components was regular, with few specific events detected during these first 10 years. Among these especial events, some algal blooms and one unusually high growth of the entire phytoplankton community were noticeable. An important observation to consider was the increase in phytoplankton biomass over the last years, related mainly to the increase in picophytoeukariotes. The knowledge obtained through this detailed study on the ultraphytoplankton in the first decade at the EPEA time series makes a substantial contribution to the characterization of the plankton community at the Bonaerense coastal system. These studies on the dynamics of this community are essential for a future prediction about the manner in which the ultraphytoplankton components can respond to natural, anthropogenic and climatic disturbances.
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