Seasonally variable riverine production in the Venezuelan llanos

We examined primary production, respiration, and nutrient dynamics in littoral areas of the mainstem and lagoons of the Cinaruco, a nutrient-poor river in the Venezuelan llanos. Gross primary productivity (GPP) was relatively high, given the poor nutrient conditions in this river. Seasonal variability in net ecosystem production (NEP) was also high, with highest values occurring in the dry season (March-April) when fish biomass and chlorophyll a (both water-column and benthic) levels were greatest (dry-season NEP = 542 mg C m ^-2 d^-1, wet-season NEP = 303 mg C m^-2 d ^-1). NEP and algal biomass (measured as chlorophyll a) were higher in lagoons than at river sites, with more pronounced differences between these 2 habitats during the dry season. Strong N limitation was evident. Dissolved inorganic N (DIN) concentrations always were <2 μM and typically were <0.5 μM. Molar ratios of DIN:SRP (soluble reactive P) varied little seasonally and were always <8. As in other Neotropical aquatic systems, water-column productivity was an important source of organic matter and was >2x benthic productivity, even in shallow regions of the riverine ecosystem. Low nutrient levels combined with high rates of autochthonous productivity in the littoral zone of this river suggest extremely rapid nutrient cycling rates and support the view that the littoral regions may be important in providing the organic matter that maintains secondary production, consistent with the Riverine Productivity Model and other observations in the Orinoco Basin.