Abstract
Primary and bacterial productivity (PP and BP, respectively) were measured in the lower Itajaí-Açu estuary and in the adjacent shelf, under low river discharge conditions. The experiment was carried out in April, 2000, when physical, chemical and biological variables were recorded in five stations: three in the estuary and two in the inner shelf. Salinity, temperature, turbidity and light intensity were determined in vertical profiles; dissolved oxygen, nutrients, chlorophyll-a, primary (PP) and bacterial production (BP) were sampled at surface and near the bottom. The BP ranged from 0.5 to 11.3 mg C.m-3 .h-1 , and the highest values were recorded at the inner stations of the estuary, associated with lower salinity and light intensity and higher dissolved nutrients concentration. On the other hand, PP was higher in the inner shelf, with PP increasing from 2.0 to 30 mg C.m-3 .h-1 . The phytoplankton biomass showed the same trend, increasing from 0.2 to 8.4 µg/L at the surface. The PP was limited by light in the inner estuary and by the PO4 concentration in the inner shelf. The higher productivity zone was formed in front of the estuarine mouth, as expected, since water transparency increases leading the consumption of the rich-nutrient waters from the estuary by autotrophic organisms. The data indicated that the ecosystem metabolism shift from heterotrophic to autotrophic in a very small length scale (few kilometers) during the sampled conditions.
Highlights
Primary and bacterial production are usually complex, since it is influenced by factors such as river correlated in aquatic systems
The bacterial and primary production rates showed a shift in the lower Itajaí-Açu estuary and the adjacent shelf present from a heterotrophic metabolism to an autotrophic metabolism under low discharge condition
The formation of a region of higher productivity occurred in the shelf just in front the estuary mouth, where there was an increase in the levels of primary productivity and phytoplankton biomass
Summary
Primary and bacterial production are usually complex, since it is influenced by factors such as river correlated in aquatic systems This behavior suggests discharge, mixing processes due to tides and the several that phytoplankton is the most important source of different effects from human activities. These factors can organic matter supporting bacterial growth in these alter the balance between production and mineralization ecosystems. The metabolism lance between autotrophy and heterotrophy in aquatic of estuarine systems is greatly influenced by the ecosystems is important for the assessment of the flux entrance of nutrients and organic matter. To the mouth of estuary, phytoplankton blooms have been recorded with the occurrence of water discoloration and high biomass (Proença, 2004)
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