Short- and Long-Term Chlorophyll a Variability in the Shallow Microtidal Patos Lagoon Estuary, Southern Brazil |
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| Authors: | Paulo C Abreu Marli Bergesch Luis A Proen?a Carlos A E Garcia and Clarisse Odebrecht |
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| Institution: | (1) Institute of Oceanography—Federal University of Rio Grande—FURG, Cx. P. 474, Rio Grande, RS, 96201-900, Brazil;(2) University of Itaja? Valley– SC, Itaja?, SC, Brazil |
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| Abstract: | In the shallow microtidal Patos Lagoon estuary, southern Brazil (32° 07′ S–52° 06′ W), chlorophyll a (Chl a) variability was studied at different time scales during the last 25 years (hourly–daily sampling in 1984/1985; weekly sampling
in 1986 and from 1988 to 1990; monthly sampling from 1993 to 2008). Phytoplankton biomass variation seems to be most influenced
by hydrology, which is primarily driven by meteorological factors like wind, rainfall, and evaporation. However, it was observed
that the hydrological driving forces play different roles at different time scales. For instance, short-term Chl a variability is mainly controlled by winds, while long-term changes are related to the freshwater input by rainfall. Significant
correlation was found between the total amount of rain in the year and the mean annual value of Chl a, though this relationship was linear until 1,500 mm of rain per year. After this threshold, mean annual Chl a values dropped significantly, probably due to a washout of the produced biomass from the estuary. Similarly, low rainfall
levels and drought years lead to small phytoplankton biomass due to scarcity of nutrient, mainly silicate, or a possible inhibitory
effect generated by high ammonium concentration. In this sense, large-scale Chl a variability would be related to the El Ni?o-Southern Oscillation climatic anomaly, which influences the rainfall levels in
Southern Brazil, though sampling periodicity has also great influence on this relationship. No Chl a or nutrient enrichment was observed in the estuarine region along the last years, indicating that this estuary is not subject
to an eutrophication process. In contrast, signals of an ongoing oligotrophication are observed, possibly a remote effect
of the eutrophication in the Northern area of the lagoon where the phytoplankton nutrients uptake may act as a biological
filter mechanism. |
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