Patos Lagoon Estuary - Phytoplankton Time Series
southeastern Brazil
Latitude:   -32.0167     Longitude:   -52.1000

 Associated Investigators:
Clarisse Odebrecht ,
Paulo Cesar_Abreu  

Related Web Sites:
[www.peld.furg.br]  

The warm temperate shallow Patos Lagoon is the world largest choked lagoon (10,360 km2; 250 km long) with a 200,000 km2 wide watershed. About 80% of the lagoon area comprises predominantly fresh to oligohaline waters, but in the southern region the water exchanges with the Atlantic Ocean give rise to the microtidal estuary (approx. 1,000 km2). The outflow and inflow of fresh and marine waters occur through a stabilized inlet bounded by 4-km long jetties that were constructed at the beginning of the twentieth century (Seeliger et al. 1997). As typical for choked lagoons, water exchange with the coastal ocean is mainly controlled by winds and freshwater discharge, while tides are of minor importance (Moller et al. 2001). In general, northeast and southwest winds generate a pressure gradient between lagoon and nearby coastal region, forcing water outflow and inflow, respectively. During flood periods, the lagoon remains fresh for months and this condition is only altered by very strong southwest winds that generate a reverse outflow (Moller et al. 2001; Fernandes et al. 2002). Peaks of river discharge are associated with El Nino episodes (negative El Nino Southern Oscillation; ENSO Index), when rainfall significantly increases in the region, as observed during 1994/1995, 1997/1998, 2002/2003 and 2010 (see Annual anomalies). In contrast, a water deficit in summer/autumn facilitates seawater penetration northward into the lagoon. During these events, the central lagoon body becomes oligohaline with salinity gradients exceeding 150 km (Moller et al. 1996; Odebrecht et al. 2005) as observed during La Nina 1999-2000 (see Annual anomalies).

The high biological productivity, large fisheries, and the presence of a maritime port and an industrial conglomerate, justify the establishment of the Patos Lagoon Estuary and adjacent coast as a SITE of the Brazilian Long Term Ecological Research (BR_LTER), funded by the Brazilian Ministry of Science and Technology (PELD_CNPQ). The long-term project aims to identify the main natural and anthropic impacts on the Estuary of Patos Lagoon and adjacent coastal area.

In the Patos Lagoon Estuary sampling site (32 01 31 S, 52 06 22 W), monthly determinations of water temperature, salinity and Secchi disk depth, concentrations of dissolved inorganic nutrients and Chlorophyll a, and the abundance and composition of phytoplankton have been carried out since January 1993. Hourly and daily samplings were performed (1984/1985; 2004/2005) in order to detect the short-term variability of environmental properties and phytoplankton. Analyses are being conducted at the Laboratory of Phytoplankton Ecology and Aquatic Microorganisms of the Institute of Oceanography of the Federal University of Rio Grande - FURG, following classical methods for nutrients (espectrophotometry) and chlorophyll (fluorometry). Phytoplankton cells abundance is estimated using the sedimentation technique under the inverted microscope equipped with phase contrast.

The shallow water column at the sampling site (1.0-1.5 m) is well mixed due the action of winds, which suspends sediments and leads to high water turbidity (Secchi disk visibility generally 10-20 cm). Water temperature variations are typical of warm-temperate systems, with lowest values (10 C to 15 C) during the austral winter (July) and highest values (22 C to 30 C) from January to March. Owing to winds, significant short-term salinity oscillations occur in the hourly/daily scale (Fujita and Odebrecht 2007; Abreu et al. 2010). The high short-scale variability tend to mask the seasonal salinity pattern, but it is clear that brackish to euryhaline water tend to be more frequent in summer/autumn (January to June) and fresh to oligohaline conditions prevail in winter/spring (July to October). The seasonal pattern of salinity is related to the annual balance between rainfall and evaporation in the lagoon and to the horizontal advection of seawater inflow caused by winds.

Chlorophyll a concentrations in the estuary (mean 7.7 ug L-1,+/-11.4) present significant short-term temporal variation, but a general pattern is observed of higher values in spring/summer (10 to 70 ug L-1), mainly due to the growth of coastal euryhaline microplankton Diatoms. During prominent low salinity periods, phytoplankton is composed of freshwater cyanobacteria and oligohaline/euryhaline Diatoms, the most conspicuous events being observed following El Nino periods and high freshwater discharge (1998, 2003). Nitrogen-fixing filamentous heterocytic cyanobacteria and the colony forming toxic Microcystis were abundant during a three months period in 2003 (Odebrecht et al. 2010).

In the long-term, a positive correlation between mean annual Chlorophyll a and annual rainfall indicates that phytoplankton growth and biomass accumulation in the estuary are closely related to freshwater discharge and water residence time (Abreu et al. 2010). This relationship disappears when rainfall exceeds 1500 mm yr-1 because the large amount of freshwater discharged from the basin flushes the phytoplankton biomass out of the estuary.

In general, statistical analyses do not show many significant correlations between Chlorophyll a and other environmental factors (see Multi-variable comparison & Correlation Plot), probably due to the high short-term variability in the estuary. However, a significant positive temporal trend along the years of Temperature and Total and Centric Diatoms becomes evident, as well as a strong correlation between the Diatoms and Temperature (see Multi-variable comparison & Correlation Plot), indicating they increase mainly during late spring/summer. An increasing temporal trend along the years is also observed for Dinoflagellates and Chlorophytes but these are not correlated with temperature, while Flagellates showed a decrease along the time series (see Annual anomalies).

It is clear that the information based on monthly sampling at one single station provides only poor resolution of the ecological processes in the estuary. However the long-term time series obtained until here in one sampling station in the Patos Lagoon estuary represents an important baseline, also providing clear responses of phytoplankton to processes related with large-scale events due to climatic changes. Moreover, this long-term study will demonstrate possible human impacts as anthropogenic eutrophication. However, so far nutrient increase due to domestic and industrial effluents were not observed at the Patos Lagoon estuary probably due to intense hydrodynamics of this ecosystem.

References:

Abreu, P.C.; Bergesch, M.; Proenca, L.A.; Garcia, C.A.E. & Odebrecht, C. 2010. Short- and Long-Term Chlorophyll a Variability in the Shallow Microtidal Patos Lagoon Estuary, Southern Brazil. Estuaries and Coasts 33: 554-569. DOI 10.1007/s12237-009-9181-9.


Fernandes, E.H.L.; Dyer, K.R.; Moller, O.O. & Niencheski, L.F. 2002. The Patos Lagoon hydrodynamics during an El Nino event (1998). Continental Shelf Research 22:1699-1713.


Fujita, C. & Odebrecht, C. 2007. Short term variability of chlorophyll a and phytoplankton composition in a shallow area of the Patos Lagoon estuary (Southern Brazil). Atlantica, Rio Grande 29(2):93-107.


Moller Jr., O.O.; Lorenzetti, J.A.; Stech, J. & Matta, M.M. 1996. The Patos Lagoon summertime circulation and dynamics. Continental Shelf Research 16(3):335-351.


Moller Jr., O.O.; Castaing, P.; Salomon, J.C. & Lazure, P. 2001. The influence of local and non-local forcing effects on the subtidal circulation of Patos Lagoon. Estuaries 24(2):297-311.


Odebrecht, C.; Abreu, P.C.; Moller, O.O.; Niencheski, L.F.; Proenca, L.A. & Torgan, L.C. 2005. Drought effects on pelagic properties in the shallow and turbid Patos Lagoon, Brazil. Estuaries 28(5):675-685.


Odebrecht, C., Abreu, P.C.; Bemvenuti, C.E.; Copertino, M.; Muelbert, J.H.; Vieira J.P. & Seeliger, U. 2010. The Patos Lagoon Estuary: Biotic responses to natural and anthropogenic impacts in the last decades (1979-2008). In: Coastal Lagoons: Critical Habitats of Environmental Change (Eds.: M.J. Kennish; H.W. Paerl) Marine Science Series, Taylor Francis, CRC Press.


Seeliger, U.; Odebrecht, C. & Castello, J.P. 1997. Subtropical Convergence Environments: the Coast and Sea in the Southwestern Atlantic. Springer Verlag. Berlin. 308 p.