Phytoplankton in a tropical estuary , Northeast Brazil : composition and life forms

We aimed verify the composition of the phytoplankton community and this life forms that occur in the Capibaribe River estuary, Pernambuco, Brazil. This is a highly impacted ecosystem by anthropic activities. We collected samples of the phytoplankton community at three stations, during three months of each season: dry, from October to December 2010; rainy, from May to July 2011. We collected samples during the low and high tide, at the spring tide. We classified the species based on life forms. We identified 127 taxa, and the majority of species were freshwater planktonic form (FP; 30%), followed by marine oceanic planktonic (MOP; 25%), marine neritic tichoplanktonic (MNT; 22%) and planktonic (MNP; 19%,), and tichoplanktonic estuarine (TE; 3%) and freshwater (TF; 1%). The majority of species identified were diatoms, since it assumes the most variability of life forms, therefore enabling its presence in the different portions at the estuary.


INTRODUCTION
Estuaries are high dynamic ecosystems affected by marine and limnetic conditions, such as changes in the river flow and marine tides, enabling different characteristic zones along the estuary.Thereby, it is observed great variations of the environmental parameters (Miranda et al. 2002).As a result, the phytoplankton community rapidly responds to these environmental changes (Cloern and Jassby 2010).Phytoplankton are one of the main primary producers in aquatic ecosystems, as well as is considered excellent bioindicators of environmental quality (Reynolds 2006).
The different life forms in the phytoplankton species are dependent, in general, on their response to the heterogeneity of habitats, besides the tolerance to oscillations in the environmental, which can resuspend or deposit cells on the bottom.The knowledge of composition and life forms of the biotic communities is a necessary tool to understand the mechanism and the ecological importance of aquatic ecosystems (Eskinazi-Leça et al. 2004;Cloern and Jassby 2010).
In this context, our study aimed to analyze the composition of the phytoplankton community and the main life forms of species occurring in the Capibaribe River estuary (Pernambuco), which is an important aquatic body in Northeast Brazil.

Study Area
The Capibaribe River estuary is located in the downtown of the Recife City (Pernambuco state, Northeast Brazil; Figure 1).Because it is located in an area of high degree of urbanization, waters are eutrophized and strongly affected by anthropic activities, mainly due to discharge of domestic and industrial effluents.As consequence, occur high concentrations of ammonia, phosphorus, heavy metals, and thermotolerant coliforms, and turning the levels of dissolved oxygen undetectable (SRH 2010).

Data sampling and analysis
We collected samples for phytoplankton analysis at three sites along the estuary: upstream (S1), downstream (S3), and one intermediate site (S2) along the river.We conducted sampling during three months of each climatic seasons, in the dry (October to December 2010) and rainy (May to July 2011) season, and during high and low tide in the same day, in spring tide.
The majority of species identified showed the life forms compatible with FP, corresponding to 30% of the total and consisting mainly of species of chlorophytes and cyanobacteria.Species of MOP represented 25% of the total, followed by MNT and MNP, with 22% and 19%, respectively.Ultimately, species of TE and TF were represented with 3% and 1% of the total, respectively (Table 1).All dinoflagellates identified in the Capibaribe River estuary marine and planktonic life forms, and also other 26 species of diatoms (Table 1).Only Fragilaria capucina Desmazières were part of the FT category (Table 1).Two species of diatoms were part of the TE, Terpsinoë musica Ehrenberg and Gyrosigma balticum (E.) Rabenhorst.
We classified the life forms to only ones the organisms identified until species level.For diatoms,

DISCUSSION
In tropical estuary ecosystems diatoms are the main components of the planktonic flora (Fujita and Odebrecht 2007;Masuda et al. 2011;Borges et al. 2012).Studies with phytoplankton species demonstrated that the predominance of diatoms in estuarine ecosystems is not only due to its high rate of division, but also to its euryhaline ability (Ribeiro et al. 2003).
More recent approaches focusing the phytoplankton community in ecosystems nearby of the Capibaribe River have shown that diatoms are the main organisms of the community.It is explained by the higher levels of silicate in these waters and thus, its benefits the diatom population (Santiago et al. 2010;Borges et al. 2012).In addition, our results suggest that the greater occurrence of diatoms in estuaries, comparing with other groups, is due to the different life forms that this population can assume (Table 1).
Many species that are present in estuaries are originally from the freshwaters, such as chlorophyceans and cyanobacteria species.These species are transported downstream by the river's flow (Masuda et al. 2011).The occurrence of some freshwater cyanobacteria species in our study show that these species are present in some freshwater ecosystems in Northeast Brazil, mostly in rivers and reservoirs that are components of the watershed of the Capibaribe River (Dantas et al. 2012).
On the other hand, dinoflagellates identified during our study were essentially marine and planktonic.These organisms are present in the estuarine environment due to the tide regime, which is responsible for their transport into the estuary (Trigueros and Orive 2000).T. musica and G. balticum are tycoplanktonic estuarine diatoms.Both species are present in other tropical estuaries in Brazil (Fujita and Odebrecht 2007;Leão et al. 2008;Santiago et al. 2010;Masuda et al. 2011), while F. capucina is the only tycoplanktonic freshwater form in the present study, commonly reported in other benthonic substrates (Roberts et al. 2004;Antoniades et al. 2005).The three diatoms mentioned are in water column due to the high dynamism of the estuarine ecosystem, which induces mixture and resuspension of organisms from the bottom (Fujita and Odebrecht 2007).