Before the construction of the Port, total cell density varied from 416,000 cells.l-1 to 5,748,000 cells.l-1 (Koening and Eskinazi-Leça, 1991). After the Port implantation, there was a strong decrease in phytoplankton density owing to the high loads of suspended material, and probably light limitation.
Low values (minimum of 121,000 cells.l-1) were registered in the rainy season, in spite of high nutrient avalability. In the dry season, when light intensity was higher, phytoplankton presented density maximum (1,474,000 cells.l-1) with low nutrients (Fig. 2).
Before the construction of the Port, 68 species of microalgae (60 species of diatoms, 5 of chlorophyceans and 3 of cyanophyceans) were registered (Eskinazi-Leça and Koening, 1985/86). The construction of the Port of Suape seemed to have altered the structure of the phytoplankton community in the estuary of the Ipojuca River, not only regarding the appearance of a greater number of species, but also in relation to the ecological. community characteristics. One hundred and thirty-three species were identified in our study: 80 species of diatoms, 28 of chlorophyceans, 12 of dinoflagellates, 7 of cyanophyceans and 6 of euglenophyceans (Table 1). These species number decreased from the station closer to the reefline to upstream. The reefline opening allowed seawater penetration and the presence of several dinoflagellate species (station 1). The high quantity of suspended material in the estuary favored the development of euglenophyceans, while considerable rainfall during the study year, contributed to the increase of chlorophyceans. The cyanophyceans, usually associated with fresh water habitats, were present in all stations. Diatoms predominated along the whole estuary, making up more than 60% of the planktonic flora. The increase in diatom species closer to the reef was due to higher salinity and nutrient concentrations, conditions favorable to the development of eurihaline diatom species. According to Levinton (1995), estuarine environments are characterized by abundant populations of few dominant species due to the great environmental variability. In the estuary of the Ipojuca River, although there was a considerable number of identified species, few of them presented high abundance: Chaetoceros lorenzianus, Climacosphenia moniligera, Cylindrotheca closterium, Gyrosigma balticum, Nitzschia sigma, Skeletonema costatum, Surirella febigerii, Terpsinoe musica and Oscillatoria tenuis. Two species (Gyrosigma balticum and Nitzschia sigma), were found in all stations and months, while most species presented low frequency. Before the Port construction, the marine dominant planktonic species were Biddulphia regia, Coscinodiscus centralis, Coscinodiscus granii, Chaetoceros lorenzianus, Chaetoceros curvisetus, Leptocylindrus danicus and Skeletonema costatum (Eskinazi-Leça and Koening, 1985/86). After the port construction, changes in the tidal cycle and in the circulation pattern caused a vertical mixing resuspending sediments together with littoral species Climacosphenia moniligera, Gyrosigma balticum, Licmophora abbreviata, Nitzschia sigma, Surirella febigerii and Terpsinoe musica.
Diversity and evenness indexes
Species diversity and evenness were high, demonstrating a relatively homogeneous distribution of species. The environmental instability, represented by various allogenic and endogenous factors, contributed to a environmental heterogeneity allowing presence of a diversified community increasing the number of niches and the incidence of species previously uncommon in the area. In some cases, high diversity was due to specialized species (k-strategists) or opportunists (r-strategists). In the last case it meant that levels of disturbance were affecting the ecosystem health (New,1995).
The specific diversity was around 3.0 bits.cel.l-1. Low value (-1) was caused by a bloom of Skeletonema costatum and Oscillatoria tenuis, which presented a relative abundance higher than 60%. The evenness oscillated between 0.4 and 0.9 (Figs. 3 and 4).
The cluster analysis of the samples showed three main groups (Fig. 5).
Group 1:all samples of the four stations gathered during the rainy season were clustered, including high and low tide, coinciding with the highest precipitation and lowest salinity. Due to precipitation, the influence of fresh water was observed up to the station closest to the reef (station 1).
Group 2: clustered samples from the dry season, and low and high tides of the four stations (flux of fresh water).
Group 3: included mainly the samples of station 1 with marine characteristics (high tide).
Clustering among three groups is evident (Fig. 6).
Group 1: This group was related to the marine flux, with higher concentrations of dissolved oxygen, transparency, salinity, pH and higher species diversity. In this group, Euglena acus, Chaetoceros lorenzianus, Bacillaria paradoxa and Campylodiscus clypeus were clustered.
Group 2:This was the biggest group, mainly constituted by phytoplankton species frequently found in estuarine environments with high concentration of chlorophyll-a and nutrients, which are partly brought in by the river and partly produced in the ecosystem itself. In this group, following species were clustered: Coscinodiscus oculusiridis, Scenedesmus quadricauda, Nitzschia sigma, Coscinodiscus sp., Terpsinoe musica, Pinnularia nobilis, Surirella febigerii, Melosira moniliformis, Nitzschia scalaris and Gyrosigma balticum. Some of these species occured practically in all months of the studied period at the four stations (Gyrosigma balticum, Nitzschia sigma, Surirella febigerii and Terpsinoe musica).
Group 3 : The species in this group were related to a greater biochemical demand of oxygen and occured mainly during the dry season, when there was a higher demand of polluting agents coinciding with low quantities of dissolved oxygen. They were typical anoxic areas influenced by mangroves. In this group, only Cylindrotheca closterium, Nitzschia longissima and not identified diatoms were clustered.
Even though the estuarine environment has high resilience, the impacts in the area of Suape were so pronounced that they caused changes in the ecosystem and related communities. The structure of the phytoplanktonic community was presently composed of littoral and fresh water species, besides a strong decrease in cell density when compared to studies before the port implantation (Koening and Eskinazi-Leça, 1991).