**4. Environmental conditions**

The lagoon has a mean depth (<60 cm) and the Secchi disk depth generally coincides with the total depth. The water temperature varies between 15.3 and 17.6°C in the colder seasons (fall and winter) and from 26.1 to 32.1°C in the hottest seasons (spring and summer). The pH varies from 7.6 to 8.8 and the oxidation-reduction potential as well as the dissolved oxygen present similar trends, with higher values in the cold seasons and lower in the hot seasons (**Table 1**).

In relation to nutrients, total phosphorus varies between 0.03 and 0.08 mg.L<sup>−</sup><sup>1</sup> in fall and summer at 0.12–0.15 mg.L<sup>−</sup><sup>1</sup> in winter and spring, from eutrophic to hypereutrophic conditions. Total nitrogen presented higher values (0.68–0.90 mg.L<sup>−</sup><sup>1</sup> ) in winter (mesoeutrophic conditions), with a decline in spring (0.01–0.09 mg.L<sup>−</sup><sup>1</sup> ) and elevation in the summer (0.55–0.65 mg.L<sup>−</sup><sup>1</sup> ), changing to mesotrophic conditions. Silica concentrations are higher in winter sampling (mean of 17.5 mg.L<sup>−</sup><sup>1</sup> ). When the

**195**

**Figure 7.**

*Subtropical Coastal Lagoon from Southern Brazil: Environmental Conditions and Phytobenthic…*

channel of connection with ocean was open, the total nitrogen concentrations at all stations elevated as well as the total phosphorus in the north and center in winter. This may have been due to the water runoff from the land around the lagoon, used

The salinity demonstrated outstanding spatial and seasonal variations. These variations were mainly related to the meteorological conditions. Spatially, the salinity varies between 1.3% in the South station in the winter (oligohaline zone) and 36.2% in the Center during the summer (euhaline zone). The station with the highest salinity variation is the North (4.5% min./winter and 34.9% max./summer), followed by the South station (1.3% min./winter and 29.5% max./summer). The Center station maintained higher values of salinity in all the climatic seasons due to its proximity with the ocean. Seasonally, salinity has the highest values in summer and the lowest in winter. These low values can be attributed to the action of the wind, predominantly northeast, that propelled the waters from the Ruivo Lake which are less saline, to the Peixe Lagoon [33]. In the summer, the decrease of the precipitation and intensity of the wind causes the outstanding increase of the salinity. This dynamic was also observed in the system from 1991 to 1996 [50]. The South

station of the lagoon presents less marine influence, therefore, lower salinity.

The wind velocity during the period of studies had the lowest averages in the fall. It intensified in the following months of winter, with a peak in August

*Average monthly wind velocity (m/s) and predominant monthly wind direction (arrows indicate direction).* 

*Data recorded by the Meteorological Station of Mostardas/RS. Source: INMET.*

to be exposed and to be submerged again in a matter of hours [50].

The variation of water levels of the lagoon is also strongly controlled by the winds regime, both intensity and direction, as well as precipitation. In the periods of predominant south wind (fall) and low precipitation (spring), the lowest levels of depth were observed. The Center is located next to the connection channel with the ocean and has a low average depth (30 cm). It is constantly saline (poly to euhaline zone). Due to the predominant northeasterly winds for most of the year, the water body of the lagoon is pushed to the west bank. The variation of the intensity of the winds can vary in the periods of day and night, causing great extensions of marginal sediment

*DOI: http://dx.doi.org/10.5772/intechopen.87776*

for livestock (**Table 1**).

*Subtropical Coastal Lagoon from Southern Brazil: Environmental Conditions and Phytobenthic… DOI: http://dx.doi.org/10.5772/intechopen.87776*

channel of connection with ocean was open, the total nitrogen concentrations at all stations elevated as well as the total phosphorus in the north and center in winter. This may have been due to the water runoff from the land around the lagoon, used for livestock (**Table 1**).

The salinity demonstrated outstanding spatial and seasonal variations. These variations were mainly related to the meteorological conditions. Spatially, the salinity varies between 1.3% in the South station in the winter (oligohaline zone) and 36.2% in the Center during the summer (euhaline zone). The station with the highest salinity variation is the North (4.5% min./winter and 34.9% max./summer), followed by the South station (1.3% min./winter and 29.5% max./summer). The Center station maintained higher values of salinity in all the climatic seasons due to its proximity with the ocean. Seasonally, salinity has the highest values in summer and the lowest in winter. These low values can be attributed to the action of the wind, predominantly northeast, that propelled the waters from the Ruivo Lake which are less saline, to the Peixe Lagoon [33]. In the summer, the decrease of the precipitation and intensity of the wind causes the outstanding increase of the salinity. This dynamic was also observed in the system from 1991 to 1996 [50]. The South station of the lagoon presents less marine influence, therefore, lower salinity.

The variation of water levels of the lagoon is also strongly controlled by the winds regime, both intensity and direction, as well as precipitation. In the periods of predominant south wind (fall) and low precipitation (spring), the lowest levels of depth were observed. The Center is located next to the connection channel with the ocean and has a low average depth (30 cm). It is constantly saline (poly to euhaline zone). Due to the predominant northeasterly winds for most of the year, the water body of the lagoon is pushed to the west bank. The variation of the intensity of the winds can vary in the periods of day and night, causing great extensions of marginal sediment to be exposed and to be submerged again in a matter of hours [50].

The wind velocity during the period of studies had the lowest averages in the fall. It intensified in the following months of winter, with a peak in August

#### **Figure 7.**

*Lagoon Environments Around the World - A Scientific Perspective*

The sediment samples (1g) were dried in an oven and cleaned with potassium permanganate and hydrochloric acid according to the Simonsen technique [55]. For light microscopy (LM) analyzed, a Zeiss Axioplan Microscope (Carl Zeiss, Oberkochen, Germany) was used. The relative abundance of the taxa was carried out in slides seeking the minimum sample efficiency of 80% [56]. Species richness was estimated by the number of taxa present in the samples. The specific diversity was assessed using Shannon index (H′) [57] and Evenness equitability (E). The analysis of variance (ANOVA) was applied to test the significance among the community attributes, since the data presented a normal distribution. The PAST® software was used for these analyzes. The relationship between biotic and abiotic variables with canonical correspondence analysis (CCA), PC-ORD® version 6.08 was used. For the construction of the biotic matrix, only species with a frequency equal to or >5% were considered in at least one sample unit and for the abiotic matrix, 10 environmental variables were included (**Table 1**). The data were transformed into log10(x + 1) in order to normalize the variances [58]. The Monte Carlo permutation test was carried out to verify the significance of the ordination axes.

The lagoon has a mean depth (<60 cm) and the Secchi disk depth generally coincides with the total depth. The water temperature varies between 15.3 and 17.6°C in the colder seasons (fall and winter) and from 26.1 to 32.1°C in the hottest seasons (spring and summer). The pH varies from 7.6 to 8.8 and the oxidation-reduction potential as well as the dissolved oxygen present similar trends, with higher values

*Physical and chemical variables analyzed in Peixe Lagoon in the four seasons, from June 2011 to February 2012, in the North (N), Center (C), and South (S) sampling stations. Depth (cm); Secchi = Secchi transparency (cm); temp = temperature (°C); ORP = oxide-reduction potential (mV); cond = conductivity (mS.cm<sup>−</sup><sup>1</sup>*

In relation to nutrients, total phosphorus varies between 0.03 and 0.08 mg.L<sup>−</sup><sup>1</sup>

eutrophic conditions. Total nitrogen presented higher values (0.68–0.90 mg.L<sup>−</sup><sup>1</sup>

winter (mesoeutrophic conditions), with a decline in spring (0.01–0.09 mg.L<sup>−</sup><sup>1</sup>

Silica concentrations are higher in winter sampling (mean of 17.5 mg.L<sup>−</sup><sup>1</sup>

in winter and spring, from eutrophic to hyper-

), changing to mesotrophic conditions.

*); PT = total phosphorus (mg.L<sup>−</sup><sup>1</sup>*

in

) in

*);* 

*); NT = total* 

). When the

) and

**3.3 Diatom analysis**

**194**

**Table 1.**

*nitrogen (mg.L<sup>−</sup><sup>1</sup>*

**4. Environmental conditions**

*).*

*DO = dissolved oxygen, salin = salinity (ppt); sil = silica (mg.L<sup>−</sup><sup>1</sup>*

fall and summer at 0.12–0.15 mg.L<sup>−</sup><sup>1</sup>

elevation in the summer (0.55–0.65 mg.L<sup>−</sup><sup>1</sup>

in the cold seasons and lower in the hot seasons (**Table 1**).

*Average monthly wind velocity (m/s) and predominant monthly wind direction (arrows indicate direction). Data recorded by the Meteorological Station of Mostardas/RS. Source: INMET.*

(6.4 m/s). A sharp decrease occurred in February 2012 (4.6 m/s). The predominant direction of the wind in the fall was south and southwest, shifting northeast in July and in the following months (**Figure 7**).
