**1. Introduction**

The shallow coastal lagoons are low depth water column mixing systems in which phytoplankton and microphytobenthos communities, microscopic eukaryotic photosynthetic algae, and cyanobacteria which live on the seabed [1] play a key role in the primary production and recycling of matter and nutrients. The role of microphytobenthos is quite important where macrophytes are absent and light radiation penetrates down to the bottom [2].

Microphytobenthos are composed of a set of microorganisms distributed in very diversified taxonomic groups, among which the diatoms are an important and often dominant component in estuarine and shallow coastal environments. These algae have varied adaptive strategies for adhesion and migration on different substrates, and there is a very large number of species sensitive to environmental changes.

Traditionally, benthic diatoms are classified according to the substrate in which they live. Those that live on thin sediment are called epipelic and those that live on sandy substrate are called epipsammic [3].

Diatoms studies in coastal lagoon were mainly concentrated in the world's largest water bodies, the Baltic, Black, and Caspian seas, which are ecosystems impacted by the anthropogenic actions and global climate changes. The eutrophication happened due to the increased nitrogen and phosphorus loads during the last century, and the increase in water temperature related to climate was detected by the changes in subfossil diatom assemblages. The accumulation of heavy metals from surrounded waters can be monitored due to the capacity of these algae to accumulate metals attached to the outside of the cell wall. There is an excellent literature review about these and other impacts; see the Snoeijs and Weckström chapter [4]. We can also find excellent information about the composition, spatial distribution of modern diatom assemblages, diversity, production, and ecology of the sediment-inhabiting diatoms in the estuaries [5–11]. In the smaller shallow lagoon from the east coast of Uruguay (South America) the diatoms studies were used to infer the paleosalinity, trophic and climate changes in relation to the sea level variation [12–16].

So far, most studies concentrated on phytoplankton at Patos Lagoon [17–24], Tramandaí-Armazém Lagoon [25–29], and Peixe Lagoon [30–34]. Regarding microphytobenthic, studies were limited to salt marshes and to the Patos Lagoon estuary [35–43].

The knowledge of the diatoms at Peixe Lagoon began with investigations on diatom assemblages in current and fossil sediments that allowed paleoenvironmental reconstruction. It demonstrated that the lagoon behaved as a deeper and more extensive lagoon system connected to the ocean by one or more permanent linking channels during the Holocene [44]. Later, studies were carried out on the taxonomic composition of diatoms in the marginal sediment of the lagoon. One study emphasizes the genus *Diploneis* Ehrenb. ex Cleve, rich in species [45]. Another investigation highlights the occurrence of *Cocconeis sawensis* Al-Handal et Riaux-Gobin, recently described for saline lakes in southern Iraq as an epiphyte in *Chara* sp. Linnaeus (1753: 1156). It was also recorded on an island in the South Pacific and epizoic on manatee in Florida Bay, USA [46]. The other species of the community were described, illustrated, and compiled with information on ecology and distribution in these coastal systems [47].

Studies about phytoplankton in subtropical coastal lagoon from south of Brazil showed that the structure and dynamic of the phytoplanktonic community were regulated by hydrological factors (inflow-outflow of continental and coastal waters in the system) as well as by meteorological conditions (wind and rainfall) and limnological variables (temperature and salinity) [18, 19, 33]. We have a set of factors that can act simultaneously while being difficult to recognize a main factor. Our question is to know if the structure and dynamic of the benthic diatoms in the Peixe Lagoon are related with these same factors. In order to answer this question, the study objectives were: (1) to know the composition of the diatoms community; (2) to verify the community structure and its spatial and temporal variation; and (3) to relate the variations of the community to environmental variables over an annual cycle.

In this chapter, firstly, we present information about the geographic, environmental, and climatic features where the Peixe Lagoon is situated. To be a case study, the methods are also included. Secondly, we describe the physical and chemical conditions of the lagoon and the benthic diatoms composition. Thirdly, we present and discuss the environmental variables related to the composition and spatial and temporal variation of the community attributes. Finally, we review the relationships of organisms occurring in plankton and sediment that should not be overlooked in studies in shallow coastal lagoons.

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**Figure 1.**

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

Peixe Lagoon is the only intermittent lagoon of the extreme south of Brazil and it is situated in the Lagoa do Peixe National Park (31°00′46″ S; 51°09′51″ W and 31°29′00″ S; 50°46′31″ W). This park is recognized by the Ramsar Convention as a Wetlands site, as well as an area of the UNESCO Atlantic Forest Biosphere Reserve, an Important Bird and Biodiversity Area (IBA) and a designated a site of international importance by the Western Hemisphere Shorebird Reserve Network (WHSRN). The coast is characterized by a microtidal regime, with a mean ampli-

Peixe Lagoon is a shallow, elongated system (35 km long and 1 km wide), parallel to the coastline (**Figure 1**) connected to the Atlantic Ocean through a single narrow channel (chocked lagoon) (**Figure 2**). The channel occlusion occurs due to deposition of sand caused by the predominance of the north and northeast winds [49, 50]. The connection with the ocean usually occurs during winter and spring, when the precipitation becomes more pronounced and the marshes and fields marginal to the lagoon are flooded. During these periods, an artificial opening of the channel is carried out by means of machines, since a natural opening only occurs sporadically [50]. The margins of the lagoon are covered by salt marshes vegetation dominated by *Paspalum vaginatum* Sw., *Cotula coronopifolia* L., *Sporobolus montevidensis* (Arechavaleta) P.M. Peterson & Saarela (= *Spartina densiflora* Brong), *Hydrocotyle bonariensis* Lam., *Androtrichum trigynum* (Spreng.) H. Pfeiff., *Bacopa monnieri* (L.) Wettst., and *Juncus acutus* L. [51]. The surface sediments at the bottom are essentially sandy. On the sites with greater depth of the lagoon, the sediments are thinner, with addition of silt and

The system is located in subtropical climate where the rainfall is distributed throughout the year. In the period of studies, the highest rainfall (145.8 mm) occurred at the end of the fall (June 2011), decreasing in the following months and then increasing (131.2 mm) in early spring (October 2011). November had the lowest cumulative precipitation (23.0 mm). The average monthly temperature varied between 12.6 and 18°C in the autumn/winter seasons and between 18.3 and 24.4°C

*Location of Peixe Lagoon area in the state of Rio Grande do Sul, southern Brazil, the sampling stations* 

*(North = N, Center = C, South = S) and the channel of connection with ocean (arrow).*

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

**2. Study area**

tude of 0.45 m [48].

clay [49].

in spring/summer (**Figure 3**).

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