2. Brazilian coastal lagoons

Coastal lagoons are considered as depressions, with depths less than 10 meters, parallel to the coast, being connected to the sea temporarily or permanently by one or more channels and separated from it by a physical barrier [19, 20]. These coastal environments, although highly productive, are very fragile and subject to strong anthropogenic pressures, such as disordered population growth [21, 22].

In the Brazilian coast, it is possible to see lagoon complexes, as well as coastal lagoons impacted by man and in the stage of advanced eutrophication. Among them are the Mundaú-Manguaba lagoon complex, Jacarepaguá, Patos lagoon, Jacuném lagoon, Açu lagoon, and Rodrigo de Freitas lagoon that have the disposal of untreated sewage in natura directly into the water bodies as the main aggravating factor of water pollution [23–25].

The Jansen lagoon is the result of successive anthropic changes in the landscape. In the 1970s, the area was an estuarine region intersected by the Jansen and Jaracati streams with extensive mangrove forest represented by species such as Rhizophora

Phytoplankton Biomass and Environmental Descriptors of Water Quality of an Urban Lagoon DOI: http://dx.doi.org/10.5772/intechopen.87955

mangle, Avicennia germinans, Avicennia schaueriana, and Laguncularia racemosa. However, the construction of roads altered the main drainage network of the Jansen stream with the sea leading to the salt water damming, forming consequently the lagoon [26].

The lagoon is located in the northwest of São Luís Island, between the coordinates 02°290 <sup>08</sup>″S and 44°18<sup>0</sup> <sup>02</sup>″W, northern Brazil (Figure 1). It covers 140 ha with an average depth of 1.5 m and is still surrounded in the east by a mangrove forest.

The lagoon is under a semidiurnal macrotidal system typical of the Amazon coastal zone [27], with amplitudes that can reach up to 7 m. It is connected with the sea and the water exchange occurs during the spring tides in the rainy season, when the freshwater supply ensures the flow toward the sea (115 m length and 3 m deep) [28]. According to the classification of Kjerfve [29] which is based on the geomorphological typology, the Jansen lagoon is classified as restricted (Figure 2), depending on fluctuations in sea level and rainfall.

Figure 1. Location of the study area with sampling points (L1–L5), Jansen lagoon, São Luís Island-Maranhão, Brazil.

Figure 2. Schematic representation of the types of coastal lagoons according to Kjerfve [29].

Regarding local climate, the region is hot and humid because it is situated in the equatorial belt with equatorial air mass influence originating in low-latitude regions where the southern boreal and trade winds converge. As a result, it has intrinsic characteristics such as high temperatures throughout the year as well as two welldefined seasonal periods strongly marked by precipitation: rainy season (January-June) and dry season (July-December).

The Jansen lagoon is a eutrophic environment, where most of the material found in it is of allochthonous origin, that is to say, originating from the release of fresh domestic sewage, which decisively influences the distribution of many nutrients, and consequently the biodiversity and water quality of the lagoon, with recurrent processes of eutrophication and algal blooms [26].
