1. Introduction

Water quality is vital when it comes to determining how society uses and values aquatic environments associated with natural resources [1] and their monitoring provides empirical evidence to support decision-making on health and environmental issues [2]. Thus, water quality can be assessed through variables such as dissolved oxygen, temperature, pH, toxic substances (heavy metals, surfactants, and phenols) and others [3, 4].

Among the aquatic ecosystems, the coastal lagoons stand out for offering important environmental services (e.g., food production, nutrient cycling,

recreational activities, and aquaculture). However, they have been degraded worldwide by human activities associated with rapid urbanization [5–8]. Therefore, coastal ecosystems need constant evaluation to provide the protection of the populations and the improvement of the quality of life.

Indexes and indicators of water quality work as tools that represent simplified models of the environmental structure, which makes it easy to understand the impacts and to present the results to the general public [9, 10]. Indicators are quantified information of environmental variables that help in the explanation of processes that undergo changes of anthropic or natural origins in time and space, thus allowing the dynamic monitoring of the real situation and identification of trends [11, 12].

The most complex indicator models integrate information from different ecosystem compartments, such as physical and chemical parameters of water, phytoplankton, zooplankton, benthos, submerged aquatic vegetation, macroalgae, sediments, etc. [13]. In this context, the indexes related to water quality are considered the most widely known water indicators in the world. They are represented by a dimensionless number, being the result of the aggregation of physical, chemical, and microbiological indicators obtained by specific methodologies [14].

The use of indexes to assess the quality of water becomes fundamental in the measurement of how and how much aquatic management is moving from the perspective of sustainability, observing the reflexes of the actions implemented in the aquatic environments as units of management of the water resources. Thus, the use of the aquatic life protection index (ALPI) is essential and therefore considers the physical-chemical quality of the water and its degree of enrichment regarding nutrients (phosphorus and nitrogen), as well as the degree of toxicity, and it can be used as an indicator of trophic status and pollution in aquatic ecosystems [9, 15].

In Brazil, studies that aim the measurement of the water quality through the application of indexes have been carried out mainly in reservoirs and rivers [16–18]. Considering the Brazilian coast, mainly in lagoon ecosystems such as the Jansen lagoon, these studies are still scarce, making the understanding of the behavior of this environment regarding the impacts that take place there evident. Thus, we propose to carry out an assessment of the water quality in the Jansen lagoon through the use of indexes and environmental descriptors as a way of assisting the management of local resources, as well as to subsidize the sustainability of this ecosystem in the global scope.
