Droughts and their socioeconomic and ecological consequences have coexisted for centuries in the Brazilian semi-arid region. However, the frequency and intensity of this phenomenon have intensified due to climate change (Marengo et al., 2020; Wu et al., 2020). This region experienced the worst drought ever recorded in Brazilian history between 2011 and 2017 (Marengo et al., 2020). A deficit of approximately 60% in the average accumulated precipitation was observed during this period compared with that in other years. Moreover, this region might experience greater increases in temperature, rainfall anomalies, and more frequent/intense dry spells and droughts in the next few decades (IPCC, 2014; Marengo et al., 2017). Regarding precipitation, decreases in the current rainfall rate of 10%–20% by 2040 and 25%–35% by 2070 (PBMC, 2014) are predicted to impact these coastal ecosystems. Such effects will have knockout effects on several small, low-inflow, and short estuaries due to the increased salinity, which can result in long-term hypersalinity.

Some reported alterations due to hypersaline waters are a reduction in species richness, with the survival of the most salt-tolerant species (Pillay and Perissinotto, 2008, 2009; Nche-Fambo et al., 2015). A previous study in a Brazilian semi-arid estuary suggested the presence of some groups that are well-adapted to deal with hypersalinity, such as phytoplankton (Barroso et al., 2018). Other detected patterns include changes in zooplankton composition (Campos, 2018; Garcia et al., 2020) and in the diversity and biomass of benthic animals in the dry period (Freitas et al., 2006; Maia et al., 2018), which are probably due to the flourishing of stress-tolerant species. Many estuaries worldwide are hypersaline and have elevated water residence times (Lancelot and Muylaert, 2011; Tirok and Scharler, 2013; Barroso et al., 2018). Under these conditions combined with nutrient enrichment, the phytoplankton biomass can increase (du Plooy et al., 2014; Hemraj et al., 2017; Barroso et al., 2018). However, this primary productivity enhancement in hypersaline conditions is not always beneficial for the general estuarine functioning, since harmful algae blooms can occur (Sahraoui et al., 2013; Wetz et al., 2017). In fact, in estuarine environments the synergistic effect of nutrient enrichment, long water residence times, and hypersaline conditions have been associated with several symptoms of eutrophication, such as high concentrations of chlorophyll, harmful algae blooms, episodic hypoxia, and fish kills (Wetz et al., 2017; Hallett et al., 2018).

Osmotic stress increases the risk of mortality of individual species and may result in a reduction in interspecific competition (Vilas et al., 2009). Some fish species experience physical stress due to changes in salinity. For example, the reactive oxygen species activity significantly increases with changing salinity (Hossain et al., 2016). Species that are poor osmoregulators or are not able to rest during periodical salinity stresses will be substituted by salt-tolerant species that are more adapted to or able to rest during unfavorable periods. This will lead to a shift in the species composition (e.g., dominance by salt-tolerant species) and community structure, thereby affecting the human economy and food security (e.g., fishing) (Feyrer et al., 2015). Hypersalinity can also cause the trophic systems to become truncated in terms of carbon flow to higher trophic levels because zooplankton predators in coastal food webs are more vulnerable to hypersalinity (Campos, 2018).

Furthermore, the longer water residence times in these Brazilian semi-arid estuaries have caused landward saline intrusion (Maia and Lima, 2004), thereby leading to the expansion of mangroves dominated by a few tolerant plant species over new areas (Godoy and Lacerda, 2015; Godoy et al., 2018). This can also lead to an increase in hypersalinity in the landward direction, thereby significantly increasing the associated marine fauna of the systems (e.g., jellyfishes and sea stars) that can be potentialized by basin drainage systems, such as multiple dams (Dias et al., 2011; Dias et al., 2013, 2018; Barroso et al., 2018; Medeiros et al., 2018; Valentim et al., 2018).

In this scenario, the irregularity of rainfall exerts a key control over the life histories (Feitosa and Rezende, 2020) and physiological characteristics of biota (Abrantes et al., 2020) because the short rainy period concentrates freshwater and riverine nutrient inputs in 3–4 months (Funceme, 2009). Thus, during most of the year, the biota face critical survival conditions related mainly to extremes of salinity, evapotranspiration, and water residence times. Therefore, the following two situations are possible: (a) rain could provide relief from suboptimal conditions for species that survived the long dry season, or (b) rain could be a disturbance because the rainfall is usually concentrated in a few months of the year. However, the limits of this adaptability, resilience, and diversification among members of the estuarine community and how semi-aridness affects the supply of resources to traditional communities are unclear.

Therefore, a major scientific challenge is to understand the responses of ecological communities to the ongoing intensification of the water deficit and increased saline intrusion. Thus, we raise the following question: how do rainfall irregularity and hypersalinity affect resource use and biological interactions on the semi-arid coast? To answer this question, long-term monitoring and research are strongly advised because changes in the stability and connectivity of communities can have key consequences for the ecosystem’s ability to resist external environmental disturbances. Also, predicting the effects of dryer scenarios and higher sea levels on saline estuarine intrusion as well as better river-dam management are essential to address adaptability and mitigation under climate change scenarios.

Global mean sea-level rise, which is caused by meltwater from glaciers and ice sheets and water expansion due to heat accumulation in the ocean, threatens different areas worldwide, including the Brazilian semi-arid coast. Along this coastline, at least three areas are at high or very high risk of flooding, and thus are more susceptible to sea-level rise (Tessler, 2008; Muehe, 2010; Nicolodi and Petermann, 2010). These areas also sustain large urban concentrations of approximately 180 K to 3000 K inhabitants. Therefore, despite the lack of precise sea-level scenarios under climate change for the region, at least 3.5 million people face a potential risk of being permanently or temporarily impacted by coastline regression by the end of this century. Regional models dealing with this sea-level rise impact are urgently needed to assess the magnitude and prevalence of potential risks, such as the destabilization of the extensive semi-arid coastline and many threats to water security, such as flooding and saltwater intrusion on coastal groundwater (Befus et al., 2020).

Coastal erosion is another consequence of sea-level rise and inappropriate land use (Section “Loss of vegetation cover and biodiversity due urbanization, aquaculture, agriculture, eutrophication, and land use change”). Current trends in coastal dynamics, combined with nearshore recession driven by sea-level rise, could result in the near extinction of almost half of the beaches worldwide by the end of the century (Vousdoukas et al., 2020). Approximately 42.4% of the Brazilian semi-arid coast is facing erosion (Morais et al., 2018; Paula et al., 2018; Vital et al., 2018). The vulnerability of this region and the continuation of this process will lead to considerable losses of coastal land in the coming decades, thereby affecting economies and low-lying human communities, increased groundwater hazards, and livelihoods (Franco et al., 2012; Befus et al., 2020). However, a non-linear increase in the erosion intensity is also expected due to the decrease in precipitation along this coast, which will increase the eolian transport of sediment from the shore to the mainland (Muehe, 2010). This lack of sediments amplifies the vulnerability of semi-arid coastal areas to erosion (Muehe, 2010), while possibly increasing the size of dune fields. This in turn may lead to siltation and burial of continental water bodies (Pinheiro et al., 2006; Tsoar et al., 2009), which will have severe consequences for estuarine and lagoon habitats. Coupled with local factors, such as river damming, these processes have already decreased the input of sediments to shallow marine areas, including sandy beaches (Morais and Pinheiro, 2011).

These ongoing processes may result in serious modifications of the coastal dynamics, thereby impacting the structure and function of adjacent marine ecosystems (e.g., seagrass beds and coral reefs) and compromising the production of vital environmental goods and services, such as coastline stabilization and freshwater provision. In such a scenario, management actions such as mapping erosion-susceptible regions, protecting coastal vegetation and regulating coastal occupation are primordial. Thus, to understand the alterations caused by environmental changes in these ecosystems, we pose the following question: how do sea-level rise and the increase in coastal erosion determine the resistance and/or resilience of these semi-arid coastal ecosystems to external disturbances and disorders?

More than half of the rivers worldwide are intermittent or ephemeral, meaning that they can naturally cease their flow or remain completely dry for a long time (Datry et al., 2016). These rivers are commonly present in hot semi-arid regions (Pusey et al., 2010). Most rivers along the Brazilian semi-arid coast are small and intermittent (Maltchik and Medeiros, 2006). During the dry season, sediments accumulate in the river channels and the freshwater flow is almost nonexistent. The connection between the land and the ocean is commonly maintained by the intrusion of saltwater caused by high tides. Therefore, transitional systems have an estuarine-lagoon morphology and are frequently subjected to hypersalinity. However, during the wet season, intense but short-lived rainfall carries sediments, nutrients, and contaminants to the ocean and establishes typical estuarine circulation. This results in intense exchanges of matter and energy between the continent-ocean interface, which is a fundamental process for local coastal dynamics in the inner continental shelf. Moreover, acidic ocean waters can alter heavy metal’s dynamics and increase their bioavailability and toxicity. These pollutants can decrease CO2 sequestration by the blue carbon ecosystems accelerating climate changes (Zeng et al., 2015). Finally, coastal regions, such as the Brazilian semiarid coast, are more vulnerable to the impact of acidification due to large influxes of pollutants from land-based sources and hypoxia levels (Melzner et al., 2013). In this sense, environmental protection from coastal pollution has a large potential for mitigating ocean acidification risk (Zeng et al., 2015; Cotovicz et al., 2020b).

However, changes in the drainage of the river basins in the past three decades (especially after the construction of multiple dams) (Molisani et al., 2006) combined with climate-change-driven reductions in rainfall (Cunha et al., 2019) have reduced the freshwater inputs during the rainy season, thereby compromising the pivotal land-ocean fluxes. This reduction increases the water residence time (Dias et al., 2011) and the sediment accumulation in the inner river areas (Morais and Pinheiro, 2011; Godoy and Lacerda, 2013), thereby perennializing the tide-dominated status of such estuaries (Pinheiro and Morais, 2010; Medeiros et al., 2018).

As the residence time of estuarine water increases, the reactivity of many substances is enhanced. Trace metals of environmental significance, such as mercury, become more reactive. Their bioavailability can also increase by complexing with dissolved organic carbon. Paradoxically, this is the opposite of the phenomenon observed in the Arctic Ocean, in which increasing flows of mercury into the ocean and bioaccumulation in biota are associated with increased river discharge due to the melting of glaciers and continental ice (Dai et al., 2009). The phenomenon of increasing pollutant mobilization due to global changes is independent of the variations in emissions and is stronger in extreme ecosystems worldwide; it has been described as an “Arctic Paradox” by Lacerda et al. (2020). Thus, the levels of a contaminant in the local biota are increasing even when the total load of the pollutant is generally decreasing.

This process is claimed to cause increasing mercury exposure in local consumers of fishery products on the semi-arid coast. Moreover, it is hypothesized that the accumulation of mercury could be an indirect consequence of the increase in semi-arid mangrove areas because it expands the metabolism-based dissimilatory SO42- reduction to a larger portion of the estuary (Lacerda et al., 2020). The longer water residence time in the dry period leads to a longer flooding time in the upper estuary, thereby leading to the production of large amounts of mercury-organic complexes due to the incomplete respiration of organic matter by anaerobes. The complexes are then transported to the lower estuarine areas where they bioaccumulate (Vaisman et al., 2005; Rios et al., 2016). Landward mangrove expansion has been reported in the Brazilian semi-arid coast as saline intrusion increases toward upstream areas (Godoy and Lacerda, 2015; Godoy et al., 2018). This increase, along with mangrove erosion next to beaches and river mouths, contributes to the remobilization of sediments and to the release of sulfur-metallic compounds, thereby dissociating them and releasing the metals to the water column, where they are much more bioavailable (Marins et al., 2020). For this reason, efficient dam management to avoid sediment retention could help to minimize such effects. Thus, to better understand the responses of these ecosystems to this changing scenario, we pose the following question: how will changes in sediment remobilization caused by low land-ocean exchanges alter biogeochemical processes in semi-arid ecosystems?

The occurrence of extreme events, such as heatwaves, temperature increases, intense and frequent droughts, and annual rainfall reduction, have been predicted and reported in the Brazilian semi-arid coast (Kundzewicz et al., 2014; Marengo et al., 2019; Soares et al., 2019). The average marine heatwave (MHW) frequency was close to 3 events per year from 1982 to 2020 in some Brazilian coastal regions, but was more than 10 events per year in this low-latitude coast. The frequency of MHW events has increased in the last 40 years, and a clear trend of a 0.2 °C temperature increase per decade was detected in the Brazilian semi-arid coast (Soares et al., 2021). These recent results indicate that the Brazilian semi-arid coast is one of the most-affected regions by climate change impacts, such as global warming, acidification, and extreme events such as heatwaves (Marengo et al., 2016, 2017, 2018; Soares et al., 2021).

A gradual increase in the temperature of 0.5 °C and 1.0 °C is expected by 2040 and 2070, respectively (PBMC, 2014). This increased warming trend and MHWs are currently impacting distinct semi-arid ecosystems, such as marginal reefs subjected to coral bleaching (Soares et al., 2019) and short low-inflow estuaries under hypersaline conditions (Barroso et al., 2018). The temperature increase is also expected to impair the trophic transfer efficiency from the individual level via changes in growth costs and carbon-use efficiency to the ecosystem level (Barneche et al., 2021).

A global-scale analysis of coastal species has shown abundant changes linked to warming, with richness declines expected in equatorward areas in the next five decades because of failures to adapt to rapid climate change (Hastings et al., 2020). Given that the ranges of marine species are closely related to their physiological thermal tolerances, global warming may lead to increases in abundance poleward and decreases in abundance toward the equator (Hastings et al., 2020), which is where the Brazilian semi-arid coast is located (Fig. 2). Acidification is another global stressor (Cotovicz et al., 2020b) that represents an additional impair mainly to calcifier organisms, some of them with key habitat-forming importance (e.g., corals and coralline algae) (Crook et al., 2012; Soares et al., 2019) and socioeconomic relevance (e.g., bivalves in brackish waters) (Waldbusser et al., 2015). Thus, determining how semi-arid coastal ecosystems respond to the increased frequency of predicted extreme events in the context of global climate change is pivotal for improving the predictions of community dynamics under climate change. Moreover, management actions, such as improving prediction models of regions susceptible to extreme events (e.g., storm surges and floods), better control of coastal occupations, and protection of coastal vegetation are key to deal with these global changes.

The Brazilian semi-arid coastal urbanization process accelerated during the 20th century owing to intense human migration from inland areas to coastal areas related to prolonged droughts and the desire to seek refuge and better living conditions. This growth became more pronounced when development activities, such as the installation of ports, shrimp farms, industries, and services, were initiated (Cerqueira et al., 2020; Lacerda et al., 2021). The intense land use and uneven distribution of governmental and private investments resulted in segregated and socially unequal access to the city, which resulted in several areas with deficient sanitation and sewage systems and high demands for housing, employment, and income. These areas put pressure on coastal ecosystems (e.g., dunes, coral reefs, estuaries, and beaches) through water and soil contamination, eutrophication, siltation, and the removal of vegetation cover.

The increase in urbanization caused strong deleterious changes. The reduction and fragmentation of vegetation cover in northeastern Brazil are prominent owing to the link between vegetation (e.g., mangrove forests) and other ecosystem components. Vegetation is a main attribute of conservation of terrestrial biodiversity and environmental services, and its loss will result in poorer ecosystems (Moro et al., 2015). Among other changes, urbanization causes soil compaction and waterproofing (caused by buildings and pavement), hydrological destabilization (resulting from hydric works or the imbalance between the withdrawal and return of water in water bodies), and water quality deterioration with eutrophication (either due to source pollution from sanitary and industrial sewage or diffuse pollution from surface runoff carrying livestock residues, mineral fertilizers, or pesticides) (Adams et al., 2001), thereby directly impacting the coastal ecosystem (Martins et al., 2012).

The main impacts on these ecosystems have been known in the Brazilian scientific literature since 2004, in which Gorayeb et al. (2004) identified mangrove deforestation as a major damage to the Pacoti estuary (Figs. 3 and 4), mainly due to the use of firewood to produce charcoal for domestic consumption and inadequate garbage disposal. Moreover, tourism and leisure activities without proper planning and mismanagement near the mouth of the rivers are other significant threats.

Some parameters are strong indicators of human influence on water quality and determinants of negative ecological impacts in coastal environments, such as biodegradable organic matter, which promotes the depletion of dissolved oxygen and hypoxia in shallow-water urbanized estuaries (MacPherson et al., 2007); lower dissolved oxygen in hypereutrophic estuaries (Dai et al., 2006); increased inorganic nutrients (mainly in the forms of NO3, NO2, NH4/NH3, PO4, and SiO4), which cause eutrophication, harmful algae blooms, and imbalances in primary productivity and the food chain (Cerqueira et al., 2020; Malone and Newton, 2020); turbidity, which causes obstruction of light and decreased photosynthetic activity (MacPherson et al., 2007); and pathogenic microorganisms, which pose risks to public health through bathing or through consumption of contaminated seafood in beaches, mangroves, and estuaries (Lipp et al., 2001). Considering this complex scenario, we raise the following multicomponent question: how do environmental variations, anthropogenic changes, and the context of global change affect ecosystem services and human communities? Answering this question is crucial to guiding stakeholders in creating accessible public policies aimed at preserving both traditional human populations and natural ecosystems.