Biogeochemistry mediators

Microorganisms are an extremely important group of organisms in mangrove ecology and functionality (Holguín et al. 2001; El-Tarabily et al. 2021; Lacerda et al. 2022a; Farrer et al. 2022). A wide range of bacterial groups are involved in cycling of carbon (C), nutrients and several metals in mangrove soils, with some groups involved in more than one cycle, for example, the nitrogen (ammonification, nitrification, denitrification), sulphur (sulphate reduction), iron (Fe) and phosphorus (P) cycles (Bashan and Holguín 2002; Alongi 2021; Ferreira et al., 2022a). Bacteria and fungi can reach around 90% of the total microbial biomass, which also includes algae and protozoa and microphytobenthos, such as Diatomacea and Cyanobacteria, that contribute to significant amounts of buried carbon and trace elements in mangrove soils and in hypersaline tidal flats (Brown et al., 2021a; Lacerda et al. 2022b).

Mangroves are highly dependent on the efficiency of specific microbial communities both in soil and waters. The microbiome of SAEC mangroves is highly diverse and displays a higher level of complexity than those in the AMMC (see e.g. Andreote et al. 2012; Tavares et al. 2021). SAEC datasets extracted from Rhizophora roots environment were dominated by Proteobacteria (reducing nitrate and sulphur compounds), mainly by Deltaproteobacteria and Gammaproteobacteria, which are part of the core microbiome of mangroves worldwide. Desulfobacterales (anaerobics involved in S and C cycling, and methane and nitrogen transformation) was the most abundant order, and Euryarcheota (Archaea) (active carbon transformation through methanogenesis) is the second most abundant group. Other ubiquitous phyla comprise Planctomycetes, Acidobacteria, Bacteroidetes and Chloroflexi. This microbiome diversity (Shannon Index) shows a significantly positive correlation with salinity, organic C, potential evapotranspiration, minimum temperatures year-round and a significantly negative correlation with annual precipitation. These responses to environmental parameters suggest adaptation to the typical stressful conditions of the SAEC and favour adaptions to a changing environment, of increasing salinity intrusion and decreasing annual rainfall, such as landward migration and increasing nutrient cycling efficiency (Tavares et al. 2021).

Bioturbators/burrowers

Several functional roles are performed by semiterrestrial Decapods (Crustacea: Decapoda), which are one of the most ecologically significant macrobenthic organisms. This group is dominated mostly by several Brachyuran crabs, such as grapsoids, ocypodoids and xanthoids. Bioturbation by burrowing is mostly driven by several Sesarmids (Grapsoidea), fiddler crabs and Ucides cordatus (Ocypodoidea), and Panopeus sp. and Eurytium limosum (Xanthoidea) (Ferreira and Sankarankutty 2002; Ferreira et al. 2019b). Ocypodid crabs tend to be richer in species in intermediate latitudes (Teles et al. 2024), but SAEC mangroves have relatively rich Brachyuran communities, mainly of Grapsids.

Mostly are fossorial species that can be found associated in communal anastomosed long-lasting tunnels (except individual-burrower fiddler crabs) between the roots of R. mangle (Ferreira et al. 2019b), contributing to oxygenating underground tree roots and adjacent soil, and also used by several juvenile and adult fishes, including cyprinodonts, gobiids, fundulids, rivulins, poeciliids and eleotrids for protection (Barletta et al. 2000; Lewis and Gilmore 2007; Lira et al. 2021). In general, the root system of Lumnitzera racemosa and Avicennia spp. seems to impair the construction of these multibranched systems, thus decreasing crab diversity (Ferreira et al. 2019b).

Through burrowing and consequent soil bioturbation these ecosystem engineers can also influence the microbiota and infaunal diversity, and through changes in sediment and porewater physical chemistry, they can influence nutrient availability and therefore tree species growth and productivity (Warren and Underwood 1986; Kristensen 2008; Ferreira et al. 2019a; Barbanera et al. 2022). In SAEC, burrowing by some fiddler crabs (Ocypodoidea) can bury small mangrove propagules, like those of L. racemosa, promoting the dominance of large R. mangle propagules, a tree species with higher biomass and C stock (Ferreira et al. 2019a).

Herbivores

Herbivory in mangroves is predominantly performed by crustaceans, insects and also gastropods and depends on the chemical characteristics of leaves of tree species. In general, mangrove leaves present chemicals to support resilience to strong ultraviolet radiation and high soil salinity, which also work as deterrents to herbivores. For example, R. mangle and L. racemosa showed significantly lower area eaten and number of leaves attacked than Avicennia schaueriana, which was attributed to leaf chemical composition of this salt-excreting species, with higher content of sodium, crude fibre, ash content and lower content of total phenols and soluble carbohydrates (Lacerda et al. 1986).

Leaf eating crabs are important for energy and carbon flow and provide a food source for predators (Ashton et al. 2003). Crab faeces are rich in nitrogen (Lee 2008) and combined with sloppy feeding (Camilleri 1989) produce smaller fragments available for deposit feeders. One of the most important herbivores at soil level is the big crab Ucides cordatus. By removing the leaf litter and storing below-ground in their burrows, carbon is retained within the mangrove system. Conversely, Aratus pisonii feed on mangrove leaves in the canopy. This crab shows a preference for the leaves of the red mangrove R. mangle over L. racemosa and Avicennia germinans.

While herbivory has not been reported to affect canopy characteristics in mangroves in the SAEC, it can, through differential propagule consumption and herbivory depending on tree species, determine the tree type that establish/remain in a site, and thus the structural/architectural features and biomass of the forest, which can indirectly influence infaunal diversity and nutrient cycling (Smith et al. 1989; Alongi and Christoffersen 1992; Ferreira et al. 2019a; Barbanera et al. 2022). The neotropical crab Goniopsis cruentata has a significant structural role in the forest through the higher consumption of propagules of L. racemosa and Avicennia sp., thus promoting the predominance of R. mangle, which is architecturally more complex and consequently richer in crab species among and over roots’ habitats. Abiotic (tides, rain, temperature, soil) and biotic (FG composition) differences between humid and semiarid mangroves are potentially able to determine differences in tree species composition (Ferreira et al. 2019a).

Studies on functional roles of insects in mangroves of the Brazilian Equatorial Margin are very scarce, limiting our knowledge on their full ecological functions in the community. The extent of herbivory depends on diverse factors affecting the palatability and nutritional value of leaves that vary with age, season and between species. Insect herbivory removes less than 5% of leaf biomass, thus with small impact on C and nutrient cycling. However, some insects can cause heavy defoliation events, despite being limited by tannin content (Lacerda et al. 1986; Hogarth 1999; Cannicci et al. 2008).

Defoliation by insect and consumption of apical buds, despite, in general, not deadly to the trees, can potentially reduce reproductive and vegetative growth, reducing reproductive output and hence influencing tree species recruitment, frequently associated with anthropogenic activities impacting on mangroves (Krauss et al. 2008; Lu et al. 2019; Maldonado-López et al. 2019). In the SAEC, insect herbivory was strongly associated with abiotic and biotic factors. Higher intensity of leaf consumption by insects occurred in the dry season, when monthly rainfall varied from 2 to 4 mm and water salinity was >54‰. In the rainy season (70–290 mm; 34–35‰) the degree of total foliar herbivory increased, mostly in L. racemosa and R. mangle (Silva and Maia 2022)

Wood borers

Several marine isopods (Crustacea), Teredinidae mollusks (shipworms) (e.g. Teredo spp.) and wood-boring coleopterans (e.g. cosmopolitan Coccotrypes rhizophorae) are wood borers in mangroves. They can affect the development and even survival of mangrove trees, hence affecting tree diversity and forest architecture (Perry and Brusca 1989; Svavarsson et al. 2002), leading to changes in live and dead biomass and thus aboveground carbon stock. Unfortunately, no study on the effects of marine isopods is known for the SAEC. Teredinids, on the other hand, burrow into mangrove wood and are important in breaking down dead wood, having a significant role in biodegradation, and when abundant can affect the amount of carbon stored in, and released by, the forest. Yet, vacant teredinid tunnels can be exploited by many macro-benthic taxa (e.g. fishes, octopus, polychaetes), enhancing trophic and functional resilience (Hendy et al. 2014, 2022). The impacts of this FG, however, have hardly been studied along the Brazilian Equatorial Margin, the few reports come from the AMMC, where teredinid molluscs are a significant component of local peoples’ traditional diet, and show they display seasonality with higher activity during the wet season (Filho et al. 2008).

Biomass, productivity and carbon stocks

Mangroves from the Equatorial Margin of Brazil are comparatively less known in terms of biomass and carbon (C) stocks than their counterparts on the southern coasts. Table 2 summarises the few most complete studies with comparable methodologies that allows an evaluation of differences between biomasses and C stocks of mangroves from the two subregions of the Equatorial coast (AMMC and SAEC). The small number of studies impedes a generalisation of the findings. However, some results are outstanding. SAEC mangroves present lower aerial biomass and aerial C stock by a factor of 2–5 relative to mangroves in the AMMC. This results from poor structural complexity due to a deficiency of freshwater supply, higher salinity and low inputs of continental-derived nutrients. However, they show similar belowground biomass and soil C stock (Table 2). Exceptionally high aboveground biomass (AGB) values and C contents are found in the Parnaíba River Delta, a 3,700 km² mangrove forest at the border between the semiarid northeast and the humid Amazon regions. The uniqueness of this area has been highlighted in previous oceanographic studies (Carvalho et al. 2017; Chielle et al. 2023a, 2023b), but the logistic and methodological challenges have hampered more detailed studies in the region and proper estimates of BGB and soil C stock are not yet available.

Meng et al. (2017) reported a positive relationship between C stocks in AGB and in BGB of mangroves in China and suggested this relationship could be applicable worldwide and thus used to obtain more accurate estimates of mangrove blue C stocks at regional or global scales. Their review, however, failed to include data from the Equatorial Western Atlantic, and the preliminary results available suggest this relationship does not hold for SAEC mangroves. Therefore, predictions of decline of C stock in AGB and BGB (C in roots and soil) under any future climate change scenario may result differently depending on the forest type and location (Singh et al. 2022).

Season is an important variable regulating forest productivity in the SAEC, being higher in the rainy season, much like any other mangroves worldwide (Portela et al. 2020; Gomes et al. 2021). Highest productivity occurs under low soil and porewater salinity and with adequate supply of nutrients and freshwater restricted to the short wet season. In pristine mangroves in the SAEC, increased fluvial discharge strongly influences nutrient concentrations and therefore availability to mangroves. Higher dissolved N and soluble reactive P concentrations occurs in the rainy season (Barroso et al. 2016; Silva et al. 2009; Nóbrega et al. 2013), a pattern also observed after storms events in other semiarid coasts of the world and attributed to increased leaching and transport of materials from river upper basins and fluvial waterways (Eyre and Ferguson 2005). This nutrient pulse during the short rainy season may be responsible for up to 85% of the total nutrients exported to the lower estuary. Unfortunately, to our knowledge, there is no estimate of mangrove litterfall rates in SAEC. However, mangrove forests under similar climatic and geological conditions shows litterfall rates in the same range of values observed in semiarid littorals; 82 gC m⁻² year⁻¹ in the Gulf of California, Mexico (Arreola-Lizarraga et al. 2004); 212 gC m⁻² year⁻¹ in Karachi, Pakistan (Farooqui et al. 2012) and from 57 to 238 gC m⁻² year⁻¹ in semiarid Caribbean mangroves (Lacerda 2002). Based on the C content of typical thyolitic gleysols, mangrove soils (Suárez-Abelenda et al. 2014; Nóbrega et al. 2019) estimated extremely high soil C stock of 8,200 ± 900 gC m⁻². Unfortunately, this extremely high estimate cannot be confirmed by field data. Year-round larger fluxes in the AMMC result in highest litterfall that vary from 51 to 203 gC m⁻² year⁻¹ (Gonçalves et al. 2006; Nascimento et al. 2006; Fernandes et al. 2007; Mehlig 2001).

Average whole ecosystem carbon stocks per unit of area in AMMC mangroves (361–746 t C ha⁻¹; at 0°40′S and annual rainfall of 2,300 mm) (Kauffman et al. 2018b); and those from the humid eastern coast, the Jaguaripe estuary in Bahia State (at 13°11′S, 1 and annual rainfall of 350 mm) are similar (250–633 Mg C ha⁻¹) (Hatje et al. 2021). This clearly confirms higher carbon stocks are more clearly associated with humid climate, rather than latitude. An exception is the urban-influenced mangroves in the SAEC, where soil carbon sequestration rates, due to high allochthonous aeolian and urban inputs of organic material rather than autochthonous production, are up to 14 times higher than the global average (Ward et al. 2023). A detailed characterisation of the mangrove soils organic matter in urban-impacted and rural areas mangroves found a significant contribution of anthropogenic sources in the total carbon accumulated in sediments, and a clear increase in importance of anthropogenic carbon in the more urbanised sites (Mounier et al. 2018). Passos et al. (2021) reported increasing accumulation rates of total organic carbon and total nitrogen in the Suape estuary, in NE Brazil following the port-industrial facilities development starting in the 1980’s, reflecting urbanisation and industrial growth. The anthropogenic contribution was clearly shown by the observed heavier δ¹⁵N values in the sediment column. This scenario, however, seems not exclusive of SAEC mangroves, since in humid coastlines, mangroves adjacent to or within metropolitan regions show increased sediment accretion rate (SAR) and carbon sequestration and contents in sediments (Sanders et al. 2014). Robust sampling undertaken along estuarine gradients strongly suggests that soil Corg stocks are considerably less variable along the sea–land estuarine gradient than across the intertidal gradient from the water edge to the border with terrestrial vegetation. Although AGB is much more variable along the longitudinal estuarine gradient, the highest AGB is observed in the lower estuary and lowest ABG in the upper estuary (Hatje et al. 2021).

Sulphur and iron soil chemistry

The AMMC is dominated by soils with significant amounts of Fe and Al, classified as Ferri-humuluvic Spodosols and Hydromorphic Arenics, a few meters in thickness, with horizons containing significant proportions of organic matter and Fe. Mangrove soils, mostly halomorphic and hydromorphic Gleisols in the AMMC, receive a large amount of Fe from upland soils. Clay and silt fractions predominate with moderate to high amounts of organic matter and soluble salts. They are weakly consolidated, greyish to black in colour, with the overwhelming presence of H₂S (Schaefer et al. 2017). Iron content is unaffected by seasonality due to a surplus of water even in the dry season. These circumstances promote nearly permanent anoxic conditions and allow the precipitation of pyrites and the accumulation of chalcophile elements, including toxic heavy metals of environmental significance, such as Cd, Pb, Hg, Zn and Cu.

SAEC mangroves soils are characterised by tertiary and quaternary deposits forming coastal plains constituted of sandy soils closer to the coast and yellow-red latosols (mostly oxisols) inland (Lacerda et al. 2008). They are relatively poorer in iron content resulting from the relatively smaller Fe input from upstream basins (Ferreira et al. 2007, 2021). During the wet season, suboxic to anoxic conditions may develop as in the AMMC. A strong water deficit in the dry season and increased flooding by oxic seawater solubilise deposited sulphides and releases Fe and heavy metals to porewaters, which are converted to Fe oxyhydroxides at the rhizosphere level.

The production of sulphides derives from the anaerobic decomposition of organic matter. Sulphides accumulate in the sediment porewater and may exceed the tolerance threshold of mangroves. In the AMMC, Fe(III) (hydr)oxides efficiently mitigate sulphide toxicity to mangroves by partially avoiding rapid sulphide accumulation by sequestering it in the sediment in the form of pyrite or jarosite (Cobacho et al. 2024). This adaptation may be impaired during the long dry season in the SAEC resulting in higher toxicity to mangrove plants. Framboidal pyrite crystals about 40 μm abound in the sediments of SAEC mangrove soils, an example is shown in a scanning electron microscopy photograph in Figure 3. The X-rays analysis of this framboid reveals the dominant presence of S (Figure 3a) and Fe (Figure 3b), elements forming the most common type of pyrites. Through the same technique, it is possible to reveal the presence of chalcophile toxic heavy metals in the framboid, such as copper (Cu). These metals can be remobilised to porewaters during the longer dry season.

The strong seasonal shifts of fluvial flux variability in the SAEC strongly affects biogeochemical processes at the soil–air interface, including evapotranspiration that significantly affect redox-sensitive biogeochemical processes, including root radial oxygen loss, iron plaque formation through iron and sulphate reduction and pyrite oxidation. These will impact on productivity, biomass growth and root exudate release, potentially affecting fauna activity, and in the case of FG (Araújo Júnior JM et al. 2016), key biogeochemical processes and ecosystem functioning and services. Precipitation of carbonates may occur, as soil contents can vary from 4 to 11%, contributing to SARs (Albuquerque et al. 2014). Redox-sensitive micronutrients, such as Fe and Mn, are particularly affected by shifts in fluvial fluxes (Lacerda et al. 2022b; Aragon and Miguens 2001) and can eventually impact on nutrient balance and availability of plant uptake as well as export to adjacent coastal areas, in particular phosphorus (Silva et al. 1998; Marins et al. 2020).

Contamination and pollution

Sulphate reduction by-products pose threats to mangroves that adapt via a range of anatomical and physiological mechanisms dependent on species, which eventually control pollutant transfer. Distribution of iron in the rhizosphere of the dominant neotropical mangrove species shows the formation of iron-plaques that are the most effective barriers to trace metal uptake and translocation by mangrove plants (Machado et al. 2005; Cheng et al. 2010). Relatively less reducing environmental conditions in mangrove sediments of the SAEC, as discussed in the previous section on Fe geochemistry, may reduce the formation and significance of iron plaques in fixating toxic metals in the rhizosphere. In the semiarid Jaguaribe River Estuary, iron plaques contribute to an average fraction of the total Cu content in roots varying from 25.8 to 42.7%. Minimum contribution, meaning more Cu being uptake by roots, occurred in less reducing Eh, whereas maximum contribution occurred in the more reducing conditions (Lacerda et al. 2024). In these mangroves, Cu concentrations are 10 times higher than those reported for these species in mangroves from humid mangroves in SE Brazil, where soil redox potential is extremely negative (−316 to −327) (Madi et al. 2015), corroborating that the less reducing conditions of the semiarid mangroves result in higher metal availability for plant uptake. In these humid areas, metal retention in iron plaques is much higher, varying from 62% in Avicennia schaueriana to 87% in R. mangle (Machado et al. 2005). In addition, Fe accumulation in the salt excretion glands of A. germinans from the SAEC suggests that salt excretion can help decrease internal plant concentrations of some toxic metals.

The capacity of mangroves to immobilise toxic metals can be used for pollution mitigation measurements and rehabilitation of mangrove as filters to protect adjacent coastal areas from metals leaching from ground water, avoiding contamination of adjacent coastal waters. Figure 4 compares the vertical distribution of selected heavy metals in mangroves from afforested and bare degraded areas sediments surrounding a landfill in a humid region. The strong capacity of mangrove rhizosphere to immobilise metals is clearly shown by the sharp increase in total metal contents at the sediment layers showing highest root biomass in the afforested site, whereas in bare, degraded mangrove sediments metal profiles show a steadily increase in concentrations towards the surface suggest transfer to pore and surface waters. This entire mechanism can be disrupted by the impact on the plant metabolism due to hyper salinity, resulting from reduced rainfall and increased saline intrusion, and smaller porewater contents of dissolved iron due to higher Eh. These are typical conditions found in SAEC mangroves (Lacerda et al. 2022b). This discussion, however, may be very preliminary, due to the scarcity of data on iron plaque formation and metal immobilisation in both the SAEC and AMMC mangroves.

Excess nutrients from anthropogenic sources trigger eutrophication in many mangrove-dominated estuaries along the SAEC, mostly due to poor sanitation, sewage treatment and inadequate waste disposal. This problem may affect the AMMC but considering the extension of their estuaries and coastal mangroves and low density of human population, no studies have quantified the eutrophic state of the region’s mangroves, except for specific locations. In addition, rapidly expanding intensive shrimp farming has greatly increased the eutrophication pressure on mangrove ecosystems in the SEAC due to excess nutrients (Lacerda et al. 2019), and today it is by far the most significant source of nutrients to SAEC mangroves, with the exception of the urban mangroves surrounding the metropolitan areas of northeastern Brazil capital cities. As a comparison, the AMMC has less than 3% of the total operating shrimp farms in the SAEC (Lacerda et al. 2020). Mangroves have been suggested to act as filters to human-derived nutrients, based on actual measurement of a net import of nutrients by mangroves, observations have shown that only a fraction of the nutrient input entering the forest is exported back to adjacent coastal areas (Sanchez-Carrillo et al. 2009; Silva et al. 1998), while others reached the same conclusion by modelling nutrient concentrations in waters as a function of dilution (Bin and Dushof 2004). Therefore, mangroves seem to actively immobilise this element either accumulating them in mangrove biomass and/or sediments. At the SAEC, although nutrient inputs may be limiting, mangroves may attain high productivity, through an efficient recycling of limiting nutrients (Holguín et al. 2001). Sediment fauna also influences this process but seems very site-specific (Ferrante and Fearnside 2019). Marins et al. (2020) and Sanders et al. (2014) reported a continuous increasing threat in total nitrogen and phosphorus, respectively, in mangrove sediments following the increasing intensity of anthropogenic drivers, similar to observations by Queiroz et al. (2020) and extended to a verified increase in GHG emissions (Queiroz et al. 2019; Cotovicz et al. 2021).

Damming

River damming and waterways diversion causes extensive changes in hydrodynamics and sedimentation in semiarid estuaries, particularly, siltation of estuaries and erosion of the coastline due to reducing sediment supply to the coast (Ward et al. 2023). Most rivers along the SAEC are small and intermittent and show a well-defined hydroperiod (Maltchik and Medeiros 2006). During the dry season, sediments accumulate in the river channels and the freshwater flow is almost non-existent. The highest fluxes in the wet season are capable of transporting large amounts of sediments to the continental shelf. Changes in the drainage of the river basins by flow diversion and the construction of multiple dams in the past three decades (Molisani et al. 2006) simultaneous to the climate change-driven reduction in annual rainfall (Cunha et al. 2019) have reduced freshwater inputs during the rainy season, thereby affecting the fundamental transfer fluxes of water and materials between the continent and the ocean.

The main effects of river damming and diversion are that the reduced and frequently regulated fluxes to estuaries are unable to wash out sediments and those accumulate along fluvial beaches, settle and create new, or enlarge existing islands and bars, creating new space for mangrove colonisation (Godoy et al. 2018). In the Pacoti river estuary, another estuary in the semiarid coast, mangrove expansion occurred over abandoned salt pans and on recently enlarged estuarine beaches and islands, also resulting from decreased and regulated fluvial flow by a sequence of dams built less than 100 km from the mouth of the river, to supply water to the metropolitan region of the Ceará (CE) State capital, Fortaleza. Natural fluxes varying from 1.0 to 19 m³ s⁻¹ were regulated to 1.7 m³ s⁻¹ year-round. These new areas were quickly occupied and fixed by mangroves, expanding the forest cover from about 71 ha in 1958 to 142 ha in 1999, following dam construction and further expanding to 144 ha in 2004, probably responding to increased ocean forcing (Lacerda et al. 2007). In addition to controls on river flows, the reduced transport capacity of rivers increased sediment retention in estuaries, which has been aggravated by a simultaneous decrease in rainfall over the SAEC of 4.8–5.6 mm year⁻¹ in the last 30 years (Moncunill 2006; Alvalá et al. 2019; Marengo et al. 2018).

Aquaculture

One of the most significant drivers of environmental impacts on the SAEC mangroves is intensive shrimp farming, mostly after recent changes to the Brazilian Forest Code, that have weakened protection for mangroves and associated salt flats (Ferreira and Lacerda 2016a, 2016b). Although the SAEC has only 4% of the total Brazilian mangrove area (ICMBio 2018), it produces over 96% of cultivated shrimp in the country. While only a small area of Northeast mangroves has been directly converted to aquaculture ponds (ICMBio 2018), this direct and indirect forest loss, reported less than 8% of the total mangrove area of the SAEC (Maia et al. 2006), may be proportionally more significant than in mangroves of the humid sector of the Equatorial coast. The reason for this being that SAEC coastal waters are highly oligotrophic and, therefore, primary productivity and fisheries are largely dependent on nutrient fluxes from mangrove-dominated estuaries.

Regional intensive shrimp aquaculture in the SAEC, although a relatively recent phenomena, has grown over 2,000% since 1997 (Figure 5), although it has stabilised to an annual production of about 70,000 tons and covering a total pond area of 30,000 ha over the past decade (Valenti et al. 2021). Typically, shrimp ponds are built in mangrove-adjacent salt flats rather than the mangroves themselves, but they maintain periodic hydrological connectivity through dammed channels, allowing the flushing of effluents to local mangrove tidal creeks. The main impacts on mangroves are, therefore, mostly indirect, due to the release of nutrient-rich, oxygen demanding effluents and changes in hydrology, which strongly affect ecosystem functioning, decrease of ecosystem service provision, reduction in nutrients, primary productivity and carbon storage capacity, and the mangrove’s efficiency as an estuarine filter (Lacerda et al. 2021).

Shrimp pond effluents are enriched not only in nutrients but also in trace elements, such as Cu and mercury (Hg) present in aquafeed and chemicals used in the production process. Emission factors of N, P, Cu and Hg are higher than all other anthropogenic sources and concentrations in excess of natural levels and ubiquitous in adjacent tidal creek waters. Mangroves within the Jaguaribe river estuary, a significant production area in the SAEC, with over 3,600 ha of shrimp ponds, have increased annual P emissions by 30% to 43.9 tons, following shrimp pond area increase between 2001 and 2006 (Marins et al. 2011). This was followed by an additional increase to 69 tons in 2013 resulting from another increase in shrimp pond area (Marins et al. 2020), effluents from the local shrimp aquaculture represent over 60% of the total phosphorus load from natural and anthropogenic sources to the lower Jaguaribe Basin (Lacerda et al. 2021). Local mangroves receiving these effluents had their efficiency to accumulate P reduced by over 50%, relative to mangroves in estuaries not affected by shrimp farm effluents, triggering algal blooms and eutrophication in the adjacent estuarine waters (Marins et al. 2020).

Changes in tidal creek hydrology also exert a significant impact on mangroves. Reduction in mangrove canopy health adjacent to shrimp farms has been reported and in certain areas has led to the complete degradation and loss of mangrove forests. Normalized Difference Vegetation Index (NDVI) comparing the photosynthetic activity related to canopy structure showed a spatial relationship between mangroves loss and increasing shrimp farm area (Alatorre et al. 2016). At the Jaguaribe estuary, nearly 30% of the total mangrove forest exhibited canopy degradation evidenced by a decreasing NDVI following shrimp farm expansion from 2003 to 2017 (Figure 6). There was a 15% reduction in NDVI between 2003 (0.78) and 2008 (0.65), following shrimp pond area increase from 340 to 1,600 ha; in 2017, there was a further decrease to 0.2, when shrimp farms area increased 10-fold to about 3,400 ha, notwithstanding no direct conversion of mangroves to shrimp ponds. This reduction in the health of mangrove forests is not added to the direct area loss (8%), when computing total area loss of mangrove forest due to shrimp aquaculture. As in the Jaguaribe estuary, this additional canopy loss of integrity would increase the actual mangrove loss up to 15% (Lacerda et al. 2021).

Decommissioning of shrimp farms is not included in Brazilian environmental legislation; soil damage and remaining infrastructure may impair or complicate mangrove restoration and their long-term existence may trigger the occupation of abandoned farms by other activities, resulting in permanent exclusion of mangroves from these areas (Ferreira and Lacerda 2016a).

Global climate change

Climate change represents an increasing direct threat to semiarid mangroves (Alongi 2022), whereas indirectly it can reinforce impacts from local anthropogenic activities (Gilman et al. 2008; Moomaw et al. 2018; Ashton 2022). Contrary to mangroves in humid regions, SAEC mangroves are already under stress from natural drivers, and as in other extreme environments, the impacts from climate change are a reality and have already altered mangrove forest’s structure, extent, distribution and functioning along this coast. Impacts of climate change are triggered, mostly from rising temperatures, sea level rise (SLR), coastal acidification, changes in precipitation patterns, increased storms and extreme weather events and rising atmospheric CO₂, among others (Ward et al. 2016)_.

While occurring and predicted, effects of climate change over mangrove forests have been extensively addressed (e.g. Ward et al. 2016; Ward and Lacerda 2021; Alongi 2022), empirical evidence is still scarce and their indirect effects through impacts on the associated biota are not yet well understood. There are concerns that climate change stressors combined with other anthropogenic stressors impact key biotic FGs resulting in functional degradation potentially eroding resilience and leading to stand dieback (Ferreira et al. 2023) and under certain circumstances, even complete loss of mangroves from certain regions of the SAEC. In addition, the increasing water demand by a growing population along the SAEC requires more damming of rivers and reservoir construction, which will further decrease water and sediment load from the continent to the sea, exacerbating the impacts of climate change.

In summary, mangrove responses to climate change are different, or at least not the same intensity when comparing in semiarid (SAEC) with humid (AMMC) mangroves. For most drivers, impacts are stronger on SAEC mangroves than at the AMMC, mostly due to the already stressful conditions of the SAEC compared to the relatively optimal environmental conditions verified in the AMMC. Table 3 summarises the main climate change impacts on the function of SAEC mangroves from drivers associated with climate change and makes a comparison with humid regions mangroves. Further, each major driver is discussed individually.

Ecosystem services provided by mangroves

Notwithstanding the relatively small area of mangroves on the SAEC, they provide critical ecosystem services that underpin environmental health and human well-being. These services include not only coastal protection (Zamboni et al. 2022) and carbon sequestration (Souza et al. 2023) but also cultural services (Queiroz et al. 2017).

The importance of mangroves as natural barriers against storm surges and coastal erosion along the SAEC has been shown to reduce shoreline exposure to coastal hazards and, therefore, helps safeguard population settlements along a mangrove area in Rio Grande do Norte state, at the easternmost sector of the SEAC (Zamboni et al. 2022).

Carbon sequestration is another crucial service provided by these mangroves. Mangroves on the SAEC contribute to carbon storage and sequestration in below- and above-ground biomass (Souza et al. 2023). The possibilities with Blue Carbon in the SEAC have been celebrated under the assumption that promoting blue carbon can be considered an environmentally responsible strategy and a key measure to ensure a sustainable and prosperous future for the region. However, the same study warns that successful implementation requires the collaboration of various stakeholders, including governments, local communities and non-governmental organisations (Tavares et al. 2023).

One ecosystem service that is frequently overlooked is the cultural aspect. Local communities in the northeast region of Brazil have identified four additional cultural services associated with the preservation of traditional ecological knowledge. These include fostering and maintaining social relationships, personal satisfaction and mental and physical relaxation (Queiroz et al. 2017). Local communities have a symbolic relationship with mangrove forests that extends beyond the typical material perspective used to value ecosystem services. This implies that policymakers should consider the socio-cultural dimension of mangrove services a crucial criterion when addressing the major challenges in coastal ecosystem conservation. However, cultural services provided by northeastern mangroves in Brazil are frequently overlooked in policy-making processes. This oversight is likely indicative of a broader global trend where mangrove ecosystems’ cultural and spiritual values are similarly undervalued.

Conservation status of Brazil’s semiarid mangroves

Despite the vast provision of ecosystem services, mangroves on Brazil’s semi-arid coast are under significant threat, from the deforestation for agriculture and urban development and the pollution from industrial activities to the expansion of shrimp farming (Ferreira and Lacerda 2016a). The detrimental impacts of shrimp farming are particularly concerning, including habitat degradation and water quality deterioration resulting from effluent discharge in northeast Brazil (Lacerda et al. 2021).

In response to these threats, the National Action Plan for the Conservation of Mangroves outlines strategies to protect and restore mangrove areas through legal protections, restoration projects and sustainable management practices (MMA 2015; ICMBio 2019). Another legal instrument protecting mangroves in Brazil is the 2012 Forest Code, which classifies mangroves as permanent protection areas. However, literature on the subject indicates that the 2012 revision of Brazil’s Forest Code has introduced changes that have weakened the protection of mangrove ecosystems (Borges et al. 2017). In present year, a special law (Decree. Nr 12.045/2024) launched in Brazil the ‘National Program of Conservation and Sustainable Use of Mangroves – ‘ProManguezal’) to promote the conservation, recovering and sustainable use of Brazilian mangroves.

Against the backdrop of environmental legislation that has been weakened by Congress and governments in Brazil (Soares-Filho et al. 2014; Ferrante and Fearnside 2019; Losekann and Paiva 2024), the National System of Conservation Units (SNUC) – which establishes protected areas and promotes sustainable natural resource use (Brasil 2000; MMA 2015) – is one of the most robust pieces of legislation to protect ecosystems in Brazil.

In Brazil, 87% of the entire mangrove environment is located within protected areas (ICMBio 2018). However, assessing the effectiveness of mangrove conservation within protected areas in the SAEC presents mixed outcomes. Despite the presence of a dedicated manager and considerable community support in a mangrove protected area in CE state, significant improvements are necessary across all management dimensions. None of the dimensions assessed achieved a ‘satisfactory’ or ‘very satisfactory’ rating (Araruna and Soares 2017). The study identified several areas for improvement, including the need to increase human and financial resources, update and refine management plans and expand environmental education initiatives within the communities.

Further research within the mangroves of marine protected areas in CE state indicated that local stakeholders support the protected areas and desire greater engagement from management institutions (Araruna and Soares 2017; Maia et al. 2019), with a need for a more inclusive approach that addresses local needs (Ternes et al. 2023).

Some of these protected areas rely heavily on co-management strategies or some other degree of involvement of local communities in mangrove conservation. In a review of community-based mangrove management worldwide, Datta et al. (2012) emphasise the effectiveness of involving local communities in conservation efforts, which has improved ecosystem health and resilience. However, the equitable distribution of accrued benefits and services among community members is also a significant concern in these initiatives. Community-led governance, which involves considering local knowledge in selecting rehabilitation and management strategies, encourages genuine participation through mutual assistance and enables independent collective decision-making. For example, the creation of the ‘Sustainable Use Reserve’ (RDS) Ponta do Tubarão in RN State was a demand of the native communities, aiming to break the expansion of aquaculture and real estate speculation (Mattos et al. 2011). However, the success of coastal protection is also influenced by geomorphological traits, indicating the need for an integrated strategy that combines physical and social aspects when shaping community participation (Damastuti et al. 2023).

These concerns extend to management instruments on Brazil’s semiarid coast, where conservation-related subsidies have and will continue to impact the living conditions of local populations positively. However, the impacts on ecosystem health are perceived as a potential concern that has not yet been realised (MDSCF 2016). Irrespective of the legal instrument in place, enforcement remains largely inadequate (Ferreira and Lacerda 2016a). Consequently, the continued occurrence of illegal activities threatens mangrove health. Therefore, it is imperative to reinforce implementation and monitoring efforts to ensure the long-term conservation of these ecosystems, particularly along the semiarid coast, where the natural extreme climate conditions, worsened by global warming, highlight their importance to local traditional populations.

Conservation challenges on the semiarid coast

The specific characteristics of these mangroves have a direct impact on the conservation and management efforts that are undertaken. These characteristics relate to the geomorphological traits of the location of these mangrove areas, the tidal regimes and the nearby environment, which may or may not be adequate for a possible landward expansion of mangroves in a sea-level rise scenario. These traits directly impact the ecological and biological features of these mangroves. A second set of characteristics has to do with the history of the colonisation of the Brazilian northeast coast, which explains the current urbanisation and other land use change patterns that directly affect northeastern mangroves.

Regarding geomorphological and geographical aspects, the available area for mangrove migration is restricted on several sites on the semiarid coast due to urban development expanding at the edge of estuaries (Ward et al. 2023). Additionally, large dune systems along the coastline are natural barriers to mangrove expansion (Lacerda 2018). This may result in mangroves, like other coastal elements, becoming encroached by mobile dunes under the current climate emergency (Maia et al. 2005).

Vis-à-vis historical degradation patterns and poverty on the semi-arid coast, the Brazilian northeast is one of the poorest regions in the country. Therefore, the significance of mangrove ecosystem services is intensified by the pervasive socioeconomic distress experienced by a considerable proportion of the population, including poverty and hunger (Ottonelli and Mariano 2014; Caldas and Sampaio 2015) and insecure employment (Silva Filho and Queiroz 2011). Consequently, the challenges associated with mangrove conservation include the consideration of the impact on local populations, who often depend on these ecosystems for their livelihoods.