Landscape biophysical constraints of the study area
The study area currently falls within the temperate Mediterranean climate with hot and dry summers, according to the Köppen & Geiger classification. The average annual precipitation is around 600 mm (IPMA 2019). The climograms of the meteorological stations show the average annual rainfall indices from 1971 to 2000; they are: 568 mm in Alcácer do Sal, 559.8 mm in SETENAVE, and 715.9 mm in Fruticultura (Fig. 2A, B and C). These stations are located at altitudes of 51 m, 4 m and 35 m, respectively. Rainfall is concentrated in the autumn-winter period, representing 60% of the annual volume, with rates above 70 mm from October through January. In the Setubal-Fruticulture station three months record precipitation above 100 mm in that period, being this volume certainly influenced by the station's geographical position.
The graphs show that the annual thermal averages vary between 10°C and 23.3°C, with a range close to 13°C. The highest temperatures occur in summer, with maximum averages between 28.8°C and 30.8°C in July and August. In the winter, the average minimum temperatures range from 4.4°C in Alcácer do Sal to 6.5°C in Setúbal.
Precipitation is the main conditioning factor of morphogenetic processes and hydrographic dynamics. Water availability influences the land use and is essential for irrigation projects, power generation, and population supply. The maritime context of the study area attenuates the thermal contrasts between the year's seasons. Locally, geographical factors such as the Serra de Grândola can also interfere in the temperature and rainfall distribution. The regional geological context encompasses lithostratigraphic units from the Upper Paleozoic to the Quaternary. The Iberian paleozoic basement witnessed the Mesozoic North-Atlantic opening and the Cenozoic Africa-Europe collision. According to Pais et al. (2013), compressive and distensive alpine phases originated several sedimentary basins with a NE-SW and E-W orientation. These basins include sedimentary records of continental and marine origin, revealing a combination of tectonic, paleogeographic, climatic and eustatic events, with which the Tertiary Sado Basin is related.
The Sado River and its tributaries drain almost permanently into detrital rocks and deposits that filled the Sado Cenozoic sedimentary basin and, locally, some metamorphic and igneous Paleozoic rocks (Pimentel 1997). In his study, Pimentel (1997) proposed three sectors for this basin: i) the northern sector (not studied by the author) - a stretch located downstream from Alcácer do Sal, towards the mouth of the river; ii) the intermediate sector – the territory to the south of this municipality and the portion to the north of the alignment of the river. Grândola fault, extending southeast to Ferreira do Alentejo, and iii) the southern sector – region between the south of the Grândola fault to close to the Messejana fault.
The Sado Basin is composed of tertiary sediments that filled the Paleozoic subsiding blocks, which may be seen around Alcácer do Sal and in some deeper tributary valleys on the right bank of the Sado River (Brito 2009). The author points out that this basin is limited to the north by the Cenozoic Tagus Basin, with Tertiary sediments, and Serra da Arrábida, made up of Mesozoic carbonate rocks – limestones, marls and dolomites. The eastern limit corresponds to a morphological high, probably associated with tectonism, which marks the contact between Tertiary sedimentary rocks and Paleozoic rocks from the South-Portuguese Zone (ZSP) and the Ossa-Morena Zone (ZOM). To the west, the basin extends to the ocean and to the southwest it is limited by the Grândola fault. The geological map of the estuarine municipalities shows the distribution of the different units (Fig. 3).
The ZSP metasediments present a low degree of metamorphism, while in the ZOM, metasedimentary and igneous rocks present a medium to high degree. The rocks of the ZSP constitute the main substrate of the Sado Basin and the surrounding reliefs, while lithologies of the ZOM are found on its eastern and northeastern edges (Pimentel 1997).
On the left bank of the Sado River, schists and greywackes from the Mértola Formation (ZSP) make up the Serra de Grândola. These rocks contact rocks from the Volcano-Sedimentary Complex of the Iberian Pyrite Belt (ZSP), with important metallic sulfides. Pebbly sands, clays and bioclastic marls of the Late Miocene Esbarrondadoiro Formation also outcrop in the area. On the right bank, between the Soberanas and Algalé streams, there are rocks from the ZOM and the Beja Complex. On the North of Alcácer do Sal, between the Santa Catarina and São Martinho streams, in addition to the lithologies of this Complex, there are schists and greywackes (Fig. 3).
Geology is a strong determinant of geomorphology. However, the sculpting of the relief results mainly from the processes linked to the climate and the fluvial dynamics, acting on the features resulting from the internal geodynamics.
The Baixo Alentejo corresponds to a plain shaped into Paleozoic terrains, with more than 3,000 km² and elevations of around 200 m (Pimentel 1997). According to this author, this morphology was considered for a long time to be a peneplain, a senile feature of the relief referring to the Davis Erosion Cycle. Subsequently, geomorphologists classified it as a pediplain, understanding that its genesis is related to sedimentation processes in a semiarid climate, probably since Early Tertiary times. Therefore, it originated from successive erosive cycles under different morphoclimatic systems, which acted on the rocks of the Hesperian Massif.
In the landscape of the studied region, a flat and smooth wavy relief dominates, with low declivity and altimetry, except for some mountains resulting from alpine tectonism and/or the action of differential erosion. The main reliefs are Grândola, Loureiro, Palma, Cordoeira, Alta and Serrinha mountains, as well as the Senhor das Chagas and Pedrógão plateaus. Bordering the mouth of the estuary, the São Luís and Louro mountains that make up the Arrábida Chain stand out, a relief associated with alpine tectonism that acted on in the mostly carbonate units of the Mesozoic Lusitanian Basin (Kullberg et al. 2006).
The geomorphological map (Fig. 4) presents a table with the hierarchical subdivision of the relief based on Ross (1992) and includes topographic profiles to make the terrain's topography evident. Three Morphostructural Units were identified: A) Hesperian Massif, B) Sado Tertiary Basin and C) Inverted Mesozoic Basin.
The Hesperian Massif has two Morphosculptural Units: A1 - Inland Residual Erosional Surface; A2 - Coastal Uplifted Erosional Surface. The Sado Tertiary Basin has three Morphosculptural Units: B1 - Sado Pediplaned Surface; B2 - Fluvial Morpho-sedimentary Surface; B3 - Coastal Surface. The Inverted Mesozoic Basin has one Morphosculptural Unit: C1 - Arrábida Mountain. To these six Morphosculptural Units correspond their equivalent Geomorphological Units, which are, by the same order: A1* Dissected Plateaus, A2* Dissected Residual upper surface; B1* Dissected surface with very smooth hills and valleys; B2* Alluvial plain; B3* Coastal plain; C1* Culminating Surface and Intermontane Arrábida depressions. The different Geomorphological Units include different types of relief, the most representative being: inland saws with narrow interfluves, coastal saw, low plateaus, dissected plateaus, tabular tops, colluvial ramps, hills, colluvial fans, river terraces, alluvial fans, fluvio-marine plain, sand dunes field, beach berm (Fig. 4).
The Morphostructural Units are products of internal geodynamics and the basis for the action of exogenous processes. They group the Morphosculptural Units that are more extensive, as they bring together sets of patterns of shapes, that is, the Geomorphological Units composed of erosion and aggradation models resulting from erosive and sedimentary fluvial, marine and wind processes.
A - Hesperian Massif Morphostructural Unit – it corresponds to the Paleozoic rocks from ZSP and ZOM, including two different Geomorphological Units (Fig. 4):
A1* - Dissected Plateau includes several “Inland saws with narrow interfluves”, such as the Alta, Palma, Loureiro, Cordoeira and Serrinha mountains, in Alcácer do Sal. They form a WNW-ESE alignment, following the Torrão fault. The average altimetry of this set is around 150 m, except for Serra Alta, with approximately 240 m. The characteristics of the lithologies favored the conservation of these features in the landscape.
A2* - Dissected Residual upper surface is represented by “Coastal saw and low plateaus”, on the left bank of the Sado. The Serra de Grândola corresponds to an uplifted and tilted Paleozoic block, acted by the alpine tectonism (Pimentel 1997). The features that stand out are the fault escarpment with a WNW-ESE orientation and the residual reliefs distributed southeast of the culmination point up to where the Sado River intersects the escarpment. They are low plateaus with decreasing altimetry from 260 m to 130 m. Between them, there are depressions produced by river erosion. The valleys follow the directions of the main regional structures – WNW-ESE and NE-SW. The tectonic efforts of the Messejana, Grândola and Torrão faults probably generated secondary faults, which are buried under sedimentary deposits and are revealed by fluvial dissection through subparallel and subdendritic drainage patterns.
B - Sado Tertiary Basin Morphostructural Unit includes three different Geomorphological Units, with reliefs developed on Tertiary detrital rocks, both continental and marine (estuarine to coastal).
B1* - Dissected surface with very smooth hills and valleys, it occupies most of the territory, extending from the Serra de Grândola escarpment to the eastern limit of the study area, heading northwest to Serra da Arrábida (Fig. 4). This flattened surface of low slope characterizes the pediplain and a smooth undulating relief with staggered levels dominates the landscape. They are dissected plateaus, hills, colluvial ramps and coalescing alluvial fans whose contact is marked by an erosive edge. Often, these features have tops with sand sheets and continental dunes.
The degree of fluvial dissection of the relief presents contrasts. The left bank compartment is more conserved, composed of extensive interfluves, gentle slopes and wide valleys, possibly related to the thicker sedimentary cover. On the right bank, the intense fluvial dissection sculpted narrow interfluves, slopes with notorious ruptures and embedded valleys, which may be an influence of the Paleozoic basement being closer to the surface. The drainage network shows aspects of structural control. In the upper course, the Grândola stream drains in the S-N direction and then inflects to the WNW-ESE, bordering the escarpment until the confluence with the Sado River. On the right bank, the main rivers and streams drain in the NE-SW direction, showing the parallelism of the channels.
B2* - Alluvial plain is formed by features and environments related to fluvial dynamics subjected to alternation of Quaternary morphoclimatic systems. In wet periods, erosion of river beds led to the abandonment of floodplains, forming terraces. The most extensive terraces are on the right bank at the confluence of the tributaries with the Sado River. They are formed by sand and gravel, with few clay levels (Zbyszewski et al. 1976). In the driest periods, depositional processes formed alluvial fans and ramps in areas with topographical contrast. These features usually make contact with the terraces. The most extensive floodplains are currently associated with the Sado River and its main tributaries.
B3* - Coastal plain gathers models and environments from marine, fluvial and wind deposits. The marine transgressive movements of the Quaternary led to the drowning of river valleys, forming estuaries, in addition to marine terraces, coastal dunes and fluviomarine or tidal plains. In the Sado Estuarine Complex (SEC) the fluvio-marine plain extends into the terminal sector of the Sado and its tributaries. It is formed by fine sediments and organic matter submitted to the hydrodynamics of the tides and the other forcings. The presence of salt marshes can identify it, vegetation composed of halophytic species adapted to environmental conditions – Spartina maritima, Halimione portucaloides, Sarcocornia fruticosa and Sarcocornia perenis. The first characterizes the low marsh and the others the high marsh (Inácio 2017). The SEC encompasses ecosystems of great biodiversity, being a nesting space for migratory birds, reproduction of marine organisms and extractive practices by traditional communities.
Waves, biological processes and anthropogenic action influence the tidal flat, and sediment input reduction can favor erosion (Gao 2019). The tidal depth is the area of the daily flooded plain, drained by narrow, small channels and without vegetation. It is usually formed by mud and may vary between mud and low-vessel sand, increasing the granulometry in depth and in the distal area of the marsh (Cunha et al. 2017).
The sandy features of Coastal plain are linked to the Tróia Peninsula, formed about 6,500 years ago B.P., which has an approximate length of 24 km and is essentially composed of parabolic dunes (Costas et al. 2015). The dunes, which in some sectors reach a height of 20 m, were created by reworking the sediments of the marine terraces and the coast. In Caldeira de Tróia, the dunes are in contact with the high marsh, whereas on the coast they border the marine terraces.
C - Inverted Mesozoic Basin Morphostructural Unit incorporates the Arrabida Mountain, characterized by the Geomorphological Unit C1* - Culminating surface and intermontane depressions of Arrábida, formed by Coastal saws and valleys. The Arrábida mountain range composed of Jurassic rocks is related to processes of the Alpine orogeny (Fonseca et al. 2015). It is characterized by a set of mountains in the territories of Setúbal and Palmela. In Fig. 4, the feature is fully represented to show how the position of Cabo Espichel protects the Sado estuary from the action of coastal currents, which move in the N-S direction.
Anthropogenic interventions in the landscape of Sado estuarine municipalities - contextualization of land use and land cover
Studies on land use and occupation patterns reveal how societies organize themselves in space and how they use natural resources. Human interventions consist of changes in natural components, which manifest through imbalances or environmental impacts directly linked to the technological development of society. The analysis of the Society-Nature relationship enables the identification of anthropogenic interventions in the geographic space and the elaboration of proposals for territorial planning and management, which harmonize the socioeconomic projects and the carrying capacity of the natural components.
The human occupation of the study area started in the Mesolithic (Arnaud 1987; Carvalho 2009; Costa et al. 2020), approximately 8400 − 7000 cal AP (Costa et al. 2020) favored by the ecological-geographical characteristics of the Sado estuary. These Mesolithic groups explored marine food resources (Carvalho 2009), collecting bivalve molluscs, fishing and hunting; and remained in the area for about a thousand years (Arnaud 1987). In the Sado valley and its tributaries upstream of Alcácer do Sal, shell middens associated with fluvial terraces and lower sectors of slopes are testimony of their presence (Costa et al. 2020).
Holocene climatic oscillations in the Portuguese territory influenced the original land cover. In the coastal region, the dominance of the sub-humid climate favored the expansion of Pinus pinaster and Quercus forests (Carvalho 2009).
Through time, the Sado estuarine municipalities were occupied by different groups - Romans, Moors and French, among others - with interest in natural resources and/or strategic geographical position. Archaeological studies in the territory of Alcácer do Sal confirm the existence of Roman amphorae ovens dating from the mid-1st century BC to the 5th century AD in the Abul and Herdade do Pinheiro. The production of amphorae was destined for the industrial complex of Tróia, where fish salting was processed (Mayet and Silva 2016). Although the use of natural resources and the occupation of the area are long-standing, the evidence of anthropogenic modifications by these people does not indicate significant environmental changes.
Salt farming has been established in Setúbal and Alcácer since the Roman period, occupying areas of the fluviomarine plain. Although there are no supporting documents, in the 14th century, a surplus of the product was recorded in these municipalities. In the following century, it expanded to the Mitrena-Setúbal peninsula due to the fishing activity in the estuary. In the 16th century, salt farms were built on the Sado marshes, mainly in Abul, Batalha, Faralhões and Gâmbia. Saliculture was prominent in the Portuguese economy until the end of the 19th century when it began to decline (Quintas 1998).
In the cartogram prepared based on the Pery Charts, the main land use and land cover types in the last decades of the 19th century and early years of the 20th are represented (Fig. 5). At that time, most of the territory of the Sado municipalities was occupied by heaths and scrubland, spontaneous xerophytic vegetation that colonized the almost flat relief of the Morphosculptural Unit - Sado Pediplaned Surface. This vegetation protected the sandy-textured, low-fertility soil from erosion by adding organic matter. According to Quintas (1998) some documents record the transformation of extensive areas of pinewoods and heaths into land with vineyards. The author highlights the large property owned by José Maria dos Santos, which included territories in Palmela, Setúbal, Alcácer do Sal and Grândola, considered the largest vineyard in the world. Quintas (1998) also points out that, since the beginning of the 18th century, rice cultivation was established in the Setúbal district, mainly in the council of Alcácer do Sal.
The dominant arable or cereal crops in the area, wheat and rice, were developed in the marginal banks of the Sado River and its tributaries due to their water demand. Therefore, the forest use corresponded to what is known today as “montado”: The forest was made of cork oak (Quercus suber) - an endemic Mediterranean species which can occur associated with holm oak (Quercus ilex) - besides the oak and pine forests (Pinus pinaster). In the 19th century, the cork oak forests occupied the territory of Grândola, Águas de Moura-Palmela, Alcácer do Sal and marginal areas of the Sado River, while the pine trees occurred in the central area, and the holm oaks predominated inland, where the climate is drier (Fig. 5).
At this time, salt pans dominated the SEC fluvio-marine plains and rice was grown on plains with a more significant influence of freshwater. Horticultural crops and vineyards predominated in the northwest (in Palmela and Setúbal). In Alcácer do Sal, vineyards were often combined with olive groves.
At present, the landscape is marked by seven land use and land cover classes: Artificial Surfaces - urbanized area, industry, commerce; Agriculture - viticulture, olive growing, rice growing and polyculture; Pasture - native and planted; Agroforestry and forestry area - cork oak forest (Quercus suber), holm oak (Quercus ilex) and pine forest (Pinus pinea L. and Pinus pinaster L.); Mixed vegetation - shrubs and forests; Coastal environment – beaches and dune fields (tourism and recreation); Coastal wetlands - salt marshes and other intertidal areas with halophytic plants (Fig. 6).
The landscape of the estuarine municipalities is marked by the following productive activities: agrosilvopastoral systems (“montado”), rice farming, salt production, mining activities, and urban development (Fig. 7).
Rice farming stands out in the floodplains and part of the fluvio-marine plain, particularly in Carrasqueira (Alcácer do Sal). Viticulture predominates in the northwestern sector, in Palmela and Setúbal, and olive growing in the southeastern sector, in the parish of Torrão-Alcácer do Sal. Polycropping systems are widespread in all the municipalities and include diverse crops; this type of management favors the conservation and replacement of soil nutrients.
Also important are the agro-silviculture-pastoral and/or agroforestry systems represented mainly by the cork oak forest, which integrates the Quercus suber species with grazing land for raising cattle or sheep. This system is managed in a way that favors landscape conservation and is a cultural trait of the Alentejo region.
The cork oak has ensured Portugal's position as the world's largest cork producer. Pine forests (Pinus pinea L. and Pinus pinaster L.) also cover a large part of the territory, as the raw material allows for different uses in the wood, furniture, cellulose and paper industries. The species are well adapted to the sandy and not very fertile soils.
The anthropogenic modifications that most impact the landscape are associated with the exploitation of mineral resources. This extractive activity produces an environmental imbalance with the mischaracterization of biotic and abiotic elements of the landscape – relief, soil, vegetation, destruction of habitats and loss of biodiversity. This activity interferes in relationships between the components of the natural dynamics. In Serra da Arrábida, the quarries of limestone for cement production led to the suppression of soil and native vegetation and the alteration of the relief. To minimize these impacts, environmental restoration is sometimes applied and consists of reforestation of the scars exposed on the slopes. Almost a dozen quarries are installed in the Arrábida chain, which is inserted in an environmental protection unit – the Arrábida Natural Park.
At Serra de Grândola, scars from the mining activity of the Caveira and Lousal mines installed in the area of the Iberian Pyrite Belt can be seen. Studies indicate that besides landscape degradation, there is evidence of soil and water contamination by heavy metals (Santinhos et al. 2010). In addition to these environmental problems, there are also records of social impacts derived from this activity (Burguete 2018). In Alcácer do Sal, the exploitation of geological resources (kaolin and quartz) occurs in the Salgada Lagoon and the sandpit of the Casal Ventoso Mine that explore the Holocene dunes and aeolian sand deposits (ERA-Arqueologia SA 2012).
Moreira et al. (2009) analyzed sediments from tidal flats and salt marshes. They found different levels of contamination by heavy metals - namely, copper, zinc and lead - between the areas closer to the margins of the estuary - Malha da Costa (Tróia Peninsula), Faralhão (Setúbal), and Carrasqueira - and those in the innermost portion of the estuary (both in Alcácer do Sal). The authors concluded that there are several sources of contaminants, such as industrial discharge, presence of ceramic material and mining activities.
In the hydrographic network of the Sado basin, the most frequent anthropogenic modifications are dams. They alter the flow dynamics and sedimentary processes. The reservoirs enable multiple uses of water - supplying the population, generating energy, and installing irrigation projects.
Public policies also contributed to anthropogenic modifications. In Portugal, in the first three decades of the 20th century, land use was strongly influenced by the Wheat Campaign, the Irrigation Plan and the Forest Population Plan. The former emphasized the importance of wheat production to the Portuguese economy. Thus, this crop occupied the low-fertility soils with the suppression of heaths and scrublands and the use of agrochemicals and mechanization. The Irrigation Plan stressed the need for dams for irrigation projects aimed at irrigated farming, and the Forest Population Plan encouraged forest expansion for the growing demand of the pulp industry, among others (Carmo 2018). These economic orientations implied changes in land use, soil management, the population's daily practices and the landscape.
The SEC is home to the Sado Estuary Nature Reserve (RNES), a protected area created in 1980 to guarantee the conservation of the estuary and the development of activities compatible with the estuarine ecosystem. To this end, the Reserve Management Plan (PORNES) was established in 2008, which defines land uses for the National Ecological Reserve (REN) - areas that are not heavily urbanized or of biological interest - and for the National Agricultural Reserve (RAN) - lands with fertile soils or propitious agricultural use (Inácio 2016) suitable for agriculture due to agroclimatic, geomorphological and pedological characteristics. This area is considered by the Directorate-General for Agriculture and Rural Development (DGADR 2022) as a territorial management instrument. However, an overlap of RAN in areas of REN is observed, with evident use of areas of the fluvio-marine alluvial plain by rice farming and salt farming.
Traditional activities, fishing and shellfish gathering are still practiced in the SEC by coastal communities. Aquaculture is developed in the fluvio-marine margins - fish farming and oyster farming. Between 1953 and 1973, the Sado and Tejo estuaries had the largest natural oyster banks, with production destined essentially for export. From the mid-1960s production began to decline due to several factors - overexploitation, diseases, and pollution of estuarine waters by industrial waste (Almeida 2019). Currently, the Portuguese Environmental Agency defines management plans for estuaries to recover habitats for threatened species. The 2014–2015 CrassoSado project recorded a recovery trend of the Portuguese oyster in SEC, Marateca and Carrasqueira (Almeida 2019).
Tourist use of the estuary is encouraged by the presence of the only bottlenose dolphin community in Portugal. Studies point out that maritime traffic in the Sado estuary probably causes physiological changes - temporary hearing loss and behavioral - changes in the emission of acoustic communication signals among individuals of this population, depending on the level of noise produced by vessels (Cruz 2012; Sobreira 2017). According to Cruz (2012), the Port of Setúbal was the site that recorded the highest noise level, above the limit considered bearable for the species.
The municipality of Setúbal stands out among the others concerning the urbanization process. It concentrates port and industrial activities at the Mitrena Peninsula, which require frequent dredging of the estuary to allow the traffic of larger ships. Dredging is a permanent threat to estuarine biotopes; to the marine prairies that support living populations, including the bottlenose dolphin community; to the salt marsh and mudflat areas due to the increase in water turbidity, among others. The Institute for Nature Conservation and Forests (ICNF 2022) recognizes that dredging may cause environmental problems and reduce the salt marsh area. However, they understand that they are carried out downstream of the RNES, i.e., outside the area included in PORNES.
The study area has a land tenure structure based on large estates, the "herdades", some of them close to the coast, characterized by traditional productive activities and typical Alentejo cultural practices. In summary, Table 1 lists the land use and coverage types of the estuarine municipalities and the percentages corresponding to the areas represented in Fig. 5.