The development and use of coastal areas have increased in recent decades, resulting in significant environmental changes [1
]. Thus, these areas have presented different population structure and growth patterns linked to global urbanization trends and demographic changes [2
]. Coastal zones account for only 10% of the Earth’s surface but are home to 54% of the world population [4
] and have a population density 2.6 times greater than interior continental areas [6
These areas offer multiple ecosystem services that contribute positively to human wellbeing, i.e., microclimates, hydrological regulation, tourism and natural resources, among others [8
], connected to a variety of ecosystems, with wetlands standing out. Coastal wetlands are areas of transition between terrestrial and oceanic ecosystems, saltmarshes and/or estuaries; they often border inner bays or low-energy intertidal zones [10
]. Land-use changes have had a substantial impact on these ecosystems, associated mainly with their fragmentation, loss of area and degradation. Indeed, urban growth has been deemed the main anthropogenic stressor, directly responsible for the loss of more than 67% of coastal wetlands, exerting determinant influences on aquatic ecosystems by modifying habitat structure, altering water quality and other actions [2
Coastal wetlands act as nutrient generators for the coastal zone, climate (temperature and humidity) stabilizers and protectors of human settlements from floods, storm surges and/or tsunamis [15
]. They are defined as complex ecosystems as a result of their hydrodynamic characteristics, in which it is possible to connect freshwater unidirectionally through precipitation, groundwater or input from rivers and then bidirectionally through saline water influenced by the tide cycle, prevailing winds and local morphology, variables that determine salinity distributions and stratification. The foregoing influences the chemical characteristics of water [17
] and therefore aquatic biota. Coastal wetlands are catalogued as biologically diverse and highly productive systems [19
], as they are inhabited by a large variety of plant and animal species, including hydrophyte plants along with woody and herbaceous species. They provide a structural habitat for epiphyte bacteria, benthic algae, macroinvertebrates, and fish and generally present dominant species along their salinity and physical stress gradients [21
]. The macroinvertebrates present in these areas provide a fundamental energy subsidy in sections of the food chain for vertebrates such as amphibians, fish and aquatic birds [23
The carrying capacity or resilience of saltmarshes and/or estuaries is determined by interactions of hydrological regimes; sedimentation rates; biomass production; nutrient generation; and processes driven by runoff, salinization and sea level rise [11
]. The seasonal changes in these ecosystems influence water residence time, flooding, pH, salinity, and temperature [30
]. As coastal wetlands are highly complex, multifactorial and geographically dependent, human activities and climate change are having drastic effects on their functioning [32
], affecting local vegetation patterns, increasing the number of and distance between patches [34
], decreasing biodiversity [14
], altering carbon flows or reserves [35
], and exacerbating ecological vulnerability [37
]. It is therefore necessary to contribute to the understanding of these ecosystems in order to design and implement suitable strategies aimed at sustainable management that ensures their preservation and/or conservation [38
]. Thus, this investigation evaluates the relationship between urbanization and the integrity and health of wetlands in order to answer the following question: How do different degrees of urbanization affect the variables linked to the functioning of a coastal saltmarsh wetland?
Chile has an extensive coastal zone, with approximately 83,850 km of coastline and around 40,000 wetlands throughout the country. However, it has a weak planning policy, along with a lack of land-use planning and coastline zoning instruments that would protect ecosystems [16
]. While a bill was recently passed to protect urban wetlands, the lack of such measures in the past allowed a significant reduction in wetland area, mainly due to urban growth and especially in the central part of the country (33–37.5° S), in which 73% of the national population is concentrated [40
]. The case of the Concepción Metropolitan Area (CMA) is especially relevant; since the 1970s, more than 23% of total wetland area has been lost, with saltmarshes and estuaries the most affected [41
]. However, wetlands in the CMA continue to exist amid different degrees of anthropogenic stressors. One of the systems under the most pressure is the Rocuant-Andalién saltmarsh (36°43′ S–73° W); 575 ha had been urbanized by 2004 and 725 ha by 2014 [16
], which has led to the loss of 40% of the wetland, mainly due to housing, road and industrial projects. Since the 1980s it has been used for fishing industry operations and to receive wastewater [15
]. In contrast, the Tubul-Raqui system (37°13′ S–73° W), located south of the CMA, is a wetland with a low degree of urbanization [44
], where the greatest anthropogenic pressure is the discharge of little-treated water directly into the estuarine system.
Assessment of the impacts of urbanization on coastal wetlands must include an understanding of the local environment and especially the factors that are responsible for the characteristics of the ecosystem. Therefore, the objective of this work was to comparatively analyze the functioning of two wetlands with different degrees of urbanization that are located in the coastal zone of the CMA: the highly anthropized Rocuant-Andalién wetland and, as a reference, the Tubul-Raqui wetland, an ecosystem with a low level of urbanization. The investigation will allow these wetlands to be described and the impact of intervention in them on water quality, sediment and aquatic biota to be assessed, providing information that will allow the creation of protection and or/conservation tools for these ecosystems in urbanized environments.
2. Materials and Methods
2.1. Study Areas
The Rocuant-Andalién (36°43′ S–73°60′ W) and Tubul-Raqui coastal saltmarsh wetlands (37°13′ S–73°26′ W), located in the CMA of the Biobío Region, were studied. This area presents a mediterranean climate, with precipitation concentrated in the austral winter, resulting in higher streamflows in winter, while in summer streamflows are diminished. Both wetlands depend on the interaction of marine water from the Pacific Ocean and continental water from their respective drainage basins (Figure 1
The Rocuant-Andalién wetland has a surface area of 767 ha, and its perimeter is under pressure from a consolidated urban area of 90,000 inhabitants, road and port infrastructure, industrial zones, and infilling for future urban projects [16
]. It is located in the Andalién River watershed (71,500 ha), where 4% of land use is urban (Table 1
). Its main freshwater source is the Andalién River, which had average annual streamflows of 4.5 m3
/s in 2016 and 6.6 m3
/s in 2017, with minimum streamflows of 1.01 m3
/s in summer and maximums of up to 565 m3
/s in winter, according to General Water Directorate (DGA, for its initials in Spanish) records (2018). In its last 6 km, the river forms an estuary that sustains the vegetation of the wetland. In this section, the river is mostly channelized and undergoes frequent dredging, contributing to a highly modified regime or hydroperiods. The river drains into the Bay of Concepción, a shallow, semi-closed, highly productive system sustained by bottom water upswelling and intrusion from oceanic zone water with high nutrient content [45
], along with nutrients and sediment that enter from the Rocuant-Andalién wetland.
The Tubul-Raqui wetland is the most important in the region due to its large surface area of 2238 ha. It is made up of a large saltmarsh lying on the coastal plain of fluvial-marine sediment, with sediment from the Quaternary [46
]. Along its perimeter are forestry plantations and a small fishing village of about 2500 inhabitants that presents high poverty rates and lacks wastewater treatment [15
]. The Tubul-Raqui River watershed (23,209 ha) has an urban land-use percentage of 0.1% (Table 1
). The main rivers that form the estuary associated with the wetland are the Tubul and Raqui Rivers, which drain into the Gulf of Arauco, which is a highly productive system maintained by upswelling and entry of oceanic bottom water, along with the input of the Tubul-Raqui wetland, all of which are rich in nutrients [48
2.2. Physical-Chemicals Parameters of Wetland Water Quality
In the center of the channels of the main rivers of each wetland, from the mouth to the upper part of the estuary (defined by the salt tide), samplings of physical, chemical and biological variables were carried out in winter of 2016 and summer of 2017. For the Rocuant-Andalién wetland, there were six sampling stations, while for the Tubul-Raqui wetland, there were six stations in the Tubul River and three in the Raqui River.
In situ observations of salinity, temperature, pH, dissolved oxygen, and turbidity were obtained with a previously calibrated Hanna HI9829 multiparameter (In-situ Inc., Ft. Collins, CO, USA). Dissolved oxygen was automatically corrected by the device with respect to electrical conductivity with combined sondes. The measurement was carried out with a luminescent sensor, preventing the errors associated with membrane sensors.
At each of the sampling points, six sediment replicates were collected from the bed using a Van Veen manual dredge (0.025 m2
capacity). Three replicates were used for granulometry analysis in the Sedimentology Laboratory of the Universidad de Concepción. The samples with high fine content (below 63 µm) were analyzed using laser diffraction with a Mastersizer, while the coarse material samples were analyzed by sieving with an AS-200 Control. The granulometric parameters were obtained by integrating the coarse and fine fractions in Gradistat v8.0 [50
], according to the method of Folk & Ward [51
2.3. Biological Sampling
One liter of water per sampling point was collected in the center of the channels for the determination of microbiological parameters and was kept cold and in darkness and transported to the laboratory to measure total and fecal coliforms using the Standard Methods for the Examination of Water and Wastewater 9221 F, E, B (APHA, 1992; Anon, 2012), which were analyzed in the Microbiology Laboratory of the School of Biochemistry and Pharmacy of the Universidad de Concepción.
2.4. Macroinvertebrate Assemblages
The macroinvertebrates were sampled from the center of each channel using a Van Veen manual dredge (0.025 m2
capacity), with three replicates per station. The samples were fixed in situ with 7% formalin and then transported to the laboratory, where the organisms were separated and preserved in 70% ethanol. All the individuals of each taxon were identified and counted under a stereomicroscope (Zeiss, model Stemi Dv4), and were identified to the lowest possible taxonomic resolution, using available taxonomic keys and reference collections [52
2.5. Statistical Analysis
For the analysis of physical-chemical variables, a two-way repeated measures ANOVA model was fitted. In the cases where interaction was not significant, the main effects were analyzed; otherwise, contrasts were carried out, fixing the level of one factor and comparing the levels of the other factor. In addition, principal component analysis (PCA) and Spearman correlation analysis of the variables for both wetlands were carried out using the R program (R Core Team, 2016). Prior to the analyses, the physical-chemical data were normalized. To analyze the macroinvertebrate community structures of the wetlands, the richness (S) and abundance (N) indices were calculated, allowing the alpha ecological diversity to be determined; to this end, various indices were used comparatively to lend robustness to the results, i.e., the Shannon–Weiner index (H’) (bits ind), Simpson index (C), Margalef index (Dmg), and Menhinick index (Dmn).
The macroinvertebrate assemblage data were used to identify differences between the sampling sites and/or seasonality. Canonical correlation analysis (CCA) was used to understand and analyze relationships in the assemblage structure and its distribution, along with the gradients of specific environmental factors. An analysis of variance was carried out to determine significant differences in abundance between the studied wetlands.
Our results suggest that the Rocuant-Andalíen and Tubul-Raqui wetlands maintain a marked marine influence. In summer, the saline influence reaches a greater area and concentration, although it does not seem to influence dissolved oxygen content given the low average depth of both systems, i.e., ≥1 m. The sediment transported by freshwater causes substantial changes in the estuary channels; this is especially significant in the mud content and high turbidity values observed in the Rocuant-Andalíen wetland, which is characteristic of urban wetlands and is also associated with high total and fecal coliform concentrations resulting from illegal discharges or possible leaks in the wastewater network of the city of Concepción.
In both watersheds, the rainfall regime determines the expansion of the flood area, such that any extreme climate event, along with urban expansion into both wetlands, can drastically alter long-term hydrological and biogeochemical responses. The benthic macroinvertebrate diversity and richness indices were lower in the highly urbanized Rocuant-Andalién wetland, which has undergone changes in the structure and composition of these organisms, compared to the wetland with less intervention, which presented an abundance 8.5 times greater and an increase in specific richness. Thus, the analysis of macroinvertebrate assemblage structure in a wetland can be a good indicator of these complex ecotones.