From blue to black: Anthropogenic forcing of carbon and nitrogen influx to mangrove-lined estuaries in the South China Sea

Effects of catchment land use on temperate mangrove forests

Human land use changes are threatening the integrity and health of coastal ecosystems worldwide. Intensified land use for anthropogenic purposes increases sedimentation rates, pollutants, and nutrient concentrations into adjacent coastal areas, often with detrimental effects on marine life and ecosystem functioning. However, how these factors interact to influence ecosystem health in mangrove forests is poorly understood. This study investigates the effects of catchment human land use on mangrove forest architecture and sedimentary attributes at a landscape-scale. Thirty sites were selected along a gradient of human land use within a narrow latitudinal range, to minimise the effects of varying climatic conditions. Land use was quantified using spatial analysis tools with existing land use databases (LCDB5). Twenty-six forest architectural and sedimentary variables were collected from each site. The results revealed a significant effect of human land use on ten out of 26 environmental variables. Eutrophication, characterised by changes in redox potential, pH, and sediment nutrient concentrations, was strongly associated with increasing human land use. The δ15N values of sediments and leaves also indicated increased anthropogenic nitrogen input. Furthermore, the study identified a positive correlation between human land use and tree density, indicating that increased nutrient delivery from catchments contributes to enhanced mangrove growth. Propagule and seedling densities were also positively correlated with human land use, suggesting potential recruitment success mechanisms. This research underpins the complex interactions between human land use and mangrove ecosystems, revealing changes in carbon dynamics, potential alterations in ecosystem services, and a need for holistic management approaches that consider the interconnectedness of species and their environment. These findings provide essential insights for regional ecosystem models, coastal management, and restoration strategies to address the impacts of human pressures on temperate mangrove forests, even in estuaries that may be relatively healthy.

Nitrogen enrichment changed the biogeochemical role of sesarmid crabs by shifting their diets in tropical mangrove ecosystems

Sesarmid crabs modulate nutrient dynamics of tropical mangroves through their leaf-eating habit. How N enrichment may alter this regulatory role, and the implications for mangrove nutrient dynamics, remain unclear. Using a mesocosm experiment, we tested how N enrichment could change the microphytobenthos (MPB) communities, thus modifying the crabs' diet and their role in nutrient dynamics. The factorial experiment combined with field investigation revealed a significant increase in the relative abundance of cyanobacteria. Stable isotope analysis suggested that the main carbon source of crabs shifted from leaf litter to cyanobacteria in mesocosms under both high (20×) and low (2×) N enrichment treatments. The significantly lower total cellulase activity of crabs in the mesocosms might explain the decreased carbon assimilation from leaf litter. The changes in the MPB and the microbiome with N enrichment in the presence of crabs may drive significantly higher carbon processing rate in tropical mangroves.

Food web dynamics in the mangrove ecosystem of the Pearl River Estuary surrounded by megacities

Global recognition that mangroves support coastal ecosystem services has increased; however, studies on trophic dynamics in mangrove ecosystems remain limited. We seasonally analysed the δ¹³C and δ¹⁵N of 34 consumers and 5 diets to elucidate the food web dynamics in the Pearl River Estuary (PRE). Fish had a large niche space during the monsoon summer, reflecting increased trophic roles. In contrast, the small niche space of benthos over seasons reflected consistent trophic positions. Consumers mainly utilized plant-derived organic matters in the dry season and particulate organic matters in the wet season. The present study with literature reviews revealed characteristics of the PRE food web with the depleted δ¹³C and enriched δ¹⁵N, indicating a high contribution of mangrove-derived organic carbon and sewage input, particularly in the wet season. Overall, this study confirmed the seasonal and spatial trophic dynamics in mangrove forests surrounding megacities for future sustainable mangrove ecosystem management.

Soil nutrient distribution and plant nutrient status in a mangrove stand adjacent to an aquaculture farm

The marine aquaculture industry has caused a suite of adverse environmental consequences, including offshore eutrophication. However, little is known about the extent to which aquaculture effluents affect nearby wetland ecosystems. We carried out a field experiment in a mangrove stand located between two effluent-receiving creeks to estimate the extent to which marine aquaculture affects the soil nutrient distribution and plant nutrient status of adjacent mangroves. Carbon (C), nitrogen (N), and phosphorus (P) contents and C isotopic signatures were determined seasonally in creeks, pore water, surface soils, and in the leaves of the dominant mangrove species Kandelia obovata. The creeks exhibited nutrient enrichment (2.44 mg N L^-1 and 0.09 mg P L^-1 on average). The soils had N (from 1.40 to 2.70 g kg^-1) and P (from 0.58 to 2.76 g kg^-1) much greater than those of pristine mangrove forests. Combined analyses of the N:P ratio, nutrient resorption efficiency, and proficiency indicated that soil P met plant demands, but plants in most plots showed N limitation, suggesting that soil nutrient accumulation did not fundamentally impact the plant nutrient status. Collectively, this case study shows that marine aquaculture farms can affect adjacent mangrove stands even though their effluents are not directly discharged into the mangrove stands, but mangrove forests may have substantial buffering capabilities for long-term nutrient loading.

Rainfall induced water and nutrient fluxes at a mangrove estuary

Deep understanding of the nutrient exchange between a mangrove estuary and the surrounding coastal region is important for preserving nearshore biodiversity and sustaining appropriate human ecotourism. This study investigated the effect of tidal changes on the water quality and nutrient fluxes of a small mangrove estuary in Japan. Temporal changes in the water temperature, salinity, dissolved oxygen, turbidity, and nutrient concentrations (NO₃, NH₄, and TN) were measured. Nutrient fluxes were calculated using data collected from the Fukido Estuary in 2016 and compared against data collected in 2007. The results showed that NO₃, NH₄, and TN exhibited a negative correlation with salinity in 2016, but not in 2007. The water and TN net fluxes changed with rainfall. The findings of this study provide useful insights into the nutrient fluxes in a mangrove estuary and facilitate the evaluation of tidal fluctuations and weather-induced effects on nutrient fluxes.

Diversity, abundance, and distribution of anammox bacteria in shipping channel sediment of Hong Kong by analysis of DNA and RNA

Anammox bacteria have been detected in various ecosystems, but their occurrence and community composition along the shipping channels have not been reported. In this study, anammox bacteria were recovered by PCR-amplified biomarker hzsB gene from the genomic DNA of the sediment samples. Phylogenetic tree revealed that Candidatus Scalindua and Ca. Brocadia dominated the anammox community of the Hong Kong channels; Ca. Scalindua spp. was present abundantly at the sites farther from the shore, whereas Ca. Jettenia and Ca. Kuenenia were detected as the minor members in the estuarine sediments near the shipping terminals. The highest values of Shannon-Wiener index and Chao1 were identified in the sediments along the Urmston road (UR), suggesting the highest α-diversity and species richness of anammox bacteria. PCoA analysis indicated that anammox bacterial communities along UR and Tai Hong (TH) channel were site-specific because these samples were grouped and clearly separated from the other samples. The maximum diversity of anammox bacteria was detected in UR samples, ranging from 6.28 × 10⁵ to 1.28 × 10⁶ gene copies per gram of dry sediment. A similar pattern of their transcriptional activities was also observed among these channels. Pearson's moment correlation and redundancy analysis indicated that NH₄⁺-N was a strong factor shaping the community structure, which showed significant positive correlation with the anammox bacterial abundance and anammox transcriptional activities (p < 0.01, r > 0.8). Also, NH₄⁺-N, (NO₃^- + NO₂^-)-N, and NH₄⁺/NO_X were additional key environmental factors that influenced the anammox community diversity and distribution. This study yields a better understanding of the ecological distribution of anammox bacteria and the dominant genera in selective niche.

A functional analysis reveals extremely low redundancy in global mangrove invertebrate fauna

Global mangrove deforestation and degradation drive the loss of the associated invertebrate fauna vital to ecosystem services. The functional diversity and resilience of this fauna has not been assessed. We show that even small mangrove patches host functionally diverse faunal assemblages and can act as biodiversity reservoirs. However, globally, functional redundancy of mangrove invertebrates (i.e., the average number of species performing a similar functional role in an assemblage) is extremely low, except in Southeast Asia. Thus, even a modest local loss of invertebrate diversity will have significant negative consequences for mangrove functionality and resilience. Current approaches to assess threats to mangroves heavily rely on loss in areal extent, but our results suggest that loss of function may be more vulnerable.

Deforestation results in habitat fragmentation, decreasing diversity, and functional degradation. For mangroves, no data are available on the impact of deforestation on the diversity and functionality of the specialized invertebrate fauna, critical for their functioning. We compiled a global dataset of mangrove invertebrate fauna comprising 364 species from 16 locations, classified into 64 functional entities (FEs). For each location, we calculated taxonomic distinctness (Δ+), functional richness (FRi), functional redundancy (FRe), and functional vulnerability (FVu) to assess functional integrity. Δ+ and FRi were significantly related to air temperature but not to geomorphic characteristics, mirroring the global biodiversity anomaly of mangrove trees. Neither of those two indices was linked to forest area, but both sharply decreased in human-impacted mangroves. About 60% of the locations showed an average FRe < 2, indicating that most of the FEs comprised one species only. Notable exceptions were the Eastern Indian Ocean and west Pacific Ocean locations, but also in this region, 57% of the FEs had no redundancy, placing mangroves among the most vulnerable ecosystems on the planet. Our study shows that despite low redundancy, even small mangrove patches host truly multifunctional faunal assemblages, ultimately underpinning their services. However, our analyses also suggest that even a modest local loss of invertebrate diversity could have significant negative consequences for many mangroves and cascading effects for adjacent ecosystems. This pattern of faunal-mediated ecosystem functionality is crucial for assessing the vulnerability of mangrove forests to anthropogenic impact and provides an approach to planning their effective conservation and restoration.

Respective contribution of urban wastewater and mangroves on nutrient dynamics in a tropical estuary during the monsoon season

Estuaries of Southeast Asia are increasingly impacted by land-cover changes and pollution. Here, our research objectives were to (1) determine the origins of nutrient loads along the Can Gio estuary (Vietnam) and (2) identify the processes that affect the nutrient pools during the monsoon. We constructed four 24-h time-series along the salinity gradient measuring nutrient concentrations and stable isotopes values. In the upper estuary, urban effluents from Ho Chi Minh City were the main input of nutrients, leading to dissolved oxygen saturation <20%. In the lower estuary, ammonium and nitrite concentration peaks were explained by mangrove export. No contribution from aquaculture was detected, as it represents <0.01% of the total river discharge. Along the salinity gradient, nutrient inputs were rapidly consumed, potentially by phytoplankton while nitrate dual-stable isotopes indicated that nitrification occurred. Thus, even in a large and productive estuary, urban wastewater can affect nutrient dynamics with potentially important ecological risks.

Fatty acids, C and N dynamics and stable isotope ratios during experimental degradation of shrimp pond effluents in mangrove water

Intensive shrimp farming generates high loads of wastewaters that are released along tropical coastlines with potential impacts on the ecosystems. In this study, we used an experimental approach to analyze the behavior of shrimp pond effluents released in the Can Gio mangrove waterways (Southern Vietnam). We incubated shrimp pond effluents (EF), river water (RV), and a mixture of both (MI; 90% RV + 10% EF) in a dark room and measured fatty acid (FA) compositions, C and N concentrations and stable isotopes ratios (δ¹³C and δ¹⁵N) of suspended particulate matter during 16 days. Fatty acid concentrations rapidly decreased in EF with a 50% loss of FA during the first 24 h of the experiment and a 75% loss after 4 days of incubation. Proportions of the FA 18:1ω7 increased in MI during incubation, suggesting that this FA may be used as a tracer of anthropogenic substances release in marine environments.

The effect of anthropogenic drivers on spatial patterns of mangrove land use on the Amazon coast

Mangroves play an essential ecological role in the maintenance of the coastal zone and are extremely important for the socioeconomics of coastal communities. However, mangrove ecosystems are impacted by a range of anthropogenic pressures, and the loss of this habitat can be attributed primarily to the human occupation of the coastal zone. In the present study, we analyzed the spatial patterns of land use in the mangrove of the Brazilian Amazon coast, and evaluated the anthropogenic drivers of this impact, using a remote sensing approach. We mapped the road network using RapidEye images, and human settlements using global data. The results of these analyses indicate that the Brazilian Amazon coast has a low population density and low rates of anthropogenic impact in most of the coastal microregions investigated, factors that contribute to the maintenance and conservation of the region’s mangrove. The study also revealed that the paved road network is one of the principal drivers of land use in the mangrove, whereas other factors, such as population density, urban centers, and the number of settlements are much less important. While the region has 2024 km of paved highways, unpaved roads (17,496 km) facilitate access to the mangrove, with approximately 90% of anthropogenic impact being recorded within a 3 km radius of these roads. While the network of paved highways is relatively reduced in extension, preventive measures are urgently required to impede any major shift in the current scenario, caused by the expansion of major development programs. The results of the study indicate that biophysical, economic, and political factors may also contribute to the reduction, stability, and development of one of the world’s largest areas of mangrove forest.

Nitrogen distribution in a tropical urbanized estuarine system in northeastern Brazil

Nitrogen enters estuaries mostly through fluvial discharge and tide, although anthropogenic sources are known to influence the amount of this element in these aquatic ecosystems. Thus, the objective of this work was to verify which river (Cachoeira, Fundão, and/or Santana) exerts greater influence on the distribution of dissolved N forms (Dissolved Organic Nitrogen and Dissolved Inorganic Nitrogen = NH₃/NH₄⁺, NO₂^-, and NO₃^-) along a tropical urbanized estuarine system in northeastern Brazil. The studies estuarine system lies with in urban municipality, and the upper portion of the Cachoeira river estuary receives the treated effluent from this municipality through a sewage treatment station and untreated effluents from nearby villages. The selected sampling stations were located near the outfall of the rivers in the estuaries to the treatment plant and the villages. Of all the nitrogen forms, dissolved organic nitrogen (DON) prevailed in the estuarine system, followed by nitrate (NO₃^-) as the main inorganic form. The highest concentrations were recorded in the fluvial portion and upper estuary of Cachoeira river in the dry season. Based on the N concentrations found in the estuarine system, Cachoeira river has the greatest anthropogenic influence due to the amount of untreated effluents from the villages and treated effluents from the sewage treatment plant (STP) in the upper portion of the estuary.

Carbon accumulation and storage capacity in mangrove sediments three decades after deforestation within a eutrophic bay

A dated sediment core from an eutrophic mangrove area presented non-significant differences in carbon accumulation rates before (55.7±10.2gm^-2yr^-1) and after three decades of deforestation (59.7±7.2gm^-2yr^-1). Although eutrophication effects appear to compensate the loss of mangrove organic matter input, the results in this work show a threefold lower carbon accumulation than the global averages estimated for mangrove sediments. The effects of increasing eutrophication and enhanced sediment dry bulk density observed after deforestation (~30% higher) did not result in higher carbon stocks. Moreover, the lower TOC:OP (<400) and C:N (~20) molar ratios, as well as increased nutrient accumulation, reflect the dominance of phytoplankton-derived organic matter after deforestation, resulting in less-efficient sedimentary carbon sinks. These results indicate that the organic material deposited from eutrophication may not compensate mangrove deforestation losses on carbon accumulation in mangrove ecosystems.

Hard science is essential to restoring soft-sediment intertidal habitats in burgeoning East Asia

Intertidal soft-sediment ecosystems such as mangrove, saltmarsh, and tidal flats face multiple stresses along the burgeoning East Asia coastline. In addition to direct habitat loss, ecosystem structure, function, and capacity for ecosystem services of these habitats are significantly affected by anthropogenic loss of hydrologic connectivity, introduction of invasive exotic species, and chemical pollution. These dramatic changes to ecosystem structure and function are illustrated by four case studies along the East Asian coast: the Mai Po Marshes in Hong Kong, the Yunxiao wetlands in Fujian, China, and the Lake Sihwa and Saemangeum tidal flats in Korea. While investment in restoration is increasing significantly in the region, the lack of key basic knowledge on aspects of the behaviour of intertidal soft-sediment ecosystems, particularly those in Asia, impairs the effectiveness of these efforts. The relationship between biodiversity and ecosystem function for relatively species-poor mangrove, seagrass, and saltmarsh systems has implications for restoration targeting monospecific plantations. The trajectory of recovery and return of ecosystem function and services is also poorly known, and may deviate from simple expectations. As many introduced species have become established along the East Asian coast, their long-term impact on ecosystem function as well as the socio-economics of coastal communities demand a multidisciplinary approach to assessing options for restoration and management. These knowledge gaps require urgent attention in order to inform future restoration and management of intertidal soft-sediment ecosystems in fast-developing East Asia.