Significant nitrate attenuation in a mangrove-fringed estuary during a flood-chase experiment

Nutrient distribution and nitrate processing in a mangrove tidal creek affected by submarine groundwater discharge (SGD)

Investigations of the spatial-temporal variations of nutrients within mangrove coastal zones are essential for assessing the environmental status of an aquatic ecosystems. However, major processes controlling nitrate cycle along the submarine groundwater discharge (SGD) pathway from the mangrove areas to adjacent tidal creek remain underexplored. A time series measurement over a 25 h tidal cycle was conducted in Qinglan Bay tidal creek (Hainan Island, China). Samples of the creek seawater, nearby terrestrial groundwater and sediment porewater were collected for analysis of nutrient concentrations and dual nitrate isotopic compositions (δ15N-NO3- and δ18O-NO3-). We observed high concentrations of nutrients in terrestrial groundwater, and their concentrations decreased with distance towards the coastal area. Using dual nitrate isotopes and a MixSIAR model, we concluded that ammonia nitrogen fertilizer (Amm-NF) is the primary nitrate sources for groundwater, while creek water nitrate originates from various sources, including manure and sewage, soil organic nitrogen, and chemical fertilizer. The isotopic ratios (δ18O: δ15N) suggest active nitrification occurs in low tide creek water, whereas assimilation and limited denitrification occurs in high tide creek water and groundwater. Based on previous report of SGD rate in the region, we calculated the flux of SGD-derived nutrient export to coastal water, and the result indicates low N: P ratio and N limitation in the tidal creek. This study highlights the complexity of different sources and biogeochemical processes that impact the nutrient distribution and nitrate dynamics in the nearshore mangrove ecosystem. Understanding these processes is essential for effectively protecting and managing of the mangrove aquatic ecosystem.

Insights into the water microbiome dynamics of a large tropical estuary transition

Tropical estuarine systems play several ecological roles, such as acting as a nursery for biodiversity and cycling nutrients and greenhouse gases. However, the phylogenetic and metabolic diversity of estuarine microbiomes is not completely known. Furthermore, it is unclear how seasonal patterns may affect microbial diversity in these systems. The aim of the present study was to determine the metagenomic diversity and its major drivers in a large tropical estuarine system dominated by a mangrove forest in the South Atlantic around the Paraiba do Sul River. In total, 12.16 million shotgun sequences were generated (dry season: n = 8; wet season: n = 5), and water-quality parameters were evaluated for all locations. Metagenomic sequences were distributed between two patterns: (1) the dry season, in which the families Rhodobactereaceae and Flavobactereaceae increased, and (2) the wet season, in which Moraxellaceae, Pseudomonadaceae, Pseudoalteromonadaceae were more abundant. The dry season was characterized by higher salinity, nitrogen fixation, nitrification, and photosynthetic potential. In contrast, the wet season had higher carbon dioxide (CO2) and methane (CH4) production and a higher abundance of methanogenic, methylotrophic, and chemoorganotrophic bacteria in samples with low salinity. These findings suggest a possible relationship with the production of greenhouse gases during the wet period.

Potential Linkages Between Submarine Groundwater (Fresh and Saline) Nutrient Inputs and Eutrophication in a Coastal Aquaculture Bay

Submarine groundwater discharge (SGD) plays a crucial role in nutrient budgets of coastal systems, encompassing both submarine fresh groundwater discharge (SFGD) and recirculated saline groundwater discharge (RSGD). Despite its significance, the specific importance of these components in mariculture bays has not been thoroughly assessed. Here, utilizing Ra isotopes and water‐salt mass balance model, we show that SFGD flux (1.1 ± 0.4 cm d−1) represented only 17% of the SGD in the Zhenzhu Bay, a typical mariculture bay along the South China Sea. Interestingly, the nutrient contribution from SFGD surpassed that from RSGD, accounting for 82% of the dissolved inorganic nitrogen (DIN) flux within the SGD. Analysis of the monthly satellite Chlorophyll‐a (Chl‐a) data confirmed that the decline in phytoplankton biomass can be linked to the limited dissolved silicate (DSi) transported by SFGD. Additionally, the elevated nitrogen to phosphorus ratio (241:1) and reduced silicon to nitrogen ratio (0.5:1) in SFGD compared to the Redfield ratio suggested that SFGD characterized by nitrogen excess and silica deficient, which likely played a role in transitioning from biogenic element constraints in coastal water. This shift may impact the proportions and functionality of the phytoplankton community, potentially mitigating water eutrophication. These findings underscore the significant influence of SGD on nutrient dynamics and the ecological environment in the Zhenzhu Bay.

High temperature and eutrophication alter biomass allocation of black mangrove (Avicennia germinans L.) seedlings

Mangrove restoration is underway along tropical coastlines to combat their rapid worldwide decline. However, restoration success is limited due to local drivers such as eutrophication, and global drivers such as climate change, yet their interactions remain unclear. We conducted a mesocosm experiment to assess the impact of increased nutrients and temperature on the photosynthetic efficiency and development of black mangrove seedlings. Seedlings exposed to high temperature and eutrophication showed reduced root growth and disproportionally long stems, with lower net assimilation rates. This architectonical imbalance between root and stem growth may increase susceptibility to physical disturbances and dislodgement. Notably, none of the experimental seedlings displayed signs of photophysiological stress, and those exposed to increased nutrients and temperature exhibited robust photosynthetic performance. The disbalance in biomass allocation highlights the importance of considering local nutrient status and hydrodynamic conditions in restoration projects, ensuring the effective anchorage of mangrove seedlings and restoration success under a warming climate.

Land use change increases contaminant sequestration in blue carbon sediments

Coastal blue carbon habitats perform many important environmental functions, including long-term carbon and anthropogenic contaminant storage. Here, we analysed twenty-five ²¹⁰Pb-dated mangrove, saltmarsh, and seagrass sediment cores from six estuaries across a land-use gradient to determine metal, metalloid, and phosphorous sedimentary fluxes. Cadmium, arsenic, iron, and manganese had linear to exponential positive correlations between concentrations, sediment flux, geoaccumulation index, and catchment development. Increases in anthropogenic development (agricultural or urban land uses) from >30 % of the total catchment area enhanced mean concentrations of arsenic, copper, iron, manganese, and zinc between 1.5 and 4.3-fold. A ~ 30 % anthropogenic land-use was the threshold in which blue carbon sediment quality begins to be detrimentally impacted on an entire estuary scale. Fluxes of phosphorous, cadmium, lead, and aluminium responded similarly, increasing 1.2 to 2.5-fold when anthropogenic land-use increased by at least 5 %. Exponential increases in phosphorus flux to estuary sediments seem to precede eutrophication as observed in more developed estuaries. Overall, multiple lines of evidence revealed how catchment development drives blue carbon sediment quality across a regional scale.

Increasing carbon, nutrient and trace metal accumulation driven by development in a mangrove estuary in south Asia

Mangrove forests sequester organic carbon, nutrients and toxic metals sorbed to fine sediment, and thus restrict the mobility of pollutants through estuarine environments. However, mangrove removal and environmental degradation caused by industrial activity and urban growth can impact the ability of mangrove communities to provide these critical ecosystem services. Here, we use sediment profiles from an impacted tropical estuary in southwest India to provide a c. 70-year record of carbon, nutrient and trace metal burial in the context of rapid urban development and the systemic removal of mangrove communities. Our results show that carbon and nutrient accumulation rates increase sharply during the 1990's in accordance with the high rates of deforestation. Nitrogen and phosphorus accumulation rates increased fourfold and twofold, respectively, during the same period. Organic carbon accumulation was fivefold higher than the global average during this period, reflecting intense deforestation during the last three decades. The enrichment of Hg, Zn, Pb, Mo, Ni, Cu and Mn demonstrate clear anthropogenic impact starting in the 1950's and peaking in 1990. Mercury, the trace metal with the highest enrichment factor, increased sevenfold in the most recent sediments due to increased fossil fuel emissions, untreated water and incineration of medical waste and/or fertilizers used in aquaculture. Organic carbon isotope (δ¹³C) and C:N molar ratios indicate shifts to more terrestrial-derived source of organic matter in the most recent sediments reflecting growing deforestation of which may be prevalent in southeast Asia due to increasing development. This study emphasizes the critical role played by mangrove ecosystems in attenuating anthropogenically-derived pollutants, including carbon sequestration, and reveals the long-term consequences of mangrove deforestation in the context of rapidly developing economies.

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.

A conceptual model of nitrogen dynamics for the Great Barrier Reef catchments

Nitrogen (N) from anthropogenic sources has been identified as a major pollutant of the Great Barrier Reef (GBR), Australia. We developed a conceptual framework to synthesise and visualise the fate and transport of N from the catchments to the sea from a literature review. The framework was created to fit managers and policymakers' requirements to reduce N in the GBR catchments. We used this framework to determine the N stocks and transformations (input, sources, and outputs) for ecosystems commonly found in the GBR: rainforests, palustrine wetlands, lakes, rivers (in-stream), mangroves and seagrasses. We included transformations of N such as nitrogen fixation, nitrification, denitrification, mineralisation, anammox, sedimentation, plant uptake, and food web transfers. This model can be applied to other ecosystems to understand the transport and fate of N within and between catchments. Importantly, this approach can guide management actions that attenuate N at different scales and locations within the GBR ecosystems. Finally, when combined with local hydrological modelling, this framework can be used to predict outcomes of management activities.

Anthropogenic nitrogen pollution in mangrove ecosystems along Dar es Salaam and Bagamoyo coasts in Tanzania

Mangroves are among the most productive coastal ecosystems; however, they are prone to anthropogenic pollution due to their land-sea interface position. We used stable nitrogen isotopes and spectrophotometric nitrate analysis to study the anthropogenic pollution in five mangrove ecosystems in Tanzania, including two polluted (Mzinga and Kizinga), one moderate (Kunduchi) and non-polluted (Mbegani and Ras Dege) sites. Also, we tested the suitability of mangrove leaves, roots, sediment, and gastropod as indicators of anthropogenic nitrogen pollution using stable δ¹⁵N isotope analysis. Results revealed higher than 10‰ δ¹⁵N values in all analysed components and the highest nitrate concentrations of 16.44 mg L^-1 in the interstitial waters at the polluted sites, indicating anthropogenic nitrogen inputs. The δ¹⁵N enrichment increased in the order: non-polluted < moderate < polluted. The polluted sites are fed by freshwater creeks and probably receive high loads of domestic sewage from the surrounding communities, industries, and agricultural effluents. Therefore, to protect mangrove ecosystems, proper waste and wastewater management upstream are recommended.

Land use and episodic rainfall as drivers of nitrogen exports in subtropical rivers: Insights from δ15N-NO3-, δ18O-NO3- and 222Rn

Ongoing land-use intensification in subtropical catchments is expected to release more inorganic nitrogen to downstream coastal waters similar to historical changes in temperate ecosystems. Here, we examined spatial and temporal drivers of stream nitrogen loads across a subtropical land-use gradient using the isotopic compositions of nitrate (NO₃^--N) and radon (²²²Rn), a natural groundwater tracer. We investigated eleven subtropical creeks/rivers over contrasting hydrological conditions in Australia. NO_x-N (nitrite (NO₂^--N) + nitrate (NO₃^--N)) accounted for 13.1%, 34.0%, and 42.6% of total dissolved nitrogen (TDN-N) in forest, peri-urban and agricultural creeks, respectively. Following an 80 mm rain event, loads of dissolved inorganic nitrogen (DIN-N) from agriculture catchments reached 368 mg N m^-2 catchment area day^-1. Forest and peri-urban catchments had aquatic TDN-N loads 17.8% and 31.1% of loads from agricultural catchments. Radon observations suggest that nitrogen and phosphorus loads were driven primarily by surface runoff rather than groundwater discharge. The δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- values in the agriculture, forest and peri-urban catchments indicate fertilisers and soil nitrogen as the main sources of NO₃^--N. However, one of the catchments (Double Crossing Creek) received a mixture of recirculated greywater and chemical nitrogen fertilisers. Isotopic signatures imply significant NO₃^--N losses via denitrification during dry conditions. Groundwater discharge played a minor role because regional aquifers were not contaminated by nitrogen. Overall, intensive agricultural land use and episodic rainfall events were the major spatial and temporal drivers of nitrogen loads.