Spatio-temporal variations in the composition of organic matter in surface sediments of a mangrove receiving shrimp farm effluents (New Caledonia)

Effects of dredging wastewater input history and aquaculture type on greenhouse gas fluxes from mangrove sediments along the shorelines of the Jiulong River Estuary, China

Dredging wastewater (DW) from aquaculture ponds is a major disturbance factor in mangrove management, and its effects on the greenhouse gas (GHG) fluxes from mangrove sediment remain controversial. In this study, we investigated GHG (N2O, CH4, and CO2) fluxes from mangrove sediment at typical aquaculture pond-mangrove sites that were stimulated by DW discharged for different input histories and from different farm types. The GHG fluxes exhibited differing cumulative effects with increasing periods of DW input. The N2O and CH4 fluxes from mangrove sediment that received DW inputs for 17 y increased by ∼10 and ∼1.5 times, respectively, whereas the CO2 flux from mangrove sediment that received DW inputs for 11 y increased by ∼1 time. The effect of DW from shrimp ponds on the N2O flux was significantly larger than those of DW from fish/crab ponds and razor clam ponds. Moreover, the total global warming potentials (GWPs) at the field sites with DW inputs increased by 29-129% of which the CO2 flux was the main contributor to the GWP (85-96%). N2O as a proportion of CO2-equivalent flux increased from 2% to 12%, indicating that N2O was an important contributor to the increase in GWP. Overall, DW increased the GHG fluxes from mangrove sediments, indicating that the contribution of mangroves to climate warming was enhanced under DW input. It also implies that the carbon sequestration potential of mangrove sediments may be threatened to some extent. Therefore, future assessments of the carbon sequestration capacity of mangroves at regional or global scales should consider this phenomenon.

Influence of wastewater discharge from dredging mariculture pond sediments on the food sources of two intertidal crab species

The present study δ13C, δ15N and fatty acid compositions of two dominant mangrove crabs, Tubuca arcuata and Parasesarma plicatum were compared between a mangrove site frequently receiving dredged wastewater from mariculture ponds and an adjacent reference site, to investigate the impact of wastewater discharge on their diets. A laboratory experiment was also conducted to further test how their diets changed with the wastewater input. The result showed no significant change in the δ13C while clear 15N enrichment of crabs in association with the wastewater discharge. Changes in 15N signature and fatty acid composition of the crabs due to the wastewater discharge indicated that the impact of wastewater discharge was related to crab species, being more apparent on the deposit feeder (T. arcuata) than the herbivorous P. plicatum. The results suggested that the discharge of dredged wastewater into mangroves resulted in the uptake of wastewater-derived materials and nutrients by mangrove crabs.

Farmed shrimp aquaculture in coastal wetlands of Latin America - A review of environmental issues

The world production of farmed Whiteleg shrimp was 5,812,180 t in 2020. Ecuador, Mexico, and Brazil produced 1,012,830 in that year according to data FishStatJ. Environmental impact studies were scarce given the rapid growth of this activity, mainly on cummulative impacts, loss of ecosystems services or resilience of wetlands and mangroves. Aspects of the management of environmental impacts published on American studies during 1986-2020 are analyzed. The main impacts included the loss of mangroves and their transformation of these ecosystems and their water bodies by receiving enriched effluents from shrimp farms. Different management measures are discussed between Ecuador, which contributes 63.76 % of American production of white leg shrimp; Brazil which published 46.2 % of the environmental studies of the analyzed period, and Mexico, in an intermediate situation. Finally, alternatives are discussed to improve the management of shrimp aquaculture with emphasis on the Ecuadorian context.

Relationship between phytoplankton community and water parameters in planted fringing mangrove area in South China

Reforestation has been commonly adopted to increase the mangrove areas lost due to developments. A diverse phytoplankton community is critical to mangrove ecosystem functions; however, its compositions in planted mangrove habitats have seldom been reported. The present study, based on the temporal and spatial variations of phytoplankton community and water parameters, evaluated their relationships in a 20-year old artificially planted fringing mangrove in South China. Thirty-one phytoplankton taxa were identified from tidal water collected from three sites along a planted mangrove shoreline: within, and at the edge of mangroves, and at bare shoreline without mangroves. In all three sites, Bacillariophyta (diatom) was the most abundant phylum, dominated by Navicula and Nitzschia at 9.82-83.76% and 2.57-33.97%, respectively. The overall diversity ranged between 0.41 and 1.94. The temporal variations of phytoplankton and water parameters were more obvious than site differences. Higher phytoplankton diversity was found in summer, with increased green algae and cyanobacteria abundance accompanied by riverine discharge. There was site-specific dominance in some phyla between June and January, noticeably Chlorophyta, Cyanophyta, Dinophyta, and Euglenophyta. A potential toxic dinoflagellate Peridinium sp. was identified during fall and winter, serving as a warning to probable harmful bloom events. Multivariate statistical analyses revealed the abundance of major phytoplankton taxa significantly related to water physiochemical parameters, especially nitrate, ammonium, total phosphorous and orthophosphate. The present study suggests that mangrove vegetation may influence water quality and shape phytoplankton composition, but temporal differences were more significant. Compared with the protected natural mangroves in the National Nature Reserve nearby, this planted fringing mangrove habitat not only had lower nutrients, especially total nitrogen in tidal water, but also had different dominant phytoplankton species and lower species richness. The 20-year old artificially planted mangroves may still not serve the same ecological function as the protected natural mangrove forest.

Screening New Xylanase Biocatalysts from the Mangrove Soil Diversity

Mangrove sediments from New Caledonia were screened for xylanase sequences. One enzyme was selected and characterized both biochemically and for its industrial potential. Using a specific cDNA amplification method coupled with a MiSeq sequencing approach, the diversity of expressed genes encoding GH11 xylanases was investigated beneath Avicenia marina and Rhizophora stylosa trees during the wet and dry seasons and at two different sediment depths. GH11 xylanase diversity varied more according to tree species and season, than with respect to depth. One complete cDNA was selected (OFU29) and expressed in Pichia pastoris. The corresponding enzyme (called Xyn11-29) was biochemically characterized, revealing an optimal activity at 40–50 °C and at a pH of 5.5. Xyn11-29 was stable for 48 h at 35 °C, with a half-life of 1 h at 40 °C and in the pH range of 5.5–6. Xyn11-29 exhibited a high hydrolysis capacity on destarched wheat bran, with 40% and 16% of xylose and arabinose released after 24 h hydrolysis. Its activity on wheat straw was lower, with a release of 2.8% and 6.9% of xylose and arabinose, respectively. As the protein was isolated from mangrove sediments, the effect of sea salt on its activity was studied and discussed.

Trace metal dynamics in soils and plants along intertidal gradients in semi-arid mangroves (New Caledonia)

Trace metal dynamics were investigated in mangroves developing in semi-arid New Caledonia, where Avicennia and Rhizophora stands grow in the upper and lower intertidal zone, respectively. We collected soil samples and mangrove tissues in an undisturbed site, a mining-influenced site and in a mining and aquaculture-influenced site. Differences in duration of immersion and organic matter (OM) cycling resulted in a sharp decrease of metal concentrations in soils and plants from landside to seaside. Both species were tolerant to metals mainly via exclusion, (i.e. metal bioaccumulation restricted to roots and leaf litter). Strong correlations (p < 0.05) were found between Na and Fe, Mn, Cu and Zn in green and senesced leaves of Avicennia marina, indicating a possible role of those metals in mechanisms to cope with hypersaline conditions. Increasing metal pollution, aridity and sea-level rise are likely to result in a decrease in mangrove efficiency in filtering trace metals seaward.

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 influence of anthropogenic organic matter and nutrient inputs on the food web structure in a coastal lagoon receiving agriculture and shrimp farming effluents

In this study, we elucidated the impacts of allochthonous organic matter (OM) and nutrients (N and P) inputs coming from agriculture and shrimp aquaculture activities on food web structure in a subtropical coastal lagoon located on the central-east of the Gulf of California. This coastal lagoon is highly influenced by OM and nutrients inputs by a large agriculture district and aquaculture development center in Mexico. We also selected a second coastal lagoon, without direct OM and nutrients inputs, considered as a 'pristine' ecosystem. We evaluated the quantity of OM, N and P in both ecosystems and reconstructed the food webs using isotopic tools of C and N from the base to top. We collected and analyzed autochthonous and allochthonous OM, and organisms including primary producers, and primary to tertiary consumers. Overall, specimens of the same species and/or functional groups showed higher δ¹⁵N values in the ecosystem receiving agriculture and shrimp aquaculture effluents than the pristine. Food webs were composed of four and five trophic levels, where fish and birds occupied the top-predator levels. Seasonal increases in OM and N and P quantities in lagoons affected by anthropogenic activities produced high δ¹⁵N values in primary producers and consumers.

Variability of CO2 emissions during the rearing cycle of a semi-intensive shrimp farm in a mangrove coastal zone (New Caledonia)

In New Caledonia, shrimp ponds are built not on cleared mangroves but on salt flats behind the mangroves. The objectives of this study were to determine the variability of CO₂ fluxes from a semi-intensive shrimp pond during active and non-active periods of the farm and to determine the carbon dynamics from the upstream tidal creek to the downstream creek, which receives the farm's effluents. CO₂ emissions from the active pond were estimated at 11.1 ± 5.26 mmol CO₂ m^-2 d^-1. By modifying the hydrodynamics of the creeks, farm practices also influenced CO₂ emissions from both the upstream and downstream creeks. After tillage, all the organic carbon deposited at the pond bottom during the active period was mineralized, resulting in CO₂ emissions to the atmosphere estimated at 7.9 TCO₂ ha^-1. Therefore, shrimp farming is an anthropogenic source of CO₂ to the atmosphere, but suitable and optimized rearing practices limit these emissions.

Influence of introduced Sonneratia apetala on nutrients and heavy metals in intertidal sediments, South China

To investigate the influence of Sonneratia apetala on nutrients and heavy metals in intertidal sediments, sediment cores of S. apetala marsh and mudflat in Shenzhen Bay, China were analyzed. The results showed that S. apetala improved sediment nutrient properties due to increased total carbon (TC), total nitrogen (TN), and total sulfur (TS). The levels of heavy metals were higher in S. apetala site than in mudflat, including chromium (Cr), nickel (Ni), copper (Cu), zinc (Zn), arsenic (As), cadmium (Cd), lead (Pb), and mercury (Hg). In S. apetala site, TC, TN, and TS were not positively correlated with Cr, Ni, As, Cd, and Pb, indicating their less important roles in trapping heavy metals. There were positive correlations among Ni, Cu, Zn, and Cd in both sites, suggesting similar anthropogenic source. Levels of As were higher than the probable effect level at both sites, indicating their toxicological importance. The geo-accumulation index and potential ecological risk index revealed higher metal contaminations in S. apetala site, especially for Cd, Hg, and As. Multivariate analysis implied that S. apetala alter the biogeochemical cycle of Cd and Cr to a certain extent. These findings indicate that S. apetala may improve soil nutrient properties and facilitate heavy metal accumulation in intertidal sediments.

Detection of environmental impacts of shrimp farming through multiple lines of evidence

In order to evaluate the impact of semi-intensive shrimp farming, comparisons between Control and Impact areas were made based on multiple lines of evidence using an asymmetrical design. Water and sediment samples were collected in four shrimp farms located in Todos os Santos Bay, Bahia, Brazil. Nutrients, trace elements and macrobenthic assemblages were evaluated using uni- and multivariate analyzes. Significant differences were observed between Impact and Control areas for the water column dataset (i.e., ancillary variables, SPM, dissolved nutrients and major and trace elements in SPM), whereas no significant differences were observed for the chemistry of sediments. Macrobenthic assemblages were negatively affected by shrimp farm activities. Impacted sites presented the lowest abundance, richness and different structure of macrofaunal benthic assemblages. Farms clearly produced negative impacts in the Todos os Santos Bay. This conclusion was only possible to be reached through the use of multiple lines of evidence. Chemistry and benthic assemblages data combined produced a better description of the quality and impacts of the evaluated environments. Different conclusions would have been reached if chemistry and ecology results were studied separately vs. together.

Meiofauna distribution in a mangrove forest exposed to shrimp farm effluents (New Caledonia)

Meiofauna abundance, biomass and individual size were studied in mangrove sediments subjected to shrimp farm effluents in New Caledonia. Two strategies were developed: i) meiofauna examination during the active (AP) and the non-active (NAP) periods of the farm in five mangrove stands characteristics of the mangrove zonation along this coastline, ii) meiofauna examination every two months during one year in the stand the closest to the pond (i.e. Avicennia marina). Thirteen taxonomic groups of meiofauna were identified, with nematodes and copepods being the most abundant ones. Meiofauna abundance and biomass increased from the land side to the sea side of the mangrove probably as a result of the increased length of tidal immersion. Abundance of total meiofauna was not significantly different before and after the rearing period. However, the effluent-receiving mangrove presented twice the meiofauna abundance and biomass than the control one. Among rare taxa, mites appeared extremely sensitive to this perturbation.

Edaphic factors controlling summer (rainy season) greenhouse gas emissions (CO2 and CH4) from semiarid mangrove soils (NE-Brazil)

The soil attributes controlling the CO2, and CH4 emissions were assessed in semiarid mangrove soils (NE-Brazil) under different anthropogenic activities. Soil samples were collected from different mangroves under different anthropogenic impacts, e.g., shrimp farming (Jaguaribe River); urban wastes (Cocó River) and a control site (Timonha River). The sites were characterized according to the sand content; physicochemical parameters (Eh and pH); total organic C; soil C stock (SCS) and equivalent SCS (SCSEQV); total P and N; dissolved organic C (DOC); and the degree of pyritization (DOP). The CO2 and CH4 fluxes from the soils were assessed using static closed chambers. Higher DOC and SCS and the lowest DOP promote greater CO2 emission. The CH4 flux was only observed at Jaguaribe which presented higher DOP, compared to that found in mangroves from humid tropical climates. Semiarid mangrove soils cannot be characterized as important greenhouse gas sources, compared to humid tropical mangroves.

Foraminiferal assemblages as bioindicators to assess potential pollution in mangroves used as a natural biofilter for shrimp farm effluents (New Caledonia)

In New Caledonia, semi-intensive shrimp farms release untreated effluents into the mangrove. Foraminiferal assemblages were analyzed for assessing the impact of effluent release on the benthic compartment. Comparison was made between samples collected (1) in an effluent receiving mangrove before and after the rearing cycle, and (2) for one-year monitoring an effluent receiving and a control mangrove. The distribution of foraminiferal assemblages was primarily driven by the gradient between Rhizophora stands and salt-flats, related to salinity and tidal elevation, and by seasonal cycles. The potential impact of effluent release was due to the combined effects of normal-saline effluents on surface salinity, and of nutrient input and microbial stimulation on food availability. Foraminiferal assemblages did not indicate a substantial impact of farm effluents and suggest that semi-intensive shrimp farming using mangrove for effluent discharge may appear as a sustainable solution in New Caledonia, when considering only the impact on the mangrove itself.