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

Deciphering environmental forcings in the distribution of meiofauna and nematodes in mangroves of the Atlantic-Caribbean-East Pacific and Indo-West Pacific regions

Mangroves develop under environmental conditions and anthropogenic pressures whose impact on benthic meiofauna remains poorly understood. It is unclear how meiofauna communities are structured according to local sedimentary conditions. This study was designed to characterize the community structure of meiofauna and nematodes (dominant taxa) and the associated environmental forcings in intertidal mangrove sediments from Mayotte (Indo-West-Pacific), Martinique and Guadeloupe (Caribbean). Sediment cores were sampled at the end of the dry season at low tide on adult mangrove stands with similar immersion time. In each sediment layer, we analyzed redox potential, pH, porewater salinity, grain size, organic matter, metals, organic contaminants, prokaryotes and meiofauna. Our results show that sediments far from cities and agricultural fields trapped site-specific contaminants due to local water transport processes. Some metals, PAHs or pesticides exceeded toxicity thresholds in most of the studied stations, thus being harmful to benthic fauna. The sedimentary environment acts as a filter selecting specific meiofauna communities at station scale only in the Caribbean. In Mayotte, horizontal homogeneity contrasts with vertical heterogeneity of the sedimentary environment and the meiofauna. Nematode genera showed particular distribution patterns horizontally and vertically, suggesting the presence of sediment patches suitable for a restricted pool of genera on each island. Results in the Caribbean are consistent with nested diversity patterns due to environmental filtering. Conversely, horizontal homogeneity at Mayotte would reflect greater dispersal between stations or more spatially homogeneous anthropogenic pressures. The nematode genera present at depth may not be the most specialized, but the most versatile, capable of thriving in different conditions. Terschellingia and Daptonema showed contrasted responses to environmental forcing, likely due to their versatility, while Desmodora showed uniform responses between study areas, except when toxicity thresholds were exceeded. Our results emphasize that a given genus of nematode may respond differently to sedimentary conditions depending on sites.

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.

First evidence of meso- and microplastics on the mangrove leaves ingested by herbivorous snails and induced transcriptional responses

Although evidence suggests the ubiquity of meso- and microplastics (MMPs) in mangrove forests, our knowledge of their bioavailability and risk on mangrove leaves is scarce. Here, we investigated MMP contamination concerning submerged mangrove leaves and herbivorous snails that mainly feed on them from the four mangrove forests located in Beibu Gulf, Guangxi Province, China. Results showed that the MMP abundance on the mangrove leaves ranged from 0.01 ± 0.00 to 0.42 ± 0.15 items cm^-2, while it ranged from 0.33 ± 0.21 to 6.20 ± 2.91 items individual^-1 in the snails. There were significant positive correlations between snails and leaves regarding the abundance of total MMPs and the proportions of MMPs with the same characteristics. Expanded polystyrene (EPS) that mainly derived from aquaculture rafts, accounted for a major component both on the leaves and in the snails in Shi Jiao (SJ). Both the detection frequency and percentage of larger EPS (2.00-17.50 mm) on the leaves in SJ were higher than other sites. Meanwhile, the detection frequency, abundance and percentage of larger EPS on the leaves had significant positive correlations with those of micro-EPS in the snails. These findings suggested that mangrove leaves may represent a viable pathway for MMPs to enter the herbivorous snails. Larger EPS with higher frequency of occurrence on mangrove leaves were more likely to be encountered and ingested by snail considering its opportunistic feeding behavior. In addition, 11 sensitive genes involved in the processes of metabolism, intestinal mucosal immune systems, and cellular transduction in the snails were significantly suppressed by MMP exposure, which may be potentially used as early biomarkers to indicate the biological effects of MMPs under realistic environmental conditions. Overall, this study provides novel insights into the fate, sources, and biological effects of MMPs on mangrove leaves.

Is halophyte species growing in the vicinity of the shrimp ponds a promising agri-aquaculture system for shrimp ponds remediation in New Caledonia?

Plant culture integration within aquaculture activities is a topic of recent interest with economic and environmental benefits. Shrimp farming activities generate nutrient-rich waste trapped in the sediments of farming ponds or release in the mangrove area. Thus, we investigate if the halophytes species naturally growing around the pond can use nitrogen and carbon from shrimp farming for remediation purposes. Halophyte biomasses and sediments influenced by shrimp farm effluents, were collected in two farms in New-Caledonia. All samples were analyzed for their C and N stable isotopic composition and N content. Higher δ¹⁵N values were found in plants influenced by shrimp farm water thus evidenced their abilities to take nutrient derived from shrimp farming. Deep root species Chenopodium murale, Atriplex jubata, Suaeda australis and Enchylaena tomentosa appears more efficient for shrimp pond remediation. This work demonstrates that halophytes cultivation in shrimp ponds with sediments, could be effective for the pond's remediation.

First Assessment of the Benthic Meiofauna Sensitivity to Low Human-Impacted Mangroves in French Guiana

Bioindicators assess the mangroves ecological state according to the types of pressures but they differ with the ecosystem’s specificities. We investigated benthic meiofauna diversity and structure within the low human-impacted mangroves in French Guiana (South America) in response to sediment variables with various distances to the main city. Contaminant’s concentrations differed among the stations, but they remained below toxicity guidelines. Meiofauna structure (Foraminifera, Kinorhyncha, Nematoda) however varied accordingly. Nematode’s identification brought details on the sediment’s quality. The opportunistic genus Paraethmolaimus (Jensen, 1994) strongly correlated to the higher concentrations of Hg, Pb. Anoxic sediments were marked by organic enrichment in pesticides, PCB, and mangrove litter products and dominance of two tolerant genus, Terschellingia (de Man, 1888) and Spirinia (Gerlach, 1963). In each of these two stations, we found many Desmodora individuals (de Man, 1889) with the presence of epibionts highlighting the nematodes decreased fitness and defenses. Oxic sediments without contaminants were distinguished by the sensitive genera Pseudocella (Filipjev, 1927) and a higher diversity of trophic groups. Our results suggested a nematodes sensitivity to low contaminants concentrations. Further investigations at different spatio-temporal scales and levels of deterioration, would be necessary to use of this group as bioindicator of the mangroves’ ecological status.

Limited impact of several years of pretreated wastewater discharge on fauna and vegetation in a mangrove ecosystem

It was hypothesized that mangroves, tropical wetlands, could be used for the finishing treatment of domestic wastewaters. Our aim was to determine if a nutrient-stressed mangrove could tolerate long-term discharges of pretreated wastewater (PW). Since 2008, in an in situ experimental system set up in Mayotte Island (Indian Ocean), domestic PW are discharged into two impacted areas (675 m²) dominated by different species of mangrove trees. Anthropogenic inputs during > 4.5 years led to an increase in vegetation growth associated with an increase in leaf pigment content, leaf surface and tree productivity. A marked increase in tree mortality was observed. There was no effect on crabs and meiofauna densities, but significant modifications of community structures. These effects may be directly linked to PW inputs, or indirectly to the modifications of the environment associated with higher tree growth. However, our results indicate that there was no major dysfunction the ecosystem.

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.