Impact of pond aquaculture effluents on seagrass performance in NE Hainan, tropical China

Changes in surface sediment carbon compositions in response to tropical seagrass meadow restoration

Seagrass meadows are declining at a global scale, threatening their capacity as blue carbon sinks. Restoration of seagrasses (via seagrass seeds or plant transplantation) may recover their carbon sequestration capacity. Previous studies have predominantly focused on sediment organic carbon (SOC), while variations in sediment carbon compositions remain poorly understood, limiting our comprehension of the influence of seagrass restoration on sediment carbon stability. Here, we researched the differences in surface (0-3 cm) sediment carbon compositions in response to tropical seagrass transplantation among species (Thalassia hemprichii and Enhalus acoroides); specifically, differences in labile, recalcitrant and refractory SOC, as well as sediment inorganic carbon (SIC) compositions variations under transplanted T. hemprichii and E. acoroides communities. It was found that seagrass transplantation enhanced suspended particle organic matter, and epiphyte and macroalgae input to surface sediment, which recovered the surface SOC concentration and stock rapidly to natural levels (increased ∼1.6-fold) within two years following transplantation. The elevated contribution of epiphyte and macroalgae significantly increased the surface labile sediment organic matter (SOM), but not the recalcitrant and refractory SOM composition after short-term transplantation. Meanwhile, surface SIC was significantly elevated, which might be mainly ascribed to allochthonous carbonate particle trapped under transplanted area with implications for carbon sequestration. The higher canopy and longer leaf seagrass species, E. acoroides, had elevated SOC, SIC and was more labile composition, compared to T. hemprichii transplant. Overall, this research suggests that tropical seagrass transplantation can increase the surface SOC, SIC concentration by increasing the labile organic matter and allochthonous carbonate particle input, respectively, with varying significantly among seagrass species.

Influence of algal organic matter on metal accumulation in adjacent sediments of aquaculture from a tropical coast region

The rapid development of coastal aquaculture in recent decades has led to excessive discharge of organic matter and nutrients into surrounding waters, which could result in eutrophication and potentially affect metal cycling. In our study, the influence of algal organic matter on metal accumulation was examined in three coastal sediment cores taken from a tropical region, Hainan Island, China. Overall, metal pollution adjacent to aquaculture ponds remained at low levels on the coast, except Zn, Cd, and Sn were moderately to highly enriched in the Dongjiao sediments. The δ¹³C values and the atomic C/N ratios indicated a major contribution of phytoplankton to sedimentary organic matter at the Dongjiao site. Moreover, both the algae-derived organic matter and effluent nitrogen were significantly associated with the enriched Zn, Cd, and Sn, suggesting that nutrient-induced phytoplankton growth and its organic matter may act as a "biological pump" to enhance the accumulation of metals. Wastewater treatment for aquaculture ponds should include the control of algal organic matter.

Use of a wastewater recovery product (struvite) to enhance subtropical seagrass restoration

Seagrasses are in decline worldwide, and their restoration is relatively expensive and unsuccessful compared to other coastal systems. Fertilization can improve seagrass growth in restoration but can also release nutrients and pollute the surrounding ecosystem. A slow-release fertilizer may reduce excessive nutrient discharge while still providing resources to the seagrass's rhizosphere. In this study, struvite (magnesium ammonium phosphate), a relatively insoluble, sustainable compound harvested in wastewater treatment plants, was compared to Osmocote™(14:14:14 Nitrogen: Phosphorus: Potassium, N:P:K), a popular polymer coated controlled release fertilizer commonly used in seagrass restoration. Two experiments compared the effectiveness of both fertilizers in a subtropical flow-through mesocosm setup. In the first experiment, single 0.5 mg of P per g dry weight (DW) doses of Osmocote™and struvite fertilizers were added to seagrass plots. Seagrass shoot counts were significantly higher in plots fertilized with struvite than both the Osmocote™and unfertilized controls (p< 0.0001). A significant difference in total P concentration was observed in porewater samples of Osmocote™vs struvite and controls (p< 0.0001), with struvite fertilized plots emitting more than controls (p ≤ 0.0001), but less than 2% of the total dissolved P (TDP) of Osmocote™fertilized plots (100+ mg/L versus x > 5 mg/L). A subsequent experiment, using smaller doses (0.01 and 0.025 mg of P per gram DW added), also found that the struvite treatments performed better than Osmocote™, with 16-114% more aboveground biomass (10-60% higher total biomass) while releasing less N and P. These results indicate the relatively rapid dissolution of Osmocote™may pose problems to restoration efforts, especially in concentrated doses and possibly leading to seagrass stress. In contrast, struvite may function as a slow-release fertilizer applicable in seagrass and other coastal restoration efforts.

Climate-Friendly Seafood: The Potential for Emissions Reduction and Carbon Capture in Marine Aquaculture

Aquaculture is a critical food source for the world's growing population, producing 52% of the aquatic animal products consumed. Marine aquaculture (mariculture) generates 37.5% of this production and 97% of the world's seaweed harvest. Mariculture products may offer a climate-friendly, high-protein food source, because they often have lower greenhouse gas (GHG) emission footprints than do the equivalent products farmed on land. However, sustainable intensification of low-emissions mariculture is key to maintaining a low GHG footprint as production scales up to meet future demand. We examine the major GHG sources and carbon sinks associated with fed finfish, macroalgae and bivalve mariculture, and the factors influencing variability across sectors. We highlight knowledge gaps and provide recommendations for GHG emissions reductions and carbon storage, including accounting for interactions between mariculture operations and surrounding marine ecosystems. By linking the provision of maricultured products to GHG abatement opportunities, we can advance climate-friendly practices that generate sustainable environmental, social, and economic outcomes.

Morphological and biochemical responses of tropical seagrasses (Family: Hydrocharitaceae) under colonization of the macroalgae Ulva reticulata Forsskål

BACKGROUND

Coastal land development has deteriorated the habitat and water quality for seagrass growth and causes the proliferation of opportunist macroalgae that can potentially affect them physically and biochemically. The present study investigates the morphological and biochemical responses of seagrass from the Hydrocharitaceae family under the macroalgal bloom of Ulva reticulata, induced by land reclamation activities for constructing artificial islands.

METHODS

Five seagrass species, Enhalus acoroides, Thalassia hemprichii, Halophila ovalis, Halophila major, and Halophila spinulosa were collected at an Ulva reticulata-colonized site (MA) shoal and a non-Ulva reticulata-colonized site (MC) shoal at Sungai Pulai estuary, Johor, Malaysia. Morphometry of shoots comprising leaf length (LL), leaf width (LW), leaf sheath length (LSL), leaflet length (LTL), leaflet width (LTW), petiole length (PL), space between intra-marginal veins (IV) of leaf, cross vein angle (CVA) of leaf, number of the cross vein (NOC), number of the leaf (NOL) and number of the leaflet (NOLT) were measured on fresh seagrass specimens. Moreover, in-situ water quality and water nutrient content were also recorded. Seagrass extracts in methanol were assessed for total phenolic content (TPC), total flavonoid content (TFC), 2,2-diphenyl-1-picrylhydrazyl radical scavenging activity (DPPH), 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid radical cation scavenging activity (ABTS), and ferric reducing antioxidant power (FRAP).

RESULTS

Seagrasses in the U. reticulata-colonized site (MA) had significantly higher (t-test, p < 0.05) leaf dimensions compared to those at the non-U. reticulata colonized site (MC). Simple broad-leaved seagrass of H. major and H. ovalis were highly sensitive to the colonization of U. reticulata, which resulted in higher morphometric variation (t-test, p < 0.05) including LL, PL, LW, and IV. Concerning the biochemical properties, all the seagrasses at MA recorded significantly higher (t-test, p < 0.05) TPC, TFC, and ABTS and lower DPPH and FRAP activities compared to those at MC. Hydrocharitaceae seagrass experience positive changes in leaf morphology features and metabolite contents when shaded by U. reticulata. Researching the synergistic effect of anthropogenic nutrient loads on the interaction between seagrasses and macroalgae can provide valuable information to decrease the negative effect of macroalgae blooms on seagrasses in the tropical meadow.

Bacterial epiphyte and endophyte communities of seagrass Thalassia hemprichii: the impact of feed extract solution

The global seagrass bed ecosystem acts as a natural ecological barrier in the littoral coastal zone. In recent years, this ecosystem has suffered from serious eutrophication and destruction caused by the continuous expansion of aquaculture. However, our understanding of the influence of aquaculture on the bacterial community remains limited. In this study, we used 16S amplicon sequencing to evaluate the impact of aquaculture feed extract solution on the composition and function of bacterial epiphytes and endophyte communities of the core seagrass from the seagrass bed ecosystem in Hainan, Thalassia hemprichii. The feed extract solution was the main factor that significantly affected the bacterial epiphyte and endophyte community structure of seagrass leaves but had no marked effect on alpha diversity was observed. Additionally, the bacterial epiphyte and endophyte community of the T. hemprichii leaves alleviated the effects of organic matter, sulfide, and nutrients caused by aquaculture wastewater. The feed extract solution promoted the proliferation of Bacteroidales, Vibrio, Desulfobulbaceae, Desulfobacteraceae, Pseudoalteromonas, Paludibacter, Marinomonas, and Pseudomonas in the leaves and root of T. hemprichii, which can effectively improve the digestibility of eutrophication. In fact, Desulfobacteraceae and Desulfobulbaceae can reduce sulfate to sulfide and oxidize sulfide to sulfur within seagrass, indicating that the increase in Desulfobulbaceae and Desulfobacteraceae facilitated the accumulation of sulfide with the treatment of feed extract solution, which may be the reason for the degradation of seagrass caused by aquaculture wastewater containing high concentrations of organic pollutants. These results suggest that although seagrass beds can withstand low concentrations of aquaculture pollutants, sulfide emissions should be minimized.

Decade changes of the food web structure in tropical seagrass meadow: Implication of eutrophication effects

Seagrass meadows are experiencing worldwide declines mainly because of nutrient enrichment. However, knowledge about how eutrophication affects its food web structure is still limited. Based on decade-scale observations in a tropical seagrass meadow, we analysed primary producer structures, isotopic niche, and the diets of consumers respond to the decade nutrient enrichment. Through decades of nutrient enrichment, the biomass of epiphytes, particulate organic matter (POM), and macroalgae significantly increased. Correspondingly, the contribution of seagrass to the entire food web decreased significantly. Meanwhile, the isotopic niches of consumers have also become more shrinking, which reflects a more concentrated diet and higher predation pressure for consumers. These findings suggest that eutrophication leads to a significant shift in the structure of primary producers, which has changed food source availability and increased predation pressure, leading to a dietary shift in consumers and a simplified food web structure.

Diversity, distribution and conservation of seagrass in coastal waters of the Liaodong Peninsula, North Yellow Sea, northern China: Implications for seagrass conservation

Seagrass beds are highly productive coastal ecosystems that are widely distributed along temperate and tropical coastlines globally. Although seagrass distribution and diversity have been widely reported on a global scale, there have been few reports on seagrass distribution and diversity in northern China, especially for coastal waters of the Liaodong Peninsula in the North Yellow Sea. In the present study, we investigated the distribution and diversity of seagrass in coastal waters of the Liaodong Peninsula in the North Yellow Sea, northern China. Field surveys of seagrass wrack were conducted along shorelines, to identify whether seagrass beds occurred in nearby waters, and sonar methods were then used to collect data relating to seagrass bed extent. Also, we analyzed the major threats facing seagrass beds. The results of the study revealed that four species (Zostera marina L., Z. japonica Aschers. & Graebn., Z. caespitosa M., and Phyllospadix iwatensis M.) were found in study area, covering a total area of 1253.47 ha. Seagrass bed area significantly decreased with increasing water depth, and most seagrass was recorded at depths of 2-5 m. Due to the steep slope of the seabed, seagrass beds exhibited a zonal distribution in most of the study areas. In addition, the amount of seagrass wrack along shorelines could be used to infer the size and distance of seagrass beds. Human activities, such as clam harvesting, land reclamation, coastal aquaculture pose a threat to the seagrass beds. This study provides new information to fill knowledge gaps regarding seagrass distribution in northern China and it provides a baseline for further monitoring of these seagrass beds.

Pond aquaculture effluents feed an anthropogenic nitrogen loop in a SE Asian estuary

Coastal aquaculture expansion resulted in mangrove area loss and ecosystem degradation in the past decades, mainly in tropical Asia. Despite increasing environmental concerns regarding nutrient and organic matter-rich effluents, little is known on the effects on adjacent estuarine and coastal food webs. To assess the impact and fate of anthropogenic nitrogen released from aquaculture facilities, we studied water quality and nitrogen (N) flow across an estuarine food web in an estuary in Hainan, China, using nitrogen stable isotopes (δ¹⁵N). We found higher δ¹⁵N values of ammonium, nitrate and suspended matter in the pond-covered inner estuary than further upstream, suggesting a strong influence of untreated pond effluents, which had a high δ¹⁵N (ammonium: ~16‰, nitrate: ~7‰, suspended matter: ~8‰). Fish and benthic invertebrates of the inner estuary had a higher δ¹⁵N than consumers further upstream and in similar aquaculture-free estuaries elsewhere, most likely due to direct or indirect uptake of ¹⁵N-enriched aquaculture effluents by phytoplankton and benthic algae. A major part of the artisanal catches from the estuary consists of small-size fish which is used as feed in the local aquaculture. Thus, estuarine fish incorporating aquaculture-effluent based food web signals are harvested and recycled as feed in aquaculture facilities, whose effluents sustain this local food web. The δ¹⁵N being at the high end of the global range on all trophic levels indicates an anthropogenic nitrogen loop in which some portion of the reactive nitrogen initially introduced into aquaculture ponds is continuously recycled and affects the estuarine food web. This recycling also indicates a shortcut in the otherwise inefficient nitrogen sink function of estuaries. Therefore, in areas with large-scale coastal aquaculture like in China and SE Asia the effect of reactive nitrogen from aquaculture sources on the performance of coastal ecosystems may be larger than previously thought.

The end of resilience: Surpassed nitrogen thresholds in coastal waters led to severe seagrass loss after decades of exposure to aquaculture effluents

Although eutrophication is considered a major driver for global seagrass loss with aquaculture effluents being a main factor, little is known about the effect on seagrass meadows in eastern Asia and their resilience to long-term nutrient impact. Seagrass meadows impacted by land-based aquaculture since the 1990s, were visited in 2008/2009 and revisited after another 9 years of effluent exposure. During that period seagrass aboveground biomass declined by 87%. Species diversity decreased with increasing effluent exposure. A δ¹⁵N of 9.0‰ of seagrass leaves and additional biogeochemical and biological indicators identify pond effluents as the driver of the observed eutrophication. When continuously exposed to dissolved inorganic nitrogen (DIN) concentrations exceeding a calculated threshold of 8 μM DIN seagrass meadows will disappear. Chronic nutrient pollution from aquaculture effluents can lead to a reduction of biodiversity and ultimately to a complete loss of seagrasses along the aquaculture-dominated coasts in E and SE Asia.

Effects of aquaculture effluents on fate of 2,2',4,4',5-pentabromodiphenyl ether (BDE-99) in contaminated mangrove sediment planted with Kandelia obovata

The problems of aquaculture effluent (AE) and polybrominated diphenyl ethers (PBDEs) are common in coastal areas. The fate of 2,2',4,4',5-pentabromodiphenyl ether (BDE-99), a dominant PBDE congener, in mangrove sediments and the effects of AE on it have never been reported. A 12-months microcosm study was conducted and more than 55% of the BDE-99 in contaminated sediment was removed at the end. The removal percentages depended on treatments, with the highest removal in the treatment planted with Kandelia obovata (Ko) and irrigated with AE (WP_AE), followed by Ko planted but without AE (WP), unplanted with AE (NP_AE) and unplanted without AE (NP). Hydroxylation of BDE-99 was observed in all treatments, with a preference in the para position bromine substitution, followed by meta position and the lowest was ortho bromine substitution. BDE-99 was also debrominated to lower brominated congeners like tri- and di-BDEs congeners. Different from parent BDE-99, ortho-substituted BDE-28 and -15 were more dominant than that of para-substituted BDE-17 and -7, suggesting that para-substituted congeners could further be debrominated. The AE addition enhanced root uptake of PBDEs in Ko. These findings suggested that the addition of AE and planting Ko could be an effective way to remedy BDE-99 in contaminated sediments.

Fine-scale genetic structure and flowering output of the seagrass Enhalus acoroides undergoing disturbance

Seagrass are under great stress in the tropical coast of Asia, where Enhalus acoroides is frequently the dominant species with a large food web. Here, we investigate the question of the fine-scale genetic structure of this ecologically important foundation species, subject to severe anthropogenic disturbance in China. The genetic structure will illuminate potential mechanisms for population dynamics and sustainability, which are critical for preservation of biodiversity and for decision-making in management and restoration. We evaluated the fine-scale spatial genetic structure (SGS) and flowering output of E. acoroides, and indirectly estimated the relative importance of sexual versus asexual reproduction for population persistence using spatial autocorrelation analysis. Results reveal high clonal diversity for this species, as predicted from its high sexual reproduction output. The stronger Sp statistic at the ramet-level compared with genet-level indicates that clonality increases the SGS pattern for E. acoroides. Significant SGS at the genet-level may be explained by the aggregated dispersal of seed/pollen cohorts. The estimated gene dispersal variance suggests that dispersal mediated by sexual reproduction is more important than clonal growth in this study area. The ongoing anthropogenic disturbance will negatively affect the mating pattern and the SGS patterns in the future due to massive death of shoots, and less frequency of sexual reproduction.

Spatial Variations in the Abundance and Chemical Speciation of Phosphorus across the River–Sea Interface in the Northern Beibu Gulf

Water samples were collected to measure dissolved and particulate phosphorus species in order to examine the dynamics of phosphorus in the water column across the river–sea interface from the lower Dafengjiang River to the open Beibu Gulf. Dissolved inorganic phosphorus concentrations were as high as 0.90 ± 0.42 μM in river water but decreased dramatically to as low as 0.02 ± 0.01 μM in open coastal waters. Total dissolved phosphorus was largely measured in the form of dissolved inorganic phosphorus in river waters (58% ± 18%), whereas dissolved organic phosphorus became the predominant species (>90% on average) in open coastal waters. Total dissolved phosphorus was the dominant species, comprising 76% ± 16% of the total phosphorus, while total particulate phosphorus only comprised 24% ± 16% of the total phosphorus pool. Riverine inputs, physical and biological processes, and particulate phosphorus regeneration were the dominant factors responsible for the dynamic variations of phosphorus species in the study area. Based on a two-end-member mixing model, the biological uptake resulted in a dissolved inorganic phosphorus depletion of 0.12 ± 0.08 μM in the coastal surface water, whereas the replenishment of dissolved inorganic phosphorus in the lower river from particle P regeneration and release resulted in an increase (0.19 ± 0.22 μM) of dissolved inorganic phosphorus in the estuarine mixing region. The molar ratios of dissolved inorganic nitrogen to dissolved inorganic phosphorus and dissolved silicate to dissolved inorganic phosphorus in the open surface waters were >22, suggesting that, although the lower Dafengjiang River contained elevated concentrations of dissolved inorganic phosphorus, the northern Beibu Gulf was an overall P-limited coastal ecosystem.

Nutrient removal in a closed silvofishery system using three mangrove species (Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle)

The removal of ammonium (NH4(+)), nitrite (NO2(-)), nitrate (NO3(-)), and phosphate (PO4(-3)) in a closed silvofishery system was examined using three mangrove species (i.e., Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle). Specifically, six closed tanks were installed for this experiment with a population of 60 Dormitator latifrons fishes per tank. We planted 40 seedlings in each of three experimental tanks separated by species, while the remaining tanks were used as control. During 15 weeks, nutrient concentrations among the three mangrove systems presented no significant differences (P>0.05). However, nutrient removal variability was minimum during the last 2-5 weeks. Mangroves presented an average efficiency of 63% for the removal of NH4(+) and NO2(-). Contrary, the average removal potential of NO3(-) and PO4(-3) was 50%. Results from this study suggest that the three mangrove species could be used in a closed silvofishery systems for the biological removal of NH4(+), NO2(-), NO3(-), and PO4(-3).