Microplastics pollution in the rivers of a metropolitan city and its estimated dependency on surrounding developed land.

The spatial distribution and abundance of suspected microplastics (SMPs) in the surface water of a metropolitan city, as represented by four Hong Kong rivers, was studied during the dry season. Shing Mun River (SM), Lam Tsuen River (LT), and Tuen Mun River (TM) are located in urbanized areas, and SM and TM are tidal rivers. The fourth river, Silver River (SR) is situated in a rural area. TM had a significantly higher SMP abundance (53.80 ± 20.67 n/L) than the other rivers. The SMP abundance increased from upstream to downstream in non-tidal rivers (LT and SR), but not in tidal rivers (TM and SM), probably due to the tidal influence and a more homogeneous urban development along the tidal rivers. Inter-site differences in the SMP abundance were strongly correlated with the built area ratio (defined as the percentage of surrounding developed land area), human activities, and the nature of the river. About half (48.72 %) of the SMPs were <250 µm. Fibers and fragments were most abundant (>98 %), with most of them being transparent (58.54 %), black (14.68 %), or blue (12.12 %). Polyethylene terephthalate (26.96 %) and polyethylene (20.70 %) were the most common polymers. However, the MP abundance could be overestimated due to the presence of natural fibers. By contrast, an underestimation of the MP abundance could result from a smaller volume of water samples collected, due to a low filtration efficiency caused by high organic content and particle concentrations in the water. A more effective solid waste management strategy and upgrading of the sewage treatment facilities for removing microplastics are recommended to ameliorate the microplastic pollution in local rivers.

Do distribution and expansion of exotic invasive Asteraceae plants relate to leaf construction cost in a man-made wetland?

Exotic species especially Asteraceae plants severely invade wetlands in Shenzhen Bay, an important part of the coast wetland in Guangdong-Hong Kong-Macau Bay Area, China. However, the reasons causing their expansion are unclear. The leaf traits and expansion indices of six invasive Asteraceae plants from the Overseas Chinese Town (OCT) wetland were studied and the results showed that nearly 45% of the total plant species (31 out of 69 species) in the OCT wetland, belonging to 15 families and 27 genera, were exotic invasive species. The expansion indices of six Asteraceae species negatively correlated with their leaf construction cost based on mass (CCM), caloric values and carbon concentration, but their relations with ash content were positive. Multiple linear regression analysis revealed that CCM was the most important factor affecting the expansion of an exotic species, indicating CCM may be an important reason causing the expansion of exotic species in coastal wetlands.

Barnacles as potential bioindicator of microplastic pollution in Hong Kong.

Microplastic (MP) pollution is an emerging problem in the marine environment and the assessment of the presence and abundance of microplastics in wild organisms is essential for risk assessment. The occurrence of microplastics in four species of barnacles at 30 sites in Hong Kong waters was investigated. The median number of microplastics ranged between 0 and 8.63 particles g-1 wet weight, or 0 and 1.9 particles individual-1, with fibers being the most abundant type of microplastics. The chemical composition of 152 pieces out of 606 potential microplastics was analyzed using micro-Fourier Transform Infrared Spectroscopy (µ-FTIR). Fifty-two of them were synthetic polymers, 95 natural cotton fibers and five unknowns. Eight types of polymer were identified with cellophane being the most abundant (58%). Correlation analysis was conducted between the abundance of MPs in sediments obtained in our previous study and that in individual barnacle species in this study, and a positive correlation was established for the barnacle Amphibalanus amphitrite, highlighting the potential of using this species as a bioindicator of microplastics.

Degradation of BDE-47 in mangrove sediments with amendment of extra carbon sources.

Polybrominated diphenyl ethers (PBDEs) are widely detected in coastal wetlands but their remediation is still difficult. In this study, different carbon sources, namely formate, acetate, pyruvate, lactate, succinate, methanol and ethanol, were added to mangrove sediments contaminated with BDE-47, a common PBDE congener, to enhance its degradation. After 2-month incubation, all carbon addition significantly enhanced degradation percentages. The residual BDE-47 percentage significantly correlated with the abundance of total bacteria and Dehalococcoides spp. The addition of methanol, acetate and succinate also achieved significantly higher degradation rates and shorter half-lives than sediments without carbon amendment at the end of 5-month incubation, although degradation percentages were comparable between sediments with and without extra carbon. The degradation pathway based on the profiles of degradation products was also similar among treatments. The results indicated the stimulatory effect of extra carbon sources on BDE-47 degradation in contaminated sediments was carbon- and time-specific.

Is Laguncularia racemosa more invasive than Sonneratia apetala in northern Fujian, China in terms of leaf energetic cost?

Laguncularia racemosa and Sonneratia apetala are fast-growing exotic mangrove species in Southern China and widely used for afforestation. However, the invasiveness of the two exotic species is still unclear. We compared structural and physiological traits and energy-use related traits between L. racemosa and S. apetala, and with two natives (Kandelia obovata and Aegiceras corniculatum) in northern Fujian. Results showed that leaf construction cost based on mass (CCM) and caloric values of L. racemosa were significantly lower than S. apetala, and the two natives had highest CCM. Because lower CCM, L. racemosa grew faster with a taller height (4.83 m) and wider ground diameter circumference (40.03 cm) than S. apetala (4.43 m tall and 35.63 cm wide) and the two natives (2.42 m tall and 26.78 cm wide). These findings indicated that L. racemosa could be more invasive than S. apetala in mangrove forests in northern Fujian, China where it still grew well, which deserves more attention.

Predator prey interactions between predatory gastropod Reishia clavigera, barnacle Amphibalanusamphitriteamphitrite and mussel Brachidontesvariabilis under ocean acidification.

Since the response to ocean acidification is species specific, differences in responses between predator and prey will alter their interactions, hence affect the population dynamics of both species. Changes in predator prey interactions between a predatory muricid gastropod Reishia clavigera and its prey, the barnacle Amphibalanus amphitrite amphitrite and mussel Brachidontes variabilis under three pCO2 levels (380, 950, and 1250 µatm) were investigated. The searching time for barnacles increased and the ability to locate them decreased at higher pCO2 levels. The movement speed and the prey consumption rate, however, were independent of pCO2. There was no preference towards either B. variabilis or A. amphitrite amphitrite regardless of pCO2. Exposure experiments involving multiple generations are suggested to assess transgenerational effects of ocean acidification and the potential compensation responses before any realistic predictions on the long term changes of population dynamics of the interacting species can be made.

Degradation of BDE-47 in mangrove sediments under alternating anaerobic-aerobic conditions.

Polybrominated diphenyl ethers (PBDEs) resistant to degradation have significant environmental impacts. Anaerobic reductive debromination and aerobic oxidation of PBDEs by microorganisms are main removal mechanisms during natural attenuation, but previous studies often focused on the process under either aerobic or anaerobic condition leading to unsatisfactory removal. The present study aims to remove PBDEs by employing alternating anaerobic-aerobic condition, which is common in inter-tidal mangrove sediments, and elucidate the degradation pathways. During 40-week experiment, BDE-47 reduced with an accumulation of tri-BDEs and di-BDEs as debromination products in all sediments. However, the removal percentages of BDE-47 and the concentrations of debromination congeners varied among flushing regimes. Sediments under less frequent flushing regime (longer duration of aerobic period) had significantly lower concentration and proportion of debromination products, especially BDE-17, than that under more frequent regime (longer anaerobic period). BDE-17 then went through aerobic degradation pathway, as evidenced by the accumulation of its hydroxylation form. Microbial analyses further revealed that less frequent regime favored accumulation of biphenyl dioxygenase gene for aerobic degradation, while more frequent tidal regime promoted growth of dehalogenating bacteria for reductive debromination. This study first time demonstrated that PBDEs in contaminated sediments could be removed under alternating anaerobic-aerobic conditions.

Contamination of polybrominated diphenyl ethers (PBDEs) in watershed sediments and plants adjacent to e-waste sites.

Polybrominated diphenyl ethers (PBDEs) are one of the persistent toxic organic pollutants in watersheds near electrical and electronic waste (e-waste) sites (EWS). Spatial redistribution, translocation and bioaccumulation of PBDEs in natural sediment-plant ecosystems, however, are still unclear. The contamination and distribution of PBDEs in core sediments and wetland plants from two EWS and two mangrove forest sites (MFS) were investigated. The eight PBDE congeners were all detected in plant tissue and sediment samples, indicating PBDE contamination was common and severe, and their spatial variations were significant. Although sediments from EWS had higher PBDE concentrations than those in MFS, with an extremely high value of 36392 ±â€¯5992 ng g-1 dw, mangroves could be the sink of PBDEs, as high concentrations (327 ±â€¯48 ng g-1 dw) were also detected in mangrove sediments. The historical usage of PBDEs was reflected by their distribution in mangrove sediment core but not so in e-waste sediment core. PBDEs were taken up and accumulated in six wetland plants, with more accumulation in mangrove plants. These results demonstrated that PBDEs were not only contaminated in sediments adjacent to e-waste sites but also plant tissues. PBDEs could enter other environments via plant littering and/or herbivorous processes that must not be neglected.

Trophic and growth baseline of dominant subtidal gastropods in contrasting subtropical marine environments.

Using 13C/12C, 15N/14N and 18O/16O isotopes, the trophic relationship and growth estimation were analyzed in gastropods Nassarius siquijorensis, Murex trapa and Turritella bacillum and their potential food sources and predators in summer and winter from estuarine and oceanic environments in subtropical Hong Kong. Results of δ13C and δ15N values and isotopic mixing model revealed N. siquijorensis and M. trapa were one trophic level higher than T. bacillum, in which its main food source was particulate organic matter (POM) whereas N. siquijorensis largely consumed POM and polychaetes and M. trapa also preyed on other gastropods. Crabs were the major predator of gastropods. Organisms collected from oceanic waters were more 13C enriched than from estuarine waters, reflecting different carbon food sources from marine or terrestrial origin. The δ18O profile from shell carbonate suggested these gastropods were one to two years old. T. bacillum exhibited faster summer growth than the other two species.

The combined effects of elevated pCO2 and food availability on Tigriopus japonicus Mori larval development, reproduction, and superoxide dismutase activity.

Previous studies have shown that ocean acidification has little effect on adult Tigriopus japonicus copepods, and mainly impairs the early development and reproduction of females. This study investigated the possible interactive effect between CO2-induced seawater acidification and food availability on larval development and reproductive output in T. japonicus. Copepods were exposed to either pH8.1 or pH7.3 under different food concentrations (0.5×104-80.0×104cells/mL). Both the development of nauplii and copepodites was delayed at pH7.3 with a greater effect at lower food concentrations. The reproductive output followed a bell-shaped curve with the highest reproductive output at food concentrations between 30×104 and 40×104cells/mL. As an indicator of oxidative stress, the activity of superoxide dismutase increased at lower pH, with a greater increase at lower food concentrations. Therefore, the effect of elevated pCO2 on T. japonicus was food dependent.

Performance and bacterial community structure of a 10-years old constructed mangrove wetland.

Constructed mangrove wetland has been used for wastewater treatment but its long-term performance has not been reported. One-year monitoring of a 10-years old horizontal subsurface-flow constructed mangrove wetland consisting of three belts, two with mangrove plants and one without, revealed that the system maintained high and stable removal percentages of organic matter and nutrients, and planted belts performed better than unplanted control. Substrates in belts planted with Aegiceras corniculatum or Kandelia obovata had higher abundance of ammonifiers, nitrifiers and denitrifiers but lower total heterotrophic bacteria than unplanted substrate. Denaturing gradient gel electrophoresis showed that microbial diversity in planted substrate was significantly lower than that in unplanted one. The bacteria in substrates, irrespective to belts, were phylogenetically related to Proteobacteria (most dominant), Acidobacteria, Firmicutes, Nitrospirae, Gemmatimonadetes, Chloroflexi and Cyanobacteria. The steady performance of this 10-year old constructed mangrove wetland was affected by the abundance and diversity of bacterial community in substrate.

Impact of habitat management on waterbirds in a degraded coastal wetland.

The loss of coastal wetlands in Hong Kong Mai Po Nature Reserve adversely affected wetland-depended species. To mitigate this impact, gei wai ponds were reconstructed according to a set of biodiversity management zones (BMZs). This study, based on Canonical Correspondence Analysis (CCA), investigated if waterbird distribution was related to BMZ characteristics. Based on habitat characteristics, ponds in the same BMZ generally clumped in the same quadrant or within a short distance on CCA scatter plots, indicating that a BMZ zone produced common habitat traits. Ponds in a close distance on the plot had similar bird abundance or community structure. Significant correlations were noted between the abundance of cormorants and tall tree, and between waders and bare ground areas within study ponds. This study indicated that the control of key habitat factors was important for the success of reconstruction of gei wais and management of waterbirds in Mai Po.

Predator-prey interaction between muricid gastropods and mussels under ocean acidification.

Predation of the muricid gastropod Thais clavigera on two-sized groups of the mussel Brachidontes variabilis was studied under three pCO2 levels, 380, 950, and 1250µatm. At 950µatm pCO2 level, the prey handling time decreased significantly and large-sized B. variabilis were preferred by T. clavigera. However, the prey consumption rate was independent of pCO2 levels, although the prey searching time increased significantly at elevated pCO2. These findings indicated that the predator-prey interaction between T. clavigera and B. variabilis was altered under ocean acidification, which will have a long-term impact on the population dynamics of the interacting species.

Microplastic ingestion reduces energy intake in the clam Atactodea striata.

The effects of microplastic concentrations (10itemsl-1 and 1000itemsl-1) on the physiological responses of Atactodea striata (clearance rate, absorption efficiency, respiration rate) were investigated. The fates of ingested microplastics and the efficiency of depuration in removing ingested microplastics were also studied. A. striata ingested microplastics and the clearance rate was reduced at high concentration of microplastics. Since the respiration rate and absorption efficiency remained unchanged in exposed A. striata, reduction in the clearance rate would reduce the energy intake. Ingestion and retention of microplastics in the body were further limited by the production of pseudofaeces and faeces, and depuration in clean water, resulting in a very small amount of microplastics stored in the body of the clam.

Physiological responses and scope for growth upon medium-term exposure to the combined effects of ocean acidification and temperature in a subtidal scavenger Nassarius conoidalis.

Physiological responses (ingestion rate, absorption rate and efficiency, respiration, rate, excretion rate) and scope for growth of a subtidal scavenging gastropod Nassarius conoidalis under the combined effects of ocean acidification (pCO2 levels: 380, 950, 1250 µatm) and temperature (15, 30 °C) were investigated for 31 days. There was a significant reduction in all the physiological rates and scope for growth following short-term exposure (1-3 days) to elevated pCO2 except absorption efficiency at 15 °C and 30 °C, and respiration rate and excretion rate at 15 °C. The percentage change in the physiological rates ranged from 0% to 90% at 15 °C and from 0% to 73% at 30 °C when pCO2 was increased from 380 µatm to 1250 µatm. The effect of pCO2 on the physiological rates was enhanced at high temperature for ingestion, absorption, respiration and excretion. When the exposure period was extended to 31 days, the effect of pCO2 was significant on the ingestion rate only. All the physiological rates remained unchanged when temperature increased from 24 °C to 30 °C but the rates at 15 °C were significantly lower, irrespective of the duration of exposure. Our data suggested that a medium-term exposure to ocean acidification has no effect on the energetics of N. conoidalis. Nevertheless, the situation may be complicated by a longer term of exposure and/or a reduction in salinity in a warming world.

Isolation and mRNA expression of hypoxia-inducible factor α (HIF-α) in two sublittoral nassariid gastropods: Nassarius siquijorensis and Nassarius conoidalis.

Hypoxia-inducible factor (HIF) is commonly found in vertebrates as an adaptation against hypoxia. In this study, partial sequences of HIF were first reported for subtidal nassariid gastropods, Nassarius siquijorensis and Nassarius conoidalis. The phylogenetic tree of HIF-α among various animal groups was inferred. Adults of both N. siquijorensis and N. conoidalis were exposed to 24 h and 1 week hypoxia (1.5 mg O2 l(-1)) and the change of HIF-α mRNA level was determined in gill, foot muscle and hepatopancreas by using quantitative real-time PCR. The expression profile of HIF-α was also investigated in veliger larvae after 1 and 24 h hypoxia exposure. The HIF-α mRNA from gills of N. siquijorensis was 2.22-fold elevated after 24 h of exposure but returned back to the baseline level after 1 week of exposure. In contrast to N. siquijorensis, both short-term and long-term exposure to hypoxia significantly enhanced the mRNA level of HIF-α in the gill of N. conoidalis. Both 24 h and 1 week of exposure resulted in a significantly higher degree of expression of mRNA of HIF-α in the foot muscle of N. siquijorensis and N. conoidalis. For hepatopancreas, the mRNA level of HIF-α remained unchanged in N. siquijorensis after both 24 h and 1 week of exposure to hypoxia but a significant enhancement was observed in N. conoidalis after 1 week of exposure to hypoxia. The mRNA level of HIF-α significantly reduced after 24 h of exposure to hypoxia in the larvae of N. siquijorensis. The different expression patterns of HIF-α mRNA between the two species reflected their different adaptation abilities to hypoxia, which will further explain the differences in the distribution of these species in Hong Kong waters with N. siquijorensis being predominant in regions suffered from summer hypoxia.

Effects of hypoxia on biofilms and subsequently larval settlement of benthic invertebrates.

Biofilms on submerged surfaces are important in determining larval settlement of most marine benthic invertebrates. We investigated if exposure of biofilms to hypoxia would alter the larval settlement pattern and result in a shift in benthic invertebrate community structure in the field. Biofilms were first exposed to hypoxia or normoxia in laboratory microcosms for 7 days, and then deployed in the field for another 7 days to allow for larval settlement and recruitment to occur. Using terminal-restriction fragment length polymorphism of the 16S rRNA gene, this study showed that hypoxia altered the biofilm bacterial community composition, and the difference between the hypoxic and normoxic treatments increased with the time of exposure period. This study also demonstrated significantly different benthic invertebrate community structures as a result of biofilm exposure to hypoxia and that the hypoxic and normoxic treatments were dominated by Hydroides sp. and Folliculina sp., respectively.

Effects of salinity on anatomical features and physiology of a semi-mangrove plant Myoporum bontioides.

The effect of different concentrations of NaCl, 0, 100, 200, 300 and 400 mM, on the anatomical features and physiology of Myoporum bontioides was investigated. The photosynthetic rates (Pn) were significantly reduced by salt stress, with the lowest values at 400 mM NaCl. The content of malondialdehyde (MDA), proline and soluble sugar, as well as the activities of peroxidase (POD) and catalase (CAT) increased at the beginning, but became similar to the control as the experiment proceeded. The NaCl effect on superoxide dismutase (SOD) was different from the other parameters, with a significant reduction at 400 mM NaCl at Day 7. Salt glands were found in both upper and lower epidermis, and the ratios of the thickness of palisade to spongy mesophyll tissues increased with NaCl concentrations. The medullary ray was clearly damaged by NaCl at levels of 200 and 300 mM. These results demonstrated that M. bontioides could adapt to a relatively low salinity, and was not a halophilous species.

Comparisons of the metabolic responses of two subtidal nassariid gastropods to hypoxia and re-oxygenation.

Changes in the levels of carbohydrate, lipid, protein and anaerobic metabolites (succinate, lactate, acetate, fumarate and propionate), upon exposure to hypoxia (1.5 mg O2 l(-1)) and after reoxygenation in subtidal gastropods Nassarius siquijorensis and N. conoidalis, were compared. A significant decrease of the glycogen content was observed under hypoxia in N. conoidalis but not in N. siquijorensis. A greater increase in the concentrations of anaerobic metabolites was observed in N. conoidalis under hypoxia, and their levels did not return to baseline after returning to normoxia for 24h. In contrast, a lower rate of accumulation of the metabolites was observed in N. siquijorensis, and complete recovery from anaerobic metabolism was observed after reoxygenation. The results lend further support to the role of hypoxia in governing the different distributional patterns between the two subtidal gastropods in Hong Kong waters.