Effects of intensive oyster farming on nitrogen speciation in surface sediments from a typical subtropical mariculture bay.

The spatial-temporal distributions of various nitrogen (N) species in surface sediments were examined in a typical subtropical mariculture bay (Maowei Sea) in the northern Beibu Gulf to assess the impact of intensive oyster culture activities on sedimentary N speciation. The results indicated that the mean contents of total nitrogen (TN), extractable (labile) nitrogen (LN) and residual nitrogen (RN) in the surface sediments were 33.3 ± 15.5 µmol g-1, 13.8 ± 1.3 µmol g-1 and 19.5 ± 15.0 µmol g-1, respectively, which lacked significant seasonal variability (P > 0.05). Four forms of LN, namely ion extractable form (IEF-N), weak acid extractable form (WAEF-N), strong alkali extractable form (SAEF-N) and strong oxidant extractable form (SOEF-N) were identified based on sequential extraction. SOEF-N was the dominant form of LN, accounting for 67.8 ± 2.5 % and 63.7 ± 5.9 % in summer and winter, respectively. Spatially, the contents of sedimentary TN, LN, RN, WAEF-N and SOEF-N in intensive mariculture areas (IMA) were significantly higher than those in non-intensive mariculture areas (NIMA) during summer (P < 0.05). Stable nitrogen isotope (δ15N) mixing model revealed that shellfish biodeposition was the predominant source of sedimentary TN in IMA with a contribution of 67.8 ± 23.0 %, approximately 5.4 times that of NIMA (12.6 ± 3.3 %). Significant positive correlations (P < 0.05) were observed between most forms of N species (WAEF-N, SOEF-N, LN and RN) and shellfish-biodeposited N in the surface sediments during summer, indicating that intensive oyster farming greatly enhanced sedimentary TN accumulation.

Spatiotemporal distributions of poorly-bound heavy metals in surface sediments of a typical subtropical eutrophic estuary and adjacent bay.

The toxicity of heavy metals is dependent on their bioavailability. This study explored the relationship existing among sedimentary nutrients such as bulk nitrogen (TN) and phosphorus (TP), organic carbon (OC), water column chlorophyll-a (Chl-a) and the poorly-bound fraction of sedimentary heavy metals (Cd, Ni, Zn, Cu, Pb and Cr) in the Dafengjiang River Estuary and adjacent Sanniang bay in 2017 and 2018. Results showed that the texture of the surface sediments was dominated by coarse sand, while sedimentary organic matter was dominated by marine phytoplankton and mariculture biodeposits. Surprisingly, concentrations of poorly-bound heavy metals in sediments were relatively high. The average contents of Cd and Ni did not vary both spatially and temporally, Cu and Pb only varied spatially, Cr varied both spatially and temporally, while Zn only varied temporally. Significant positive correlations occurred between sedimentary TN, TP, and OC, including water column Chl-a and poorly-bound heavy metals in sediments. As sediments are important sources of nutrients for primary productivity, the results of this study suggest that the remobilization of sequestered poorly-bound heavy metals in surface sediments deposited in shallow eutrophic estuaries and coastal waters enriched by labile organic matter can enhance by nutrients. The relationship between the poorly-bound heavy metals and nutrients in surface sediments and water column Chl-a is concerning and requires further in-depth investigation. This is because estuaries are economically important ecosystems rich in bioresources, characterized by dynamic biogeochemical conditions.

Sources, burial flux and mass inventory of black carbon in surface sediments of the Daya Bay, a typical mariculture bay of China.

The contents of chemothermal oxidation (CTO)-derived black carbon (BC) and organic carbon (OC) and their stable isotopes (δ13CBC and δ13COC), including major elemental oxides, and grain sizes were measured to constrain the sources, burial flux, and mass inventory of BC in surface sediments of the Daya Bay. Surface sediments were mainly clayey silt (>90%) and contained 0.28-1.18% OC and 0.05-0.18% BC. Fossil fuel emission and physical erosion contributed to the sedimentary BC sources. High BC/OC ratio (6-30%), burial flux (154.88-922.67 µg cm-2 y-1), and mass inventory (22-34 Gg y-1) of BC in the upper 5 cm of surface sediments indicated that the Daya Bay is a significant sink of BC. The high accumulation of BC in sediments is attributed to a strong affinity to fine-grained sediments due to the enrichment of muddy biodeposits excrements from the cultured species in the bay.

Co-occurrence of chromophytic phytoplankton and the Vibrio community during Phaeocystis globosa blooms in the Beibu Gulf.

Accumulating research evidence has revealed that harmful algal blooms (HABs) can substantially affect the community structures of phytoplankton and heterotrophic bacteria in marine ecosystems. However, little is known about their species-specific interactions between phytoplankton and heterotrophic bacteria during the HABs period and about their interaction shifts in response to blooms. From this perspective, we investigated the co-occurrence of chromophytic phytoplankton and Vibrio during Phaeocystis globosa blooms in the Beibu Gulf. The results showed that Vibrio communities were distinct during the blooms, and P. globosa blooms resulted in a decline in phytoplankton alpha diversity, revealing that the blooms could affect their community compositions. The regression lines between the Shannon indices and Bray-Curtis distances of phytoplankton and Vibrio showed positive correlations with each other (p < 0.001), suggesting that they may have intrageneric symbiotic interactions overall. In addition, network analysis further demonstrated that relationships between phytoplankton and Vibrio were dominated by positive correlations, and more interaction modules were observed during the blooms, revealing that the blooms intensified synergistic association and mutual symbiotic interactions between them. Environmental factors (SiO32-, NH4+, NO3- and TN,) and P. globosa density more deeply affected network interactions between phytoplankton and Vibrio during the periods of P. globosa blooms than those before the blooms and after the blooms. This study provided new insight to elucidate community structure and interaction relationships between phytoplankton and Vibrio in response to P. globosa blooms and their ecological effects in marine ecosystems.

Pigment Characterization of the Giant-Colony-Forming Haptophyte Phaeocystis globosa in the Beibu Gulf Reveals Blooms of Different Origins.

The giant-colony-forming haptophyte Phaeocystis globosa has caused several large-scale blooms in the Beibu Gulf since 2011, but the distribution and dynamics of the blooms remained largely unknown. In this study, colonies of P. globosa, as well as membrane-concentrated phytoplankton samples, were collected during eight cruises in the Beibu Gulf from September 2016 to August 2017. Pigments were analyzed by high-performance liquid chromatography coupled with a diode array detector (HPLC-DAD). The pigment 19'-hexanoyloxyfucoxanthin (hex-fuco), generally considered a diagnostic pigment for Phaeocystis, was not detected in P. globosa colonies in the Beibu Gulf, whereas 19'-butanoyloxyfucoxanthin (but-fuco) was found in all colony samples. Moreover, but-fuco in membrane-concentrated phytoplankton samples exhibited a similar distribution pattern to that of P. globosa colonies, suggesting that but-fuco provided the diagnostic pigment for bloom-forming P. globosa in the Beibu Gulf. Based on the distribution of but-fuco in different water masses in the region prior to the formation of intensive blooms, it is suggested that P. globosa blooms in the Beibu Gulf could originate from two different sources. IMPORTANCE Phaeocystis globosa has formed intensive blooms in the South China Sea and even around the world, causing huge social economic losses and environmental damage. However, little is known about the formation mechanism and dynamics of P. globosa blooms. 19'-Hexanoyloxyfucoxanthin (hex-fuco) is often used as the pigment proxy to estimate Phaeocystis biomass, while this is challenged by the giant-colony-forming P. globosa in the Beibu Gulf, which contains only 19'-butanoyloxyfucoxanthin (but-fuco) but not hex-fuco. Using but-fuco as a diagnostic pigment, we traced two different origins of P. globosa blooms in the Beibu Gulf. This study clarifies the development process of P. globosa blooms in the Beibu Gulf, which provides a basis for the early monitoring and prevention of the blooms.

Stochastic processes dominate marine free-living Vibrio community assembly in a subtropical gulf.

Understanding the effects of eutrophication on heterotrophic bacteria, a primary responder to eutrophication, is critical for predicting the responses of ecosystems to marine environmental pollution. Vibrio are indigenous in coastal water and of significance to geochemical cycling and public health. In this study, we investigated the diversity and assembly features of Vibrio, as well as their relationship with the environmental factors in the subtropical Beibu Gulf. We found that the alpha diversity of Vibrio increased in parallel with the trophic state they occupy. A Mantel test indicated that the trophic state was correlated to Vibrio beta diversity and the correlation gradually strengthened at higher trophic states. Variation partitioning analysis suggested that the geographic distance was an important factor impacting the variables of Vibrio communities in all the samples, but nutrients exerted more influence in the more highly eutrophic samples. Our results demonstrated that stochastic processes govern the turnover of marine Vibrio communities in the Beibu Gulf and that ecological drift was the most important process for assembly of the Vibrio communities.

Phylogenetic Responses of Marine Free-Living Bacterial Community to Phaeocystis globosa Bloom in Beibu Gulf, China.

Phaeocystis globosa blooms are recognized as playing an essential role in shaping the structure of the marine community and its functions in marine ecosystems. In this study, we observed variation in the alpha diversity and composition of marine free-living bacteria during P. globosa blooms and identified key microbial community assembly patterns during the blooms. The results showed that the Shannon index was higher before the blooming of P. globosa in the subtropical bay. Marinobacterium (γ-proteobacteria), Erythrobacter (α-proteobacteria), and Persicobacter (Cytophagales) were defined as the most important genera, and they were more correlated with environmental factors at the terminal stage of P. globosa blooms. Furthermore, different community assembly processes were observed. Both the mean nearest relatedness index (NRI) and nearest taxon index (NTI) revealed the dominance of deterministic factors in the non-blooming and blooming periods of P. globosa, while the bacterial communities in marine waters after the blooms tended to be controlled by stochastic factors. Our findings revealed that the assembly of the bacterial community in marine P. globosa blooms is a complex process with mixture effects of marine microbiomes and environmental parameters.

Compositions and spatio-temporal distributions of different nitrogen species and lability of dissolved organic nitrogen from the Dafengjiang River to the Sanniang Bay, China.

Field investigations and incubation experiment were carried out in the Dafengjiang River (DFJR) and Sanniang Bay (SNB) surface water from September 2017 to July 2018 to study the composition and spatio-temporal distributions of different nitrogen (N) species, lability of dissolved organic N (DON), and contributions of different N species from the DFJR to the dissolved inorganic N (DIN) level in the SNB. The spatio-temporal distributions of different N species exhibited significant seasonal variation (p < 0.05). The average contribution of DIN, DON, and particulate nitrogen (PN) to TN was 31.91%, 46.57%, and 21.52%, respectively. Comparatively being the dominant form of N in the study area, the average lability of DON across the incubation sites D1, D5 and D7 ranged from 72%-79%. DIN, DON, and PN from the DFJR respectively contributed to 55.95%, 7.03%, and 5.63% of water quality in the SNB.

Bulk sedimentary phosphorus in relation to organic carbon, sediment textural properties and hydrodynamics in the northern Beibu Gulf, South China Sea.

Bulk sedimentary phosphorus (P) is studied to evaluate its source, distribution, preservation and enrichment in relation with organic carbon (OC), sediment textures and moisture contents in the northern Beibu Gulf. Approximately 80% of surface sediments in the investigated sites were composed of coarse sandy texture (>63 µm). Total P (TP), inorganic P (IP) and organic P (OP) contents were lower to medium range compared to the levels reported for other marginal seas. Sedimentary OC and P were derived from mixed sources, with high terrestrial influence in the coastal areas (molar OC/OP ratios >250:1). The distribution of P corroborated with the variation tendency of fine-grained sediments, moisture contents and OC. Both IP and OP may significantly influence the trophic state of seawater if released from surface sediments. Influenced by hydrodynamics, frequent resuspension and high abundance of sand, TP is less preserved, and shows low to moderate enrichment in surface sediments.

Phosphorus chemical speciation and seasonal variations in surface sediments of the Maowei Sea, northern Beibu Gulf.

This study presents the distribution, seasonal variations and factors influencing phosphorus (P) forms in surface sediments from the Maowei Sea. P forms were measured using the sequential extraction (SEDEX) procedures. Inorganic P (IP) was the predominant chemical form of total P (TP). Fe-bound P (FeP) was the main IP form. Sediment particle sizes, organic matter distribution, terrestrial input and aquaculture activity were responsible for the seasonal variations of different forms of P in sediment. In summer, the average proportions of P fractions in TP followed the order of organic P (OP) > Fe-P > authigenic P (CaP) > detrital P (De-P) > exchangeable P (Ex-P); in winter, the corresponding order was OP > Fe-P > De-P > Ca-P > Ex-P. The potential bio-available P accounted for 71.1 ±â€¯4.9% and 70.6 ±â€¯6.3% of TP in summer and winter, respectively. Sedimentary organic matter mainly came from land-based sources in winter.

Distribution characteristics and ecological evaluation of chlorobenzene compounds in surface sediment of the Maowei Sea, Guangxi, China.

To enhance our understanding on environmental conditions of the Maowei Sea in Guangxi province, China, the concentration and distribution of 22 chlorobenzene compounds (CBs) in the surface sediment were determined by gas chromatography-mass spectrometry (GC-MS). The relationship of the sediment between CBs and total organic carbon (TOC) was also investigated. The results showed that a total of eight kinds of CBs compounds were detected in the sediment samples which were collected from the coastal environment of the Maowei Sea, with an average concentration of 15.3 ng·g-1 (the concentration range, 2.5-61.5 ng·g-1). The rank of their average concentrations was as follows: 2,3,4,5,6-pentachlorotoluene > hexachlorobenzene, 2-chlorotoluene, 3-chlorotoluene and 2,3-dichlorotoluene > 2,4-dichlorotoluene > 4-chlorotoluene > pentachlorobenzene. Most CBs were distributed in sediments along the east coast of the Maowei Sea. For total TOC content in sediments, the concentration in the sampling locations was similar, with a mean concentration of 8.83 g·kg-1 (the concentration range, 4.87-20.13 g·kg-1). However, there was no significant correlation between the concentration of TOC and total CBs. Compared to the corresponding CBs in the sediment of other coastal areas in mainland China and other countries, the value of CBs in the Maowei Sea was low.

Microplastic pollution in the Maowei Sea, a typical mariculture bay of China.

The ingestion of microplastics by organisms presents a potential exposure route for humans via seafood consumption. Although mariculture has become an essential source of seafood worldwide, the content of microplastics in the mariculture zone has received less scrutiny than in the wild environment. The Maowei Sea is a semi-closed bay that is rich in fishery resources. The specific levels of microplastics in the Maowei Sea and its fishery products remain undetermined. In this paper, we detail the distributions and characteristics of microplastics in the aquaculture water and biota of the Maowei Sea. Microplastics were detected in the range of 1.2-10.1 particles/L in Maowei Sea surface water, with high microplastic content in estuarine oyster nursery (10.1 particles/L) and Qinzhou harbor (8.8-9.5 particles/L) sites. In water samples from the three inflowing rivers, the abundances ranged from 2.9 to 4.5 particles/L, which is comparable to that in Maowei Sea surface water. Of 66 collected fish belonging to 12 species, microplastics were observed in all of the gastrointestinal tracts (GITs) and in the gills of 40 individuals. In the GIT and gill tests, the abundances of microplastics ranged from 2.0 to 14.0 and from 0.0 to 8.5 particles/individual, respectively. The enhanced particles/individual figures in the GIT relative to the gill are particularly noteworthy. Demersal species showed significantly higher abundances of microplastics than pelagic species (p < 0.05). Microplastics were also detected in the soft tissues of all oyster samples, with abundances ranging from 3.2 to 8.6 particles/individual. The microplastic composition was dominated by rayon and polyester and tended to be white in color and fibrous in shape. Altogether, these results correspond to high levels of microplastics in the Maowei Sea. As the study region is a mariculture bay, the observed microplastics contamination in its fishery products presents a route for human exposure. CAPSULE ABSTRACT: Microplastics are widespread in the aquaculture water and biota in the Maowei Sea, a mariculture bay.

High Specificity of a Quantitative PCR Assay Targeting a Saxitoxin Gene for Monitoring Toxic Algae Associated with Paralytic Shellfish Toxins in the Yellow Sea.

The identification of core genes involved in the biosynthesis of saxitoxin (STX) offers a great opportunity to detect toxic algae associated with paralytic shellfish toxins (PST). In the Yellow Sea (YS) in China, both toxic and nontoxic Alexandrium species are present, which makes it a difficult issue to specifically monitor PST-producing toxic algae. In this study, a quantitative PCR (qPCR) assay targeting sxtA4, a domain in the sxt gene cluster that encodes a unique enzyme involved in STX biosynthesis, was applied to analyze samples collected from the YS in spring of 2012. The abundance of two toxic species within the Alexandrium tamarense species complex, i.e., A. fundyense and A. pacificum, was also determined with TaqMan-based qPCR assays, and PSTs in net-concentrated phytoplankton samples were analyzed with high-performance liquid chromatography coupled with a fluorescence detector. It was found that the distribution of the sxtA4 gene in the YS was consistent with the toxic algae and PSTs, and the quantitation results of sxtA4 correlated well with the abundance of the two toxic species (r=0.857). These results suggested that the two toxic species were major PST producers during the sampling season and that sxtA-based qPCR is a promising method to detect toxic algae associated with PSTs in the YS. The correlation between PST levels and sxtA-based qPCR results, however, was less significant (r=0.552), implying that sxtA-based qPCR is not accurate enough to reflect the toxicity of PST-producing toxic algae. The combination of an sxtA-based qPCR assay and chemical means might be a promising method for monitoring toxic algal blooms.

Tracing the origin of paralytic shellfish toxins in scallop Patinopecten yessoensis in the northern Yellow Sea.

Some dinoflagellate species within the genera Alexandrium, Gymnodinium and Pyrodinium are well-known producers of paralytic shellfish toxins (PST), which led to many poisoning incidents around the world. In the northern Yellow Sea, an important mariculture zone for scallop Patinopecten yessoensis, PST have been frequently detected from scallops. However, there is little knowledge concerning PST-producing microalgae in this region so far. In cruises carried out in 2011 and 2012, scallop and phytoplankton samples were collected from the northern Yellow Sea. PST were detected from scallops by high-performance liquid chromatography with fluorescence detection (HPLC-FLD). Toxin content and profile were remarkably different among the four tissues, i.e. viscera, adductor muscle, mantle and gonad, suggesting apparent toxin transfer and transformation in scallops. Viscera always had the highest content of PST dominated by low-potency N-sulfocarbamoyl toxins C1 and C2, which closely resembled the toxin profiles of net-concentrated phytoplankton samples in spring. Based on the morphological features, cells of Alexandrium spp. in net-concentrated phytoplankton samples were picked out and a partial sequence of the large subunit ribosomal RNA gene (LSU rDNA) was amplified using a single-cell polymerase chain reaction (PCR) method. Cells of both toxic A. tamarense species complex and non-toxic A. affine were identified from the phytoplankton samples based on the partial LSU rDNA sequence information. According to these findings, it is implied that A. tamarense species complex is the major toxic species related to PST contamination in scallops of the northern Yellow Sea. The presence of both toxic and non-toxic Alexandrium spp. in this region requires for a species-specific method to monitor the distribution and dynamics of A. tamarense species complex.