Effects of physical-biochemical coupling processes on the Noctiluca scintillans and Mesodinium red tides in October 2019 in the Yantai nearshore, China

Key environmental predictors of Noctiluca scintillans distribution in the China sea and its climate change response

Noctiluca scintillans is one of the most common harmful algal species worldwide. In this study, a MaxEnt model was constructed to calculate the present and future habitat suitability of N. scintillans in the China Sea. A comprehensive evaluation index of variable importance was defined to measure the importance of key predictors in the model, and offshore distance, long-term average minimum primary productivity, water depth, long-term average minimum temperature, and minimum salinity were determined as the dominant drivers. The HAB index that was constructed by integrating the carrying capacity and habitat suitability characteristics was used to measure the risk of harmful algal blooms (HABs). The index indicated that high-risk areas of HABs caused by N. scintillans occurred around Hainan Island, Taiwan Island, the coastal areas of Guangdong, Fujian, and Zhejiang provinces, and the eastern nearshore area of Weihai in Shandong. Regardless of the greenhouse gas emission scenario, the occurrence of N. scintillans red tides was predicted to persist until 2100. Moreover, the total area of the HABs high zone was predicted to increase under RCP2.6 and decrease under RCP8.5, and the center of the integrated HABs high zone was predicted to be concentrated in the central area of the entire China Sea spanning 15°N to 33°N.

Analysis on Bacterial Community of Noctiluca scintillans Algal Blooms Near Pingtan Island, China

Noctiluca scintillans, known as a global red tide species, is a common red tide species found in Pingtan Island. To examine the bacterial community structure in different environments during the red tide period of N. scintillans on Pingtan Island, samples were collected from the Algal Bloom Area (ABA), Transition Area (TA), and Non-Algal Bloom Area (NBA) on 6 April 2022, and the environmental physicochemical factors and bacterial community were determined. The outbreak of N. scintillans red tide significantly impacted the water quality and bacterial community structure in the affected sea area. The water quality in the ABA has deteriorated markedly, with the contents of COD, NH4+-N, and PO43- in the ABA being significantly higher than those in the TA and NBA, while the pH is significantly lower than that in the TA and NBA. The richness, diversity, and evenness of the bacterial community in the ABA are all lower than those of the TA and NBA. For instance, the Shannon index values of the three sampling points are 4.41, 5.41, and 6.37, respectively. At the phylum level, the dominant bacterial phyla in the ABA are Proteobacteria, Firmicutes, and Cyanobacteria; in the TA, they are Proteobacteria, Bacteroidetes, and Firmicutes; and in the NBA, they are Proteobacteria, Bacteroidetes, and Cyanobacteria. At the genus level, the dominant bacterial genera in the ABA are Vibrio, Carnobacterium, Candidatus_Megaira, Planktomarina, and Pseudoalteromonas; in the TA, they are Vibrio, Planktomarina, Lentibacter, Glaciecola, and Jannaschia; and in the NBA, they are Planktomarina, Amylibacter, NS5_marine_group, Aurantivirga, and marine_metagenome. In the ABA, the combined proportion of Vibrio and Carnobacterium exceeds 50%, with Vibrio_splendidus accounting for 93% of the total Vibrio population. These research results can provide a scientific basis for clarifying the environmental characteristics and bacterial composition during the large-scale N. scintillans red tide in Pingtan Island.

Distribution of protoporphyrin IX during Prorocentrum donghaiense blooms and its relationship with particle-attached and free-living bacterial communities

Particle-attached (PA) and free-living (FL) bacterial communities are essential for nutrient cycles and metabolite production and serve as a food source in aquatic systems. However, our understanding of how biotic factors influence community interactions, co-occurrence patterns, and niche occupancy remains limited. This study investigated the influence of protoporphyrin IX (PPIX) on bacteria with different lifestyles during Prorocentrum donghaiense bloom. The findings revealed that PPIX distribution responded variably to changes in physicochemical parameters induced by red tide bloom. Large-sized or particle-attached (PA) phytoplankton (cell size >3 μm) were identified as the primary contributors to environmental PPIX, while small-sized plankton or free-living (FL) microorganisms (<3 μm) contributed less. In red tide-affected areas, PPIX and its derivatives were significantly more abundant than in non-red tide areas, indicating an increased demand for porphyrins by plankton during red tides. Additionally, the red tide also significantly influenced the preference of bacterial lineages for PA or FL lifestyles, highlighting a close interaction between bacteria with different lifestyles and PPIX levels. This study quantitatively analyzed the distribution of PPIX across different cell sizes in red tide and non-red tide marine environments, providing insights into microbial interactions and dynamics in changing ecosystems and offering a reference for using PPIX to predict red tide ecological disasters.

Long-term changes in spatiotemporal distribution of Noctiluca scintillans in the southern North Sea

To assess the spatiotemporal evolution of the heterotrophic dinoflagellate Noctiluca scintillans in the North Sea, the Helgoland Roads time series and Continuous Plankton Recorder survey were analysed using generalized additive models. Over the last decades, blooms of N. scintillans have occurred more frequently and intensively in many regions. This harmful algal bloom forming species can alter food webs, reduce ecosystem productivity, and lead to economic losses while causing lower aquacultural yields. After the 1990s, N. scintillans abundances have significantly increased by 1.65-fold and a significant prolongation of the bloom window was found (from 27.5 to 98 days in recent decades) off the island of Helgoland, Germany. Significant correlations were found between bloom initiation and nutrients, as well as light availability since these factors lead to increased prey availability. Highest abundances of N. scintillans were associated with water temperatures around 17 °C and wind speed below 6 ms-1 causing dense surface accumulations. Solar radiation of more than 200 Wm-2 was identified as a main driver for post-bloom conditions as it can deteriorate the cells and lead to the decline of N. scintillans abundances. In the southern North Sea, N. scintillans occurrences have intensified and spread since the 1980s with hotspots identified as the coastal waters adjacent to the estuaries of the Elbe and Rhine rivers.

Biodiversity and distribution patterns of blooming jellyfish in the Bohai Sea revealed by eDNA metabarcoding

BACKGROUND

The mass occurrence of scyphozoan jellyfish severely affects marine ecosystems and coastal economies, and the study of blooming jellyfish population dynamics has emerged in response. However, traditional ecological survey methods required for such research have difficulties in detecting cryptic life stages and surveying population dynamics owing to high spatiotemporal variations in their occurrence. The environmental DNA (eDNA) technique is an effective tool for overcoming these limitations.

RESULTS

In this study, we investigated the biodiversity and spatial distribution characteristics of blooming jellyfish in the Bohai Sea of China using an eDNA metabarcoding approach, which covered the surface, middle, and bottom seawater layers, and sediments. Six jellyfish taxa were identified, of which Aurelia coerulea, Nemopilema nomurai, and Cyanea nozakii were the most dominant. These three blooming jellyfish presented a marked vertical distribution pattern in the offshore regions. A. coerulea was mainly distributed in the surface layer, whereas C. nozakii and N. nomurai showed a upper-middle and middle-bottom aggregation, respectively. Horizontally, A. coerulea and C. nozakii were more abundant in the inshore regions, whereas N. nomurai was mainly distributed offshore. Spearman's correlation analysis revealed a strong correlation between the eDNA of the three dominant blooming jellyfish species and temperature, salinity, and nutrients.

CONCLUSIONS

Our study confirms the applicability of the eDNA approach to both biodiverstiy evaluation of blooming jellyfish and investigating their spatial distribution, and it can be used as a supplementary tool to traditional survey methods.

Physical drivers of Noctiluca scintillans (Dinophyceae) blooms outbreak in the northern Taiwan Strait: A numerical study

The red Noctiluca scintillans (RNS) blooms often break out near Pingtan Island, in the northern Taiwan Strait from April to June. It is essential to gain insights into their formation mechanism to predict and provide early warnings for these blooms. Previous studies and observations showed that RNS blooms are the most likely to occur when winds are weak and shifting in direction. To explore this phenomenon further, we employed a high-resolution coastal model to investigate the hydrodynamics influencing RNS blooms around Pingtan Island from April to June 2022. The model results revealed that seawater exhibited weak circulation but strong stratification during RNS blooms. Residence time were examined through numerical experiments by releasing passive neutrally buoyant particles in three bays of Pingtan Island. The results showed a significantly longer residence time during RNS blooms, indicating reduced flushing capabilities within the bays, which could give RNS a stable environment to multiply and aggregate. This hydrodynamic condition provided a favorable basis for RNS blooms breakout near Pingtan Island. The shifts and weakening of the prevailing northeast wind contributed substantially to weakening the flow field around Pingtan Island and played a crucial role in creating the hydrodynamics conducive to RNS blooms. Our study offers fresh insights into the mechanisms underpinning RNS blooms formation near Pingtan Island, providing a vital framework for forecasting RNS blooms in this region.

New insights into estimation of bioavailable inorganic phosphorus in natural coastal seawater

Considering the impact of the high salinity and high turbidity of coastal seawater on phosphorus forms, a new method was proposed to determine bioavailable inorganic phosphorus (BIP). The phosphorus most relevant to eutrophication is BIP, and traditional analysis methods may underestimate the degree of eutrophication. In this study, a microelectrode of multigold (AuμE) was fabricated for direct voltammetric determination of BIP without filtration, and BIP environmental characteristics including distribution and correlation relationships with environmental factors in typical coastal seawater of Northern China were analyzed. The proposed AuμE showed a low detection limit of 0.03 μM. The surface and bottom BIP concentrations ranged from 1.00 to 2.13 and from 0.88 to 2.05 μM, respectively. BIP dominated the total P (TP) accounting for 48.5-67.5 % in the surface layer samples, and 32.6-92.7 % in the bottom layer samples, respectively. The concentrations of BIP were obviously higher than those of DIP, indicating that DIP may underestimate the probability of eutrophication occurring. And BIP was positively correlated with dissolved oxygen (DO) (P < 0.05). BIP may be a promising indicator of eutrophication potential in coastal areas with high salinity and high turbidity. The proposed reliable voltammetry method provides a new indicator for environmental assessment and represents a significant step in the comprehensive analysis of P species.

Feeding Behavior Responses of the Small Copepod, Paracalanus parvus, to Toxic Algae at Different Concentrations

The feeding relationship between copepods and phytoplankton has immense ecological significance. This study investigated the feeding behavior of copepods by studying the feeding selectivity of Paracalanus parvus, a key small copepod species, using a high-speed camera. The feeding behavior of P. parvus separately fed on three algae, Prorocentrum minimum, Alexandrium minutum, and Thalassiosira weissflogii, was studied at five different concentrations. The factors characterizing feeding behavior, including the beating frequency (BF), beating time (BT), and rejection behavior, were analyzed. The average BT and BF of P. parvus fed on toxic algae were significantly lower than those of copepods fed on nontoxic algae, indicating that the toxic algae negatively affected their feeding behavior. There were no significant differences in feed rejection among the three algae during the short period of experimentation, indicating that the rejection behavior was insignificant in the early period (within 20 min) of feeding on toxic algae. The feeding behavior was inhibited when the concentration reached 250 cells/mL. The BT was initially affected at increasing concentrations followed by the BF, and P. minimum and A. minutum reduced the BF at concentrations of 250 and 1000 cells/mL, respectively. Analysis of the average BFs revealed that P. parvus was more significantly affected by P. minimum containing diarrheal shellfish poison than by A. minutum containing paralytic shellfish poison. The BF of copepods fed on P. minimum was significantly lower than that of copepods fed on A. minutum at 250-500 cells/mL but was not significantly different from that at 1000 cells/mL. This indicated that the inhibitory effect of P. minimum on the feeding behavior was more significant at concentrations observed at the onset of red tide blooms (0.25-0.5 × 102 cells/mL), but insignificant at concentrations reaching those in advanced red tides (>103 cells/mL). This study demonstrates that toxic dinoflagellates alter the feeding behavior of copepods and describes the variations in their feeding response to different algal species and concentrations. The findings provide crucial insights for further studies on the feeding relationship between copepods and phytoplankton and on functional assessment of plankton ecosystems.

Profiling of the spatiotemporal distribution, risks, and prioritization of pharmaceuticals and personal care products in coastal waters of the northern Yellow Sea, China

Pharmaceuticals and personal care products (PPCPs) have aroused global concerns due to their ubiquitous occurrence and detrimental effects. The spatiotemporal distributions of 64 PPCPs and their synergetic ecological risks were comprehensively investigated in the seawater of Yantai Bay, and 1 H-benzotriazole (BT), ethenzamide, phenazone, propyphenazone, 4-hydroxybenzophenone and N, N'-diphenylurea were first determined in the seawater of China. Fifty-six PPCPs were detected and their concentrations were 27.5-182 ng/L, with BT contributing around 58.0%. Higher PPCP concentrations were observed in winter and spring, with the concentrations of antioxidants, analgesic/anti-inflammatory drugs and human-used antibiotics significantly higher in winter, while those of aquaculture-used antibiotics and UV filters significantly higher in summer, which was closely related with their usage patterns. Positive correlations were observed for PPCP concentrations between surface and bottom water, except summer, during which time the weak vertical exchange and varied environmental behaviors among different PPCPs resulted in the distinct compositions and concentrations. Terrestrial inputs and mariculture resulted in higher PPCP concentrations in the area located adjacent to the coast and aquaculture bases. The PPCP mixtures posed medium to high risk to crustaceans, and bisphenol A was identified as a high-risk pollutant that needs special attention.

Long-term changes in Noctiluca scintillans blooms along the Chinese coast from 1933 to 2020

Noctiluca scintillans is one of the most common harmful algal species and widely known due to its bioluminescence. In this study, the spatial distribution, seasonal variations, and long-term trends of N. scintillans blooms in China and the related drivers were analyzed and discussed. From 1933 to 2020, a total of 265 events of N. scintillans blooms were recorded in Chinese coastal waters, with a total duration of 1052 days. The first N. scintillans bloom occurred in Zhejiang in 1933, and only three events were recorded before 1980. From 1981 to 2020, N. scintillans caused harmful algal blooms (HABs) almost every year, both the average duration and the proportion of multiphase HABs showed an increasing trend. 1986-1992, 2002-2004, and 2009-2016 were the three peak periods with a frequency of no less than five events of N. scintillans blooms per year. In terms of spatial distribution, N. scintillans blooms spread from the Southeast China Sea to the Bohai Sea after 2000, Guangdong, Fujian, and Hebei were the three provinces with the highest numbers of recorded events of N. scintillans blooms. Moreover, 86.8% of the events of N. scintillans blooms occurred in spring (March, April, and May) and summer (June, July, and August). Among environmental factors, the dissolved inorganic phosphate, dissolved silicate and chemical oxygen demand were significantly correlated with the cell density of N. scintillans during N. scintillans blooms, and most of N. scintillans blooms were recorded in the temperature range of 18.0-25.0°C. Precipitation, hydrodynamics, water temperature, and food availability might be the main factors affecting the spatial-temporal distribution of N. scintillans blooms along the Chinese coast.

The dynamics of red Noctiluca scintillans in the coastal aquaculture areas of Southeast China

Noctiluca scintillans (NS) adds an aesthetic appeal to many coastal areas because of their bioluminescence. An intense bloom of the red NS frequently occurs in the coastal aquaculture region of Pingtan Island in Southeastern China. However, when NS exceeds in abundance, it causes hypoxia which has devastating impacts on the aquaculture. This study was conducted in the Southeastern part of China with an aim to examine the relationship between the profusion of NS and its impacts on marine environment. Samples from four stations on Pingtan Island were collected for 12 months (January to December 2018) and were later analyzed in laboratory against five parameters, namely temperature, salinity, wind speed, dissolved oxygen, and chlorophyll a. Results showed that the NS blooms were particularly active during the months of May and June in the Pingtan Island area. The seawater temperatures during that time were recorded between 20 and 28.8 °C indicating the optimum survival temperature for NS. The NS bloom activity ceased above 28.8 °C. A principal component analysis (PCA) indicated that the richness of NS was positively associated with temperature and salinity, whereas there was a significant reverse correlation between NS accumulation and wind speed. NS is a heterotrophic dinoflagellate and relies on the predation of algae for reproduction; therefore, a significant correlation was observed between NS abundance and chlorophyll a concentration, and an inverse correlation was observed between NS and phytoplankton abundance. Additionally, red NS growth was observed immediately following the diatom bloom, suggesting that phytoplankton, temperature, and salinity are the essential factors in the evolution, progression, and termination of NS growth.

The joint effects of atmospheric dry and wet deposition on organic carbon cycling in a mariculture area in North China

In this research, the atmospheric dry and wet deposition fluxes of particulate organic carbon (POC) over the coastal waters around the Yangma Island in North Yellow Sea were investigated. Combining the results of this research and previous reports about the wet deposition fluxes of dissolved organic carbon (DOC) in precipitation (F_DOC-wet) and dry deposition fluxes of water-dissolvable organic carbon in atmospheric total suspended particles (F_DOC-dry) in this area, a synthetic assessment of the influence of atmospheric deposition on the eco-environment was conducted. It was found that the annual dry deposition flux of POC was 1097.9 mg C m^-2 a^-1, which was approximately 4.1 times that of F_DOC-dry (266.2 mg C m^-2 a^-1). For wet deposition, the annual flux of POC was 445.4 mg C m^-2 a^-1, accounting for 46.7 % that of F_DOC-wet (954.3 mg C m^-2 a^-1). Therefore, atmospheric POC was mainly deposited through dry process with the contribution of 71.1 %, which was contrary to the deposition of DOC. Considering the indirect input of organic carbon (OC) from atmospheric deposition, that is, the new productivity supported by nutrient input from dry and wet deposition, the total OC input from atmospheric deposition to the study area could be up to 12.0 g C m^-2 a^-1, highlighting the important role of atmospheric deposition in the carbon cycling of coastal ecosystems. The contribution of direct and indirect input of OC through atmospheric deposition to the dissolved oxygen consumption in total seawater column was assessed to be lower than 5.2 % in summer, suggesting a relatively smaller contribution to the deoxygenation in summer in this region.

Why do red tides occur frequently in some oligotrophic waters? Analysis of red tide evolution history in Mirs Bay, China and its implications

The process and management of red tide in oligotrophic waters are poorly understood as most studies on red tide were focused on eutrophic areas. In this study, 404 red tide events together with the historical water quality dynamics during 1991-2020 were investigated in an anthropogenically influenced bay in China - Mirs Bay, whose most region is oligotrophic except small inshore areas. Red tides of oligotrophic offshore accounted for 20 % of all. With the effective governmental management on inshore areas, concentration of PO₄ and DIN has been decreased to a low level (PO₄ <0.01 mg/L while DIN <0.1 mg/L) in the bay since about 2000. However, the reduction of nutrients was still accompanied by the frequent outbreaks of red tides, as well as a shift of dominant algae from diatoms to dinoflagellates, which might be due to the unbalanced nutrient reduction, such as N:P ratio fluctuation and organic nutrient increase. This shift might trigger more red tide events and even some super ones (long-duration or large-scale) in oligotrophic areas. Detailed analysis on red tide events combined with model simulation proved that the outbreak of red tide in Mirs Bay was caused by the joint contribution of nutrients and hydrodynamics. Nutrients of inshore area supported the red tides there, and with the help of physical conditions, red tides inshore could be transferred to offshore areas and then were likely to bloom again or be preyed to support blooms of other organisms. This study acknowledged that the reduction of both N and P either inorganic or organic nutrients was essential to control red tides, even in oligotrophic waters, but a balanced strategy considering the dual reduction of both nitrogen and phosphorus was of pivotal role to restore the health of coastal water systems disturbed by human.

Seasonal and spatial variations in nutrients under the influence of natural and anthropogenic factors in coastal waters of the northern Yellow Sea, China

Analysis of the common and most influential natural and anthropogenic activities on the spatiotemporal variation in nutrients at a multiannual scale is important. Eleven cruises from 2015 to 2017 were carried out to better elucidate the seasonal and spatial variations in nutrients, as well as the impact factors on dissolved inorganic nitrogen (DIN), phosphorus (DIP) and silicate (DSi). Both nutrient concentrations and forms showed similar and significant seasonal variations over the 3 years, and were closely related to the biomass and species of phytoplankton. Terrestrial inputs had significant effects on the spatial distribution of nutrients throughout the year, especially in the surface water, which showed DIN > DIP>DSi. In summer, shellfish aquaculture and hypoxia jointly affected the spatial distribution of nutrients. The bottom water nutrient concentrations in the aquaculture area were 1.1-2.3 times higher than those outside of the aquaculture area. Seasonal hypoxia can increase the release of DSi and NH₄⁺ from the sediment to the water. In summary, anthropogenic activities and physical conditions jointly influenced the nutrient distributions.

Massive shellfish farming might accelerate coastal acidification: A case study on carbonate system dynamics in a bay scallop (Argopecten irradians) farming area, North Yellow Sea

Seven cruises were carried out in a bay scallop (Argopecten irradians) farming area and its surrounding waters, North Yellow Sea, from March to November 2017 to study the dynamics of the carbonate system and its controlling factors. Results indicated that the studied parameters were highly variability over a range of spatiotemporal scales, comprehensively forced by various physical and biological processes. Mixing effect and scallop calcification played the most important role in the seasonal variation of total alkalinity (TAlk). For dissolved inorganic carbon (DIC), in addition to mixing, air-sea exchange and microbial activity, e.g. photosynthesis and microbial respiration processes, had more important effects on its dynamics. Different from the former, the changes of water pH_T, partial pressure of CO₂ (pCO₂) and aragonite saturation state (Ω_A) were mainly controlled by the combining of the temperature, air-sea exchange, microbial activity and scallop metabolic activities. In addition, the results indicated that massive scallop farming can significantly increase the DIC/TAlk ratio by reducing the TAlk concentration in seawater, thereby reducing the buffering capacity of the carbonate system in seawater especially for Ω_A. Preliminary calculated, ~75.7 and ~45.5 μmol kg^-1 of TAlk were removed from the surface and bottom waters respectively in one scallop cultivating cycle. If these carbonates cannot be replenished in time, it is likely to accelerate the acidification process of coastal waters. This study highlighted the control mechanism of the carbonate system under the influence of bay scallop farming, and provided useful information for revealing the potential link between human activities (shelled-mollusc mariculture) and coastal acidification.

Perpetual atmospheric dry deposition exacerbates the unbalance of dissolved inorganic nitrogen and phosphorus in coastal waters: A case study on a mariculture site in North China

The monthly magnitudes of dissolvable nutrients through atmospheric dry deposition (ADD) and their ecological effects to the coastal waters around the Yangma Island, North Yellow Sea, were investigated for one year. The results indicated that anthropogenic activities were the major sources of dissolvable inorganic and organic nitrogen (DIN and DON); dust events were the major sources of inorganic phosphorus (DIP) and silicate (DSi); however, organic phosphorus (DOP) could be originated from marine biological activities. The annual ADD fluxes of DIN, DON, DIP, DOP and DSi were 21.8, 2.7, 0.10, 0.30 and 0.73 mmol m^-2 yr^-1, respectively. Overall, the new production supported by the bioavailable nitrogen through ADD in winter was up to 9.14 mg C m^-2 d^-1. Notably, the annual molar ratio of DIN/DIP through ADD was 216 ± 123, which was much higher than that of the dissolved inorganic nitrogen to phosphorus in seawater and might exacerbate their unbalance in some coastal waters.