Spatiotemporal Patterns and Morphological Characteristics of Ulva prolifera Distribution in the Yellow Sea, China in 2016–2018

Estimating Ulva prolifera green tides of the Yellow Sea through ConvLSTM data fusion

Green tides, a worldwide problem, are harmful to aquaculture, tourism, marine ecosystems, and maritime traffic. Currently, green tide detection relies on remote sensing (RS) images, which are often missing or unusable. Thus, the observation and detection of green tides cannot be performed daily, which makes it difficult to improve environmental quality and ecological health. To address this problem, this study proposed a novel green tide estimation framework (GTEF) through convolutional long short-term memory, which learned the historical spatial-temporal seasonal and trend patterns of green tides from 2008 to 2021 and fused the previously observed or estimated data and biological (optional) and physical (optional) data over the preceding seven days when RS images were absent or unusable for daily observation and detection tasks. The results showed that the overall accuracy (OA), false-alarm rating (FAR), and missing-alarm rating (MAR) of the GTEF were 0.9592 ± 0.0375, 0.0885 ± 0.1877 and 0.4315 ± 0.2848, respectively. The estimated results described the green tides in terms of attributes, geometry and position features. Especially in the latitudinal features, the Pearson correlation coefficient of the predicted data and observed data were over 0.8 (P < 0.05), which showed a strong correlation. In addition, this study also discussed the role of biological and physical factors in the GTEF. Sea surface salinity may be the dominant factor in the early stages of green tides; in the late stage, solar irradiance may be the dominant factor. Sea surface winds and sea surface currents also played a significant role in green tide estimation. Results showed the OA, FAR and MAR of the GTEF which, with physical factors but without biological factors, were 0.9556 ± 0.0389, 0.1311 ± 0.3338 and 0.4297 ± 0.3180, respectively. In short, the proposed approach could generate a daily map of green tides, even if RS images were missing or unusable.

Assessment of global habitat suitability and risk of ocean green tides

Green tides, which are widespread problems, are harmful issues that affect the protection of ocean ecosystems and natural resources. Scientific assessment and prevention of the green tides are essential for sustainable planning and the utilization of maritime traffic, tourism, and industry. However, the suitable or risk habitats and their dominant factors of green tides from global perspective are unknown. Here, this study proposed a novel framework to show the habitat suitability and risk of ocean green tides by considering marine environmental factors (i.e., sea surface temperature, sea surface salinity, solar irradiance, chlorophyll-a concentration, and sea surface wind). Through global remote sensing images and marine environmental factor data, this study found that (1) suitable and at-risk green tides areas are located in the north and south temperate zones; (2) marine physical factors are expected to weaken the green tide risk globally and enhance the green tide risk in coastal areas; (3) the green tides in the North Atlantic Ocean and the West Pacific Ocean are dominated by environmental factors and physical factors, respectively; and (4) when reducing carbon to promote sustainability, more potentially suitable green tide areas may appear at high latitudes. The results demonstrate the at-risk location and future trend of green tides, which are helpful for sustainable planning of ocean ecosystems.

Changes in dissolved organic pool and regulation of associated nutrients during green tides: A case study of Ulva prolifera bloom in the southern Yellow Sea

To explore the spatial and temporal distributions of dissolved organic matter (DOM) affected by Ulva prolifera bloom and the regulation of DOM released by the addition of nitrate (NO₃-N) nutrients, the variation in DOM was analyzed by three cruises in the coastal waters of the southern Yellow Sea (SYS) off Qingdao during a 2018 U. prolifera bloom. A mesocosm experiment was conducted during the decay of U. prolifera. The U. prolifera bloom resulted in higher concentrations of the bulk DOM except for total dissolved amino acids (TDAA), compared with levels recorded during non-bloom periods. Three different fluorescent substances of chromophoric dissolved organic matter (CDOM) were identified, which were also affected by the bloom. Concentrations of total dissolved carbohydrates (TCHO), monosaccharides (MCHO), and polysaccharides (PCHO) peaked during the late bloom, with average levels of 49.78 ± 9.73, 14.23 ± 3.32, and 35.55 ± 9.19 μmol C L^-1, respectively, accompanied by an increase of DOC, indicating release of DOM with U. prolifera decay. In contrast, the concentrations of CDOM and TDAA declined during the late bloom, with averages of 0.10 ± 0.10 m^-1 and 1.16 ± 0.82 μmol L^-1, suggesting that the yields of CDOM and TDAA were regulated by the U. prolifera bloom and other mechanism. These findings were confirmed by the mesocosm experiment. DOM production was obviously improved by NO₃-N addition except for TDAA and CDOM. The release of protein-like CDOM was primarily regulated by NO₃-N nutrients, however, it was not obvious with humic-like CDOM. Our findings provide evidence supporting the role of U. prolifera blooms in the coastal DOM pool and its significant effect on the marine carbon cycle.

Variations in Marine Bacterial and Archaeal Communities during an Ulva prolifera Green Tide in Coastal Qingdao Areas

Green tides caused by Ulva prolifera occur annually in the Yellow Sea, potentially influencing the marine microorganisms. Here, we focused on the variations in marine bacterial and archaeal communities during an U. prolifera green tide in coastal Qingdao areas with Illumina high-throughput sequencing analysis. Our results revealed that the diversity and structure of bacterial and archaeal communities, as well as the organization and structure of microbial co-occurrence networks, varied during the green tide. The decline phase may be favorable to the bacterial and archaeal diversity and richness. The bacterial community, as well as the archaeal community, showed clear variations between the outbreak and decline phases. A simpler and less connected microbial co-occurrence network was observed during the outbreak phase compared with the decline phase. Flavobacteriales and Rhodobacterales separately dominated the bacterial community during the outbreak and decline phase, and Marine Group II (MGII) dominated the archaeal community during the green tide. Combined with microbial co-occurrence network analysis, Flavobacteriales, Rhodobacterales and MGII may be important organisms during the green tide. Temperature, chlorophyll a content and salinity may have an important impact on the variations in bacterial and archaeal communities during the green tide.

Investigating Spatial Distribution of Green-Tide in the Yellow Sea in 2021 Using Combined Optical and SAR Images

Optical remote sensing is limited to clouds and rain. It is difficult to obtain ground object images in severe weather. Microwave remote sensing can penetrate clouds and rain to obtain ground object images. Therefore, this paper combines optical and microwave data to analyze the time and space of the green-tide in the Yellow Sea in 2021. Compared with a single data source, the distribution characteristics increase the frequency of time observation and show the green-tide changes in more detail. The continuous remote sensing observation time is 80 days. Ulva prolifera has experienced discovery (mid-late May), development (mid-late May to early June), outbreak (early June to mid-late June), decline (late June to mid-July), and extinction (late July to mid-August) in five stages; the development period drifts along the northeast direction, the outbreak period drifts along the northwest direction, the decline and extinction periods are mainly in the Rizhao and Qingdao waters. Ulva prolifera has a tendency to drift northward as a whole, drifting through Yancheng, Lianyungang, Linyi, Rizhao and Qingdao waters eventually landing on the coast of Qingdao and gradually disappearing.

Succession of marine bacteria in response to Ulva prolifera-derived dissolved organic matter

Increasing macroalgal blooms as a consequence of climate warming and coastal eutrophication have profound effects on the marine environment. The outbreaks of Ulva prolifera in the Yellow Sea of China occurring every summer since 2007 to present have formed the world's largest green tide. The green tide releases huge amounts of dissolved organic matter (DOM) to the seawater, causing an organic overload. However, how marine bacteria respond to this issue and the potential impact on the marine environment are still unclear. Here, we monitored the highly temporally resolved dynamics of marine bacterial community that occur in response to Ulva prolifera-derived DOM by performing a 168-h microcosm incubation experiment. DOM inputs significantly increased bacterial abundances within 6 h, decreased bacterial diversity and triggered clear community successions during the whole period of incubation. Vibrio of Gammaproteobacteria robustly and rapidly grew over short timescales (6-24 h), with its relative abundance accounting for up to 52.5% of active bacteria. From 24 to 48 h, some genera of Flavobacteriia grew rapidly, which was more conspicuous at a higher DOM concentration than at a lower concentration. The genus Donghicola of Alphaproteobacteria was predominant at later time points (>48 h). This bacterial community succession was accompanied by significant variations in the activity of 12 different extracellular enzymes, resulting in a rapid reduction of dissolved organic carbon by 74.5% within the first 36 h. In summary, our study demonstrates rapid successions of bacterial community and extracellular enzyme activity after DOM inputs, suggesting that the bacterial response to Ulva prolifera-derived organic matter may contribute to environmental restoration and may pose a health threat due to the bloom of potential pathogenic Vibrio.

Red Sea MODIS Estimates of Chlorophyll a and Phytoplankton Biomass Risks to Saudi Arabian Coastal Desalination Plants

Harmful algal blooms (HABs) and the high biomass associated with them have afflicted marine desalination plants along coastal regions around the world. Few studies of HABs have been conducted in the Red Sea, where desalination plants along the Saudi Arabian Red Sea coast provide drinking water for millions of people. This study was conducted along the Saudi Arabian Red Sea coast from 2014 to 2015 to assess the potential for using Moderate Resolution Imaging Spectroradiometer (MODIS) remote sensing of chlorophyll a (Chl a) or fluorescence line height (FLH) to identify risks for biofouling at these desalination plants. Ship-based surveys of phytoplankton were conducted along the Saudi Arabian coastline offshore of desalination plants at Jeddah, Al Shoaibah and Al Qunfudhuh to assess the density of phytoplankton populations and identify any potential HAB species. Ship-based surveys showed low to moderate concentrations of phytoplankton, averaging from 1800–10,000 cells L−1 at Jeddah, 2000–11,000 cells L−1 at Al Shoaibah and 1000–20,500 cells L−1 at Al Qunfudhuh. Sixteen different species of potentially toxigenic HABs were identified through these surveys. There was a good relationship between ship-based total phytoplankton counts and monthly averaged coastal MODIS Chl a (R2 = 0.49, root mean square error (RMSE) = 0.27 mg m−3) or FLH (R2 = 0.47, RMSE = 0.04 mW m−2 µm−1 sr−1) values. Monthly average near shore Chl a concentrations obtained using MODIS satellite imagery were much higher in the Red Sea coastal areas at Al Qunfudhuh (maximum of about 1.3 mg m−3) than at Jeddah or Al Shoaibah (maximum of about 0.4 and 0.5 mg m−3, respectively). Chlorophyll a concentrations were generally highest from the months of December to March, producing higher risks of biofouling desalination plants than in other months. Concentrations decreased significantly, on average, from April to September. Long-term (2005–2016) monthly averaged MODIS Chl a values were used to delineate four statistically distinct zones of differing HAB biomass across the entire Red Sea. Sinusoidal functions representing monthly variability were fit to satellite Chl a values in each zone (RMSE values from 0.691 to 0.07 mg m−3, from Zone 1 to 4). December to January mean values and annual amplitudes for Chl a in these four sinusoidal functions decreased from Zones 1–4. In general, the greatest risk of HABs to desalination occurs during winter months in Zone 1 (Southern Red Sea), while HAB risks to desalination plants in winter months are low to moderate in Zone 2 (South Central Red Sea), and negligible in Zones 3 (North Central) and 4 (Northern).

DOC dynamics and bacterial community succession during long-term degradation of Ulva prolifera and their implications for the legacy effect of green tides on refractory DOC pool in seawater

Under climate warming and coastal eutrophication, outbreaks of green tides have increased in recent decades; e.g., the world's largest green tide caused by Ulva prolifera has occurred in the Yellow Sea for 13 consecutive years. The massive assemblage of macroalgae absorbs large amounts of atmospheric CO₂ and converts it into biomass. After the green tide, millions of tons of the macroalgal biomass sink to the seabed to be degraded eventually; this inevitably has a significant impact on the coastal organic carbon pool and microbial community. However, this impact is poorly understood. Here, the degradation of Ulva prolifera over 520 days revealed that relatively sufficient degradation of the macroalgae occurred at ca. 7 months. The rapid release of dissolved organic carbon (DOC) mainly occurred in the first week, which not only increased the size and diversity of the DOC pool in a short time but also promoted the rapid growth of bacteria and led to hypoxia and acidification of the seawater. After that, the labile portion of DOC was gradually used up by bacteria within one month, while the degradation of semi-labile or semi-refractory DOC occurred in half a year. The remaining DOC existed in the form of refractory DOC (RDOC), resisting bacterial consumption and remaining stable for 10 months. During the long-term degradation process, bacterial community structure and metabolic function showed obvious successional characteristics, driving the gradual transformation of DOC from labile to refractory through the microbial carbon pump mechanism. After the long-term degradation, the remaining RDOC accounted for approximately 1.6% of the macroalgal carbon biomass. As RDOC can maintain long-term stability, we propose that the frequent outbreaks of green tides not only affect microbial processes but also may have an important cumulative effect on the coastal RDOC pool.

Intra- and inter-seasonal variations in the hydrological characteristics and nutrient conditions in the southwestern Yellow Sea during spring to summer

Based on field survey in the southwestern Yellow Sea (SWYS) during April-September 2017, the spatiotemporal variations in the hydrological characteristics and nutrient conditions were coupled and analyzed; the intra-seasonal variations in the upwelling in the front of the Yellow Sea Cold Water Mass (YSCWM) and impacts on nutrient transport were explored. The coastal area was controlled by the low-salinity high-nutrient Lubei Coastal Current, Subei Coastal Current, and Yangtze River Diluted Water from north to south; at bottom, the northeastern SWYS was controlled by the low-temperature high-salinity high-nutrient YSCWM. Temperature, salinity and nutrient fronts formed around YSCWM. The upwelling velocity in the front increased during April to late June and decreased in early September; the upwelled fluxes of dissolved inorganic nitrogen (0.29×10³-7.77×10³ μmol·m^-2d^-1), phosphate (0.02×10³-0.27×10³ μmol·m^-2d^-1) and silicate (0.98×10³-8.75×10³ μmol·m^-2d^-1) showed similar variations during April-September. The upwelled nutrients could potentially contribute to local green tide development and phytoplankton growth during spring-summer.

An anomalous bi-macroalgal bloom caused by Ulva and Sargassum seaweeds during spring to summer of 2017 in the western Yellow Sea, China

An unprecedented bi-macroalgal bloom caused by Ulva prolifera and Sargassum horneri occurred from spring to summer of 2017 in the western Yellow Sea (YS) of China, where annual large-scale green tides have prevailed for a decade. The distinct genesis and blooming dynamics of the two seaweed species were detected and described. Unlike the consistent raft-origin of the floating Ulva biomass, the massive pelagic S. horneri was derived from multiple sources (residual seaweeds from the previous winter bloom and those drifting from offshore water in the south). The scale of the green tide in 2017 was found smaller than the previous four years. We then discussed a number of hypotheses attributing to this reduction, including reduced epiphytic green algae from aquaculture rafts and the influences of the massive pelagic S. horneri. However, further research is needed to identify the origin of the pelagic S. horneri in the western YS and any affiliations with the benthic populations, and to elucidate the interactions of this species with the annual green tides and the ensuing consequences.

Remotely-Observed Early Spring Warming in the Southwestern Yellow Sea Due to Weakened Winter Monsoon

The seasonal warming over the southwestern Yellow Sea (YS) in the spring is of vital importance to the local ecologic environment, especially to the massive green algae blooms of the YS in late spring and early summer. Based on daily optimum interpolation sea surface temperature (SST) data consisting of satellite derived SST from Advanced Very High Resolution Radiometer (AVHRR) and in situ measurements, this study analyzed the spring SST variation over the southwestern YS (SWYS) from 1982 to 2018. The results show that the recent warming trend of spring SST over the SWYS is four-to-six times that of the global average, and as a result, sea water over the Subei Shoal (SBS) shifts about 10–13 days earlier to reach 10 °C in early April. This implies that, accordingly, the micro-propagules of green algae over the SBS may have the chance to germinate earlier. SST variability in early April significantly correlates with northerly wind and exhibits a general warming over the SWYS with an intensified warming anchored along the axis of the submarine canyon off the Yangtze estuary. The Moderate Resolution Imaging Spectroradiometer (MODIS) red–green–blue composite images captured the intrusion of the Taiwan Warm Current (TWC) into the SWYS through the submarine canyon during northerly wind relaxation in early April. Ocean remote sensing provides important clues for understanding the regional SST variability in the SWYS. Following this clue, this study finds that the weakening of winter monsoon in the spring leads to northward migration of the TWC and results in enhanced spring warming over the SWYS. The attendant advanced warming in spring, resulting in a favorable temperature condition for early development of green alga, may have contributed to the green tide blooms in the Yellow Sea in the recent decade.