Dominant species of red tide in Jiangsu coastal area from 2000 to 2016.
The studies in this chapter are focused on marine ecological disasters in Jiangsu coastal area. Three kinds of algal blooms occurred in this region, namely, red tide associated with Dinoflagellate, green tide associated with Ulvaprolifera and golden tide associated with Sargassum. Numerical model results demonstrated that red tides in Haizhou Bay originated locally, because most of Dinoflagellates near Zhoushan Islands would be transported northeastward by the Changjiang diluted water, and even the lucky ones that entered the south of Jiangsu coastal area would die in the Subei Shoal due to high turbidity there. Due to the Changjiang diluted water and the prevailing southerly wind, Ulvaprolifera could not drift southward, either. Seawater with high turbidity in the Subei Shoal limited sunlight penetration into deep water column, and further inhibited the growth of Ulvaprolifera suspending in the water column. In this chapter, we use drift bottles and satellite-tracked Argos drifters to provide solid direct dynamic evidence that Ulvaprolifera could drift from the Subei Shoal to Qingdao coastal area and even further north. The sand ridges limited the traveling path of Ulvaprolifera in the Subei Shoal, and wind-driven currents and other baroclinic processes helped Ulvaprolifera travel farther to the north.
- red tide
- green tide
- golden tide
- physical controlling mechanism
- Jiangsu coastal area
1. Marine ecological disaster and research contents of this chapter
Since the end of the last century, marine environmental quality has become worse and worse as local economy developed rapidly in Jiangsu . Chinese national water quality distribution showed that the pollution problems in China were especially serious in the Changjiang estuary and Jiangsu coastal area. Water pollutions have a series of negative effects on inshore cultivation, wetland protection, among others. Similar as water quality situation, eutrophication problems along the Jiangsu coast were conspicuous. Seawater pollution in China was mainly caused by the discharge of land-sourced pollutants, and coastal cities including Nantong city, Yancheng city and Lianyungang city were affected mostly. The most polluted coastal waters were near estuaries, sewage outlets and their adjacent seas. It was obvious that the seawater near the coast was much more polluted than that of farther offshore. The main pollutants were inorganic nitrogen, active phosphate and petroleum. As the equation of eutrophication index showed , the most polluted seawaters were the most eutrophic seawaters. Increment of inorganic nitrogen and active phosphate caused eutrophication in the coastal waters, and this situation has been going on for a long time. There was no obvious inter-annual variation of the polluted sea area in Jiangsu, with the largest area of seriously polluted waters to be 14,371 km2 in 2012. Generally, eutrophication gradually worsened from spring to autumn during a year. In spring, the seawaters were in the critical state of eutrophication; eutrophication gradually accelerated during summer and finally seawaters became seriously polluted along the Jiangsu coast . Nutrients were sufficient for algal growth in the Jiangsu coastal region. Some algae can produce toxin poisoning shellfish, fish and other marine organisms. Even for nontoxic algal blooms, the excessive reproduction of algae can also cause blockage or damage to gills, and marine organisms can be asphyxiated in the poor oxygen waters . At present, the major marine disasters suffered in the Jiangsu coast were red tide and green tide; but in 2017, golden tide seemed to join in.
1. Red tide
Red tide is a kind of algal bloom with a red or brown color caused by some species of
2. Green tide
|Dominant species of red tide||Frequency||Year|
|5||2001, 2005, 2010, 2011, 2012|
|3||2005, 2006, 2010|
|3||2007, 2008, 2013|
Algal blooms caused by excessive growth of green algae, such as
Since 2007, green tide erupted in 11 consecutive years and has become a common marine disaster in Jiangsu and Shandong coastal seas. According to some studies [6, 7],
3. Golden tide
1.2. Research contents in this chapter
Though being the two main kinds of algal blooms in Jiangsu province, red tide and green tide rarely caused related algal blooms in Zhejiang province, especially in the north of Zhejiang coastal area, which is adjacent to the southern Jiangsu coastal area. Red tides of Zhejiang coast, a province with the most frequent red tides in the country, usually happened around Zhoushan Islands near the Changjiang estuary, while Haizhou Bay in the northern Jiangsu province was a place with frequent red tides. These two provinces are close to each other, but their algal bloom distribution patterns are so different. What separates these red tides is the first question we want to answer in this chapter.
Another emphasis in this chapter is on
In the following subsections, we will answer these questions and reveal the physical mechanisms for the drifting and development of
2. Observation and research plans
2.1. Hydrological and meteorological data collection
Data used in this study were from field observations and satellite remote sensing products. The data of temperature, salinity, currents, transparency, suspended particulate matter (SPM) and photosynthetically available radiation (PAR) were collected from the field observations. Conductivity-temperature-depth (CTD) instruments deployed at two mooring stations collected long time series of temperature and salinity data. Current data were obtained using the acoustic Doppler current profilers (ADCPs) installed at two anchored and two moored stations.
MODIS-Aqua images from April to June 2012 were used to retrieve monthly averaged SPM data . The wind data were from a blended sea wind data product supported by the US National Oceanic and Atmospheric Administration (http://www.ncdc.noaa.gov/oa/rsad/air-sea/seawinds.html).
2.2. Drift bottles and satellite-tracked Argos drifters
The Subei Shoal is too shallow to deploy satellite-tracked Argos drifters. As a result, house-made drift bottles were used, and 80 were released at 33°13.3′N, 121°10.2′E in 2012 (Figure 5). Half of the bottles were empty, to insure that they can float near the surface, and the rest were filled with sand to make them submersed under the surface as some
Outside the Subei Shoal, four satellite-tracked Argos drifters were deployed in June 2011 in the deep waters, and six were released in June 2011 (Figure 5). Location information collected hourly was transmitted to the laboratory via satellite.
2.3. Numerical model
The Regional Oceanic Modeling System (ROMS; citations needed) was used to build the 3D hydrodynamic model for the East China Sea with three-layer nested grids. Table 2 shows the domains and related information of the three-layer nested models. After being validated, the model can reproduce main currents and their annual variation in the East China Sea, including the Kuroshio current, the Taiwan warm current, the Min-Zhe coastal currents, and so on. In addition, a coastal numerical model built for the Subei Shoal will be described in the following paragraphs.
|Model||Domain||Resolution ()||Depth (m)||Vertical layers|
An unstructured grid, finite-volume, primitive equation community ocean model (FVCOM) was used to build the Subei Shoal coastal numerical model. The model domain was large enough to ensure that the open boundary was far from the Subei Shoal. The resolution in the ridge area was refined to be ~140 m, while the grid was 15,000 m near the open boundary. The model included 56,548 elements and 28,456 nodes in the horizontal direction and 11 sigma levels in the vertical direction. Tidal forcing along the open boundaries was added hourly, which was derived from the Oregon State University Tidal Inversion Software (OTIS) Regional Tidal Solutions and included tidal constituents of M2, S2, N2, K2, K1, P1, O1 and Q1. Time step was 1 s for the external mode, and the time split was 10. Finally, the results from this model were validated by observations .
3. Main results
3.1. Controlling effects of Changjiang diluted water on the algal distribution in the East China Sea and the Yellow Sea
The Changjiang is the largest river in China; its average annual sediment discharge of 4.86 tons and runoff of 924 billion cubic meters ranked the third and fourth, respectively, in the world. Such large amounts of sediment and runoff will inevitably have important impacts on the physical environment of the East China Sea. The Changjiang River is also a main source of nutrients for our study domain. The Changjiang diluted water also plays an important role on nutrient distribution and its variation trend and affects distribution pattern of algal disaster.
Based on the observation data, the ROMS numerical model was applied to study the extension of the Changjiang diluted water and its effect on nutrient distribution pattern. The results in Figure 6 show that the Subei coastal current, the Changjiang diluted water and the Min-Zhe coastal current flow southward under the strong northeast winter monsoon; furthermore, the Changjiang diluted water and the Min-Zhe coastal current flow close to the shore. The Subei coastal current appeared to invade the northern part of the East China Sea, and the Min-Zhe coastal current still tended to move northward but was obviously slowed down. In summer, the Changjiang diluted water turned toward northeast, heading to Jeju Island. Both the Min-Zhe coastal current and Taiwan warm current moved northeastward with speeds larger than those in the other months. The Subei coastal current had an obvious tendency to move northward along the coast. Spring and summer were transition seasons. The diluted water in the offshore area was limited in the surface layer, and basically there was no evidence of diluted water in the 20-m layer. This was consistent with the characteristics of observed diluted water distribution pattern.
In addition, algal migration paths in the red tide were studied. Assuming that at the end of April, red tide appeared constantly in the Zhoushan coastal area. Simulation through September 1st showed the algal drifting as neutral particles. The trajectories of all particles are shown in Figure 7. Most (~89%) of them moved to the Changjiang estuary and the Kuroshio region, and a few (6%) of them were transported to the Tsushima Strait. Satellite images and
For green tides, it is assumed that
3.2. Direct dynamic evidence for
Ulvaproliferamoving from south to north
Many (~80) drift bottles were deployed on May 2, 2012, and two were retrieved (Figure 10a) with one being an empty bottle near the Jiaozhou Bay mouth on May 28, 2012 and the other being sand-filled bottle at 121°15.2′E, 36°30.4′N (near Haiyang) on June 11, 2012. If only looking at the start and the end points, the empty bottle and the sand-filled bottle drifted north by west and east, respectively. They all landed on the coast of Shandong province. This means
Numerous small-scale spiral oscillations were observed in the trajectories, indicating strong tidal currents or meso-scale eddies. Net movement of drifters was partly covered by the periodical movements. A low-pass filter at a cutoff period of 25 h was applied to obtain the tide-filtered velocity vectors of the drifters (Figure 10b). In the south of 34°30.0′N, most of the vectors nearly kept the same pace as others pointed toward the northwest. This means these vectors are dominated by the same Lagrangian residual current direction. After crossing 34°30.0′N, the consistent pace was broken. The vectors in the north were likely to be affected by complex dynamics there, with the wind being one of the important factors.
To explain the potential relationship with wind, we compared the wind speed data and the clockwise direction deflection angle between wind and drifter velocity (Figure 11). Theoretically, in terms of the Ekman theory, the surface current should be 45° to the right of the wind in the Northern Hemisphere. In reality, the angle is less than 45° in the shallow coastal waters. But in our case, the comparison results show only 33% of the angle was between 0 and 45° (Figure 11a). It suggests that the trajectories cannot be totally explained by the wind-driven Ekman theory, and other baroclinic processes must also influence the trajectories.
In Figure 11a, when wind speeds were larger than 7 m s−1, the wind-driven component dominated the drifter direction, and more data fell within (or close to) 0–45°. For those vectors with angles between 0 and 45°, correlation analysis was done between wind speed and drifter velocity (as a proxy for the wind drift current). The results show that they have significant linear relationship through the origin (
With the field experiments, we obtained first solid evidence that
Ulvaproliferadrifting patterns in the Subei area
Figure 12a comes from the comparison of the satellite images related to raft distribution of
The FVCOM Subei Shoal coastal numerical model showed that, without wind forcing,
3.4. Physical controlling mechanisms of spatial and temporal distribution of
Many studies have been carried out in the southwestern Yellow Sea. But specific to the Subei Shoal and its northern waters, field data of physical oceanography were few and precious, not to mention the field data during green tides. In this study, field data were collected in the Subei Shoal and its northern waters during green tides. More importantly, the survey domain included the Subei Shoal and the relatively deep waters. The results given in the following paragraphs are about currents, temperature, salinity, SPM and PAR.
The currents in the Subei Shoal were dominated by M2 tides. The current pattern was significantly limited by the topography, and the back-and-forth current directions were highly consistent with the channel directions. At the four stations, the maximum current speed reached 1.73 m s−1. Harmonic analysis was done for one-month-long current data from Stations M1 and M2. The lengths of the semimajor axes of the surface tidal current ellipses for the M2 constituent at both stations were about 0.74 m s−1, while the values for the semiminor axes were 0.05 m s−1 at Station M1 and 0.32 m s−1 at Station M2. This result means that the tidal currents were typical alternating and rotational for Station M1 and Station M2, respectively. The maximum subtidal current speed at Stations M1 and M2 was less than the observed total current by one order of magnitude. The direction of subtidal current was mainly affected by the wind. During spring and summer with the prevailing southerly wind,
The temperature pattern at the two mooring stations shows that temperature was going up from late-April to early June with the increasing rate of 0.15°C/day (Figure 14). The diurnal temperature variation was less than 2.21°C. The field observation data during all four periods indicate that a zone with cold surface water existed around the Jiangsu coast between isobaths of 20 m and 30 m (Figure 15). These cold zones were speculated to be produced by strong mixing from strong tidal currents and upwelling. The optimum temperature for
The opposite change between salinity and temperature in Figure 15a illustrates this station was influenced by the intrusion of Changjiang diluted water. The horizontal salinity pattern shows the intrusion path of Changjiang diluted water (Figure 15b). The local diluted water around the Subei Coast was produced by the river discharge in Jiangsu province, which was slowly converging to the north end of Jiangsu coast from late-April to early June with its initial pattern evenly distributed along the coast. From this, we can see that there existed certain kind of relation between Subei diluted water and Changjiang diluted water. When the tongue of Changjiang diluted water went further northward, Subei diluted water was concentrated in the north. Otherwise, Subei diluted water was evenly distributed along the coast. Saline water with salinity more than 5 psu was a hospitable situation for
The sea surface SPM concentration determined the penetration of PAR through the seawater column.
4. Summary and discussion
Three marine disasters caused by algal blooms in the Jiangsu coastal region were described in detail in this chapter. Red tide was the first kind of algae bloom in the region, green tide has lasted for 11 years since 2007 and golden tide was new but likely to be another common algal bloom. Now,
The Subei Shoal area is seldom affected by red tides in the Changjiang estuary and Zhejiang province where red tides are frequent. This is because the Changjiang diluted water acts like a barrier, which prevents
Red tides in the Subei area often happened in Haizhou Bay, which was shown to originate locally. Since 2014, red tide has disappeared from Jiangsu province. This may be caused by the growth inhabitation of
The study in this chapter also provided solid direct dynamic evidence that
Physical controlling mechanisms were studied here, and answered the question why
This study was carried out within the framework of the National Key Research and Development Program of China (grant 2017YFC1404300). It was jointly supported by the National Nature Science Foundation of China (grant 41506027), by the Strategic Priority Research Program of the Chinese Academy of Sciences (grant XDA11020304), by the National Programme on Global Change and Air-Sea Interaction(grand GASI-IPOVAI-04), and the Project of the State Key Laboratory of Satellite Ocean Environment Dynamics, the Second Institute of Oceanography (grants SOEDZZ1503 and SOEDZZ1805). We thank Zuojun Yu for improving English writing.