As shown in Table 1, the number of observation points in this paper exceeds 300 every month, far more than that in previous studies25,26,27,28,29. Based on the comparisons using substantive dataset, we have selected RBF-Linear as the most effective interpolation method. In this paper, we seek to scrutinize the appropriateness of this approach within practical applications.
Spatial and temporal distribution of Chl-a concentration in the Bohai Sea
The spatial distribution of Chl-a concentration in the Bohai Sea during March, May, August, and October of 2016 to 2018 was investigated using RBF-Linear interpolation (Figs. 4, 5). The results show that Chl-a concentration in the coastal area was generally higher than that in the offshore area, and the concentration in the surface layer (0.5 m) was higher than that in the middle layer (~ 10 m). Chl-a concentrations over 10 μg/L were found primarily in surface water of BHB and LDB, particularly in estuaries such as the Haihe River, Luanhe River, Fuzhou River, and Daqing River, and in regions used for mariculture, e.g., Qinhuangdao coastal waters. This pattern is attributed to the influence of nutrient inputs from coastal rivers and maricultural activities that encourage the growth of phytoplankton in coastal seas30,31,32,33,34. Transparency of the water also plays a crucial role in determining the depth of the euphotic layer, which in turn limits the depth at which phytoplankton can grow25,35. Thus, the primary growth of phytoplankton takes place in surface and subsurface water. In some sections of the Bohai Sea’s 10 m layer, scattered throughout the BHB, LDB, and Qinhuangdao offshore water, Chl-a concentration is near to 10 μg/L.
The temporal distribution of Chl-a in the Bohai Sea exhibits seasonal variation, as evident from Figs. 4 and 5. The Chl-a concentration is notably higher in March and August relative to the other months, owing to the availability of key factors that promote phytoplankton growth. Specifically, sufficient nutrients and appropriate levels of photosynthetically active radiation (PAR) during March promote the growth and development of phytoplankton, thereby contributing to the elevated levels of Chl-a observed during this period. The Chl-a concentration in August is influenced by the availability of nutrients, particularly rainfall, which stimulates phytoplankton bloom25,36. The inter-annual variability in the Chl-a concentration of the Bohai Sea is also evident during the same period. The Bulletin on the State of China’s Marine Ecology and Environment reports a significant decline in the concentration of Chl-a during March and August of 2017 and 2018 as compared to the corresponding months in 2016. This decline may be attributed to a comparative decrease in the input of nutrients during 2017 and 2018.
In summary, the investigation of the spatial and temporal pattern of Chl-a concentration in the Bohai Sea reveals a substantial improvement in the marine ecological condition during the years 2017–2018. Nevertheless, areas proximal to aquaculture farms and estuaries have not witnessed similar progress. The findings demonstrate the correspondence between the interpolated Chl-a distribution and a range of actual physical and biochemical variables, hence confirming the validity of the RBF-Linear interpolation technique for reconstructing the spatial and temporal Chl-a distribution.
Marine ecological environment assessment in four sub-regions of Bohai Sea
Total amounts of Chl-a and high Chl-a area are two crucial indicators for evaluating the marine ecosystem. Comparing the total amounts of Chl-a and the high Chl-a area in four Bohai Sea subregions (BHB, LDB, LZB, and CBS), this section examines whether it is reasonable to calculate these two indicators using RBF-Linear interpolation results.
Figure 6 illustrates the temporal variations of total amounts of Chl-a in four subregions of the Bohai Sea. The investigation employs two distinct methods for computing the total amounts of Chl-a. The first approach involves the multiplication of the average Chl-a concentration obtained from observations by each subregion’s area, while the second method involves the integration of the interpolated Chl-a results. The findings (Fig. 6a, b) reveal that the first method yields remarkably higher values than the second method. The overestimation of total amounts of Chl-a using the first method can be attributed to the extensive range of Chl-a concentrations in the dataset, with maximum and minimum values varying by more than two orders of magnitude. In contrast, the interpolation method offers a more accurate approach to mitigate this issue.
The study examines the temporal fluctuation of total amounts of Chl-a in the surface of four sub-regions. The results are presented in Fig. 6c, which illustrates significant variations in total amounts of Chl-a among the sub-regions from 2016 to 2018. Specifically, the total amounts of Chl-a in BHB exhibits a yearly increase in March from 2016 to 2018, while LDB experiences a yearly decrease. Meanwhile, LZB and CBS display slight fluctuations. In May, the total amounts of Chl-a show little variation across all sub-regions. In August, LDB shows minimal change throughout the three-year period, while the total amounts of Chl-a in BHB declines substantially in 2017 and 2018, compared to 2016. Further, it is observed that the typhoon “Wimbiya” in August 2018 results in a significant increase in Chl-a concentrations in LZB and CBS, with subsequent rise of dissolved inorganic nitrogen and silicon in the surface29. Finally, the total amounts of Chl-a in BHB and LDB dramatically decreases in October over the three-year period, with no significant changes noted in LZB and CBS.
Figure 6d depicts total amounts of Chl-a at 10 m layer in the four sub-areas, which is noticeably less than that at the surface. The findings demonstrate that BHB experienced an upward trend in total amounts of Chl-a in March, while LZB witnessed a decline over time. In contrast, LDB and CBS exhibited no significant changes in total amounts of Chl-a. Notably, the total amounts of Chl-a in CBS increased dramatically in 2018, while the other three sub-regions remained relatively stable. In August, the total amounts of Chl-a in three sub-regions remained relatively constant except for a notable fluctuation in CBS. However, in October, the total amounts of Chl-a showed a yearly decline across all four sub-regions.
In light of the total amounts of Chl-a concentrations, it is evident that the marine ecological environment underwent significant changes among different regions and seasons during the years 2016 to 2018. During the onset of spring, the marine ecological environment of BHB deteriorated, while that of LDB improved considerably. Additionally, the marine ecological environment of CBS was at risk of deterioration towards late spring. During the summer, the marine ecological environment of BHB witnessed substantial improvement, while in certain areas, remarkable ecological improvements were observed in autumn. These shifts in primary production in the three bays and CBS are primarily attributed to variations in water clarity and nutrient composition, as documented in previous research26. The findings also emphasize the significant impact of typhoons on the marine ecology. It is worth noting that marine ecological studies, primarily focusing on the surface and subsurface layers, tend to ignore the concentration of Chl-a in deep layers. However, our research reveals notable differences in Chl-a concentrations in the deep layer among areas, which might relate to water depth. Therefore, while disregarding the Chl-a concentration in the deep layer may be possible in several places during assessing the marine ecological environment, it is necessary to consider it in particular areas such as CBS.
The area where algal blooms occur is a crucial indicator of the quality of the marine biological environment in a region. Chl-a concentrations that exceed the threshold of 10 μg/L are widely considered a critical limit for phytoplankton blooms in the marginal seas of China37. This investigation focuses on scrutinizing and assessing the marine ecological environment in four sub-regions of the Bohai Sea by evaluating the extent of areas with high concentrations of Chl-a (> 10 μg/L).
In Fig. 7, the high Chl-a concentration region and its four sub-regions in the Bohai Sea are presented. The results revealed that the area with high Chl-a concentration has decreased annually from 2016 to 2018. It is worth noting that the area calculated in this study demonstrates the extent of the region reaching the critical value of the bloom rather than the area of the disastrous bloom. Consequently, the area estimated in this study was more extensive than the data provided in the China Marine Disaster Bulletin (Table 3). Nonetheless, the changes observed in the high Chl-a concentration area estimated by interpolation results were consistent with the variation in data given in the China Marine Disaster Bulletin. These results confirm the validity of RBF-Linear interpolation findings and the use of the high Chl-a concentration area to evaluate the marine ecological environment in the Bohai Sea. Moreover, the Bulletin of the State of the Marine Ecological Environment of China reported a consistent yearly decrease in total nitrogen and phosphorus discharged into the Bohai Sea from 2016 to 2018, which aligned with the variation in high Chl-a concentration region. These findings indicate that the reduction in nutrient input played a significant role in ameliorating the water quality of the Bohai Sea.
Figure 7 also displays the changes in Chl-a concentrations exceeding 10 μg/L on the surface of the four sub-regions of the Bohai Sea from March, May, August, and October in 2016 to 2018. The surface waters of LDB and BHB exhibited prevalent Chl-a concentrations exceeding 10 μg/L. BHB is surrounded by various megacities, including Tianjin, and is fed by 16 rivers. Water exchange in the region can be impacted by reclamation and mariculture activities, along with the combined influence of land runoff and offshore reclamation. As a result, BHB’s coastal water quality is declining, and red tides are frequent 27. On the other hand, LDB, which is encircled by numerous industrial cities, is mainly affected by land-based pollution, leading to significant phytoplankton proliferation 36. The surface waters of BHB exhibited a high Chl-a content over a large area (> 800 km2) in March and August from 2016 to 2018, with no noticeable interannual fluctuations. Conversely, there was a significant decrease in LDB, stemming primarily from the slow growth of heavy industry in Hebei and Liaoning Province and the significant reduction of land-based pollution, leading to a reduction in Chl-a concentration 36. The high Chl-a content area on the surface waters of LZB and CBS is frequently small. LZB is situated in the southern Bohai Sea, bordered by the Yellow River, the Xiaoqing River, the Weifang River, and the Jiaolai River, and the ecological environment in the region is primarily influenced by river runoff, domestic sewage, and mariculture. Consequently, both the Chl-a concentration and the area with high Chl-a concentration are generally lower than those in BHB and LDB 38. Furthermore, because the dissolved inorganic nitrogen in CBS is relatively low compared to other regions 39, the Chl-a concentration and the area with high Chl-a concentration in this region are relatively low 40. The investigation reveals that the extent of the high Chl-a concentration area, exceeding 10 μg/L, in the 10 m layer is primarily localized in the BHB and LDB sub-regions of the Bohai Sea, encompassing an area which is typically less than or equal to 100 km2, despite a noteworthy anomaly in March 2016 where the high Chl-a concentration zone in LDB is calculated to be approximately 300 km2. Owing to the relatively diminutive magnitude of high Chl-a concentration areas encountered in the 10 m layer across each sub-region of the Bohai Sea, we refrain from comprehensive analysis in the present study.
Based on the investigation conducted into the marine ecological environment of the Bohai Sea and its four sub-regions, our findings reveal that the BHB sub-region shows no significant improvement, whereas the LDB sub-region has exhibited notable improvements. In contrast, the LZB and CBS sub-regions have consistently maintained a favorable marine ecological environment. These results indicate a correlation between the state of the marine ecological environment in each sub-region and the level of development of the adjacent urban centers. Additionally, the assessment of the marine ecological environment presented here further endorses the practicability and rationality of using the Chl-a result with RBF-Linear interpolation as a suitable evaluation method for the marine ecological environment. Meanwhile, this will provide a new technical method for the research of Chl-a in the future.