Impacts of typhoons on environmental conditions of marginal seas
Typhoon events usually cause stronger wind and thus upwelling, which brings cold water into the surface layer, resulting in a decrease in SST (Liu et al. 2019; Zheng and Tang 2007). Our results showed a decrease in SST associated with enhanced upwelling (Additional file 1: Fig. S3), particularly in the western YBS near to the tracks of Typhoon Lekima, which was consistent with other studies that reported surface cooling under strengthened upwelling (Liu et al. 2019, 2020a). Apart from the impact of upwelling, there was also evidence of further SST decrease due to input of extra freshwater (with lower temperature) from heavy rainfall and enhanced runoff (Fu et al. 2016a, b). On the other hand, the freshwater input could also enhance ocean stratification and suppress vertical mixing due to decreased sea surface salinity, which could reduce the decline of SST (Liu et al. 2020b). Our analysis also showed a decrease of salinity in most nearshore waters (Additional file 1: Fig. S4), reflecting the dilution effect caused by heavy rainfall and enhanced freshwater inputs from coastline runoff.
Previous studies have reported significant increases in nutrients following typhoon events (Hung et al. 2013; Jiang et al. 2020), which often results from strengthened upwelling that brings more nutrients from bottom waters into surface layer (Liu et al. 2019; Zheng and Tang 2007). In addition, enhanced runoff could also deliver more nutrients into coastal waters (Fu et al. 2016a, b). Indeed, a recent field study showed that total inorganic nitrogen concentrations increased by > 90% in coastal waters of the NYS post the Typhoon Lekima 2019 (Lu et al. 2020), implying that there would be consequently biological responses.
Biological responses to typhoon-induced environment changes
There was evidence of increased Chl-a following typhoon events, which was attributed to enhanced nutrient supply associated with typhoon-enhanced terrestrial runoff and upwelling (Shiah et al. 2000). Although our study showed lower Chl-a levels during 14–21 August in 2019 (post the Typhoon Lekima event) than in non-typhoon years, the relative increase of Chl-a (from 1–8th to 14–21st August) was greater in 2019 (16–46%) than in non-typhoon years (6–39%) in the YS (Table 1), indicating that phytoplankton growth increased in association with the passage of Typhoon Lekima. In addition, the increase of Chl-a post-typhoon events could be partly due to upwelling of phytoplankton from subsurface into surface in the sections with subsurface Chl-a maximum (Chen et al. 2017; Liu et al. 2019), suggesting that the increase of Chl-a in the surface of SYS might be partly attributable to the upwelling of high-Chl-a water from subsurface (Fu et al. 2018). There was also evidence of changes in community structure post-typhoon events, i.e., more abundance in large size of phytoplankton that often has a higher Chl-a:carbon ratio (Frenette et al. 1996; Ma et al. 2021; Sun et al. 2002; Wei et al. 2017).
Earlier studies reported a large increase of Chl-a (usually by ~ 50–100%) after typhoon events in shallow waters of the Northwest Pacific, e.g., in the northern South China Sea due to enhanced nutrient supply. However, we found that the increase of Chl-a was much smaller (16–46%) in the YS following the passage of Typhoon Lekima that lasted for only ~ 9 h. In addition, the intensity of Typhoon Lekima was weak in the YS, causing much weaker Ekman upwelling (0.5–3.5 × 10–5 m s−1), relative to those (1.0–480 × 10–5 m s−1) with other typhoon events (Pan et al. 2017; Sun et al. 2010; Zhao et al. 2008), which was largely responsible for the small increase in Chl-a. The obvious difference in the relative increase of Chl-a was also attributed to the difference in Chl-a baseline, i.e., much higher in the YS (> 0.8 mg m−3) but lower in those oligotrophic seawaters (< 0.25 mg m−3) (Liu et al. 2019; Zhao et al. 2008).
Our results also revealed the decreases of Chl-a in the middle BS and northwestern coastal SYS shortly after the passage of Typhoon Lekima, where strong upwelling prevailed during the Typhoon Lekima event. Previous studies reported decreased Chl-a post-typhoon events, which was partly due to perturbation caused by bottom water entrainment (Shih et al. 2020), or attributed to dilution and flushing induced by elevated freshwater discharge (Huang et al. 2011; Wiegner et al. 2012). Apparently, the heavy rainfall brought by the Typhoon Lekima event would have dilution effects on Chl-a, particularly in the BS (Fig. 3e). In addition, enhanced surface current associated with the Typhoon Lekima transported low Chl-a seawater from the NYS to the central BS (Additional file 1: Fig. S1e), as indicated by the co-occurrence of a significant decrease in Chl-a (Fig. 4f) and increase in salinity (Additional file 1: Fig. S4d). Moreover, water column was not stable due to increased current velocity (Additional file 1: Fig. S1e), which could affect phytoplankton growth and depress the pre-existing bloom (Long et al. 2011; Mitrovic et al. 2003). The decline of Chl-a in the SYS was also partly caused by dilution and flushing due to massive freshwater discharge. In addition, there was evidence that enhanced terrestrial runoff caused further phosphorus limitation in the SYS due to the low-phosphorus concentration in runoff from adjacent lands (Guo et al. 2020; Lian et al. 2020), which could lead to lower Chl-a. While phytoplankton growth could be restrained by light limitation due to sediments resuspension caused by typhoon events (Ding et al. 2012; Hung et al. 2010), our analyses did not show light limitation in most typhoon-affected sections, as indicated by deepened euphotic depth (Additional file 1: Fig. S5).
Impacts of typhoon on organic carbon in marginal seas
Previous studies reported significant increases of POC (> 60%) post-typhoon events in marginal seas in the Northwest Pacific, e.g., the south East China Sea, which were largely caused by typhoon-induced phytoplankton blooms (Hung et al. 2010; Shiah et al. 2000). Our analyses showed a modest increase of POC (22–46%) in the YBS post the Typhoon Lekima event, which might be attributable to the small increase of phytoplankton biomass [indicated by the small increase of Chl-a (16–46%)], due to the short duration and weak intensity of Typhoon Lekima in the YBS and its less extent of impacts.
The increase of POC from 1–8th to 14–21st August was much greater in 2019 (41–79 mg m−3, or 22–46%) than in non-typhoon years (9–45 mg m−3, or 4–16%) in the YBS (Table 1). The BS revealed increased POC with decreased Chl-a post the Typhoon Lekima event (Table 1), and a larger intercept (83 vs. 20) in the correlation between the change of POC and the change of Chl-a (ΔPOC–ΔChl-a) in 2019 than in non-typhoon years (Fig. 7), indicating that there were other sources rather than marine ecosystem responsible for the increase of POC associated with the Typhoon Lekima event. A recent study also reported that typhoon-induced changes in physical processes had effects on POC level in the coastal Japan Sea (Tsuchiya et al. 2017). Strong wind in association with the Typhoon Lekima event could increase injection of sedimentary POC to the surface layer via enhanced resuspension and wind driven upwelling in the BS, and similar findings were reported in other marginal seas (Dickey et al. 1998; Shiah et al. 2000). In addition, enhanced runoff associated with heavy rainfall (as indicated by the decrease of salinity, see Additional file 1: Fig. S4d) could also bring more terrigenous POC into the coastal waters of BS, as discussed in a previous study (Yu et al. 2018). On the other hand, the reduction of POC in the central BS near the Bohai Strait was attributed to enhanced water exchange with the low-POC water from the NYS (Additional file 1: Fig. S1e).
There were considerable differences in the response to the Typhoon Lekima event between POC and Chl-a in the YS (Figs. 4f, 5f). We evaluated the variation of POC:Chl-a ratio (an index used to assess the sources of POC in the oceans), in which a small POC:Chl-a ratio (< 200) indicates a large contribution of phytoplankton to POC (Hung et al. 2010; Yu et al. 2019). Our analyses showed that there was a decrease in POC:Chl-a ratio (by 10–23) in the YS from 1–8th to 14–21st August in non-typhoon years, indicating that biological production was the main driver responsible for the increase of POC in summer (Yu et al. 2019). However, POC:Chl-a ratio revealed an increase post-Typhoon Lekima in the YS except in the coastal waters of the NYS. There was evidence that large-size phytoplankton bloom post-typhoon events could result in lower POC:Chl-a ratio, particularly in nearshore waters (Lee et al. 2020).
Our further analyses demonstrated that there was a significantly positive correlation between the increase of POC and the increase of Chl-a from 1–8th to 14–21st August; the slope of ΔPOC–ΔChl-a correlation (as shown in Fig. 7) was much greater in 2019 (63–81) than in non-typhoon years (32–41) in the YS, suggesting that there were other processes in addition to biological production contributing elevated POC. Previous studies reported that apart from biological contribution, the dynamics of POC in the YBS was largely regulated by terrestrial inputs, sediment resuspension, and water exchange (Fan et al. 2018; Yu et al. 2018). For example, sedimentary resuspension is one of the major processes causing high level of the POC in the YBS (Fan et al. 2018). There was also evidence of significant sedimentary POC supply during typhoon events in other marginal seas, owing to stronger upwelling and enhanced resuspension resulting from strong winds (Dickey et al. 1998; Shiah et al. 2000).
There was evidence that terrigenous POC could be transported via large rivers and coastline runoff into nearshore waters of marginal seas (Qiao et al. 2020; Trefry et al. 1994; Wang et al. 2012). Our study showed an increase in POC:Chl-a ratio in the coastal waters of SYS (Fig. 6f), indicating that there might be other sources of POC during the Typhoon Lekima event. The typhoon-induced heavy rainfall could enhance runoff along the coastline, which would inject extra terrigenous POC into the nearshore waters. In addition, the further northward extension of Yangtze River Diluted Water during typhoon events (Oh et al. 2014) could also transport more terrigenous POC into the SYS. On the other hand, elevated POC:Chl-a ratio (due to greater increase in POC) in the central sections of YS might reflect strengthened current resulting from Typhoon Lekima, which could transport high-POC waters from nearshore to offshore.
Previous studies reported lower levels of Chl-a and POC in summer than in spring and autumn in the YBS due to poor nutrient supply and weak sediment resuspension via restrained vertical mixing caused by stronger stratification (Fan et al. 2018; Zhao et al. 2019). The increases of Chl-a and POC in the YBS caused by Typhoon Lekima were lower than the seasonal increases (from summer to autumn), indicating that the influences of Typhoon Lekima on the biogeochemical processes were insignificant. The YBS had been impacted by various typhoon events over the recent decades, which occurred in > 50% of the summer seasons with 1–4 passages in each season. There was another typhoon event (Typhoon Danas) prior to Typhoon Lekima, which could cause changes in environmental conditions, thus alter the responses of biological and chemical processes. Apparently, the interactive responses of physical and biological processes to typhoon events were complex, which would have significant impacts on the nutrients and carbon cycle in the YBS. Future studies with in situ measurements of critical carbon cycle parameters and process-orientated modeling studies warrant better understanding the impacts of typhoons on the carbon cycle in marginal seas.