Air-sea interactive forcing on phytoplankton productivity and community structure changes in the East China Sea during the Holocene

doi:10.1016/j.gloplacha.2019.05.008

Global and Planetary Change

Volume 179, August 2019, Pages 80-91

https://doi.org/10.1016/j.gloplacha.2019.05.008 Get rights and content

Highlights

•
We reconstruct phytoplankton productivity and community structure in the ECS.
•
Biomarker records reveal increased phytoplankton productivity during the Holocene.
•
Variations of productivity and community structure relate to air-sea forcing.

Abstract

Phytoplankton productivity and community structure in the East China Sea (ECS) play an important role in marine ecology and carbon cycle, but both have been changing rapidly in response to recent oceanic and atmospheric circulation changes. However, the lack of long-term records of phytoplankton productivity and community structure variability in the region hinders our understanding of natural forcing mechanisms. Here, we use the phytoplankton biomarker (brassicasterol, dinosterol and alkenones) contents as well as the ratios between these biomarkers in three sediment cores from the ECS shelf to reconstruct the spatiotemporal variations of productivity and community of diatoms, dinoflagellates and coccolithophores during the Holocene, respectively. During 9–7 ka, the ECS shelf was characterized by low phytoplankton productivity with low coccolithophore contribution, caused by the oligotrophic condition mainly owing to the restricted Kuroshio Current (KC) intrusion under low sea-level conditions, thus the lack of nutrient input. Phytoplankton productivity generally increased during 7–4.6 ka, in response to the initial intrusion of the Yellow Sea Warm Current (YSWC, a branch of the KC), bringing nutrient from the subsurface KC to the upper layer of the ECS for phytoplankton growth. Phytoplankton productivity continuously increased during 4.6–1 ka, due to an enhanced circulation system (YSWC and Yellow Sea Coastal Current (YSCC)) driven by strong East Asia Winter Monsoon (EAWM). Significantly, high alkenone contents and coccolithophore contribution in the eastern core F11A was associated with its location closer to the warm and saline YSWC, which was suitable for coccolithophore growth. Beyond diagenetic processes which could partly account for higher biomarker contents near core tops, elevated phytoplankton productivity during the last 1 ka might be induced by more nutrient supply from the intensified circulation system driven by enhanced KC and anthropogenic activities. The latter also resulted in high dinoflagellate proportions in all three cores. These temporal and spatial changes of phytoplankton productivity and community structure in the ECS during the Holocene corresponded to different mechanisms by the air-sea interaction, providing insights into distinguishing natural forcing and anthropogenic influences on marine ecology.

Introduction

Phytoplankton productivity and community structure play an important role in the global carbon cycle, regulating climate by controlling CO₂ level in the atmosphere (Chisholm, 2000; Rost and Riebesell, 2004; Schneider et al., 2008). As the basis of food web in the oceans, changes in phytoplankton productivity and community structure could also influence the marine ecosystem (Cloern and Dufford, 2005). Phytoplankton productivity is significantly related to sea surface nutrient concentration (cf. Chen, 2000). Since the industrial revolution, anthropogenic activities have led to increasing nutrient input, thus resulting in widespread coastal and marginal sea phytoplankton blooms, such as those during the 1980s and 1990s in the Aegean Sea and Black Sea (Moncheva et al., 2001), frequent red tides/harmful algal blooms from 2002 to 2005 in the East China Sea (ECS) (Zhou et al., 2008) and the world's largest green tides in the summer of 2008 in the Yellow Sea (Liu et al., 2010a). Especially some toxic dinoflagellates are becoming dominant in the coastal habits of many countries, causing hypoxia in water column, significant fish kill and harming human health (Kirkpatrick et al., 2004; Walsh et al., 2006). However, recent studies reported that increased diatom productivity was a result of the increased natural forcing of upwelling in Zhejiang coastal area (Duan et al., 2014). Similarly, Florida red tides were also a natural phenomenon caused by dense aggregations of unicellular organisms (Kirkpatrick et al., 2004). In order to understand mechanisms controlling phytoplankton blooms, it is essential to determine the spatial and temporal changes in phytoplankton productivity and community structure and their natural variability in the past.

Phytoplankton species such as diatoms and dinoflagellate/dinocysts in water column and surface sediments have been used to indicate the abundance of phytoplankton (cf. Chiang et al., 2004; Cho and Matsuoka, 2001). However, the use of diatom and dinoflagellate cyst for paleo-environment reconstruction was limited due to their poor preservation in marine sediments, e.g., dissolution of siliceous and carbonate frustule (Mudie et al., 2001). Biomarkers, fossil molecules, could be relatively well preserved in marine sediments and have been used to reconstruct long-term natural climate and ecological changes (Schubert et al., 1998; Werne et al., 2000; Zhao et al., 2006). Previous studies pointed out that lipid biomarkers in surface suspended particles can reflect relative phytoplankton biomass, as revealed by similar distribution patterns between lipid biomarkers and Chl a and/or phytoplankton cell counting (Sicre et al., 1994; Dong et al., 2012; Wu et al., 2016a). A recent study in the ECS also validated the applicability of brassicasterol, dinosterol, and C₃₇ alkenones as proxies of productivity and community structure of the three phytoplankton taxa: diatoms, dinoflagellates, and coccolithophores (Wu et al., 2016a). Therefore, community structures with relative contributions of diatoms, dinoflagellates and coccolithophores to phytoplankton productivity can be represented by brassicasterol/Sum (B/Sum, Sum = ∑brassicasterol+dinosterol+alkenones), dinosterol/Sum (D/Sum) and alkenones/Sum (A/Sum), respectively (Schubert et al., 1998; Xing et al., 2016). Brassicasterol/dinosterol ratio (B/D) is used to indicate the relative contribution of diatoms compared to dinoflagellates (Duan et al., 2014; Xing et al., 2016). In the ECS, water mass properties (temperature, salinity, and inorganic nutrient) were important factors controlling phytoplankton biomass and community structure spatial variations (Wang and Cheng, 1988; Bi et al., 2018).

Today, primary productivity in the ECS is high with an average of 390–529 (mg C m²)/d (Ning et al., 1995; Gong et al., 2003), which has become an ideal area for environment and ecosystem change studies recently. When the Yellow Sea Warm Current (YSWC) intrudes on the ECS shelf as a branch of the Kuroshio Current (KC), the upwelling of the KC subsurface waters brings nutrients (especially phosphate and silicate) to the euphotic zone for phytoplankton growth (Chen, 2000; Zhang et al., 2007). Thus, primary productivity of upwelling zones on the ECS continental slope is even higher than that of the terrestrial-nutrient-prevailing coastal areas (Chen, 2000). In addition, East Asia Winter Monsoon (EAWM) was reported as an essential driver of ecosystem changes in the ECS by driving the circulation system. In winter, the northerly EAWM drives the Yellow Sea Coastal Current (YSCC) and causes the pressure gradient along the Yellow Sea Trough, which forces the deeper northwestward intrusion of the YSWC as a compensating current of the YSCC (Yang, 2007; Xu et al., 2009; Yuan and Hsueh, 2010). Thus the intensified circulation system of the YSWC and the YSCC can induce the upwelling of nutrients to surface layer (Li et al., 2014 and references therein), although the enhanced EAWM somehow might weaken the strength of the KC (Jian et al., 2000). Biomarker records reveal that the ecosystem change has been driven by both natural (Pacific Decadal Oscillation) and anthropogenic forcing (terrestrial nutrient) over the last 100 years in the coastal areas of the ECS (Xing et al., 2016). However, lack of long-term and spatially-resolved records of phytoplankton productivity and community structure in the region hinders the understanding of ecosystem variation and its forcing mechanisms.

In this study, three sediment cores from the mud area on the ECS shelf with high sedimentation rate (Fig. 2) and relatively high organic carbon contents (average 0.7%) in surface sediments (Zhu et al., 2011), were used to reconstruct variations of phytoplankton productivity and community structure by multiple biomarker analyses. We compared the spatiotemporal variations of biomarker contents and their ratios to reveal the mechanisms controlling the phytoplankton productivity and community structure changes in the ECS during the Holocene, with the main focus on the air-sea interactive forcing.

Section snippets

Regional setting

The ECS, one of the largest shelf seas in the world, is located between the West Pacific and the East Asia Continent (Fig. 1). This region is influenced by both high-latitude climate forcing through the East Asian Monsoon and tropic-subtropical ocean circulation through the KC (Chen, 2009; Lie and Cho, 2016; Li et al., 2017). The East Asian Monsoon results from the different potential heating between the West Pacific Warm Pool and the Asian continent, which is fundamental to climate dynamics

Sediment cores and age model

The three cores for this study were recovered from the ECS (Fig. 1), using a gravity sampler on R/V Dongfanghong 2 in 2011, F11A (31°53′ N, 126°21′ E, water depth: 93 m, core length: 206 cm), F10C (31°45′ N, 126°07′ E, water depth: 65 m, core length: 205 cm) and B3-1A (31°37′ N, 125°45′ E, water depth: 79 m, core length: 289 cm). All sediment samples were taken at 1 cm intervals and preserved frozen at −20 °C.

Benthic foraminifers from 7 depths of core B3-1A and from 5 depths of core F11A were

Chronology

The ¹⁴C-dated core depths for B3-1A covered a time span of the last 9 ka. Linear interpolation between radiocarbon dates yielded sedimentation rates between 10 and 149.1 cm/kyr (Fig. 2A). The dated core interval for F11A spanned the Mid- and Late Holocene (4.6 ka) with sedimentation rates between 29.8 and 100.5 cm/kyr (Fig. 2C). For core F10C, the ¹⁴C-dated covered a time span of the last 12.6 ka (Fig. 2B). Linear interpolation between radiocarbon dates yielded sedimentation rates between 7.4

Factors influencing sedimentary lipid biomarker contents

When using the lipid biomarkers for reconstruction of phytoplankton productivity and community structure, information of the source of phytoplankton biomarker is needed. According to Volkman et al. (1998), brassicasterol is not specifically derived from diatoms and is also abundant in some coccolithophores. In the ECS, diatoms and dinoflagellates are the dominant phytoplankton (Guo et al., 2014; Xiao et al., 2017), suggesting that the contribution of brassicasterol from coccolithophores could

Conclusions

Based on the contents and ratios of phytoplankton biomarkers of brassicasterol, dinosterol and alkenone, the records of phytoplankton productivity and community structure during the Holocene have been reconstructed in the ECS. Our results from three cores provided high spatiotemporal resolution evidence for air-sea interactive forcing on the variations of marine ecology in the study area. A comparison with the Holocene records of the KC strength, EAWM variability and ENSO frequency is presented

Acknowledgments

We thank Rong Xiang and Liping Zhou for help with core chronology, Hailong Zhang and Li Li for technical assistance of the organic geochemical analyses. This work was supported by the National Natural Science Foundation of China (Grant No. U1606404, 4130966, 41520104009) and the “111” Project (No. B13030). This is MCTL contribution #163.

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The precision (standard deviation, 1σ) of δ13C for duplicate samples is better than 0.2‰. Lipid biomarker analysis was performed following the procedures described by Wang et al. (2019). In brief, after adding an internal standard of C19 n-alcohol, approximately 3 g freeze-dried sediment sample was extracted with dichloromethane/methanol mixture (3:1, v/v).
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Research articleAir-sea interactive forcing on phytoplankton productivity and community structure changes in the East China Sea during the Holocene

Highlights

Abstract

Introduction

Section snippets

Regional setting

Sediment cores and age model

Chronology

Factors influencing sedimentary lipid biomarker contents

Conclusions

Acknowledgments

Quat. Sci. Rev.

Geochim. Cosmochim. Acta

J. Mar. Syst.

Mar. Micropaleontol.

Org. Geochem.

Deep Sea Research Part II Topical Studies in Oceanography

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Earth Planet. Sci. Lett.

Prog. Oceanogr.

Geomorphology

Earth Planet. Sci. Lett.

Quat. Sci. Rev.

Mar. Micropaleontol.

Harmful Algae

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Quat. Int.

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Prog. Oceanogr.

Mar. Geol.

Nucl. Inst. Methods Phys. Res. B

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Estuar. Coast. Shelf Sci.

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J. Asian Earth Sci.

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Quat. Sci. Rev.

Diatoms, temperature and climatic change

Eur. J. Phycol.

Observations of Kuroshio flow variations in the East China Sea

J. Geophys. Res. Oceans

The significance of extant coccolithophores as indicators of ocean water masses, surface water temperature, and palaeoproductivity: a review

Paläontol. Z.

Water mass control on phytoplankton spatiotemporal variations in the northeastern East China Sea and the western Tsushima Strait revealed by lipid biomarkers

J. Geophys. Res.

Water mass analysis and end–member mixing contribution using coupled radiogenic Nd isotopes and Nd concentrations: Interaction between marginal seas and the northwestern pacific

Geophys. Res. Lett.

The Kuroshio intermediate water is the major source of nutrients on the East China Sea continental shelf

Oceanol. Acta

Comparisons of primary productivity and phytoplankton size structure in the marginal regions of southern East China Sea

Cont. Shelf Res.

Patterns of thermal limits of phytoplankton

J. Plankton Res.

Research article
Air-sea interactive forcing on phytoplankton productivity and community structure changes in the East China Sea during the Holocene