Introduction

China ended the feudalism of the Qing dynasty in 1911 A.D., and then experienced short-term civil war in the first half of twentieth century (Wasserstrom 2015). From the 1950s, China entered the stage of a planned economy until the early 1990s. In 1995, the economic mode was adjusted to a market economy after the 5th Plenary Session of the 14th CPC Central Committee (Wu 2003). Obviously, economic development in China was closely related to policy guidance in the past 150 years. Mostly coastal areas benefitted from these policies, and gained rapid development (Lemoine et al. 2015). Meanwhile, coastal environmental and ecological systems have deteriorated, resulting in an unsustainable development especially in the last 30 years (Liu and Diamond 2005; Liang et al. 2015; Liu and Mu 2016). Although the worsening status in some bays and coastal seas of China are extensively reported (Liang et al. 2015), it is still difficult to disentangle the effects of different policies on coastal ecosystems (Yuan et al. 2016). This situation prevents governments from taking target-directed measurements to correct the problems corresponding to different economic policies used in earlier years.

We selected Jiaozhou Bay to check whether different Chinese economic policies and their environmental effects were archived in the sediment cores. Jiaozhou Bay is a semi-enclosed bay located in the southern Shandong Peninsula, China with a total area of 390 km2. It represents one of the most developed coastal regions and has witnessed several economic policies in the past 150 years. From 1860s, the sea area of Jiaozhou Bay decreased more than one-third with the influence of anthropogenic activities (Yuan et al. 2016). About 20 % of the bay is intertidal area, which decreased ~70 % in the last half century (Yang et al. 2003). Anthropogenic activities have significantly changed the input of nutrients (i.e. N, P, and Si) into Jiaozhou Bay (Shen 2001) and resulted in the frequent red tides (Dai et al. 2007). Sediment discharges from surrounding rivers also decreased from 1950s to 1980s (Wang 1986; Wang and Gao 2007). These observations indicated that the sedimentary environments probably changed to a large degree in Jiaozhou Bay. Fortunately, there are relatively complete historical records (Ma et al. 2014; Yuan et al. 2016) in Jiaozhou Bay over the last 150 years, which would help us resolve the confounding effects of the various policies.

Materials and methods

Jiaozhou Bay has an average tidal range of 2.7–3.0 m (Liu et al. 2007). Recently, the bridge construction (e.g. Jiaozhou Bay Bridge) has changed the hydrodynamic conditions (Zhao et al. 2015). In order to avoid the influence of bridge construction, two stations (i.e. C4 and C3) in the southeast of Jiaozhou Bay were occupied (Fig. 1), where sediment is well-preserved. The water depths at C4 and C3 ranged between 10 m and 20 m. The sediment cores were collected using a gravity corer on 11 June 2015, and sectioned into subsamples of 0.5 cm thickness. The weights of the wet sediments were measured to calculate the water contents. After freeze drying, the sediment weights, radioactive activities of 210Pb and 137Cs were determined in subsamples. 210Pb was measured via its granddaughter 210Po. In brief, 0.6 g of sediment was digested with concentrated HNO3 after adding an accurate 209Po spike. Both 210Po and 209Po in solution were finally auto-deposited on an Ag-plate under specific conditions (Yang et al. 2011, 2015). The activities of Po isotopes (i.e. 210Po and 209Po) were counted using α-spectrometry until the counting errors were less than ±1σ. 137Cs activities were determined by non-destructive gamma counting using a Canberra ultra-high purity germanium well detector. The counting efficiency at 661.3 keV for 137Cs was calibrated using standard material from the National Institute of Metrology (No. 9MLSG) with the same geometry as the samples in the counting vial. Both 210Pb and 137Cs in Bq kg−1 dry sediment were used to calculate the 210Pb-chronology and label 137Cs-chronology (Tables S1-S7, and Fig. 2).

Fig. 1
figure 1

Sampling coordinates in Jiaozhou Bay connected to the Yellow Sea. The coastlines in 1935 and 2013 are from Yuan et al. (2016)

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Fig. 2
figure 2

Comparison between 137Cs-labeled 1963 and 210Pb-chronology (CRS model) in the sediment cores at C4 and C3. The consistencies between the two methods support the validity of 210Pb-chronology

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The constant rate of supply model (i.e. CRS, Goldberg 1963), also called the constant flux model (i.e., CF, Robbins 1978), of 210Pb-chronology was adopted in this study. 210Pb in coastal seawater is derived mainly from atmospheric deposition (Baskaran 2011), and this source usually shows little inter-annual variation on the western Pacific coasts (Huh et al. 2006; Wang et al. 2014). 210Pb in coastal seawater is usually scavenged into local sediments because coastal waters have very high particle concentrations, which efficiently remove atmosphere-derived 210Pb and even 210Pb transported from open seawater, i.e. boundary scavenged-210Pb (Nozaki et al. 1997; Huh and Su 1999). Hence, the flux of 210Pb at a specific site in coastal seas would, to a large extent, meet the precondition of the CRS model rather than other 210Pb-chronology models (i.e. Appleby and Oldfield 1983; Sanchez-Cabeza and Ruiz-Fernández 2012). In addition, the rapid development of the economy in the past 30 years probably resulted in large variations of the sedimentation rate in Chinese coastal seas, either in cm yr−1 or g cm−2 yr−1. Combining the small variability in 210Pb flux and the large variability in sedimentation rate, the constant activity of 210Pb in the last century does not seem to be valid for the constant activity model, constant sedimentation model, or constant flux constant sedimentation model (Sanchez-Cabeza and Ruiz-Fernández 2012). In contrast, this factor has little influence on the application of the CRS model. To further validate the CRS model, 137Cs-chronology was concurrently used in the present study.

Results and discussion

The maximum of the 137Cs specific activity in sediment cores, witnessing the highest 137Cs content in the atmosphere during the 1960s, is widely used to label 1963 A.D. and validate the 210Pb-chronology (Sanchez-Cabeza and Ruiz-Fernández 2012). At stations C4 and C3, 137Cs clearly showed the highest value at 11.5–12.0 cm and 6.0–6.5 cm, although it fluctuated in the cores (Fig. 2). Concurrently, 210Pb-chronology confirmed 1963 A.D. was observed at 11.0–11.5 cm and 6.0–6.5 cm in C4 and C3 cores. These consistencies verified that the CRS model derived 210Pb-chronologies at the two stations reflected the archived sediment mass accumulation rate (MAR) in sediments.

In the past 150 years, the MARs varied largely at stations C4 and C3 (Fig. 3). However, they showed similar patterns in most cases. Owing to the low time resolution of the MAR at C3, C4 was mainly used here to illustrate the temporal evolution of sedimentation dynamics in Jiaozhou Bay. Overall, five periods were identified based on MAR variation (Fig. 3). Before 1935, the MAR changed little with an average of 0.03 ± 0.01 g cm−2 yr−1. From 1935, the MAR increased to 0.19 g cm−2 yr−1 in 1949 and then decreased to 0.12 g cm−2 yr−1 in the late 1960s. From the late 1960s to around 1990, the MAR exhibited a second round of increase and decrease, varying from 0.13 to 0.26 g cm−2 yr−1 and peaking in 1981. The third round cycling of the MAR occurred between 1990 and 2002. After this, the MAR experienced a rapid increase again. The temporal pattern revealed complicated variations of the MAR in Jiaozhou Bay rather than the monotone increasing and decreasing of the MAR in some areas (Sun et al. 2008), probably due to the influence of anthropogenic activities in addition to natural processes.

Fig. 3
figure 3

Variations of sedimentation rates in g cm−2 yr−1 at stations C4 and C3 in the past 150 years, as well as the sea area of Jiaozhou Bay. The data relating to sea area are from Dai et al. (2007), Zhou et al. (2010), and Ma et al. (2014)

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Either increase of sediment loads or decrease of sea area could result in the increase of the MARs. From the 1950s to 1980s, annual sediment discharges from seven major rivers entering Jiaozhou Bay decreased (Wang 1986; Wang and Gao 2007), indicating that the decrease of sediment loads was not the reason behind the increased MARs. However, the sea area of Jiaozhou Bay also shows an overall decrease from the 1860s to 2010s (Zheng et al. 1991; Bian et al. 2001; Wu et al. 2008; Dai et al. 2007; Zhou et al. 2010). Before 1935, the sea area showed a small decrease (from 579 to 559 km2) (Fig. 3). After 1940, the sea area decreased; especially after 1960 when it was reduced rapidly. Interestingly, the sea area appears to show asymmetric temporal patterns compared to the MARs during the past 150 years, supporting the dominant control of sea area rather than sediment loads (Fig. 3). By analyzing the variations of the MAR with the sea area, a significant inverse correlation was observed (r 2 = 0.93, p < 0.0001, Fig. 4), indicating an exponential increase of MAR instead of a linear increase accompanying the decrease in sea area. Similar relationship was also observed at C3 (Fig. 4) with a comparable coefficient of 0.010 ± 0.003 to that of 0.015 ± 0.005 at C4. These results indicated that sedimentation at C4 and C3 show similar dependence on the sea area in Jiaozhou Bay.

Fig. 4
figure 4

Relationships between sediment mass accumulation rates at station C4, C3 and the sea area of Jiaozhou Bay

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The decrease of the sea area in Jiaozhou Bay results mainly from coastal utilization directed by the government (Zhou et al. 2010; Ma et al. 2014). From 1863 to 1935, anthropogenic reclamation decreased only 1.42 km2 of sea area (Fig. 5), while, in contrast, natural processes diminished the sea area by 13.7 km2 (Ma et al. 2014). In this period, natural processes dominated the little variability in MAR. During 1935–1969, saltern development was introduced by the government, accounting for more than 80 % of the decreased sea area during this period. Correspondingly, the MAR showed a rapid increase (Fig. 3). From 1969, the economic policy surrounding Jiaozhou Bay changed into mariculture, which became the dominating factor for the decrease of sea areas during 1969–1987, following saltern development. Hence, mariculture industry, to a large extent, controlled the second increase of the MAR in Jiaozhou Bay (Fig. 3). Mariculture lasted as the main industry until the late 1990s (Liang et al. 2015). Then, tourism and the manufacturing industries quickly replaced the mariculture industry and became the pillar industries after the late 1990s (Shen 2003; Li and Jia 2006). At the same time, land reclamation became the predominant reason for the decrease of sea area as a result of population and industry expansion (Ma et al. 2014). Notably, at the later stage of each policy, i.e. 1950–1960s, 1980s, and 1990s, the MAR decreased rather than increased (Fig. 3), probably because of the end of the implementation of the old policy.

Fig. 5
figure 5

Impacts of Chinese policies on the variations of sediment mass accumulation rates in Jiaozhou Bay

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To discriminate the different speed of MAR increase induced by different sea area usage, the equation between the MAR and sea area at C4 (Fig. 4) was used to calculate the accelerating speed of MAR increase, i.e.

$$ \frac{d^2y}{d{x}^2}=42.16\times {\left(-0.015\right)}^2\times {e}^{-0.015x} $$
(1)

where y is the MAR, and x is the sea area.

Before 1935, the accelerating speed of MAR dominated by natural processes averaged 0.20 (in 10−5 mg cm−2 yr−1 km−4, and the same unit hereafter) (Fig. 5). Saltern usage, on average, resulted in an accelerating speed of 0.98 from 1935 to 1969. From 1969 to 1987 and 1987–2002, the accelerating speed averaged 2.73, mainly influenced by the mariculture industry. The highest acceleration occurred after 2002, corresponding to 4.71. Overall, the influence of saltern development was a slow process, while land reclamation in the last two decades appeared to be very fast. Such a difference was attributed to the direct sediment input during reclamation. A considerable amount of the anthropogenic addition of solid material would be re-distributed in Jiaozhou Bay with tides and circulation (Zhao et al. 2015; Yuan et al. 2016). In contrast, saltern development did not introduce a sediment load increase.

The close relationships between the MAR increase and sea area decrease in Jiaozhou Bay proved that economic policies, to a large extent, dominated the sedimentary dynamics over the past 150 years. The sediment MAR quickly increased especially in the last 30–40 years accompanying the rapid development of economy surrounding Jiaozhou Bay. The increase of MAR would enhance the siltation in the inner bay, and then weaken the circulation in Jiaozhou Bay (Yuan et al. 2016). Consequently, the exchange of nutrients and pollutants between Jiaozhou Bay and Yellow Sea could be suppressed. Thus, the MAR increase may be responsible for the enhanced eutrophication observed in Jiaozhou Bay.

Conclusions

Chinese policies on the utilization of coastal seas in order to develop the economy exerted considerable influence on coastal environments in the twentieth century, especially in the last 40–50 years. The sediments in Jiaozhou Bay archived the environmental impacts of these policies. High temporal resolution research on bay sediments would provide an objective passage for understanding the interactions between intensive anthropogenic activity and coastal environmental evolution in China or even other developing countries.