热带海洋学报, 2020, 39(5): 109-116 doi: 10.11978/2019118

海洋环境保护

南海与周边海域表层塑料颗粒交换的拉格朗日示踪研究

孟钊,1,3, 李宁5, 管玉平,1,2,4, 冯洋1,2

1.热带海洋环境国家重点实验室(中国科学院南海海洋研究所), 广东 广州 510301

2.南方海洋科学与工程广东省实验室(广州), 广东 广州 511458

3.中国科学院大学资源与环境学院, 北京 100049

4.中国科学院大学地球与行星科学学院, 北京 100049

5.澳门科技大学资讯科技学院, 澳门特别行政区 999078

Exchanges of surface plastic particles in the South China Sea through straits using Lagrangian method

MENG Zhao,1,3, LI Ning5, GUAN Yuping,1,2,4, FENG Yang1,2

1. State Key Laboratory of Tropical Oceanography (South China Sea Institute of Oceanology, Chinese Academy of Sciences), Guangzhou 510301, China

2. Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou 511458, China

3. College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China

4. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

5. Faculty of information Technology, Macau University of Science and Technology, Macao Special Administrative Region 999078, China

通讯作者: 管玉平。E-mail: guan@scsio.ac.cn

责任编辑: 殷波

收稿日期: 2019-11-20   修回日期: 2020-02-24   网络出版日期: 2020-09-10

基金资助: 国家自然科学基金项目.  41676021
广东省实验室重大专项.  GML2019ZD0306
广东省实验室重大专项.  GML2019ZD0303

Corresponding authors: GUAN Yuping. E-mail: guan@scsio.ac.cn

Editor: YIN Bo

Received: 2019-11-20   Revised: 2020-02-24   Online: 2020-09-10

Fund supported: National Natural Science Foundation of China.  41676021
Major Project of Guangdong Province Laboratory.  GML2019ZD0306
Major Project of Guangdong Province Laboratory.  GML2019ZD0303

作者简介 About authors

孟钊(1994—), 男, 山东省菏泽市人, 硕士研究生, 主要从事海洋环境和可视化研究。E-mail: mengzhao@scsio.ac.cn

摘要

塑料污染已成为国际海洋界关注的海洋环境问题之一。文章探讨海洋环流对南海及其周边海域表层塑料颗粒交换的影响。在南海周边多个海域, 分别在4个季节投放塑料颗粒。一年后, 用拉格朗日颗粒示踪方法考察投放颗粒的运动轨迹和最终停留位置。结果表明, 在秋、冬季, 大部分塑料颗粒会进入南海和爪哇海, 极少部分颗粒北输送到太平洋; 在春、夏季, 仅有部分颗粒进入南海和爪哇海, 而多数颗粒流到太平洋。南海洋流具有季节特征, 塑料颗粒轨迹特征与之较为符合。

关键词: 塑料 ; 环流 ; 季节变化 ; 南海

Abstract

Plastics have caused serious pollution in the ocean, and become one of the marine environmental issues of concern to the international marine community. The South China Sea is a major area of marine plastic pollution. The areas around the South China Sea are major sources of plastic pollution, and previous studies have focused on the offshore environment. In this study, we discuss the exchange of plastic particles in the surface of the South China Sea and surrounding waters by ocean circulations. First, the particles were placed at the junction of the South China Sea and its adjacent seas, and were released at the initial time of every season Using Lagrangian particle tracing method, we analyzed the trajectories and final locations of the particles within one year after release. The results show that plastic particles enter the South China Sea mainly through the southern strait. The transport of particles at the same starting point varies a lot with season. During autumn and winter, most particles will enter the South China Sea and Java Sea, and little particles will be transported to the Pacific Ocean. However, only a few particles enter the South China Sea and Java Sea during spring and summer, while most of them are transported to the Pacific Ocean .The South China Sea current is mainly affected by monsoon and has significant seasonal characteristics. This research will help understand the impact of surface plastics around the South China Sea on the plastic pollutant in the South China Sea.

Keywords: plastic ; circulation ; seasonal change ; South China Sea

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本文引用格式

孟钊, 李宁, 管玉平, 冯洋. 南海与周边海域表层塑料颗粒交换的拉格朗日示踪研究. 热带海洋学报[J], 2020, 39(5): 109-116 doi:10.11978/2019118

MENG Zhao, LI Ning, GUAN Yuping, FENG Yang. Exchanges of surface plastic particles in the South China Sea through straits using Lagrangian method. JOURNAL OF TROPICAL OCEANOGRAPHY[J], 2020, 39(5): 109-116 doi:10.11978/2019118

塑料在当今世界中扮演着重要的角色(Cózar et al, 2014), 由于在环境中难于自然降解, 也导致了严峻的污染, 塑料废物污染已成为世界环境难题, 受到广泛关注。截止到2015年, 人类已经产生了63亿吨的塑料垃圾(Geyer et al, 2017)。塑料通过河流等途径进入海洋, 由海流长距离输送进入大洋和深海海底(Eriksen et al, 2014; Chiba et al, 2018)。在2010年进入海洋的塑料垃圾有480万至1270万吨, 预计到2025年, 从陆地进入海洋的废弃塑料累积量将增加一个数量级(Jambeck et al, 2015)。

全球公海的表层塑料主要汇聚在5个亚热带环流的辐合带中。这些聚集带主要是由热带东风(约 0—30°N)和中纬度西风(约30°—60°N)引起的Ekman迁移所致, 它们将地表水和漂浮碎片输送到位于南北半球约 30°N的海洋盆地中心(Maximenko et al, 2012; Van Sebille et al, 2012)。另有研究表明, 北太平洋西部的黑潮海域、大洋洋底、偏远无人居住的岛屿(Yamashita et al, 2007; Lavers et al, 2017)以及南北极都已经出现了塑料的身影(Hand, 2014; Waller et al, 2017), 甚至在北冰洋区域, 新的塑料聚集区已经形成(Cózar et al, 2017)。来源多样的海洋塑料废弃物随洋流进行环球输运, 带来了海洋视觉污染、海洋生物、航道安全等问题(Bjorndal et al, 1994; Lamb et al, 2018; Savoca et al, 2016)。

南海位于亚洲大陆, 通过巴士海峡、马六甲海峡、苏禄海等与大洋相通, 珠江、红河和湄公河等水系为南海带来充沛的陆源物质。南海环流受多种因素影响, 季风更迭使南海环流总在不断演变之中, 冬季是西北季风, 夏季是东南季风, 而黑潮经吕宋海峡的作用则使南海环流更加复杂(Hu et al, 2000)。南海拥有宽广的珊瑚礁水域, 经济鱼类众多, 保护南海的生态环境刻不容缓。国外有研究指出中国是废弃塑料产量大国, 南海周边的入海河口和沿岸水域塑料含量较高(Lebreton et al, 2017; Hurley et al, 2018)。南海与外部水域相通, 本文基于拉格朗日方法, 对南海周边海洋表层塑料颗粒随表层洋流的长期输运进行示踪分析, 可以了解到达南海周边的海洋表层塑料废物是否会进入南海, 以及由于南海表层流受季风控制, 那么南海与周边海域对表层塑料颗粒交换的季节特征是什么, 为进行南海区域海洋塑料污染治理提供理论基础。

1 数据和方法

1.1 海表洋流数据

本文采用的海表洋流数据来自亚太数据研究中心(Asia-Pacific Data-Research Center, APDRC), 由日本地球模拟器模拟计算得到的长时间序列的涡解高精度海洋模式(OGCM for the Earth Simulator, OFES)数据(Masumoto et al, 2004; Sasai et al, 2004; Sasaki et al, 2004, 2008)。我们选取1999—2001年, 水深2.5m, 空间尺度为0.1°×0.1°的全球表层洋流流速月平均数据。

1.2 方法

1.2.1 结构

本文搭建了一个拉格朗日颗粒追踪系统, 包括数据读取、轨迹计算、图形显示3个部分。颗粒在选定区域内, 颗粒位置随机分布, 颗粒属性初始化; 根据拉格朗日颗粒追踪方法, 生成颗粒轨迹; 在颗粒起始位置和终止位置处分别赋予不同的标记。

基于月平均的全球欧氏网格速度场, 每个颗粒的位置对应着一个由时间和颗粒空间位置决定的矢量速度U(x,t), 其中x是位置, t是时间。每个被放置的颗粒都有自己的运动轨迹${{x}_{\text{path}}}\text{(}t\text{,}\ {{x}_{\text{0}}}\text{,}\ {{t}_{\text{0}}}\text{)}$, 即基于给定的初始条件: t0时刻, x0位置, 轨迹曲线随着参数发生变化。颗粒的轨迹${{x}_{\text{path}}}\text{(}t\text{,}\ {{x}_{\text{0}}}\text{,}\ {{t}_{\text{0}}}\text{)}$是由常微分方程控制的:

$\frac{\text{d}{{x}_{\text{path}}}\text{(}t\text{,}\ {{x}_{\text{0}}}\text{,}\ {{t}_{\text{0}}}\text{)}}{\text{d}t}\text{=}U\text{ }\!\![\!\!\text{ }{{x}_{\text{path}}}\text{(}t\text{,}\ {{x}_{\text{0}}}\text{,}\ {{t}_{\text{0}}}\text{),}\ t\text{ }\!\!]\!\!\text{ }$

我们在南海的周边(与周边海域或海峡的交接处), 分别放置一定数量的颗粒, 用颗粒来模拟海洋表层塑料颗粒, 并将颗粒在其后一年的运动轨迹和一年后的最终停留位置进行分析。同时我们假定颗粒的运动只受表层洋流的作用, 不考虑下降流、上升流等作用, 只探讨表层塑料颗粒水平方向输运规律, 并且用初始月份是3月、6月、9月和12月分别代表春夏秋冬4个季节的起始时间。我们共选择8个代表性的交界处, 分别是台湾海峡(119°36′—119°42′E, 24°15′— 24°21′N), 吕宋海峡(120°58′12″—121°4′12″E, 20°35′24″— 20°41′24″N), 南海与苏禄海的交界处一(119°52′12″—119°58′12″E, 12°52′12″—12°58′12″N), 交界处二(116°28′48″—116°34′48″E, 7°12′—7°18′N), 加里曼丹岛沿岸(109°56′24″—110°2′24″E, 3°51′— 3°57′N), 卡里马塔海峡(108°18′—108°24′E, 2°0′36″— 2°6′36″S), 马六甲海峡(104°51′—104°57′E, 0°21′36″— 0°27′36″N)和印度支那半岛东部沿岸(109°7′48″— 109°13′48″E, 9°27′— 9°33′N)。

1.2.2 颗粒控制

颗粒主要具有如下属性: 位置值, 即颗粒所处的经纬度; 年龄值, 即颗粒已经经历过的帧数值; 最大年龄值, 即颗粒可以存在的最大帧数值; 月帧数, 即表示一个月时间的帧数值。

在选定区域内, 随机投放多个颗粒, 计算颗粒初始属性值。文中基于拉格朗日方法计算颗粒的运动轨迹, 在计算的过程中, 用于速度采样的流场数据随时间变化, 时间跨度根据所选数据集确定。本文中用于速度采样的流场数据变化的时间跨度为一个月。

颗粒所对应的速度坐标(px, py)的计算公式为:

$({{p}_{x}},{{p}_{y}})=[\frac{({{S}_{y}}-la{{t}_{1}})}{la{{t}_{2}}-la{{t}_{1}}}\times {{n}_{\text{line}}},\frac{({{S}_{x}}-lo{{n}_{1}})}{lo{{n}_{2}}-lo{{n}_{1}}}\times {{n}_{\text{column}}}]$

式中: Sx为颗粒位置的经度; Sy为颗粒位置的纬度; (nline, ncolumn)为洋流速度场原始数据的行列数; (lon1~lon2), (lat1~lat2)为流场速度原始数据可表示的经纬度范围。

本文中坐标(px, py)所对应的速度U(x,t)的计算采用双线性插值方法。颗粒的年龄属性值随着帧数的增加而增大。海洋塑料颗粒随着洋流运动到陆地近海区域时, 若比较靠近海岸, 我们假定近岸颗粒会停滞在沿岸海域, 直到生命值达到最大值, 然后被初始化。

1.2.3 动态示踪轨迹生成

海洋表层塑料颗粒在随海流运动的过程中, 不会因为在某一时刻、某一位置的速度为零值而消失, 所以记录每一帧所有颗粒的相关属性值。年龄值达到最大的颗粒(即非活性颗粒)不会被记录位置。

当颗粒从时间ti运动到ti+1, 为了确定ti+1时刻的位置, 需要从titi+1进行积分。由生成${{x}_{\text{path}}}(s,{{x}_{0}},{{t}_{0}})$的常微分方程(1)得到迹线方程:

${{x}_{\text{path}}}(t,{{x}_{0}},{{t}_{0}})={{x}_{0}}+\underset{{{t}_{0}}}{\overset{t}{\mathop \int }}\,u[{{x}_{\text{path}}}(s,{{x}_{0}},{{t}_{0}}),s]\text{d}s$

2 结果与分析

春季, 在台湾海峡中部投放的颗粒在接下来的一年时间内大部分在台湾海峡内运动, 并且一年后仍停留在亚洲大陆东南沿岸, 只有极少部分颗粒随着黑潮流动, 向北流动并最终停留在日本岛南部(图1a)。在吕宋海峡投放的颗粒进入东海, 随着黑潮进行输送, 向东北方向输送至日本岛南岸后向东输送, 最远输送到北太平洋中部。颗粒的运动轨迹很好地表现了黑潮延伸体的运动轨迹和范围。其中大部分颗粒最终停留在北太平洋, 极小部分颗粒由于地形因素进入了日本海并停留在日本岛西南沿岸(图1b)。由于地形的因素, 在南海与苏禄海交界处一投放的大部分颗粒向北经由苏禄海进入南海, 并在南海内进行输送, 小部分颗粒经由亚洲大陆沿岸向南输送, 最远进入爪哇海, 颗粒停留在菲律宾群岛沿岸、台湾岛沿岸、印度支那半岛东部沿岸和中南半岛最南端。小部分颗粒沿台湾岛东岸向东北方向输送, 随着黑潮输送至北太平洋中部并停留(图1c)。在南海与苏禄海交界处二投放的颗粒, 大部分会停留在海南岛沿岸、菲律宾群岛沿岸和台湾岛沿岸, 少部分会随黑潮进入太平洋(图1d)。在加里曼丹岛沿岸投放的颗粒, 一部分停留在加里曼丹岛沿岸、南海和爪哇海内部, 一部分穿过卡里马塔海峡进入爪哇海, 并在沿岸停留(图1e)。在卡里马塔海峡投放的颗粒直接进入爪哇海, 并停靠在印度尼西亚群岛沿岸(图1f)。在马六甲海峡投放的颗粒, 大部分停留在马六甲海峡附近, 极少部分进入南海, 并停留在印度支那半岛东部沿岸(图1g)。在印度支那半岛东部沿岸投放的颗粒, 则会随着南海内部向东北方向的流在南海内部进行输送并停留扩散至整个南海, 部分颗粒进入泰国湾, 部分颗粒向南进入爪哇海, 并在沿岸停留(图1h)。

图1

图1   春季颗粒输运

a. 台湾海峡; b. 吕宋海峡; c. 南海与苏禄海交界处一; d. 南海与苏禄海交界处二; e. 加里曼丹岛沿岸; f. 卡里马塔海峡; g. 马六甲海峡; h. 中南半岛沿岸。红色五角星代表颗粒的初始位置点, 蓝色线代表颗粒运动轨迹, 黑色三角形代表颗粒的最终停留点。依据审图号GS(2016)1667底图制作

Fig. 1   Particle transport in spring


夏季, 在台湾海峡投放的颗粒, 向北进入东海和黄海, 部分颗粒停留在黄海和东海, 部分颗粒在日本岛的作用下进入日本海并最终停留在朝鲜半岛和日本岛沿岸, 小部分颗粒向东北方向输送至日本岛东部沿海(图2a)。在吕宋海峡投放的颗粒, 向北输送至台湾岛南岸并停留(图2b)。在南海与苏禄海交界处一投放的颗粒, 由于复杂的地形因素, 颗粒向北停留在菲律宾群岛沿岸(图2c)。在南海与苏禄海交界处二投放的颗粒, 向南进入西里伯斯湾后, 部分颗粒进入望加锡海峡, 也有部分颗粒向东进入菲律宾海。大部分颗粒最终停留在苏禄海和西里伯斯湾之间的岛屿附近以及望加锡海峡沿岸, 小部分颗粒停留在菲律宾海(图2d)。在加里曼丹岛沿岸投放的颗粒, 向南输送并停留在加里曼丹岛沿岸, 不再扩散(图2e)。投放在卡里马塔海峡和马六甲海峡的颗粒, 两者的输运轨迹相似, 一部分颗粒向北进入南海, 在南海内部输运并停留在南海和加里曼丹岛沿岸; 一部分颗粒向南输送, 进入爪哇海并在沿岸停留(图2f, 2g)。在印度支那半岛东部沿岸投放的颗粒, 部分颗粒在南海内部向北输运并停留,大部分颗粒受地形影响, 会停留在巴拉望岛沿岸(图2h)。

图2

图2   夏季颗粒输运

a. 台湾海峡; b. 吕宋海峡; c. 南海与苏禄海交界处一; d. 南海与苏禄海交界处二; e. 加里曼丹岛沿岸; f. 卡里马塔海峡; g. 马六甲海峡; h. 中南半岛沿岸。红色五角星代表颗粒的初始位置点, 蓝色线代表颗粒运动轨迹, 黑色三角形代表颗粒的最终停留点。依据审图号GS(2016)1667底图制作

Fig. 2   Particle transport in summer


秋季, 在台湾海峡投放的颗粒, 一部分向北沿岸线输运并停留, 小部分颗粒进入东海, 向东北方向输送, 最终停留在日本岛南岸。另一部分颗粒则向南沿岸线输送, 在沿岸停留, 最远到达亚洲大陆南部沿岸(图3a)。在吕宋海峡投放的颗粒, 向北输送至台湾岛南岸后停留(图3b)。在南海与苏禄海交界处一和交界处二处投放的颗粒, 在秋季的输送相似, 只在投放点附近输送并停留在投放点周边(图3c, 3d)。在加里曼丹岛沿岸投放的颗粒, 也并未向外输送, 颗粒最终停留在投放点沿岸(图3e)。在卡里马塔海峡处投放的颗粒, 向南进入爪哇海并停留在印度尼西亚群岛沿岸(图3f)。在马六甲海峡投放的颗粒分别向南向北输送, 向北最远输送至泰国湾与南海交界处; 向南进入爪哇海, 并且大部分颗粒最终停留在印度尼西亚群岛沿岸(图3g)。投放在印度支那半岛东部沿岸的颗粒, 在整个南海内部进行输送并在海域内部和南海沿岸停留, 部分颗粒向南进入爪哇海并停留(图3h)。

图3

图3   秋季颗粒输运

a. 台湾海峡; b. 吕宋海峡; c 南海与苏禄海交界处一; d. 南海与苏禄海交界处二; e. 加里曼丹岛沿岸; f. 卡里马塔海峡; g. 马六甲海峡; h. 中南半岛沿岸。红色五角星代表颗粒的初始位置点, 蓝色线代表颗粒运动轨迹, 黑色三角形代表颗粒的最终停留点。依据审图号GS(2016)1667底图制作

Fig. 3   Particle transport in autumn


冬季, 投放在台湾海峡的颗粒与其他3个季节不同, 颗粒并不向北输送, 而是只向南输送并停留在亚洲大陆东南沿岸(图4a)。投放在吕宋海峡的颗粒只向南输送进入南海, 最终大部分颗粒停留在印度支那半岛东部沿岸(图2b)。在南海与苏禄海交界处一和交界处二投放的颗粒, 颗粒只在投放点周边运动并停留(图2c, 2d)。投放在加里曼丹岛岛屿沿岸的颗粒则比较活跃, 在南海内部和泰国湾进行输送, 并有相当部分的颗粒在海域内部和沿岸停留。小部分颗粒向北穿过台湾海峡进入东海, 一部分停留在东海和日本海交界处, 一部分则继续向东北方向输送至太平洋(图4e)。投放在卡里马塔海峡和马六甲海峡的颗粒, 输送轨迹相似, 都是向南进入爪哇海, 并最终停留在印度尼西亚群岛沿岸(图4f, 4g)。投放在印度支那半岛东部沿岸的颗粒, 同样也是沿着岸线向南输送, 并最终停留在马来西亚群岛沿岸(图4h)。

图4

图4   冬季颗粒输运

a: 台湾海峡; b: 吕宋海峡; c: 南海与苏禄海交界处一; d: 南海与苏禄海交界处二; e: 加里曼丹岛沿岸; f: 卡里马塔海峡; g: 马六甲海峡; h: 中南半岛沿岸。红色五角星代表颗粒的初始位置点, 蓝色线代表颗粒运动轨迹, 黑色三角形代表颗粒的最终停留点。依据审图号GS(2016)1667底图制作

Fig. 4   Particle transport in winter


3 结果与讨论

基于OFES表层流场数据, 本文对南海表层塑料颗粒的水平运动进行拉格朗日示踪, 探究在仅受海洋表层环流影响下, 塑料颗粒的输运情况。分析结果表明, 进入南海的颗粒主要是来自南部海峡(如加里曼丹岛沿岸)。在不同的季节, 同一个投放点颗粒的输送情况大不相同。在秋、冬两季, 大部分颗粒会进入南海, 但是春、夏两季,投放的颗粒大部分会向南海外输送, 这一情况和南海的表层环流息息相关。在秋季和冬季投放的颗粒几乎都会进入南海和爪哇海; 春季投放在南海东北方向海域(如台湾海峡、吕宋海峡等)的颗粒会进入太平洋, 其他投放点的颗粒会进入南海与爪哇海; 夏季在台湾海峡投放的多数颗粒会进入东海甚至日本海, 在其他投放点投放的颗粒多会进入南海和爪哇海。这一结果表明, 南海的表层塑料不仅仅来自于陆地, 也有相当一部分塑料是由外界海域输入南海。

图1~图4图5进行对比发现, 在指定位置投放的塑料颗粒主要受到表层流场的作用, 颗粒的轨迹与平均流场符合较一致。

图5

图5   春季(a)、夏季(b)、秋季(c)、冬季(d)的平均流场(数据来源: 地球模拟器模拟的海洋模式数据OFES)

依据审图号GS(2016)1667底图制作

Fig. 5   Mean currents in spring (a), summer (b), autumn (c), and winter (d) (Data Source: OGCM for the Earth Simulator, OFES)


文章探讨了不同季节南海与周边海域交汇处的表层塑料垃圾的输送轨迹和最终停留位置, 利用OFES模式数据和拉格朗日法, 我们可以对只受表层环流影响的海洋表层塑料进行定点定时追踪, 这对海洋表层塑料垃圾的扩散研究具有启发意义。

尽管本文较为清晰地给出了不同季节到达南海与周边海域交汇处的表层塑料垃圾随表层洋流的输运趋势, 但仍然存在一些不足之处。首先, 本文仅采用了一种数据。一方面是因为全球范围的高精度的数据较少, 另一方面不同的数据之间由于采用的同化或建模的方法不同, 也会导致结果出现一些差别, 但是这种差别对总体趋势的呈现没有太大影响。另外, OFES数据本身存在一定的误差, 但是这个问题超出了我们现在讨论的范围。这些原因都可能带来一定的误差, 我们将在接下来的工作中能够更详细的探讨这些问题。

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