Numerical simulation of the influence of biofilm on the dynamic geomorphological evolution of tidal flats
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摘要: 河口海岸潮滩湿地是一个复杂的生态系统，其地貌的形成和演变是水动力、泥沙输移和生物过程等多种因子相互作用的结果，特别是，探究潮滩生物过程并阐明其生物−物理效应是当前海洋科学领域研究的热点和难点。本文聚焦微生物生物膜，构建了耦合生物膜与水动力、沉积物输移、地貌演变的二维生物动力地貌模型，探究了生物膜在潮滩泥沙输移和地貌演变中发挥的作用。利用文献数据验证生物动力地貌模型，模型结果与文献数据吻合较好，表明所构建的模型可以较好地模拟出生物膜的增长规律及年际变化情况。结果表明，当水动力较弱时，在有生物膜作用的潮滩上，潮沟向陆侧延伸更充分，呈现出树杈状分布，潮间带区域的潮沟两侧分布有生物膜。通过对潮沟形态进行定量分析，发现生物膜的存在促进了潮沟数量增加，并向纵深方向发展，同时限制了其宽度的增加。相较于没有生物膜影响的潮滩，潮沟的平均深度增加，总面积减小，总长度增加，平均宽度减小，总体积增加。研究结果有助于加深对生物膜在潮滩地貌塑造中的作用机制认识，为海岸带保护与生态修复工程提供科学依据。Abstract: Tidal flats maintain a complex ecosystem, while its formation is driven by multi-factor interaction, including hydrodynamics, sediment transport, and biological processes. In particular, investigating tidal flat biological processes and elucidating their biological-physical effects are current research hotspots and challenges in the field of marine science. This study focused on intertidal biofilms, constructed a two-dimensional biomorphodynamic model which coupled biofilms with hydrodynamics, sediment transport, and bed level change, to explore the role of biofilms in sediment transport and geomorphological evolution. The biomorphodynamic model was validated using literature data, indicating that the constructed model can simulate the growth pattern and interannual variation of biofilms well. Model results show that tidal creeks with biofilm attachment are more fully extended towards the landward side, showing a branching distribution when hydrodynamics are weak, and biofilms were distributed on both sides of the intertidal zone. Through quantitative analysis of tidal creek morphology, it is found that the presence of biofilms promoted an increase in the number of tidal creek and their development in the vertical direction, while limiting the increase in their width. Compared to tidal flats without the influence of biofilms, the average depth of tidal creeks increases, the total area decreases, the total length increases, the average width decreases, and the overall volume increases. The research outcome of this study deepens the understanding of the role of biofilms on tidal flat evolution and provides a scientific basis for coastal zone protection and ecological restoration projects.
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图 2 生物量与温度的年际变化（a）和黏土组分的临界起动切应力年际变化（b）
Fig. 2 Interannual changes in biomass and temperature (a) and interannual variation of critical shear stress of clay components (b)
图 3 生物量年际变化不同泥沙组分（a）；不同温度（b）；水深对光照强度的不同衰减作用（c）
Fig. 3 Interannual variation of biomass: different sediment components (a); different temperatures (b); different attenuation effects of water depth on light intensity (c)
图 5 波流作用下1年、3年和10年后潮滩地貌（a–c）和潮滩上生物量分布（d–f）
Fig. 5 The tidal flats landforms (a–c) and biomass distribution (d–f) on tidal flats after 1 year, 3 years and 10 years under wave flow, respectively
图 6 生物膜存在时的潮滩地貌演变过程（a–c）和无生物膜存在时的潮滩演变过程（d–f）
Fig. 6 The evolution process of tidal flat landform in the presence of biofilm (a–c) and the evolution process of tidal flat without biofilm (d–f)
图 7 垂直于岸线断面x = 5 km高程变化情况（a）和平行于岸线断面y = 10 km高程变化情况（b）
Fig. 7 The elevation change of the section perpendicular to the shoreline x = 5 km (a) and the elevation change of y = 10 km parallel to the shoreline section (b)
图 8 生物膜潮滩和无生物膜潮滩10年后平行于岸线断面高程变化
a. y = 10 km断面；b. y = 12 km断面
Fig. 8 Elevation changes of parallel shoreline section after 10 years for biofilm and non-biofilm tidal flat
a. y = 10 km section; b. y = 12 km section
图 9 波流作用下有无生物膜作用时潮沟发育的形态参数统计
a. 潮沟总体积；b. 潮沟总面积；c. 潮沟平均深度；d. 潮沟总长度；e. 潮沟平均宽度
Fig. 9 Statistics of morphological parameters of tidal gully development with or without biofilm under the wave
a. Total volume of channel; b. total area of channel; c. average depth of channel; d. total length of channel; e. average width of channel
表 1 参数设置及取值
Tab. 1 Parameter setting and value
变量 单位 取值范围 默认取值 含义 取值依据 μmax d–1 0.007 8～1.11 1.07 参考温度下的最大生长速率 Uehlinger等[34]
Labiod等[35]Ks（以Chl a计） (mg/m2)–1 0.016 2～0.508 0.02 半饱和常数 Uehlinger等[34]
Labiod等[35]I μE/(m2·s–1) – – 每日平均光强 Mariotti和Fagherazzi[29] KI μE/(m2·s–1) 0.1～50 25 半饱和光系数 Boulêtreau等[36] Io μE/(m2·s–1) 0～2 000 300 水面上的每日平均光强 Uehlinger等[34] kd m–1 0.1～3 1.5 光强随水深的衰减系数 Lawson等[37] β °C–1 –0.205～0.022 4 0.01 温度对生物膜发育的影响系数 Uehlinger等[34] To °C – 20 参考温度 Uehlinger等[34] Tmax °C – 33 最高温度 江苏盐城 Tmin °C – –1 最低温度 江苏盐城 ε d–1 ～(0.001～0.1)u* 0.2 整体衰减系数 Uehlinger等[34]
Labiod等[35]Xb（以Chl a计） mg/m2 4.4 × 10−5～1.68 1 最小生物量 Mariotti和Fagherazzi[29] α（以Chl a计） Pa/(mg·m−2) 0.001～0.02 0.001 随生物膜生长τcr的增长系数 Le Hir等[33] τcr,o Pa 0.05～1 0.2 无生物膜的泥沙临界起动切应力 Whitehouse等[38] MC – 0～1 0.5 某一泥沙组分下生物量变化的系数 Riechmüller等[32] mct – 0～1 0.5 底床中值粒径小于63 μm泥沙的总含量 Riechmülle等[32] Dmin m 0～0.1 0.01 生物膜生存的平均高潮下的最小深度 Mariotti和Fagherazzi[39] D m – – 河床高度与高潮位的差 根据模型设置计算取值 
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表 2 模型参数设置汇总表
Tab. 2 Summary of model parameter settings
参数项 设置值 时间步长 0.3 min 谢才系数 65 m1/2/s 水平涡黏系数 1 m2/s 水平涡流扩散系数 10 m2/s 潮流边界条件 M2，S2 粉砂 干容重 1 600 kg/m3 中值粒径 50 μm 底床厚度 5 m 黏土 沉速 0.5 mm/s 临界起动切应力 0.2 N/m2 临界沉降切应力 1 000 N/m2 冲刷系数 5 × 10−4 kg/(m2·s) 底床厚度 10 m
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表 3 水动力条件工况设置
Tab. 3 Hydrodynamic condition setting
变量 单位 取值 含义 μmax d−1 0.9 参考温度下的最大生长速率 Ks（以Chl a计） (mg/m2)−1 0.02 半饱和常数 KI μE/(m2·s−1) 25 半饱和光系数 Io μE/(m2·s−1) 300 水面上的每日平均光强 kd m−1 1.5 光强随水深的衰减系数 β °C−1 0.022 4 温度对生物膜发育的影响系数 ε d−1 0.2 整体衰减系数 Xb（以Chl a计） mg/m2 1 最小生物量 α（以Chl a计） Pa/(mg·m−2) 0.001 6 随生物膜生长τcr的增长系数 To °C 20 参考温度 Tmax °C 33 最高温度 Tmin °C −1 最低温度
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表 4 第10年潮沟形态参数
Tab. 4 Morphological parameters of tidal channel in the 10th year
形态参数 无生物膜−潮沟 生物膜−潮沟 同比相差 潮间带下部 潮间带中部 潮滩 潮间带下部 潮间带中部 潮滩 潮间带下部 潮间带中部 潮滩 总体积/（106 m3） 0.32 2.66 4.57 0.46 3.50 5.58 45.2% 31.6% 22.1% 总面积/（106 m2） 0.27 2.44 4.12 0.32 2.22 3.83 19.8% −9.0% −7.0% 平均深度/m 1.19 1.09 1.11 1.44 1.57 1.46 21.2% 44.6% 31.5% 总长度/104 m 0.21 4.13 5.30 0.24 4.18 5.42 13.5% 1.1% 2.3% 平均宽度/m 127.45 59.20 77.87 134.53 53.26 70.70 5.6% −10.0% −9.2%
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