摘要
为了优化稳定性同位素估算消费者营养级的计算过程,降低算法和样品质量对其的影响,应用贝叶斯算法对中国水域已发表文献中的鲢(Hypophthalmichthys molitrix)、鳙(Hypophthalmichthys nobilis)营养级进行了估算和分析。结果显示:贝叶斯算法下鲢、鳙营养级与总磷(TP)、总氮(TN)2种环境因素无显著相关性。部分研究存在采样个数过少、采样频率过低,忽略了基准季节性和种内的变异性等问题,从而无法准确代表目标生物的营养特征,可能导致了营养级估计中出现异常值。贝叶斯算法与传统算法之间无显著性差异,表明其依然无法冲淡采样设计所带来的影响。曲线拟合的结果表明,维持采样数在6个以上有助于提高贝叶斯算法的计算结果的准确性。因此,在试验设计中,建议采样样品数多于6,并考虑季节与空间的差异,避免单次、少量的采集。
稳定性同位素技术因对生态系统的影响小,能很好地反映生态系统中的能量流动和物质循环,被广泛应用于记录生态系统的特征和过程,如物种的饮食结构、营养位置、资源获取和分配模式及生态位特征
另外,传统算法的估算结果往往会受到样品采集质量的影响。在试验设计中,每个样点的采样个数一般为10~20。低于这个标准可能会导致数据的离散程度增大,结果的准确性降低,同时也无法反映出消费者营养级连续、线性的变化过程。除此之外,基准营养级在时间和空间上存在变异性,最终会通过食物链作用,影响生态系统中所有营养级生物的同位素特
贝叶斯算法作为一种高效、便捷的计算方式,能在模型建立和参数后验估计中引入个体变异性和抽样误
本研究以鲢(Hypophthalmichthys molitrix)、鳙(Hypophthalmichthys nobilis)2种滤食性鱼类作为研究对象。鲢、鳙具有简短且清晰的食物链,广泛分布于我国各个流域,了解其不同区域的营养级状况对整个生态系统有重要的意义。国内外对鲢、鳙的食性、营养级都有一定程度的研究,目前主流观点是鲢的食性偏向浮游植物,而鳙更偏向浮游动物。受食物来源营养等级的影响,鲢、鳙营养级一般在2~3之间波动,且鳙略高于鲢(

图1 鲢、鳙在浮游食物网中相互作用的示意图
Fig. 1 Schematic illustration of trophic interaction in a pelagic food web dominated by silver carps and bighead carps
2019年10月利用中国知网,搜索模式设为“全文”,搜索关键词“鲢”“鳙”“稳定同位素”“食物网”“营养级”,共搜集了15个样点的鲢、鳙、基线(baseline)三类的
对搜集到的数据进行整理,表格数据直接录入Excel,图形数据经过GetData Graph Digitizer软件提取后录Excel。当基准是初级生产者时λ值应为2;当基准是初级消费者时λ值应为1。检查原文献中的计算结果,对基准类型与λ值不对应的情况进行更正。通过软件R(version 4.0.0)中tRophicPosition包对鲢、鳙以及基线生物的同位素数据进行处理,处理后生成营养级,所有数据总结于
样点 Locations | 传统算法 Traditional algorithm | 贝叶斯算法 Bayesian algorithm | 鱼样本数 Sampling number of fish | 基准类型 Baseline | 基准样本数 Sampling number ofbaseline | 总氮/(mg/L) Total nitrogen | 总磷/(mg/L) Total phosphorus | 参考文献 References | |||
---|---|---|---|---|---|---|---|---|---|---|---|
鲢Sliver carp | 鳙Bighead carp | 鲢Sliver carp | 鳙Bighead carp | 鲢Sliver carp | 鳙Bighead carp | ||||||
城陵矶 Chenglingji | 2.58 | 2.94 | 2.50 | 2.67 | 3 | 3 |
颗粒有机物 POM | 3 | 1.71 | 0.09 |
[ |
淀山湖 Dianshan Lake | 2.8 | 3.2 | 2.77 | 3.06 | 15 | 5 |
河蚬 Corbicula fluminea | 18 | 2.78 | 0.06 |
[ |
鄂州 Ezhou | 2.85 | 1.95 | 1.87 | 1.30 | 3 | 1 |
颗粒有机物 POM | 3 | 2.27 | 0.17 |
[ |
高阳 Gaoyang | 2.25 | 2.69 | 2.29 | 2.55 | 3 | 3 |
颗粒有机物 POM | 3 | 1.35 | 0.06 |
[ |
黄石 Huangshi | 3.03 | 4.56 | 2.85 | 4.53 | 1 | 1 |
颗粒有机物 POM | 6 | 1.48 | 0.07 |
[ |
湖口 Hukou | 1.59 | 2.98 | 1.63 | 2.98 | 3 | 1 |
颗粒有机物 POM | 3 | 2.04 | 0.09 |
[ |
荆州 Jingzhou | 2.48 | 3.62 | 2.04 | 3.50 | 3 | 3 |
颗粒有机物 POM | 3 | 2.27 | 0.17 |
[ |
老虎潭 Laohutan | 2.37 | 2.63 | 2.41 | 2.62 | 12 | 12 |
铜锈环棱螺 Bellamya aeruginosa | 30 | 2.75 | 0.02 |
[ |
千岛湖 Qiandao Lake | 2.5 | 2.7 | 2.47 | 2.70 | 10 | 10 |
铜锈环棱螺 Bellamya aeruginosa | 10 | 0.91 | 0.02 |
[ |
双江 Shuangjiang | 3.94 | 4.92 | 4.10 | 5.03 | 1 | 1 |
颗粒有机物 POM | 3 | 1.58 | 0.08 |
[ |
太湖 Taihu | 1.9 | 1.4 | 1.95 | 1.40 | 3 | 3 |
藻类 Algae | 3 | 2.35 | 0.19 |
[ |
万州 Wanzhou | 1.57 | 1.17 | 1.64 | 1.35 | 3 | 1 |
颗粒有机物 POM | 3 | 2.35 | 0.90 |
[ |
养鹿 Yanglu | 3.32 | 3.83 | 3.88 | 4.58 | 1 | 1 |
颗粒有机物 POM | 1 | 1.39 | 0.06 |
[ |
宜昌 Yichang | 2.98 | 3.29 | 2.89 | 3.47 | 3 | 3 |
颗粒有机物 POM | 3 | 1.67 | 0.09 |
[ |
小兴凯湖 Xiaoxingkai Lake | 3.8 | - | 3.80 | - | 4 | - |
东北田螺 Viuiparus chui Yen | 6 | 1.60 | 0.16 |
[ |
使用分位数图(quantile-quantile plot)检验用于配对样本t检验和相关分析的数据是否符合正态分布。采用配对样品t检验对传统算法和贝叶斯算法下的鲢、鳙营养级进行差异性分析;对贝叶斯算法下鲢、鳙营养级后验估计的区间大小(95% CI)分别和鱼样本数、基准样本数进行Pearson相关性分析,并计算相关系数;对贝叶斯算法下鲢、鳙的营养级后验估计的众数和水体环境中TP、TN数据进行Pearson相关性分析,并计算相关系数。选取一次函数、反比例函数、对数函数、指数函数及复合函数等作为候选模型,对贝叶斯算法下后验分布的置信区间范围与采集样本数的相关性进行拟合,使用赤池信息准则(Akaike information criterion,AIC)筛选出最优模型。
在所有的15个样点中,鲢的
传统算法下鲢的营养级范围在1.57~3.94、鳙为1.17~4.92,最小值和最大值均分别出现在万州和双江;贝叶斯算法下鲢的营养级范围在1.64~4.10,最小值、最大值出现地点与传统算法下一致,鳙的营养级范围在1.30~5.03,最小值出现在鄂州,不同于传统算法,最大值出现地(双江)与传统算法一致。
经检验用于配对样本t检验和相关分析的数据符合正态分布,传统算法下与贝叶斯算法下的鲢、鳙营养级没有显著性差异(分别为P鲢=0.501 7,P鳙=0.897 6,
物种 Species | 样点数 Sample size | t 值 t value | 自由度 df | 成对差分均值 Mean of the difference | P 值 P value |
---|---|---|---|---|---|
鲢 Sliver carp | 15 | -0.69 | 14 | -0.058 5 | 0.501 7 |
鳙 Bighead carp | 14 | -0.13 | 13 | -0.010 6 | 0.897 6 |
项目Item | 样本数 Sampling number | 营养级范围(后验95%置信区间的大小) Range of trophic position (posterior 95% CI) |
---|---|---|
自由度 df | 鱼类 Fish | 27 |
基线 Baseline | 27 | |
t 值 t value | 鱼类 Fish | -6.042 |
基线 Baseline | -3.293 | |
相关系数 Correlation coefficient | 鱼类 Fish | -0.758 |
基线 Baseline | -0.535 | |
P 值 P value | 鱼类 Fish |
1.89×1 |
基线 Baseline |
0.002 7 |
注Note:***:0<P<0.001; **:0.001<P<0.01.
项目 Item | 氮磷含量/(mg/L) Nitrogen and phosphorus content | 营养级(后验众数) Trophic position (posterior mode) | |
---|---|---|---|
鲢 Sliver carp | 鳙 Bighead carp | ||
自由度 DOF | 总磷 Total phosphorus | 13 | 12 |
总氮 Total nitrogen | 13 | 12 | |
t 值 t value | 总磷 Total phosphorus | -1.471 | -1.861 |
总氮 Total nitrogen | -1.793 | -1.685 | |
相关系数 Correlation coefficient | 总磷 Total phosphorus | -0.378 | -0.473 |
总氮 Total nitrogen | -0.445 | -0.437 | |
P 值 P value | 总磷 Total phosphorus | 0.165 | 0.087 |
总氮 Total nitrogen | 0.096 | 0.118 |
通过选用不同模型对贝叶斯算法下营养级后验分布的置信区间范围与采集样本数之间的相关关系进行拟合,发现指数函数对观测结果的解释能力最强(AIC=63.676,
模型公式 Model formula | AIC值 AIC value | a | b |
---|---|---|---|
y= a x+b | 128.993 6 | -0.619 3 | 6.689 |
y =(a/x) + b x | 70.286 4 | 8.621 6 | -0.022 |
y = a + b log x | 91.218 7 | 7.751 4 | -3.472 |
y = exp (1) ^ (a + b x) | 63.676 0 | 2.612 9 | -0.481 |
y = a / x + b | 70.944 7 | 8.505 5 | 0.042 |

图2 贝叶斯算法中营养级95%置信区间的大小随鱼样本数变化的模拟曲线
Fig. 2 Simulation curve of correlation between the size of 95% confidence interval of trophic position in Bayesian algorithm and sampling number of fish
在2016年武汉东湖的一项试验中显示,主要摄食浮游生物的鲢、鳙营养级的准确范围应该在2~3之
传统方法计算的鲢、鳙营养级误差较大,而使用贝叶斯算法计算的鲢、鳙营养级与传统方法比较前后无明显差异,说明贝叶斯算法无法提升传统计算方法的结果,或者说即使是利用贝叶斯算法也无法冲淡试验设计对研究结果的影响。
本研究中,贝叶斯算法的置信区间与鱼类和基线的采集样本数目均具有显著相关性,说明采样的强度确实会影响后续分析的结果。在本研究所涉及的15处样点中,除老虎潭、千岛湖、淀山湖外,其他样点采样数量总体偏少,多数为单次采样、无周期性采样,个别样点采样数量为1,如双江、养鹿、黄石,这可能导致了在鲢、鳙营养级估计上的偏差。另外在少数位点,如鄂州、养鹿等,2种算法获得的结果有较大差异。这可能是因为较低的样本量使得传统算法中所依赖的均值和贝叶斯算法所拟合的样本分布之间出现了偏差。而曲线拟合的结果表明,维持采样数在6个以上有助于提高贝叶斯算法的计算结果的准确性。
另外,有10处样点使用了POM进行标定。POM作为一种高周转率的基准,短期或者某一时间点的样品采集分析无法反映出基准作
贝叶斯算法下鲢、鳙营养级与环境因子TN、TP存在相关关系,这说明鲢、鳙所处的水体环境对其营养级具有一定程度的影响。Xu
综上所述,稳定同位素技术为研究生态系统各种群之间的能量流动、营养关系提供了新的技术手段,但是该方法的应用还有赖于科学的试验设计和对同位素组成的时间、空间、种内的全面理解,从其结果和采样个数来看,原文献的数据还不能很好地代表目标生物的营养特征,使鲢、鳙的营养级计算结果出现了较大的偏差。为展示研究对象连续、完整、准确的营养特征,今后可展开多月份连续、适当数量的采样,同时还要考虑样点的水体环境的影响。
采样之前,要明确研究整体所代表的时间尺度,基于不同的时间尺度,分析系统中、短期和长期的结构与功能变化也要依据不同检测手段,进行相互补充、相互优
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