Department of Earth, Ocean & Atmospheric Science, Florida State University and National High Magnetic Field Laboratory, Tallahassee, FL 32306-4100, USA.
South Florida Water Management District, West Palm Beach, FL 33406, USA.
Sci Total Environ. 2014 Jul 15;487:557-64. doi: 10.1016/j.scitotenv.2014.04.060. Epub 2014 May 8.
Quantifying and predicting the food web consequences of anthropogenic changes is difficult using traditional methods (based on gut content analysis) because natural food webs are variable and complex. Here, stable and radioactive carbon isotopes are used, in conjunction with nitrogen isotopes and mercury (Hg) concentration data, to document the effects of land-use change on food webs and Hg bioaccumulation in the Everglades - a subtropical wetland ecosystem in the US. Isotopic signatures of largemouth bass and sunfish in reference (relatively pristine) wetlands indicate reliance on the food supply of modern primary production within the wetland. In contrast, both fish in areas impacted by agricultural runoff had radiocarbon ages as old as 540 years B.P., and larger isotopic variability than counterparts in reference wetlands, reflecting differences in the food web between impacted and reference wetlands. Consistent with this difference, particulate and dissolved organic matter in impacted areas had old radiocarbon ages (>600 years B.P.), indicating that old carbon derived from historic peat deposits in the Everglades Agricultural Area was passed along the food chain to consumers. Significant radiocarbon deficiencies in largemouth bass and sunfish, relative to mosquitofish, in impacted areas most likely indicate a reduced dependence on small fish. Furthermore, largemouth bass and sunfish from impacted areas had much lower Hg contents than those from reference wetlands. Taken together, these data suggest a shift toward lower trophic levels and a possible reduction in mercury methylation in impacted wetlands. Our study provides clear evidence that hydrological modification and land-use change in the Everglades have changed the system from one driven primarily by in-situ productivity to one that is partially dependent on allochthonous carbon input from peat soils in the agricultural area and altered the Hg biogeochemical cycle in the wetlands. The results have implications for the restoration and management of wetland ecosystems.
量化和预测人为变化对食物网的影响是困难的，因为自然食物网是多变和复杂的。在这里，我们使用稳定和放射性碳同位素，结合氮同位素和汞（Hg）浓度数据，记录土地利用变化对食物网和 Hg 生物累积的影响，该研究地点为美国亚热带湿地生态系统——大沼泽地。参考湿地（相对原始）中的大口黑鲈和太阳鱼的同位素特征表明，它们依赖于湿地内现代初级生产力的食物供应。相比之下，受农业径流影响的鱼类的放射性碳年龄高达 540 年前，并且与参考湿地中的对应物相比具有更大的同位素变异性，反映了受影响和参考湿地之间食物网的差异。与这种差异一致的是，受影响地区的颗粒态和溶解态有机物具有古老的放射性碳年龄（>600 年前），这表明源自大沼泽地农业区历史泥炭沉积物的古老碳通过食物链传递给消费者。与食蚊鱼相比，受影响地区的大口黑鲈和太阳鱼的放射性碳明显不足，这很可能表明它们对小鱼的依赖减少。此外，受影响地区的大口黑鲈和太阳鱼的 Hg 含量明显低于参考湿地。综上所述，这些数据表明，受影响湿地的营养水平降低，Hg 甲基化可能减少。我们的研究提供了明确的证据，表明大沼泽地的水文变化和土地利用变化已经改变了该系统，使其从主要由原地生产力驱动的系统转变为部分依赖于农业区泥炭土壤输入的异源碳的系统，并改变了湿地的 Hg 生物地球化学循环。这些结果对湿地生态系统的恢复和管理具有重要意义。
