基于附石硅藻的三峡水库入库支流氮、磷阈值

为全面了解河流附石硅藻群落对水体氮、磷变化的响应规律,并找出氮、磷指示物种,本研究采用非参数突变点分析法(nCPA)及临界指示物种分析法(TITAN)探讨了三峡水库入库支流附石藻类叶绿素a、硅藻物种丰富度、Shannon多样性指数以及硅藻群落组成的总氮(TN)、总磷(TP)阈值.结果表明: 据nCPA得到的所有藻类参数的TP阈值均接近0.03 mg·L^-1;除硅藻群落组成外,其他参数的TN阈值也较接近(约为0.8 mg·L^-1).TITAN区分了群落中敏感种、耐受种对TN、TP变化的响应,且阈值范围涵盖了其他参数的阈值;还确定了9种TN指示种和10种TP指示种,其中TN敏感种1种,耐受种8种;TP敏感种5种,耐受种5种.因此,本研究以TITAN结果为依据确定河流的TN、TP阈值.即:当河流TN浓度低于0.382 mg·L^-1或TP浓度低于0.016 mg·L^-1时(负响应阈值),河流附石硅藻群落组成相对稳定;超过这一浓度阈值后,敏感种密度减少;当河流的TN浓度超过1.298 mg·L^-1或TP浓度超过0.065 mg·L^-1时(正响应阈值),耐受种也将受到明显影响,附石藻类的群落组成会发生显著变化.据此标准,调查样点中约87%的样点TN浓度超过TN负响应阈值,22%的样点超过正响应阈值;而超过TP负响应阈值的点占94%,超过TP正响应阈值的点占14%,说明三峡水库大部分入库河流虽受到了一定程度的干扰,但干扰程度并不严重.本研究结果可为三峡水库入库河流生态管理决策提供科学依据.

Total nitrogen and total phosphorus thresholds for epilithic diatom assemblages in inflow tributaries of the Three Gorges Reservoir,China.

To study the relationships between in-stream total nitrogen (TN) and phosphorus (TP) loads and responses of epilithic algal metrics, nonparametric change-point analysis (nCPA) and thresholds indicator taxa analysis (TITAN) were used to detect possible TN and TP thresholds for algal chlorophyll a, diatom richness, Shannon diversity index, and diatom community composition for tributary rivers of the Three Gorges Reservoir (TGR). We found that nCPA simulated similar TP thresholds for all the four algal metrics (-0.03 mg·L^-1), with similar TN thresholds (-0.8 mg·L^-1) for all metrics but community composition. In contrast, TITAN provided specific nutrient thresholds for both sensitive and tolerant taxa within the whole community, and the threshold ranges covered thresholds for the other metrics. Additionally, nine (one sensitive and eight tolerant taxa) and ten indicator taxa (five sensitive and five tolerant taxa) for TN and TP were detected by TITAN, respectively. Therefore, we used the results of TITAN to define nutrient thresholds for the study rivers. That is, when in-stream TN concentration exceeded 0.382 mg·L^-1 or TP concentration exceeded 0.016 mg·L^-1(i.e. negative response threshold), sensitive taxa within river epilithic diatom assemblages would respond with decrease in density. When TN exceeded 1.298 mg·L^-1 or TP exceeded 0.065 mg·L^-1(i.e. positive response threshold), even tolerant taxa would be influenced and the whole assemblage would be changed significantly. Based on such criteria, we found that 87% sites had TN concentrations higher than the negative response threshold, with 22% sites being higher than the TN positive response threshold. As for TP, 94% and 14% sites exceeded the negative and positive response threshold, respectively. Our findings suggest that although most of the inflow tributaries of Three Gorges Reservoir were disturbed by human disturbance, yet the disturbance was not so extensive. Our findings provided significant information for managing river ecosystems of the TGR region.