Phosphorus and thermotolerant coliforms’ loads in Brazilian watersheds with limited data: considerations on the integrated analysis of water quality and quantity

ABSTRACT Integrated analyses of the qualitative and quantitative aspects of aquatic ecosystems play an important role in decision making, which is linked to water resources management and promotion of ecosystem services. However, the possible advantages of such integration are still hampered by basic aspects, such as high uncertainties due to the partial and non-synchronized monitoring of discharges and observed concentrations of water quality variables. The lack of data has been limiting the adequate representation of a more accurate hydrological regime, which is in turn possibly affected by the potential effects of land use and size of drainage areas. The aim of this research was to discuss the advantages of the integrated analysis of qualitative and quantitative data from water bodies. Under different specific discharge regimes (L.s -1.km-2), we assessed the total phosphorus (TP, t.day-1 .km-2) and thermotolerant coliforms’ (TC, CFU.day-1.km -2) specific loads in rivers with different drainage areas (27-26,500 km 2) and land uses. We compared such loads with the admissible specific loads for these variables, which were estimated based on long-term average discharges and water quality guidelines (i.e., Brazilian environmental standards). Based on data from 2001-2013, we sought to contribute to the qualitative and quantitative analysis of surface waters, focusing on the discussion of the results on critical thresholds of duration and critical loads and discharges. The results showed the existence of significant time intervals with conflicts between the specific admissible and observed loads. This non-compliance varied between 10 and 100% of the duration of the regime for TP, and between 25 and 100% for TC, with a more critical situation observed in rivers with smaller drainage areas. The critical loads ranged from 1 10-4 to 8 10-3 t.day-1.km-2 (TP) and 1 10-2 to 1 10-1 CFU.day-1.km-2 (TC). Although a relatively limited dataset was considered (total N=1,402), the significant variations in discharges and critical loads highlighted the need for considering how these dynamics would affect the traditional methodology for estimating ecological flows, whose current approach is almost exclusively quantitative.

摘要 对水生生态系统（aquatic ecosystems）的定性与定量特征开展综合分析，在与水资源管理（water resources management）及生态系统服务（ecosystem services）提升相关的决策制定中发挥着重要作用。然而，此类综合分析的潜在优势仍受限于若干基础性问题，例如因流量（discharges）与水质变量（water quality variables）的监测工作不全面、不同步所带来的高度不确定性（uncertainties）。数据匮乏制约了对精准水文情势（hydrological regime）的充分表征，而水文情势本身又可能受土地利用（land use）与流域面积（drainage areas）潜在效应的影响。本研究旨在探讨水体定性与定量数据综合分析的优势。本研究针对不同流域面积（27~26500 km²）与土地利用类型的河流，在不同单位面积流量（specific discharge regimes）情景（单位：L·s⁻¹·km⁻²）下，评估了其总磷（total phosphorus, TP，单位：t·d⁻¹·km⁻²）与耐热大肠菌群（thermotolerant coliforms, TC，单位：CFU·d⁻¹·km⁻²）的单位面积负荷（specific loads）。将上述实测单位面积负荷与基于长期平均流量（long-term average discharges）及水质准则（water quality guidelines，即巴西环境标准（Brazilian environmental standards））估算得到的对应变量允许单位面积负荷（admissible specific loads）进行对比。本研究依托2001-2013年的监测数据，旨在为地表水（surface waters）的定性与定量分析提供支撑，重点围绕持续时长临界阈值（critical thresholds）、临界负荷与临界流量的相关研究结果展开讨论。研究结果显示，存在显著时段内实测单位面积负荷与允许单位面积负荷冲突的情况。总磷的违规时长占该流量情景总时长的比例介于10%~100%之间，耐热大肠菌群的该比例则介于25%~100%之间；流域面积较小的河流违规情况更为严峻。临界负荷范围分别为1×10⁻⁴~8×10⁻³ t·d⁻¹·km⁻²（总磷）与1×10⁻²~1×10⁻¹ CFU·d⁻¹·km⁻²（耐热大肠菌群）。尽管本研究采用的数据集规模相对有限（总样本量N=1402），但流量与临界负荷的显著变化凸显了需要考量上述动态如何影响传统生态流量（ecological flows）估算方法的必要性——当前该类方法几乎仅采用定量分析手段。