Department of Civil and Environmental Engineering, University of California, Davis, CA, 95616, USA.
Department of Civil and Environmental Engineering, Portland State University, Portland, OR, 97207, USA.
Water Res. 2019 Sep 15;161:262-273. doi: 10.1016/j.watres.2019.05.090. Epub 2019 Jun 1.
The underlying physical and biogeochemical mechanisms associated with low dissolved oxygen (DO) levels below 5 mg L were examined through field data analyses and water quality modeling of the lower 40 miles of the Minnesota River. Insights into flow and water quality data of nineteen years (1999-2017) at five sites demonstrate that low DO levels parallel the obvious longitudinal (upstream-to-downstream) decline in phytoplankton biomass and increase in ammonium nitrogen (NH4) and dissolved orthophosphate (PO4) in the last 22-mile river reach (i.e., navigation channel) during late summer low flow conditions. River discharge is inversely related to the magnitude of the longitudinal change in DO, phytoplankton biomass, NH4 and PO4, indicating that the late summer low flow hydrodynamics in the navigation channel with a longer residence time, deeper water and slower velocity provide an extended opportunity for the biogeochemical reactions involving phytoplankton, DO and nutrients. Moreover, the ratio of the longitudinal decline in DO versus the longitudinal increase in NH4 is particularly close to the Redfield O:N ratio, suggesting that the decline in DO and increase in nutrients most likely result from the decomposition of phytoplankton detritus under aerobic conditions. This is further proved by the water quality modeling of the lower Minnesota River. The primary reasons for impaired water quality are substantially elevated sediment oxygen consumption and nutrient release derived from the decomposition of settled phytoplankton detritus in the navigation channel. Therefore, we recommend that active prevention of abrupt phytoplankton blooms and collapses through regulation of river discharge and local hydrodynamics may assist in maintaining acceptable water quality in eutrophic rivers with a high level of phytoplankton biomass.
本研究通过对明尼苏达河下游 40 英里河段的野外数据分析和水质建模，探讨了与溶解氧（DO）水平低于 5mg/L 相关的潜在物理和生物地球化学机制。对 1999 年至 2017 年 19 年的 19 个站点的流量和水质数据的深入分析表明，在夏季低流量条件下，下游 22 英里河段（即航道）中 DO 水平明显呈纵向（上游到下游）下降，浮游植物生物量降低，铵氮（NH4）和溶解正磷酸盐（PO4）增加。河川流量与 DO、浮游植物生物量、NH4 和 PO4 的纵向变化幅度呈反比关系，表明在航道中，夏季低流量条件下具有更长的停留时间、更深的水和更慢的流速，为涉及浮游植物、DO 和营养物的生物地球化学反应提供了更长的机会。此外，DO 纵向下降与 NH4 纵向增加的比值特别接近 Redfield O:N 比值，表明 DO 的下降和营养物的增加很可能是由于好氧条件下浮游植物碎屑的分解所致。这一点进一步通过对明尼苏达河下游的水质建模得到了证实。水质变差的主要原因是航道中沉降的浮游植物碎屑分解导致的沉积物耗氧和营养物释放大量增加。因此，我们建议通过调节河川流量和局部水动力来积极预防浮游植物的爆发和崩溃，这可能有助于维持富营养化河流中高浮游植物生物量的可接受水质。
