Shifting of phytoplankton assemblages in a regulated Chinese river basin after streamflow and water quality changes

Highlights

• Phytoplankton deterioration results in high pollution and regulation.

• Water quality factors contribute more than flow regimes indices.

• High flow indices contribute to the deterioration and low flow indices restrain it.

• Nitrogen nutrients take the most important role in phytoplankton communities.

Abstract

Phytoplankton is critical to river ecosystems. These organisms are sensitive to streamflow and water quality changes and, therefore, used to determine stability of river ecosystems, especially in regulated rivers. However, exactly how such disturbances alter spatial distribution of phytoplankton remains unclear, particularly during different seasons. A thorough understanding of these mechanisms is required to better analyze impact of environmental factors on regulated rivers. Given this, phytoplankton communities, streamflow, and water quality factors were assessed in areas sampled four times from 2015 to 2016 in upper and middle Huai River Basin. Biodiversity indices, as well as cluster and rank analyses, were used to (1) determine phytoplankton composition and distribution and (2) clarify impacts of key streamflow and water quality factors on such communities. It was found phytoplankton composition deteriorated over time, with phyla number decreasing from six to three. Moreover, proportion of Bacillariophyta increased from 51.83% to 68.13%. Phytoplankton in three regions, upstream region (Shannon-Wiener index 1.39–2.95), midstream region (0.70–4.55), and downstream region (0.22 to 2.97), were spatially clustered. The most impact factors impacting variation in composition and distribution were water quality factors and then hydrological factors. Of these, the most important factors in wet seasons were total nitrogen and maximum runoff, while ammonia nitrogen and low flow discharge were the most important factors during dry seasons. Streamflow and water quality contributed the most in midstream region, which was significantly affected by numbers of high and low flow. Contributions of these factors to downstream region were the strongest during dry seasons, which were significantly affected by numbers of low flow. Collectively, these results reveal significant impact of streamflow and water quality factors on phytoplankton deterioration in upper and middle Huai River Basin. Critically, this study provides scientific and technological support for increased biomonitoring and ecohydrological studies in regulated river basins.

Introduction

Global rivers are most affected by intense anthropogenic activity, including flow alterations by dams, a booming human population, land use changes, as well as diffuse and point pollutant sources (Hering et al., 2015; Ouyang et al., 2015, Ouyang et al., 2018). Point and non-point source pollution and transportation in the aquatic system are the main threats to water quality and are seriously affected by rainfall patterns, land use, agricultural tillage practices, and hydrological response features (Ouyang et al., 2010, Ouyang et al., 2016; Hao et al., 2013; Wei et al., 2016). Regulated rivers are those subjected to anthropogenic controls. Critically, dam construction disrupts river continuity and alters the geomorphology of river channels and their attendant hydrological processes. These changes can cause significantly disturbance to natural aquatic systems (Arthington et al., 2006). Moreover, intensive dam regulations can severely aggravate water pollution and cause additional damage to river ecosystems (Zhang et al., 2010).

Previous studies had shown 60% of the world's rivers were regulated by large reservoirs and small dams. This was particularly true in China, which had the highest number of water projects in the world (Lehner et al., 2011). As the most densely populated river basin in China, the Huai River Basin had experienced serious disturbances due to intensive human regulation and other anthropogenic activities (Zhai et al., 2017). As a result, phytoplankton communities had been damaged and even entire ecosystems degraded. Given the importance of water protection, as well as the intensifying environmental supervision of river ecosystem stability, it was important to identify the spatial and temporal changes in aquatic biological communities and key factors impacting changes in regulated rivers.

Phytoplankton played an irreplaceable role as a primary producer and in trophic interactions in water systems. These organisms, especially for Bacillariophyta, Cyanophyta and Chlorophyta, not only determined the development of ecosystems and their stability, but were also exquisitely sensitive to changes in discharge and phosphorus and nitrogen in rivers (Erga et al., 2014; Thomas et al., 2016; Zeng et al., 2017). To this end, the temporal and spatial distribution of phytoplankton communities could become rapidly altered by changes in water ecosystems. The biodiversity indices can fully describe the composition of phytoplankton and have been used in many studies to analysis the water ecosystem health (D'Alessandro et al., 2018; Smeti et al., 2019). Thus, the phytoplankton biodiversity indices are more suitable as the bio-indicator, rather than some functional groups or species, to assess water conditions and to understand the relationships between phytoplankton and streamflow and water quality factors (Fuhrman et al., 2015).

Past work had sought to identify key drivers and mechanisms underlying how these different water quality factors influence phytoplankton communities, focusing mainly on nutrients, e.g., nitrogen and phosphorus (O'Donnell et al., 2017) and dissolved oxygen (Vajravelu et al., 2017). Nutrients promoted river phytoplankton growth (Paerl and Huisman, 2008). However, an excess of nutrient discharge facilitated extreme phytoplankton overgrowth to the point of community suffocation (Dai et al., 2017). Additionally, the physical properties of water in a given river system could affect the amount and multiplication rate of phytoplankton. For instance, the temperature exceeding a certain range had a weak positive correlation with nutrients and phytoplankton (Rigosi et al., 2014). Growing water consumption led to increasing anthropogenic hydrodynamic changes. These changes imposed new restrictions on freshwater and estuarine ecosystems, impacting ecosystem structure and functioning (Roelke et al., 2013; Zhang et al., 2018b). Finally, changes to hydrological processes may also modify physical habitat conditions. Conditions were further affected by other physiochemical elements that could even affect entire water ecosystems (Richter et al., 1998; Lytle and Poff, 2004; Paerl et al., 2011). For example, water levels and residence time played roles in phytoplankton biomass (Bowes et al., 2012; Bere et al., 2016).

The significant influence exerted by hydrological and physiochemical factors on the distribution and composition of biological communities in regulated basins had been demonstrated in many previous studies. Poff et al. (1997) showed natural hydrological processes were disturbed by dams and reservoirs, which could change flow regimes and seriously interfere with the stability of river ecosystems. The impact of flow discharge on the amounts of Cyanobacteria in rivers under reservoir regulation was more obvious than the impact of water chemical factors (Grabowska and Mazur-Marzec, 2016). Cha et al. (2017) found the carrying capacity of Cyanobacteria increased along with the weir inflow level. Cao et al. (2018) underscored the serious impact of different types of pollution on phytoplankton succession, which was also significantly affected by rainfall runoff. As indicated by Yuan et al. (2018), phytoplankton communities were majorly affected by river water depth and transparency and hydrological connectivity increased with community similarity. Meanwhile, a strong relationship between nutrients and phytoplankton and the rapid response of these factors to storm runoff was present in rivers regulated by several reservoirs (Chen et al., 2018). Interspecific interactions between environmental factors and zooplankton communities significantly shaped the phytoplankton community structure in ecosystems regulated by reservoirs (Yang et al., 2018). Meanwhile, the influence of dams was reflected in changes in biodiversity indices, which also increased along the watercourse in regulated rivers (Chi et al., 2018). Despite this knowledge base, the impact of streamflow regimes and water quality factors on phytoplankton distribution and the relationship between seasonal changes, e.g., wet and dry seasons, in regulated rivers remained to be explored.

Therefore, we sought to characterize the impact of key streamflow and water quality drivers on the spatial variation of phytoplankton communities during the wet and dry seasons in regulated rivers. As an exemplar river, we chose the upper and middle Huai River Basin as our study area, as it was regulated by over 10,000 dams and sluices. Our objectives included: (1) to characterize phytoplankton community structures and reveal their temporal and spatial distribution patterns, (2) to identify decisive hydrological and physiochemical drivers in different regions during wet and dry seasons, and (3) to quantify the impact of key drivers on phytoplankton distribution. The results of this study provided a basis for water resource management and protection with consideration of the conservation of biological diversity in highly regulated rivers.