Identifying the effects of environmental factors on benthic diatom assemblages in typical tributaries of Taizi River, northeast China using the CCA method Open Access

Rivers are one of the important sources of water supply. The ecological security of rivers is crucial for sustainable water supply. Benthic diatom assemblages have received great attention in past years due to their wide distribution, variety, easy storage, and sensitivity to environmental changes (Guo et al. 2020; Pajunen et al. 2020; Dalu et al. 2022). To ensure the reliability of diatom river water quality monitoring and evaluation, it is necessary to clarify the effects of human disturbance and natural factors on the characteristics of the river benthic diatom community. Understanding the effects of environmental factors on river benthic diatom assemblages is an important basis for maintaining the health of the river ecosystem (Dalu et al. 2022).

Diatom community structure is not only affected by water quality factors, but also by altitude, geology, and other environmental factors. Some scientists argue that microscale factors, e.g. physical–chemical, are the determining factors that affect diatom assemblages (Bellinger et al. 2006; Andrén & Jarlman 2009), while other scientists have found that the large-scale factors, e.g. geomorphology, altitude, and land use, strongly affect diatom assemblages (Smucker & Vis 2011). It is important to consider the spatial component explicitly when studying assemblage–environment relationships (Bottin et al. 2014). Besides chemical variables (nutrients and metal ions), hydrological variables also show remarkable effects on diatom community variation (Sun et al. 2018). So, it is arbitrary to use diatoms to assess environmental change directly. This issue has attracted great attention, leading scientists to view the biological integrity index (IBI) as a robust indicator of the health of the river ecosystem in each ecozone (Weilhoefer & Pan 2007; Carlisle et al. 2008; Hawkins et al. 2010). As to the relationships between river benthic diatoms and environmental factors, previous research focused on a river with fewer sampling sites (O'Driscoll et al. 2012; Mirzahasanlou et al. 2020) or multiple river tributaries with sampling sites widely spaced (Urrea & Sabater 2009). So, in this study, it is hypothesized that sampling sites that are closely set may give a clear picture of the relationships between the benthic diatoms and the environmental factors in a river system.

Taizi River with multiple tributaries is the main river of northeast China. Its catchment basin is an important economic development region of China. Cities that are well developed industrially and agriculturally, such as Benxi city, Anshan city, and Liaoyang city, are incorporated into the river basin, and it has been listed as an important region to protect aquatic ecology in the future. The typical rivers located in the upper, middle and lower Taizi River tributaries were selected to intensively investigate the benthic diatom and environmental factors. The aims were: (1) investigate the benthic diatom community structure in this area, because the benthic diatom has been scarcely studied in this area; (2) explore how basin-scale factors and physical–chemical factors affect the benthic diatom community. The results will give a clear indication that multi-scale factors affect benthic diatom assemblages and will lead to the construction of an effective benthic–diatom-based monitoring tool for the river.

Benthic diatom assemblages were sampled every three kilometres from the sources to the estuary of each river. So, there were 18 sites, 15 sites, 27 sites, and 20 sites located at North Taizi, South Taizi, Xi, and Haicheng, respectively. At each site, a typical reach of almost 50 m was selected, and then three stones with a diameter sized 5–15 cm were randomly picked up in each of the upper, middle, and lower of the stream reach. So, a total of nine stones were collected in the reach. Then, a rubber ring of 3.5 cm in diameter was fixed on each stone, and sedimentation on the outside of the rubber ring was scraped and rinsed away completely. Then the rubber ring was removed and the remaining sedimentation was scraped with a knife and rinsed with algae-free water into a 500 ml plastic bottle. When all the nine circles of sedimentation were scraped and washed into the same bottle, the bottle was volumed to 400 ml with algae-free water. Then it was preserved with 4% formalin for identifying the taxa. The sample was allowed to settle for at least 24 hours and the upper liquid with no algae was discarded, then the remaining turbid liquid was volumed with water containing no algae to 100 ml, and it was agitated to get a homogenized sample. An amount of 0.1 ml of homogenized sample was drawn to a 0.1 ml of counting chamber to evaluate the total number of diatoms in the bottle at a magnification of 400×, and three replicate samples were analyzed. After that, a 20 ml homogenized sample was drawn for diatom identification. The diatom sample was concentrated by centrifuge and digested by concentrated hydrochloric acid and hydrogen peroxide, and then rinsed repeatedly with purified water until the pH was approximately neutral. Diatoms were mounted on slides with Naphrax (refractive index 1.73). The slides with diatoms were scanned using a Nikon ECLIPSE E500-type microscope with a DIC lens at 1,000× until at least 600 diatom valves were identified. Duplicate diatom slides were prepared and counted. The primary references for diatom taxonomy were Zhu & Chen (2000) and You (2006).

At each sampling site, water samples were taken in the upper region (the same reach of the river) of the benthic diatom sampling station. Water temperature, pH, dissolved oxygen (DO), conductivity (EC), and total dissolved solids (TDS) were directly measured in situ using a multi-parameter water quality monitoring instrument (YSI Incorporated, Yellow Springs, OH, USA). The sensors of the instrument were calibrated before measurement. Water samples of each site were preserved in polyethylene plastic bottles which were rinsed at least three times with distilled water, and then kept at 4 °C in insulation boxes before analysis for suspended solids (SS), chemical oxygen demand (COD_cr), ammoniacal nitrogen (NH₄⁻-N), nitrate nitrogen (NO₃⁻-N), total nitrogen (TN) and total phosphorus (TP). Water pretreatment and chemical parameter measurement were all executed according to national standard methods (State Environmental Protection Bureau 2002). Altitude was measured by portable GPS. Distance to the stream source (the distance from the sampling site to the river source) was measured by an odometer. Current velocity at 60% of the water depth and the total water depth were measured at the sampling sites by a current meter (FP101 type readable current meter).

Canonical correspondence analysis (CCA) was used to examine the distribution of the diatom assemblage along the major environmental gradients. To reduce the influence of rare species in the analysis, only those species with relative abundance greater than 1% and occurring at more than two sites were selected in the data analyses. All data were log(x + 1) transformed, expected for pH. Before CCA, the environmental factors with high partial correlation coefficients (r > 0.8) and variance inflation factors >20 were eliminated. Forward selection of environmental variables was used. At each step, only variables significantly related (p < 0.05, Monte Carlo randomization test with 999 permutations) to assemblage structure were included in the model. CCA was run using Canoco (ver. 4.5).

A total of 105 diatom taxa were identified in the four rivers. The dominant diatom assemblages were Diatoma vulgare Bory, Cocconeis placentula Ehrenberg, Gomphonema parvulum Kützing and Cocconeis placentula var. lineatalineata (Ehrenb.) Van Heurck, based on their individual occurrence frequency (as seen in Table 1). These species in the Taizi River tributaries were like those in the Xiangxi River of China (Tang et al. 2004), in which Cocconeis placentula, Achnanthes linearis, and Diatoma vulgare were listed as the dominant species, but different from those in the Gangqu River of China, in which Achnanthes linearis and Achnanthes lanceolata var. elliptica were listed as the dominant species (Wu et al. 2009). This difference may be ascribed to the highly canopied habitat of the Gangqu River (Wu et al. 2009).

The water quality and physical habitat of the four rivers are summarized in Table 2. The values of TDS and SS were high in the Xi and the Haicheng, indicating the extensive sand dredging activities in these two rivers. The value of COD was highest in the Haicheng, indicating Haicheng was the most polluted river by organic matter. As for the concentrations of nutrients, total nitrogen was high, but total phosphorus was very low throughout the four rivers.

These suggest that the diatom assemblage is a good indicator of the environment, as it is not only sensitive to physical–chemical factors but also significantly sensitive to large-scale factors (e.g., altitude, distance to the river source) in the Taizi River.

Gradient–altitude streams in the Taizi River basin have gradient land use, anthropogenic disturbances, and climate conditions. Land-use influences various physical–chemical factors that favor certain diatom assemblages (Stevenson et al. 2008; Tu 2010). Anthropogenic disturbances, e.g., disturbing the riverbed by sand dredging and water removal for land irrigation, are common practices in the Taizi River tributaries, which can affect diatom assemblages (Angeli et al. 2010). Climate also affects the growth and propagation of diatoms. Urrea & Sabater (2009) and Leira & Sabater (2005) found altitude and water temperature affect diatom assemblages significantly. Total nitrogen and total phosphorus concentrations influence diatom abundance and composition. Results by Schönfelder et al. (2002) in northeastern German lakes and rivers, and Wu et al. (2009) in Ganqu River of China, obtained similar results to this study that total nitrogen and total phosphorus were the important factors that affect diatom assemblages. Distance to river source affects hydrology and nutrition, and then affects diatom assemblages. Water depth and suspended solids can shield sunshine and then affect diatom photosynthesis. Diatoms distributed along regions of organic pollution can also be traced by COD_cr (Sládeček 1986). In this study, water velocity did not affect diatom assemblages significantly because the water velocities were relatively low at all the sampling sites (as can be seen in Table 2). The issue that diatom assemblages are not only sensitive to physical–chemical factors but also significantly sensitive to large-scale factors (such as altitude, and distance to river source) has attracted great attention from ecological scientists (Leira & Sabater 2005; Urrea & Sabater 2009). So, the environmental scientists who use diatoms to assess river ecosystem conditions should consider the importance of the natural factors that determine the aquatic biology community.

The dominant species were Diatoma vulgare Bory, Cocconeis placentula Ehrenberg, Gomphonema parvulum Kützing and Cocconeis placentula var. lineatalineata (Ehrenb.) Van Heurck in typical tributaries of the Taizi River, northeast China. Altitude, distance to river source, water temperature, TN, SS, average water depth, TP, and COD_Cr were the environmental factors that determine the community structure of diatoms. These suggest that the diatom assemblage is a good indicator of environmental change, and it is not only sensitive to physical–chemical factors but also significantly sensitive to large-scale factors (e.g., altitude, and distance to river source). So, environmental scientists who use diatoms to assess river ecosystem conditions should consider the importance of the natural factors that determine the aquatic biology community.

We sincerely thank the editors and anonymous reviewers for their outstanding comments.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.