Deep Reservoir Study Reveals the Seasonal Tug-of-War Between Cyanobacteria and Diatoms

Beneath the surface of Lushan West Reservoir, a vast deep-water basin in China’s southern mountainous Jiangxi Province, a year-long microscopic battle has been playing out. Cyanobacteria seize control in the warm, nutrient-rich months, only to cede dominance to silica-armored diatoms when winter chills the water and clarity improves. A new study published in Frontiers in Water has now mapped this seasonal tug-of-war in unprecedented detail, identifying the precise environmental switches that determine which group of algae wins and when.

The research team, led by Ling Su of the Jiangxi Academy of Eco-Environmental Sciences and Planning, monitored twenty sampling sites across the reservoir’s Jinkou and Sima areas every month from May 2022 to April 2023. Their goal was to fill a conspicuous gap in freshwater ecology: while shallow, nutrient-polluted lakes in China have been studied extensively, deep reservoirs with complex thermal layering, engineered water-level swings, and steep underwater terrain remain poorly documented. Lushan West Reservoir, with a storage capacity of 7.92 billion cubic meters, an average depth of 18.59 meters, and a maximum depth of 45 meters, offered an ideal natural laboratory.

What the team found was a community of remarkable richness and volatility. In total, 116 phytoplankton species belonging to eight phyla were identified. Green algae of the phylum Chlorophyta led in sheer species count, accounting for 43.97 percent of the catalog, followed by diatoms of Bacillariophyta at 28.45 percent and cyanobacteria at 17.24 percent. Yet species richness told only part of the story. When it came to actual abundance, cyanobacteria dominated throughout the year, making up between 48.64 and 62.26 percent of cells in spring and surging to as much as 94.72 percent in winter.

The seasonal rhythm of total cell density was equally striking. Numbers peaked at 9.98 million cells per liter in September, crashed to roughly 1.72 million cells per liter by December, and bottomed out below one million cells per liter in the early spring months of February and April. A curious dip occurred in June, when density plummeted from 7.86 million to 1.08 million cells per liter. The researchers attribute this collapse to pre-flood dam operations, which flush water through the reservoir, shorten the time phytoplankton spend in productive zones, and dilute populations before they can recover.

To untangle which environmental variables were steering these shifts, the team employed redundancy analysis, a multivariate statistical technique suited to linear species-environment relationships. The model explained 62.4 percent of the total variation in community structure, with the first two ordination axes accounting for 57.5 percent. Four factors emerged as the principal drivers: water temperature, transparency, total nitrogen, and total phosphorus. Transparency carried the strongest conditional effect, followed closely by temperature, both statistically significant at the one percent level.

The thresholds the study identified read like an ecological rulebook. When water temperature climbed above 25 degrees Celsius, total phosphorus reached 0.22 milligrams per liter, and total nitrogen hit 1.29 milligrams per liter, cyanobacterial biomass peaked at 0.42 milligrams per liter and cell density hit its annual maximum. Conversely, when temperatures fell below 13 degrees Celsius and transparency exceeded 320 centimeters, diatom relative abundance climbed to 41.80 percent. These numbers align with the reservoir’s own seasonal extremes: summer water temperatures reached 28.42 degrees Celsius, while winter lows dipped to 12.68 degrees.

One of the study’s most intriguing findings concerns a seeming paradox. Total phosphorus showed no statistically significant seasonal variation when measured in integrated water-column samples, yet cyanobacterial dominance fluctuated dramatically with the seasons. The authors resolve this by pointing to thermal stratification. In summer, a stable thermocline forms in the deep reservoir, creating oxygen-poor conditions in the bottom layer that trigger the release of phosphorus from sediments. This internally loaded phosphorus accumulates below the thermocline and cannot fully mix into surface waters, which is why whole-column measurements miss it. But bioavailable dissolved phosphorus diffuses upward into the sunlit zone, where warm temperatures supercharge cyanobacteria’s phosphorus uptake efficiency, giving them a decisive competitive edge.

Niche analysis added a competitive dimension to the picture. Using Levins’ modified niche breadth index, the researchers found that diatoms maintained relatively broad ecological niches, with values ranging from 1.75 to 2.54, reflecting their ability to exploit a wide range of conditions. Cyanobacteria, however, achieved even wider niche widths, from 1.25 to 2.78, with the filamentous species Pseudanabaena sp. topping the scale at 2.78. This broad adaptation, combined with efficient resource utilization, explains how cyanobacteria sustain year-round dominance despite seasonal environmental swings. Green algae, by contrast, occupied narrower niches between 0.40 and 1.58, limiting their competitive reach.

The study also carries a sobering management message. Pollution source census data cited in the paper reveal that urban domestic wastewater is the dominant external nutrient source, contributing 936.15 tons of total nitrogen and 51.33 tons of total phosphorus annually to the watershed. Industrial discharges add another 44.43 tons of nitrogen and 1.11 tons of phosphorus per year. The authors recommend prioritizing sewage collection and treatment upgrades, agricultural buffer strips, riparian restoration, and controls on tourism-related discharges to curb the external nutrient inputs that fuel summer cyanobacterial blooms.

For a reservoir that serves as both a drinking water source and a scenic tourism destination, the stakes are considerable. Cyanobacterial blooms can release toxins, suppress zooplankton communities, and disrupt energy flow through the food web, while diatom dominance tends to stabilize primary productivity and aid nutrient recycling through the deposition of siliceous shells. By pinpointing the temperature and nutrient thresholds at which the balance tips, this research offers water managers a quantifiable early-warning framework, not just for Lushan West Reservoir but for the many deep, regulated reservoirs scattered across the mountainous landscapes of southern China.

Subject of Research: Seasonal dynamics of phytoplankton community structure and environmental drivers in a deep freshwater reservoir

Article Title: Spatiotemporal dynamics of phytoplankton community structure in freshwater reservoir and its relationship with environmental factors

Article References: Su, L., Li, L., Ji, Y., Xiong, P., Cheng, W., Wan, F., & Liu, H. (2026). Spatiotemporal dynamics of phytoplankton community structure in freshwater reservoir and its relationship with environmental factors. Frontiers in Water, 8, Article 1906226. https://doi.org/10.3389/frwa.2026.1906226

Image Credits: AI Generated

DOI: 10.3389/frwa.2026.1906226

Keywords: phytoplankton, cyanobacteria, diatoms, freshwater reservoir, eutrophication, water quality, thermal stratification, niche analysis, redundancy analysis, total phosphorus, total nitrogen, China