Dams can bind and gag rivers
Structures such as locks, dams, barrages, and weirs are known to drastically reduce the longitudinal connectivity of rivers, impact downstream flow, and alter riverine ecosystems.
This study titled 'Impact of barrages on assemblage pattern of phytoplankton in tropical river Ganga, India' published in Environmental Monitoring Assessment authored by Jeetendra Kumar, Absar Alam, Dharm Nath Jha, Pranab Gogoi, Simanku Borah and Basanta Kumar Das informs that the construction of dams and barrages can not only lead to a decline in the river flows and ecology, but also affect river biodiversity negatively. Amount of water discharged from dams plays an important role in sustaining and managing river ecosystems through flow regulation, mixing, nutrient supply, light regimes, and volume of suspended matter.
The Ganga River basin, one of the most important river basins and the lifeline of India is regulated by more than 300 such hydraulic structures (planned, commissioned and under construction) for hydropower generation and meeting the water needs for agriculture. As a result, the upstream–downstream linkages of hydrological, geomorphological and ecological processes in the river basin have been severely impacted affecting the river health negatively.
Read this interesting interview with a researcher who has written a book on the bound Ganga
Assessing the health of rivers is essential to save them
Assessment of river health is crucial for the survival of the river and includes an integration of physical, chemical and biological factors that maintain the structure and function of the natural ecosystem of the river. It also includes the inherent ability of a river to recover after disturbance, to support local plants, animals and human populations and to maintain key processes such as sediment transport, nutrient cycling and energy exchange.
What is river health assessment. Know more here
Phytoplankton can act as bioindicators for assessing river health
Several natural plant and animal communities in the river ecosystem can be used for health assessment and aquatic environment monitoring. Phytoplankton can be greatly useful as indicators of river health.
What are phytoplankton
The term “plankton” has its roots in the Greek language, and it literally means “drifters” – referring to the many minute organisms which are carried around by water currents. Plankton comprises of both plant and animal groups referred to as phytoplankton and zooplankton respectively, and includes microscopic organisms like bacteria, some single-celled plants as well as multi-cellular animals.
Phytoplankton form the base of the aquatic food web by functioning as primary producers that also serve as food for both the tiny zooplankton and also for larger animals such as whales. What they lack in size, the plankton make up in numbers, which are crucial for maintaining the balance of freshwater ecosystem functioning.
Phytoplankton can reproduce quickly in a short period and serve as excellent indicators of ecological changes in the water besides acting as food for numerous aquatic organisms.
Phytoplankton in rivers
Phytoplankton growth depends on the availability of carbon dioxide, sunlight, and nutrients. Phytoplankton require nutrients such as nitrate, phosphate, silicate, calcium etc depending on the species. Phytoplankton growth is normally regulated in rivers because a flowing river aids in the dilution of different types of discharges and nutrients mixed in its waters.
When rivers get contaminated with unprocessed sewage, agricultural and industrial wastes and have lower flows, long residence periods and a low dilution due to decline in river flows can lead to the proliferation of suspended phytoplankton.
When availability of nutrients such as nitrogen and phosphate exceeds normal limits in the water bodies, phytoplankton such as algae and cyanobacteria can rapidly grow out of control and bloom when the water is warm and slow moving and there is adequate amount of sunlight and carbon dioxide. This process is referred to as eutrophication.
Algal blooms can occur in freshwater, marine water as well as brackish (a mixture of fresh and salt) water and can appear like foam or scum on the surface of the water and can change the appearance of water to green, blue, brown, red.
Harmful algal blooms (HABs)
Certain species of phytoplankton produce powerful biotoxins leading to harmful algal blooms that can cause damage to animals living in water. After massive blooms, dead phytoplankton sink to the ocean or lake floor. The bacteria that decompose the phytoplankton deplete the oxygen in the water, suffocating animals that live in the water resulting in a dead zone devoid of oxygen.
Algal blooms also limit light penetration, reduce growth and cause death of plants near the shores of the water bodies (littoral zones) while also affecting the ability of organisms living in the water to access food. They thus gradually disturb the ecological balance of the water body, gradually leading to deterioration of the water body.
Know more on phytoplankton and their role in sustaining the ecology of freshwater bodies such as rivers here
Phytoplankton and health of the river Ganga: the study
The paper informs that while intensive studies on seasonal changes in composition, density, and diversity of phytoplankton in river Ganga and its basins have been carried out, those on assemblage patterns and diversity of phytoplankton upstream and downstream of the barrages on river Ganga are scarce. The present study was carried out upstream and downstream of river Ganga at three barrages namely Bijnor, Narora, and Kanpur.
The main aim of this study was to assess seasonal changes and assemblage patterns of phytoplankton along with community structure and to examine the possible impact of the physico-chemical parameters on the health of river Ganga.
The study found that:
- The water quality of the Ganga has deteriorated
The natural low of the Ganga has gradually decreased from its origin to the confluence point at the Bay of Bengal due to several anthropogenic barriers such as dams, barrages, and weirs which are installed for diversion and use of river water for industries, agriculture, and domestic purpose.
The water quality of the river has also degraded over the years by tributaries carrying industrial effluents, household waste, and agricultural surface runoff from different sources that have contaminated the river water. The water quality shows deterioration from the first to the third barrages indicating obstructions in the uniform flow of riverine water and increasing residence time.
- Industrial effluents, agricultural chemicals, and pesticides have polluted the water
Chloride, specific conductivity, and total dissolved solids are low during the monsoon season due to high dilution by monsoonal water and runoff from catchment areas. Total hardness is highest in the monsoon season possibly due to incorporation of turbid water with abundant calcium and magnesium ions.
Phosphate is highest in the post-monsoon season, possibly due to the enhancement of phytoplankton richness and density. Dissolved organic matter is highest during summer season possibly due to persistent release of domestic sewage and a greater decline in the monsoon season due to dilution by monsoonal water.
- Pollution is more during the postmonsoon season as can be seen from proliferation of phytoplankton
Physicochemical parameters greatly influenced the phytoplankton assemblage pattern and diversity. Surface runoff from catchment areas during the monsoon season added nutrients to the water which gradually leached during the post-monsoon season and was utilised by phytoplankton to proliferate indicating high pollution during postmonsoon season. High diversity in phytoplankton during the post-monsoon season possibly occurred due to high phosphate in water.
Pollution of the river water was low during the monsoons that led to low abundance and diversity of phytoplankton due to the high flow of water current, and only well-adapted phytoplankton species showed their presence.
- Barrages obstruct the river flow and increase pollution
The series of barrages on river Ganga obstructed the water flow and enhanced the pollution due to effluents coming in from the tributaries. The species richness and density of phytoplankton increased from the first barrage (Bijnor) to the third barrage (Kanpur) suggesting lower river flows. The highest phytoplankton abundance upstream and downstream was found at Kanpur.
The higher abundance of phytoplankton downstream indicated lower river flows from the barrages during the summer season coupled with nutrients from catchment areas. Kanpur barrage of the river Ganga showed very high pollution levels with eutrophic conditions during summer and post-monsoon seasons due to reduced flows, higher residence time, and inflow of industrial effluents.
The study emphasises the need for regular tracking of environmental flow and ecological status to determine the impact of dams and barrages on the river in the future.
This paper is an open access article and can be downloaded from the journal site here