Waterways can disrupt riverine ecosystems 

How does barge trafficking/movement affect the ecology and biodiversity of riverine ecosystems? A study explains.
River Hooghly at Kolkata (Image Source: Yercaud-elango via Wikimedia Commons)
River Hooghly at Kolkata (Image Source: Yercaud-elango via Wikimedia Commons)

Rivers, as waterways

Through the ages, rivers have helped people and goods travel over long distances and in recent times, inland transport has been done using mechanised vessels to transport large cargo. India has river systems, canals, backwaters, creeks, and tidal inlets that extend to about 14,500 km that are increasingly being promoted for navigation by mechanised vessels to carry loads of cargo for trade, transport and tourism . The navigable waterways are confined to a few States and are location specific. The Inland Water Transport (IWT) is functionally important in regions covered by the Brahmaputra and the Ganges in the North-East and Eastern parts of the country, Kerala, Goa and in the deltas of the rivers of Krishna and Godavari.

The Ganga-Bhagirathi-Hooghly river system from Allahabad to Haldia port, the National Waterway No. 1 of India, is considered to be an important route through which cargo, passenger and cruise vessels are used for Inland Water Transport (IWT).

However, experts in India have raised serious questions on the dependability of waterways and the deleterious impacts that they have on river ecosystems, biodiversity and livelihoods of local communities. 

Read this report by Manthan Adhyayan Kendra to know more

How do barge movements affect river biodiversity

While waterways are exposed to various natural and anthropogenic pressures over the years, how does barge trafficking/movement affect the ecology and biodiversity of riverine ecosystems?

This paper titled 'Impact assessment of barge trafficking on phytoplankton abundance and Chl a concentration, in River Ganga, India' published in 2019 in PLOS ONE informs that barge trafficking and navigation through water ways can cause deleterious impacts on chemical and biological components of the river ecosystem along with hydrodynamic alterations, habitat destruction, changes in water quality, reduction of fish catch and loss of production of larval and adult fish.

The paper discusses the findings of a study that explores the impact of barge movements on phytoplankton in rivers.

What are phytoplankton

Phytoplankton in rivers are crucial for the river ecosystem as they are primary producers and form the basis of the ecological pyramid of the river. 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.

These floating communities are susceptible to the altered environmental conditions. Chlorophyll present in phytoplankton is a photosynthetic pigment, which plays an important role in photosynthesis, needs sunlight to produce food and is useful for the survival and the biomass of phytoplankton. 

Phytoplankton can be vulnerable to turbulence

Phytoplankton are affected by episodic turbulences that are of high intensity in water, generated by the anthropogenic stresses such as rotational movement of boat propellers and natural causes like strong winds and breaking waves.

The study

The study explored phytoplankton diversity and cellular density and the impact of barge movements in the Bhagirathi-Hooghly river system on phytoplankton. The shipping channel of the Bhagirathi-Hooghly river system was selected to study the impact of barge movements on the phytoplankton dynamics in the river.

The study area was divided into three zones and initial zones were subjectively chosen by inspecting visually the extent of disturbance activities, including commercial goods carrying vessels, tourist ships/ferry-boats, oil tankers, fishing activities, fishing boats. The final zonation was done objectively on the basis of a measurable quantity, the number of passenger ferry-boats per day.

The study found that:

  • The area showed high phytoplankton diversity with a high number of diatoms. 

Read more on diatoms here

  • Phytoplankton that experienced intermittent turbulence showed lower growth rate as compared to those exposed to continuous turbulence. 
  • The adverse impact of ‘barge movement’ on phytoplankton was evident and phytoplankton abundance and diversity was found to be much lower after barge movements than before. This was due to the effect of propeller turbulence in the barge passage. 
  • Thus, 52 types of phytoplankton were found before barge movements while those decreased to 38 after barge movements. Maximum density of phytoplankton was observed in the sample collected before barge movement followed by after and during barge movement. Phytoplankton abundance was relatively more in zone I (lower stretch) followed by zone III (upper stretch) and the zone II (middle stretch) of the river.
  • The magnitude of water turbulence induced by boat propeller was more intense than the natural flow turbulence, and led to mortality and physical stress on phytoplankton resulting in  lesser abundance of phytoplankton during and after barge movements with a 44 percent decrease in phytoplankton abundance during barge movement as compared to before. 
  • Phytoplankton cells, especially the frustules of diatoms broke down or were damaged due to high speed rotational motion and the cells were distorted into tiny parts and lost their original frustules shape.

Most phytoplankton species have a regeneration period of 2–4 days for increase of the population and this is lesser in standing aquatic ecosystems while the time needed is longer in flowing aquatic ecosystems such as rivers, which allows increase of quickly growing populations and exclusion of slow regenerating species due to horizontal flow movement. This condition can worsen with frequent barge trafficking and thus thirty minutes interval after barge passage may not regain the normal condition of phytoplankton population. 

Intensive barge movement generates turbulence in the water which causes disturbance like alluviation and loose soil formation along the shoreline of the rivers. Those loose soils are carried downstream by the river and increase the suspended sediment loads. This unconsolidated soil is also accompanied with tree roots, clumps of grass from collapsed river banks.

This eroded river bank causes low light permeability under water, and prevents or limits photosynthesis and phytoplankton development as well as micro-zooplanktonic growth. Turbulence also triggers extracellular release of phytoplankton derived organic matter and trace metals. This limiting factor damages the functionality of the primary producers which play an important role in the aquatic food chain informs the paper.

The paper identifies the need for more detailed studies to explore the impact of barge movements on phytoplankton and flowing aquatic ecosystems in the future. 

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