Seasonal Variations in Gross and Net Primary Productivity in Upper Lake, Bhopal Using the Light-Dark Bottle Method

Abstract

This study investigates seasonal variations in Gross Primary Productivity (GPP), Net Primary Productivity (NPP), and community respiration in Upper Lake, Bhopal, using the Light–Dark Bottle method. Monthly sampling was conducted at two ecologically distinct sites, Boat Club and Khanugaon from September 2023 to August 2024. Physico-chemical parameters including temperature, pH, dissolved oxygen, nitrate, phosphate and transparency were analyzed following APHA (2017) protocols. Results indicated the highest primary productivity during summer, associated with elevated temperature and enhanced light penetration, while the lowest productivity occurred in winter. Khanugaon exhibited higher nutrient concentrations and biochemical oxygen demand, reflecting greater anthropogenic influence compared to Boat Club. Correlation analysis revealed that GPP had a strong positive relationship with dissolved oxygen and transparency, and a negative association with nitrate, phosphate and BOD. The findings suggest that Upper Lake is moderately productive but prone to nutrient enrichment and emerging eutrophication risk. Effective catchment management and nutrient load reduction strategies are essential for sustaining ecological health and fisheries potential.

Introduction

Primary productivity in freshwater ecosystems—driven by phytoplankton and aquatic plants—is a key determinant of food-web dynamics and ecosystem health. Metrics such as Gross Primary Productivity (GPP), Net Primary Productivity (NPP), and community respiration (CR) help assess energy flow and environmental impacts. Urban lakes in India, including Upper Lake in Bhopal, are particularly sensitive to seasonal fluctuations influenced by monsoon cycles, nutrient loading, water-level changes, and shoreline development.

This study aimed to quantify seasonal variations in GPP, NPP, and CR in Upper Lake using the light-dark bottle method, linking productivity to environmental parameters such as temperature, dissolved oxygen (DO), nutrients (NO??, PO?³?), and water transparency. Water samples were collected monthly from two sites—Boat Club and Khanugaon—over four seasons (Sept 2023–Aug 2024). Standard APHA methods were used to measure physico-chemical parameters, and oxygen changes in light and dark bottles were used to calculate NPP, CR, and GPP.

Key Findings:

• Physico-chemical patterns:

  • Water temperature and air temperature peaked in summer and dropped in winter.

  • pH remained slightly alkaline (7.5–8.0), favorable for phytoplankton growth.

  • DO was highest in summer; BOD and nutrient levels were higher at Khanugaon, reflecting greater organic and nutrient loading.

  • Transparency was highest in summer (~80 cm) and lowest during monsoon due to turbidity.

• Productivity trends:

  • Peak GPP occurred in summer (3.36 g C/m³/day at Khanugaon), with monsoon values lower due to reduced light penetration.

  • NPP followed a similar seasonal pattern, with slightly higher values at Boat Club in winter.

  • Respiration rates paralleled productivity, increasing in summer and monsoon.

  • NPP/GPP ratios were lower during high productivity periods, indicating more carbon was respired during these times.

The study highlights the influence of seasonal changes and site-specific conditions on Upper Lake’s productivity, linking environmental parameters to ecosystem health, potential eutrophication, and fisheries management.

Conclusion

The present study on seasonal variations in phytoplankton productivity and water quality of Upper Lake, Bhopal highlights the close interdependence between physicochemical conditions and primary productivity. Seasonal fluctuations in temperature, dissolved oxygen, and nutrient concentrations significantly influenced phytoplankton dynamics, with maximum productivity during summer and minimum in winter. Contrasts were observed between the two study sites. Boat Club exhibited comparatively higher dissolved oxygen, greater water transparency and moderate nutrient levels, reflecting better circulation and lower pollution influence. Khanugoan showed higher biochemical oxygen demand, elevated nitrate and phosphate concentrations, and reduced oxygen and transparency attributes indicating greater organic enrichment and nutrient loading from agricultural runoff. Upper Lake can be characterized as a moderately productive but nutrient-stressed ecosystem where sustained anthropogenic pressures may lead to eutrophication. Effective management strategies should focus on controlling nutrient inflow, improving sewage treatment, and conserving the lake’s catchment vegetation to maintain balanced phytoplankton productivity and safeguard the ecological health of this vital freshwater resource.

References

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Copyright

Copyright © 2025 Piyush Soni, Mukesh Dixit, Vipin Vyas. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.