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Despite Processing Huge Amounts of CO2 through Photosynthesis, the Total Biomass of Phytoplankton Remains Small Due to Several Factors

Phytoplankton play a pivotal role in global photosynthesis and carbon cycling, yet their total biomass remains surprisingly low. This apparent paradox can be attributed to several interrelated factors, including short life cycles, zooplankton predation, sinking and sedimentation, and decomposition in the euphotic zone. This article explores these factors in detail, providing insight into why phytoplankton biomass stays minimal despite their extensive ecological contributions.[^1][^2]

Short Life Cycles​

Phytoplankton exhibit rapid turnover rates, characterized by life cycles that often span only a few days to weeks. This rapid reproduction and subsequent death mean that, despite their high productivity, the standing biomass of phytoplankton at any given moment remains relatively low. The swift turnover ensures a dynamic ecosystem where nutrients are constantly cycled.[^3]

Predation by Zooplankton​

Zooplankton play a crucial role in curbing phytoplankton populations. A substantial portion of phytoplankton biomass is consumed by these predators, making zooplankton grazing a significant factor in limiting phytoplankton biomass. Studies estimate that approximately 50-60% of phytoplankton biomass is transferred to higher trophic levels through consumption by zooplankton and other marine herbivores. This predation not only regulates phytoplankton populations but also facilitates energy transfer throughout the marine food web.[^4]

Sinking and Sedimentation​

The biological pump is another crucial process affecting phytoplankton biomass. When phytoplankton die, they often aggregate into larger particles that sink to the deep ocean. This sinking process sequesters carbon in the ocean's interior and sediments, reducing the standing biomass of phytoplankton in the euphotic zone. Although only a small fraction of phytoplankton biomass typically sinks out of the euphotic zone, this process still plays a significant role in reducing overall biomass and in global carbon sequestration.[^5][^6][^7][^8][^9]

Decomposition in the Euphotic Zone​

Many phytoplankton die and decompose within the euphotic zone, where sunlight penetrates the ocean surface. This decomposition process, facilitated by bacteria and other microorganisms, releases nutrients and CO2 back into the water, effectively reducing phytoplankton biomass. Estimates suggest that roughly 20-30% of phytoplankton biomass undergoes microbial decomposition in the euphotic zone, further contributing to the low standing biomass of these primary producers.[^10][^11]

Conclusion​

Phytoplankton are immensely productive organisms, vital to global carbon cycling through their substantial CO2 processing via photosynthesis. Nevertheless, their total biomass remains low due to several key factors: short life cycles leading to rapid turnover, intense predation by zooplankton, the sinking and sedimentation of organic matter, and decomposition within the euphotic zone. These processes govern the dynamics of phytoplankton populations, ensuring their consistent but modest standing biomass. Despite these constraints, phytoplankton maintain a crucial role in marine ecosystems and global biogeochemical cycles, underscoring their importance in sustaining marine food webs and sequestering atmospheric carbon.

Sources​

[^1] Azam, Farooq, et al. "The Ecological Role of Water-Column Microbes in the Sea." Marine Ecology Progress Series, vol. 10, 1983, pp. 257-263. DOI:10.3354/meps010257.

[^2] Behrenfeld, Michael J., and Paul G. Falkowski. "Photosynthetic rates derived from satellite-based chlorophyll concentration." Limnology and Oceanography, vol. 42, no. 1, 1997, pp. 1-20. DOI:10.4319/lo.1997.42.1.0001.

[^3] Sohma, A., Imada, R., Nishikawa, T., & Shibuki, H. (2022). Modeling the life cycle of four types of phytoplankton and their bloom mechanisms in a benthic-pelagic coupled ecosystem. Ecological Modelling, 467, 109882. https://doi.org/10.1016/j.ecolmodel.2022.109882

[^4] Calbet, A., & Landry, M. R. (2004). Phytoplankton growth, microzooplankton grazing, and carbon cycling in marine ecosystems. Limnology and Oceanography, 49(1), 51-57. https://doi.org/10.4319/lo.2004.49.1.0051

[^5] Kiørboe, T. (2009). A mechanistic approach to plankton ecology. ASLO Web Lectures, 1(2), 1-91. Wiley Online Library.

[^6] Boyd, Philip W., and Trull, Thomas W. "Understanding the Export of Marine Biogenic Particles: Is There Consensus?" Progress in Oceanography, vol. 72, no. 4, 2007, pp. 276-312. DOI:10.1016/j.pocean.2006.10.007.

[^7] Siegel, David A., et al. "Global Assessment of Ocean Carbon Export by Combining Satellite Observations and Food-Web Models." Global Biogeochemical Cycles, vol. 28, no. 3, 2014, pp. 181-196. DOI:10.1002/2013GB004743.

[^8] Honjo, Susumu, et al. "Understanding the Role of the Biological Pump in the Global Carbon Cycle: An Imperative for Ocean Science." Oceanography, vol. 21, no. 1, 2008, pp. 46-55. DOI:10.5670/oceanog.2008.08.

[^9] Ducklow, Hugh W., et al. "Biogeochemical Provinces: Towards a JGOFS Synthesis." Deep Sea Research Part II: Topical Studies in Oceanography, vol. 48, no. 1-3, 2001, pp. 107-120. DOI:10.1016/S0967-0645(00)00035-X.

[^10] Buchan, Alison, et al. "Master Recyclers: Features and Functions of Bacteria Associated with Phytoplankton Blooms." Nature Reviews Microbiology, vol. 12, no. 10, 2014, pp. 686-698. DOI:10.1038/nrmicro3326.

[^11] Azam, Farooq, et al. "The Ecological Role of Water-Column Microbes in the Sea." Marine Ecology Progress Series, vol. 10, 1983, pp. 257-263. DOI:10.3354/meps010257.