Seasonal shifts in ocean color may aid in predicting fluctuations in Earth's climate.

The results of the research, supported by a grant from the Russian Science Foundation (RSF), have been published in the journal Science of The Total Environment.

Microscopic marine plants — phytoplankton — produce oxygen and serve as food for a multitude of aquatic animals ranging from zooplankton to fish and whales. The quantity of phytoplankton determines the diversity of marine inhabitants, their population size, and biomass in a specific area of the ocean. Previously, scientists believed that in the tropics and subtropics, the reproductive cycles of phytoplankton were little affected by the seasons, as water temperature and light levels in these regions are consistently similar.

Researchers from the P.P. Shirshov Institute of Oceanology of the Russian Academy of Sciences (Moscow) utilized the Aqua MODIS satellite to study seasonal changes in chlorophyll concentration — the primary pigment in phytoplankton that gives water its green hue — in tropical and subtropical regions. Daily ocean photographs were compiled into a comprehensive map and analyzed using complex mathematical algorithms. This approach allowed for the filling of data gaps when satellite images were obstructed by clouds or other interferences, thereby generalizing information about phytoplankton reproductive cycles on a planetary scale, taking seasonal dynamics into account.

It turned out that 78% of the World Ocean's area in the studied latitudes experiences significant seasonal fluctuations in chlorophyll concentration. In these zones, peaks of blooms — periods of active microalgae reproduction — were observed, during which zooplankton could not consume the abundant food supply, resulting in uneaten plant remnants sinking to the ocean floor. These organic flows contribute to the growth of deep-sea populations. Additionally, during bloom periods, phytoplankton consumes carbon dioxide from the atmosphere, and in processing it, transfers carbon in the form of organic matter to the deeper layers of the ocean and ultimately to the seabed. This carbon becomes sequestered in sediments, leading to a reduction in the greenhouse effect on Earth.

By analyzing the seasonal cycles of phytoplankton, the authors identified four types of zones. In 22% of the areas, primarily near the equator, a year-round balance was maintained, meaning that the quantity of phytoplankton changed little throughout the year and remained in constant equilibrium with its consumers — zooplankton. Other zones experienced bloom peaks. In 4% of the ocean's area, phytoplankton reproduction spikes were linked to the influence of water flows from major rivers such as the Orinoco, Amazon, and Congo, as well as monsoons — winds that change direction depending on the season.

The influence of monsoons was observed in the northern Indian and eastern Atlantic Oceans. In other regions, chlorophyll levels varied based on the thickness of the water layer mixed by wind and waves. When this layer expanded, nutrients for phytoplankton were drawn up from deeper waters, stimulating its seasonal reproduction. In this context, 55% of the marine area saw minor peaks, while 18% were characterized by significant phytoplankton bloom surges.

The researchers published a detailed map of zones with different types of ecosystems. This classification will assist scientists in better understanding the biogeochemical and biological processes occurring in the sea, as well as in forecasting deep-sea biological resource stocks and climate change on a global scale, given that the ocean is a crucial component of the Earth's climate system. The proposed zonation could serve as a foundation for ecological initiatives aimed at mitigating the impacts of climate fluctuations and human activities on ocean life. Furthermore, the new structure will enable the identification of areas with rich deep-sea ecosystems, thereby facilitating more effective management of marine resources, such as in fisheries.

“This time we only studied the indicators from the ocean's surface, based on images obtained from satellites. We are now planning to utilize data from Argo floats that monitor chlorophyll levels, biogenic nutrients for phytoplankton, and other environmental parameters at different depths in the water column. Additionally, we plan to use data from sediment traps — devices for measuring carbon flows that passively settle into the ocean's depths. These parameters are crucial for modeling climate processes, and it is essential to verify how they relate to the zonation we proposed,” says the project leader, supported by the RSF grant, corresponding member of the Russian Academy of Sciences Alexander Vereshchaka, head of the Laboratory of Structure and Dynamics of Plankton Communities at the P.P. Shirshov Institute of Oceanology of the Russian Academy of Sciences.