Bloom of green Noctiluca in Northern Arabian Sea may be harbinger of climate-driven change | POGO

Bloom of green Noctiluca in Northern Arabian Sea may be harbinger of climate-driven change

The Northern Arabian Sea experiences intense blooms of the heterotrophic dinoflagellate, green Noctiluca scintillans, during the winter monsoon. Although not known to produce organic toxins, these blooms are still categorized as a harmful due to due to their association with massive fish mortalities attributed to toxic levels of ammonia. In addition, green Noctiluca is a voracious predator of diatoms, and thus competes with zooplankton, having adverse impacts on the food chain and fisheries. The decay phase of green Noctiluca blooms can result in the decline of dissolved oxygen levels in nearshore subsurface regions, leading to fish mortality and evasion in the region of the blooms.

 

Recent work has attributed these blooms to the vertical expansion of the oxygen minimum zone, driven by cultural eutrophication from major coastal cities in western India. However, this link is not supported by results of a new joint Indo-US study that was recently published in the journal Harmful Algae, titled “Characterization of oceanic Noctiluca blooms not associated with hypoxia in the Northeastern Arabian Sea”. This collaborative study between scientists from Indian National Centre for Ocean Information Services (INCOIS) under the Ministry of Earth Sciences (MoES), Govt. of India and National Oceanic and Atmospheric Administration (NOAA), National Marine Fisheries Service (NMFS) under the U.S. Dept. of Commerce, resulted from the Memorandum of Understanding, ‘Development of Predictive Capabilities for Marine Fisheries and Harmful Algal Blooms in Indian Seas'. Chlorophyll sensors on satellites were used to observe the distribution of both diatoms and harmful Noctiluca. In addition, sea conditions, nutrients, and oxygen concentrations were studied using both sensors attached to special free-floating, self-propelling Argo floats and water samples collected from research vessels.

 

Near surface, oceanic waters in the northeastern Arabian Sea were observed to have sufficient oxygen, clearly refuting any linkage between low oxygen and Noctiluca abundance and success. There was no evidence that cultural eutrophication from major cities along the west coast of India has contributed to the decadal scale shifts in plankton composition in the Northeastern Arabian Sea oceanic waters. Conversely, climatic warming of surface waters is thought to have intensified stratification, thereby reducing net nutrient flux to the photic zone and decreasing silicate to nitrate ratios (Si:N); thereby increasing the competitive advantage of green Noctiluca over diatoms. If so, the decadal-scale trajectory of phytoplankton community composition in the Northeastern Arabian Sea may be a harbinger of climate-driven change and thus may be expected to disrupt the food chain by causing a decline in regional fisheries yields in the future.

 

Citation: Lotliker, A.A., Baliarsingh, S.K., Trainer, V.L., Wells, M.L., Wilson, C., Udaya Bhaskar, T.V.S., Samanta, A., Shahimol, S.R. 2018. Characterization of oceanic Noctiluca blooms not associated with hypoxia in the Northeastern Arabian Sea. Harmful Algae, 74: 46-57, https://doi.org/10.1016/j.hal.2018.03.008.

 

Field photograph of Noctiluca bloom in Northern Arabian Sea taken during February 2017

(Credit: FORV Sagar Sampada, cruise no. 356)

 

Schematic showing the association of Dissolved Oxygen (DO), measured using bio-argo profiling float, with high biomass green Noctiluca bloom in oceanic waters of Northern Arabian Sea. Phytoplankton class products (Noctiluca and Diatom) were generated using the algorithm developed by Dwivedi et al. (2015)

(Source: Lotliker, A.A., Baliarsingh, S.K., Trainer, V.L., Wells, M.L., Wilson, C., Udaya Bhaskar, T.V.S., Samanta, A., Shahimol, S.R. 2018. Characterization of oceanic Noctiluca blooms not associated with hypoxia in the Northeastern Arabian Sea, Harmful Algae, Volume 74, 46-57, https://doi.org/10.1016/j.hal.2018.03.008)

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