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. 2021 Jun 29;11(1):13448.
doi: 10.1038/s41598-021-92897-3.

Physicochemical controls on the initiation of phytoplankton bloom during the winter monsoon in the Arabian Sea

R S Lakshmi  1   2 , Satya Prakash  3 , Aneesh A Lotliker  1 , Sanjiba K Baliarsingh  1 , Alakes Samanta  1 , Teesha Mathew  1   2 , Abhisek Chatterjee  1 , Biraja K Sahu  4 , T M Balakrishnan Nair  1
Affiliations

Physicochemical controls on the initiation of phytoplankton bloom during the winter monsoon in the Arabian Sea

R S Lakshmi et al. Sci Rep. .

Abstract

Occurrence of phytoplankton bloom in the northern Arabian Sea (NAS) during the winter monsoon is perplexing. The convective mixing leads to a deeper and well-oxygenated (> 95% saturation) mixed layer. We encountered low chlorophyll conditions though the nutrient conditions were favorable for a bloom. The mean ratio of silicate (Si) to DIN (Dissolved Inorganic Nitrogen: nitrate + nitrite + ammonium) in the euphotic zone was 0.52 indicating a "silicate-stressed" condition for the proliferation of diatoms. Also, the euphotic depth was much shallower (~ 49 m) than the mixed layer (~ 110 m) suggesting the Sverdrup critical depth limitation in the NAS. We show that the bloom in this region initiates only when the mixed layer shoals towards the euphotic zone. Our observations further suggest that two primary factors, the stoichiometric ratio of nutrients, especially the Si/DIN ratio, in the mixed layer and re-stratification of the upper water column, govern the phytoplankton blooming in NAS during the later winter monsoon. The important finding of the present study is that the Sverdrup's critical depth limitation gives rise to the observed low chl-a concentration in the NAS, despite having enough nutrients.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
Map of the Arabian Sea showing sampling locations (S1 to S10). Map created in ODV.
Figure 2
Figure 2
Vertical sections of Temperature (deg C), Salinity (PSU), chlorophyll (µg/l), Nitrate (µM), Silicate (µM) and Phosphate (µM) along 21°N (left panel) and 20°N (right panel) transects. Plots created using ODV.
Figure 3
Figure 3
Vertical sections of Si/DIN and DIN/P along 21°N and 20°N transects. DIN includes nitrate, nitrite and ammonium. Plot created in ODV.
Figure 4
Figure 4
(a) Bar diagram of euphotic depth (red) and mixed layer depth (blue) at 10 sampling stations, (b) time-series of simulated mixed layer depth (INCOIS- GODAS), satellite derived euphotic depth and surface chl-a.
Figure 5
Figure 5
Schematic diagram which shows the interplay between mixed layer depth and euphotic depth on the initiation of phytoplankton blooms.
See this image and copyright information in PMC

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