Characterizing the seasonality of vertical chlorophyll-a profiles in the Southwest Indian Ocean from the Bio-Argo floats

Effects of an eastward shift in the Agulhas retroflection region on chlorophyll-a bloom on its south flank

“…A high amount of sunlight enters the euphotic layer [ 15 ], which may be a significant influencing factor for the Chl-a bloom event on the south side of the Agulhas Current. Similar conclusion was also reached by Gu et al, in their study on the relationship between Chl-a and mixed layer depth, where Chl-a might be limited by light in spring-autumn-winter and by the nutrient in summer [ 8 ]. Swart et al have also suggested that the upper ocean stratification can induce springtime bloom by increasing the mean light level for the phytoplankton [ 22 ].…”

“…The main controlling factors on retroflection and leakage are the latitude of maximum westerlies and the southward inertia of the Agulhas Current at the separation time, which are both largely determined by the strength and position of the wind field over the Indian Ocean [ 3 ]. The strong Agulhas Return Current and the complex front system (including the Agulhas Return Front, the Subtropical Front and the Subantarctic Front) make the Southwest Indian Ocean one of the most energetic regions across the global oceans [ 7 ], which then have significant impacts on phytoplankton production [ 8 ].…”

First description of in situ chlorophyll fluorescence signal within East Antarctic coastal polynyas during fall and winter

“…By carefully assessing the fluorescence profiles recorded in this area individually, we found that 46% of them had a fluorescence signal below the MLD that was greater than 0.12 mg.m -3 and were recorded between the end of March and April, contributing to the subsurface fluorescence maxima observed in the average April profiles (Figure 2D). During the mixing of the water column, the fluorescence peak is expected to remain above the MLD rather than below (Gu et al, 2020;Prakash et al, 2020;Louw et al, 2022). Two hypotheses, not mutually exclusive, might explain the phenomenon observed in SP: the dynamic hypothesis centered around the physical structure; and the biological hypothesis which focuses on the biological activity and vertical movement of phytoplankton (Figure 8A).…”

“…Once sea ice melts, polynyas do not necessarily lead to more phytoplankton blooms than surrounding areas, but the primary production comes earlier in the season which makes them ecologically attractive after the dark winter months (Tremblay and Smith, 2007). In autumn, light decreases and vertical mixing of the water column increases which reduces the phytoplankton bloom and deepens the phytoplankton cells (Tremblay and Smith, 2007;Arteaga et al, 2020;Gu et al, 2020). During the austral autumn/winter seasons, coastal polynyas are exposed to one of the harshest climate on Earth, which makes them extremely difficult to access and to sample by ships (Tremblay and Smith, 2007).…”

“…We expect to observe a decrease of the fluorescence signal with time due to a decline in irradiance and a deepening of the fluorescence maximum due to a reduction in stratification (i.e. deepening of the mixing layer) caused by sea ice production, salt release and winds mixing the surface layer (Mitchell and Holm-Hansen, 1991;Gu et al, 2020;von Berg et al, 2020). We posit a strong impact of the season on the identified biological and environmental signals given the light dependence of the chlorophyll-a growth cycle (Smith et al, 2000) and the dependence of temperature and salinity with seasonal sea ice production.…”