Subsurface algal blooms of the northwestern Arabian Sea

Abstract

ABSTRACT:In situ plankton sampling, combined with remotely sensed and ocean Seaglider observations, provided insight into the termination of the winter monsoon bloom and subsequent evolution into a subsurface fluorescence maximum in the northwestern Arabian Sea. This subsurface maximum gradually descended, presenting increased fluorescence between 25 and 55 m depth during the spring inter-monsoon season. Species diversity decreased by half within the deep fluorescence maximum relative to the bloom. The dinoflagellate Noctiluca scintillans dominated by biomass in all samples collected from the depth of the subsurface fluorescence maximum. We show that the subsurface algal bloom persists throughout inter-monsoon seasons, linking algal blooms initiated during the southwest and northeast monsoons. In situ samples showed a net decrease in Noctiluca cell size, illustrating a shift towards a deep chlorophyll maximum adapted community, but did not exhibit any increases in chlorophyll-containing endosymbionts. We propose that the plankton biomass and estimates of the northwestern Arabian Sea productivity are much greater than estimated previously through remote sensing observations, due to the persistence, intensity and vertical extent of the deep chlorophyll maximum whichusing remote means -can only be estimated, but not measured.

Living on the edge: biofilms developing in oscillating environmental conditions

“…Additionally, seasonal changes (i.e. increase of temperature from March to June) have affected vertical profiles of the physical and biological parameters (Piontkovski et al 2017).…”

“…A total of 712 dives with 1424 vertical profiles of environmental parameters covering over 2080 km and repeating the survey transects 24 times out over a period of 91 days were carried out. The ocean glider was equipped with a Seabird free-flushing CT sail, an Aanderaa 4330F oxygen optode, a Biospherical QSP-2150 PAR sensor (spectral region -400-700 nm), a Wetlabs Triplet ECO sensor measuring chlorophyll a (based on fluorescence intensity) and backscatter at 470 and 700 nm (Piontkovski et al 2017). Satellite communication was used for retrieval of the data in near real time after every dive at a speed of about 25 cm s -1 .…”

“…Abundances of phytoplankton eukaryotes in water were determined by direct count in the Niskin bottle samples using a Zeiss inverted microscope (Germany, 50× and 100× magnification). The taxonomic composition of the phytoplankton eukaryotic community was characterised according to Piontkovski et al (2017). Abundance of prokaryotes in each sample was estimated using flow cytometry (FC).…”

“…The circulation is driven by reversing summer and winter monsoons, impacting the depth of high salinity Persian (Arabian) Gulf outflow water and exchanging at the eastern boundary with the Arabian Sea (Vic et al 2015). The unique circulation and high production in the Sea of Oman create an oxygen minimum zone where there is almost no oxygen in the water column (< 2 µmol kg -1 , Banse and Piontkovski 2006;Piontkovski et al 2017). The Sea of Oman provides a unique opportunity to investigate formation of microbial biofilms on ocean gliders at the gradients of salinity, temperature, oxygen and pressure.…”

Present past and future of the OMZ in the northern Indian Ocean

“…Additionally, prior to their appearance as surface blooms, Noctiluca were observed in large numbers at depth in association with the oxycline (Goes & Gomes, 2016). In their study, Piontkovski et al (2017) were able to show a gradual descent of Noctiluca cells into the water column towards the oxycline following peak blooms at the surface. More recently based on observations that showed that Noctiluca blooms of the eastern Arabian Sea were not associated with hypoxic waters Lotliker et al (2018) argued that low oxygen waters were not the cause of Noctiluca blooms.…”