Noctiluca blooms in the Arabian Sea

SUDDEN APPEARANCE OF BLOOMS OF NOCTILUCA SCINTILLANS IN THE ARABIAN SEA – AN ABRUPT SHIFT IN BIODIVERSITY OF WINTER MONSOONAL BLOOMS

In the last decade there has been a radical shift in  winter monsoonal phytoplankton blooms in the northern Arabian Sea  with large blooms of the dinoflagellate Noctiluca scintillans replacing diatoms, the unicellular, siliceous photosynthetic organisms that were observed as the main blooming phytoplankton when waters were enriched with nutrients from enriched waters from winter convective mixing

 

Winter (Nov-Mar) blooms as seen by NASA's ocean color satellite Aqua MODIS.
Winter (Nov-Mar) blooms as seen by NASA’s ocean color satellite Aqua MODIS.

THE CONSISTENCE AND LARGE-SCALE APPEARANCE OF NOCTILUCA SCINTILLANS BLOOMS HAS BEEN CAPTURED BY NASA’S OCEAN COLOR SATELLITES
Our recent studies show that it is now possible to characterize the presence of Noctiluca scintillans blooms from space using optical inversion models.

NOCTILUCA SCINTILLANS, AN UNUSUAL DINOFLAGELLATE WHICH SUSTAINS FROM AUTROPHY OF ITS GREEN SYMBIONT AND PHAGOTROPHIC FEEDING

Cells of Noctiluca scintillans

Noctiluca scintillans is  a large green mixotroph that combines photosynthesis from its chlorophyll containing endosymbiont Pedimonas noctilucae with ingestion of prey. Wide spread basin-scale blooms of Noctiluca scintillans are now being seen with predictable regularity every year from Jan. to Mar.

WHAT IS THE CAUSE FOR THE RECENT OUTBREAK OF N. SCINTILLANS AND THE CHANGING BIODIVERSITY OF WINTER BLOOMS IN THE ARABIAN SEA?

schematic-NoctilucaCopyRightIn a recent study in Nature Communications we have shown that these massive outbreaks of N. scintillans in winter are being facilitated by an unprecedented influx of oxygen deficient waters into the euphotic zone and by the extraordinary ability of its endosymbiont Pedinomonas noctilucae to fix carbon more efficiently than other phytoplankton such as diatoms under hypoxic conditions.

THE ARABIAN SEA’S EXPANDING OXYGEN MINIMUM ZONE

From: Stramma et al (2008), 320, 655-658
From: Stramma et al. (2008), 320, 655-658

The Arabian sea has one of the most severe of Oxygen Minimum Zones of the worlds’ oceans and like other oceans in the world is experiencing a decline in oxygen levels. Our study using historic and current dissolved oxygen data shows a decline in dissolved oxygen levels from the 1980s to the 2000s when blooms of N. scintillans began making an appearance.

WHAT IS CAUSING THE RECENT EXPANSION OF THE ARABIAN SEA’S OXYGEN MINIMUM ZONE ?

  • Increased delivery of organic matter to depth. Our recent findings show that the Arabian sea is becoming more productive because the summer monsoon has strengthened, increasing upwelling and favoring enhanced phytoplankton blooms. More details are available here
  • Sewage and industrial outfall from countries bordering the Arabian Sea
  • Explosive growth in the use of synthetic fertilizers in South Asia
  • Atmospheric deposition from burning of fossil fuels and industrial emissions

REPERCUSSIONS FOR THE FOOD CHAIN

noctliluca-amphipod
Amphipod feeding on Noctluca

The emergence of N. scintillans as the dominant bloom forming organism  portends changes in the food web structure  of the Arabian Sea. As its large size precludes it from being grazed conventional grazers of phytoplankton, it is mainly food for salps and jellyfish both minor components of fish diet.

Salps-NoctilucaRinged
The salp Pegea confoederata feeding on a dense Noctiluca bloom

During our ship expeditions we observed that salps fed voraciously on Noctiluca with clearance rates of 200 to 800 ml per hour suggesting that high concentrations of salps (up to hundreds of zooids per m2) could rapidly consume dense aggregations of N. scintillans.