Algal blooms in the Arabian Sea

Marine algae are important to life on earth because

  1. Control atmospheric carbon dioxide (CO2)

  2. Basis of marine food web

  3. Produce marine biotoxins

If conditions are favourable, phytoplankton can sometimes grow and reproduce at such a high rate that they create dense, highly colored patched in the water, popularly known as algal bloom and red tides in marine environment.

Red tide blooms are not only red but can be brown, yellow, green or milky in colour. These blooms can be made up of Harmful Algal Blooms (HAB) or made up of non toxic species of algae.

In the Arabian Sea blooms of diatoms, dinoflagellates and cyanobacteria are frequently noticed

Diatom blooms:

Following species of the diatom blooms were recorded during our study.

Coscinodiscus spp.

Thalassiosira spp.

Rhizosolenia spp.

Chaetoceros spp.

Skeletonema spp.

Thalassiothrix spp.

Melosira spp.

Nitzschia spp.

An extensive bloom of the Navicula spp. was recorded in the Arabian Sea during November 2001.

Navicula sp. II

Vertical profile of Navicula sp. counts

 

 

 

 

 

 

 

 

 

 

The Navicula blooms in the Arabian Sea were most important mostly non-toxic, however the anoxic conditions getting created can not be ruled out.

Dinoflagellate blooms

Dinoflagellate blooms appear in Arabian Sea as response to monsoon upwelling and winter cooling in Central and NE Arabian Sea. The bloom of Noctiluca scintillans appears annually and detected by IRS-P4 (February-March 2003 & 2004). Noctiluca cells range from 200-2000m in diameter, but most commonly found in the range of 500-600m. The flagellum present does not propel the cell, and thus it is considered to be non- motile. It can regulate its vertical position in the water column by controlling its buoyancy. Colour of Noctiluca bloom was intense geen. The extent of bloom noticed was basin scale and harmful effect on flora and fauna was not noticed. Other dinoflagellates blooms were

Gymnodinium spp.

Alexandrium spp.

Gyrodinium spp.

Ceratium spp.

Peridinium spp.

Dinophysis spp.

Prorocentrum spp.

  

IRS-P4 OCM image during Noctiluca scintillans bloom

Noctiluca scintillans

Cyanobacteria

Cyanobacterial blooms (Trichodesmium spp.) in the Arabian Sea were recorded during November to May. The two species of Trichodesmium were T. erythraeum and T. thiebautii. Trichodesmium erythreaum cells were wider than long i.e. from 7 to 11m wide, rarely to 21m, are slightly constricted at the crosswalls and have cap at the end of the trichome. Cells of T. thiebautii are either long as or up to twice as long as wide i.e. from 7 to 16m wide, are not constricted at the cross walls and have no terminal cap. Trichodesmium is a colonial cyanobacterium that is large enough to be visible to the naked eye. Colonies of Trichodesmium are made up to 50 to 200 trichomes-filaments, each made up of about 100 cells. Each colony is about 5mm in length and 1mm across. Estimates of Trichodesmium vary greatly, the colony concentration varies from 1 colony to many colonies. They grow in clean, optical transparent and pristine environment with warm temperature of >250C. It grows when other phytoplankton does not grow due to paucity of nutrients.
Trichodesmiumbloom in the NE Arabian sea Trichodesmium erythraeum

Other organisms like Prymnesiophyta (Phaeocystis globosa) from Arabian Sea reported by Madhupratap et al., 2000 and holococcolithophore (Helladosphaera sp.) from the Southern Malabar Coast reported in the September 2004 (Ramaiah et al., 2004) also form bloom in the Arabian Sea which requires more attention. During bloom of holococcolithophore fish kills were recorded.

Pheocystis globosa colony

Vertical profile of Pheocystis globosa counts (Madhupratap et al., 2000)

by:

Dr S.G. Prabhu Matondkar

Scientist, National Institute of Oceanography, India