Zoological Studies

Vol. 54, 2015

Biogeochemistry and ecology of Pyrosoma spinosum from the Central Arabian Sea

Mangesh Gauns*, Sunita Mochemadkar, Anil Pratihary, Rajdeep Roy and Syed Wajih Ahmad Naqvi

National Institute of Oceanography (Council of Scientific and Industrial Research), Dona Paula, Goa 403004, India

Background: A swarm of pelagic tunicate (Pyrosoma spinosum) was found in the surface open waters of the Arabian Sea during late southwest monsoon (September 2007). The swarm site was characterized by moderate southwesterly wind (approximately 7 m s−1), relatively low sea-surface temperature (approximately 26C), shallow mixed layer (approximately 50 m), and substantial macro-nutrient concentrations (surface values: 2.5 μM nitrate, 0.3 μM phosphate, 0.9 μM silicate, and 5.0 μM ammonium). Despite adequate macronutrient availability, the swarm site was characterized by low diversity of phytoplankton (> 5 μm) and mesozooplankton in the upper 200 m. Low chlorophyll a concentration (27.3 mg/m2 in the upper 120 m) at the swarm site was dominated (90% to 95% in the upper 40 m) by the Synechococcus (20 106 /ml).
Results: Silicate deficiency in surface waters upwelled or entrained from the thermocline may be a key factor for the dominance of smaller autotrophs (flagellates and cyanobacteria) that seems to offer favorable conditions for episodic occurrence of swarms of these filter feeders. Low carbon content (37% of total dry weight) and the lower molar (carbon-to-nitrogen) ratio (5) in P. spinosum suggest growth of these organisms is carbon-limited.
Conclusions:We describe various physicochemical and biological conditions at the P. spinosum swarm location and at two other nearby sites not affected by it. The biological factors predominantly high densities of Synechococcus and flagellates were best suited conditions for the proliferation of pyrosome biomass in the central Arabian Sea.

Key words: Pyrosoma spinosum; Arabian Sea; Biogeochemistry; Phytoplankton; Zooplankton.

*Correspondence: E-mail: gmangesh@nio.org