Ecological Observation of a Trichodesmium erythraeum Bloom in the Lagoon Waters of Minicoy Island, Lakshadweep Archipelago

Abstract

A widespread bloom of the filamentous cyanobacterium Trichodesmium erythraeum was observed in the lagoon waters of Minicoy Island, Lakshadweep Archipelago. This genus is widely distributed in tropical and subtropical oceans and is known for forming dense, yellowish-brown, mat-like surface aggregations commonly referred to as “sea sawdust.” The bloom event in Minicoy Lagoon was notable due to its extensive surface coverage and the visible discoloration of lagoon waters, a phenomenon seldom reported from the Lakshadweep Islands. Hydrographic and nutrient analyses indicated that the prevailing environmental conditions were conducive to bloom development, with warm temperatures (28–30°C), high salinity (35–36.5 psu), and a stable water column. A marked reduction in nitrate concentration (0.04–0.28 µM) during the bloom period suggested active nitrogen fixation by Trichodesmium, while relatively high phosphate levels (up to 0.35 µM) supported its continued growth. Silicate concentration ranged from 2.1 to 3.3 µM, consistent with oligotrophic lagoon environments. Microscopic observations revealed that T erythraeum dominated the phytoplankton assemblage, comprising about 67% of the total community, with cell densities reaching 3.09 × 10⁵ cells L⁻¹ and filament counts ranging between 75 and 4,808 filaments L⁻¹. The findings suggest that the combination of warm, saline, and nutrient-modified conditions within the lagoon facilitates Trichodesmium proliferation, emphasizing its ecological role in nitrogen fixation and primary productivity within tropical coral reef-associated environments like the Minicoy Lagoon.