Algal species dynamics in North Arabian Sea using long term ocean colour satellite data

摘要

North Arabian Sea experiences massive proliferation of variable algal species. The study presents variability of Noctiluca and its association with hydrographic parameters such as sea surface temperature (SST) and water column stability using ten years of satellite data. The area was categorized into three regions, North (23 to 26°N & 56 to 70°E), West (18 to 23°N & 56 to 62°E) and East (18 to 23°N & 62 to 74°E). The Noctiluca dominated area was extracted following approach of Dwivedi et. al. (2015) based on slope of Remote Sensing Reflectance (R_(rs)) between 488 to 443nm and 488 to 531nm. The data used in the present study depicted two distinct clusters based on regression between difference of R_(rs)(488) & R_(rs)(443) with R_(rs)(488) & R_(rs)(531). The major clusters representing Noctiluca falls within the range of 0.0004 to 0.0015 (R_(rs)488-R_(rs)443) and -0.0012 to -0.0004 (R_(rs)488-R_(rs)531). The occurrence of Noctiluca showed bi-modal distribution at an annual scale with the dominance in the northern region during winter monsoon (February-March). In western and eastern region higher frequency of Nuctiluca was during post monsoon having lag of one month from western (September) to eastern (October) region. The periodicity of Noctiluca, carried out using Fourior analysis, showed predominance at annual scale in Northern and semi-anual scale in Western and Eastern region. This indicates that the Noctiluca bloom in the northern region is primarily triggered by winter mixing whereas in western and eastern part of northern Arabian Sea it has combined effect of summer upwelling as well as winter mixing.