Environmental influences on the cyanobacterial mat formation in the mangrove ecosystems along the southwest coast of India

Abstract

Mangrove forests are highly diverse niches playing fundamental roles in ecosystem sustainability, particularly in the tropics. Benthic realms in mangrove forest sustain a wide variety of autotrophs among which cyanobacteria forms a major component. The present study discusses the spatiotemporal variabilities in benthic cyanobacterial assemblage in selected mangrove ecosystems (Kannur—S1, Kochi—S2 and Kollam—S3 were sampled for 1 year, 2018 to 2019) along the south-west coast of India. A total of nine cyanobacterial species belonging to six genera were identified, which included mainly non-heterocystous forms like Oscillatoria irrigua, Phormidium chalybeum, Geitlerinema bigranulatum, Oscillatoria sancta, Spirulina major, Trichodesmium sp., whereas heterocystous cyanobacteria included Nostoc sp. Oscillatoria irrigua was the major species distributed in all the mangrove ecosystems. Temporal variations were observed in the distribution with pre-monsoon (PRM) and monsoon (MON) seasons having higher numerical density and lesser diversity of benthic cyanophytes with lesser abundance and comparatively higher diversity during post-monsoon (POM). Regionally, S2 observed higher numerical abundance with higher dominance, whereas S3 observed less abundance with high diversity. Physico-chemical parameters of sediment and pore water analysis observed that sediment temperature positively correlated with cyanobacterial abundance, whereas a negative correlation was observed with dissolved oxygen and pore water dissolved nitrate. Pore water salinity also influenced the community composition of cyanobacteria with heterocystous cyanobacteria in comparatively less saline conditions. Hence, the study emphasis spatiotemporal variabilities in the benthic cyanobacterial assemblage of mangrove ecosystems in relation to environmental variables.