Responses of Phytoplankton Benthic Propagules to Macronutrient Enrichment and Varying Light Intensities: Elucidation from Monsoon-Influenced Mandovi and Zuari Riverine System

Abstract

The ecological importance of phytoplankton-benthic-propagules (PBP) from coastal sediments, except tropical monsoon-influenced rivers and estuaries, is well documented. PBP in such systems is often transported from upstream (near freshwater) to downstream (estuary) through freshwater discharges during monsoon and thereby experiences higher salinities (>30 PSU) and nutrients with varying light conditions due to reducing discharges during the monsoon-break/withdrawal-phase. However, the PBP responses (germination and subsequent growth) to such changes are unknown and are examined here at ~35 PSU salinity. For the study, macronutrients (nitrate, phosphate, silicate, and nitrate+phosphate+silicate) and light intensities were considered to assess the response of PBP representing estuarine, brackish, and near freshwater locations of monsoon-influenced Mandovi and Zuari rivers (Goa, India). Diatoms dominated the viable PBP community, but the maximum growth and sustained photosynthetic activity were observed when all macronutrients were supplied than individually. Here, variable fluorescence technique utility in PBP resurrection (detection of viability and responses) was also explored. The PBP growth was similar for macronutrients but increased with light intensity indicating a longer growth response during monsoon. For PBP (germination and photosynthetic activity), light intensities drive the rate of improvement/development, whereas the nutrients are essential for maintaining vegetative growth upon germination in the region. The PBP dominance of common planktonic species (Skeletonema and Thalassiosira) along the river also signifies the role of seawater intrusion (up to upstream locations) in distribution. Skeletonema and Thalassiosira, which contribute significantly to the total community, are light-sensitive with a similar response and cause single species blooms during monsoon and non-monsoon, respectively, depending on the species' tolerance to salinity