Assessment of phytoplankton photo-physiological status from a tropical monsoonal estuary

Abstract

Assessment of phytoplankton physiological responses to abiotic changes provides valuable insights into the health of an ecosystem. The variable fluorescence (Fv = Fm-F0), photosynthetic efficiency (Fv/Fm), and functional absorption cross-section estimated using active fluorometry, are the commonly used parameters to gauge the physiological status or stress or photosynthetic efficiency (PE). However, in general, physiological assessments at longer time scales (seasonal to annual) are scarce but no information from monsoon-influenced estuarine systems in the tropical environment. Here phytoplankton physiological responses to abiotic changes were evaluated based on daily observations (October-2008 to May-2010) of variable fluorescence, chlorophyll and environmental parameters from a monsoon-influenced Dona Paula Bay (west-coast of India). Minimum fluorescence (F0) was an excellent proxy to track biomass changes. During monsoon (June-September) and non-monsoon (October-May), four (lasting up to 6-days) and two (lasting up to 8-days) bloom respectively were observed. For physiological assessment following scenarios low (<0.30), moderate (0.30–0.50) and high (0.51–0.65) levels of Fv/Fm suggesting stressed, transitional and growing conditions, respectively for phytoplankton were adapted. Low-to-moderate and moderate-to-high Fv/Fm were observed during monsoon and non-monsoon seasons, respectively. The dominance of photosynthetically inactive cells due to benthic-resuspension, bloom-phases (lag/decline), salinity-stress, and high-turbulence resulted in reduced Fv/Fm during monsoon. Photoinhibition and low-nutrient concentrations resulted in reduced Fv/Fm during non-monsoon. On an annual scale, Fv/Fm was moderate and was in accordance with the oceanic measurements suggesting that the phytoplankton community operates approximately half of maximum PE as reported for upper oceans. However, unlike oceans, the influencing factors may not be limited to nutrient-limitation. Nevertheless, the incorporation of such physiological scaling in monitoring programs from the perspective of the impact of variable monsoons/environment and anthropogenic activities will be crucial in ecological surveillance.