Fine-scale responses of phytoplankton to freshwater influx in a tropical monsoonal estuary following the onset of southwest monsoon

Abstract

In May of 2007, a study was initiated by the National Institute of Oceanography (NIO), Goa, India, to investigate the influence of monsoonal rainfall on hydrographic conditions in the Mandovi River of India. The study was undertaken at a location ∼2 km upstream of the mouth of this estuary. During the premonsoon (PreM) in May, when circulation in the estuary was dominated by tidal activity, phytoplankton communities in the high saline (35-37 psu) waters at the study site were largely made up of the coastal neritic species Fragilaria oceanica, Ditylum brightwellii and Trichodesmium erythraeum. During the later part of the intermonsoon (InterM) phase, an abrupt decline in salinity led to a surge in phytoplankton biomass (Chlorophyll a ∼14 mg m −3 ), of a population that was dominated by Thalassiosira eccentricus. As the southwest monsoon (SWM) progressed and the estuary freshened salinity and Chlorophyll a (Chl a) concentrations decreased during the MoN, Skeletonema costatum established itself as the dominant form. Despite the low biomass (Chl a <2 mg m −3 ), the phytoplankton community of the MoN was the most diverse of the entire study. During the postmonsoon (PostM), the increase in salinity was marked by a surge in dinoflagellate populations comprising of Ceratium furca, Akashiwo sanguinea, and Pyrophacus horologium.

Distribution and sources of particulate organic matter in the Indian monsoonal estuaries during monsoon

“…Microscopic analysis in the Godavari estuary suggested that these blooms were mainly contributed by freshwater phytoplankton [ Bharati , 2014]. Similar phenomenon was also observed in Mandovi‐Zuari estuarine system [ Pednekar et al ., ] and Cochin back waters (N. Madhu, personal communication, 2014) on the west coast of India. Hence, the positive relation of OC with Chl a suggests that discharge driven phytoplankton blooms might have contributed significantly to the OC and ON in the Indian estuaries during the study period.…”

“…Furthermore, higher discharge also introduced instability of water column that further decreases phytoplankton biomass as such conditions do not favor phytoplankton growth [Cloern et al, 1985]. Occurrence of phytoplankton blooms were observed in Indian estuaries in association with lean discharge when water column contains low SPM and stratified water column [Gupta et al, 2008;Pednekar et al, 2011;Acharyya et al, 2012].…”

“…The river‐borne nutrients may have a significant impact on the phytoplankton biomass in the monsoonal estuaries. Although high nutrient concentrations are observed during peak discharge period, lower phytoplankton biomass is noticed in several Indian estuaries (Cochin estuary [ Gupta et al ., ], Godavari estuary [ Sarma et al ., ], and Mandovi‐Zuari estuary [ Pednekar et al ., ]) than during dry period due to high turbidity that inhibits light penetration. The concentrations of SPM and photic depth are crucial parameters controlling phytoplankton biomass in estuaries [ Cloern et al ., ; Cloern and Jassby , ; Sin et al ., ].…”

“…The concentrations of SPM and photic depth are crucial parameters controlling phytoplankton biomass in estuaries [ Cloern et al ., ; Cloern and Jassby , ; Sin et al ., ]. On the other hand, enhanced phytoplankton biomass was noticed during the dry period associated with lower SPM and nutrient concentrations than peak discharge period [ Sarma et al ., ; Pednekar et al ., ], suggesting that the clarity of water is more important than nutrients in the Indian estuaries.…”

Impact of river discharge on distribution of dimethyl sulfide (DMS) and its fluxes in the coastal Bay of Bengal

“…The influence of river discharge on phytoplankton composition was previously studied with reference to DMS production (Chan and Hamilton, 2001;Shenoy and Patil, 2003). It was observed that the shift in the phytoplankton composition from diatoms to dinoflagellates occurred due to river discharge in the Zuari estuary and this shift was associated with an increase in concentrations of DMS and DMSP t (Pednekar et al, 2011;Shenoy and Patil, 2003). Shenoy et al (2000) found that phytoplanktons alone did not seem to control the DMS concentration but changes in salinity effectively regulated the extent of DMSP t production by phytoplankton in the Bay of Bengal.…”