Response of phytoplankton to heavy cloud cover and turbidity in the northern Bay of Bengal.

An interesting physiological response of phytoplankton to large fluctuations in underwater photosynthetically active radiation (PAR) levels in the northern Bay of Bengal has been presented here. This study is primarily based on a 12-day time series observation in the northern Bay of Bengal during the peak Southwest Monsoon (July 2012), when the study region was recurrently exposed to alternating cloudy and sunny sky conditions. On overcast days, the PAR available underwater at the time series location (TSL) drastically decreased, with the noontime PAR at the surface water (2 m) usually being ~600 µmol m-2 s-1 on sunny days and declining to ~50 µmol m-2 s-1 on heavily overcast days. Closely linked with the sunny and cloudy days at TSL, chlorophyll a concentration in the water column showed noticeable features; it increased in the upper water column (surface-40 m) and decreased in the lower water column (41-80 m) on cloudy days, while the reverse was the case on sunny days. Based on in-situ and laboratory experimental data, it was observed that these temporal changes in the vertical distribution of chlorophyll a in the northern Bay of Bengal were due to the short-term physiological acclimation of phytoplankton to large changes in underwater PAR.

Summer monsoon onset-induced changes of autotrophic pico- and nanoplankton in the largest monsoonal estuary along the west coast of India.

This study presents the response of autotrophic pico- and nanoplankton to southwest monsoon-associated hydrographical transformations in the Cochin backwaters (CBW), the largest monsoonal estuary along the west coast of India. By the onset of the southwest monsoon, the euhaline/mesohaline conditions in the downstream/upstream of CBW usually transform into oligohaline/limnohaline. The flow cytometer analysis revealed the dominance of picoeukaryotes > Synechococcus > nanoautotrophs, with Prochlorococcus either very low or entirely absent. Synechococcus abundance was high during the pre-southwest monsoon (10(6) L(-1)), which dwindled with heavy fresh water influx during the southwest monsoon (10(5) L(-1)). The drastic drop in salinity and faster flushing of the CBW during the southwest monsoon replaced the euhaline/mesohaline strain of Synechococcus with an oligohaline/limnohaline strain. Epifluorescence microscopy analyses showed that, among the two strains of Synechococcus, the phycoerythrin-rich (PE-rich) one was dominant in the mesohaline/euhaline conditions, whereas the phycocyanin-rich (PC-rich) strain dominated in oligohaline/limnohaline conditions. Although Synechococcus abundance diminished during the southwest monsoon, the total abundance of picoplankton community remained virtually unchanged in the upstream due to an increase in the abundance of picoeukaryotes. On the other hand, the autotrophic nanoplankton abundance increased from pre-monsoon levels of av. 3.8 × 10(6)-av. 9.5 × 10(6) L(-1) at the onset of the southwest monsoon. Utilizing suitable multivariate analyses, the study illustrated the differential response and niche preference of various smaller communities of autotrophs to the southwest monsoon-associated hydrographical ramifications in a large monsoonal estuary, which may be applicable to similar such estuaries situated along the Indian coastline.

Residual fluxes of water and nutrient transport through the main inlet of a tropical estuary, Cochin estuary, West Coast, India.

Determining robust values for estuarine material fluxes has been a complex task and an interdisciplinary research challenge. With the advent of acoustic Doppler profilers (ADPs) having bottom-track capability and which provides three-dimensional current velocity profiles, more accurate estimation of cross-sectional fluxes is far accomplished in unsteady and bidirectional flow conditions of estuaries. This paper reports for the first time the discharge measurements conducted across Cochin inlet using ADP to examine the spring-neap variability in residual fluxes of water and nutrients during dry season. Cross-sectional current velocity profiles and salinity profiles were captured using ADP and conductivity temperature depth (CTD) instrumentation. Samples of surface and bottom water were also collected at 3-h intervals. The results indicated that there is a distinct transition from the neap to spring tides related to flow and salinity structure. The neap tide was partially mixed with large diurnal inequalities whereas the spring tide was well-mixed with symmetric tides. During ebb, an increase in the concentrations of nitrate, nitrite, phosphate, and silicate was noticed indicating upstream sources for their inputs. In contrast, elevated levels of ammonia were found in the estuary throughout the period of observation. There was net residual outflow during both tides, and the computed residual water fluxes of neap doubled that of spring. The strong ebb currents and the increased nutrient concentrations during ebb resulted in the export of all nutrients (except ammonia during spring) into the sea. The findings of this study highlight the consequences of anthropogenic interventions in the estuary and their effects on the fluxes of ecologically relevant substances.

Plankton food web and its seasonal dynamics in a large monsoonal estuary (Cochin backwaters, India)-significance of mesohaline region.

The paper presents the ecology and dynamics of plankton food web in the Cochin backwaters (CBW), the largest monsoonal estuary along the west coast of India. The data source is a time series measurement carried out in the CBW during the Spring Intermonsoon (March-May) and the Southwest Monsoon (June-September). The plankton food web consisting of autotrophic/heterotrophic picoplankton, autotrophic/heterotrophic nanoplankton, microzooplankton, and mesozooplankton was quantified in relation to the seasonal hydrographical settings in the CBW. The study showed that significant changes in the abundance and dynamics of plankton food web components were governed mostly by the spatial and seasonal changes in hydrography rather than short-term changes induced by tide. During the Spring Intermonsoon, all plankton consumers in the CBW was higher than the Southwest Monsoon, and the trophic interaction was more effective in upstream where there was a close coupling between all prey components and their consumers. During the Southwest Monsoon, on the other hand, the trophic interaction was more effective downstream where the abundance of all plankton consumers was significantly higher than the upstream. Based on statistical analyses NMDS/SIMPROF and RDA, we demarcated the spatial difference/mismatch in the prey and consumer distribution in the CBW and showed that a more efficient plankton food web exists in the mesohaline regions during both seasons. This suggests that a noticeable spatial shift occurs seasonally in the active plankton food web zone in the CBW; it is upstream during the Spring Intermonsoon and downstream during the Southwest Monsoon.

Copepod carcasses in a tropical estuary during different hydrographical settings.

Dead copepods (carcasses) are widespread in aquatic systems, but their scientific quantification is rare due to the difficulty in discriminating them from live ones. In this paper, we hypothesized that due to large spatial and temporal changes in hydrography in the Cochin backwaters, the percentage of copepod carcasses in the system could also change significantly on a spatial and temporal scale. In order to understand this aspect, we quantified the live and dead copepods in the Cochin backwaters under different hydrographical settings based on live and mortal staining technique. The most prominent temporal hydrographical feature during the study period was the large decline in salinity across the system, which was more pronounced downstream (15-20 units) and was caused by the large freshwater influx associated with the southwest monsoon. During the entire sampling period, copepod carcasses were pervasive all over the study area with large spatial and temporal variations in their percentage contribution (2.5-35.8 %) to the total community abundance. During all sampling, carcasses concentrated more in the downstream region, with maximum turbidity (16.5-35.8 %), than in the upstream region (2.5-14.5 %). The percentage of carcasses was the highest during the onset of the southwest monsoon (av. 23.64 ± 8.09 %), followed by the pre-southwest monsoon (av. 13.59 ± 6.72 %) and southwest monsoon (av. 8.75 ± 4.14 %). During the onset of the southwest monsoon, copepod carcasses in the downstream were contributed by ∼80 % high saline and ∼15 % low saline species, indicating a salinity shock-induced mortality. On the other hand, the cumulative effect of the long residence time of the Cochin backwaters and high partial predation rate of carnivores contributed to the high abundance of carcasses during the pre-monsoon.

Transport of dissolved nutrients and chlorophyll a in a tropical estuary, southwest coast of India.

Intra-tidal variability in the transport of materials through the Cochin estuary was studied over successive spring and neap tides to estimate the export fluxes of nutrients and chlorophyll a into the adjoining coastal zone. The results showed that there was a substantial increase in the freshwater flow into the estuary following heavy rains (~126 mm) prior to the spring tide observations. The estuary responded accordingly with a relatively larger export through the Cochin inlet during spring tide over neap tide. Despite an increased freshwater discharge during spring tide, the export fluxes of phosphate and ammonia were high during neap tide due to their input into the estuary through anthropogenic activities. The significance of this study is that the export fluxes from the Cochin estuary could be a major factor sustaining the spectacular monsoon fishery along the southwest coast of India.