Alarming waterweeds proliferation in the Vembanad Lake System might significantly increase water loss through transpiration.

Eichhornia crassipes and Monochoria vaginalis are waterweeds, and their uncontrolled proliferation in fresh and brackish water habitats is a serious ecological problem in many parts of the world. These weeds are quite common in the Vembanad Lake System (VLS), India's second-largest Ramsar wetland. During the non-monsoon season, the Thannermukkom saltwater barrage divides the VLS into two zones: saline water downstream and freshwater upstream. The field sampling of the current study was carried out in the upstream zone of the VLS during the Pre-Monsoon (March 2017). Fresh Eichhornia and Monochoria samples were collected, transported to the lab, and experiments were conducted under natural light conditions to determine how much extra water they transpire into the atmosphere. The results showed that the water loss in experimental tanks with Eichhornia (evapotranspiration) is roughly twice that in control tanks without them (only evaporation). Monochoria transpires fairly more water to the atmosphere than Eichhornia. These results reveal that the proliferation of waterweeds has a significant adverse effect in conserving water in all freshwater bodies infested with them. The current study also points out that the expansion of waterweeds has the potential to worsen drought conditions as they cause excess water loss into the atmosphere and a faster drying up of freshwater reservoirs. Two possible approaches for managing the waterweeds in the VLS include reducing nutrient loading upstream and more frequent opening of the Thannermukkom saltwater barrage to allow saltwater intrusion, which could inhibit the growing waterweeds.

Impact of a large hydraulic barrage on the trace metals concentration in mesozooplankton in the Kochi backwaters, along the Southwest coast of India.

This study delineates the impact of a man-made hydrological barrage [Thannermukkom barrage (TB)] on the concentration of selected trace metals (Fe, Zn, Cu, Co, Cr, Mn, Ni, Pb) in mesozooplankton from the Kochi backwaters (KBW). The overall results point out that during Pre-Southwest Monsoon (PRM), the concentration of trace metals in mesozooplankton was high in the upstream due to the closure of the barrage, which essentially causes stagnancy of the waters. Trace metal concentrations in the downstream and upstream regions of KBW were found to be lower during Post- Southwest Monsoon (PSWM) compared to the rest of the seasons. In general, trace metals in mesozooplankton from the KBW showed the following order of concentration: Fe > Mn > Zn > Ni > Cu > Cr > Co > Pb. A comparison with earlier data across the globe showed that trace metal concentration in mesoplankton in the KBW is lower than the values reported from other parts of the world.

Impact of salinity on the grazing rate of a cladocera (Latonopsis australis) in a large tropical estuarine system.

Cladocerans are ecologically important as active grazers at the secondary trophic level, and they are economically important in aquaculture as potential live feed for many commercially valuable fishes. This study deals with the effect of salinity on grazing of the rare cladocera Latonopsis australis. The experimental specimens were collected from the lower reaches of the Kochi backwaters, the largest estuarine system along the west coast of India, during the Pre-Southwest Monsoon (May 2015), and their cultures developed in the laboratory. Eight experimental salinity treatments (freshwater, 2, 4, 6, 8, 10, 12 and 14 salinity) with three life phases of the specimens (phase I: neonates with a developed digestive tract, phase II: adults carrying egg and phase III: adults carrying developing neonates) were carried out to measure their grazing rates. Two different approaches, namely individual-specific and biovolume-specific grazing measurements, were adopted in the study. The results showed a significant influence of salinity on the size and grazing rates of L. australis, irrespective of their life phases. Filtration and ingestion rates of the specimens also varied significantly between different life phases with the lowest rates in phase III. Irrespective of the life phases, grazing rates were the highest in freshwater conditions, which decreased to 25-84% in medium salinity (4-8) and 65-93% in high salinity (10-14). Further, the study demonstrated that size/biovolume/weight-based quantification of cladocera grazing provides a more precise method for the estimation of feeding rates compared to the individual-basic approach. This study emerges as the first attempt to quantify the grazing measurements of a cladocera from Indian waters.

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.

Role of rotifers in microzooplankton community in a large monsoonal estuary (Cochin backwaters) along the west coast of India.

The distribution ecology of microzooplankton in the Kochi (Cochin) backwaters has been presented. Emphasis has been given to the micro-rotifers present in the environment, considering they were a hitherto ignored component of the microzooplankton in the past studies. Three seasonal samplings were carried out at six locations along the salinity gradients in the Kochi backwaters during the Pre-Monsoon (March), Southwest Monsoon (August), and Northeast Monsoon (December). A total of 48 species of microzooplankton were recorded, of which 35 were ciliates, 10 were rotifers, and 3 were heterotrophic dinoflagellates. The study also reports the swarm of a microzooplankton species from the Kochi backwaters, which was formed by a tintinnid ciliate, Tintinnopsis uruguayensis, during the Northeast Monsoon. Very high microzooplankton density (11,990 No. L-1), as swarm in the downstream location was associated with the mesohaline condition and high availability of food. Rotifers were the major component of microzooplankton in the limnohaline/oligohaline region, whereas ciliates dominated in the polyhaline/mesohaline region. Hence, in the present study, salinity appeared to be a major factor affecting the composition of the microzooplankton community in the Kochi backwaters. As rotifers have a wide food spectrum, they can feed on almost all components of the microbial food web, including small ciliates. They also share the same food spectrum with larger ciliates and crustacean nauplii. The present study, for the first time, recorded the importance of rotifers in the microzooplankton community in the plankton food web in the Kochi backwaters.

Mud Banks along the southwest coast of India are not too muddy for plankton.

Considering Alappuzha Mud Bank in the southern Kerala coast as a typical case of biologically productive Mud Banks that form along the southwest coast of India during the Southwest Monsoon (June - September), the present study addresses several pertinent missing links between the physical environment in Mud Banks and their influence on plankton stock. This study showed that very strong coastal upwelling prevails in the entire study domain during the Southwest Monsoon, which manifests itself in the form of significantly cool, hypoxic and nitrate-rich waters surfacing near the coast. The upwelled water persisting throughout the Southwest Monsoon period was found to have fuelled the exceptionally high phytoplankton stock in the entire study area, including the Mud Bank region. Having accepted that Mud Banks are special because of the calm sea surface conditions and relatively high turbidity level in the water column around them, the present study showed that except at points close to the sea bottom, turbidity level in the Alappuzha Mud Bank was below the critical level to inhibit the plankton stock. The suspended sediments that form in the Mud Bank occasionally could be attributed to the disturbance of the bottom fluid muddy layer and their vertical spurts.

Feeding preference and daily ration of 12 dominant copepods on mono and mixed diets of phytoplankton, rotifers, and detritus in a tropical coastal water.

Results of the experimental studies on the feeding habit and daily ration (DR) of 12 dominant copepods from a tropical coastal water (off Kochi, Southwest coast of India) on different food items (phytoplankton, rotifers, and detritus) are presented. Even though, all species of copepods consumed all types of food items in the experiments, they showed noticeable feeding preferences, having important ecological implications. Calanoid Paracalanus parvus and Acrocalanus gracilis consumed phytoplankton and rotifers equally in mono diets (74-89% of DR) and mixed diets (53-82% of DR), which indicated their ability to shift their diet in natural environment based on the availability of food items. Calanoid Acartia erythraea and A. danae consumed more phytoplankton (DR 83 and 72%, respectively) than rotifers (DR 51 and 46%, respectively) in mono diets, and in mixed diets, their consumption was high in phytoplankton combined food mixtures (P + R DR and P + D DR) rather than the R + D food type, indicated their preference for mixed diets of phytoplankton. Similarly, Calanoid Temora turbinata, Pseudodiaptomus serricaudatus, and Centropages tenuiremis preferred a herbivorous diet as evidenced by their high ingestion rate on phytoplankton mono (70 to 87% to their DR) and mixed diets (58 to 80% of DR). On the other hand, Cyclopoid Oithona similis and Poecilostomatoid Corycaeus danae preferred a carnivorous diet, consuming more rotifers (> 80% of DR) than phytoplankton (18-20% of DR) and detritus (5-6% of DR). Harpacticoids Macrosetella gracilis and Euterpina acutifrons equally preferred phytoplankton (78-92% of DR) and detritus (65-89% of DR). The study showed that the dominant copepods in the coastal waters off Kochi occupy different trophic niches available in the environment, which may be applicable in other similar environments as well.

Trichodesmium blooms and warm-core ocean surface features in the Arabian Sea and the Bay of Bengal.

Trichodesmium is a bloom-forming, diazotrophic, non-heterocystous cyanobacteria widely distributed in the warmer oceans, and their bloom is considered a 'biological indication' of stratification and nitrogen limitation in the ocean surface layer. In the first part of this paper, based on the retrospective analyses of the ocean surface mesoscale features associated with 59 Trichodesmium bloom incidences recorded in the past, 32 from the Arabian Sea and the Bay of Bengal, and 27 from the rest of the world, we have showed that warm-core features have an inducing effect on bloom formation. In the second part, we have considered the environmental preferences of Trichodesmium bloom based on laboratory and field studies across the globe, and proposed a view about how warm-core features could provide an inducing pre-requisite condition for the bloom formation in the Arabian Sea and the Bay of Bengal. Proposed that the subsurface waters of warm-core features maintain more likely chances for the conducive nutrient and light conditions required for the triggering of the blooms.

Noctiluca and copepods grazing on the phytoplankton community in a nutrient-enriched coastal environment along the southwest coast of India.

The relative grazing impact of Noctiluca scintillans (hereafter referred only Noctiluca) and copepods (Acrocalanus gracilis, Paracalanus parvus, Acartia danae and Oithona similis) on the phytoplankton community in an upwelling-mudbank environment along the southwest coast India is presented here. This study was carried out during the Pre-Southwest Monsoon (April-May) to the Late Southwest Monsoon (August) period in 2014. During the sampling period, large hydrographical transformation was evident in the study area (off Alappuzha, Southwest coast of India); warmer Pre-Southwest Monsoon water column condition got transformed into cooler and nitrate-rich hypoxic waters during the Southwest Monsoon (June-August) due to intense coastal upwelling. Copepods were present in the study area throughout the sampling period with a noticeable increase in their abundance during the Southwest Monsoon. On the other hand, the first appearance of Noctiluca in the sampling location was during the Early Southwest Monsoon (mid-June) and thereafter their abundance increased towards the Peak Southwest Monsoon. The grazing experiments carried out as per the food removal method showed noticeable differences in the feeding preferences of Noctiluca and copepods, especially on the different size fractions of phytoplankton. Noctiluca showed the highest positive electivity for the phytoplankton micro-fraction (av. 0.49 ± 0.04), followed by nano-fraction (av. 0.17 ± 0.04) and a negative electivity for the pico-fraction (av. -0.66 ± 0.06). In total ingestion of Noctiluca, micro-fraction contribution (83.7%) was significantly higher compared to the nano- (15.7%) and pico-fractions (0.58%). On the other hand, copepods showed the highest positive electivity for the phytoplankton nano-fraction (av. 0.38 ± 0.04) followed by micro- (av. -0.17 ± 0.05) and pico-fractions (av. -0.35 ± 0.05). Similarly, in total ingestion of copepods, nano-fraction (69.7%) was the highest followed by micro- (28.9%) and pico-fractions (1.37%). The grazing pressure of Noctiluca on the total phytoplankton was found to be 27.7% of the standing stock and 45.6% of the production, whereas in the case of copepods, it was 9.95% of the standing stock and 16.6% of the production. The study showed that the grazing pressure of Noctiluca on the total phytoplankton as well as larger phytoplankton fraction was 2.8- and 8-folds higher than that of the copepods. This suggests the leading role of Noctiluca as an effective grazer of larger phytoplankton along the southwest west coast of India, especially during the Peak/Late Southwest Monsoon.

Copepod grazing and their impact on phytoplankton standing stock and production in a tropical coastal water during the different seasons.

The grazing rate of copepods on the total and size-fractionated phytoplankton biomass in a coastal environment (off Kochi, southwest coast of India) were measured during pre-monsoon (PRM), peak southwest monsoon (PKSWM), late southwest monsoon (LSWM) and post-southwest monsoon (PSWM). The phytoplankton standing stock (chlorophyll a-Chl. a) and growth rate (GR) were less during the PRM (Chl. a 0.58 mg m-3; GR 0.23 ± 0.02) and PSWM (Chl. a 0.89 mg m-3; GR 0.30 ± 0.05) compared to PKSWM (Chl. a 6.67 mg m-3; GR 0.43 ± 0.02) and LSWM (Chl. a 4.09 mg m-3; GR 0.40 ± 0.04). The microplankton contribution to the total Chl. a was significant during the PKSWM (41.83%) and LSWM (45.72%). Copepod density was lesser during the PRM (1354 No m-3) and PSWM (1606 No m-3) than during PKSWM and LSWM (4571 and 3432 No m-3, respectively). Seasonal changes in phytoplankton biomass, phytoplankton size structure, and copepod community were closely related to the hydrographical transformations in the study domain. Dominant calanoid copepods in the study region ingested 8.4 to 14.2% of their daily ration from phytoplankton during the PRM and PSWM, which increased to >50% during the PKSWM and LSWM. The cyclopoid Oithona similis was abundant during the PKSWM, ingesting only 21% of their daily ration from phytoplankton. Temporal variation in the phytoplankton biomass and copepod species composition caused differences in community level top-down control. The copepod community ingestion on phytoplankton was high during the LSWM (18,583 µg C m-3d-1), followed by PKSWM (9050 µg C m-3d-1), PSWM (1813 µg C m-3d-1), and PRM (946 µg C m-3d-1). During the low Chl. a period (PRM and PSWM), dominant calanoid copepods showed a positive selectivity for the micro- and nano-phytoplankton size fractions, whereas during the high Chl. a period (PKSWM and LSWM), they showed a positive selection for nano-phytoplankton fractions. Irrespective of the seasons, dominant calanoid copepods showed a negative selection of pico-phytoplankton fraction. The cyclopoid O. similis and Poecilostomatoid Corycaeus danae showed a positive selection of nano- and pico-phytoplankton fractions rather than micro-fraction. The grazing pressure of copepod community ingestion on micro-fraction was less (0.56% of the phytoplankton biomass and 1.06% of the phytoplankton production) during the PKSWM. This study provides, for the first time, clear findings on the seasonal variation in the top-down control of phytoplankton by copepods in a tropical coastal water ecosystem and discusses its implications on phytoplankton blooming, plankton food web, and biogeochemistry.

Tumour-like anomaly of copepods-an evaluation of the possible causes in Indian marine waters.

Globally, tumour-like anomalies (TLA) in copepods and the critical assessment of their possible causes are rare. The exact causative factor and ecological consequences of TLA in copepods are still unclear and there is no quantitative data available so far to prove conclusively the mechanism involved in developing TLA in copepods. TLA in copepods are considered as a potential threat to the well-being of the aquatic food web, which prompted us to assess these abnormalities in Indian marine waters and assess the possible etiological agents. We carried out a focused study on copepods collected from 10 estuarine inlets and five coastal waters of India using a FlowCAM, advanced microscopes and laboratory-incubated observations. The analysis confirmed the presence of TLA in copepods with varying percentage of incidence in different environments. TLA was recorded in 24 species of copepods, which constituted ~1-15 % of the community in different environments. TLA was encountered more frequently in dominant copepods and exhibited diverse morphology; ~60 % was round, dark and granular, whereas ~20 % was round/oval, transparent and non-granular. TLA was mostly found in the dorsal and lateral regions of the prosome of copepods. The three suggested reasons/assumptions about the causes of TLA such as ecto-parasitism (Ellobiopsis infection), endo-parasitism (Blastodinium infection) and epibiont infections (Zoothamnium and Acineta) were assessed in the present study. We did find infections of endo-parasite Blastodinium, ecto-parasite Ellobiopsis and epibiont Zoothamnium and Acineta in copepods, but these infectious percentages were found <1.5 % to the total density and most of them are species specific. Detailed microscopical observations of the samples collected and the results of the incubation experiments of infected copepods revealed that ecto-parasitism, endo-parasitism and epibiont infections have less relevance to the formation of TLA in copepods. On the other hand, these studies corroborated the view that wounds on the exoskeleton caused by partial predation as the potential reason for the TLA of copepods in Indian waters.

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.

Influence of salinity on the life table demography of a rare Cladocera Latonopsis australis.

Latonopsis australis is a rare Cladocera inhabiting the entire stretch of the Cochin backwaters, the largest monsoonal estuary along the West Coast of India, during the summer monsoon, but restricted to the upper reaches during the non-monsoon periods. Here, we present the results of an experimental study, which assessed the influence of salinity on the life table demography of the species at different salinity levels. The life table demographic parameters such as net reproduction rate, generation time, intrinsic growth rate, gross reproductive rate, and survivorship of the species were measured in different salinities ranging from freshwater to mesohaline levels (salinity 14). The study showed that higher salinity had a significant negative effect on all life table demography parameters of the species, whereas freshwater to low saline conditions (salinity up to 8) favored the survivorship, life expectancy, net production, and growth rate. It was also noticed that salinity above 8 caused a significant decrease in the survivorship, life expectancy, and reproduction rate of the species, which clearly explained the seasonal distribution pattern of the species in the Cochin backwaters. The present study suggests salinity 2 to 6 as the optimum range for the large-scale production of L. australis for purposes like live feed in aquaculture.

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.

Why do satellite imageries show exceptionally high chlorophyll in the Gulf of Mannar and the Palk Bay during the Norteast Monsoon?

The Gulf of Mannar (GoM) and the Palk Bay (PB) are two least studied marine environments located between India and Sri Lanka. Exceptionally high chlorophyll a concentration in the GoM and the PB during the Northeast Monsoon (November-February) is a consistent feature in satellite imageries, which has been attributed to the intrusion of the Bay of Bengal (BoB) waters. The analyses of the Moderate Resolution Imaging Spectroradiometer (MODIS) and field chlorophyll data collected from 30 locations in the Indian sector of the GoM and the PB in January 2011 showed significant overestimations in the satellite data. This error was much higher in the PB (60-80 %) as compared to the GoM (18-28 %). The multivariate analyses evidenced that the exceptionally high satellite chlorophyll in the PB is contributed largely by turbidity, colored dissolved organic matter (CDOM), and bottom reflectance. The paper cautions that though MODIS is superior in estimating chlorophyll a in optically complex waters, there are still chances of overestimations in regions like the PB.