Surge in the jellyfish population of a tropical monsoonal estuary: A boon or bane to its plankton community dynamics?

Recurrent jellyfish blooms and their impacts on ecosystem deliverables of coastal habitats have become a major ecological concern. In view of this, repercussions of a surge in the jellyfish population on the plankton community were studied in Cochin estuary (CE), the largest tropical estuary along the southwest coast of India. Evaluation of hydrographic attributes and plankton community of the CE during early and late pre-monsoon revealed a marked disparity in its hydrography which favoured an increase in jellyfish abundances during late pre-monsoon, eliciting distinct impacts on the plankton community. The escalation in the jellyfish abundance and their subsequent predation on the crustacean plankton released the phytoplankton community from the grazing pressure resulting in a trophic cascade in the planktonic food web. The indiscriminate feeding of jellyfishes on the ichthyoplankton, decapod larvae, and Copepoda, the primary diet component of forage fishes evoked a potent threat to the fishery potential of CE.

Impact of a catastrophic flood on the heavy metal pollution status and the concurrent responses of the bentho-pelagic community in a tropical monsoonal estuary.

Consequences of a catastrophic flood on the habitat quality and the concurrent responses of the bentho-pelagic community were studied in Cochin estuary, a eutrophic estuary along the southwest coast of India. The episodic flood in 2018 led to a marked decline in the dissolved nutrients and heavy metal concentrations in water and sediments of the estuary. The pre-flood phytoplankton abundance dominated by a bloom-forming species Cerataulina bicornis experienced a significant drop after the flood. Contrarily, zooplankton and macrobenthos responded favorably towards the flood-imposed habitat alterations. Higher susceptibility to heavy metal pollution and increased grazing pressure from gelatinous carnivores restricted the abundance of Copepoda, the dominant zooplankton taxon during pre-flood. The lower heavy metal concentration in the sediment after the flood favored higher macrobenthic abundance and diversity with a conspicuous change in the community structure from opportunistic polychaetes, indicators of pollution to molluscans and crustaceans, indicators of the healthy benthic zones.

Spatio-temporal variability in macrobenthic communities and trophic structure of a tropical estuary and its adjacent coastal waters.

Spatio-temporal distribution of the macrobenthic community (> 500 µm) and the trophic ecology of polychaetes were studied for a year in the Cochin estuary (CE) and its adjacent coastal waters. A profound influence of the southwest monsoon (SWM) was evident in the CE, a tropical monsoonal estuary, during the present study. The sediment texture and macrobenthic density exhibited a pronounced spatial variation in the estuary, while in the coastal waters temporal changes were more prominent. Polychaetes formed the dominant taxa in terms of density and diversity in the estuary (53 species belonging to 25 families) as well as in the adjacent coastal waters (39 species belonging to 20 families). The polychaete species Diopatra neapolitana dominated during nonmonsoonal periods was found to be replaced by the higher abundance of Pisione sp. in the CE during the monsoon period. On the other hand, in the coastal waters, Cossura coasta, the dominant species during the pre-monsoon period, was replaced by the surface deposit feeder Paraprionospio pinnata during the monsoon and post-monsoon periods. The study evidenced the combined influence of sediment texture, salinity, and food availability in determining the distribution and abundance of the macrobenthic fauna in the CE and adjacent coastal waters. Feeding guild analysis and trophic importance index showed that carnivores and surface and subsurface deposit feeders were the dominant and trophically important feeding guilds in the study area. Redundancy analysis showed that the sandy substratum supported a diversified functional composition in the estuary and its adjacent coastal waters, and motile, discretely motile polychaetes and filter feeders were more associated with the sandy substratum. Results of the present study revealed that the seasonally fluctuating polychaete community structure in the study area was predominantly controlled by their diversified feeding habits and the ecological descriptors of the polychaete feeding guilds were linked primarily to the substratum type and food availability.

Responses of the zooplankton community to peak and waning periods of El Niño 2015-2016 in Kavaratti reef ecosystem, northern Indian Ocean.

The study addressed the impact of the El Niño 2015-2016 on the ecosystem functioning and the subsequent effects on the distribution and community structure of zooplankton in the Kavaratti reef, a prominent coral atoll in the tropical Indian Ocean. The elevated ocean temperature (SST) associated with El Niño resulted in a mass bleaching event affecting > 60% of the live corals of the Kavaratti atoll. The concomitant changes observed in the nutrient concentration, coral health, and phytoplankton of the reef environment during the course of the El Niño led to discernible variations in the zooplankton community with markedly higher abundance and heterogeneity in distribution during the peak period of El Niño compared to its waning phase. A notable shift was also evident in the community structure of Copepoda, the dominant zooplankton taxon, with a predominance of calanoids and poecilostomatoids in the peak period and by harpacticoid copepods in the waning phase of the El Niño. The harpacticoid, Macrosetella gracilis, dominated in the waning phase because of their unique adaptability in the utilization of Trichodesmium erythraeum, both as nutritional and physical substrates in the nutrient-depleted environment of the reef ecosystem.

Variability in the phytoplankton community of Kavaratti reef ecosystem (northern Indian Ocean) during peak and waning periods of El Niño 2016.

El Niño, an interannual climate event characterized by elevated oceanic temperature, is a prime threat for coral reef ecosystems worldwide, owing to their thermal threshold sensitivity. Phytoplankton plays a crucial role in the sustenance of reef trophodynamics. The cell size of the phytoplankton forms the "master morphological trait" with implications for growth, resource acquisition, and adaptability to nutrients. In the context of a strong El Niño prediction for 2015-2016, the present study was undertaken to evaluate the variations in the size-structured phytoplankton of Kavaratti reef waters, a major coral atoll along the southeast coast of India. The present study witnessed a remarkable change in the physicochemical environment of the reef water and massive coral bleaching with the progression of El Niño 2015-2016 from its peak to waning phase. The fluctuations observed in sea surface temperature, pH, and nutrient concentration of the reef water with the El Niño progression resulted in a remarkable shift in phytoplankton size structure, abundance, and community composition of the reef waters. Though low nutrient concentration of the waning phase resulted in lower phytoplankton biomass and abundance, the diazotroph Trichodesmium erythraeum predominated the reef waters, owing to its capability of the atmospheric nitrogen fixation and dissolved organic phosphate utilization.

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.

Seasonal dynamics of the copepod community in a tropical monsoonal estuary and the role of sex ratio in their abundance pattern.

BACKGROUND: As salinity is considered the prime "ecological master factor" governing the zooplankton distribution and abundance in estuaries, a spatio-temporal interactive approach was followed to assess whether the responses of the estuarine copepod community towards the salinity are always direct with a strong positive affinity or whether there exist any complexities in their interrelationship. The study, also for the first time, addressed the role of sex ratio in governing the abundance and the population structure of copepods in the tropical monsoonal estuarine system. RESULTS: The ecological scenario in the Cochin estuary revealed that irrespective of the season, higher zooplankton abundance occurred in the mesohaline zone (MSZ; salinity 5-18) of the estuary, despite the pronounced spatial shift of the MSZ from the lower reaches of the estuary to upstream locations, in conjunction with the varying seasonal fluvial influx and marine water intrusion. In the case of the sex ratio of copepods, the seasonal scenario revealed that the dominant copepods had a lower sex ratio during the period of higher abundance, and a negative relation was observed between the abundance and the sex ratio of copepod species during most of the seasons. CONCLUSIONS: The preponderance of the mesohaline and euryhaline species of the copepod community formed the key contributing factor towards the maintenance of higher abundance in the MSZ of the estuary. The bias in the sex ratio towards females favored higher reproductive output, resulting in the higher abundance during the favorable pre-monsoon period.