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The Gulf of Mannar (GoM), located between India and Sri Lanka, has astonishing faunal richness and diversity. Two oceanographic data sets supplemented with satellite remote sensing observations are discussed here to show the unique ecological setting in the GoM sustaining a rich and diverse fauna. We tested the hypothesis that a specific stretch of a large marine environment behaves differently from the rest of the region due to its peculiar geographical position. Primarily, unlike the adjacent Indian southwestern shelf in the Southeastern Arabian Sea, oxygen deficiency associated with coastal upwelling imparting physiological stress to marine fauna does not occur in the GoM. Secondly, the GoM along the Indian coastline receives an adequate amount of primary (plankton) food from the Arabian Sea and the Bay of Bengal through the advected water associated with the seasonally reversing surface currents. Thirdly, the GoM water has high transparency, aerated sandy seafloor conducive for the growth of diverse corals and much sensitive fauna. All these indicate that an astonishingly rich and diverse aquatic fauna in the GoM is a biological manifestation of a conducive geographical setting and propose that similar other environments worldwide, protected from oxygen deficiency, might also be functioning as a refuge for marine life.

Based on a monthly field sampling over a year in the Kochi backwaters (KBW), this study presents the larval ecology of black clam and discusses how an extensive saltwater barrage [Thannermukkom barrage (TB)] impacted the natural black clam resource distribution. Spatial variations in salinity were found minimal during the Southwest Monsoon (June–September) due to the predominance of the freshwater associated with heavy monsoonal rainfall. Conversely, significant spatial changes in salinity were evident during the Pre-Southwest Monsoon (March–May) and Post-Southwest Monsoon (October–February). Monthly sampling exercises revealed that the black clam stock in the KBW breeds throughout the year, as their larvae were found (8 indiv. m–3–494 indiv. m–3) in all the locations. This observation is the modification of the traditional belief that black clam in the KBW breeds only twice a year. Mesohaline condition (salinity 5–18) is the most conducive for peak spawning and larval production. There were two peaks of larval production in the KBW over a year, mainly associated with the prevalence of the optimum salinity conditions on different spatial scales. The closing of the TB after the Southwest Monsoon (September) causes shrinkage of the area of the oligohaline and mesohaline conditions, the most conducive environment for the peak spawning and larval production of black clam in the KBW. This study presents a clear case of how human alterations of the natural environment impact valuable biological resources, which may apply to many similar aquatic ecosystems across the globe.

This study presents how human-altered hydrographical settings (flow restrictions) impacts the natural distribution and community structure of copepods in the Kochi Backwaters (KBW), the largest monsoonal estuary along the southwest coast of India. This study is primarily based on an extensive seasonal sampling in the KBW and their comparison with a historical data set. Thannermukkom Barrage (TB) was built in the southern section of the KBW in the 1970s to prevent saline water intrusion to the upstream during the non-monsoon periods. Thirteen locations (1–4 in the downstream, 5–9 in the midstream, and 10–13 in the upstream) were sampled in this study over the entire stretch of the KBW during the Pre-Southwest Monsoon (PRM), Southwest Monsoon (SWM), and Post-Southwest Monsoon (PSWM). The overall effect of TB in the KBW is a seaward push of mesohaline conditions during all seasons with varying intensities. In response to the seaward push of mesohaline conditions, copepods Acartiella keralensis, Acartia plumosa, Acartia sp., Pseudodiaptomus annandalei, Pseudodiaptomus serricaudatus, Euterpina acutifrons and Oithona brevicornis showed a corresponding spatial shift for their highest abundance and diversity from midstream during PRM to the downstream during the SWM/PSWM. Multivariate and IndVal analysis demarcated many indicator species of copepods of different hydrographical settings in the KBW. A comparison with the historical data set showed that there is an apparent long-term change in hydrography, copepod composition and community structure in the upstream of the KBW due to TB.

This study deals with the macrobenthic assemblages and the biotic environmental quality of Kochi backwaters (KBW), India. Due to the heavy river discharge, extensive limnetic and turbid conditions prevailed in the KBW during the southwest monsoon (June to September). This exerted a profound adverse effect on the abundance, richness, and diversity of macrobenthic assemblages. Overall, mesohaline conditions with a clayey sand bottom substratum favored the high macrofaunal abundance during the southwest and northeast monsoon seasons. But mesohaline condition and sandy silt bottom were found to support high macrofaunal abundance in the KBW during the pre-monsoon season. Polychaete dominated the macrobenthic community, regardless of seasons. Capitella capitata, Heteromastus similis, Paraheteromastus sp., Prionospio cirrobranchiata, Minuspio cirrifera, Pagurapseudopsis kochindica, P. gymnophobia, Ctenapseudes indiana, C.chilkensis, Tanais sp., Villorita cyprinoides, Grandidierella sp., Ampelisca sp., and Littorina sp. were the dominant ones observed during the study. The sediment organic carbon, in general, showed a positive correlation with polychaete abundance during all three seasons. The ecological status of KBW during all three seasons was assessed as per BO2A index, which ranged from 0.05 to 0.18, suggesting a healthy to a moderately polluted bottom environmental condition.

The primary ecological effects of the eutrophication of upwelling in the nearshore waters are the triggering and sustenance of phytoplankton (diatom) blooms. Here, we present the changes in the morphological characteristics and body size of microautotrophs associated with the nutrient enrichment of coastal upwelling along the southwest coast of India. The FlowCAM data of microautotrophs representing two time series locations in the coastal environment along the southwest coast of India at 18 weekly/biweekly frequencies showed the following features. The most significant response of the microautotrophs to the eutrophication associated with coastal upwelling during the Southwest Monsoon (June-September) was an increase in their mean bio-volume, either by increasing the individual cell size or by forming large colonies. During the Southwest Monsoon, large colonies of Thalassiosira (~ 8 cells/colony), Thalassionema (> 16 cells/colony), and Asterionellopsis (> 8 cells/colony) become very abundant. Streptotheca formed large mat-like colonies (av. 192350 ± 348 μm3/individual), and Fragilariopsis grew as very long ribbons (av. 272997 ± 5071 μm3/individual). Large-sized Chaetoceros (av. 45715 ± 1273 μm3/individual) with longer setae were also plentiful during the Southwest Monsoon. All these changes in the morphology and life forms of microautotrophs would help them to rapidly assimilate nitrate under turbulent conditions during the Southwest Monsoon.