The proportion of blue carbon contribution by benthic fauna with regard to climate change issue is a relevant topic recent times. The present study is focusing on the carbon potential and blue carbon contribution of meiobenthic nematodes in the intermediate zone of Arctic Kongsfjord during 2015–2017 and the study is the first of its kind from this region. Foraminifera and Nematoda were the dominant fauna in 2015; whereas it was Foraminifera in 2016 and Tintinnida and Nematoda in 2017. Significant difference was noticed in the community structure of meiofauna in this three consecutive years. Twenty six nematodes were identified from the intermediate zone and their individual carbon potential and total blue carbon during the above-mentioned period is estimated. Parasphaerolaimus paradoxus (7.14 $\pm$ 0.05 $\mu$g/mg) recorded the highest carbon potential and the lowest value was recorded in Terschellingia longicaudata (0.11 $\pm$ 0.3 $\mu$g/mg). The sediment factors such as sediment granulometric composition, salinity, pH, Eh and total organic carbon (TOC) were the leading factors influencing the meiofaunal distribution as well as the distribution of nematodes based on carbon potential in the intermediate zone. Investigation of blue carbon deposition by nematodes turned out to be relevant in mitigating the climatic change as the fjord has a tranquil nature.

The accumulation trends of Cu, Zn, Pb and Ni and their effects on two native species, Etroplus suratensis and Villorita cyprinoides from Cochin estuary and their biomarker responses were explored. Bioaccumulation and metal selectivity index (MSI) in E . suratensis for gill and liver showed the highest accumulation for Zn (209.33 $\pm$ 17.14 mg kg$^{-1}$) followed by Cu (64.16 $\pm$ 8.07 mg kg$^{-1}$), while in V . cyprinoides Cu showed the highest accumulation (80.78 $\pm$ 6.92 mg kg$^{-1}$) in gill tissue followed by Zn (65.28 $\pm$ 7.06 mg kg$^{-3}$). Histological alterations were evaluated in gill and liver tissues of E . suratensis using histopathological index (I$_{h}$) method. Lamellar hyperplasia and deformed lamellar architecture were the evident changes in gill tissue, while the alterations in liver tissues were marked by the presence of melanomacrophage centers and necrotic lesions in liver parenchyma. The high target hazard quotient (THQ) obtained for Pb (0.94) indicated that the daily consumption of aquatic products contaminated with metals Pb, Cu and Zn has obvious health risk to human consumption. Thus, the elevated metal accumulation rate in the tissues with supportive histological changes suggests that these alterations can serve as definite signature of heavy metal contamination in native organisms of Cochin estuary.

$\bf{Highlights}$

$\bullet$ The tissue-specific bioaccumulation and metal selectivity index (MSI) of E. suratensis and V. cyprinoides revealed significantly higher accumulation of Cu and Zn compared to Pb and Ni throughout the study with higher rates during non-monsoonal periods.

$\bullet$ The histopathological studies in fish, E. suratensis from field conditions portrayed significant histopathological alterations such as hyperplasia, deformed secondary lamellae in gill tissues; melanomacrophage centres and necrotic lesions in liver.

$\bullet$ The estimation of human health risk assessment of heavy metals (Cu, Zn and Pb) of E. suratensis from Cochin estuary denoted a possible risk from consumption of fishes from the Cochin estuary.