Benthic foraminiferal assemblages are widely used as bioproxies for coastal environment monitoring. The present study investigated seasonal variations in live benthic foraminiferal assemblages over a period of 12 mo from the largest coastal lagoon in Asia, which is strongly influenced by tropical monsoons. The investigation revealed an extremely low diversity of benthic foraminiferal assemblages, comprising 12 species, of which 8 were agglutinated. The most dominant taxa throughout the study period belonged to the genus Ammonia. The living assemblage was restricted to the topmost 4 cm of the sediment, with the majority of assemblages occurring in the top 2 cm. Data analysis revealed the presence of a significant variation in the biotic assemblage, indicating a patchy distribution. Overall, some of the benthic foraminiferal species, such as Ammomarginulina sp. and Miliammina obliqua, showed strong seasonal variation, while the dominant Ammonia spp. did not show any temporal variation. The environmental parameters displayed significant correlations with certain species, although no single factor could be identified as being the major force driving assemblage composition. The present findings of seasonality and correlations observed between certain factors with certain species may be utilized to improve biomonitoring in similar settings on a global scale.
KEY WORDS:Benthic foraminifera · Coastal lagoon · Live assemblage · ANOSIM · UPGMA-based cluster · Ammonia OPEN PEN ACCESS CCESS
The present study undertaken in the largest coastal lagoon of Asia, Chilika, deals with monthly monitoring of benthic foraminifera assemblages in terms of distribution pattern, diversity and variations in taxonomic composition spanning over a period of 20 months. In total, 13 species of benthic foraminifera represented by eight families were identified in the lagoon. The stations in the Southern sector of the lagoon showed relatively low foraminifera abundance yet high diversity whereas higher abundance and lower diversity were observed in stations located in the Central sector which indicates the spatial patterning of the assemblage. Live foraminifera abundance was sparse in the study area indicating the stressed nature of the lagoon environment. The dissolved nutrient concentration of bottom water reflected significant seasonal variation. The stressed nature of the lagoon is further indicated by the dominance of the genus Ammonia across the inner sectors of the lagoon, a genus known to inhabit impacted habitats. Overall these data can serve as a baseline proxy for understanding palaeontological assemblages of foraminifera in similar shallow-water settings globally.
Nutrient loading from land masses pose a serious threat to coastal ecosystem functioning. In this study the hypothesis involving applicability of a benthic foraminiferal genus, Ammonia as a reliable proxy for tracking sediment organic carbon load was evaluated in Chilika, Asia's largest lagoon. In situ environmental parameters namely water temperature, pH, dissolved oxygen and salinity were measured from surface water and from sediment water interface along with measurements of dissolved form of nutrients from six stations of Chilika for a period of 12 months. Besides, sediment samples were collected and analyzed for elucidating total benthic foraminifer communities representing 0-2 cm fraction from all the above stations. Along with benthic foraminifera, pore water was extracted from 0 to 2 cm sediment from each station and subsequently analyzed to determine salinity, pH and dissolved form of nutrients as well as collected sediment (0-2 cm) from each station was analyzed for organic carbon (C org) content including source characterization (stable isotope of δ 13 C) for the entire study period. Ammonia population representing live and dead ones was also enumerated from the surface of the sediment column. The investigation revealed that concentration of dissolved nitrate were high in both surface (Max. 86.25 µM, Min. 19.5 µM) and bottom (Max. 90.17 µM, Min. 26.75 µM) while in pore water it was higher in pre-monsoon month of June (Max. 131.67 µM, Min. 68.33 µM). Stable isotopic ratios of carbon (δ13C) from sediments varied within a narrow range (Max. −20.7, Min. −24.2) and did not exhibit any seasonal pattern but supported the idea of nutrient driven increased primary production. Organic carbon (C org) content showed trends with seasonal oscillation during pre-monsoon (Max. 5.66%, Min. 2.87%) and post-monsoon months (Max. 3.88%, Min. 2.66%) in one of the studied stations. Out of 72 sediment samples, 46 samples had stained individuals of Ammonia spp. which were considered living at the time of collection, while 53 samples had dead specimens. Pearson's correlation coefficient analysis showed that assemblages of Ammonia spp. were linked with sedimentary organic carbon and could be used as a reliable proxy for tracking sedimentary organic carbon loading in lagoons.
Modern benthic foraminiferal assemblages are instrumental in providing information regarding changes in relative sea level as well as prevailing environmental conditions in marine environments. Marginal marine environments are coastal environments that are in most cases characterized by high influx of terrestrially originated nutrients. Inventorizing of modern benthic foraminiferal assemblages from such habitats can act as biotic indicators of water quality variations along with any changes in relative sea level. The present study documents the modern benthic foraminiferal assemblage from three major marginal marine habitats located along the North West coast of Bay of Bengal, in the Indian Ocean. Sediment samples for the purpose were thus collected from the Indian Sundarbans Delta, Chilika lagoon and the Gautami Godavari estuarine zone which encompasses the Kakinada bay. A total of 32 species of benthic foraminifera were documented during the study. The present observations were compared with previous reports of benthic foraminiferal diversity from these habitats and exhibited variability.
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