The relative contribution of key diatom genera to export flux in the Western Indian shelf
Medhavi Pandey*, Dr. Haimanti Biswas
National Institute of Oceanography
SAS 20, Nio Colony Dona Paula, GOA, India Pincode: 403004 Mobile: 918765555855 E-mail: firstname.lastname@example.org
In the productive coastal areas, marine phytoplankton, particularly diatoms support the pelagic fisheries as well as benthic production by transporting organic matter to the sediment. They are potential carbon exporters as the silica frustules provide excellent ballasting that facilitates the sinking speed of phytodetritus. The eastern Arabian Sea or Western Indian shelf is characterized by high spatial and seasonal biogeochemical variability that directly impacts phytoplankton production and community composition. There is a significant quantity of literature available to date describing the surface water phytoplankton variability in the shelf region. However, the contribution of the key diatoms in transporting organic materials to the sediment has never been addressed. To fill this gap, we had collected three short sediment cores (up to 5 cm at 0.5 cm intervals) from coastal stations i.e, Kerala (C1), off Goa (C2), and off Gujarat (C3) along the western Indian Shelf (200 m isobaths). The results depict high diatom diversity from all stations with substantial variability among the stations. Out of a total of 39 diatom genera identified (size range 30-60 µm), 15 belonged to centric and 24 were pennate. Centric diatoms were the major contributors to the surface sediments and the most frequently occurring genera were Thalassiosira sp. followed by Coscinodiscus sp. suggesting their role as an efficient carbon exporter in this shelf region. The ability of Thalassiosira to form a long chain and the large frustule size of Coscinodiscus could be the reasons for their ability to escape grazing followed by consequent sinking. In general, C1 was majorly dominated by centric diatoms Thalassiosira (52%) Coscinodiscus (18%), and Cyclotella (9%) with the least contribution from pennate. On the contrary, in C2 Thalassiosira contributed 28% and Coscinodiscus 12% while chain forming pennates like Fragilaria, Fragilariopsis, Nitzschia, Pseudo-nitzschia and Thalassionema were also prominent in this region. Centric diatom Thalassiosira (36%) and Coscinodiscus (33%) dominated C3 region while pennates like Nitzschia and Surirella were also found. This study is the first of its kind from the Indian Shelf region and could help infer more about the type of phytoplankton contributing to carbon export with rapidly changing environmental conditions.