Copepod carcasses in the western Bay of Bengal and associated ecology

Although a portion of all copepods in the natural environment are physiologically inactive or dead, there is little data on this aspect in Indian seas. Using the classical neutral red live staining technique, the dead percentage of copepods in the western Bay of Bengal (BoB) is determined for the first time in this study, which takes into account hydrography and zooplankton samples collected on April 2015 (Spring Intermonsoon [SIM]) from 6 inshore and 8 offshore locations in the western BoB. The offshore parts of the research area were infested with a persistent and extensive warm-core eddy/gyre during the sampling time, as demonstrated by satellite data of sea surface temperature, mean sea level anomaly, and surface currents. As a result, the mixed layer depth in the offshore zone was higher, whereas nutrients, chlorophyll a, and turbidity were significantly lower than inshore. Copepods dominated the zooplankton community (> 80%), with abundances ranging from 112 to 2580 No m^-3 in the study area. Calanoids (particularly Paracalanidae) dominated nearshore waters (35-65%), whereas cyclopoids (notably Corycaeidae and Oncaeidae) dominated offshore (49-71%). There were 48 species of copepods identified in total, and all of them had dead individuals (carcasses). Offshore waters had a larger percentage of copepod carcasses (27-39%) than inshore seas (18-28%). The larger percentage of carcasses is possibly related to a higher top-down feeding pressure on cyclopoids in offshore waters due to extreme oligotrophy and the dominance of the microbial food web.