Seasonality and diversity variations of planktonic tintinnids (Ciliophora) in the tropical Cochin estuary, southwest coast of India

Tintinnids are ciliate microzooplankton that play a vital role in estuarine ecosystems, serving as critical links in food webs and contributing to nutrient regeneration. They act as both prey for larger organisms and grazers of phytoplankton and bacteria, and although less common, they also serve as bioindicators. Seasonal variations in abundance and diversity of tintinnids in the highly productive tropical Cochin Estuary were investigated from March 2019 to February 2020. A total of 22 species of tintinnids belonging to 7 genera were identified. Tintinnopsis was the most dominant genus, with Tintinnopsis uruguayensis being the most abundant tintinnid species observed during the study period, with occurrences reported in all months. The total abundance of tintinnids ranged from 1.1 × 10 2 to 3.4 × 10 3 cells L −1 , with the peak during the post-monsoon season (3.4 × 10 3 cells L −1 ). In contrast, the maximum diversity of tintinnids was recorded during pre-monsoon (Hʹ 2.465) and monsoon season (Hʹ 2.322). Routine monitoring of tintinnids in the estuarine waters showed a positive correlation between tintinnid abundance and factors like phytoplankton, chlorophyll a , salinity, temperature, nitrate, phosphate and a negative correlation with pH and silicate in the Cochin estuary. This suggests that these factors, directly and indirectly, influence the tintinnid community, with variations in tintinnid abundance shaping the estuarine food web and productivity.