An investigation was conducted on the occurrence and abundance of phytobenthos in the intertidal mudflats and sandbars of the Calabar River, Nigeria, for three month (May – July 2021). Total of 12 samples (6 each from mudflats and sandbars) were collected on bi-weekly basis for the three months and used for this study. The Samples were collected during lowtides when the mudflats and sandbars were exposed, by scrapping about 1cm² of the surface layers of the mudflats and sandbars using a hand trowel at Nsidung beach located between Latitude 4⁰ 57^’ 0^” - 4⁰ 58’ 30” N and Longitiude 8⁰ 18” 0’ - 8⁰ 19” 30’ E. The collected samples were stored in 250ml amber glass bottles, preserved in 10% formaldehyde solution and transported to the Biological Oceanography Laboratory, University of Calabar Nigeria for analysis. In the laboratory samples were analysed by addition of 1-3mls of Lugols iodine solution and subjected to microscopy using x10 objectives. The present study is aimed at identifying i) the different species of the phytobenthos in the mudflats and sandbars, ii) the commonest and most abundant phytobenthos in the mudflats and sandbars, iii) the month of high abundance of the phytobenthos in the mudflats and sandbars, and iv) to assess which of the two ecotones has higher abundance of the phytobenthos in the Calabar River, Nigeria, during the period of study. Phytobenthos cells were identified using standard schemes and atlases and data analysed empirically and statistically. Phytobenthic cells abundance in each of the two ecotones were recorded during each month of sampling. The intertidal mudflats were richer in phytobenthos with 903 (86.91%) cells than the sandbars with 136 (13.09%) cells. The month of May was generally richer in phytobenthos in both ecotones with 344 cells (mudflats) and 58 (sandbars). This was followed by June with 317 cells in the mudflats and 46 cells in the sandbars and July with 242 cells in the mudflats and 32 cells in the sandbars. The Bacillariophyceae were the most abundant phytobenthos with 394 (43.63%) cells in the mudflats and 59 (43.38%) cells in the sandbars, with the least abundant being the Rhodophyceae with 52(5.76%) cells in the mudflats and 7 (5.15%) cells in the sandbars.

The present study strongly recommends further studies of the mudflats and sandbars of the Calabar River, particularly the rate of primary productivity, to enhance the ecological understanding of the primary productivity of these very important ecotones.