Intertidal discolouration driven by phytobenthic blooms and organic matter in a tropical estuary of Goa, central west coast of India
Chinnadurai Karnan, Sambath Kumar R, Biraja Kumar S, Aswathy S, Harshan M.K.Abstract
Algal blooms and discolouration events are commonly reported in coastal waters; however, intertidal blooms and discolouration remain poorly documented. High-frequency observations (daily to fortnightly) conducted from May 2023 to February 2024 documented discolouration events in the intertidal zones of the Caranzalem–Miramar and Dona Paula within the Mandovi–Zuari estuarine system. These discolouration events were primarily observed during low tide and exhibited clear seasonal variability. During early May, the intertidal zones appeared pale cream with moderately turbid waters. By late May, the Dona Paula intertidal zone exhibited a green-dominant discolouration (dark khaki), while in late August, the Caranzalem shore transformed into a yellow-brown discolouration (peru) accompanied by amber-coloured waters. A dark brown-black patch was also observed along southern Caranzalem and the overlying waters. Additionally, during December–January, the zone displayed a green-dominant discolouration. Microscopic examination of surface sediments and water samples revealed that the green-dominant discolouration at Dona Paula was associated with a phytobenthic bloom of Thalassiosira spp., whereas the peru-coloured yellow-brown discolouration at Caranzalem resulted from dense mats of Navicula spp. The dark slate grey patch was linked to the accumulation of semi-degraded organic matter. The green-dominant discolouration observed during the northeast monsoon coincided with a high abundance of Euglena spp., which are commonly associated with organic-rich environments. We also examined the associated hydrographic conditions that may have facilitated these discolouration events. These observations highlight the ecological significance of episodic discolouration in estuarine intertidal zones and emphasize the need for sustained monitoring to better understand their drivers, frequency, and ecological implications.