Hydrodynamics, nutrient concentrations, and phytoplankton biomass in a shallow and restricted coastal lagoon under different tidal and monsoonal environmental drivers

Abstract The influences of environmental forces under different tidal and monsoonal settings on the variability of temperature and salinity, nutrient concentrations, and phytoplankton biomass (chlorophyll-a) in a shallow and narrow Setiu Lagoon were studied during the southwest monsoon, wet period of northeast monsoon, and dry period of northeast monsoon. Vertical profiles of temperature and salinity were measured at 10 stations. Water samples were collected at the surface layer of all stations for the determination of nutrient and chlorophyll-a concentrations. The analyses showed that during the southwest monsoon, low chlorophyll-a concentrations were recorded due to limited nutrients supply as a result of strong vertical mixing of the water column and little freshwater input. Meanwhile, large river discharges and heavy rainfalls appeared to be the pushing factors that generated the salt wedges formation, which affected the scale of nutrients availability and phytoplankton biomass during the northeast monsoon. This freshwater discharge was also believed to be the responsible factor that gave rise to different magnitude of nutrient concentrations between low and high tides. The presence of anthropogenic activities in the vicinity of Setiu Lagoon contributed to elevated nutrient concentrations in the study area. However, the stoichiometric criteria of the major inorganic nutrients suggested the phytoplankton growth in the study area was possibly limited by phosphate or dissolved inorganic nitrogen (DIN), thus preventing further increase of chlorophyll-a concentrations.