Temporal and spatial variation in primary production in Obama Bay

Abstract

Submarine groundwater discharge (SGD) is an important nutrient source for the coastal ecosystems of Obama Bay as well as river water. However, the response of phytoplankton to nutrients supplied from freshwater is unknown. In this study, we evaluated the spatial and temporal variability of primary production from phytoplankton in Obama Bay. From May 2011 to March 2012, we made monthly field observations of Chlorophyll-a (Chl-a) concentration and calculated potential primary productivity (PP) using a simple mathematical model. Chl-a concentrations and PP were each higher in the upper layer from November to February, when the river discharge was high. During the months from March to October, when the nutrients in the upper layer were almost depleted, Chl-a concentrations and PP were higher in the lower layer. In May 2011, we observed significantly higher concentrations of Chl-a (51.8 μg l⁻¹) in the bottom layer at an area 2 km offshore from the river mouth. We also observed low-salinity water in the bottom layer at around the same area in June 2011, one week after a notable flood. Together these observations strongly suggest that SGD led to the phytoplankton bloom observed in the bottom layer during the spring.