Equatorial Subsurface Water and the nutrient seasonality distribution of the Gulf of Nicoya, Costa Rica

Abstract

A dry season nutrient survey was conducted in the Gulf of Nicoya. Costa Rica. The highest concentrations of nitrate and phosphate over the surveyed range ocurred at the deep waters off the gulf mouth. Vertical stratification kept the nutrient rich Equatorial Subsurface Water (ESW) at the deeper levels below the thermocline over most of the lower (outer) section of the Gulf. Higher tidal energy dissipation on the mid Gulf appears to be mixing this nutrients over the entire water column. Additionally two anchor stations were set at the western and eastern lower (outer) Gulf. There was a net flux into the Gulf of nitrate and phosphate at the west end, while at the east side nutrients are exported offshore. Survey data suggests that the major source of nitrate and phosphate for the embayment is the ESW, whereas silicate would be mostly provided by rivers. Our data and that of previous workers indicate that this pattern does not change in the rainy season. The influx of the ESW water into the Gulf of Nicoya may be dampening the seasonal signal due to runoff. at least for inorganic nitrogen and phosphorus, keeping relatively steady levels of these nutrients throughout the year over most of the Gulf.