Seasonal variability of land-ocean groundwater nutrient fluxes from a tropical karstic region (southern Java, Indonesia)

In tropical karstic regions, knowledge about the timing and quantity of land-ocean groundwater nutrient fluxes is important, as those nutrients may affect sensitive coastal ecosystems such as coral reefs. High permeability of karst aquifers, combined with high discharge during heavy rain events, lead to a close connectivity between groundwater in the hinterland and the coastal zone. Alteration between dry and wet periods can lead to a temporal variability of groundwater discharge volume and its associated nutrient fluxes. We studied the seasonal variability of land-ocean groundwater nutrient fluxes in the tropical karstic region of Gunung Kidul (southern Java, Indonesia) from November 2015 to December 2016. Satellite thermal infrared imagery revealed two major areas of direct submarine and coastal groundwater discharge. Nutrient fluxes were estimated at the largest coastal spring using a discharge dataset from a subsurface river dam and a monthly record of nutrient concentrations sampled from the spring. Nitrate fluxes ranged from 6 × 106-245 × 106 mol/day, dissolved silicon fluxes from 58 × 106-546 × 106 mol/day and phosphate fluxes from 17 × 103-1571 × 103 mol/day. Nutrient fluxes are mostly controlled by discharge and show a high variability through time. Extraordinarily high nitrate and phosphate fluxes were observed after a period of constant and heavy rain. Most likely a nutrient pool in the top soil in the hinterland from untreated sewage or fertilizer is flushed during rain events through the aquifer to the coast. In tropical karstic regions sudden inputs of large amounts of nutrients via groundwater discharge may affect coastal ecosystems such as coral reefs making them especially vulnerable during high discharge events.