Karst-channelled intertidal submarine groundwater discharge (SGD) conditions the form of the rock pool sessile assemblage

Abstract

Freshwater submarine groundwater discharge (SGD) frequently occurs in areas of karst coastline where karst aquifers are hydraulically connected to the sea. Though up to 25% of the world's coastline is karst, little work has been done to determine the ecological impacts of karst-channelled freshwater SGD. Reported herein are the effects of karst-channelled freshwater SGD on the rock pool sessile assemblage in the locale of Olhos de Agua on the south Portuguese coast. Two quadrat surveys were conducted in eight intertidal rock pools, which ranged in freshwater SGD content from 0 to 57.3%. Salinity and nitrogen (NO3− and NO2−) concentrations were measured in the eight surveyed rock pools, as well as a further three rock pools on the same plateau, three inland groundwater boreholes and the adjacent coastal marine environment. There was a strong negative correlation between salinity (used as a proxy for freshwater SGD) and nitrogen concentration (r2 = 0.98; p < 0.01), with rock pool nitrogen concentrations ranging from 15.3 μM at salinity 36 (0% freshwater SGD) to ∼515 μM at salinity 4.27 (88% freshwater SGD). The concentration of nitrogen in the inland boreholes was elevated (∼258–∼450 μM), though lower than that of the rock pool with the highest freshwater SGD content. Freshwater SGD was associated with altered percentage cover of sessile organisms (p = 0.03), decreased organism cover (n.s.), and decreased species number (n.s). There was a negative correlation between freshwater SGD and the percentage cover of the coralline red algae Ellisolandia elongata (r2 = 0.811; t = 4.88; df = 8; p < 0.01). The most abundant algae, Ulva spp., was found in all rock pools except the rock pool with the highest fraction of SGD. These results provide evidence for a direct influence of karst-channelled freshwater SGD over the composition of the rock pool sessile assemblage. These results suggest a greater role of salinity than nitrogen additions in conditioning the form of the rock pool sessile assemblage.