THE ROLE OF SUBMARINE GROUNDWATER DISCHARGE IN MAINTAINING UNIQUE SUBMARINE SPRING BENTHIC ECOSYSTEMS IN THE ADRIATIC SEA, CROATIA

The karstified Velebit Mountains along the coast of the Adriatic Sea, Croatia are hydrogeologically connected to submarine springs on the adjacent sea floor. The submarine groundwater discharge from these springs are rich in carbonate and nutrients. As a result, they are sites of unique and geographically limited but diverse marine ecosystems adapted to life in and around the submarine spring plumes. The dominant animals at these springs are colonies of the massive, fast growing, benthic, marine bryozoan Pentapora foliacea. The question we hope to address is: Are these animals living in fresh submarine spring water, ambient seawater, or a mixture of the two? In one spring off the coast of Grmac, Croatia, we measured the temperature and oxygen isotopic composition of the submarine groundwater discharge and compared those data to the equivalent values from the ambient seawater as well as an adjacent terrestrial freshwater spring. These data had a temporal range up to 1 year. We also took over 250 samples of biogenic skeletal carbonate from two bryozoan colonies growing in the submarine spring plume. In a previous study, seasonal isotope profiling was used to determine the ages of the bryozoan colonies. These data had a range up to five years and thus provide a longer term record of the submarine groundwater discharge. The in-situ temperatures of the ambient seawater adjacent to the spring fluctuated seasonally ~10 ^oC as expected for that part of the Adriatic Sea. The in-situ temperatures of the submarine groundwater discharge varied less (~2 ^oC) as expected for groundwater. The oxygen isotope value of the terrestrial spring (d¹⁸O = -10.04 ‰ VSMOW) was the lightest, and that for the ambient seawater was the heaviest (d¹⁸O = 0.49 ‰ VSMOW). The submarine groundwater discharge was intermediate (d¹⁸O = -3.92 to -6.46 ‰ VSMOW). Oxygen isotope values from the bryozoans (mean d¹⁸O = 0.69 ‰ VPDB) as well as the calculated temperatures reveal a range and seasonality more like ambient sea level but dampened by groundwater. Thus, the range, mean, and coefficient of variation of the temperature and oxygen isotope data indicate mixing of fresh groundwater with ambient seawater in the subsurface before discharging at the submarine springs where the animals live.