Characterization and potential causes of a whiting event in the Mar Menor coastal lagoon (Mediterranean, SE Spain)

The Mar Menor is a hypersaline coastal lagoon in the southeast of Spain, which has experienced several eutrophication episodes in recent years. In 2022, an area of milky white turbid water appeared covering about 10 % of its surface, and it has been there ever since. A comprehensive field research was set up to assess the origin of this possible whiting event, including identification of continental water inputs, satellite imagery and analyses of physico-chemical properties of the water column, sediment, and the planktonic and benthic communities. Our observations indicate that the water turned white due to precipitation of authigenic (formed where it is currently located) calcite as described in whiting events occurring in other marine areas. In addition to very high turbidity, the affected area showed increased nutrient and chlorophyll a concentrations compared to the rest of the lagoon. The altered conditions in this area also affected the planktonic communities and led to a complete disappearance of the previously abundant benthic vegetation (Caulerpa prolifera), thus influencing the biogeochemical cycles. Satellite images coupled with sediment core analysis suggest that this whiting event, which appears to have begun in 2022, is unprecedented in the Mar Menor. The enhanced alkalinity due to the semi-enclosed nature of the lagoon, along with the inflow of continental water may be producing favorable conditions for calcium carbonate supersaturation since rising groundwater levels in the adjacent aquifer seem to have intensified the groundwater discharge, which contains high concentrations of HCO₃^-. This rise is likely due to a long-term increase in recharge, induced by the expansion of irrigation practices, in combination with oscillations in rainfall cycles.