Dissolved organic matter distribution in the water column and sediment pore water of a highly anthropized coastal lagoon (Mar Menor, Spain): characteristics, sources, and benthic fluxes

Abstract

Coastal lagoons are among the most productive and biodiverse systems in the world, as well as important sentinels of climate change. The Mar Menor is one of largest coastal lagoons in the Mediterranean Sea providing a variety of ecosystem services and resources for the community. However, in the last decades, this lagoon, has suffered drastic changes and deterioration (e.g., eutrophication, anoxic events) induced by human activities (mainly urban and agricultural). Here, we studied the processes involving the dissolved organic matter (DOM) pool with the aim of improving our understanding of the functioning of the lagoon as well as helping to implement strategies to prevent the deterioration of these systems. We analyzed the dissolved organic carbon (DOC) concentration and the optical properties of DOM (absorbance and fluorescence) in the column water during summer and winter 2018 and during eighteen month covering from 2016 to 2018. We further analyzed the DOM distribution in the sediments pore water and estimated benthic fluxes. Overall, we found that the composition of DOM was mainly associated and/or enhanced by the anthropogenic activities occurring around the lagoon. Thus, this DOM enters the lagoon via urban and agricultural runoff and drainage systems and wastewater treatment plants, and it is also affected by microbial activity in the lagoon. We observed a strong seasonal variability related with precipitations and the break down of the system in 2016 (phytoplankton bloom) that lead to the dead of 80% of the macrophytes. The sediments of the lagoon acted as a source of DOM and the estimated benthic fluxes were relatively higher than those reported in other estuarine systems, probably due to the relatively elevate content of organic matter and the intense microbial activity. DOM distribution was mainly related with the specific conditions occurring during the sampling period and reflects the anthropogenic impact that this lagoon is suffering.