Long-term sediment records reveal over three thousand years of heavy metal inputs in the Mar Menor coastal lagoon (SE Spain)

Changes in macrozoobenthic assemblages of a coastal lagoon in the context of long-term human pressures and a eutrophication process

Coastal lagoons have long been perceived as simple, uniform and naturally stressed habitats, but recent research reveals that these ecosystems may be more complex, heterogeneous, and structured than anticipated, with an unexpected homeostatic and resilience capacity to respond to anthropogenic pressures. In addition, they are considered among the ecosystems with the highest biological productivity and among the most valuable in terms of socioeconomic importance. However, they face numerous challenges and an almost general lack of planning that hinders effective protection against anthropogenic pressures like agriculture, fishing, aquaculture, mining, tourism, coastal works, industrial pollution or eutrophication. This study represents a long-term analysis of changes in the composition and structure of the macrozoobenthic assemblages of the main habitats of the Mar Menor coastal lagoon, and their response to a severe eutrophic break episode, framed in the eutrophication process and human pressures that the lagoon has been suffering for the last 40 years. Four time periods have been studied, since the 1980s, established as a reference for the quality of lagoon benthic communities. 99 taxa (3 Phyla, 1 Class and 95 Families) have been identified. The main environmental characteristics and assemblage descriptors in the seven more representative hard and soft bottom habitats of the lagoon have served to discuss the process of change that they have been undergoing and their spatiotemporal variability. The dynamics of the communities is the result of multiple processes that interact with human impacts. The mechanisms that have permitted maintaining the ecosystem integrity for more than two decades of nutrient input are mainly based on the channelling of primary production towards benthic communities, and the export of excess production. During the periods of high and more or less abrupt increases in the input of nutrients, the ecosystem responds by increasing the populations of some species and trophic strategies in a sequence of filter-feeders, herbivorous and deposit feeders. All this in a complex scenario of spatial and temporal heterogeneity of the environmental conditions and biological assemblages, and species replacement in a framework of restricted connectivity and random colonization from the open sea.

Direct laser infrared microscopy for the monitoring of microplastics in Holothuria poli and sediments of the Mar Menor coastal lagoon

Plastics represent a major threat to marine and terrestrial ecosystems if they are not recycled or disposed properly. Plastics undergo processes of physical erosion and chemical degradation, generating one of the most worrying emerging pollutants: microplastics (MPs), which may be incorporated into the food chain through ingestion by the different organisms that live in that habitat. This study investigates the possible accumulation of MPs in the gut content and integument of deposit feeder sea cucumbers, Holothuria poli Delle Chiaje, 1824 from the Mar Menor lagoon, as well as in the sediments of the areas they inhabit, using laser direct infrared microscopy (LDIR) technique. LDIR is a novel methodology based on illuminating samples with a mid-infrared quantum cascade laser, which was applied to analyse the chemical composition of MPs, as well as their size and shape. This technique offers high sensitivity and selectivity in the measurement of MPs. The most effective sample procedures involved the addition of 3 mL of HNO3 for 48 h at 65 °C for integument samples (1 g) and 4 mL of H2O2 for 2 h at 65 °C for sediment and gut content samples (1 g), with subsequent density flotation process for these two last matrices. The presence of MPs in all analysed samples was confirmed through the proposed methodology, especially polyamide and polyethylene particles, confirming the accumulation of these materials in the marine organism and the sediments. These results are key to understand MPs pollution in marine ecosystems and its possible impact on marine fauna.

Effects of dissolved oxygen changes in the benthic environment on phosphorus flux at the sediment-water interface in a coastal brackish lake

In semi-enclosed coastal brackish lakes, changes in dissolved oxygen in the bottom layer due to salinity stratification can affect the flux of phosphorus (P) at the sediment-water interface, resulting in short- and long-term water quality fluctuations in the water column. In this study, the physicochemical properties of the water layers and sediments at five sites in Saemangeum Lake were analyzed in spring and autumn for four years, and phosphorus release experiments from sediments were conducted for 20 days under oxic and anoxic conditions during the same period. Sediment total phosphorus (T-P) decreased in autumn compared to spring due to mineralization of organic bound phosphorus, which was the most dominant P fraction. This may be related to the increase in the ratio of PO4-P to T-P in bottom waters in autumn, when hypoxia was frequently observed. The difference in P fluxes between oxic and anoxic conditions indicated that during autumn, as compared to spring, the release of phosphorus could have a more immediate impact on the water column during the formation of hypoxia/anoxia. The main factors influencing changes in P fluxes from sediments were identified through redundancy analysis. Additionally, based on the results of multiple regression analysis, sediment TOC, sediment non-apatite phosphorus, porewater pH, and porewater PO4-P were determined to be the most significant factors affecting P fluxes from sediments, depending on the season or redox conditions. Recently, the increased influx of seawater into Saemangeum Lake has been shown to contribute to water quality improvements in the water column due to a strong dilution effect. However, the sediment environment has shifted towards a more reduced state, leading to increased P release under anoxic conditions. Therefore, for future water quality management within the lake, it is necessary to consistently address the recurring hypoxia and continuously monitor phosphorus dynamics.