Ecological role of foraminifera as bioindicators of eutrophication and heavy metal pollution: a case study of Estany dels Ponts Lagoon (Mallorca, Spain)

The Estany dels Ponts is a coastal lagoon in northeastern Mallorca, Spain, currently undergoing a restoration project focused on cataloging its biota and characterizing the ecological and physicochemical conditions of its water column and sediments. This study analyzes the total foraminiferal assemblage in 16 sediment samples collected throughout the lagoon and investigates its relationship with nutrient and element concentrations, including heavy metals and rare earth elements (REEs). Foraminifera, as key bioindicators, were examined (2900 individuals belonging to 140 species) to assess their distribution patterns and environmental interactions. Despite significant anthropogenic modifications since the mid-twentieth century, the lagoon maintains a relatively good conservation status, exhibiting meso-eutrophic to eutrophic conditions, likely driven by wastewater infiltration during peak tourist seasons. These conditions influence sediment chemistry through organic matter deposition, favoring the dominance of tolerant species. The sediment composition reveals spatial variability in nutrient and heavy metal concentrations, with urban wastewater identified as the primary pollution source. Although most metal concentrations remain within safe thresholds, nickel levels exceed ecological limits, posing potential risks, particularly in the inner lagoon. The foraminiferal community, dominated by Ammonia aberdoveyensis, reflects adaptability to fluctuating salinity and temperature. Species diversity is comparable to other Cymodocea nodosa-dominated habitats. Despite the high organic matter content and presence of heavy metals in sediments, no significant pollution-related effects were detected, as no foraminiferal test deformities were observed.