Benthic Foraminiferal Response to Trace Elements in a Tropical Mesotidal Brazilian Estuary

Living benthic foraminifera have been widely used as ecological indicators in coastal ecosystems. There is, however, a lack of studies on their response to trace element pollution in tropical estuarine systems. Here we analyze the living assemblages of benthic foraminifera, collected in 2016, in the Cachoeira River Estuary (CRE) in northeastern Brazil, to understand their response to natural and anthropogenic stressors, including trace element pollution. Some species were good bioindicators of specific environmental conditions, such as the agglutinant Paratrochammina clossi which preferred mangrove areas and anoxic conditions. In addition, the calcareous Ammonia tepida and Cribroelphidium excavatum, dominant within the whole system disregarding organic or trace element pollution, seem to resist even in the areas most polluted by trace elements. Interestingly, C. excavatum showed a particular positive relationship with trace element pollution (specifically by Cu and Pb), outnumbering the opportunistic A. tepida in the areas with higher pollution of these metals. However, for other species, it is still difficult to constrain to which parameters they respond (i.e., Haynesina germanica and Elphidium gunteri, which in the present study seem to respond to natural conditions, whereas in the literature they are regarded as indicators of trace element and organic pollution, respectively). Therefore, our findings shed light on the response of benthic foraminiferal species in a highly polluted and highly mixed tropical estuarine system and highlight the need to understand the complexity of these environments when applying foraminiferal biological indexes to avoid imprecise conclusions.