Sánchez, A. & A., Gómez-León (2024). Azoic sediments and benthic foraminifera: Environmental quality in a subtropical coastal lagoon in the gulf of California. Environmental Research. 244: 117924. DOI: 10.1016/j.envres.2023.117924.
Alberto Sánchez 1 y Adriana Gómez-León 1
Marine transitional environments play an important role in human sustainability. Around these ecosystems,coastal lagoons are subject to high anthropogenic pressure from population growth. The increased demand forgoods and services is associated with the elevated discharge of untreated and treated wastewater into lagoonsystems. The absence of benthic organisms in lagoon environments has been linked to extreme natural conditionsand severe anthropogenic impact at both spatial and temporal scales. However, the mechanisms that lead to thepresence of azoic sediments in lagoon environments have yet to be studied. This study aimed to determine thevertical variability of textural groups, geochemistry, and benthic foraminiferal fauna to understand how naturaland anthropogenic components generate a vertical sediment sequence with low or absent benthic foraminifera ina subtropical coastal lagoon in the southwestern end of the Gulf of California. A 41 cm-long sediment core wascollected from La Paz Lagoon at a 1-m depth. The core was sectioned every centimeter, and sediment subsampleswere dried and homogenized for grain size, calcium carbonate, elemental and isotopic carbon and nitrogenanalyses, and benthic foraminifera quantification. Muds with fine sands towards the core’s base characterized thesedimentary sequence. Organic carbon and total nitrogen increased from the base (1.4% and 0.06%, respectively)to the core-top (CT, 3.0% and 0.14%, respectively), significant from the 27 cm interval. Calcium carbonatecontent was very low (<0.8%). The relationship of d13C vs. C:N ratio indicated that sedimentary organiccarbon was derived from the marine and sewage source mixture. The d15N of organic matter increased by 3.7‰,starting from the 27 cm interval towards the CT. The nitrogen sewage input source was relatively more significantthan nitrogen fixation. The few individuals (<18 ind. in 10 g) and genera (Ammonia and Elphidium), aswell as the absence of foraminifera in 19 of 41 intervals in the core, indicated that environmental conditions wereunfavorable, even for colonization of environmentally stress-tolerant genera. The frequency of azoic sedimentswas higher from the 25 cm interval to the CT vs. from the base to the 25 cm interval. Moreover, the AEI revealedsevere to moderate hypoxia in the study area. The limited presence of benthic foraminifera and calcium carbonatepreservation corroborated that the quality of the lagoon’s environment has deteriorated along with
population growth, which requires strategic programs to sustain this transitional ecosystem.
Palabras clave: Ammonia; organic carbon; stable nitrogen isotopes; Ammonia-elphidium index; Azoic sediment
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