Aquatic community structure as sentinel of recent environmental changes unraveled from lake sedimentary records from the Atacama Desert, Chile

Abstract

The Atacama Desert (21-26°S) is currently one of the driest places on Earth and metal(loid)s are of special concern for this region, which hosts the largest-known porphyry copper deposits produced in Chile. Evidence of past environmental conditions is commonly preserved in natural archives, such as lacustrine sediments. Sediment records obtained from Inca Coya Lake (22°20'S-68°35'W, 2534 m.a.s.l.), a small lake located in the Atacama Desert, reflected the evolution of regional mining activity during the 20th century and sedimentation associated with decadal climate variability. We studied the aquatic community structure changes recorded in sediment records from Inca Coya Lake. By analysis of magnetic properties (susceptibility, hysteresis curves and Curie temperatures), grain size and geochemical composition of the sediments, we identified environmental periods and changes in the community of benthic and planktonic organisms (diatoms and diapausing egg bank). We identified three detrital episodes that we interpret as dry/wet phases during the last 90 years associated with the increase of flash flood events promoting hypoxia oscillations; anthropogenic (mining activity) signals were also identified. Invertebrate community structure (primary consumers) reflected the metal exposure, measured as changes in assemblage composition through species turnover. Diatom community composition was best associated with variables related to wetter/drier alternation and consequent changes in oxygen availability. Bioindicators analyzed (diatoms, diapausing egg bank and invertebrate community) demonstrated to be excellent indicators of the bioavailability of compounds in the aquatic ecosystem of Inca Coya Lake, allowing the environmental impact assessment of the water resources due to flash floods and mining activity in the driest desert of the world.

Fig 1. The study site location.

Inca Coya Lake (22°20’S-68°35’W), in the context of different mining activities in the zone (DATUM WGS 1984 UTM Zone 19S).

Fig 2. Analysis of CORE1.

Grain size, magnetic properties χlofi: low-field susceptibility, χhifi: high-field susceptibility, Ms: saturation magnetization, Mr: remnant magnetization, Hc: coercive force, and Hcr: remnant coercive force).

Fig 3. Geochemical analysis of CORE2.

Elemental composition of sediment core CORE2 from Inca Coya Lake by XRF analysis and environmental periods identified by CONISS analysis.

Fig 4. Diatom assemblage.

Diatoms species and their abundance identified from Inca Coya core.

Fig 5. Redundancy analysis.

Plot of redundancy analysis (RDA) ordination (axes 1 and 2) for 19 samples with diatom data. The arrows represent the significant environmental variables explaining variation in the diatom assemblages. Species names are in cyan colour and symbolized by initials (Cc: Campylodiscus clypeus; Ns: Nitzschia semirobusta; Ea: Epithemia argus; Cp: Cocconeis placentula; Nv: Navicula veneta; Np: Navicymbula cf. Pusilla; Ch: Craticula halophila; Ms: Mastogloia smithii; Am: Achnanthidium cf. Minutissimum; Asp: Achnanthes sp.; Dt: Denticula cf. thermalis). The points represent depth of samples in the core: white points are top core samples and bottom core samples in black.

Fig 6. Planktonic and benthic abundance in sediment record.

Planktonic (diapausing eggs in the eggs bank) and benthic invertebrates and their abundance identified from Inca Coya core.

Fig 7. Rates of change of bioindicators.

Euclidean distance between contiguous samples in terms of their ordination on the DCA rescaled ecological space; units are standard deviation (SD) as a metric for rate of change (RoC) calculated for the Inca Coya record for (a) benthic invertebrates, (b) plankton (diapausing eggs in the egg bank) and (c) diatoms identified in Inca Coya core.