Influence of precipitation, landscape and hydrogeomorphic lake features on pelagic allochthonous indicators in two connected ultraoligotrophic lakes of North Patagonia

Using CDOM spectral shape information to improve the estimation of DOC concentration in inland waters: A case study of Andean Patagonian Lakes

For the last two decades different scientific disciplines have focused on lacustrine dissolved organic matter (DOM) given its importance in the biogeochemistry of carbon and in ecosystem functioning. New satellites supply the appropriate resolutions to evaluate chromophoric dissolved organic matter (CDOM) in inland waters, opening the possibility to estimate DOM at appropriate spatiotemporal scales. This requires, however, a robust relationship between CDOM and dissolved organic carbon (DOC). In this work, we evaluated the use of CDOM as a proxy of DOC in 7 Andean Patagonian lakes. Considering the entire data set, CDOM absorption coefficients (a₃₅₅ and a₄₄₀) were linearly related with DOC. Shallow lakes, however, drove this relationship showing a moderate relationship, whereas, deep lakes with lower colour presented a weaker relationship. Therefore, we assessed the use of CDOM spectral shape information to improve DOC estimates regardless of observed DOM differences due to climatic seasonality and lakes' morphometry. The use of well-known CDOM spectral shape metrics (i.e., S_275-295 and a₂₅₀:a₃₆₅ ratio) significantly improved DOC estimation. Particularly, using a Gaussian decomposition approach we found that much of the variation in the spectral shape, associated with the variability of CDOM:DOC ratio, was explained by differences in two dynamic regions centred at 270 and 320 nm. A strong nonlinear relationship was found between the a₂₇₀:a₃₂₀ ratio and the DOC-specific absorption coefficients a*₃₅₅ and a*₄₄₀. This was translated into a further improvement in DOC estimation yielding the higher R² and lower mean absolute differences (MAPD < 16%), either considering the entire data set or shallow and deep lakes separately. Our results highlight that incorporating the CDOM spectral shape information improves the characterization of the DOC pool of inland waters, which is particularly relevant for remote and/or inaccessible sites and has significant implications for the environmental management, biogeochemical studies and future remote sensing applications.

Natural levels of hydrogen peroxide (H2O2) in deep clear South temperate lakes: Field and laboratory evidence of photo- and biotic production

Hydrogen peroxide (H₂O₂) is a ubiquitous reactive oxygen species (ROS) in aquatic systems and is produced mainly in surface water by the interaction of ultraviolet radiation (UVR) and natural dissolved organic carbon (DOC). Andean Patagonian lakes are ultraoligotrophic, clear systems with extended photic zones (~40 m), and are exposed to challenging UVR levels due to their lati-altitudinal situation and extremely low DOC levels. This investigation describes the seasonal levels of H₂O₂ in relation to DOC quality in the water column of lakes Moreno East (ME) and Moreno West (MW), two deep (ca. 100 m), ultraoligotrophic, low-DOC (<0.7 mg L^-1) systems of Andean Patagonia. H₂O₂ concentrations recorded in the lakes were below 60 nM, ranging from ~3 to ~60 nM in Lake ME and from ~5 to ~35 nM in Lake MW. In most of the samples of both lakes, the H₂O₂ levels were higher in the photic zone (surface to 30-40 m) than the aphotic zone (from 30-40 m to 90-100 m), particularly in summer samples. Laboratory experiments evaluated the abiotic (photochemical) and biotic (microbial) production of H₂O₂ in seasonal (summer, autumn) samples which varied DOM quality due to lake (ME, MW) and depth (photic and aphotic lake layers) provenance. Abiotic and biotic production of H₂O₂ attained higher levels in summer samples from the photic zones of both lakes. Humic DOM from deep layers (particularly from summer samples) was more susceptible to both photo- and bio-degradation than DOM from upper lake layers, which was characterized by stronger signs of degradation and progress in diagenesis.

Linking landscape heterogeneity with lake dissolved organic matter properties assessed through absorbance and fluorescence spectroscopy: Spatial and seasonal patterns in temperate lakes of Southern Andes (Patagonia, Argentina)

Hydrological connectivity between terrestrial and aquatic systems is influenced by landscape features. Topography, vegetation cover and type, lake morphometry and climate (seasonality, precipitation) drive the timing, concentration and quality of allochthonous dissolved organic matter (DOM) inputs to lakes, influencing lake metabolism. The impact of climate changes on terrestrial-aquatic linkages depends on regional trends and ecosystems properties. We examined how landscape heterogeneity affects lake DOM in pristine temperate headwater lakes located in sharp bioclimatic gradients at the leeward side of the southern Andes (Patagonia, Argentina), and predicted their potential responses to forecasted changes in regional climate. We assessed DOM properties of deep and shallow lakes spotted along precipitation and altitudinal gradients which reflect on vegetation heterogeneity. Lake DOM (concentration, and chromophoric and fluorescent properties) was related to terrestrial bioclimatic conditions, addressing also DOM bio- and photodegradation processes. Co-effects of climate and vegetation determined the quantity and quality of allochthonous DOM inputs. Higher terrestrial signs showed up at the wettest extreme of the gradient and during the rainy season, being attributable to higher hydrological land-water connectivity, and dense vegetation cover. Under drier conditions, DOM displayed higher photobleaching signs at spatial and temporal scales. The ratio between non-humic and terrestrial humic substances indicated that DOM biodegradation dominates in shallow forested lakes and photodegradation prevails in deep ones, whereas coupled photo- and biological processing shaped the DOM pool of high altitude lakes. Overall, DOM optical metrics captured landscape heterogeneity. Under the forecasted climate changes for Patagonia (decreasing precipitation and increasing temperature), piedmont lakes may experience lower hydrological connectivity, lower terrestrial inputs and, enhanced photobleaching usually associated with longer water residence time. In high altitude lakes, terrestrial DOM inputs are expected to increase due to the upward expansion of native deciduous forests, thus becoming more similar to lakes located lower in the landscape.

The microbial mercury link in oligotrophic lakes: Bioaccumulation by picocyanobacteria in natural gradients of dissolved organic matter

Andean Patagonian lakes are oligotrophic systems characterized by low dissolved organic carbon (DOC) levels and moderate to high Hg concentration that determine naturally high Hg/DOC ratios and bioavailability. In these lakes, microbial food webs are extremely important in Hg trophodynamics, being that the picophytoplankton fraction is a major entrance path of Hg²⁺ into pelagic food webs. This study analyzed the bioaccumulation of Hg²⁺ by the picocyanobacteria Synechococcus sp. using the radiotracer ¹⁹⁷Hg²⁺ and water from four Andean Patagonian lakes presenting a natural gradient of DOM concentration and quality. Hg²⁺ bioaccumulation by Synechococcus was calculated as the uptake of Hg²⁺ per biovolume unit (volume concentration factor VCF; pL μm^-3). Hg uptake showed a wide variation (13 < VCF< 300 pL μm^-3) in the natural DOC gradient tested (0.7-4 mg L^-1; Hg²⁺/DOC ratio: 1.8-14 ng mg^-1). The bioaccumulation of Hg²⁺ in Synechococcus decreased exponentially with DOC concentration. Differences in the quality of dissolved organic matter (DOM) among lake water influenced also Hg²⁺ bioaccumulation. Naturally degraded DOM, with low molecular weight/size, promoted higher Hg uptakes in Synechococcus compared to humic DOM, rich in high molecular weight/size aromatic compounds, that retained Hg in the dissolved phase. In Andean Patagonian lakes picocyanobacteria are pivotal organisms in the Hg cycling, taking dissolved Hg²⁺ and transferring it to pelagic food webs, as well as fueling the benthic Hg pathway through sedimentation.

Natural levels and photo-production rates of hydrogen peroxide (H2O2) in Andean Patagonian aquatic systems: Influence of the dissolved organic matter pool

In aquatic environments the reactive oxygen species hydrogen peroxide (H₂O₂) is produced through photochemical reactions involving chromophoric dissolved organic matter (CDOM). Andean Patagonian freshwaters experience challenging underwater UV levels, which promote high levels of photochemical weathering. In this investigation, we study natural H₂O₂ levels and experimentally address the photochemical formation of H₂O₂ in stream and lake water with a range of dissolved organic matter (DOM) concentrations and quality. The screening of different pristine aquatic systems of Patagonia revealed that H₂O₂ concentration fluctuates between 8 and 60 nM. Laboratory incubation of different water types in PAR + UV showed photo-production of H₂O₂. The H₂O₂ formation rate increased linearly with dissolved organic carbon (DOC) in streams (13.5-20.5 nM h^-1) and shallow lakes (25.7-37.8 nM h^-1). In contrast, the H₂O₂ formation rate in deep lakes was much lower (2.1-7.1 nM h^-1), and decreased with DOC. The natural potential for H₂O₂ formation was related to the concentration and quality of the DOM pool. At higher DOC levels, such as those present in shallow lakes, H₂O₂ production was directly related to DOC, whereas at low DOC levels in deep lakes and streams, two patterns were distinguished in relation to their DOM pool quality. Stream DOM, composed of high molecular weight/size humic compounds, proved to be a reactive substrate, as reflected by their high H₂O₂ formation rates. On the other hand, deep lake DOM, with its higher relative contribution of small and more processed compounds, was found to be a less reactive substrate, affording lower H₂O₂ formation rates.

Climate-driven terrestrial inputs in ultraoligotrophic mountain streams of Andean Patagonia revealed through chromophoric and fluorescent dissolved organic matter

Fluvial networks transport a substantial fraction of the terrestrial production, contributing to the global carbon cycle and being shaped by hydrologic, natural and anthropogenic factors. In this investigation, four Andean Patagonian oligotrophic streams connecting a forested catchment (~125km(2)) and draining to a double-basin large and deep lake (Lake Moreno complex, Northwestern Patagonia), were surveyed to analyze the dynamics of the allochthonous subsidy. The results of a 30month survey showed that the catchment supplies nutrients and dissolved organic matter (DOM) to the streams. The eruption of the Puyehue-Cordón Caulle at the beginning of the study overlapped with seasonal precipitation events. The largest terrestrial input was timed with precipitation which increased particulate materials, nutrients and DOM through enhanced runoff. Baseline suspended solids and nutrients were very low in all the streams (suspended solids: ~1mg/L; total nitrogen: ~0.02mg/L; total phosphorus: ~5μg/L), increasing several fold with runoff. Baseline dissolved organic carbon concentrations (DOC) ranged between 0.15 and 1mg/L peaking up to three-fold. Chromophoric and fluorescent analyses characterized the DOM as of large molecular weight and high aromaticity. Parallel factor modeling (PARAFAC) of DOM fluorescence matrices revealed three components of terrestrial origin, with certain degree of microbial processing: C1 and C2 (terrestrial humic-like compounds) and C3 (protein-like and pigment derived compounds). Seasonal changes in MOD quality represent different breakdown stages of the allochthonous DOM. Our survey allowed us to record and discuss the effects of the Puyehue-Cordón Caulle eruption, showing that due to the high slopes, high current and discharge of the streams the volcanic material was rapidly exported to the Moreno Lake complex. Overall, this survey underscores the magnitude and timing of the allochthonous input revealing the terrestrial subsidy to food webs in Patagonian freshwaters, which are among the most oligotrophic systems of the world.

Source and trophic transfer of mercury in plankton from an ultraoligotrophic lacustrine system (Lake Nahuel Huapi, North Patagonia)

The incorporation and trophic transfer of total and methyl mercury (THg, MeHg) were examined in three size classes of plankton (10-53, 53-200, and >200 μm size range) and a small planktivorous fish, Galaxias maculatus, from the large multi-branched Lake Nahuel Huapi (North Patagonia, Argentina). Three sites representing a large range of lake benthic-pelagic structures (based on depth and shoreline characteristics) and precipitation regimes were sampled. Nitrogen and carbon stable isotopes (δ(15)N, δ(13)C) were analyzed to assess Hg trophodynamics. Selenium concentrations were determined together with THg in order to consider its potential effect on Hg trophodynamics. High THg concentrations (0.1-255 µg g(-1) dry weight (DW)) were measured in plankton, largely in inorganic form (MeHg: 3-29 ng g(-1) DW, 0.02-7% of THg, in the two larger size classes). A trend of increasing THg concentrations, varying in two to three orders of magnitude, with decreasing plankton size was associated with precipitation measured prior to each sampling event. Passive adsorption of dissolved Hg(2+) from wet deposition and runoff is considered to be the principal Hg uptake mechanism at the base of the pelagic food web. Despite the initially high THg uptake in the smaller plankton classes, the transfer to G. maculatus, and consequently to the entire food web, is likely limited due to low proportion of MeHg to THg in plankton. Furthermore, evidence of G. maculatus with benthic feeding habits having higher impact on MeHg trophic transfer compared to the same species with more pelagic (e.g., zooplankton) feeding habits, was observed. Although there is a high THg uptake in plankton, limited amounts are incorporated in the entire food web from the pelagic compartment.

Influence of dissolved organic matter character on mercury incorporation by planktonic organisms: an experimental study using oligotrophic water from Patagonian lakes

Ligands present in dissolved organic matter (DOM) form complexes with inorganic divalent mercury (Hg2+) affecting its bioavailability in pelagic food webs. This investigation addresses the influence of a natural gradient of DOM present in Patagonian lakes on the bioaccumulation of Hg2+ (the prevailing mercury species in the water column of these lakes) by the algae Cryptomonas erosa and the zooplankters Brachionus calyciflorus and Boeckella antiqua. Hg2+ accumulation was studied through laboratory experiments using natural water of four oligotrophic Patagonian lakes amended witht'97Hg2+. The bioavailability of Hg2+ was affected by the concentration and character of DOM. The entrance of Hg2+ into pelagic food webs occurs mostly through passive and active accumulation. The incorporation of Hg2+ by Cryptomonas, up to 27% of the Hg2+ amended, was found to be rapid and dominated by passive adsorption, and was greatest when low molecular weight compounds with protein-like or small phenolic signatures prevailed in the DOM. Conversely, high molecular weight compounds with a humic or fulvic signature kept Hg2+ in the dissolved phase, resulting in the lowest Hg2+ accumulation in this algae. In Brachionus and Boeckella the direct incorporation of Hg from the aqueous phase was up to 3% of the Hg2+ amended. The dietary incorporation of Hg2+ by Boeckella exceeded the direct absorption of this metal in natural water, and was remarkably similar to the Hg2+ adsorbed in their prey. Overall, DOM concentration and character affected the adsorption of Hg2+ by algae through competitive binding, while the incorporation of Hg2+ into the zooplankton was dominated by trophic or dietary transfer.

Differential mercury transfer in the aquatic food web of a double basined lake associated with selenium and habitat

Food web trophodynamics of total mercury (THg) and selenium (Se) were assessed for the double-basined ultraoligotrophic system of Lake Moreno, Patagonia. Each basin has differing proportions of littoral and pelagic habitats, thereby providing an opportunity to assess the importance of habitat (e.g. food web structure or benthic MeHg production) in the transfer of Hg and Se to top trophic fish species. Pelagic plankton, analyzed in three size classes (10-53, 53-200, and >200 μm), had very high [THg], exceeding 200 μg g(-1) dry weight (DW) in the smallest, and a low ratio of MeHg to THg (0.1 to 3%). In contrast, [THg] in littoral macroinvertebrates showed lower values (0.3 to 1.8 μg g(-1) DW). Juvenile and small fish species feeding upon plankton had higher [THg] (0.2 to 8 μg g(-1) muscle DW) compared to large piscivore fish species (0.1 to 1.6 μg g(-1) muscle DW). Selenium concentrations exhibited a much narrower variation range than THg in the food web, varying from 0.5 to 2.7 μg g(-1) DW. Molar Se:Hg ratios exceeded 1 for the majority of organisms in both basins, with most ratios exceeding 10. Using stable nitrogen isotopes as indicator of trophic level, no significant correlations were found with [THg], [Se] or Se:Hg. The apparent lack of biomagnification trends was attributed to elevated [THg] in plankton in the inorganic form mostly, as well as the possibility of consistent Se supply reducing the biomagnification in the food web of the organic portion of THg.