High frequency monitoring in high mountain lakes of the Spanish Sierra Nevada after intense Saharan aerosol inputs

Study Extent

This dataset comprises weekly (Laguna de la Caldera, Laguna de Río Seco, and Laguna-embalse de las Yeguas) and fortnightly (Laguna Larga) lake monitoring in Spanish Sierra Nevada during the ice-free period of 2022. This was an anomalous year characterized by the input of very intense aerosol deposition events, a large part of which were dragged with the melting ice and snow into the lakes. See location in the map of the shallow lakes in https://lagunasdesierranevada.es/lagunas/

Sampling

1. Sampling: sampling was conducted over the entire ice-free period from late June to early November in 2022 with a total of 14 (Laguna de la Caldera), 15 (Laguna de Río Seco), 17 (Laguna-embalse de las Yeguas), and 6 (Laguna Larga) sampling days. The chemical and biological samples were taken with a Van Dorn sampler at the deepest point of the lake (discrete sampling depths) in Laguna de la Caldera and Laguna-embalse de las Yeguas, and with a integrated water sampler (integrated sample) in Laguna de Río Seco and Laguna Larga. Underwater irradiance–depth profiles were obtained for PAR (400–700 nm) using a waterproof spectrometer (Ocean Insights Inc). Multiparametric probes were used to measure in situ physico-chemical parameters (temperature, pH, dissolved oxygen, conductivity, and salinity). Subsamples of lake water were collected for total phosphorus (TP), total nitrogen (TN), and after filtration through GF/F Whatman filters for total dissolved phosphorus (TDP) and total dissolved nitrogen (TDN). Zooplankton samples were taken after sieving 10-12 L through a 40 µm mesh size and immediately preserving the zooplankton in 2% lugol. An aliquot of 1-2 L was transported cold and dark to the laboratory for bacteria, chlorophyll a, alkalinity, and major cation and anion determinations. 2. Nutrients, major cations/anions, and alkalinity: total nitrogen and total phosphorus samples were persulfate digested and measured as nitrate and as soluble reactive phosphorus, respectively, by means of standard spectrophotometric methods (APHA, 1998). These methods were also used to measure total dissolved nitrogen and total dissolved phosphorus after filtration through Whatman GF/F filters. Major cations and anions were measured in ion chromatography. Total alkalinity was measured by the acid tritation method (APHA, 1998). 3. Bacteria and zooplankton: for the quantification of bacteria aliquots were fixed with paraformaldehyde, stained with SYBR Green I DNA and determined using a Becton Dickinson FACScan flow cytometer and Yellow-green-1 µm beads (Lozano et al. 2022). Zooplankton were identified and counted with the aid of an inverted microscope. 4. Chlorophyll-a (chl-a) analysis: water samples were filtered through pre-combusted glass fibre filters (Whatman GF/F, pore size = 0.7 µm). Filters were frozen at -20ºC until the analysis. For analysis, chl-a was extracted with 7 ml 99% absolute ethanol for analysis during 24 h in refrigerated and dark conditions. Chl-a concentration was determined spectrophotometrically using a Perkin Elmer UV-Vis 25 spectrophotometer with 5-cm path-length cuvettes (Jeffrey & Humphrey, 1975; Ritchie, 2006). 5. Total solids: The water sample (Vol filter, between 300-500 mL) was filtered through precombusted (500°C) and preweighed GF/F filter (Pi filter), and dried for one hour at 105 °C. A volume of 250 ml of the filtrate (Vol beaker) collected in a preweighed beaker (Pi beaker) and dried in an oven (temperature was initially set at 95 °C to prevent the water from boiling, and when all the water had evaporated, further dried at 105 °C for one hour). Both, the beakers and filters were kept in the desiccator and allow them to cool to room temperature before reweighed (Pf beaker and Pf filter, respectively) (Elosegi & Sabater, 2009). To calculate the different fractions of total solids the following equations were applied: - Total suspended solids (TSS, g m-3) = (Pf filter – Pi filter) / (Vol filter) - Total dissolved solids (TDS, g m-3) = (Pf beaker – Pi beaker) / (Vol beaker) After the described weighing, the beakers and filters were burned in the muffle for 30 min at 500 °C, and reweighed (Pm beaker and Pm filter, respectively) to calculate volatile solids as: - Volatile suspended solids (VSS, g m-3) = (Pf filter – Pm filter) / (Vol filter) - Volatile dissolved solids (VDS, g m-3) = (Pf beaker – Pm beaker) / (Vol beaker) Total solids (TS) were calculated as: - TS = TSS + TDS 6. Sediment traps: to investigate the sinking flux of suspended solids (TSS), sediment traps were set in three of the four studied lakes. Each sediment trap was composed of two bottom-closed cylinders that allowed the acquisition of two field replicates. A sediment trap was deployed at 1 m below the surface water in the three lakes (Laguna de la Caldera, Laguna de Río Seco, Laguna-embalse de las Yeguas), and one additional trap was deployed at 8 m depth in Laguna-embalse de las Yeguas. The sediment traps consisted of twin-plexiglass cylinders, of 1.5 L volume each, which were sampled weekly to calculate sinking flux of total suspended solids (STSS) and volatile suspended solids (SVSS) according to the equations proposed by de Vicente et al. (2009): - STSS = (Pf – Pi) * VT * VF-1 * A-1 * T-1 where Pi is the initial weight of the precombusted GF/F filter, Pf is the filter weight alter filtering a known volume (VF) of the homogenized entrapped suspension; VT is the trap volume, 1.5 L; A is the collection area, 33.18 cm2; and T is the time of trap exposure. - SVSS = (Pf – Pm) * VT * VF-1 * A-1 * T-1 where Pf is filter weight after filtering a known volume (VF) of the homogenized entrapped suspension, Pm is the filter weight after ignition at 500°C for 30 min; VT is the trap volume, 1.5 L; A is the collection area, 33.18 cm2; and T is the time of trap exposure.

Quality Control

1. Sampling: researchers and field technicians carried out the sampling, analysis and processing of the data. 2. Digitalisation: all data has been revised by experts before their introduction in the dataset. 3. Storage: data is stored in Linaria (https://linaria.obsnev.es/), the institutional data repository of the Sierra Nevada Global-Change Observatory. Linaria is a normalised database focused on ecology and biodiversity related-data and it is developed in a PostgreSQL/PostGIS relational database management system (RDBMS). 4. Taxonomic validation: scientific names were reviewed by experts and were checked with the GBIF backbone taxonomy using the species matching tool (https://www.gbif.org/tools/species-lookup). 5. Coordinates validation: the sampling event’ coordinates are the same as those that locate the lakes on this official website https://lagunasdesierranevada.es/lagunas/. 6. Standardisation: the standardisation to Darwin Core was done according to the practices recommended by the TDWG guidelines (https://dwc.tdwg.org/terms/).

Method steps

1. 1. Field sampling and measurement of environmental variables (see Sampling Description section). 2. Sampling processing in the laboratory (see Sampling Description section). 3. Data is stored in Linaria (https://linaria.obsnev.es/), the institutional data repository of the Sierra Nevada Global-Change Observatory. 4. The dataset was standardised to the Darwin Core structure (De Pooter et al., 2017) as sampling event data. It contains, specifically: 149 events (53 parent events and 96 child events), 2,412 occurrences, and 3,339 records of associated measurements (32 variables). The parent events represent the integrated samplings, whereas the child events the samplings at different depths. The Darwin Core elements included in the Event Core are: eventID, modified, language, institutionCode, ownerInstitutionCode, datasetName, license, eventDate, year, month, day, continent, country, countryCode, highergeography, waterBody, minimumElevationInMeters, maximumElevationInMeters, samplingProtocol, eventRemarks, decimalLatitude, decimalLongitude, geodeticDatum. For the Occurrence Extension are: occurrenceID, catalogNumber, collectionCode, eventID, eventDate, organismQuantity, organismQuantityType, basisOfRecord, scientificName, taxonRank, kingdom, phylum, class, order, family, genus, specificEpithet, scientificNameAuthorship, occurrenceStatus. For the Measurement or Fact Extension table, the Darwin Core elements included are: measurementID, eventID, measurementType, measurementValue, measurementUnit, measurementMethod. For each child event, the sampling depth is indicated in meters in the eventRemarks element. Special values in the measurementValue element: “BDL” (Below Detection Limit). 5. The resulting dataset was published through the Integrated Publishing Toolkit of the Spanish node of the Global Biodiversity Information Facility (GBIF) (http://ipt.gbif.es).

This dataset includes a total of 2,412 occurrence records of bacteria and zooplankton, the latter represented by 15 families, 20 genera and 12 species.

1. Bacteria
   rank: kingdom
2. Arthropoda
   rank: phylum
3. Rotifera
   rank: phylum

The Sierra Nevada is a mountain range located in the southern part of Spain, primarily within the autonomous community of Andalusia. It is one of the most extensive mountain ranges in Spain and covers a significant portion of the province of Granada, although it but it also extends into the neighboring province of Almería. The highest peak in the Sierra Nevada is Mulhacén, which stands at 3,479 meters (11,414 feet) above sea level, making it the highest mountain in mainland Spain. This mountain range is known for its stunning landscapes, including snow-capped peaks, and unique flora and fauna. But also, Sierra Nevada is known for hosting numerous high mountain lakes, some of which are called "lagunas" in Spanish. The most remarkable feature of these lakes is that, due to their low latitude, they are among the highest temporarily thawed glacial lakes on the continent. Most high lakes are oligotrophic (Chl a <1 μg L−1), highly transparent (>10% of photosynthetically active radiation [PAR, 400–700 nm] penetrate to maximum depth) and with low dissolved organic carbon (<1 mg C L−1). These pristine lakes are often nestled in scenic alpine settings and contribute to the region's natural beauty. Some of the most prominent high mountain lakes in Sierra Nevada are the ones included in this study: Laguna de la Caldera, Laguna de Río Seco, Laguna Larga and Laguna-embalse de las Yeguas.