Lake Sanabria ecosystem regime shift (1986-2019)

This dataset has been used to study the ecosystem regime shifts of Lake Sanabria, the largest natural glacial lake in Spain, in a situation of climate change that may affect compliance with ecological quality objectives, even with no significant water quality pressures. It comprises basic data from long-term (1986-2017) and intensive short-term (2015-2017) limnological monitoring of a few relevant state variables related to nutrients balance, primary production of phytoplankton and thermal structure of the water column. Data about recent history of the lake productivity, reconstructed by high-resolution palaeolimnological analysis of a surface sediment core, is provided. Time series analysis over several decades has detected significant conditional heteroscedasticity in the concentrations of parameters such as chlorophyll and oxygen in recent years in relation to lake turnover rates, coinciding with exceptional episodic single-species blooms of some planktonic diatoms. We include precipitation data, inflow measurements and land use modeling data that indicate how external nutrient loadings have declined during the last decades, with reduced precipitation and progressive afforestation of the catchment, but the lake is shifting to a more productive regime enhancing the relevance of internal loading and processes.

Methods
Analytical methods are described in more detail in the associated publications. 

1.       Description of the methodology used to generate the dataset.

1.1   Site:

·         Lake Sanabria, of glacial origin, is the largest natural lake in the Iberian Peninsula (maximum depth 50 m, area 3.536 km2, volume 99.114 hm3, watershed 122.16 km2) located at 1004 m a.s.l. on granitic bedrock. D02 sampling station coordinates: 42° 7' 12.2" N 6° 42' 27.9" W.

1.2   Meteorological stations:

·         Puente Porto reservoir (M02 station, 42˚ 7' 1" N, 6º 49' 52" W, 1645 m a.s.l)

·         Puebla de Sanabria (AEMET 2770B station, 42˚ 3' 15" N, 6º 38' 2" W, 960 m a.s.l)

1.3   Lake physico-chemical and biological data:

·         Long-term lake monitoring (1986-2018) included monthly inlet and outlet samples and profiles in the eastern sub-basin (station D02) for temperature, conductivity, dissolved oxygen, Secchi disk depth, soluble reactive phosphorus (SRP), total phosphorus (TP), nitrate, silica, and chlorophyll. Water samples were collected at 2.5-m intervals from surface to 50-m depth, and the same lab made measurements throughout the period. Additionally, there was a period (2015-2017) of more comprehensive sampling at both sub-basins, including phytoplankton samples and littoral points.

·         Chemical parameters were analyzed according to standardized methods, as described in the associated publications.

·         Phytoplankton taxa were identified and counted under light and SEM microscopy, and biovolume was determined followed European standards (EN 15204:2006, EN 16695:2015).

·         The physical structure of the water column was studied following the methodology and resolution described in the associated publications.

·         Cylindrical sediment traps were installed at three depths (6, 14, and 40 m) in the deepest area of the western sub-basin. Several surface sediment cores were obtained at the deepest area of the eastern sub-basin using a Glew-type gravity corer and were sliced at 2 mm intervals. Chemical and biological analyses were performed according to standardized methods, as described in the associated publications.

1.4   Hydrological and external load modelling:

·         The water inflow and nutrient loads to the lake were assessed by dynamic modeling on a 5-m spatial resolution digital elevation model (CNIG, 2015) validated with an intensive biennial (2015-2017) sampling of catchment tributaries and atmospheric deposition. Details of the methodology used in the hydrological modelling and the calculation of external loads can be found in the associated publications.

1.5   Lake trends analyses:

·         The Lagrange multiplier test for conditional heteroscedasticity (Seekell et al., 2012) was used to analyze lake ecosystem trends.

2.       Procedures followed to ensure the quality of the data.

In the field sampling and laboratory analyses, official or standardized methodologies and protocols have been used or, failing that, those most widely accepted by the scientific community, adapting them to better meet the objectives of the work. The laboratories where the analyses have been carried out are certified for the ISO 9001 quality system for the design, development and performance of physical and chemical analyses using chromatography, spectrometry, spectrophotometry, elemental analysis, and toxicity analysis in inland waters.