Lake Atitlan water quality 2010-2024

Field data were collected monthly from a pelagic point identified as Weiss G (14°42’23.182” N 91°11’13.901” W), close to the lake center and with a depth of 325 m. From 2009 to early 2017, temperature (ºC) and dissolved oxygen (mg L-1) were measured monthly using a YSI 51B and YSI 85 probe that measures to 60 m depth. Additional depths were sampled by raising water to the surface with a 2.2 L opaque Van Dorn bottle and immediately reading the temperature and dissolved oxygen. From early 2017 continuous measurements were made using a RBR maestro probe that reached the lake bottom. We collected all samples for laboratory analyses with a 2.2 L opaque van Dorn bottle at 0, 5, 10, 15, 20, 30, 40, 60, 80, 100, 150, 200, 225, 250, 275, and 305 m depths, except for 2010 to 2013, when sampling was usually limited to 100 m maximum depth. For chemical analyses, we transferred samples to acid-washed 250 ml glass BOD bottles and kept them on ice in darkness until processed in the laboratory within the next 9 hours. We manually filtered samples within 9 h of collection using 25 mm diameter 0.7 µm glass fiber filters (Whatman GF/F) and then refrigerated the filtered samples at 4 ºC until analysis within 48 h of filtration. Nutrient concentrations were determined using the methods: - Ammonia (NH4+–N µg L-1) concentrations with the phenate method (detection limit (DL) = 3 µg L-1) (APHA, 2005) - Nitrate plus nitrite (NO3-–N + NO2-–N µg L-1) concentrations with the hydrazine reduction method (DL = 2 µg L-1) (Kamphake et al., 1967) - Soluble reactive phosphate (SRP) (PO4-3–P µg L-1) concentrations with the ascorbic acid reduction method (DL = 1 µg L-1) (Murphy and Riley, 1962) - Total phosphorus (TP µg L-1) with the SRP method after digestion with the method 4500-P J (APHA 2005). When concentrations were below DL, we used one-half of the DL value for statistical analysis. All recipients, as well as all glassware used, were washed with a 0.1N HCl solution, and rinsed three times with demineralized water. Acid washing followed by a triple rinse was repeated before each sample collection. Basic scripts for data analysis are included.