Lake water samples were collected at specific depths with a five-liter Niskin bottle during normal LTER limnological sampling. Sub-samples were decanted into three 1 L Nalgene bottles (two light and one amber), two 500 mL amber Nalgene bottles, three 150 mL borosilicate glass bottles, two 20 mL scintillation vials, and one 30 mL serum vial. A 500 mL sample from the one liter amber Nalgene bottle was decanted and filtered through a combusted and acidified Whatman 25 mm GF/F filter. The filters were pre-combusted at 475 °C for 4 hours to remove organic phosphorus, soaked in 1% HCl, rinsed with DI water, and then brought to pH 8 with sodium hydroxide before being dried and transported to the field. After filtering, the filter was placed onto an aluminum weighing pan and air dried. Once the filters were dry, the pans were stacked together and frozen for later analysis.
The filter analysis method of total particulate phosphorus in natural waters is based on the digestion procedure of Solórzano and Sharp (1980), and on a modification developed by Murphy and Riley (1962) and Strickland and Parsons (1972). Residue collected on a filter is treated with magnesium sulfate and baked at high temperature to decompose organic phosphate. The residue is then treated with hydrochloric acid to hydrolyze polyphosphates and the orthophosphate is measured by the Molybdate method. Analysis of orthophosphate depends on the reduction of a stable phosphomolybdate complex by ascorbic acid in the presence of antimony. A blue sol is produced; our standard curves showed linearity in absorbance up to 9.5 µM P (294 µg P L-1) using an Ocean Optics Jaz Spectrometer with a 10 cm cell at 885 nm wavelength. Solórzano and Sharp (1980) and Priscu (MCM LTER Limno Methods Manual, 2019) revealed that the method gives 100% recovery with refractory phosphorus compounds (apple leaf standard) in marine and fresh waters.
All samples that were analyzed in 2021 (2012-2013 to 2019-2020 field seasons) were centrifuged at 5000 rpm for ten minutes after the addition of the mixed reagent. Our tests indicated that this centrifugation step reduced the absorbance of the blanks. Samples analyzed in previous years (2006-2007 to 2011-2012 field seasons) were centrifuged at 2000 rpm for five minutes to decrease turbidity if the sample looked cloudy.
L2 samples from the 2007-2008 season were analyzed in Crary Laboratory in McMurdo Station with a different spectrophotometer and a modified protocol. These samples were diluted ten times before the addition of mixed reagent. The turbidity of each sample at 660 nm was evaluated on the spectrophotometer. If absorbance at 660 nm exceeded 0.002, the samples were syringe filtered before being analyzed at 885 nm. These samples were not centrifuged, and no filter blanks were subtracted from the sample absorbances.
No particulate phosphorus data is available for the 2010/2011 field season.
Particulate phosphorus is no longer collected or analyzed as of the 2022-2023 field season. Samples collected during the 2021-2022 and 2022-2023 austral summers have not been analyzed. These samples are in storage, but there is no plan to analyze those samples as of March 2025.
