After each 24-hour stagnation period, cells were mixed by inverting five times. Aliquots of 10 mL were then decanted into polypropylene tubes, acidified to pH < 2 with concentrated trace metal grade nitric acid, and held for a minimum of 24 hours before analysis. Separate 10 mL aliquots were filtered, immediately after collection, using 0.45 µm membrane filters in a syringe-mounted apparatus.