Methods
Sample collection (2012–2013). Water samples were collected at Upper Mystic
Lake (Medford, MA), from one location in the middle of the lake (∼42° 26.155′ N,
71° 08.961′ W) where the total water depth is 23 m.Water samples were collected in
2012 (2 October) and 2013 (26 March, 10 May, 17 June, 17 July and 15 August).
Water samples were collected at approximately 1 to 2 m intervals through 25 m of
plastic Tygon tubing using a peristaltic pump. Two volumes of water at each depth
were pumped through the tubing before 50 ml was filtered through an in-line
Swinnex filter holder onto sterile 0.22 μm filters (Millipore) and the filtrate collected
in a 50 ml conical tube. Filters and filtrate were placed on dry ice immediately and
transported back to the laboratory where they were placed at −80 °C until processing.
To determine the influence of contamination from the tubing, sampling method and
carryover from the previous depth, we collected blanks by pumping 2 l of sterile
water through the tubing before and after sampling. Blanks were distinct from other
samples. Samples (1 ml) of both filtered and unfiltered water were put into a 1.5 ml
microcentrifuge tube with 43 μl of concentrated HCl and placed in the dark during
transport back to the laboratory for ferrous (Fe2+) and total iron analysis,
respectively. Samples were stored at −20 °C until iron was measured.