Total, dissolved, total reactive and ferrous reactive iron concentrations (ug/L).

This study looked for relationships between cyanobacterial abundance and iron in western Lake Erie and Saginaw Bay, Lake Huron, with an emphasis on total reactive iron, during a cruise on the R/V Laurentian August 21-26, 2005. These samples are from Saginaw Bay, Lake Erie, Lake St. Clair, the St. Clair River and the Detroit River.

Total unfiltered iron	ICP-mass spectrometry
Total filtered iron	ICP-mass spectrometry
Total reactive iron	TPTZ (2,4,6-tris(2-pyridyl)-s-triazine) colorimteric method
Ferrous reactve iron	TPTZ (2,4,6-tris(2-pyridyl)-s-triazine) colorimteric method

Methods

Water samples were collected for iron analyses from the near surface (approx 0.2 m) with a Niskin bottle. Stations sampled consisted of standard NOAA sampling stations in western Erie and Saginaw Bay, as well as two stations in Maumee Bay (outer bay, T3 and inner bay at the mouth of the Maumee River, T4), two stations just outside Maumee Bay (T1 and T2), Sandusky Bay (X1), and a transect between the mouth of the Detroit River and Lake Huron where X2 is the mouth of the St. Clair River, X3 is the Detroit River south of Lake St. Clair, X5 is in Lake St. Clair and X6 is near the start of the St. Clair River in Lake Huron (Table 1). GPS derived coordinates, nutrient chemistry, and phytoplankton counts were not recorded for stations X1-X6, nor were samples for nutrient chemistry and phytoplankton collected for transect T.

Samples were collected in acid washed polyethylene containers. Ten ml of unfiltered and filtered (0.45 um Millipore cellulose membrane filter) water were placed in acid washed vials, acidified with 1 drop of trace metal grade hydrochloric acid and stored for analysis of iron by ICP-mass spectrometry. Particulate iron is the difference between total and ‘dissolved’ (i.e., filtered) iron.

Duplicate samples were taken for measurement of total reactive and ferrous iron using a modification of the TPTZ (2,4,6-tris(2-pyridyl)-s-triazine) method (developed by the Fisheries Ocean Canada laboratory at the Experimental Lakes Area in northwestern Ontario). Immediately upon collection, reagents were added to 25 ml of unfiltered sample in an acid-washed syringe. The fixed samples were kept refrigerated in the dark and the absorbance at 595 nm (10 cm path length) was measured within 3 hours of collection (absorbance of standard solutions remains stable for up to 3 hours at room temperature, varying less than 4% for total reactive Fe > 10 ug/L (Molot, unpublished data)). Ferrous iron was measured with 5 ml of buffer without reductant and 2.5 ml of TPTZ solution added to 25 ml of sample. The buffer consisted of 287g sodium acetate trihydrate and 115 mL glacial acetic acid in 1.0 L of deionized water. The TPTZ solution consisted of 0.15 g TPTZ and 25 mL 3N HCl in 500 mL of deionized water. Total reactive iron was measured as above with 5 ml of buffer that included a reductant (10 g hydroxylammonium chloride). Since dissolved organic carbon (DOC) and particulate matter absorb at 595 nm, the absorbance of water samples without reagents corrected for dilution was subtracted from the absorbance with reagents before calculating iron concentration. All chemicals were reagent grade. Samples were calibrated using a certified iron solution (1000 µg ml-1) from Anachemia.

Samples from a depth of 1 m were analyzed for chlorophyll a, organic nitrogen and total phosphorus (TP) by NOAA at stations A-E.

For more information, contact:

Lewis Molot
Faculty of Environmental Studies
York University
4700 Keele Street
Toronto, Ontario M3J 1P3
Canada
email:
Tel: 416-736-2100, ext 22613
Fax: 416-736-5679