Biologically relevant macronutrients, nitrate + nitrite, silicate, phosphate and ammonia, were measured at all sites throughout the study. Nitrate + nitrite values (NOx) at the surface showed clear seasonal trends, peaking over winter and drawing down to near zero in summer and autumn. Phosphate concentrations also reached a peak in winter, which was associated with Southern Ocean influence. Median ammonium concentrations at all sites were generally <0.5 μM, with no clear peaks in any season or month. Overall, the lowest values were measured in August and other months showed reasonable spread around the median. Median silicate concentrations were consistently highest at sites 1 and 9, followed by site 5. Water from the River Derwent flows through site 1, then tracks east towards site 9 then site 5. Seasonally, silicate was generally highest in winter when the River Derwent outflow is also greatest.
Water samples were collected, using a 6 L or 8 L Niskin bottle, from 0.5 – 1 m below the surface, at 10 m depth, and within 5 m of the seabed. At site 3 an intermediate sample from 50 m was also collected. Subsamples were taken at each depth for macronutrients (unfiltered), and were frozen and stored in the dark until processing at the CSIRO Marine and Atmospheric Laboratories, Hobart. The biologically relevant nutrients, dissolved nitrate+nitrite, phosphate, ammonia and silicate concentrations (μM) were measured by CSIRO Hydrochemistry laboratory using standard colorimetric methodology adapted for flow injection analysis on a 5-channel Lachat Quick Chem 8000 series Automated Ion Analyser. Method detection limits were 0.02 μM P, 0.05 μM N, 0.05 μM Si, and 0.05 μM ammonia.