YELLOW SUBSTANCE IN COASTAL AND MARINE WATERS ROUND THE SOUTH-ISLAND, NEW-ZEALAND

Yellow substance, the single largest component of dissolved organic matter in natural waters, is often the most important blue-light-absorbing constituent. The spectral absorption of yellow substance was measured by spectrophotometry in coastal (including fiord) and marine waters round the South Island of New Zealand on Cruise 2027 (May 1989) of the RV Rapuhia; this examined a wide range of optical variables in relation to primary productivity and water mass characteristics. The spectral absorption by yellow substance fitted an exponential model, as is typical, and the spectral slope parameter varied only slightly (mean = 0.015 nm-1, standard deviation = 0.002 nm-1) for oceanic waters in the range 300-460 nm. The concentration of yellow substance, indicated by the absorption coefficient of filtrates at 440 nm (g440), varied more than two orders of magnitude, from 0.014 m-1 in the cold subantarctic water south-east of the South Island, to 1.8 m-1 in humic-stained surface water in Doubtful Sound. In marine waters, g440 was fairly tightly distributed around a mean of 0.06 m-1 - a value typical in sea waters. g440 did not correlate with phytoplankton pigment content of marine waters, probably because most of the yellow substance was old material, derived from terrestial production or long past marine production. Nor was g440 correlated with salinity. But there was a weak (probably incidental) correlation with temperature (r = +0.57), with higher values of yellow substance in the relatively warm waters along the South Island west coast than in the colder waters off the east coast. The concentration of yellow substance explained nearly half of the total variation in light (PAR) absorption at the stations visited.