CAUSES OF LIGHT ATTENUATION IN 9 NEW-ZEALAND ESTUARIES

Optical properties and the constituent composition of the waters of nine northern New Zealand estuaries were determined during 1988-1989. Substantial variation was found both within and between the estuaries. For characteristics such as beam attenuation, turbidity and suspended solids, sources of variation in particular estuaries included: state of the tide (causing three-fold differences); seasonal changes (five to ten-fold); incomplete mixing, both horizontal (two to three-fold) and, in one case, vertical (about two-fold); and the presence of flood-waters (at least five-fold). The waters at the estuary mouths were fairly turbid (Secchi depths 1·2-4·4 m), and were comparatively bright (PAR reflectance 6-12%), because the constituents tended to scatter more light than they absorbed (7-17 times more on average). Phytoplankton biomass was low (chlorophyll α, 0·5-5 mg m-3) and suspended particulates were mostly inorganic (82%). Equations were derived which described how the water constituents affected the estuaries' optical properties. Reasonably reliable predictions resulted when the equations were tested against a separate data set. The inorganic suspensoids (3-11 g m-3) caused 56% of beam attenuation, four times that caused by phytoplankton. Scattering per unit mass of suspensoids was several times lower than that found in some New Zealand lakes because of the larger average particle size of the estuarine material. Oxidation pond effluent discharged to one of the estuaries had little effect on beam attenuation there. © 1990.