Nitrates in eastern Australian coastal waters

Abstract

Mean monthly temperature, salinity and nitrate concentration of eastern Australian coastal waters, at 0 and 50 m depth, for periods of up to 36 years have been determined. Using the seasonal changes in the mean properties it has been shown that the prime source of nitrates in coastal waters of eastern Australia between Evans Head and Eden is the intrusion, at various times in the spring-summer seasons, of subsurface slope waters, which are relatively rich in nitrates. Once present in the bottom layers of these coastal waters, these nitrates form a reservoir from which local processes of mixing in the autumn-winter seasons create a surface nitrate maximum, which persists for several months into the following spring. Off Evans Head and particularly off Laurieton, upwelling later in the spring creates a secondary nitrate maximum but these are of a transient time scale and little residual surface nitrate remains after the upwelling ceases. In summer, thermal capping, aided, particularly in the north, by surface salinity dilution, greatly limits the vertical mixing of nitrates to the surface, resulting in very low surface nitrate concentrations in summer. Off Eden, the spring introduction of nitrates by slope-water intrusions is impeded by a northward flow of Bass Strait waters which, because of their low nitrate content, act as a nutrient diluent. It is only when this northward flow ceases in early summer that these slope-water intrusions occur off Eden and provide the nitrate reservoir for the winter turnover. Off Maria I., slope-water intrusions are not an important contribution to the nitrate economy. The winter maximum in nitrate concentration at the Maria I. site is attributed to an onshore drift of nitrate-richer offshore waters, containing Sub-Antarctic elements. In spring, snow-melt waters mix with these winter waters off Maria I. and decrease both the salinity and nitrate content of these waters. In summer, subtropical waters of low nitrate content occur off Maria I.