Biological properties across the Tasman Front off southeast Australia

Abstract

Physical, geochemical and biological observations across the Tasman Front off southeast Australia provide the first detailed view of the relationship between physical forcing and biological properties within the frontal system. At the beginning of the austral spring of 2004, high resolution measurements were taken using a CTD and a towed undulating vehicle along transects perpendicular to the Tasman Front at 152 ∘ 00 ′ E , 153 ∘ 00 ′ E and 153 ∘ 30 ′ E . The front was characterised by a sharp surface gradient in physical and biological properties and a sub-surface intrusion of low-salinity water. In general the surface temperature changes across the front from 19 ∘ C in the Coral Sea waters to the north to 17 ∘ C in the Tasman Sea waters over ∼ 10 km . Over the same distance we observed (1) an increase of the mixed layer depth from ∼ 40 to ∼ 100 m ; (2) a 3–8-fold increase in the depth-integrated chlorophyll; (3) an order of magnitude increase in biovolume of particulate matter in the size range of 357– 2223 μ m ; (4) a 2-fold increase in filtered particulate organic matter; (5) a 3 ‰ increase in δ 13 C POM ; and (6) a 4 ‰ increase in δ 15 N POM . The particulate matter in the warmer Coral Sea waters is well approximated by a linear fit of the normalised biomass size spectrum (NBSS) with a slope of between - 0.95 and - 0.99 , while the Tasman Sea waters have a more non-linear and less negative ( - 0.59 to - 0.8 ) spectrum. The low-salinity intrusion that penetrates to within 40 m of the surface between the Coral Sea and Tasman Sea waters is biologically unproductive, with low oxygen, fluorescence and particulate matter counts. The unproductive low salinity intrusion of the Tasman Front contrasts with the highly productive intrusion observed at the Gulf Stream Front off Cape Hatteras, USA. Observations are consistent with Coral Sea and Tasman Sea waters being found in close proximity with steep gradients in biological properties across the front suggesting minimal cross-front mixing. North of the front, the stratified, oligotrophic Coral Sea waters are relatively unproductive, while the vertically well-mixed waters south of the front exhibit strong biological activity.