The effect of the Leeuwin Current on phytoplankton biomass and production off Southwestern Australia

Abstract

[1] Unlike most eastern boundary currents, the Leeuwin Current off the west coast of Australia flows poleward and is therefore warm, nutrient-poor and suppresses upwelling. As a result, the waters off Western Australia are relatively oligotrophic. Primary productivity and concentrations of chlorophyll are particularly low in summer, when the water column is stratified and most chlorophyll is concentrated in a layer above the nutricline at approximately 100 m depth. The phytoplankton blooms in late autumn and winter, coinciding with the period of strongest Leeuwin flow. The bloom in winter is maintained by cooling and storms, which promote convective mixing of the upper water column and a shoaling of the nutricline and chlorophyll maximum layer. However, the late autumn bloom coincides with the initial intensification of the Leeuwin Current. Eddies and meanders spin up just beyond the shelf break, and Leeuwin Current water was observed at this time to destratify the water column and flood the shelf with relatively nutrient-rich water. Several potential sources for the nutrients are proposed: upwelling by the eddies and meanders as they induce cross-shelf motions; advection from the north, where the nutricline is shallower; and seasonal cooling.

Oceanographic forcing of nutrient uptake and release over a fringing coral reef

“…However, the LC autumn bloom is now known to be the single largest regional productivity event annually (Koslow et al 2008), and recent work has suggested that the LC may actually increase nutrient supply to Ningaloo during the autumn, albeit in the form of POM rather than as NO x or PO 4 . The ratio of PON from phytoplankton (PON p ; Wyatt et al [2010]) to nitrate, PON p : NO x , was significantly higher during both May 2007 (2.7 6 0.3) and May 2008 (20 6 9) than during November (0.52 6 0.1; A.S.J.W.…”

Long-term changes in temperate Australian coastal waters: implications for phytoplankton

“…As a result, phytoplankton biomass and primary production in southwest Australian waters are relatively low compared to the east coast, and show a winter rather than a spring maximum (Jitts 1969, Koslow et al 2008. Interannual variability in Leeuwin Current flow and sea surface temperature is related to the El Niño-Southern Oscillation (ENSO) phenomenon (Feng et al 2003).…”

“…extinction coefficient for PAR ≈ 0.05 m -1 offshore of Rottnest, Thompson et al 2007), such that remote sensing 4 Table 2. Description of the spatial and temporal sampling for phytoplankton at or near coastal monitoring stations (see Table 1) and areas assessed for chlorophyll a (chl a) biomass using SeaWiFS would detect chl a to~20 m. A deep chlorophyll maximum (DCM) is likely in summer (Koslow et al 2008), although a DCM should not bias the assessment of temporal trends over time spans >1 yr. At Maria Island, the growth rate of the spring bloom was calculated from a linear regression of the natural log of the remotely sensed mean monthly chl a concentrations over the period from July to October (4 mo) over the area from 42.2 to 43°S by 148.2 to 149.0°E (Table 2). One growth rate (=1 regression) was estimated for each year from 1997 to 2006.…”

Particulate nutrient fluxes over a fringing coral reef: relevant scales of phytoplankton production and mechanisms of supply

“…Correspondingly, the integrated flux of phytoplankton to the reef during , respectively). There are several possible mechanisms for increased phytoplankton biomass during the non-upwelling season in May, including (1) autumn acceleration of the LC transporting nutrients from the north, from below the photic zone, and in eddies and meanders; (2) convective mixing due to cooling and storms activity (Koslow et al 2008); and (3) changes in near-reef biogeochemistry related to autumn coral spawning (i.e. high concentrations of nutrients).…”