Seasonal and inter-annual variability of water column properties along the Rottnest continental shelf, south-west Australia

Abstract

A multi-year ocean glider dataset, obtained along a representative cross-shelf transect along the Rottnest continental shelf, south-west Australia, was used to characterise the seasonal and inter-annual variability of water column properties (temperature, salinity, and chlorophyll fluorescence distribution). All three variables showed distinct seasonal and inter-annual variations that were related to local and basin-scale ocean atmosphere processes. Controlling influences for the variability were attributed to forcing from two spatial scales: (1) the local scale (due to Leeuwin Current and dense shelf water cascades, DSWC) and (2) the basin scale (El Niño-Southern Oscillation, ENSO, events). In spring and summer, inner-shelf waters were well mixed due to strong wind mixing, and deeper waters ( > 50 m) were vertically stratified in temperature that contributed to the presence of a subsurface chlorophyll maximum (SCM). On the inner shelf, chlorophyll fluorescence concentrations were highest in autumn and winter. DSWCs were also the main physical feature during autumn and winter. Chlorophyll fluorescence concentration was higher closer to the seabed than at the surface in spring, summer, and autumn. The seasonal patterns coincided with changes in the wind field (weaker winds in autumn) and air-sea fluxes (winter cooling and summer evaporation). Inter-annual variation was associated with ENSO events. Lower temperatures, higher salinity, and higher chlorophyll fluorescence (> 1 mg m −3 ) were associated with the El Niño event in 2010. During the strong La Niña event in 2011, temperatures increased and salinity and chlorophyll fluorescence decreased (< 1 mg m −3 ). It is concluded that the observed seasonal and inter-annual variabilities in chlorophyll fluorescence concentrations were related to the changes in physical forcing (wind forcing, Leeuwin Current, and air-sea heat and moisture fluxes).

Substantial kelp detritus exported beyond the continental shelf by dense shelf water transport

“…In this study, we mainly focused on the export of simulated kelp detritus beyond the continental shelf edge at 200 m depth. This is below the depth of the Leeuwin Current 15 as well as the typical mixed layer depth in the region 33,34 . Kelp detritus transported past the shelf edge can therefore potentially be sequestered as there is little exchange with surface waters below 200 m. However, sequestration time scales vary widely depending both on the depth and the geographic location 35 , and biogeochemical models combined with ocean circulation models are required to determine potential sequestration timescales of tens to hundreds of years 36 .…”

Including the invisible: Deep depth-integrated chlorophyll estimates from remote sensing may assist in identifying biologically important areas in oligotrophic coastal margins

“…Euphotic zone depth estimates based on these alternative definitions do not vary in lockstep with those based on Z1% PAR (Wu et al, 2021), so using a more appropriate definition of the euphotic zone may decrease the scatter observed. A lesser-known source of variation may have come from inter-annual variation in productivity, which in our study area is primarily related to fluctuations in the Leeuwin Current strength following the El Niño Southern Oscillation (ENSO; Feng et al, 2009;Chen et al, 2019). Three strong ENSO events occurred during the study period (i.e., one El Niño and two La Niña's; Australian Bureau of Meteorology, 2023), and we recommend that their effects on the relationships found are further assessed.…”

“…For such an assessment to be of value to future inferences of phytoplankton productivity (e.g., hotspots or anomalous productivity events) from satellite-derived surface chlorophyll data, it is imperative to ensure that depthintegrated values capture DCMs and that satellite remote sensing accurately reflects in situ conditions. While previous studies have touched on these topics, they were either spatiotemporally restricted (e.g., Hanson et al, 2005aHanson et al, , b, 2007Fearns et al, 2007;Twomey et al, 2007;Koslow et al, 2008;Chen et al, 2019) or based on discontinued satellite data (i.e., SeaWIFS, concluded in 2010; Koslow et al, 2008). Hence, this study assessed temporal patterns in water column stratification, DCM formation, and DCM characteristics (i.e., type, depth, width); analysed the relationship between surface and depth-integrated chlorophyll values; and validated satellite-derived against in situ chlorophyll measurements.…”

True blue: Temporal and spatial stability of pelagic wildlife at a submarine canyon

“…La Niña strengthens the Leeuwin Current’s flow, driving marine heatwaves, the tropicalization of temperate communities, and changes in productivity (Feng et al 2013, Pearce and Feng 2013, Pearce et al 2016, Hewitt et al 2018). For example, the 2011 La Niña event raised temperatures by over 4°C and reduced chlorophyll concentrations from 1 to <0.25 mg/m 3 (Chen et al 2019). As such, the 2018 La Niña may have affected the productivity and other characteristics of the Perth Canyon, with knock‐on effects on taxonomic richness and abundance.…”