Decadal and seasonal changes in temperature, salinity, nitrate, and chlorophyll in inshore and offshore waters along southeast Australia

Abstract

Sixty years of oceanographic in situ data at Port Hacking (348S) and Maria Island (428S) and 15 years of satellite-derived chlorophyll a (chl a) in inshore and offshore waters of southeast Australia show changes in the seasonality and trend of water properties consistent with long-term intensification and southerly extensions of East Australian Current (EAC) water. Decadal analyses reveal that the EAC extension water at Maria Island increased gradually from the 1940s to 1980s, followed by a rapid increase since the 1990s. This acceleration coincided with enhanced winter nitrate, implying increased injections of subantarctic water at Maria Island. Satellite-derived chl a at six coastal sites and offshore companion sites in the western Tasman Sea showed significant inshore-offshore variations in seasonal cycle and long-term trend. After 2004-2005, the Maria Island seasonal cycle became increasingly similar to those of Bass Strait and St. Helens, suggesting that the EAC extension water was extending further southward. Comparative analyses of inshore-offshore sites showed that the presence of EAC extension water declined offshore. Seasonal cycles at Maria Island show a recent shift away from the traditional spring bloom, toward increased winter biomass, and enhanced primary productivity consistent with extensions of warm, energetic EAC extension water and more frequent injections of cooler, fresher nitrate-replete waters. Overall, we find complex temporal, latitudinal, and inshore-offshore changes in multiple water masses, particularly at Maria Island, and changes in primary productivity that will profoundly impact fisheries and ecosystems.

Phytoplankton absorption predicts patterns in primary productivity in Australian coastal shelf waters

“…In contrast, our results show that NPP peaks during winter at Port Hacking, when cooling waters create deeper mixed layer depths (Everett and Doblin, 2015) and have higher dissolved nitrate concentrations (Kelly et al, 2015). Chlorophyll-a concentration, although not always synchronous with NPP, is also negatively correlated to in-situ water temperature, and winter and spring concentrations have been shown to be comparable (Everett and Doblin, 2015;Kelly et al, 2015).…”

“…In-situ observations and satellite (a PHY derived) estimates of surface NPP were comparable to those reported (for 13 C and 14 C incubations) in the literature for the North-west Kimberley region and in the North (0.6-8 mmol C m -3 d -1 ; Furnas and Carpenter, 2016 and references therein), GBR (0.2-4.1 mmol C m -3 d -1 ; Furnas and Carpenter, 2016;Furnas and Mitchell, 1987) and South-east (0.1-5 mmol C m -3 d -1 ; Everett and Doblin, 2015; See also Figure 7). We observed the highest in-situ daily NPP rates at stations in the South-east (V2 and PHNRS) and expected that the mean in-situ daily NPP would also peak in this region (Everett and Doblin, 2015) given the propensity for wind-driven and EAC driven upwelling (Roughan and Middleton, 2002) which encourages the growth of nanoplankton and microplankton (Ajani et al, 2014;Kelly et al, 2015). Generally, the oligotrophic Coral Sea region of the North-east supports very low phytoplankton biomass (Hayes et al, 2005).…”

Observed multi-decadal trends in subsurface temperature adjacent to the East Australian Current

“…show that the shelf warming in the EAC southern extension is almost solely advection-driven (Malan et al, 2021), changes in remote forcing effects seem more likely to be the main drivers of the trends that we see at Port Hacking and Maria Island. The accelerating warming at Maria Island, particularly since the 1980s, is also consistent with Kelly et al (2015) who showed that the amount of EAC extension water at this site rapidly increased over the same time period.…”

“…In order to provide a comprehensive record of temperature trends in our region, we compare our results with previous studies that have investigated some aspect of temperature trends at or close to the sites used in this study (Wijffels et al, 2018;Malan et al, 2021;Thompson et al, 2009;Ridgway, 2007;Hill et al, 2008;Shears and Bowen, 2017;Holbrook and Bindoff, 1997;Kelly et al, 2015;Foster et al, 2014). Each of these studies have used linear methods to estimate the trends, and most studies have used surface data only.…”

Variability and Long‐Term Trends in the Shelf Circulation Off Eastern Tasmania

“…Tasman Sea Blocking (TSB) events also significantly affect the atmospheric circulation and rainfall patterns in the region (Pook & Gibson, ), and may influence the nearshore marine climate but this has yet to be demonstrated. Nearer to the Tasmanian coast, observations of trends and variability have been restricted to the relatively long Maria Island record, which exhibits significant multidecadal increases in temperature, salinity, and nitrate (Kelly et al, ). On the other hand, very little is known about the trends or variability in the ZC or the spatial distribution of multidecadal trends in temperature and salinity over the shelf.…”