Upwelling linked to warm summers and bleaching on the Great Barrier Reef

Abstract

We investigate a range of indices to quantify upwelling on the central Great Barrier Reef (GBR), Australia, so that environmental and biological relationships associated with upwelling in this area can be explored. We show that ''Upwelling days'' (the number of days of upwelling) and diurnal variation in subsurface temperature (maximum-minimum, 20-m depth) are satisfactory metrics to describe the duration and intensity of upwelling events, respectively. We use these to examine key characteristics of shelf-break upwelling in the central GBR. Our results show, somewhat paradoxically, that although upwelling involves cold water being brought near to the surface, it is linked to positive thermal anomalies on the GBR, both locally and regionally. Summers (December to February) with strongest upwelling occurred during the GBR-wide bleaching events of 1997-1998 and 2001-2002. Upwelling in the GBR is enhanced during doldrums conditions that were a feature of these summers. During these conditions, the poleward-flowing East Australian Current flows faster, lifting the thermocline closer to the surface, spilling more sub-thermocline waters onto the shelf. Doldrums conditions also result in intense local heating, stratification of the water column, and, when severe, coral bleaching. Upwelling intrusions are spatially restricted (central GBR), generally remain subsurface, and are often intermittent, allowing GBR-wide bleaching to occur despite conditions resulting in enhanced upwelling. Intense upwelling events precede anomalous seasonal temperature maxima by up to 2 months and bleaching by 1-3 wk, leading to the prospect of using upwelling activity as a seasonal forecasting index of unusually warm summers and widespread bleaching.

Satellite-Derived Photic Depth on the Great Barrier Reef: Spatio-Temporal Patterns of Water Clarity

“…During austral winter, or the "dry season" on the GBR, the prevailing southeasterly trade winds dominate, limiting cross-shelf surface flow. Furthermore, seasonal strengthening of the southward-flowing EAC in early spring (September-October), when the opposing trade winds begin to subside [26][27][28], results in strong intrusions of clear, oligotrophic EAC waters through the Palm and Magnetic Passages in the central GBR [29], with southward flow along the mid-shelf channel (Figure 3(b)). At the southern end of the GBR, the Capricorn Eddy forms most strongly during spring when the EAC strengthens, with resultant intrusions of clear EAC waters into the Capricorn and Curtis Channels [15], further enhancing separation of the inner and outer shelf regions (Figure 3(b)).…”

“…Furthermore, water clarity is generally greatest during early spring ( Figure 7(b)), when the EAC strengthens and the southeast trade winds relax. This springtime acceleration of the EAC is known to lead to intrusions in the central GBR [27,29] and further to the south, the Capricorn Eddy forms most strongly and regularly in spring, with associated intrusions up the Capricorn Channel [15,28]. These intrusions onto the mid-shelf would somewhat separate the water bodies of the inner-shelf from the outer shelf waters.…”

Symbiosis and microbiome flexibility in calcifying benthic foraminifera of the Great Barrier Reef

“…On the GBR, factors such as nutrient concentration and temperature vary across an inshore-offshore gradient. Inshore reef sites are more prone to temperature fluctuations and influx of dissolved inorganic nutrients from terrestrial runoff, whereas offshore sites display more stable temperature conditions and low nutrient concentrations [36, 37]. The capacity to acquire different eukaryotic symbionts (i.e., photosynthetic algae) and prokaryotic associates is likely to be a potential driver of the ability of LBFs to occupy a broad range of habitats in reef environments.…”

“…Samples were collected by SCUBA divers from the back slope of reefs with similar habitat located on the (1) inner-shelf—Martin Reef (14° 45′ 19.2″ S; 145° 20′ 07.9″ E), (2) mid-shelf—Lizard Island (14° 14′ 22.3″ S; 145° 27′ 58.1″ E), and (3) outer-shelf—Yonge Reef (14° 35′ 50.1″ S; 145°37' 26.3"E) at depths of 6 to 8 m (corrected to lowest astronomical tide levels). Reef sites are located along a water quality gradient [36, 49] and also experience differing patterns of temperature fluctuation [37, 47, 50]. Pieces of dead coral rubble were brought to the laboratory located at the Lizard Island Research Station and were scrubbed to remove LBF.…”

Environmental and biological cues for spawning in the crown-of-thorns starfish

“…Sea surface temperatures can vary > 4°C throughout the summer spawning season on the GBR, but within the course of a single day, temperatures usually vary within 1°C in the relatively deeper reef slope and within 1–2°C in the shallower reef flats [31]. Although rare, abrupt temperature changes have been reported in some parts of the GBR [32,33,61]. Temperature spikes from normal diurnal temperature variation have been associated with intense summer upwelling events in the GBR [61].…”