Variations in Mid‐ to Late Holocene Nitrogen Supply to Northern Great Barrier Reef Halimeda Macroalgal Bioherms

Abstract

The northern Great Barrier Reef (GBR) Halimeda bioherms have accumulated on the inter-reef seafloor of the outer continental shelf from CaCO 3 algal sediments since at least ∼10,000 years ago (G. R. Orme & Salama, 1988). Biophysical processes interacting with the Halimeda bioherms influence the northern GBR system across 6° of latitude (Figure 1), covering an area >6,000 km 2 and up to 25% of the continental shelf north of 16°S latitude (McNeil et al., 2016). Therefore, Halimeda bioherm sediments represent a potential archive of Holocene marine biogeochemistry and paleoclimate. Given their vast distribution and CaCO 3 volume, Halimeda bioherms likely play a key role in the tropical shallow marine carbon cycle (Rees et al., 2007) via C fixation (photosynthesis) and mineralization (precipitation of CaCO 3). Yet the biogeochemical processes regulating Halimeda bioherm formation and distribution are poorly understood (Erler et al., 2018; McNeil et al., 2016). The expanse of Halimeda bioherms behind the GBR Ribbon Reefs, and their persistence over time, suggests a direct and long-term relationship between Halimeda growth and the source of nutrient supply (Wolanski et al., 1988) to bioherms in this region.

Australia’s Two Great Barrier Reefs: What Can ~360 Million Years of Change Teach Us?

“…Major frequent changes in the correction occurred prior to stabilization at ~5.5 kyr BP, when more modern ENSO behavior engaged, altering the degree of upwelling of deep older waters that reached the GBR. Halimeda bioherms in the GBR lagoon record changes in δ 15 N that reflect changes in terrestrial versus upwelled water sources that also correlate well with ENSO frequency and intensified El Niño activity after ~5 kyr BP [317]. The data also correlate with monsoon (IASM) reconstructions based on independent speleothem data [318].…”

“…Importantly, CCA thickness decreases significantly below 6 m depth, as does the abundance of vermetid gastropods encrusted within the thalli [217]. In the back-reef lagoon, extensive Halimeda bioherms occur in water depths of 20 to 50 m and with an area of >6000 km 2 , they occupy more than twice the area covered by coral reefs [317,373]. The bioherms reach as much as 20 m in height and initiated ~10,000 yr ago, when the shelf was first flooded during the deglacial [374].…”

Australia’s Two Great Barrier Reefs: What Can ~360 Million Years of Change Teach Us?

“…Major frequent changes in the correction occurred prior to stabilization at ~5.5 kyr BP, when more modern ENSO behavior engaged, altering the degree of upwelling of deep older waters that reached the GBR. Halimeda bioherms in the GBR lagoon record changes in δ 15 N that reflect changes in terrestrial versus upwelled water sources that also correlate well with ENSO frequency and intensified El Niño activity after ~5 kyr BP [317]. The data also correlate with monsoon (IASM) reconstructions based on independent speleothem data [318].…”

“…Importantly, CCA thickness decreases significantly below 6 m depth, as does the abundance of vermetid gastropods encrusted within the thalli [217]. In the back-reef lagoon, extensive Halimeda bioherms occur in water depths of 20 to 50 m and with an area of >6000 km 2 , they occupy more than twice the area covered by coral reefs [317,373]. The bioherms reach as much as 20 m in height and initiated ~10,000 yr ago, when the shelf was first flooded during the deglacial [374].…”

Crustose Coralline Algae as a Proxy for Marine Nitrogen Cycling and Coral Trophic Strategies

“…In experimental systems, CCA exhibit a greater a nity for NH₄⁺ than for nitrate at a given concentration 60 . This is a common observation across algal taxa 71 and presumably re ect the lower energetic cost of assimilating reduced nitrogen species 72 . Ammonium is central to the regenerative N cycle of the euphotic zone.…”

Australia’s Two Great Barrier Reefs: What Can ~360 Million Years of Change Teach Us?

“…Major frequent changes in the correction occurred prior to stabilization at ~5.5 kyr BP, when more modern ENSO behavior engaged, altering the degree of upwelling of deep older waters that reached the GBR. Halimeda bioherms in the GBR lagoon record changes in δ 15 N that reflect changes in terrestrial versus upwelled water sources that also correlate well with ENSO frequency and intensified El Niño activity after ~5 kyr BP [317]. The data also correlate with monsoon (IASM) reconstructions based on independent speleothem data [318].…”

“…Importantly, CCA thickness decreases significantly below 6 m depth, as does the abundance of vermetid gastropods encrusted within the thalli [217]. In the back-reef lagoon, extensive Halimeda bioherms occur in water depths of 20 to 50 m and with an area of >6000 km 2 , they occupy more than twice the area covered by coral reefs [317,373]. The bioherms reach as much as 20 m in height and initiated ~10,000 yr ago, when the shelf was first flooded during the deglacial [374].…”

Crustose Coralline Algae as a Proxy for Marine Nitrogen Cycling and Coral Trophic Strategies

“…In experimental systems, CCA exhibit a greater a nity for NH₄⁺ than for nitrate at a given concentration 60 . This is a common observation across algal taxa 71 and presumably re ect the lower energetic cost of assimilating reduced nitrogen species 72 . Ammonium is central to the regenerative N cycle of the euphotic zone.…”

Australia’s Two Great Barrier Reefs: What Can ~360 Million Years of Change Teach Us?

“…Major frequent changes in the correction occurred prior to stabilization at ~5.5 kyr BP, when more modern ENSO behavior engaged, altering the degree of upwelling of deep older waters that reached the GBR. Halimeda bioherms in the GBR lagoon record changes in δ 15 N that reflect changes in terrestrial versus upwelled water sources that also correlate well with ENSO frequency and intensified El Niño activity after ~5 kyr BP [317]. The data also correlate with monsoon (IASM) reconstructions based on independent speleothem data [318].…”

“…Importantly, CCA thickness decreases significantly below 6 m depth, as does the abundance of vermetid gastropods encrusted within the thalli [217]. In the back-reef lagoon, extensive Halimeda bioherms occur in water depths of 20 to 50 m and with an area of >6000 km 2 , they occupy more than twice the area covered by coral reefs [317,373]. The bioherms reach as much as 20 m in height and initiated ~10,000 yr ago, when the shelf was first flooded during the deglacial [374].…”

Crustose Coralline Algae as a Proxy for Marine Nitrogen Cycling and Coral Trophic Strategies

“…In experimental systems, CCA exhibit a greater a nity for NH₄⁺ than for nitrate at a given concentration 60 . This is a common observation across algal taxa 71 and presumably re ect the lower energetic cost of assimilating reduced nitrogen species 72 . Ammonium is central to the regenerative N cycle of the euphotic zone.…”