Ecophysiology of coral reef primary producers across an upwelling gradient in the tropical central Pacific

Abstract

Upwelling is an important source of inorganic nutrients in marine systems, yet little is known about how gradients in upwelling affect primary producers on coral reefs. The Southern Line Islands span a natural gradient of inorganic nutrient concentrations across the equatorial upwelling region in the central Pacific. We used this gradient to test the hypothesis that benthic autotroph ecophysiology is enhanced on nutrient-enriched reefs. We measured metabolism and photophysiology of common benthic taxa, including the algae Porolithon, Avrainvillea, and Halimeda, and the corals Pocillopora and Montipora. We found that temperature (27.2-28.7˚C) was inversely related to dissolved inorganic nitrogen (0.46-4.63 μM) and surface chlorophyll a concentrations (0.108-0.147 mg m -3 ), which increased near the equator. Contrary to our prediction, ecophysiology did not consistently track these patterns in all taxa. Though metabolic rates were generally variable, Porolithon and Avrainvillea photosynthesis was highest at the most productive and equatorial island (northernmost). Porolithon photosynthetic rates also generally increased with proximity to the equator. Photophysiology (maximum quantum yield) increased near the equator and was highest at northern islands in all taxa. Photosynthetic pigments also were variable, but chlorophyll a and carotenoids in Avrainvillea and Montipora were highest at the northern islands. Phycobilin pigments of Porolithon responded most consistently across the upwelling gradient, with higher phycoerythrin concentrations closer to the equator. Our findings demonstrate that the effects of in situ nutrient enrichment on benthic autotrophs may be more complex than laboratory experiments indicate. While upwelling is an important feature in some reef ecosystems, ancillary factors may regulate the associated consequences of nutrient enrichment on benthic reef organisms.

Differential resistance and acclimation of two coral species to chronic nutrient enrichment reflect life‐history traits

“…While laboratory experiments are critical for identifying the mechanisms behind species‐specific sensitivities, extending similar studies to natural nutrient gradients will enable more accurate quantification of the consequences of nutrient enrichment on coral reefs at ecosystem scales. Across a natural upwelling gradient in the Southern Line Islands, Pocillopora meandrina and Montipora aequituberculata generally increased endosymbiont density and chl‐ a concentrations proportionally to nutrient availability (Fox et al., 2018; Johnson et al., 2020) but the consequences for calcification or differential bleaching sensitivities on these reefs are unknown. Across the Pacific basin, elevated nutrients associated with upwelling resulted in increased bioerosion of coral skeletons (DeCarlo et al., 2015), while seabird derived nutrients enhanced skeletal growth in Fiji (Savage, 2019), yielding opposing consequences for reef carbonate budgets.…”

Rapid ecosystem-scale consequences of acute deoxygenation on a Caribbean coral reef

“…Coral samples were thawed and then processed for symbiont densities and pigment content 45 . The blastate was vortexed and then centrifuged at 1800 × g for 4 min to separate the host coral animal tissue and symbiont fractions.…”

Symbiont regulation in Stylophora pistillata during cold stress: an acclimation mechanism against oxidative stress and severe bleaching

“…Coral reefs are frequently exposed to these local episodes, especially to upwellings, as seen in the Great Barrier Reef (Andrews and Gentien, 1982), Central Pacific islands (Kelly et al, 2014), Pacific coast of Central America (D'Croz and O'Dea, 2007), Florida Keys (Leichter et al, 1996), Caribbean (Diaz-Pulido and Garzon-Ferreira, 2002) and Seychelles (Novozhilov et al, 1992). Although cooling events can be beneficial to corals during periods of increased SST by creating thermal refuges and reducing bleaching intensity (Glynn, 1996;Riegl and Piller, 2003;West and Salm, 2003;Riegl et al, 2019;Johnson et al, 2020;Sawall et al, 2020;Storlazzi et al, 2020), this is not always the case. For example, around the northern island of the Galapagos reefs, internal waves, forcing cool water upwards above the thermocline, led to a 12°C drop in temperature over a 6-day period, and induced coral bleaching and mortality (Banks et al, 2009;Glynn et al, 2017a,b).…”

“…For example, around the northern island of the Galapagos reefs, internal waves, forcing cool water upwards above the thermocline, led to a 12°C drop in temperature over a 6-day period, and induced coral bleaching and mortality (Banks et al, 2009;Glynn et al, 2017a,b). In addition to reduced SST, upwellings and wind-driven mixing are also associated with nutrient enrichment of surface waters by up to 15 µmol l −1 dissolved inorganic nitrogen and 1.2 µmol l −1 phosphorus (D'Croz and O'Dea, 2007;Stuhldreier et al, 2015;Radice et al, 2019;Riegl et al, 2019;Johnson et al, 2020). Importantly, despite the fact that nutrient enrichment and ratios play a major role in the oxidative metabolism and extent of bleaching that corals undergo during heat stress (Wiedenmann et al, 2013;Marangoni et al, 2020), the combination of nutrient enrichment and cold stress on the physiology of reef-building corals has been poorly studied (Johnson et al, 2020).…”