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.
Highlights
The availability of inorganic nutrients in the environment influences organismal physiology and can have cascading effects on ecosystem-scale processes [1]
Average daily Photosynthetically active radiation (PAR) levels did not vary significantly across islands (S2 Table), though the lowest daily average intensity between the hours of 10:00–14:00 was at Vostok on a cloudy day (428 ± 43) and the highest was at Malden (884 ± 1) (Table 1)
While our findings corroborate well-established patterns of increasing surface ocean primary production near the equator [10,56,57], they indicate that provisioning of nutrients from equatorial upwelling does not necessarily enhance autotroph physiology on the benthos, as we observed in the SLI
Summary
The availability of inorganic nutrients in the environment influences organismal physiology and can have cascading effects on ecosystem-scale processes [1]. Persistent upwelling caused by long-standing currents [7], or periodic upwelling caused by episodic processes (e.g., internal tides) [8], deliver subthermocline water enriched in dissolved inorganic nitrate, phosphate, and carbon to the ocean surface [5]. These deep, cooler waters mix with warmer surface waters and replenish depleted inorganic nutrients that are important for regulating biological activity [9,10]. Understanding how the delivery of essential nutrients by upwelling can influence autotroph ecophysiology will shed light on how natural oceanographic processes shape ecosystem structure and function
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