Search for: All records

Abstract Uncertainties in the temporal and spatial patterns of marine primary production and respiration limit our understanding of the ocean carbon (C) cycle and our ability to predict its response to environmental changes. Here we present a comprehensive time‐series analysis of plankton metabolism at the Hawaii Ocean Time‐series program site, Station ALOHA, in the North Pacific Subtropical Gyre. Vertical profiles of gross oxygen production (GOP) and community respiration (CR) were quantified using the¹⁸O‐labeled water method together with net changes in O₂to Ar ratios during dawn to dusk in situ incubations. Rates of¹⁴C‐bicarbonate assimilation (¹⁴C‐based primary production [¹⁴C‐PP]) were also determined concurrently. During the observational period (April 2015 to July 2020), euphotic zone depth‐integrated (0–125 m) GOP and¹⁴C‐PP ranged from 35 to 134 mmol O₂m⁻²d⁻¹and 18 to 75 mmol C m⁻²d⁻¹, respectively, while CR ranged from 37 to 187 mmol O₂m⁻²d⁻¹. All biological rates varied with depth and season, with seasonality most pronounced in the lower portion of the euphotic zone (75–125 m). The mean annual ratio of GOP to¹⁴C‐PP was 1.7 ± 0.1 mol O₂(mol C)⁻¹. While previous studies have reported convergence of GOP and¹⁴C‐PP with depth, we find a less pronounced vertical decline in the GOP to¹⁴C‐PP ratios, with GOP exceeding¹⁴C‐PP by 50% or more in the lower euphotic zone. Variability in CR was higher than for GOP, driving most of the variability in the balance between the two. 

The supply of nutrients is a fundamental regulator of ocean productivity and carbon sequestration. Nutrient sources, sinks, residence times, and elemental ratios vary over broad scales, including those resulting from climate-driven changes in upper water column stratification, advection, and the deposition of atmospheric dust. These changes can alter the proximate elemental control of ecosystem productivity with cascading ecological effects and impacts on carbon sequestration. Here, we report multidecadal observations revealing that the ecosystem in the eastern region of the North Pacific Subtropical Gyre (NPSG) oscillates on subdecadal scales between inorganic phosphorus (P i ) sufficiency and limitation, when P i concentration in surface waters decreases below 50–60 nmol⋅kg −1 . In situ observations and model simulations suggest that sea-level pressure changes over the northwest Pacific may induce basin-scale variations in the atmospheric transport and deposition of Asian dust-associated iron (Fe), causing the eastern portion of the NPSG ecosystem to shift between states of Fe and P i limitation. Our results highlight the critical need to include both atmospheric and ocean circulation variability when modeling the response of open ocean pelagic ecosystems under future climate change scenarios. 

From June to August 2018, the eruption of Kīlauea volcano on the island of Hawai‘i injected millions of cubic meters of molten lava into the nutrient-poor waters of the North Pacific Subtropical Gyre. The lava-impacted seawater was characterized by high concentrations of metals and nutrients that stimulated phytoplankton growth, resulting in an extensive plume of chlorophyll a that was detectable by satellite. Chemical and molecular evidence revealed that this biological response hinged on unexpectedly high concentrations of nitrate, despite the negligible quantities of nitrogen in basaltic lava. We hypothesize that the high nitrate was caused by buoyant plumes of nutrient-rich deep waters created by the substantial input of lava into the ocean. This large-scale ocean fertilization was therefore a unique perturbation event that revealed how marine ecosystems respond to exogenous inputs of nutrients.