Abstract
Global analyses of satellite-and modeled data suggest decreased phytoplankton abundance and primary productivity in oligotrophic gyres as they expand in response to increased surface temperatures, shoaling of surface mixed layers, and decreased supply of subsurface macronutrients. However, concomitant changes have not been evident in situ at Hawaii Ocean Time-series (HOT) Station ALOHA, suggesting physiological or structural reorganization not observed from space, uncharacterized spatiotemporal variability, or uncorrected sensor drift. To address the spatiotemporal variability hypothesis, we compared interannual patterns of in situ data to gyre geography based on multiple metrics including dynamic topography, satellite surface chlorophyll (chl a), and multivariate seascapes using modelled or satellite data. There was only weak evidence of secular increases in the extent of the subtropical gyre; rather, interannual oscillations were evident in physical, biological, and multivariate biophysical definitions of the gyre. Modelled and satellite-based multivariate seascapes agreed well in terms of expansion (surface area of seascapes) and isolation of Station ALOHA (distance to seascape boundary) resulting in combined data record of nearly three decades. Isolation was associated positively with the North Pacific Gyre Oscillation (NPGO), and negatively with Multivariate ENSO Index (MEI) and Pacific Decadal Oscillation (PDO); the converse was true for the gyre’s expansion. This expansion followed a shoaling and freshening of the surface mixed layer and declines in in situ 14C assimilation rates measured over 12 hours in ambient light suggesting that Station ALOHA may serve as an leading indicator of gyre biogeographic patterns. Lags between geographic indicators and in situ conditions appear to partially explain past observed discrepancies between patterns from satellite remote sensing and those from in situ conditions at Station ALOHA appear to be partially explained by lags between geographic indicators and in situ conditions as well as satellite sensor drift and data record length.
Highlights
Covering over 20 million square kilometers, the North Pacific Subtropical Gyre (NPSG) is the largest ecosystem on the planet surface (Sverdrup et al, 1942; Karl, 2010)
To understand the role of mesoscale variability on basinscale geography, we examined the relationship between mean Eddy kinetic energy (EKE) and the difference in extent between the gyre defined by circulation and that defined by oligotrophy
The area of low-level chl a was dependent on algorithm, with the lower sensitivity of the OCX resulting in a larger expanse of oligotrophy (Figure 3A, Table 1)
Summary
Covering over 20 million square kilometers, the North Pacific Subtropical Gyre (NPSG) is the largest ecosystem on the planet surface (Sverdrup et al, 1942; Karl, 2010). Accurate estimation of global ocean production and export, will require reliable estimation of NPSG ecosystem processes (Karl, 2010) and a detailed understanding of the oceanographic context in which these processes are occurring, including an accurate characterization of gyre geography and the dynamics of gyre areal extent and boundary location Because of their immense size and age, oligotrophic gyres have been considered historically to support pelagic ecosystems in a climax state relatively resulting from stable environmental forcing in space and time; they are recognized to display substantial spatial, seasonal, and interannual variability (Venrick, 1995; Karl, 2010; Karl and Church, 2017). These increases have been linked to a shift in the phase of the Pacific Decadal Oscillation (PDO, Karl et al, 2001) or El Niño Southern Oscillation (ENSO)/PDO interactions (Corno et al, 2007) and resulting changes in stratification
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