Abstract
In Eastern Boundary Current systems, wind-driven upwelling drives nutrient-rich water to the ocean surface, making these regions among the most productive on Earth. Regulation of productivity by changing wind and/or nutrient conditions can dramatically impact ecosystem functioning, though the mechanisms are not well understood beyond broad-scale relationships. Here, we explore bottom-up controls during the California Current System (CCS) upwelling season by quantifying the dependence of phytoplankton biomass (as indicated by satellite chlorophyll estimates) on two key environmental parameters: subsurface nitrate concentration and surface wind stress. In general, moderate winds and high nitrate concentrations yield maximal biomass near shore, while offshore biomass is positively correlated with subsurface nitrate concentration. However, due to nonlinear interactions between the influences of wind and nitrate, bottom-up control of phytoplankton cannot be described by either one alone, nor by a combined metric such as nitrate flux. We quantify optimal environmental conditions for phytoplankton, defined as the wind/nitrate space that maximizes chlorophyll concentration, and present a framework for evaluating ecosystem change relative to environmental drivers. The utility of this framework is demonstrated by (i) elucidating anomalous CCS responses in 1998–1999, 2002, and 2005, and (ii) providing a basis for assessing potential biological impacts of projected climate change.
Highlights
Moderate wind speeds are optimal for nearshore phytoplankton populations[17,18], they are either idealized or geographically limited, and do not explicitly consider variability in the subsurface nitrate field relative to wind forcing or the interaction between the nearshore and offshore environments
Using data from 1998 to 2010, we define the individual and combined influences of nearshore wind stress and subsurface nitrate concentration on chlorophyll concentrations in both the nearshore and offshore environments, and use this framework to elucidate the bottom-up forcing behind three periods of highly anomalous ecosystem responses in the California Current System (CCS): the delayed upwelling season of 2005, strong subarctic influence in 2002, and the El Niño/La Niña conditions of 1998–1999
Vertical velocities in the offshore region are weak and of variable sign, and much of the offshore nutrient and phytoplankton biomass is derived through advection from the nearshore region rather than from local processes[16]
Summary
Moderate wind speeds are optimal for nearshore phytoplankton populations[17,18], they are either idealized or geographically limited, and do not explicitly consider variability in the subsurface nitrate field relative to wind forcing or the interaction between the nearshore and offshore environments. We use a regional ocean model to derive estimates of subsurface nitrate (based on observed temperature-salinity-nitrate relationships) and surface wind stress, and combine them with satellite chlorophyll measurements to explore physical and chemical controls on phytoplankton biomass during the California Current System (CCS) upwelling season (see Methods for details). Using data from 1998 to 2010, we define the individual and combined influences of nearshore wind stress and subsurface nitrate concentration on chlorophyll concentrations in both the nearshore and offshore environments, and use this framework to elucidate the bottom-up forcing behind three periods of highly anomalous ecosystem responses in the CCS: the delayed upwelling season of 2005, strong subarctic influence in 2002, and the El Niño/La Niña conditions of 1998–1999. As wind and nitrate are commonly invoked to explain bottom-up ecosystem control, we focus our analysis on them
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