Ocean fronts drive marine fishery production and biogeochemical cycling

Abstract

Long-term changes in nutrient supply and primary production reportedly foreshadow substantial declines in global marine fishery production. These declines combined with current overfishing, habitat degradation, and pollution paint a grim picture for the future of marine fisheries and ecosystems. However, current models forecasting such declines do not account for the effects of ocean fronts as biogeochemical hotspots. Here we apply a fundamental technique from fluid dynamics to an ecosystem model to show how fronts increase total ecosystem biomass, explain fishery production, cause regime shifts, and contribute significantly to global biogeochemical budgets by channeling nutrients through alternate trophic pathways. We then illustrate how ocean fronts affect fishery abundance and yield, using long-term records of anchovy-sardine regimes and salmon abundances in the California Current. These results elucidate the fundamental importance of biophysical coupling as a driver of bottom-up vs. top-down regulation and high productivity in marine ecosystems.

Keywords: Reynolds decomposition; aggregation; fronts; trophic interactions.

Fig. 1.

Effects of spatial covariance between predators and prey. (A) Evenly dispersed condition currently assumed in models. (B) Highly aggregated condition representative of most ocean environments. (C) Magnitude of the effect of spatial covariance on species production.

Fig. 2.

Front parameterized ecosystem model results. (A–C) Productivity vs. frontal strength $\left(\mathrm{\Delta }U/\mathrm{\Delta }x\right)$ and nutrient supply (μM N⋅d⁻¹) for (A) phytoplankton community structure, (B) ardine/anchovy regimes, and (C) salmon production. Dashed black line in C represents production in an ecosystem model of California Current (20). (D) Relationship of phytoplankton size structure, zooplankton abundance, and anchovy–sardine regimes to front parameter.

Fig. 3.

Effects of fronts on ecosystem dynamics. (A) Spatial distribution of the first EOF of front probability, the front probability index (FPI). (B) Anchovy–sardine ratio [AS, normalized (sardine biomass − anchovy biomass)/(combined biomass); − indicates anchovy dominated, + indicates sardine dominated] with best-fit model and predictors. (C) Climate and environmental indexes used in Generalized Linear Models for anchovy–sardine regimes and salmon abundance. (D) Salmon abundance (Sacramento index, SI) with best-fit model and predictors. Dashed red lines in B and C show 95% confidence intervals. Symbols in equations refer to the Pacific Decadal Oscillation (PDO), El Nino Southern Oscillation (ENSO), North Pacific Gyre Oscillation (NPGO), Sacramento River outflow (OUT), front probability index (FPI), front strength index (FSI), and previous year anchovy–sardine ratio (PREV).

Fig. 4.

Rates of carbon export vs. frontal strength $\left(\mathrm{\Delta }U/\mathrm{\Delta }x\right)$ and nutrient supply (μM N⋅d⁻¹).