Delayed upwelling alters nearshore coastal ocean ecosystems in the northern California current

Abstract

Wind-driven coastal ocean upwelling supplies nutrients to the euphotic zone near the coast. Nutrients fuel the growth of phytoplankton, the base of a very productive coastal marine ecosystem [Pauly D, Christensen V (1995) Nature 374:255-257]. Because nutrient supply and phytoplankton biomass in shelf waters are highly sensitive to variation in upwelling-driven circulation, shifts in the timing and strength of upwelling may alter basic nutrient and carbon fluxes through marine food webs. We show how a 1-month delay in the 2005 spring transition to upwelling-favorable wind stress in the northern California Current Large Marine Ecosystem resulted in numerous anomalies: warm water, low nutrient levels, low primary productivity, and an unprecedented low recruitment of rocky intertidal organisms. The delay was associated with 20- to 40-day wind oscillations accompanying a southward shift of the jet stream. Early in the upwelling season (May-July) off Oregon, the cumulative upwelling-favorable wind stress was the lowest in 20 years, nearshore surface waters averaged 2 degrees C warmer than normal, surf-zone chlorophyll-a and nutrients were 50% and 30% less than normal, respectively, and densities of recruits of mussels and barnacles were reduced by 83% and 66%, respectively. Delayed early-season upwelling and stronger late-season upwelling are consistent with predictions of the influence of global warming on coastal upwelling regions.

Fig. 1.

Alongshore wind stress off the U.S. west coast. (A–C) Alongshore wind stress from three west-coast buoys: Newport, OR (A), Monterey Bay, CA (B), and Point Conception, CA (C). Values for 2005 (blue) are plotted on top of the climatological mean (black) ± 1 SD (gray shading) for 1985–2005. In A, the dashed curve is the north–south position of the jet stream, and the two arrows indicate the times of maps shown in Fig. 2. (D) Cumulative alongshore (north–south) wind stress from National Oceanic and Atmospheric Administration NDBC Buoy 46050 offshore of Newport, OR, starting from the spring transition. The black curve and shading represent the mean ± 1 SD for 1985–2005, and the blue curve is for 2005. At zero cumulative wind stress, the black curve and shading represent the mean and ±1 SD of the date of the spring transition, and the blue line represents the date of the 2005 spring transition. The 2005 curves are dark blue when absolute values exceed any observed values during the previous 20 years. Data locations are indicated at lower left.

Fig. 2.

Maps of 200-hPa surface height (m, blue contours) and wind speed at 300 hPa (m·s⁻¹, color shading), from six hourly National Centers for Environmental Prediction reanalyses (25) when the jet stream is located anomalously south (May 20, 2005, 1200 Coordinated Universal Time; A) and in its more typical summer position (July 19, 2005, 0000 Coordinated Universal Time; B). Wind speeds >35 m·s⁻¹ are shown, with color increments at 45, 55, and 65 m·s⁻¹.

Fig. 3.

Surface (0–2 m) temperatures during 2005 (solid lines) compared with climatological means (dashed lines) with ±1 SD (shaded) from inner-shelf moorings in 15 m of water off of central Oregon (44.25°N, 124.13°W) (1998–2004 mean) (A), 21 m of water off of Monterey Bay, CA (36.97°N, 122.16°W) (1999–2004 mean) (B), and 15 m of water off of Santa Barbara, CA (34.46°N, 120.29°W) (1999–2004) (C). Temperatures during 2005 that are warmer (colder) than the climatological mean ± 1 SD are shaded in red (blue).

Fig. 4.

Surf-zone chlorophyll and nutrients measured along the Oregon and north/central California coasts. (A–D) Chl-a measured along the coast during May-August 2005 (●), the long-term climatological mean (○) with 95% C.I. (bars), and 2005 anomalies from the mean (colored bars). (E–H) As in A–D, but for nitrate plus nitrite (N + N). The arrows at 44.25°N indicate the location of time series shown in Fig. 5.

Fig. 5.

Time series of surf-zone chlorophyll and nutrients off the central Oregon coast. (A) Time series of chl-a (circles) measured at the coast off of central Oregon (44.25°N): 2005 (filled symbols); 1993–2004 climatological mean (open symbols) with 95% C.I. (bars). (B) As in A, but for nitrate plus nitrite (diamonds).

Fig. 6.

Mussel (Mytilus spp.) (Left) and barnacle (B. glandula) (Right) recruitment at sites along the Oregon coast: 2005 (●) compared with climatological monthly means (○) computed for 8–17 years (including 2005) depending on site. Means and SEM are shown for all data. Dotted vertical lines mark the “early” and “late” recruitment seasons used in data analysis. The absence of dotted lines indicates sites where no differences between years or season occurred.