Abstract
The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) provides global monthly measurements of both oceanic phytoplankton chlorophyll biomass and light harvesting by land plants. These measurements allowed the comparison of simultaneous ocean and land net primary production (NPP) responses to a major El Niño to La Niña transition. Between September 1997 and August 2000, biospheric NPP varied by 6 petagrams of carbon per year (from 111 to 117 petagrams of carbon per year). Increases in ocean NPP were pronounced in tropical regions where El Niño–Southern Oscillation (ENSO) impacts on upwelling and nutrient availability were greatest. Globally, land NPP did not exhibit a clear ENSO response, although regional changes were substantial.
Temporal changes in the physical environment are manifested in the light-harvesting capacity of plant communities throughout the biosphere and can be monitored remotely by changes in surface chlorophyll concentration (C sat) in the oceans and the Normalized Difference Vegetation Index (NDVI) on land. A continuous, 20-year global record of satellite NDVI has permitted characterization of interannual, climate-driven changes in terrestrial photosynthesis (1–5). Coincident changes in ocean productivity have not been assessed because an analogous long-term globalC sat record does not exist. The firstC sat measurements were made with the Coastal Zone Color Scanner (CZCS: 1978–86), but this proof-of-concept sensor collected data on a highly irregular basis that yielded incomplete global coverage even after integration over the entire 8-year mission. Eleven years later, SeaWiFS was launched, marking the beginning of the first multiyear satellite measurements of phytoplankton biomass since CZCS. SeaWiFS now provides greater global coverage ofC sat each month than was achieved throughout the lifetime of CZCS. In addition, SeaWiFS is the first satellite instrument with the spectral coverage and dynamic range necessary to derive both C sat and NDVI. Here we report spatial and temporal changes in the photosynthetic biosphere for an El Niño to La Niña transition period, as recorded during the first 3 years of the SeaWiFS mission.
We analyzed global, 4-km resolution, monthly SeaWiFSC sat and NDVI data collected between September 1997 and August 2000. Stability of the sensor was characterized from monthly lunar-based calibrations and derived products verified by comparison with field measurements (6–8). Biospheric net primary production (NPP) was estimated following the approach of Fieldet al. (9), which integrates the Vertically Generalized Production Model (VGPM) for the oceans (10) with the Carnegie-Ames-Stanford Approach (CASA) for land (11, 12). Variations in NPP for the CASA-VGPM model arise from changes in three factors: (i) incident photosynthetically active radiation (PAR), (ii) the fraction of radiation absorbed by plants (related to C satand NDVI), and (iii) light use efficiency (ɛ). Coincident changes in these factors collectively control NPP. Unlike previous calculations that used C sat, NDVI, and climate data from different periods (9), all data used in the current NPP estimates were collected during the SeaWiFS period (13). The CASA-VGPM model was operated on a monthly time step.
SeaWiFS measurements began near the peak of the 1997–98 El Niño event (by some measures, one of the strongest on record) (14) and then continued through an equally strong La Niña period. A pronounced seasonal cycle dominated temporal variability in global mean C sat throughout the SeaWiFS record (Fig. 1A). Summer phytoplankton blooms in the Northern Hemisphere exceeded those in the Southern Hemisphere, causing minima in global meanC sat between November and March and maxima between May and September. Superimposed on this prominent seasonal cycle was a clear El Niño–Southern Oscillation (ENSO)–related change in ocean productivity, as illustrated by the monthly C sat anomaly record (Fig. 1A) (15). The El Niño to La Niña transition altered ocean nutrient distributions, causing nearly a 10% increase in global mean C sat between September 1997 and December 1998. Changes in C sat during this period were not restricted to the equatorial belt but rather were global in extent. During the subsequent La Niña period of January 1999 to August 2000, C sat continued to increase at the reduced rate of 2.2% per year, primarily reflecting increased phytoplankton biomass in the Pacific Ocean.
Global monthly means and anomalies in (A) surface ocean chlorophyll (C sat: mgChl m−3) and (B) land NDVI (dimensionless) for SeaWiFS measurements between September 1997 and August 2000. Anomalies were calculated as the difference between C sat or NDVI for a given month and the average value for that month during the 3-year time series. (A) •, monthly mean C sat (left axis); ⋄, monthly anomaly (right axis). (B) •, monthly mean NDVI (left axis); ⋄, monthly anomaly (right axis).
Temporal changes in ocean NPP exhibited seasonal and interannual patterns similar to C sat, increasing from 54 to 59 Pg C year−1 (Pg = 1015 g) over the 3-year SeaWiFS period. Regionally, NPP was highest near equatorial and eastern margin upwelling centers, at high latitudes in the Northern Hemisphere, and within the southern subtropical convergence zone (Fig. 2, A and B). Seasonal changes in Southern Hemisphere NPP mirrored those of the Northern Hemisphere, except between 40° and 75°S latitude from October to April (Fig. 3). At >40°N, phytoplankton growth is restricted by deep mixing and low PAR during winter months and then increases markedly in the summer when surface waters rich in nutrients become stratified and PAR is high. Consequently, NPP was strongly seasonal in this region, varying from 0 to 49 g C m−2month−1 (Fig. 3). In contrast, seasonality in NPP was greatly dampened poleward of 40°S, with summer values decreasing from 27 to 7 g C m−2 month−1 between 40° and 70°S (Fig. 3). This absence of a high-latitude, Southern Hemisphere bloom results from weak seasonality in factors limiting phytoplankton growth, particularly iron and vertical mixing (16–19). We calculated that a 9 Pg C year−1increase in NPP would result if seasonal changes in phytoplankton biomass between 40° and 75°S paralleled those in the Northern Hemisphere (20).
Seasonal average and interannual differences in biospheric NPP (g C m−2month−1) estimated with SeaWiFS data and the integrated CASA-VGPM model (9). Average NPP for (A) the La Niña Austral summer of December 1998 to February 1999 and (B) the La Niña Boreal summer of June to August 1999. (A and B) White, ice cover during (A) January and (B) July; tan, near-zero NPP for terrestrial regions not permanently covered by ice. (C) Transition from El Niño to La Niña conditions resulted in substantial regional changes in NPP, as illustrated by interannual differences in Austral summer NPP (i.e., average NPP for December 1998 to February 1999 minus average NPP for December 1997 to February 1998). (D) Changes in NPP between two La Niña Boreal summers (1999 minus 1998). (C and D) Red, increase in NPP; blue, decrease in NPP; white, no substantial interannual change in NPP.
Seasonal changes in the latitudinal distribution of ocean NPP (g C m−2 month−1) for the 3-year SeaWiFS record. Solid line, average Austral summer (December through February) NPP; dashed line, average Boreal summer (June through August) NPP. The vertical dotted line marks the equator.
On land, temporal changes in global mean NDVI were dominated by strong seasonal fluctuations, with minima of 0.44 ± 0.01 (dimensionless) between December and February and maxima of 0.55 ± 0.01 between June and September (Fig. 1B). Land NPP peaked between 15°S and 10°N, reaching 87 g C m−2 month−1, and varied seasonally at >35°N from 0 to 75 g C m−2month−1 (Fig. 2, A and B). Despite the strong El Niño and La Niña, monthly anomalies indicated little systematic impact on global mean NDVI for the 3-year SeaWiFS record (Fig. 1B) (15). Land NPP was nearly constant for both climate regimes, ranging from 57 to 58 Pg of C year−1between September 1997 and August 2000. Substantial ENSO-related regional changes, however, are hidden in these global integrals.
Biospheric distributions of NPP register spatiotemporal variations in light, soil moisture, nutrient availability, and temperature. Interannual variability in NPP is thus linked to regional changes in physical forcings that regulate these resources and environmental conditions. Particularly striking examples of this relation during the 1997–98 El Niño to La Niña transition included an increase in equatorial Pacific NPP resulting from enhanced upwelling (21) and reduced terrestrial NPP in eastern Africa related to decreased precipitation (Fig. 2C). Additional features of the transition included (i) a change in Indian Ocean circulation that increased NPP in the northeast while decreasing productivity west of Indonesia (22), (ii) precipitation-related changes in NPP over Amazonia and Argentina, and (iii) nutrient-driven increases in ocean NPP east of Argentina and in the Mauritanian upwelling plume off western Africa (Fig. 2C). Persistent La Niña conditions between the Boreal summers of 1998 and 1999 led to spatially heterogeneous changes in NPP, including a large equatorial decrease and off-equatorial increase in the Pacific Ocean that likely reflected broad-scale shoaling of the thermocline (23) (Fig. 2D).
The CASA-VGPM model gave biospheric NPP estimates of 111 to 117 Pg C year−1 for the September 1997 to August 2000 period (24). Using the same model and remote sensing data collected between 1978 and 1990, Field et al. (9) estimated biospheric NPP at 105 Pg C year−1. Their estimate for the land component (56 Pg C year−1) was about the same as that reported here. However, their estimate of ocean NPP (49 Pg C year−1) was considerably lower than our results, largely because of higher C sat values from SeaWiFS (1997–2000) than from CZCS (1978–86) (9,25).
Since September 1997, SeaWiFS has provided the first multiyear measurements of ocean plant biomass in over a decade, as well as the first single-sensor global observations of the photosynthetic biosphere. SeaWiFS NDVI and C sat data provide a basis for quantifying temporal changes in NPP, which is a critical component of global carbon and nutrient cycles. Land and ocean productivity responds to changes in the physical environment across the temporal continuum of climate variability, with seasonal cycles dominating over interannual changes (Fig. 1). Our initial analysis of the first 3 years of SeaWiFS data suggests that this sensor will have the capacity to detect longer time scale, lower amplitude responses of the photosynthetic biosphere to climate change. Achieving this goal will require a long-term commitment to intercalibrated global observations and improved ɛ models (26) and remote sensing algorithms (27). As these developments are realized, the SeaWiFS record will provide a basis against which future estimates of Earth system elemental cycling can be compared.
↵* To whom correspondence should be addressed. E-mail: mjb{at}neptune.gsfc.nasa.gov
