KEY FINDING 18.Primary productivity has increased on more than 20% of the vegetated land area of Canada over the past 20 years, as well as in some freshwater systems. The magnitude and timing of primary productivity are changing throughout the marine system.
Primary productivity is the conversion of the sun's energy into organic material through photosynthesis. On land, it is driven by temperature and availability of water and nutrients modified by land use. In aquatic ecosystems, primary productivity is driven by the availability of nutrients and light and, to a lesser extent, by temperature and other factors. Primary productivity is important because it is the process that forms the foundation of food webs in most ecosystems.
Changes in primary productivity on land
Trends in annual peak NDVI, 1985 to 2006
Note: trends shown are statistically significant.
Source: Ahern et al., 2010,1 adapted from Pouliot et al., 20093
Primary productivity increased significantly on 22% of Canada's vegetated land area between 1985 and 2006 and decreased on less than 1% of land.1 This trend in primary productivity is based on changes in the normalized-difference vegetation index (NDVI), a remote-sensing based measurement of photosynthetic activity – it is a good indicator of the amount of healthy green vegetation.2-4
The largest increases in primary productivity were found in the North where temperatures have risen the most. Changes in vegetation that correspond with this "greening" in northern Canada include a transition to shrubs and grasses where lichens and mosses once dominated,5 and changes in tree growth and density at mountain and northern treelines.6-8
In southern Canada, increases in primary productivity are likely more strongly related to changes in land use than they are to climate change.3 For example, increases in primary productivity in the Prairies are related to increases in crop area.3 The small decreases in primary productivity seen in some areas may be associated with urban and industrial development, or, as in interior British Columbia, forest insect infestations. Some increases in primary productivity may also be associated with fire, as burns can have positive or negative NDVI trends,depending on the age of the burn.3
Photosynthetic activity was estimated to have increased on about 25% of the Earth’s vegetated area and decreased over 7% of this area from 1982 to 1999. The greatest increases were in the tropics, as a result of fewer clouds and increased exposure to the sun, and in high latitudes of the Northern Hemisphere, attributed to increased temperature and water availability.9
The figure shows chlorophyll a reconstructions from Lost Pack Lake, one of six Baffin Island lakes examined for long-term trends. All lakes show dramatic increases of inferred primary production within the most recently deposited sediment, following prolonged periods of comparatively low values.10 Dating of the sediment cores indicates that these rapid increases started in the late 19th century and continue to the present. The increases are a departure, in most lakes, from relatively stable levels of primary production that persisted for millennia. A widespread increase in freshwater production over much of northern Canada is also inferred from major shifts in species composition of algae in ponds and small lakes in many areas (also detected from studies of sediment cores).11, 12 The best explanation for this change in algae is climatic warming leading to longer ice-free growing seasons and associated changes in lake ecosystems.13, 14 The changes are most pronounced in the High Arctic, but similar shifts in algal species are found in many locations in the Northern Hemisphere – with changes being more recent in temperate latitudes.15
Satellite measurements of ocean colour have shown variable decade-scale trends in marine primary production, including a short-term increase in primary production in the Arctic Ocean from 1998 to 2008.17, 18 A recent study16 extended the record by also using longer-term measurements of water transparency and chlorophyll concentrations. This study concluded that, over the past 110 years, primary production has declined in most of the world’s ocean regions.16 High-latitude regions, including the North Pacific, showed the greatest long-term declines. The global decline in the amount of phytoplankton is estimated at 1% per year, with a total decline of 40% since 1950. Shorterterm trends were related to climate oscillations, while the long-term declines were most strongly related to increasing sea-surface temperatures – which leads to less mixing of ocean waters, reducing the nutrient supply for phytoplankton. The exceptions are the Arctic and Antarctic oceans, where the causes of the observed long-term decreases in primary production are less clear, but may be related to increased wind intensity.16