One of the most important processes in nature is primary productivity

Ocean Colour

 
Our planet is constantly changing. Some changes are fast – such as seasons and phytoplankton blooms – while others are slow and occur over decades to millennia. Some changes are caused by natural phenomena, while others, including global warming and ocean acidification, are caused by human activity. To see changes beyond the normal seasonal variability that is caused by our planet’s weather patterns, it is necessary to collect data over long periods of time. We have been using satellite sensors to collect such data and to observe our planet from space for more than two decades.

One of the most important processes in nature is primary production, which occurs when plants take up carbon dioxide (CO2) from the atmosphere or the ocean and use the energy from the sun to fix the carbon that is necessary for their growth. Primary productivity is at the base of our food chain and key to all life on Earth. In the oceans, microscopic plants called phytoplankton are responsible for the majority of primary production.

Many things affect primary productivity, including sunlight, CO2, and nutrients, which are the essential ingredients for photosynthesis and growth. In the oceans, phytoplankton primary productivity is mostly controlled by the amount of sunlight and the availability of nutrients. Using satellite observations, we have measured how ocean primary productivity – which produces oxygen for us to breathe, supports marine food webs, and helps to store atmospheric CO2 in the deep ocean – has changed over the past 25 years.

Even with this long record, it is difficult to disentangle natural drivers of change from those induced by humans, such as climate change. Why is this so hard? The ocean and the atmosphere are connected, and some of the natural changes take place over decades. For example, the Atlantic Multidecadal Oscillation (AMO) – which causes cool and warm fluctuations in sea surface temperature (SST) – may last for 20-40 years. The El Niño-Southern Oscillation (ENSO), a major phenomenon that causes warming in the equatorial Pacific Ocean, can be as frequent as every 3 years, with variable intensity. These natural changes, or oscillations, affect SST, precipitation, and wind patterns, leading to changes in the water column conditions on which phytoplankton and primary production depend. Human activities, especially the increase of CO2 in the atmosphere due to the burning of fossil fuels, have also impacted global temperatures and water column conditions, which in turn impact ocean primary production.

Our ocean is vast and complex, which results in regional differences in primary production. For example, in coastal areas, particularly those close to rivers, primary productivion is higher due to the runoff of nutrients from land. The western coasts of the continents, where wind and topography bring nutrient-rich deep water to the surface (i.e. upwelling), also show high primary productivity. These areas are important for fisheries, particularly in South America. In contrast, in large areas of swirling ocean currents, known as gyres (Fig. 1), primary production is low and these regions are considered ocean deserts. A 16-year (1998-2013) analysis of trends in the five subtropical ocean gyres revealed warming and expansion of these ocean deserts, which means a decrease for the ability of our ocean to absorb CO2 from the atmosphere (Signorini et al. 2015, Leonelli et al, 2022).

Scientists also study global trends in annual primary productivity because of its importance to ecosystems and human welfare. Satellite data, in combination with in situ measurements and mathematical models to estimate primary productivity for the entire planet, allow us to take the pulse of our marine ecosystems.

Figure 2 shows the trends in annual primary production from different data sets. The first striking characteristic is the differences in trends across the ocean, reflecting its complexity.

In general, we see a decrease in primary production in temperate oceanic regions in both the Northern and Southern Hemisphere (up to -3% change per year) and an increase at higher latitudes near the Polar Regions (up to +4.5% change per year). This does not sound like much, but over 24 years, these changes in primary production are in the order of tonnes of carbon. There is uncertainty in the direction, magnitude and differences of changing primary productivity in shelf and coastal regions, which are affected by a variety of natural and anthropogenic factors. In addition, the trends observed are not all linear, meaning that they are not all changing at the same rate. The local differences are especially important for organisms that depend on primary production for their food.

Together, the regional increases and decreases in primary production largely balance each other out. Global primary productivity shows an increase in rates from 1998 to 2003 (Fig. 3), relatively stable rates between 2003 and 2011, and a subsequent decrease in rates until 2015, after which rates show a small increase. This interannual variability appears to be strongly correlated to ocean-atmospheric connections: the initial increase in global annual primary production between 1998 and 2003 was related to ENSO and other ocean-atmosphere connections like the Indian Ocean Dipole (IOD), while the decrease in global annual primary production after 2011 was related to ENSO and the Pacific Decadal Oscillation (PDO) (Kulk et al., 2021).

Long-term climate records allow us to see how our planet is changing and can provide the necessary information to mitigate negative effects to our ecosystem and way of life.

Figure 3: Annual global primary production for each year in the period between 1998 and 2018 (from Kulk et al., 2021)

The Interactive map below provided by NASA shows Global Ocean Primary Productivity

 
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References:

• Kulk, G.; Platt, T.; Dingle, J.; Jackson, T.; Jönsson, B.F.; Bouman, H.A.; Babin, M.; Brewin, R.J.W.; Doblin, M.; Estrada, M.; Figueiras, F.G.; Furuya, K.; González-Benítez, N.; Gudfinnsson, H.G.; Gudmundsson, K.; Huang, B.; Isada, T.; Kovač, Ž.; Lutz, V.A.; Marañón, E.; Raman, M.; Richardson, K.; Rozema, P.D.; Poll, W.H.v.d.; Segura, V.; Tilstone, G.H.; Uitz, J.; Dongen-Vogels, V.v.; Yoshikawa, T.; Sathyendranath, S. Primary Production, an Index of Climate Change in the Ocean: Satellite-Based Estimates over Two Decades. Remote Sens. 2020, 12, 826. https://doi.org/10.3390/rs12050826

• Signorini S. R., Franz Bryan A., McClain Charles R., .: Chlorophyll variability in the oligotrophic gyres: mechanisms, seasonality and trends.Frontiers in Marine Science 2015 Vol. 2, https://www.frontiersin.org/article/10.3389/fmars.2015.00001, DOI=10.3389/fmars.2015.00001

• Leonelli F. E., M. Bellacicco M., Pitarch Portero J., Organelli, E.Buongiorno Nardelli B., V. de Toma V., C. Cammarota C., S. Marullo S., R. Santoleri R. , Ultra-oligotrophic waters expansion in the North Atlantic Subtropical Gyre revealed by 21 years of satellite observations. Submitted to GRL, 2022.

• Behrenfeld, M.J., Falkowski P.G.. Photosynthetic rates derived from satellite-based chlorophyll concentration. Limnology and Oceanography, 1997 Volume 42: 1-20