Climate-driven trends in contemporary ocean productivity

Abstract

Contributing roughly half of the biosphere's net primary production (NPP), photosynthesis by oceanic phytoplankton is a vital link in the cycling of carbon between living and inorganic stocks. Each day, more than a hundred million tons of carbon in the form of CO2 are fixed into organic material by these ubiquitous, microscopic plants of the upper ocean, and each day a similar amount of organic carbon is transferred into marine ecosystems by sinking and grazing. The distribution of phytoplankton biomass and NPP is defined by the availability of light and nutrients (nitrogen, phosphate, iron). These growth-limiting factors are in turn regulated by physical processes of ocean circulation, mixed-layer dynamics, upwelling, atmospheric dust deposition, and the solar cycle. Satellite measurements of ocean colour provide a means of quantifying ocean productivity on a global scale and linking its variability to environmental factors. Here we describe global ocean NPP changes detected from space over the past decade. The period is dominated by an initial increase in NPP of 1,930 teragrams of carbon a year (Tg C yr(-1)), followed by a prolonged decrease averaging 190 Tg C yr(-1). These trends are driven by changes occurring in the expansive stratified low-latitude oceans and are tightly coupled to coincident climate variability. This link between the physical environment and ocean biology functions through changes in upper-ocean temperature and stratification, which influence the availability of nutrients for phytoplankton growth. The observed reductions in ocean productivity during the recent post-1999 warming period provide insight on how future climate change can alter marine food webs.

Holocene and Eemian sea surface temperature trends as revealed by alkenone and Mg/Ca paleothermometry

“…Possible is that in the permanently stratified tropical ocean where light is no limited low-nutrient surface waters are influenced seasonally by upwelling that acts synchronously to increase the surface water nutrient content -favouring primary productivity -and to decrease SST, potentially making coccolithophorids susceptible to blooms when SSTs are below the meanannual average. A feature that is in agreement with recent satellite observations for an inverse relationship at low latitudes between net primary productivity and temperature, the link between these processes being the upper ocean stratification (Behrenfeld et al, 2006). However, changes in phytoplankton assemblages due to coccolithophorids vs. diatoms competing for nutrients also need to be considered, so that tropical alkenone warming trends have to be analyzed with models involving biogeochemistry to fully encompass the suite of climatological and biological processes ultimately driving the alkenone-based SST signal.…”

Long-term sedimentary recycling of rare sulphur isotope anomalies

“…In addition, dissolved O 2 concentrations in the modern ocean are rarely drawn down to values approaching zero despite the fact that the vast majority of respired carbon on the modern Earth is redox-coupled to O 2 . This simple observation, combined [61][62][63] . However, when integrated over large areas the overall rates of production in open ocean settings account for ~85-90% of global marine primary production 64,65 and ~70% of carbon export flux from the photic zone 65 .…”

Southern Ocean phytoplankton physiology in a changing climate

“…They are a highly diverse group of microscopic photosynthesising protists that inhabit the sunlit surface waters of the ocean and are key to the health and productivity of the marine ecosystem, influencing nutrient cycling, food web dynamics and global biogeochemical cycling (Buesseler, 1998;Garibotti et al, 2003a,b). Global carbon biomass of phytoplankton equates to less than 1% of the total photoautotrophic biomass on the planet (Bryant, 2003), yet accounts for 40-50% of global carbon fixation (Field et al, 1998), with more than 100 million tonnes of inorganic carbon fixed by phytoplankton on a daily basis (Behrenfeld et al, 2006). This organic carbon is transferred through the food web, re-released to the atmosphere or sequestered to ocean depths, through a process known as the biological pump.…”