Ocean Photosynthesis May Be Decreasing

My previous blog post noted that global land photosynthesis is clearly increasing in recent decades.  However, when we turn to the ocean, the story is different (albeit more ambiguous).

About half of global photosynthesis takes place in the ocean.  Much of the resulting biomass production (net primary production) is consumed, thus supporting a food web that includes large marine animals like fish and whales.  Billions of people eat wild caught ocean fish each day as a source of protein.

Photosynthesis in the ocean is frequently constrained by nutrient availability.  Hence, areas where photosynthesis is high are often where upwelling brings nutrient-rich deep water to the surface, or runoff from the land includes nutrients. 

Earth system scientists now monitor global ocean photosynthesis using a combination of satellite remote sensing, direct measurements, and modeling.  As always with global scale processes, there is significant uncertainty about the estimates and some regions show increasing net primary production (NPP) while other regions show decreases.  Various studies have reached differing conclusions about trends in the global total, but a recent study suggested that ocean NPP is in decline (1998 – 2015).

Figure 1.  Trend in ocean primary production.  Estimates are based on a biogeochemical model (coupled to a full ocean model) that assimilates ocean color satellite data. 

Oceanographers are beginning to get an understanding of what is driving the decline.

A key factor appears to be reduced delivery of nutrients to the ocean’s surface.  The causes are related to global warming, a process driven by rising concentrations of greenhouse gases in the atmosphere. 

An important mechanism that is slowing delivery of nutrients to the surface ocean is an increase in stratification associated with the general warming of ocean surface waters.  A cap of warm water tends to reduce vertical mixing, which in turn reduces recharge of surface nutrients from deeper waters where much decomposition and nutrient release takes place.

Figure 2.  Trend in ocean mixed layer depth.  Estimates are based on a global physical three-dimensional model of the ocean driven by geophysical observations.

Another process related to nutrient supply involves the cycling of water from the surface to the deep ocean and back to the surface (the thermohaline circulation).  The descending arm of this Earth-girdling loop of ocean circulation is based on warm water brought north by the Gulf Stream.  That water cools, densifies, and sinks in the North Atlantic Ocean and eventually returns to the surface elsewhere bringing with it nutrients from the deep ocean.  Recent measurements suggest a weakening of that descending arm cause by a freshening of North Atlantic waters driven mostly by melting of the Greenland ice cap.

A decline in ocean photosynthesis − the base of the ocean food chain − likely translates into lower fish production.  Fisheries all over the planet are already under stress from many factors, not least of which is overharvesting.  Ocean warming causes decline of coral (a source of NPP), ocean acidification reduces NPP of calcifying plankton, decreases in ocean oxygen from reduced mixing and excess nutrient runoff (coastal dead zones) force fish to migrate, and toxic waste inputs (including macro-, micro-, and nano- plastics) reduce feeding efficiency.  After decades of fish harvest increases, the global catch peaked in the 1990s.  Model-based projections of ocean animal biomass suggest continuing declines with further ocean warming.

Despite the immensity of the ocean, human impacts on it are piling up.  A new narrative about ocean management is needed.

We (as a part of the technosphere) cannot directly change ocean circulation in an attempt to restore declining primary production and fish production.  We can only slow the emissions of greenhouse gases, which would slow global warming and its associated impacts on ocean mixing and circulation.  Stabilizing, then reducing, the atmospheric CO2 concentration would also slow ocean acidification.

The time is now to support leaders who understand the realities of global environmental change and are committed to working domestically and internationally to implement policies that change the current trajectory of the Earth system.

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