Phytoplankton in Precipitous Decline – what this means for our food chain

A few weeks ago, there were a number of articles about the precipitous decline of phytoplankton in the oceans.   A team in the journal Nature reported phytoplankton has declined on an average 1% per year over the past 100 years, declining about 40% since 1950.

The researchers, and most of the reporting of the study, focused on increased warming of the ocean surface, leading to more stratification, and less nutrient for the plankton.

Phytoplankton need sunlight to grow, so they’re constrained to the upper layers of the ocean and depend on nutrients welling up from below. But warmer waters are less likely to mix in this way, which starves the phytoplankton and limits their growth.

At the time, I suspected the story might be more significant, and more complicated than originally reported.  The more I look, the more I am convinced that my suspicions were correct.   But before we get into that, here is a little background for any of you that may have missed it.

What is plankton?

On Spongebob, Plankton might be evil, but in our oceans plankton plays a crucial role.  There are two main types of plankton.  Phytoplankton (the plants of the sea) and zooplankton,  which are typically microscopic animals living near the surface of aquatic environments.  While both are important, it is the phytoplankton where the decline has been documented.  Since phytoplankton form the base of the entire aquatic food chain,  we’ll focus our discussion here.  According to NASA

Derived from the Greek words phyto (plant) and plankton (made to wander or drift), phytoplankton are microscopic organisms that live in watery environments, both salty and fresh.

Some phytoplankton are bacteria, some are protists, and most are single-celled plants. Among the common kinds are cyanobacteria, silica-encased diatoms, dinoflagellates, green algae, and chalk-coated coccolithophores.

Like land plants, phytoplankton have chlorophyll to capture sunlight, and they use photosynthesis to turn it into chemical energy. They consume carbon dioxide, and release oxygen. All phytoplankton photosynthesize…

The scale of phytoplankton is staggering.  (phytoplankton) account for approximately half the production of organic matter on Earth. Phytoplankton act as the first step in a huge carbon pump that takes carbon dioxide out of the atmosphere, and either holds it in the marine biosphere or traps it in the deep ocean.

Worldwide, this “biological carbon pump” transfers about 10 gigatonnes of carbon from the atmosphere to the deep ocean each year.

And this is why the decline in phytoplankton is so troubling.   From Real Climate

Over the last 150 years, carbon dioxide (CO2) concentrations have risen from 280 to nearly 380 parts per million (ppm)….The roughly 500 billion metric tons of carbon we have produced is enough to have raised the atmospheric concentration of CO2to nearly 500 ppm. The concentrations have not reached that level because the ocean and the terrestrial biosphere have the capacity to absorb some of the CO2 we produce.* However, it is the fact that we produce CO2 faster than the ocean and biosphere can absorb it that explains the observed increase.

OK  – so the concentration of CO2 in our atmosphere has increased over 35%, and one of the mechanisms of keeping it from rising faster is disappearing.  As we add more CO2 into the atmosphere, the biosphere (particularly the marine portion of it) will be less and less able to process it.

Another factor for us to consider  is the “carbon sink” properties of higher marine life forms.  After all, plankton do more than live and die, they are also eaten forming the base of the food chain that supports schools of deepwater fish and mammals including cod, bass, sharks, and all the way up to whales.  A pod of whales, for example, is a significant carbon sink.

In addition to being a carbon sink, these larger animals also serve as a holding area for the other nutrients needed by any eco system.  While there are a lot of nutrients involved, for this discussion, we will focus on nitrogen, one of the most basic building blocks.

In many ways, the nitrogen cycle is similar to the carbon cycle.   Nitrogen (along with carbon, oxygen, and other nutrients)  is used by plants to create more complex molecules such as sugars, which are then eaten by animals.  The animals excretion contains nitrogen, and nitrogen is released when the plants and animals die and decompose.

But there is an important difference.  Atmospheric carbon (CO2) is readily used by plants.  In fact, it is a crucial element in photosynthesis.  But atmospheric nitrogen (N2) is not readily used by plants.

This is because the strong triple bond between the N atoms in N2 molecules makes it relativelyinert. In fact, in order for plants and animals to be able to use nitrogen, N2gas must first be converted to more a chemically available form such as ammonium (NH4+), nitrate (NO3), or organic nitrogen (e.g. urea – (NH2)2CO). The inert nature of N2 means that biologically available nitrogen is often in short supply in natural ecosystems, limiting plant growth and biomass accumulation.

And that leads me to why I suspected the story is more complex than originally reported.   We are actively removing organic nitrogen, in the form of fish, from our oceans.  This section draws heavily on the work and research of Debbie MacKenzie.  For the past 10 years MacKenzie has been writing that overfishing has been causing the oceans to starve.  At first, her work was dismissed, but recently more and more researchers are acknowledging that overfishing is a contributing factor in decline of the overall health of the ocean bioshpheres.

Sea creatures are now underfed and algae growth slowed, showing declining ocean carbon uptake. This seems to be an unanticipated, indirect impact of fishing, which is an important new insight. Fish lift nutrients to the surface water, fertilizing algae and powering a natural carbon sink. But our massive removal of fish, sea birds and marine mammals over centuries has damaged this carbon sink, like deforesting the land. If sea animals made a comeback, the fish-powered carbon sink would mitigate atmospheric carbon dioxide. But this idea – that fish boost ocean carbon uptake, and that science has overlooked it – challenges accepted ideas and threatens the fishing industry.
2010 – UPDATE – Scientists are starting to report the carbon sink ‘value’ of living, moving whales, plus the carbon ‘cost’ of whaling, and suggest financial ‘carbon credits’ could now be earned by stopping fishing and whaling, supporting the theme of this website:

MacKenzie provides multiple examples of declining marine ecosystems. Bleached seaweed, tidal pools bereft of barnacles, but most dramatic, the disappearance of large predators in the oceans

Centuries ago, before fishermen began removing fish and their predators, seas were unimaginably full of large healthy animals. The first explorers of the Northwest Atlantic ocean reported that the noise of the great numbers of whales in the Gulf of St. Lawrence prevented sleeping at night, that large cod could be caught in baskets lowered into the sea, that fish were so numerous they slowed the progress of ships, that oysters and other invertebrates grew to huge sizes and were extremely abundant, that there were massive colonies of big fat seabirds, and that rivers were so full of giant sturgeon it was dangerous to get in a canoe.

Fish and their predators are subdued today. The living biomass of sharks and other large fish is estimated to have fallen below 10% of original levels. Some have been driven to extinction. Really big specimens of any fish species have become very rare, and there are fewer small fish. Numbers of whales and seabirds are far below former levels, while seals have made only a partial recent recovery after near eradication by humans.

This great loss of animal life has been accompanied by a lowering of food production in the ocean. This can be seen in the increasingly poor condition of fish as they grow to larger sizes. Mature fish become spent today much more quickly than they were in the past.

Consider cod in Atlantic Canada, a fish that once grew to be a formidable predator itself at 6 feet long, 200 pounds, and living 40 or 50 years. Today, virtually all cod die before age 7. This is occurring with no human cod fishery. The size trap for cod has been set much lower than before, now at less than two feet long. Cod that approach this size become weak and emaciated and are killed by natural predators. The surviving natural predators are mostly seals, that now try to eat bigger spent cod in addition to thinning the numbers of small cod as was their traditional role. Less effective than sharks in eating bigger fish, seals often manage only to bite out the bellies from spent cod.

All this tells us that we have been pulling nutrients from the oceans, and not replacing them. As we deplete the oceans of organic nitrogen the entire ecosystem is in rapid decline.  Up until recently, many thought that the nitrogen taken from the ocean was being replaced by human waste and agricultural runnoff in rivers.  It was a fairly common assumption that the fixed nitrogen entering the oceans this way equaled or was greater than the fixed nitrogen taken from the oceans by fishing.  Mackenzie has this response:

First, what becomes of the huge quantities of nitrogen that are now entering the sea via rivers? (The contributors are human sewage, agricultural runoff of fertilizers and manure, increased input due to erosion, and nitrates arriving there via aerial deposition/fuel burning.) One effect is very well known, called “eutrophication” it’s a syndrome wherein the oversupply of nutrients stimulates overgrowth of algae, which then die and sink to the bottom where they undergo decomposition by bacteria which depletes the water of oxygen, killing fish and being generally unhelpful. But the nutrients in this scenario don’t get very far, they become part of the nearshore bottom sediment rather than effectively stimulating production in the food web as a whole or moving offshore to become “food” for fish on the offshore banks. Another thing that happens in the polluted rivers is that they become much more effective as functioning septic systems – i.e. conditions of high nitrates and low oxygen stimulate higher rates of denitrification by the bacteria that normally carry out that function. (It has recently been discovered that some “nitrifiers” will actually switch over and function as “denitrifiers” in these conditions, amplifying the nitrogen-removing effect.) This bacterial activity has the effect of changing much of the organic nitrogen into the inorganic form, which then no longer qualifies as a “nutrient.” Actually denitrification is a well-known process that occurs in the seabed, both nearshore and offshore, so it represents one of the normal “sinks” but it is greatly accelerated by the practice of dumping large quantities of concentrated, liquid nutrients into estuaries and coastal waters

This process has resulted in dead zones in many coastal waters.  A large area of the Gulf of Mexico is a dead zone resulting from agricultural runoff from the midwest through the Mississippi River.  The Baltic Sea currently has 7 of the 10 largest dead zones in the world.

Conclusion

OK – It has taken a while to get here, but let’s look at how this picture plays out

1. Overfishing has caused a decline in the overall level of fixed nitrogen in the oceans,

2. The warming of the oceans has led to a stratification, meaning that the nutrients still in the sea (particularly fixed or organic nitrogen) are not rising to the top where they are needed by phytoplankton.

3 The result is a precipitous decline in phytoplankton, the base of the entire oceanic food chain

4.  As to volume of phytoplankton decrease, the ocean biosphere is less able to take up and use atmospheric carbon (CO2.) accelerating the increased level of CO2 in the atmosphere, leading to further warming.

So, where does this leave us?

For numerous reasons listed above, we need to improve the health of the oceans.  If for no other reason than to have another carbon sink, but also because the oceans represent a huge source of food for us.

Unfortunately, “fertilizing” the deep ocean is probably an expensive thing to do, and one where the benefits don’t necessarily go to the people making the investment.  If a farmer fertilizes his land (hopefully with natural, organic fertilizer) the increased yields benefit the farmer.  But, if someone adds nutrients to the deep ocean, anyone fishing will benefit.

This is probably a case where we need to think and act globally, but is anyone paying enough attention?



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