The reduction in phytoplankton is not due to oceanic warming, but instead to overfishing. Guest writer Ed Caryl digs into the subject of phytoplankton.
The Phytoplankton Are Starving
By Ed Caryl
A recent press release from Dalhousie University, Nova Scotia, announced an article published in Nature (behind a pay-wall) that we’ve been losing 1% of our phytoplankton each year dating back to 1899, meaning 40% since 1950. The article blames it on ocean warming. The article was discussed in July on WUWT and here. Let’s pull our heads back a bit more and look beyond warming.
Why are phytoplankton important?
Phytoplankton or algae are single-celled, photosynthesizing creatures at the “bottom” of the ocean food chain. They take in sunlight and CO2 and produce carbohydrates, just like plants do on land. They are the source of the biomass that all larger animals and fish feed upon. Without them, life in the ocean would not exist. Yet, much of the literature makes no mention of any dependence in the other direction, i.e. phytoplankton depend on the creatures higher up in the food chain.
Phytoplankton are a hybrid of plant and animal. Some have locomotion capability as if they were single-celled animals. They need the same nutrients as all plants: CO2, nitrogen (as nitrates or ammonia), and phosphorus (as phosphate); iron, zinc and manganese are also needed in trace quantities. When there is an excess of these nutrients, such as in agricultural runoff, “blooms” can occur where runaway reproduction produces so many that during the night when they are not photosynthesizing they can consume all the dissolved oxygen in the water, and thus suffocate themselves and the fish.
Why are phytoplankton disappearing in deep ocean areas?
In the deep ocean, away from nutrient-feeding rivers and streams, the phytoplankton population is generally low. This can be seen in the graphic above from satellite data where blue indicates low chlorophyll.
Upwelling from the deep-ocean delivers nutrients from the bottom and increases the population in areas close to shore where waters are shallow, and where rivers and streams bring nutrients from land. The phytoplankton population seems to have increased in shore areas in recent years, probably due to nutrient runoff from agriculture, while the mid-ocean gyres are still seeing population decreases.
The questions then are:
1) Why is the mid-ocean so generally poor in phytoplankton?
2) Why has the overall phytoplankton population dropped?
Let’s discuss the possibilities: Is increasing CO2 in the atmosphere causing it?
This is unlikely for three reasons. First, CO2 started increasing in a significant way many years after the reduction in phytoplankton started. Second, CO2 is a nutrient that should stimulate growth, not inhibit it. Three, the recent inshore population increase seems to go against the CO2 influence.
Is ocean warming the culprit?
Note that planktons are plentiful in very warm equatorial waters where nutrients are available. Even though the far north and south seem to have more phytoplankton, warm temperatures do not seem to be the limiting factor. Phytoplankton are not a single monolithic species. There are thousands of species of phytoplankton, adapted to various environments, including temperature. Some phytoplankton also diurnally move vertically in the water column through large temperature gradients.
So, what could be limiting phytoplankton? The likely culprit is the limitation of nutrients. Phytoplanktons are starving in the mid-ocean. Why?
The oceans are mostly a closed ecosystem. Inputs to the system are sunlight and nutrients from rivers and streams. 95% of the biomass in the ocean is phytoplankton. Everything else in the ocean depends directly or indirectly on phytoplankton. Zooplankton: copepods, krill, and shrimp eat phytoplankton. Small fish, such as herring, sardines, and menhaden eat the zooplankton. Large fish eat the small fish. At the top of the food chain are: squid, tuna, salmon, sharks, whales, porpoise, etc…and finally us humans. But remember, this is a closed system. Except for what man removes, everything else recycles in the ocean. Everything in the ocean lives, excretes, dies, or is eaten by something. Even if it falls to the bottom, bacteria, worms, and crabs consume what is left. Only the non-digestible mineral skeletons and exoskeletons remain in the ocean-bottom sediments. Except for what man removes, the nutrients remain in the ocean. And when man does remove those nutrients, it’s forever.
Man’s removal from the oceans
The problem is that we have fished out the oceans. Only 10% of the large fish found in the oceans in 1950 remain. And we have been over-fishing the oceans far longer than just the last 60 years. Even backe in 1950, people were already noticing a reduction in stocks of whales, salmon, cod, halibut, and other fish. We probably have less than 10% of the large fish and whale stocks that were present before we began harvesting the seas in earnest.
Where does the nitrogen that phytoplankton require come from? At the the mid-ocean levels, some comes from nitrogen fixing bacteria, but the rest comes from excrement in the form of urea and ammonia from bacteria breaking down protein as carcasses decompose. Iron, molybdenum, and phosphates come from the same sources. But remember we are removing large amounts of fish protein from the ocean, especially whale, tuna, shark, and other large fish. 90% of what was there is now gone. We are removing more every year. The fish products are no longer available to the phytoplanktons. Their food supply has diminsihed. They are starving.
What does that mean for the carbon cycle?
Why is this important? If we have lost 40% of our phytoplanktons, then we have lost a significant part of the photosynthetic biosphere. Photosynthesis binds 100 to 115 million metric tons of carbon each year. Phytoplanktons are responsible for half the photosynthesis that occurs on the planet. If we have lost 40% of the phytoplanktons, then we have lost 20% of our total photosynthesis capability, or the conversion of more than 20 million metric tons of carbon per year, in carbon dioxide, into oxygen and water. This is a huge loss, but it pales in comparison to a larger loss.
Phytoplanktons have silica and calcium carbonate skeletons. Phytoplanktons are eaten by copepods and other zooplankton, which also have calcium carbonate skeletons and exoskeletons. 40% of the carbon is converted into calcium-carbonate shells and excreta that sink to the bottom, and 60% to dissolved organic carbon originating from flesh. In 1950 planktons were sequestering 16.6 billion metric tons of carbon. Today, planktons convert 10 billion metric tons of carbon. If we have lost 40% of the phytoplanktons since 1950, i.e. 40% of the carbon sequestration capability, then 6.6 billion metric tons of carbon should appear in the atmosphere.
Currently we are adding more than 5.5 billion tons of carbon yearly to the atmosphere from fossil fuel and cement production (here). In 1950, we were adding more than 1.5 billion tons of carbon per year. That’s a 4 billion ton increase. Four billion tons a year is the yearly average atmospheric carbon-in-CO2 increase. But if in that same period, 40% of plankton have starved out, they are no longer sequestering 6.6 billion tons of carbon. These figures don’t agree. This is probably because the plankton loss is not quite 40%, but somewhat below 30%. Still, the plankton loss alone can account for the rising atmospheric CO2 levels.
Maybe it’s time we curtailed eating fish for a while, and focus on consuming poultry, pork or beef.