QUESTIONS AND COMMENTS RE: Nutrient Flow patterns and the Nitrogen Cycle in the Marine Ecosystem

Many scientists and others have informed me that adequate amounts of nutrients enter the sea to replace the fish that are removed. Also that there is an increasing amount of bio-available nitrogen in the global system overall...basically concluding that it is therefore not possible that marine creatures can be suffering from nitrogen-deficit/starvation. Typically...

”...suspended particulates in rivers or, more appropriately, fertilizers in run-off from farm land or sewage - which, without tertiary treatment, conveniently returns all that fish nitrogen we ate minus the 10% we assimilated of course...”

(nitrogen input) “..don’t forget volcanism and organic-inorganic cycling occurring within sediments along shorelines”

“Apart from all of the obvious nutrient sources, there is a net positive gain of nutrients by the sea because in tropical areas the nitrogen locked into the soil for centuries is being washed into the sea as the forests are cut and the soil leaches into the rivers.”

“Much usable nitrogen is from terrestrial sources and from atmospheric deposition (dry and wet).” (One atmospheric source is from lightning which also changes some amount of inorganic nitrogen in the atmosphere to an organic form.)

“Amounts of usable nitrogen are increasing, and eutrophication in semi-enclosed systems can be a serious issue.”

The flipside of the “how much nutrients/nitrogen goes into the sea?” is “how much leaves the sea?” with the added question of whether or not the amount of fish/biomass removed by fishing could actually make a dent in it overall, or perhaps is the “pool” of bio-available nitrogen so vast that fishing removals are merely trivial to such a system. This appears to be very difficult, maybe impossible, to answer with any certainty...

“While the rate of new protein production is related to the supply of fixed nitrogen as you correctly state, the principal supply comes from the simply enormous stock of nitrate in the deep sea. Its sources ultimately are nitrogen fixation by cyanobacteria (formerly called blue-green algae), weathering on the continents, and atmospheric discharges (lightning). All of these processes have acted for at least a billion years, yet the ocean has not become a cesspool because of the removal of fixed nitrogen by sedimentation (burial in sediment) and bacterial denitrification...While the measurements of the rates of the various gain and loss rates are still afflicted by large error margins so that we do not know whether or not there is long-term (centuries and longer) equilibrium, and the rate of removal of nitrogen by our fisheries cannot be compared with them, it is certain that the total stock of fixed nitrogen is so large that a few hundred years of fishing (intense only for about half a century) cannot have made a dent.”

“The continental shelf environment presents a formidable challenge to those who wish to understand the role of bottom sediment in the carbon (and nitrogen) cycle.”

“I would think that the ‘response time,’ relative to total oceanic fixed nitrogen would make it almost impossible that the biotic, fish compartment could respond to fishing that has occurred over a few to several decades. By that I mean that the fish-nitrogen removed in the last century would not have a perceptible effect on total nitrogen or nitrogen concentrations.”

“I suspect that the removal of nitrogen in the form of fish landed is within the natural range of variability in processes such as fixing, denitrification and exchange between organic and inorganic fractions (e.g. through variations in upwelling driven by climate variability). I don’t know of any calculations to confirm or refute this.”

The process of “denitrification” is interesting. Certain bacteria work to “undo” the nitrogen fixation work of others, with the result that they render bio-available nitrogen into forms no longer usable by most living organisms. They change it back to an inorganic form (some of which is in gaseous form so it bubbles to the surface and re-enters the atmosphere). This is recognized as a naturally occurring process in the system and seems to be something of a compensatory mechanism to counteract situations where nitrates become too concentrated. The “denitrifiers” are much more active in water contaminated with sewage (high nitrate levels), for instance, and they also work better in conditions of low oxygen levels (this describes the waters affected by eutrophication). So, as we increasingly “contribute” nutrients to the ocean in the form of sewage and agricultural runoff in such quantities that it pollutes estuaries and coastal waters...we might not be providing as much “usable” nitrogen as we think. Denitrifiers are likely much more active in the ocean system overall now that it includes so many more eutrophied areas than historically were normal. The “denitrifying” bacteria obviously cannot work fast enough to actually clean up the mess that we have made in these areas, but they might make a dent in the numbers when guessing how much “nutrient” is actually being “contributed” to the ocean. (Note: I also read recently that it has been discovered that some of the bacteria that are “nitrifiers” will actually switch to being “denitrifiers” in conditions of low oxygen tension. Natural processes seem to try to work to counteract the effects of “eutrophication.” (Reference: )

Besides the effect - to whatever extent that it occurs - of denitrification in polluted estuaries, I wonder how much nutrients end up locked into the bottom sediment in some of these areas. Some rivers lay down a lot of sediment so significant amounts of organic material could “leave” the food web this way. These are just some of the uncertainties in the system as it occurs naturally and as it has been altered by the activities of humans.

“In many of the world’s inland and coastal waters, too much nitrogen is seen as a problem, not too little, as your arguments suggest. Eutrophication control is one of the major activities of water boards and environmental agencies in the industrialized countries. In other areas, nutrients are perceived to limit productivity (e.g. Norwegian fjords). As with all these issues, the big picture looks different if you disaggregate it.”

“The net effect of removing nitrogen by fishing has little effect on the total nitrogen in the ocean. The amount of nitrogen tied up in micro-organisms to that in fish is probably greater than 1000:1. Thus depletion of all the fish in the world would have little effect on the total amount of dissolved or particulate nitrogen in the ocean. Furthermore, the amount of biomass due to fish in the ocean is orders of magnitude smaller than the amount in micro-organisms...However, there does seem to be a change in the past few decades of the role of nitrogen-fixing bacteria in the Central Pacific, but this is presumed to be due to changes in physical forcing associated with changes in climate.”

So organic runoff is up, nitrogen-fixation may be down, denitrification may be up, fishing is pretty intense, upwelling is down, aerial deposition is up...trying to sort out the actual nutrient balance in the ocean by only looking at numbers like these, may prove to be impossible. Here’s a little excerpt on this topic from my book:

”..I have been assured by the biologists who did write back to me that “adding food is not the solution for the depleted ground fishery,” and that “plankton and primary producers seem just as healthy as ever.”

So I claim that the ocean is depleted of nutrients and the scientists claim that it is not. How can this be? And who is right? Well, one place where we differ is in the definition of terms. When they say that the concentration of nutrients is fine, they are referring to the levels of dissolved available nutrients like nitrates and phosphates, and the amounts of microscopic life forms (plankton of both the plant and animal varieties). When I discuss the level of nutrients, I am considering the points of view of such things as carnivorous fish. From their point of view the principle can be stated this way: “If you have a heartbeat, then you are a nutrient...or a potential nutrient.” A hungry fish simply cannot wait for nutrients to make their way up the “food chain” (or “food web” if you prefer) because it just takes too long. A hungry fish needs something it can sink its teeth into, today.

Sure, nutrients are taken up by microscopic forms and then consumed by successively larger creatures until they can be eaten by the top predators, (most of us have seen the diagrams), but the progression is not as neat and orderly as that, and not so one-directional. Most of the nutrient exchange goes on in little cycles inside the web and the amount that actually starts out “new” at the bottom and becomes available to the guy at the top is very small. For example, a given bit of protein could experience this sequence of events: For the sake of argument, it starts off incorporated into the body of a capelin (a small fish, commonly eaten by many others), the capelin/protein bit is then eaten by a codfish, then the cod uses it to produce an egg, then the egg hatches, the little cod larva is then eaten by a capelin, which is eaten by a cod, and the cycle goes around indefinitely, with the little protein bit never actually reaching the bottom level (microscopic nutrient) or the top level (eaten by a top predator such as a shark or swordfish.) Only considering the levels of plankton and dissolved nutrients will never reveal the fact that this little protein bit is missing. And many bits are missing. And we took them all...”

I was trying to convey the idea that the FORM that the nutrients are incorporated in is very significant (i.e. solid food is necessary for creatures with teeth). Particulate organic matter and dissolved organics from sewage, along with soot and dissolved organics from the skies...are utterly inappropriate foods for actual swimming fish. And TIME is a major factor in the equation: I think it takes much longer than many presume for the ecosystem to transform micro-nutrients into fish flesh in any significant quantities. There are many routes that can be taken by nutrients in the web, doubling back repeatedly, and why wouldn’t they?...I can’t see where there is any particular force pushing them to find their way straight to the “top.” (But the success of the top level absolutely depends on the health of the levels below it, and that’s why top predators must disappear first as the overall system is depleted.)

Besides the form of the nutrients and the time factor, LOCATION is obviously another key factor regarding nutrient input into the ocean. Dumping all the “input” from run-off near the coastlines, spreading the atmospheric “input” randomly across the globe - fails to target or provide for the specific needs of the fish on the depleted offshore fishing banks. Marine life (and nutrients) was formerly very concentrated in these specific areas, and these are the areas that need to be deliberately targeted for nutrient input or “feeding.”

A comment along these lines from one scientist:

“One idea I think may be interesting is that changes in the form and timing of nutrient inputs may be important. Commercially important organisms may be declining and being replaced by other organisms less favored by humans. This could be happening even if overall nutrient levels and food levels have not declined (the food may just be present in a different form). If true, desirable fish may be starving not because there is too little food overall, just too little of the food that they can utilize. I think this is mostly consistent with your main argument(?)”

Besides the question of how much usable nitrogen is actually in the system, there is also the issue of how “available” the nitrogen is to the living things. Availability to the lower end of the web (phytoplankton, and therefore “primary productivity”) depends on the nutrients being present in the upper layer of the water where the sunlight reaches. Since organic matter constantly falls to the bottom (dead organisms and excrement), currents that raise bottom water to the surface are critical to recycling them through the plankton level of the web. Environmental factors (like temperature and climate change) can affect these currents, resulting in less strong upwelling currents, with the end result of less nutrients being made available at the surface where photosynthesis occurs. This is certainly one way that climate changes affect productivity in the ocean, but how can the magnitude of the effect be determined? Do the upwelling currents stop altogether or are they present at a decreased intensity? Climate change appears to be one factor among many affecting the system. There appears to be a connection between temperature and degree of upwelling and consequently availability of nutrients. I don’t deny this or that the system is very complicated...

“It may not be that the warm water CAUSES the fish to grow more slowly, but that the warm water and the slow growth are themselves both associated with another phenomenon. Coastal upwelling is an important factor in growth and survival of fish off the west coast of the U. S. This upwelling is caused by winds blowing offshore, moving the surface layer of water offshore with them. That water has to be replaced by something, and what it is replaced by is colder nutrient-rich water drawn up from the depths. The upwelled water provides nutrients for blooms of phytoplankton, which feed zooplankton...and on up the (fairly short) food chain to salmon. So it isn’t the warm water slowing growth, but the colder water encouraging production of feed, which promotes growth in fish.”

Regarding the amount of nutrients brought to the surface by upwelling, one thing besides the strength of the currents that would have an impact, would be the amount of organic material that sinks to the bottom. A study in the Pacific found the following:

“...Dr. Smith and Ronald S. Kaufmann, a former Scripps post-doctoral researcher now at the University of San Diego, found that the rainfall of food in the North Pacific had declined over seven years.

About 135 miles off the central California coast, in a region more than two and a half miles deep, they measured the rain in sediment traps, cone-shaped devices that catch falling particles. The traps were suspended off the bottom at elevations of roughly 150 and 2,000 feet. Trapped particles were collected periodically over the seven-year study period and analyzed to determine organic carbon content.

The amount of sinking particulate matter (a measure of food supply) was compared to oxygen consumption by marine creatures in the seabed (a measure of food demand). The scientists reported that between 1989 and 1996 the ratio decreased by more than 50 percent.

"The findings of Smith and Kaufmann will have far-reaching implications," Ellen R. M. Druffel of the University of California at Irvine and Bruce H. Robison of the Monterey Bay Aquarium Research Institute, in Moss Landing, Calif., said of the research in a Science commentary. "We used to think of the deep sea as a highly stable, steady-state system."

Dr. Smith and Dr. Kaufmann said the decline in the food supply might be related to an increase in surface temperatures during the same period. The ocean warming, they said, may have prompted a decline in zooplankton -- tiny creatures that eat the tiny plants of the sea and in turn become food for a vast web of life. And their diminishment in turn may have cut the total amount of food that falls into the depths.

"A long-term reduction in surface productivity," Dr. Kaufmann said in a Scripps statement, "could severely impact the amount of food delivered to the deep ocean."

The article quoted above is on the internet at:

So the amount of food raised by upwelling depends on the productivity at the surface which depends on the amount of food raised by upwelling...this is true, no doubt, but if the overall system suffered a decline, then both of these would also likely become diminished as a result.

Regarding the effects on food levels of fish-biomass removal from the deep sea, several sources agree that it could be significant:

“Another aspect of deep-sea fisheries that deserves more study is the impact of removing fishes from the food web. Deepwater environments are more food-limited, and industrial-scale fisheries that remove large biomasses may have ecological costs. Data on biomass, trophic interactions, and productivity of non-target species are rarely available (even for coastal waters).” - Jon A. Moore and Pamela M. Mace. 1999. Challenges and prospects for deep-sea finfish fisheries. Fisheries 24(7): 22-23

“The annual removal of 100 million metric tons of consumers from the world’s oceans is an impressive amount, orders of magnitude larger than losses by any toxic pollutant and much more likely to lead to long-term, large-scale changes. Fishing fleets have removed a substantial fraction of biomass from the upper levels or marine food webs in many parts of the oceans. Too little is known as to the potential consequences of this massive alteration. The intensive fisheries at once remove consumers, affecting possible top-down controls, and also reduce the abundance of food items for top predators, thus interfering with bottom-up controls.” - Ivan Valiela. Marine Ecological Processes, Second edition. New York: Springer, 1995, pp 513-514.

So, in conclusion regarding nutrient levels and overall biomass in the sea - it would appear to be very difficult to state anything with absolute certainty. I have been told frequently that “There is no evidence that the total marine biomass is now severely depleted.” But on the other hand it looks to many as if it is “down” a bit lately at least in the Northwest Atlantic ocean. And there is also “no evidence” that the overall biomass generally stays the same, or experiences cyclical climate-driven fluxes in level. The idea that the overall biomass stays the same seems to be an assumption underlying many theories about the dynamics of the marine ecosystem. I think it is time to take a serious and critical look at this idea. Regardless of the “total” it is obvious to me that from the point of view of such creatures as carnivorous fish, that the “form” and the “location” of the nutrient inputs (that are meant to compensate for the fishing removals) are very significant factors in determining their real “availability” as fish food.

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Debbie MacKenzie


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