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Institute for Environmental Studies
University of Southern California
|(Partially drawn from a paper with Dave Karl and Tony Knap and from our Biocomplexity Proposal to NSF)
Our current conceptual model for the cycling of carbon in the ocean assumes that reactive nitrogen, mostly nitrate, limits plant growth and that the cycling of carbon can be linked to the nitrogen cycle by the Redfield stoichiometry. This is expressed in models by a simplified version of the new production hypothesis (Dugdale and Goering, 1967) that concentrates on the dynamics of nitrate and ammonia as a reflection of new and regenerated processes respectively. The carbon cycle is then reflected as a simple multiplication of the nitrogen fluxes by 6.6 (moles/mole). This is computationally tractable and, in many cases, seems to represent the dynamics of the drawdown of a spring bloom or upwelling in highly productive areas. However, the assumption that this behavior can be abstracted globally has important and perhaps un-intended consequences for the dynamics of the ocean and how we represent them in models.
The simple assumptions of a fixed Redfield stoichiometry and a nitrate-based new production system basically imply that biological processes are largely irrelevant in the global carbon dynamics that control atmospheric CO2. In a world that operated with these simple assumptions, all of the DIC that is required to fuel phytoplankton growth is introduced to the surface when the nutrients are upwelled or mixed into the euphotic zone. It was put their by the same remineralization processes that produced the nutrient. Since the simple N cycle has no gas exchange component, a simple carbon system that is linearly linked to nitrogen will have no net gas exchange either. Most of the global biogeochemical models of the ocean are represented by these dynamics. Thus it is not surprising that they have trouble articulating the role of biological processes in the uptake of CO2 by the ocean. Either these simple assumptions are true and ocean biology is minimally important (in which case we can probably invest our global change effort elsewhere), or we have to study the biology of the ocean from a different perspective.
The important processes for the net role of biology in the air-sea carbon balance will be those deviations from the simple assumptions that allow for a net exchange of carbon with the atmosphere. Many of these have been described, but are not usually the focus of research. If the Redfield ratio assumption is relaxed in a non-steady state world, the carbon dynamics become more interesting. An increased C:N ratio of export compared to uptake has been described in many studies and seen at both BATS and HOT. Dissolved organic carbon and nitrogen may be even more out of Redfield balance. If this dynamic is added to the link between the simple nitrogen dynamics and the resultant carbon dynamics, there is a net transfer of carbon from the atmosphere to the deep sea on the time-scales of the ventilation of those deeper watermasses. This same dynamic can also occur if the lengthscale for the depth of remineralization of carbon is larger than that of nitrogen. On millennial time-scales, this enriched deep carbon will ultimately ventilate back to the surface, however on interannual, decadal and centennial time-scales it means that a change in the rate of biological activity (and export in particular) can have a net effect on the atmospheric carbon concentration.
The other obvious modification of these assumptions is the inclusion of a gas phase or air-sea exchange to the nitrogen cycle. This will then require a gas exchange for carbon at some appropriate stoichiometry. Nitrogen deposition and nitrogen fixation are two processes where nitrogen is introduced from the atmosphere or from a dissolved gas phase that is ultimately equilibrated with the atmosphere. On a global scale, N deposition may play a modest, but increasing role (Galloway et al., 1995). We are just coming to realize that nitrogen fixation may be very important in the carbon dynamics on the interannual scales of climate variability and on the centennial to millenial scales of global change.
Oceanic N2 fixation has recently been identified as a much more significant part of the oceanic nitrogen (N) cycle than previously thought. The global rate of nitrogen fixation is now estimated to exceed 100-150 Tg N/y and global denitrification rate estimates are even higher. The balance of these two rates may directly influence the sequestration of atmospheric CO2 in the oceans by providing a new source of N to the upper water column. On longer time-scales, they change the total amount of nitrate in the ocean within the permissible bounds of the variability of N:P ratios in organisms. This also changes the air-sea partitioning of carbon. The prokaryotic microorganisms that convert N2 gas to reactive N are an unique subcomponent of planktonic ecosystems and exhibit a variety of complex dynamics including the formation of microbial consortia and symbioses and, at times, massive blooms. Accumulating evidence indicates that iron (Fe) availability may be a key controlling factor for these planktonic marine diazotrophs. The primary pathway of Fe delivery to the upper oceans is through dust deposition.
Perhaps the most exciting implication of the role of diazotrophy is that N2 fixers may be directly involved in global feedback cycles with the climate system and these feedbacks may exhibit complex dynamics on many different time-scales. The hypothesized feedback mechanisms will have the following component parts: The rate of N2 fixation in the world?s oceans can have an impact on the concentration of the greenhouse gas, carbon dioxide (CO2), in the atmosphere on time-scales of decades (variability in surface biogeochemistry) to millennia (changes in the total NO3- stock from the balance of N2 fixation and denitrification). CO2 concentrations in the atmosphere influence the climate. The climate system, in turn, can influence the rate of N2 fixation in the oceans by controlling the supply of Fe on dust and by influencing the stratification of the upper ocean. Humans also have a direct role in the current manifestation of this feedback cycle by their influence on dust production, through agriculture at the margins of deserts, and by our own production of CO2 into the atmosphere. The circular nature of these influences can lead to a feedback system, particularly on longer time-scales.
The key point is that the examination of the simple nitrogen dynamics and rate processes in the upper ocean that come from the basic paradigm (new production, export) may be less important than understanding how the deviations in our assumptions affect the links between elements. One of the strongest lessons from the two time-series stations and the other JGOFS investigations is that an intensive, multi-year study of each of these systems has revealed processes that are at odds with the assumptions of the simplest versions of our basic paradigms. When we include these new processes and changed assumptions, we come up with air-sea carbon dynamics that differ from the patterns in our simple models. Biology may be more important for global processes when it breaks the rules and we have to look for these unique patterns to understand their impacts.
|My comments are not specific to carbon cycling, but nutrient cycling in the ocean more generally, including fixed nitrogen cycling....
Dear Marine Scientists,
Taking a fish out of the sea does NOT leave a type of ""vacuum"" that nature is somehow compelled to fill in with another fish!
...PLEASE TAKE A FEW MINUTES TO CONSIDER THIS IDEA.
I ask you to consider a new explanation for the collapsing fisheries and other recent changes in the marine ecosystem. My theory is that the TOTAL MARINE BIOMASS IS NOW SEVERELY DEPLETED (it is obviously a direct result of fishing, and is in fact a CUMULATIVE effect, instead of something that happened because of recent ""mismanagement."") If you look at this theory closely, all of the trends that we are seeing in marine populations begin to make logical sense. Before I discuss the evidence for ""starvation,"" please consider how and why this overall biomass depletion has occurred.
WHAT IS THE NATURAL ""PRODUCTIVITY"" OF THE OCEAN? (The answer to this is fairly well understood.)
Carbon fixation and nitrogen fixation naturally occur in the ocean and are the mechanisms by which new ""life"" is constantly produced. Nitrogen fixation is more important in determining the absolute quantity that can be ""made"" each year since very few organisms (only some blue-green algae) are able to accomplish it. Fixed nitrogen is the limiting nutrient for phytoplankton (at least in the Northern hemisphere) and the rate of nitrogen-fixation therefore determines how much new protein will ultimately be added to the marine ecosystem each year.
WHICH FACTORS AFFECT THE ""RATE"" OF THIS NATURAL ""PRODUCTION"" IN THE OCEAN? (The answer to this question is NOT well understood.)
The productivity of the blue-green algae probably varies with climate change, but in the last millenium has likely changed very little, other than a possible recent decrease. Since the total area of the sea and the number of hours of sunlight in a day have been the same, and temperature has really varied very little, it is reasonable to assume that the rate of nitrogen fixation by blue-green algae is basically the same now as it has been for many thousands of years. It is the means by which life slowly accumulated in the sea over many millions of years. ""SLOWLY"" is the key word, and ""slowly"" is how it STILL is being produced. It was able to accumulate (over a VERY long time) and become very rich in life since the fixed nitrogen was constantly recycled through the marine ecosystem, only a small amount of ""new"" stuff was added each year, just a tiny annual gain in the total biomass - removals by land animals were insignificant until humans became adept at fishing. The amount of fixed nitrogen (protein) that we have removed from the sea has increased dramatically in the last century - there is no reason to believe that the blue-green algae have responded to this by stepping up their ""production.""
THE RATE OF PRODUCTION IN THE SEA IS NOT RELATED TO THE RATE AT WHICH WE CATCH FISH! ...And it never has been....Annual worldwide fish harvests are around 100 million tons - if no-one went fishing next year would the blue-green algae respond to this change by putting the brakes on and NOT fixing the nitrogen required for 100 million tons of fish? No, the blue-green algae are relatively unaffected by fishing and will only do what they have always done, that is: produce a SMALL annual gain in protein. ""IF"" they were capable of producing LARGE amounts of protein on an annual basis they would have done so a long time ago, and the result, over the millions of years of the ocean's existence, would have been a sea overloaded with life that would have sickened and died as a consequence long ago.
Life has been slowly accumulating in the sea this way for MILLIONS of years. Over the last 500+ years humans have removed what probably amounts to half of the original biomass that was there before the fishing industry began. We have run up a huge debt with the sea because our relationship has essentially been ""all take and no give.""
HOW DID FISH STOCKS WITHSTAND FISHING PRESSURE IN THE PAST? ...HOW DID THEY REPLENISH THEMSELVES?
Stocks like the northern cod on the Grand Banks of Newfoundland supported major fisheries for many years, apparently up to 30% of that cod stock was removed repeatedly on an annual basis and for a long time it seemed to have a great ability to ""rebound"" and ""rebuild"" itself. It took many years of heavy exploitation before the northern cod appeared to suffer significant losses. The cod rebuilt itself all those years at the expense of it's neighbours, it drew on the other compartments of the ecosystem, including it's many prey species and affected all lower levels, even the plankton. So the catching of the cod depleted not only the actual cod stock but drained the system as a whole. All fishing and whaling ultimately has this effect. People were, and still are, largely unaware of this fact. An old myth is still widely believed: ""there are lots of other fish in the sea.""
STARVATION - the end result of the depleted biomass - why?...and what evidence supports this?
The reduction of the total marine biomass has resulted in marine life now being relatively diluted or ""watered down"" compared to conditions years ago. Individual animals today are therefore faced with diluted prey, which leads to difficulty in getting enough to eat, which ultimately leads to starvation. For many species this now appears to be a critical problem. Plankton is also similarly depleted.
The most convincing piece of evidence of starvation that I see is a widespread decline in ""weight-at-age"" in fish over the last two decades. Nothing except food shortage would cause this in whole populations. It's not a case of too many of one species competing for the normal amount of food, since the abundance of these fish is rapidly declining along with their individual size. Other evidence...
- decline in abundance of virtually all fish stocks, except for a few of the smaller ones and some crustaceans which are at a temporary advantage because their predators have disappeared first.
- decline in the average size of fish that are caught, virtually all fisheries are no longer catching any ""big"" ones.
- fish are maturing at smaller sizes than they did in the past
- widespread failure of Atlantic and Pacific salmon stocks to return from their time at sea
- gray whales dying of starvation in the Pacific Ocean
- malnourished right whales in the Atlantic Ocean
- orcas and belugas in decline, not reproducing normally
- Sea lions, seals and seabirds dying in the Bering Sea due to starvation
- Sea otters (California and Alaska) in decline
- zooplankton in decline; this has been recorded in many places
- coccolithophores ""blooming"" in the Bering Sea: a new development, a type of plankton that thrives in ""nutrient poor"" water in a normally nutrient rich part of the sea..
THE CAUSE OF THESE CHANGES? ...WHY NOT ""EL NINO"" AND OTHER RECENT TEMPERATURE/CLIMATE ABNORMALITIES?
Temperature changes doubtlessly affect marine organisms, putting some species at a relative advantage and others at a disadvantage, so the species mix in the system as well as the ranges of species could well change with the temperature. But the SUM TOTAL OF LIFE in the ecosystem should not be decreasing from the small-scale temperature changes that we have seen recently from global warming. It's not logical. The overall decline in sea life is simply the end result of centuries of fishing.
FURTHERMORE...since fishing has been dragging down the whole system for centuries, plankton levels have also been in decline for that length of time. This includes phytoplankton...and the result of the gradual drop in phytoplankton has been a gradual increase in atmospheric carbon dioxide...so it looks most likely that declining fish stocks may be the CAUSE and not the RESULT of global warming...
(This view is very much against the grain of current thinking but makes logical sense, especially considering the time frame of the CO2 rise...it was up significantly before the industrial revolution...and lots of fishing and whaling had already been done by that time...)
WHAT CAN BE DONE ABOUT THE PROBLEM? ONE THING ONLY: ""FEED THE FISH""
Regardless of what you think about my final conclusion, I am really interested in your opinion on the ""starving ocean"" theory, and would love to hear your comments. If you see a flaw in the logic, please tell me; I am trying to stimulate a serious debate on this topic. I have done a lot of research into this theory and have recently made a website on the theme if you are interested in taking a look at it - my main ""starvation"" argument is at http://www.fisherycrisis.com/starving.html
of Maryland Center for Environmental Science
Mar 06 2000
Ocean Carbon Cycle Studies: The Importance of Ocean
Time-Series and Mesopelagic Process Studies
Future ocean carbon studies, whether OCTET, EDOCC or some
combination of the two should consider conducting process
studies in the context of established time series stations.
Our present ocean time-series stations, BATS and HOTS, have
proven to be a tremendous resource to assess seasonal and
annual variability in carbon cycle processes; to conduct
focused process studies; and, to test various types of
Given this background of data and understanding, it is
surprising that we did not conduct JGOFS-type process studies at the HOTS and BATS sites
with all of the detailed measurements of trophic processes,
nutrient cycling and flux measurements that were done at the
JGOFS process study sites. Detailed process studies imbedded
within time-series studies allow one to assess the physical,
chemical and biological conditions of the study site before
and after the process measurements and thus provide an
essential framework for interpreting rate measurements in the
context of seasonal, annual and decadal changes.
Future carbon studies in both the open ocean and on coastal
margins should establish time-series stations with comparative
approaches in different ecosystems. Long-term studies are
essential to understand the changes that are occurring in the
ocean carbon cycle. While previous expeditionary processes
studies have resulted in many new insights into important
biogeochemical processes, these “snapshots” do not allow
us to confidently extrapolate rate measurements that vary as
the scalar properties of the ecosystem change. An integral
component of future process studies should be the
establishment of time-series stations that provide an
essential context of variability of the system.
processes studies were concentrated primarily in the euphotic
zone. Through these studies we have greatly increased our
understanding of the food-web processes that recycle carbon
within, or export carbon from the euphotic zone. Thorium and sediment trap studies have estimated the
gravitational flux at different depths in the water column,
however we have little notion of the mechanisms responsible for
the utilization and transformation
of carbon below the euphotic zone. JGOFS process studies in the
Arabian Sea and Equatorial Pacific have shown that roughly 90%
of the carbon fixed by autotrophs is recycled within the water
column with approximately 10% exported as the gravitational flux
at the base of the euphotic zone. Sediment traps at 1000 m have
shown that approximately 90% of this gravitational flux from the
euphotic zone is recycled between 100 and 1000 m. Thus the
percentage of the carbon that is recycled (90%) is roughly the
same in the euphotic zone and in the 100 to 1000 m depth
horizon. Is this recycling distributed homogeneously between 100
and 1000 m? How do
the bacteria, protozoa, mesozooplankton and fish determine the
rates of carbon utilization, transformation and export in the
mesopelagic zone? We might guess that there are particular depths horizons that
are “hot spots” for recycling, regeneration and the
utilization and repackaging of sinking particles. If one
“average” particle concentration (POC) from bottle casts to
the carbon requirements for the resident mesopelagic
mesozooplankton, the zooplankton should all starve to death.
Clearly this is not the case, implying that sinking particles
may be concentrated at particular density interfaces where the
mesozooplankton could not only meet their nutritional needs but
also repackage sinking material into faster sinking fecal
pellets. In the case of meso- and macrozooplankton , there are similar
species and genera worldwide in the mesopelagic depths. Thus there are particular species that could be important
in the interception and utilization of sinking carbon particles.
University of Washington
|To Steering Committee EDOCC
(Ecological Determinants of Ocean Carbon Cycle)
I have before me the Workshop Prospectus (report of the first
organizational committee meeting, updated 01/10/00). Let me first and
foremost write that I applaud your initiative wholeheartedly: In my
opinion, the current large programs and those of the recent past (here,
JGOFS in particular) will not solve key problems of understanding the
marine carbon cycle based on mechanistic understanding, but at best,
provide statistical fits to the new data, coupled with modeling in terms
of C, N, Si, perhaps Fe, and physics. For example, I doubt that global or
regional predictions of the CONCENTRATIONS of phytoplankton chlorophyll
within a factor of two, including the seasonal changes will become
available (although the prediction of the timing of, e.g., blooms may be
in hand). Without this, the zooplankton concentration and production, and
(next level of complication) the vertical flux out of the photic zone and
(a further level) the fraction that arrives at the deep-sea bed, cannot be
calculated with useful accuracy, based on mechanistic understanding.
From the questions listed in the Prospectus (all of which I regard as
valid, although some may not be amenable to testing in the field), I
suppose that there are others who share this pessimistic view. I am
writing, therefore, whether a workshop agenda exists now. Because the
members of the steering committee principally are studying microbial
processes, I just want to make sure that items like to so-called "top-down
processes" are not being forgotten.
University of Hawaii
reading the EDOCC document, I had some comments on Section 4:
Web Structure and Physical Processes.
Not only do physical processes
influence the species composition of a given oceanic
environment, but as
far as carbon cycling, the physics can overwhelm any
contribution by the
biology. This is
very evident in the equatorial Pacific.
I do find the
attempts to link keystone species to specific physical regimes
interesting, because this may be one way to model global oceanic
cycling without the use of ecosystem models that don't work in
than the region they were tuned to.
Webpage by Jasmine S. Bartlett, Oregon State University.