Ocean nourishment in the Humboldt Current
Ian S F Jones
University of Sydney
otg@otg.usyd.edu.au
ABSTRACT
Ocean nourishment is the concept of controlling the climate and enhancing the marine food chain
by providing additional nutrients to the world’s oceans. This paper looks at the consequences of
carrying out large scale ocean nourishment activities in the Humboldt Current area off the coast of
South America. Portions of this current are associated with a very productive fishery but one
known for its wild fluctuations that have social consequences for the peoples in the developing
countries that rely on this fishery. The use of ocean nourishment to stabilise this fishery relies on a
better understanding of the role of primary production in modulating fish catch. By assuming fish
catch responds to primary production it is found that ocean nourishment is an economic way of
providing additional food from this fishery when combined with the sale of carbon credits.
INTRODUCTION
The rising world population is leading to an increasing amount of carbon dioxide dumping in the
atmosphere and to a rising demand for food. Ocean nourishment is a technology aimed at
addressing both these issues by providing the nutrients to the photic zone of the ocean to sequester
additional atmospheric carbon and to enhance the marine food chain. This was classified as macronutrient fertilisation in a discussion of CO2 sequestration options by Ormerod and Angel (1998).
There will be many regions of the ocean where both micro and macro nutrients are needed.
Land based factories using natural gas to produce the primary nutrient, nitrogen, have been
discussed by Jones and Otaegui (1997) while the benefits of floating platforms able to exploit
stranded gas have been explored in Jones and Cappelan Smith (1999). The cost of providing the
primary nutrient from shore based factories and delivering it to the edge of the continental shelf
have been estimated in Shoji and Jones (2001). The global impact of large scale nourishment on
the atmospheric carbon dioxide has been addressed in Matear (2000a and 2000b). The importance
of micro nutrients in an ocean nourishment strategy is yet to be resolved. The role of one micro
nutrient, iron, has received much attention. Coale et al (1996) has shown that both phytoplankton
and zooplankton respond to iron fertilization and as a consequence draw down the carbon dioxide
in the upper ocean.
Figure 1 The ocean nourishment process showing the fate of carbon with time. Sequestered carbon
recycles in and out of the atmosphere for a small fraction of the time.
Ocean nourishment has the prospect of producing large scale permanent sequestration of carbon in
the deep ocean. Such sinks of carbon could be used to generate tradable carbon dioxide credits
more economically than any other large scale proposal. It is well suited for Activities Implemented
Jointly under the United Nations FCCC. Jones and Young (1998) discuss how ocean nourishment
is genuinely additional to planned activities and has a low opportunity cost, two issues that bedevil
AIJ proposals.
The sequestration process is illustrated in Fig. 1. The reactive nitrogen (and other nutrients)
combine with dissolved carbon dioxide to produce organic carbon. A fraction of this sinks out of
the photic zone of the ocean while the rest is recycled for further phytoplankton growth. This
recycling is not shown since it does not matter how many times the introduced nutrient recycles as
long as a fraction is sequestered in each reworking. A problem would arise if this recycling went
on so long that the surface waters were subducted before the nutrients carried their carbon out of
the photic zone. The export-to-recycle fraction depends upon the size of the phytoplankton and on
the nature of the zooplankton faecal pellets. However the size of the phytoplankton induced seems
of secondary issue for carbon sequestration. When the sequestered carbon again comes to the
surface by upwelling, as shown in the middle of Fig. 1, it is again in contact with the atmosphere.
The nitrogen on the other hand remains trapped in the upper mixed layer. A little nitrogen is lost to
the atmosphere by denitirification and this is shown as the thin arrow. The nutrients again take up
an amount of carbon dioxide the equivalent to the upwelled carbon, less the loss due to
denitrification.
A small amount of organic carbon settles to the seafloor, shown as a broken arrow in Figure 1.
THE OCEANIC FOOD WEB
We propose to look at the Humboldt Current system, known for the large catches of small pelagic
(open ocean) fish such as sardine and anchovy. The relative abundance of these two clupeids seem
to fluctuate as well as the total catch, a phenomena documented by Alheit and Bernal (1993).
Figure 2 reproduces the total catch for Peru from Carr and Broad (2000). Sardine-like fish have a
mixed diet of phytoplankton and zooplankton according to James (1988). They display a high
degree of opportunism in fulfilling their dietary requirements. James (1988) states that for
“intermediate microphagous clupeids (it is their) flexible and opportunistic feeding behaviour,
which enables them to forage efficiently over a wide range of particle sizes that has led to their
success in unstable (upwelling) regions.”
Figure 2 Annual catch of small pelagic fish off Peru showing the large fluctuations. Important El
Nino years were 1972, 1982, and 1997. Reproduced from Carr and Broad (2000)
Carr and Broad found a good correlation, r=0.5, between Peruvian fish catch and modelled
mesozooplankton grazing. The model follows that of Moloney and Field (1991) where high
nutrient levels encourage the preferential growth of mesozooplankton over smaller zooplankton.
There are many reasons why plankton levels should be weak indicator of fish catch. The small
pelagic fish here have growth times of a few years while the zooplankton grow to maturity in a few
weeks. Fishery management also makes fish catch not well correlated with biomass. Thus the
emergence of a correlation suggesting food limitations strongly influences fish catch, indicates that
the small pelagic fish catch would respond if extra plankton was produced by ocean nourishment.
How important is the size distribution of the primary production induced by the addition of
nutrients? It is often found that at low nutrient levels, small phytoplankton dominate. This is not at
all clear and the study of the surface plankton in the North Pacific by Han and Takahashi (2000)
showed no clear trend for the fraction of netplankton greater than 30 micron, to depend upon the
total phytoplankton abundance. They found from regression that 4.3% of the chlorophyll was in
the larger phytoplankton. The total chlorophyll varied from 0.15 g Chl l-1 to 6.63 g Chl l-1. Will
the phytoplankton species in an environment low in trace nutrients be more or less suitable for
clupeids? This question is not addressed in this paper, but is related to the design strategy for the
nutrient mix of an ocean nourishment plant. There will be differing optimum designs in different
locations.
The economics of fish production can be estimated by the following simple analysis. Clupeids
have short food webs with only one or two steps. Lasker (1988) citing data by Villavicencio
suggests 1 t of anchovy have a metabolic requirement of 3.4 tC/yr. If we assume they consume
zooplankton (a conservative assumption) and they are step 2 in the food chain, we can calculate the
amount of primary production needed to support them. If the efficiency of energy transfer, from
phytoplankton to zooplankton is 10%, we see that 1 t of small pelagic fish requires 34 tC/yr of
primary production. Ocean nourishment is measured in terms of new primary production, while the
above is the primary production involving extensive upper ocean recycling of the nutrients. Let us
estimate the export of carbon at 20% per cycle, that is 0.8 of the carbon is recycled to provide
further primary production. The nitrogen produces 1 + 0.8 + 0.64 + ..... = 5 times the new primary
production.
Thus 1 t of fish needs 34 tC/yr of primary production or about 7 tC/yr of new production.
Assuming a Redfield ratio of 7:1, one tonne of fish needs 1 tN/yr. Sustainable fishing allows about
40% of the biomass to be harvested giving 2.5 tN/yr. Nitrogen costs about US$150 tonne so to
harvest 1 tonne small pelagic fish requires about US$375. This is broadly in agreement with, but
less than, the estimate of US$1,100 per tonne of fish catch by Jones and Young (1997). They took
the global annual fish catch and compared it with estimates of the current level of new primary
production. The lower cost of ocean nourishment for clupeids possibly comes from a shorter food
chain than for harvestable fish in general. In the costing we have not addressed the issue of the
other macro or micro nutrients that might be needed.
Those regions which have tradable fishing quota will be able to control the financial return from the
fish stocks produced as a result of ocean nourishment.
THE ROLE OF OCEAN NOURISHMENT
If the fish stocks are limited by primary production, the ocean nourishment technology could be
used to modulate the fluctuations. Alheit and Bernal (1993) show the total pelagic fishery of the
Humbolt system fluctuating by about 10 Mt/yr from epoch to epoch. We can calculate the macro
nutrients needed to eliminate this fluctuation based on the assumptions about the food web
developed above.
While maintaining the fish catch at a sustainable constant level has much societal value to the
people adjacent to the Humboldt current, the uptake of carbon dioxide by enhanced primary
production will be of benefit to all people in mitigating climate change.
In those years where 25 MtN/yr were supplied, the sequestering of carbon can be estimated from
the calculations of Jones and Otaegui (1997). They suggest, after allowing for biological uptake
efficiency and the carbon dioxide produced by the ocean nourishment system, that 1 tN sequesters
12 tCO2. Thus 25 MtN would sequester 300 MtCO2 (avoided). Based on a cost of $US10 per
tonne carbon dioxide, suggested as a mid range by Shoji and Jones (2001), the carbon credits would
cost of US$3,000M to create. As US$10 per tonne carbon dioxide avoided is a very modest cost
for large scale abatement, this would be the main revenue stream and would represent for example
about ¼ of the carbon dioxide the USA needs to avoid if it is to meet the Kyoto Protocol in 2008.
The capital costs extracted from Shoji and Jones (2001) needed to produce this much ocean
nourishment capacity would be of order 12 billion dollars. Private investment in electricity plants
in Latin America was, for example, about US$20 billion in 1997 alone (World Bank). As the ocean
nourishment capacity would be installed over a few years the capital cost is not prohibitive. The
capital investment in fishing infrastructure has already been made on the South American coast.
RISKS
When the nutrient is introduced to closely mimic the naturally occurring upwelling in area adjacent
to historical upwelling regions, such as the Humboldt Current region, the risks to the environment
will be small. As the ocean plants would be introduced progressively the impact on the
environment could be monitored and the program terminated if an unacceptable change was
occurring. As with any increase in primary production there is a corresponding increase in oxygen
consumption in the thermocline region of the ocean. If these waters are distributed over the ocean
basin the impact should be moderate.
CONCLUSIONS
The financial benefits of increased commercial fish stocks expected from ocean nourishment is a
substantial supplement to the income that might be generated by carbon credits. It is very suitable
for low-income, food-deficient countries of the world (LFDC) who depend on fishing for their
protein and their livelihoods. In the present case the role of providing additional fish catch in years
of low yield provide the opportunity to maintain the livelihoods of people in an already established
industry.
In ocean nourishment we seem to have a scheme that can contribute to the problem of managing the
climate and generating additional food for the rising world population.
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