7.4 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Ocean fertilization | 2/4 | https://en.wikipedia.org/wiki/Ocean_fertilization | reference | science, encyclopedia | 2026-05-05T07:35:53.616567+00:00 | kb-cron |
== Approaches == Phytoplankton require a variety of nutrients. These include macronutrients such as nitrate and phosphate (in relatively high concentrations) and micronutrients such as iron and zinc (in much smaller quantities). Nutrient requirements vary across phylogenetic groups (e.g., diatoms require silicon) but may not individually limit total biomass production. Co-limitation (among multiple nutrients) may also mean that one nutrient can partially compensate for a shortage of another. Silicon does not affect total production, but can change the timing and community structure with follow-on effects on remineralization times and subsequent mesopelagic nutrient vertical distribution. High-nutrient, low-chlorophyll (HNLC) waters occupy the oceans' subtropical gyre systems, approximately 40 per cent of the surface, where wind-driven downwelling and a strong thermocline impede nutrient resupply from deeper water. Nitrogen fixation by cyanobacteria provides a major source of N. In effect, it ultimately prevents the ocean from losing the N required for photosynthesis. Phosphorus has no substantial supply route, making it the ultimate limiting macronutrient. The sources that fuel primary production are deep water stocks and runoff or dust-based.
=== Iron ===
=== Phosphorus === In the very long term, phosphorus "is often considered to be the ultimate limiting macronutrient in marine ecosystems" and has a slow natural cycle. Where phosphate is the limiting nutrient in the photic zone, addition of phosphate is expected to increase primary phytoplankton production. This technique can give 0.83 W/m2 of globally averaged negative forcing, which is sufficient to reverse the warming effect of about half the current levels of anthropogenic CO2 emissions. One water-soluble fertilizer is diammonium phosphate (DAP), (NH4)2HPO4, that as of 2008 had a market price of 1700/tonne−1 of phosphorus. Using that price and the C : P Redfield ratio of 106 : 1 produces a sequestration cost (excluding preparation and injection costs) of some $45 /tonne of carbon (2008), substantially less than the trading price for carbon emissions.
=== Nitrogen (urea) === This technique proposes to fertilize the ocean with urea, a nitrogen rich substance, to encourage phytoplankton growth. Concentrations of macronutrients per area of ocean surface would be similar to large natural upwellings. Once exported from the surface, the carbon remains sequestered for a long time. An Australian company, Ocean Nourishment Corporation (ONC), planned to inject hundreds of tonnes of urea into the ocean, in order to boost the growth of CO2-absorbing phytoplankton, as a way to combat climate change. In 2007, Sydney-based ONC completed an experiment involving one tonne of nitrogen in the Sulu Sea off the Philippines. This project was criticized by many institutions, including the European Commission, due to lack of knowledge of side effects on the marine ecosystem. Macronutrient nourishment can give 0.38 W/m2 of globally averaged negative forcing, which is sufficient to reverse the warming effect of current levels of around a quarter of anthropogenic CO2 emissions. The two dominant costs are manufacturing the nitrogen and nutrient delivery.
In waters with sufficient iron micro nutrients, but a deficit of nitrogen, urea fertilization is the better choice for algae growth. Urea is the most used fertilizer in the world, due to its high content of nitrogen, low cost and high reactivity towards water. When exposed to ocean waters, urea is metabolized by phytoplankton via urease enzymes to produce ammonia.
CO
(
NH
2
)
2
+
H
2
O
→
u
r
e
a
s
e
NH
3
+
NH
2
COOH
{\displaystyle {\ce {CO(NH_2)_2 + H_2O ->[urease] NH_3 + NH_2COOH}}}
NH
2
COOH
+
H
2
O
⟶
NH
3
+
H
2
CO
3
{\displaystyle {\ce {NH_2COOH + H_2O -> NH_3 + H_2CO_3}}}
The intermediate product carbamate also reacts with water to produce a total of two ammonia molecules. Another cause of concern is the sheer amount of urea needed to capture the same amount of carbon as eq. iron fertilization. The nitrogen to iron ratio in a typical algae cell is 16:0.0001, meaning that for every iron atom added to the ocean a substantial larger amount of carbon is captured compared to adding one atom of nitrogen. Scientists also emphasize that adding urea to ocean waters could reduce oxygen content and result in a rise of toxic marine algae. This could potentially have devastating effects on fish populations, which others argue would be benefiting from the urea fertilization (the argument being that fish populations would feed on healthy phytoplankton).
=== Pelagic pumping === Local wave power could be used to pump nutrient-rich water from hundred- metre-plus depths to the euphotic zone. However, deep water concentrations of dissolved CO2 could be returned to the atmosphere. The supply of DIC in upwelled water is generally sufficient for photosynthesis permitted by upwelled nutrients, without requiring atmospheric CO2. Second-order effects include how the composition of upwelled water differs from that of settling particles. More nitrogen than carbon is remineralized from sinking organic material. Upwelling of this water allows more carbon to sink than that in the upwelled water, which would make room for at least some atmospheric CO2 to be absorbed. the magnitude of this difference is unclear. No comprehensive studies have yet resolved this question. Preliminary calculations using upper limit assumptions indicate a low value. 1,000 square kilometres (390 sq mi) could sequester 1 gigatonne/year. Sequestration thus depends on the upward flux and the rate of lateral surface mixing of the surface water with denser pumped water.