Changes to Ocean Nutrient Cycles

Another experiment maintaining natural plankton communities in mesocosms at 1x pre-industrial CO2, 2x pre-industrial CO2 and 3x pre-industrial CO2 showed that primary productivity increased by as much as 39% under high CO2 while nutrient uptake remained the same. This excess carbon consumption was associated with a more efficient biological pump and increasing C: N ratios [73]. If these findings were transferrable to the natural environment this could lead to an expansion of deep ocean oxygen minimum zones. Increasing C:N ratios would also lower the nutritional value of organic matter produced by primary producers thereby having further implications for marine ecosystem dynamics.

Nutrients such as nitrogen, phosphorus and iron often limit phytoplankton growth in major parts of the worlds' oceans. The lower pH expected over the next hundred years can theoretically impact the speciation of many elements

[15,29,74]. These include biologically important nutrients (nitrogen, phosphate and silica) and micronutrients (iron, cobalt, manganese, etc.). For instance, a decrease in pH of 0.3 could reduce the fraction of NH3 by around 50% [75]. In addition, the key process of nitrification is sensitive to pH with rates reduced by —50% at pH 7 [76]. This may result in a reduction of ammonia oxidation rates and the accumulation of ammonia instead of nitrate. Using this data to parameterise a shelf sea ecosystem model about a 20% decrease in pelagic nitrification by 2100 was predicted [10]. Trichodesmium cyanobacteria play a key role in sustaining primary production in the large low nutrient areas of the worlds' oceans through nitrogen fixation and show a >35% increase in rates of nitrogen fixation under elevated CO2 of 750 ppm [77]. In addition, the proportion of soluble iron may increase which might be beneficial to the 10% of the oceans where iron is thought to limit primary production.

Depending on their nutrient requirements and uptake abilities, primary producers may respond differently to the effects of ocean acidification and nutrient speciation. Each response has the potential to impact the biodiversity and nutritional value of phytoplankton and the food webs and biogeochemical cycles that depend on them. Clearly, unravelling the combined impacts of declining pH on critical seawater constituents, such as nutrients and key bio-geochemical processes such as nitrification, denitrification, nitrogen fixation and nutrient uptake will be a challenge.

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