How does rising atmospheric CO2 affect marine organisms?

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Biodiversity (N-Fixers vs. Non-N-Fixers) -- Summary
Will nitrogen-fixing (N-fixing) plants benefit more from atmospheric CO2 enrichment than non-N-fixers and thus obtain a competitive advantage over them that could lead to some non-N-fixers being excluded from certain plant communities, thereby decreasing the biodiversity of those ecosystems?

In a two-year glasshouse study of simulated low-fertility ecosystems composed of grassland species common to Switzerland, Stocklin and Korner (1999) found that atmospheric CO2 enrichment did indeed give nitrogen-fixing legumes an initial competitive advantage over non-N-fixers.  However, it would be expected that, over time, a portion of the extra nitrogen fixed by these legumes would become available to neighboring non-N-fixing species, which would then be able to use it to their own advantage, thereby preserving the species richness of the ecosystem over the long haul.  Indeed, in a four-year study of an established (non-simulated) high grassland ecosystem located in the Swiss Alps, Arnone (1999) found that there was no difference between the minimal to non-existent growth responses of N-fixing and non-N-fixing species to elevated levels of atmospheric CO2.  In addition, in a study of mixed plantings of the grass Lolium perenne and the legume Medicago sativa, Matthies and Egli (1999) found that elevated CO2 did not influence the competition between the two plants, either directly or indirectly via its effects upon the root hemiparasite Rhinanthus alectorolophus; and in a study of mixed plantings of two grasses and two legumes, Navas et al. (1999) observed that plant responses to atmospheric CO2 enrichment are more dependent upon neighboring plant density than they are upon neighboring plant identity.

In the few studies of this question that have been conducted to date, therefore, it would appear that there is little evidence to suggest that N-fixing legumes will out-compete non-N-fixing plants and eliminate them from any ecosystems.

Arnone III, J.A.  1999.  Symbiotic N2 fixation in a high Alpine grassland: effects of four growing seasons of elevated CO2Functional Ecology 13: 383-387.

Matthies, D. and Egli, P.  1999.  Response of a root hemiparasite to elevated CO2 depends on host type and soil nutrients.  Oecologia 120: 156-161.

Navas, M.-L., Garnier, E., Austin, M.P. and Gifford, R.M.  1999.  Effect of competition on the responses of grasses and legumes to elevated atmospheric CO2 along a nitrogen gradient: differences between isolated plants, monocultures and multi-species mixtures.  New Phytologist 143: 323-331.

Stocklin, J. and Korner, C.  1999.  Interactive effects of elevated CO2, P availability and legume presence on calcareous grassland: results of a glasshouse experiment.  Functional Ecology 13: 200-209.