7.3.4.4 Carbon Cycle Feedbacks Induced by Nutrient Cycling and Land Ocean Coupling

Rivers deliver carbon (DIC, DOC) and nutrients to the ocean. Rising CO₂ levels in the atmosphere and land use may lead to increased chemical and physical weathering, resulting in increased carbon and alkalinity loads in rivers (Clair et al., 1999; Hejzlar et al., 2003; Raymond and Cole, 2003; Freeman et al., 2004). Depending on the lithology and soil composition of the catchment areas, increased levels of alkalinity, DIC or DOC can lead to local positive or negative feedbacks. Mobilisation of silicate carbonates from soils and transfer to the ocean would lead to a negative feedback to atmospheric CO₂ on long time scales (Dupre et al., 2003). Variations in nutrient supply can lead to species shifts and to deviations from the large-scale average Redfield ratios mainly in coastal waters, but also in the open ocean (Pahlow and Riebesell, 2000). Nutrient supply to the ocean has been changed through increased nitrate release from land due to fertilizer use as well as nitrogen deposition from the atmosphere in highly polluted areas (De Leeuw et al., 2001; Green et al., 2004).

Dust deposition to the ocean provides an important source of micronutrients (iron, zinc and others, e.g., Frew et al., 2001; Boyd et al., 2004) and ballast material to the ocean. Areas where iron is not supplied by aeolian dust transport in sufficient amounts tend to be iron-limited. A warmer climate may result on the average in a decrease of dust mobilisation and transport (Werner et al., 2002; Mahowald and Luo, 2003) although increased dust loads may result as well due to changes in land use (Tegen et al., 2004) and in vegetation cover (Woodward et al., 2005). A decrease in dust loads could result in a net positive feedback, further increasing CO₂ through a weakening of marine biological production and export of aggregates due to clay ballast (Haake and Ittekkot, 1990; Ittekkot, 1993). Changes in plankton species composition and regional shifts of high production zones due to a changing climate could lead to a series of further feedbacks. Light absorption due to changes in bio-optical heating may change and induce a respective temperature change in ocean surface water (Sathyendranath et al., 1991; Wetzel et al., 2006). An increase in blooms involving calcifying organisms as indicated for the high northern latitudes (Broerse et al., 2003; Smyth et al., 2004) can temporarily increase surface ocean albedo, though the effect on the radiation budget is small (Tyrell et al., 1999).