| Summary: | Forests of the giant kelp Macrocystis pyrifera found on coastal rocky reefs lack the large reservoirs for nutrient storage found in many terrestrial environments. Supporting their high year-round growth rates requires a continuous supply of nitrogen. Complementary timing of nutrient supply associated with the physical processes that deliver nitrate to reefs largely achieves this goal, but modeling studies indicate that the magnitude of nitrate delivery is inadequate to support the measured nitrogen demand of kelp forests during summer. Ammonium, from sediment efflux and excretion by reef consumers, likely fills the deficit. Together, the varied sources of inorganic nitrogen supplied to kelp forests support their high growth rates throughout the year. Kelp compensates for diminished nitrogen supply during summer by decreasing tissue nitrogen content, resulting in a doubling of kelp C:N ratios.
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