Enhancement of marine primary production by nitrogen-enriched acid rain

Abstract

The effect of acid rain on the pH of freshwater habitats is well known¹. The acidity of rainwater is largely due to hydrated oxides of sulphur, nitrogen and carbon², which also constitute nutrients essential for plant growth. Most freshwater and marine environments contain sufficient quantities of inorganic carbon and sulphur to satisfy plant growth needs, but nitrogen inputs can at times fall short of growth demands^3,4. Nitrogen limitation of phytoplankton growth is widespread and chronic in some marine waters^5–7. I report here that nitrogen-limited conditions in North Carolina near-surface waters are ameliorated following rainfall inputs; the most acidic rainfall events led to the greatest stimulation of phytoplankton growth. Continentally-derived acid rain consistently contained more inorganic nitrogen (NO⁻₃, NO⁻₂ and NH⁺₄) than near neutral rainfall generated from oceanic fronts. Marine primary production can thus be influenced by the source of rainfall in marginal environments.