Coastal eutrophication and harmful algal blooms: Importance of atmospheric deposition and groundwater as “new” nitrogen and other nutrient sources

Abstract

Nitrogen‐limited estuaries, shallow coastal waters, and continental shelf waters cover only 15% of the world’s ocean area, but account for nearly half the global oceanic primary production. This disproportionality is partly attributed to accelerating and geographically expanding anthropogenic N loading and eutrophication. Among accelerating N inputs, atmospheric deposition (AD) (as wet‐ and dryfall) and groundwater (GW) discharge are of considerable and growing importance. AD contributes from 300 to >1,000 mg N m−2 yr−1 to coastal waters as the biologically available forms NOx−, NH3/NH4+, and dissolved organic N. GW‐N inputs have not been extensively characterized and quantified but in certain regions may be comparable to AD. AD and GW can jointly account from 20 to >50% of total exogenous, or “new,” N loading and may uniquely mediate coastal eutrophication by bypassing estuarine filters of terrigenous N inputs. Direct AD and offshore GW inputs may impact harmful algal bloom dynamics of coastal and pelagic waters downwind of emissions and downstream of discharges. Example include North American Atlantic coastal waters, the Baltic and North Seas, the western Mediterranean, and the Yellow Sea. Impacts include enhanced N availability and alteration of stoichiometric ratios of nutrients (N, P, Si). Trace metal enrichment (e.g. Fe) in AD and GW may interact synergistically with N to stimulate coastal production. Growing urban, industrial, and agricultural AD and GW inputs to coastal and offshore waters may be linked to a purported expansion of harmful algal blooms.