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

Nitrogen-limited csluaries, shallow coastal waters, and continental shelf ivatcrs 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 h-lading and eutrophication. Among accelerating N inputs, atmospheric deposition (AD) (as wetand dryfall) and groundwater (GW) discharge are of considerable and growing importance. AD contributes from 300 to > 1,000 mg N m -2 yr -I lo coastal waters as the biologically available forms NO, , NHJNH,‘, and dissolved organic N. GW-N inputs have not been extensively characterized and quantified but in certain regions may be comparable to 11D. AD and GW can -jointly account from 20 lo >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 Norlh American Atlantic coastal waters, the Baltic and North Seas, the western Mediterranean, and the Yellow Sea. Impacts include enhanced N availability and alteration of stoicl-iometric 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. Nitrogen limitation characterizes large segments of the world’s oceanic, coastal, and estuarine waters (Dugdale 1967; Ryther and Dunstan 1971; Codispoti 1989). The rate of biologically available nitrogen supply to these waters is a key control of primary production and resultant trophic state (Ryther and Dunstan 197 1; Eppley and Peterson 1979; D’Elia et al. 1986; Nixon 1986, 1988). Biologically available N originates from either endogenous or exogenous sources. Primary production supported by endogenous N is referred to as “regenerated production,” while exogenous production is termed “new” production. In deep (>200 m) oceanic regions away from land masses, much of the primary production is dependent on regenerated ammonium derived from organic matter mineralization. Exceptions include production supported by N, fixation or NO, from upwelled deep water, both of which are considered new N sources. Coastal and estuarine environments are heavily influenced by new N, supplied either naturally by weathering of minerals, decomposition, lightning, and geothermal emissions or anthropogenically. Anthropogenic sources include urban and Acknowledgments Research and logistic support were provided by the National Science Foundation (OCE 90-l 2496, 9115706, DEB 92-10495, and DEB 92-20886), The National Sea Grant Office (NOAA) and North Carolina Sea Grant College Program (NC R/EHP-1 and R/MER23), the U.S. Geological SurveyAJNC Water Resources Res. Inst. (UNC-79-2) and the U.S. Dept. of Agriculture (93-37 102-9103), the U.S. EPA, Nessling Foundation of Finland, and Danish Ministry of the Environment. Technical assistance was provided by H. Barnes, J. Pinckney, M. Fitzpatrick, M. G., and C. Donahue. J. Pinckney, J. Willey, S. Nixon, and several anonymous reviewers provided valuable discussions, critiques, and reviews during the course of manuscript preparation. rural wastewate; nonpoint source agricultural (i.e. fertilizers, animal waste, sediments), atmospheric deposition of fossil fuel and other combustion products (NO,, DON : dissolved organic N) ant agricultural emissions (NH,, DON), and N-enriched groL[ndwater. In response to growing exogenous N inputs, new production is often enhanced, leading to eutrophication (Nixon 1995), increased frequencies and magnitudes of phytoplankton blooms, including harmful (toxic, hypoxiaand anoxiainducing, and food-web-altering) taxa (Paerl 1988; Smayda 1990; Hallegraeff 1993). An analogous scenario has long been recognized in freshwater systems, where excessive phosphorus loading is directly linked to eutrophication (Vollenweider 1982). The coastal zone and waters beyond represent the frontier of nutrient(specifically N) driven new production and eutrophication (Nixon 1986, 1995; Legendre and Gosselin 1989; Paerl 1993a,b). Within only a few decades, numerous previously pristine, oligotrophic estuarine and coastal waters have undergone a remarkably troubling transformation to more mesotrophic and eutrophic conditions. In some instances, this transformation has been accompanied by th= appearance and persistence of harmful algal blooms (HABs) to the extent that it has recently been suggested that mtrient-impacted coastal and offshore waters are experiencing an epidemic of harmful phytoplankton blooms (Smayda 1990). Phytoplankton bloom dynamics rely on the synergistic interactions of favllrable physical, chemical, and biotic conditions (Paerl 1988). Of particular importance in nutrientsensitive waters sre the rates of new N and other nutrient supply-fundamental determinants of bloom development, maintenance, and proliferation. From a practical perspective, regulating nutrient supply is often the only feasible management approach to bloom control. The spatio-temporal pat-