Throwing fuel on the fire: synergistic effects of excessive nitrogen inputs and global warming on harmful algal blooms.

A worldwide proliferation of harmful (toxic, food-web altering, hypoxia-generating) algal blooms (HABs) has been linked to human nutrient (phosphorus (P) and nitrogen (N)) overenrichment. In addressing this pressing environmental and human health issue, reducing P inputs has received the most attention, in part because this nutrient was first identified as driving eutrophication (1), and because P is simpler and less expensive to remove from pollution sources (e.g., wastewater, industrial effluents). Furthermore, P reductions have been prescribed to control a particularly noxious group of HABs, the N2 fixing cyanobacteria, which can obtain N2 from the atmosphere, thereby minimizing ecosystem-level nitrogen (N) limitation (1). However, less than 50%, of primary production demands are typically met by N2 fixation, even when P supplies are sufficient. Furthermore, denitrification, the microbially mediated reduction of nitrate to N2 gas, often offsets fixed N input (Table 1). The net balance of these N transformations in eutrophic waters can result in sustained ecosystem N limitation (2). Also, eutrophying freshwater, estuarine, and marine systems are increasingly plagued with non-N2 fixing Cyanobacterial HABs (CyanoHABs) and eukaryotic bloom-forming groups (dinoflagellates, chrysophytes, prymnesiophytes), indicating that these systems may be increasingly influenced by accelerating N loads. Nutrient loading dynamics have changed substantially over the past several decades. While P reductions have been actively pursued, human population growth has been paralleled by accelerating N inputs, often at rates much higher than those of P. Sources of N inputs include fossil fuel combustion, agricultural fertilizers, stormwater discharge, groundwater pollution, and urban, agricultural, and industrial wastes (3). Biologically available reactive nitrogen (Nr), includes reduced (ammonium, organic N compounds) and oxidized (nitrate, nitrite) forms; inputs of all have increased dramatically. On the global scale, human activities now create approximately 2-fold more Nr than natural ecosystems, while in the U.S., anthropogenically generated Nr is about 5-fold larger than natural processes (3). A large proportion of Nr loading, generally exceeding 50%, is from diffuse, nonpoint sources (agricultural and urban runoff, atmospheric deposition, groundwater), which complicates remediation of this vast and rapidly growing supply of N pollution. Excessive N loading has been recognized as promoting marine (estuarine and coastal) eutrophication and HAB expansion. However, the “N problem” is not isolated to these waters. Increasingly Nr plays either a primary or secondary (i.e., colimiting) role as a limiting nutrient in freshwater ecosystems. For example, oligotrophic, alpine, tropical, and subtropical, and other lakes having small watersheds relative to the lake surface/volume, and lakes experiencing various stages of eutrophication, tend to be N-limited (4). Additionally, numerous reservoirs, rivers, and fjords worldwide exhibit N limitation and N and P