Anthropogenic nutrient sources rival natural sources on small scales in the coastal waters of the Southern California Bight

Anthropogenic nutrients have been shown to provide significant sources of nitrogen (N) that have been linked to increased primary production and harmful algal blooms worldwide. There is a general perception that in upwelling regions, the flux of anthropogenic nutrient inputs is small relative to upwelling flux, and therefore anthropogenic inputs have relatively little effect on the productivity of coastal waters. To test the hypothesis that natural sources (e.g., upwelling) greatly exceed anthropogenic nutrient sources to the Southern California Bight (SCB), this study compared the source contributions of N from four major nutrient sources: (1) upwelling, (2) treated wastewater effluent discharged to ocean outfalls, (3) riverine runoff, and (4) atmospheric deposition. This comparison was made using large regional data sets combined with modeling on both regional and local scales. At the regional bight-wide spatial scale, upwelling was the largest source of N by an order of magnitude to effluent and two orders of magnitude to riverine runoff. However, at smaller spatial scales, more relevant to algal bloom development, natural and anthropogenic contributions were equivalent. In particular, wastewater effluent and upwelling contributed the same quantity of N in several subregions of the SCB. These findings contradict the currently held perception that in upwelling-dominated regions anthropogenic nutrient inputs are negligible, and suggest that anthropogenic nutrients, mainly wastewater effluent, can provide a significant source of nitrogen for nearshore productivity in Southern California coastal waters. Eutrophication of coastal waters has greatly increased in the last several decades throughout the world, with demonstrated linkages to anthropogenic nutrient loads (see reviews Howarth 2008; Paerl and Piehler 2008). Human population growth, development of coastal watersheds, agricultural and aquaculture runoff into the coastal oceans, and burning of fossil fuels are among the many factors contributing to increased eutrophication of coastal waters (Anderson et al. 2002; Howarth 2008). Anthropogenic inputs of agricultural runoff, wastewater and sewage discharge, and groundwater discharge have all been shown to provide significant sources of nitrogen (N) that have been linked to increased primary and macroalgal production (Lapointe et al. 2004, 2005) and harmful algal blooms (HABs) (Anderson et al. 2002; Glibert et al. 2005; Heisler et al. 2008). Anthropogenic nutrient inputs are considered the most significant factor contributing to the global increase in the frequency and intensity of HABs (Hallegraeff 2004; Glibert et al. 2005). Although many studies have focused on agricultural runoff, wastewater has also been found to promote HABs and increase primary productivity (Jaubert et al. 2003); in some regions, wastewater has been shown to be more important than upwelling as a N source (Chisholm et al. 1997; Thompson and Waite 2003; Lapointe et al. 2005). Nitrogen has been the focus of most coastal eutrophication studies because it has been shown to be the primary limiting macronutrient for algae in coastal waters (Dugdale 1967; Ryther and Dunstan 1971) including California (Eppley et al. 1979). However, previous research has shown that the N form, not just quantity, is important for HABs and algal blooms (Glibert et al. 2006), particularly in California coastal waters (Howard et al. 2007; Cochlan et al. 2008; Kudela et al. 2008). Recent studies within the Southern California Bight (SCB) have documented chronic algal bloom hot spots that coincide with areas that have potentially significant anthropogenic nutrient inputs (Nezlin et al. 2012). Before 2000, toxic outbreaks of Pseudo-nitzschia (an algal diatom that produces domoic acid) were considered rare (Lange et al. 1994); however, in recent years, frequent occurrences (Seubert et al. 2013) and high concentrations of this toxin have been documented in the SCB (Trainer et al. 2000; Schnetzer et al. 2007; Caron et al. 2010) and have been attributed to upwelling (Lewitus et al. 2012; Schnetzer et al. 2013). Increased awareness of toxic HAB events served as the primary motivation for establishment of the Harmful Algae and Red Tide Regional Monitoring Program by the Southern California Coastal Ocean Observing System (SCCOOS). This ongoing program collects weekly HAB species and toxin information from five pier locations in Southern California (SC; data available online, http://www.sccoos.org/data/habs/ index.php). * Corresponding author: [email protected] Present addresses: a Remote Sensing Solutions, Pasadena, California b University of Washington, Seattle, Washington c King Abdullah University of Science and Technology, Thuwal Kingdom of Saudi Arabia Limnol. Oceanogr., 59(1), 2014, 285–297 E 2014, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2014.59.1.0285