Over the past 150 years, nitrogen (N) deposition to the ocean has doubled (Duce et al., 2008). Atmospheric phosphorus (P) deposition, in contrast, has remained relatively steady (Mahowald et al., 2008). Recent modelling studies indicate that because the deposition fluxes of these two common limiting nutrients are diverging, P depletion in surface waters will increase (e.g., Zamora et al., 2010). The highly oligotrophic North Atlantic subtropical gyre (NASG) is particularly susceptible to changes in atmospheric nutrient flux. Due to its proximity to strong continental sources, the NASG receives higher anthropogenic N deposition flux than other ocean basins. Phosphate levels in surface waters are already very low, reaching subnanomolar levels (Wu et al., 2000). Therefore, diazotrophs in the subtropical North Atlantic are thought to be limited or co-limited by P (e.g., Mills et al., 2004). Recent studies indicate that the greater phytoplankton community is P stressed as well (based on multiple physiological responses of local organisms to low phosphate concentrations). To better understand the magnitude of atmospheric N and P deposition on the NASG, we undertook a variety of field and modelling experiments. First, we estimated the flux of atmospheric P and N to the NASG. Due to the paucity of data, models commonly estimate total phosphorus (TP) deposition to the ocean based on dust deposition. We validated this technique by measuring dust and TP aerosol concentrations in Barbados and Miami (Figure 1 & 2), confirming a very strong correlation between the two. Using the dust-TP relationship in combination with a 20-year aerosol dust and inorganic N record at Barbados and Miami, we calculated historic deposition rates of excess inorganic N (the amount of atmospheric N deposited in excess of Redfield molar ratio expectations of P; i.e., 16N:1P). For two years, we also measured the contribution of water soluble organic N (WSON) in aerosols and precipitation, finding that WSON adds an additional 6-14% of total soluble N deposition in Miami and Barbados (Zamora et al., submitted). Water soluble organic P was very low in comparison. The flux of excess N to the sea surface represents the potential for P depletion in surface waters. Conservatively assuming that TP is fully bioavailable and that WSON will contribute 10% of soluble nitrogen in atmospheric deposition, we estimate that most (72-94%) nutrient deposition to the western subtropical NA is in excess of Redfield ratios, contributing 12-14 and 61-62 mmol.N.m-2yr-1 in Barbados and Miami, respectively. If these sites are representative of the larger region, atmospheric N deposition rates would be comparable with regional N fluxes from lateral Ekman transport, diapycnal mixing, and N2 fixation, which provide 30-60, 15- -2 -1 50, and 45-149 mmol.m yr , respectively (e.g., Williams and Follows, 1998; Oschlies, 2002; Hansell et al., 2007).
Phosphorus stress induced by atmospheric deposition to the surface waters of the subtropical North Atlantic
A Landolfi;
2011
Abstract
Over the past 150 years, nitrogen (N) deposition to the ocean has doubled (Duce et al., 2008). Atmospheric phosphorus (P) deposition, in contrast, has remained relatively steady (Mahowald et al., 2008). Recent modelling studies indicate that because the deposition fluxes of these two common limiting nutrients are diverging, P depletion in surface waters will increase (e.g., Zamora et al., 2010). The highly oligotrophic North Atlantic subtropical gyre (NASG) is particularly susceptible to changes in atmospheric nutrient flux. Due to its proximity to strong continental sources, the NASG receives higher anthropogenic N deposition flux than other ocean basins. Phosphate levels in surface waters are already very low, reaching subnanomolar levels (Wu et al., 2000). Therefore, diazotrophs in the subtropical North Atlantic are thought to be limited or co-limited by P (e.g., Mills et al., 2004). Recent studies indicate that the greater phytoplankton community is P stressed as well (based on multiple physiological responses of local organisms to low phosphate concentrations). To better understand the magnitude of atmospheric N and P deposition on the NASG, we undertook a variety of field and modelling experiments. First, we estimated the flux of atmospheric P and N to the NASG. Due to the paucity of data, models commonly estimate total phosphorus (TP) deposition to the ocean based on dust deposition. We validated this technique by measuring dust and TP aerosol concentrations in Barbados and Miami (Figure 1 & 2), confirming a very strong correlation between the two. Using the dust-TP relationship in combination with a 20-year aerosol dust and inorganic N record at Barbados and Miami, we calculated historic deposition rates of excess inorganic N (the amount of atmospheric N deposited in excess of Redfield molar ratio expectations of P; i.e., 16N:1P). For two years, we also measured the contribution of water soluble organic N (WSON) in aerosols and precipitation, finding that WSON adds an additional 6-14% of total soluble N deposition in Miami and Barbados (Zamora et al., submitted). Water soluble organic P was very low in comparison. The flux of excess N to the sea surface represents the potential for P depletion in surface waters. Conservatively assuming that TP is fully bioavailable and that WSON will contribute 10% of soluble nitrogen in atmospheric deposition, we estimate that most (72-94%) nutrient deposition to the western subtropical NA is in excess of Redfield ratios, contributing 12-14 and 61-62 mmol.N.m-2yr-1 in Barbados and Miami, respectively. If these sites are representative of the larger region, atmospheric N deposition rates would be comparable with regional N fluxes from lateral Ekman transport, diapycnal mixing, and N2 fixation, which provide 30-60, 15- -2 -1 50, and 45-149 mmol.m yr , respectively (e.g., Williams and Follows, 1998; Oschlies, 2002; Hansell et al., 2007).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
