Abstract
Abstract. Sea salt (NaCl) has recently been proven to be of the utmost importance for ecosystem functioning in Amazon lowland forests because of its impact on herbivory, litter decomposition and, thus, carbon cycling. Sea salt deposition should generally decline as distance from its marine source increases. For the Amazon, a negative east–west gradient of sea salt availability is assumed as a consequence of the barrier effect of the Andes Mountains for Pacific air masses. However, this generalized pattern may not hold for the tropical mountain rainforest in the Andes of southern Ecuador. To analyse sea salt availability, we investigated the deposition of sodium (Na+) and chloride (Cl−), which are good proxies of sea spray aerosol. Because of the complexity of the terrain and related cloud and rain formation processes, sea salt deposition was analysed from both, rain and occult precipitation (OP) along an altitudinal gradient over a period between 2004 and 2009. To assess the influence of easterly and westerly air masses on the deposition of sodium and chloride over southern Ecuador, sea salt aerosol concentration data from the Monitoring Atmospheric Composition and Climate (MACC) reanalysis data set and back-trajectory statistical methods were combined. Our results, based on deposition time series, show a clear difference in the temporal variation of sodium and chloride concentration and Na+ ∕ Cl− ratio in relation to height and exposure to winds. At higher elevations, sodium and chloride present a higher seasonality and the Na+ ∕ Cl− ratio is closer to that of sea salt. Medium- to long-range sea salt transport exhibited a similar seasonality, which shows the link between our measurements at high elevations and the sea salt synoptic transport. Although the influence of the easterlies was predominant regarding the atmospheric circulation, the statistical analysis of trajectories and hybrid receptor models revealed a stronger impact of the north equatorial Atlantic, Caribbean, and Pacific sea salt sources on the atmospheric sea salt concentration in southern Ecuador. The highest concentration in rain and cloud water was found between September and February when air masses originated from the north equatorial Atlantic, the Caribbean Sea and the equatorial Pacific. Together, these sources accounted for around 82.4 % of the sea salt budget over southern Ecuador.
Highlights
Poor substrate and intense leaching by precipitation make tropical forests prone to nutrient deficiency
Chloride concentration peaks in occult precipitation (OP) were highest at the lowest meteorological stations (MSs), Estación Científica San Francisco (ECSF) (Fig. 2d)
Similar seasonal patterns were observed by modelling at a larger scale, using Monitoring Atmospheric Composition and Climate (MACC) sea salt concentration data and ERAInterim air mass back trajectories, confirming the influence of the medium- to long-range transport at higher elevations
Summary
Poor substrate and intense leaching by precipitation make tropical forests prone to nutrient deficiency. While phosphorus is mainly considered a limitation to net primary productivity (NPP) in lowland Amazonian tropical forests, phosphorus and nitrogen co-limit growth in the tropical mountain rainforests, as in southern Ecuador (Homeier et al, 2012; Koehler et al, 2009; Tanner et al, 1998; Vitousek, 1984; Wolf et al, 2011; Wullaert et al, 2010). Because of a worldwide increase in nitrogen and phosphorus emissions and a pronounced increase in emissions from developing countries, where the majority of tropical forests are located, atmospheric deposition in these countries has gained attention (Dentener et al, 2006; Galloway et al, 2008; Phoenix et al, 2006). Makowski Giannoni et al.: Atmospheric deposition of sea salt in southeastern Ecuador
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