Elsevier

Chemical Geology

Volume 511, 20 April 2019, Pages 178-189
Chemical Geology

Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond
Evaluation of atmospheric dry deposition as a source of nutrients and trace metals to Lake Tahoe

https://doi.org/10.1016/j.chemgeo.2019.02.005Get rights and content

Highlights

  • TSP loads and trace metal solubilities in the Tahoe Basin are seasonally variable.

  • Trace metal concentrations and Pb isotopes are homogenous in Lake Tahoe water.

  • TSP is not a significant source of soluble metals to Lake Tahoe.

  • Riverine and groundwater inputs are the major Pb sources to Lake Tahoe.

Abstract

Atmospheric deposition can be an important source of nutrients and trace metals to oligotrophic alpine lakes, affecting their biogeochemistry. We measured trace metal concentrations and lead (Pb) isotope ratios in lake water, river water, ground water, and aerosol total suspended particles (TSP), as well as nutrient (NO3, NH4+, PO43−) concentrations in TSP in the Tahoe Basin. The contribution of TSP deposition to the lake trace metal budget was assessed. Our results show seasonality in TSP and associated trace metal concentrations with higher concentrations during Oct – April. However, trace metal solubilities are higher during May – Sept, resulting in a higher contribution of soluble trace metals to the lake water. The source of most of the trace metals in TSP in the Lake Tahoe Basin is mineral dust; however, Zn, Cu, and Cd also have an anthropogenic origin. Among major nutrients, NO3 concentrations are slightly higher during Oct – April, while NH4+ and soluble reactive phosphorus (SRP) are higher during May – Sept. The distributions of trace metal concentrations and Pb isotopic ratios are homogenous throughout the lake water column, suggesting that the residence time of the trace metals in the lake is longer than the lake water mixing time. The contribution of atmospheric TSP deposition to the upper 20 m of lake water trace metal inventory is low, ranging from 0.03% for V to 5.7% for Mn. A triple-isotopes plot of Pb indicates that riverine and groundwater inputs are the major Pb sources, but aerosols still contribute some Pb to the lake.

This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Introduction

Atmospheric deposition is an important source of nutrients and trace metals to remote water bodies like the open-ocean and alpine lakes (Camarero et al., 2009; Duce et al., 1991; Mladenov et al., 2012; Prospero et al., 1996). As a result atmospheric deposition can affect water quality by contributing major and minor nutrients for phytoplankton growth with implications to lake trophic conditions (Morales-Baquero et al., 2006). For example, anthropogenic nitrogen (N) deposition has been linked to increased nitrate (NO3) levels in lakes and to changes in phytoplankton community structure (Saros et al., 2003; Wolfe et al., 2001; Wolfe et al., 2003), as well as changes in nutrient cycling, with negative impacts on lake ecosystems (Driscoll and Newton, 1985; Greaver et al., 2012). In fact, it has been suggested that an observed decrease in N deposition rates in the last decades contributed to the recovery of community dynamics and species richness in several aquatic ecosystems (Arseneau, 2011; Finlay, 2003) and specific changes were linked to altered lake N:P ratios (Gerson et al., 2016). Trace metals can also have impacts on freshwater organisms. For example Cd concentrations above 0.1 μg L−1 have been related to reduction in reproductive function (Tarvainen et al., 1997) and Zn above 0.5 μg L−1 or Mn above 50 μg L−1 have been deemed harmful to trout (Lydersen et al., 2002; Sayer et al., 1989). Aluminum (Al) at levels >50 μg L−1 may also be toxic to some aquatic organisms (Baker and Schofield, 1982; Lydersen et al., 2002; Schofield and Trojnar, 1980). Copper (Cu) has been shown to inhibit chlorophyll a levels, photosynthesis, and nitrogen fixation of fresh water algae at concentrations of 5–10 μg L−1 (Elder and Horne, 1978). Notably, Cd, Zn, Mn and Pb in many lakes are associated with atmospheric deposition particularly related to anthropogenic origins and increases in their concentration in lake water or sediment have been documented in some lakes in Europe (Burton et al., 2013; Rippey et al., 2008). On the other hand, many trace metals are required by organisms for a wide range of metabolic processes (e.g., Fe) and low concentrations of these micro-nutrients can limit or co-limit lake phytoplankton productivity, hence affecting lake biology and ecosystem structure (Sterner et al., 2004).

Lake Tahoe is a remote subalpine lake situated at an elevation of 1897 m and surrounded by mountains including the Carson Range and the Sierra Nevada (39°05.5′N 120°02.5′W). The unique characteristics of the watershed, including nutrient-poor soils, erosion resistant substrate, and dense forest, minimize terrestrial nutrient inputs from runoff to the lake. These watershed properties, along with the large lake size of 490 km2 (150 km3) compared to the size of the watershed (1300 km2), make Lake Tahoe oligotrophic, resulting in high water clarity. However, the clarity of Lake Tahoe has declined dramatically since the 1960s (TERC, 2016). There is evidence that increasing population, changes in land use and anthropogenic emissions contribute to the decreasing water clarity (Huang et al., 2013; Jassby et al., 1994; Juma et al., 2014). Specifically, atmospheric deposition has been suggested as an important source of nutrients that support phytoplankton growth and affect water quality (Brahney et al., 2014; Morales-Baquero et al., 2006). Accordingly, several studies including the Lake Tahoe Atmospheric Deposition Study (LTADS) by the California Air Resources Board (CARB) have been conducted to monitor gases (Tarnay et al., 2005; Zhang et al., 2002), atmospheric particulate matter (Gertler et al., 2006a; Sahoo et al., 2013; Zhang et al., 2002) and wet deposition (Jassby et al., 1994) at representative sites throughout the Lake Basin. These along with other studies provided important information on total particle deposition (Dolislager et al., 2012; Sahoo et al., 2013; VanCuren et al., 2012) and specifically on N (Jassby et al., 1994; Tarnay et al., 2001) and phosphorus (P) inputs (Dolislager et al., 2012; Jassby et al., 1994) and the distribution of these constituents within atmospheric particulate matter of different size (Dolislager et al., 2012; Tarnay et al., 2001). Other studies focused on organic composition of TSP in the region (Cahill, 2010; Datta et al., 1998). All studies collectively suggested that airborne pollutants which may influence water clarity mainly originate from within the Tahoe Basin (Bytnerowicz et al., 2013; Gertler et al., 2006b).

A bioassay incubation experiment using aerosol TSP addition has been carried out to understand how dry atmospheric deposition may affect phytoplankton growth and ecosystem dynamics in Lake Tahoe (Mackey et al., 2013). This study showed that atmospheric TSP deposition provides nutrients with a high ratio of N:P that favors the growth of picoplankton, thus can increase primary productivity without causing a substantial increase in chlorophyll (Chl a) or biomass. The study demonstrated the importance of understanding nutrient inputs from TSP deposition and, more importantly, the fate of TSP after deposition onto the lake surface. However, no detailed study evaluating the contribution of trace metals from TSP deposition relative to other sources has been conducted in Lake Tahoe. Specifically, seasonal changes in water quality may be controlled by temporal changes in TSP sources and their chemical composition (Dolislager et al., 2012). To our knowledge, none of the previous studies in the Tahoe Basin have reported multi-year seasonal variations of nutrients and trace metals in dry atmospheric deposition TSP or evaluated if and how TSP sources change seasonally. Considering the potential impacts of nutrients and trace metals on lakes biogeochemistry and the potential influence of atmospheric deposition (including TSP deposition) on their concentration, it is important to assess the contribution of TSP associated trace metals and nutrients to Lake Tahoe. To fill this gap, we use weekly collected TSP to determine seasonal changes in concentrations and solubility of trace metals and nutrients and calculate deposition rates and contributions to lake water. We also use this information along with Pb isotopes to shed light on the sources of TSP in the basin (Véron and Church, 1997; Witt et al., 2006).

Section snippets

Total suspended particles (TSP) collection and treatment

TSP samples were collected between 2005 and 2010. Weekly integrated samples were collected on acid washed quartz fiber filters (10″ × 8″, Whatman®) using a Graseby Andersen TSP High Volume Sampler. Between November 2005 and May 2007, the sampler was located near the lake at the UC Davis Field Station (Hatchery) away from any local source of disturbance. After May 2007, the sampler was relocated about 300 m south to reduce local impacts due to remodeling at the Hatchery. The TSP sampler was

Temporal variation in TSP concentrations and in their chemical composition

Between December 2005 to February 2010, TSP concentrations in the air at the Tahoe Basin ranged between 2.7 and 76 μg m−3 of air, with an average and standard deviation of 26.3 ± 14.2 μg m−3 (Table S3 and Fig. 2). In general, TSP concentrations in the Tahoe Basin air were higher in the cold periods during late Fall to early Spring (October to April) than in the warm periods during late Spring to early Fall (May to September). Average seasonal concentrations were 29.8 ± 16.1 μg m−3 and

Conclusions

Our results indicate that TSP concentrations in the air in the Tahoe Basin are higher during October to April than during May to September, likely due to seasonal differences in atmospheric mixing layer thickness that result from different sensible heat conditions. Consistent with the TSP higher concentration, bulk trace metal concentrations (total concentrations) in the atmosphere are also higher during October to April. The solubilities of trace metals in TSP are, however, higher during May

Acknowledgment

We thank the staffs in Tahoe Environmental Research Center for providing accommodation and assistance on TSP and water sampling. This work was supported by NSF-OCE grant 0850467 to Adina Paytan.

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