Nutrient Source Identification in Groundwater and Periphyton Along the Nearshore of Lake Tahoe

In oligotrophic lakes, such as Lake Tahoe, excessive N and P degrade water quality by stimulating algal growth.The USGS recently evaluated seasonal trends in periphyton biomass, along with Ward Creek nutrient loads, and other physical and chemical explanatory variables measured in the Lake Tahoe nearshore area⁴. Findings from this study indicate groundwater contributions of nutrients to the nearshore are significant and contribute to development of algal biomass. The proximity of recharge from streamflow can also control the timing of nutrient and groundwater flux which trigger the algal growth response. At the Pineland site, significant correlation between lake and groundwater N and dissolved phosphorus (DP) concentrations indicate nutrient-rich groundwater seeping into the nearshore area. The timing of nutrient discharge and response of periphyton along the nearshore near the mouth of Ward Creek is related to early winter runoff following a period of low-flow conditions. Further from the influence of Ward Creek, nutrient discharge at Pineland is relatively constant, and the rate is controlled more by diffuse recharge and hydraulic gradient between lake level and nearshore groundwater. 

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The contribution of N and P from anthropogenic and natural sources entering the nearshore environment is largely unknown. This research will distinguish and quantify the contributions of natural and anthropogenic sources of N and P using a multi-isotopic approach by analyzing stable isotopes δ¹⁵N, δ¹⁸O, and Δ¹⁷O in groundwater and δ¹⁵N, δ³⁴S, δ¹³C in periphyton to identify sources of N and P.

Unique sources of nitrate NO₃ can be identified in water using the characteristic isotopic signatures of δ¹⁵N, δ¹⁸O, and Δ¹⁷O^{5, 6}. Furthermore, δ¹⁵N and δ¹⁸O can also provide insight about denitrification (or assimilation) which causes the enrichment of δ¹⁵N and δ¹⁸O in the remaining NO₃. Additionally, unique isotopic signatures of δ¹⁸O can be used as a proxy for phosphate ( δ¹⁸O_p) contamination^{7, 8}. Thus, known isotopic signatures of anthropogenic sources of N and P identified by Kendall⁵ can then be used as proxies for identifying anthropogenic nutrient sources.

A multi-isotopic analysis of δ¹⁵N, δ³⁴S, δ¹³C in periphyton is necessary to account for changes associated with possible isotopic fractionation caused by nitrate or sulfate reduction occurring in groundwater. Isotopic fractionation is a natural process that is the result of bacterial nitrate and/or sulfate reduction enriching the δ¹⁵N signatures and/or depleting δ³⁴S signatures. For example, transformations of N in groundwater caused by denitrification and ammonia volatilization can result in isotopic fractionation preferentially retaining the heavier stable isotope. Thus, the changes detected in the δ¹⁵N and δ³⁴S signature can be used to demonstrate whether the groundwater has been subjected to reduced conditions, which would enrich the isotopic composition from the original source contribution⁹. Through the examination of all three isotopes, we can more clearly distinguish the change in isotopic values associated with anthropogenic sources of nutrients rather than changes associated with fractionation caused by bacterial reduction.

δ¹³C and δ³⁴S in periphyton can also be used to identify anthropogenic sources such as sewage effluent and fertilizer input that are distinct from natural sources¹⁰. The use of δ¹³C and δ³⁴S is therefore important for distinguishing different sources coming from the groundwater and potentially distinguishing chemical reduction that may be occurring within the aquifer.

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Interpreting Stable Isotope Data to Answer Nutrient Source Questions

This work will interpret stable isotope data to answer the following questions: 

Question 1: What are the sources of N and P in groundwater?

Elevated concentrations of N and P in groundwater will be related to anthropogenic enrichment from fertilizer and effluent; however, seasonal variations in hydrological processes may contribute natural sources to groundwater and periphyton at various times. Nutrient transport to the nearshore environment is dependent on physical drivers within the watershed that are temporally and spatially variable. Studies have shown that N and P concentrations in Lake Tahoe streams are typically greatest during first flush events where runoff from the landscape, channel, and urban areas contribute to increased nutrient concentrations in the lake ^{11, 12, 13, 14}. Precipitation and recharge within the landscape mobilize natural N and P from forest soils, leaf litter and alders10. Nutrient inputs from recharge also increase concentrations in groundwater and stimulate periphyton growth along the nearshore ^{12, 4, 14.} Thus, characterizing the relative contributions of nutrient sources to streams and groundwater during the first flush and later stages of snow-melt periods can be used by water resource managers to mitigate anthropogenic influences to nutrient enrichment.

Question 2: What are these sources identifiable in nearshore periphyton?

Sources identified in groundwater and periphyton will be similar where groundwater plays a role in the transport of nutrients to the nearshore. However, groundwater may not be important at every periphyton hot spot along the shore of Lake Tahoe. This study is an important first step in gathering multi-isotopic nutrient source information from both periphyton biomass and groundwater.

Approach

Research will be implemented in four tasks.

References

¹Goldman, C. R. ,1988, Primary productivity, nutrients, and transparency during the early onset of eutrophication in ultra-oligotrophic Lake Tahoe, California-Nevada. Limnology and Oceanography, 33(6), 1321-1333.

²Goldman, C. R., Jassby, A. D., & Hackley, S. H., 1993, Decadal, interannual, and seasonal variability in enrichment bioassays at Lake Tahoe, California-Nevada, USA. Canadian Journal of Fisheries and Aquatic Sciences, 50(7), pp. 1489-1496.

³Loeb S.L., Reuter J.E., Goldman C.R. (1983) Littoral zone production of oligotrophic lakes. In: Wetzel R.G. (eds) Periphyton of Freshwater Ecosystems. Developments in Hydrobiology, vol 17. Springer, Dordrecht.

⁴Naranjo, R.C., Niswonger, R.G., Smith, D., Rosenberry D.O. and S. Chandra. 2019, Linkages between hydrology and seasonal variations of nutrients and periphyton in a large oligotrophic subalpine lake. Journal of Hydrology Vol. 568, pp. 877-890.

⁵Kendall, C., 1998, Tracing sources and cycling of nitrate in catchments C. Kendall, J.J. McDonnell (Eds.), Isotope Tracers in Catchment Hydrology, Elsevier Science, Amsterdam (1998), pp. 519-576.

⁶Michalski, G. and M. Thiemens, 2006, The use of multi-isotope ratio measurements as a new and unique technique to resolve NOx transformation, transport and nitrate deposition in Lake Tahoe. Final Report Contract No. 03-317. Prepared for The California Air Resources Board and the California Environmental Agency. August 15, 2006.

⁷McLaughlin, K., Silvia, S., Kendall, C., H. Stuart-Williams, and A. Paytan, 2004, A precise method for the analysis of O18 of dissolved inorganic phosphate in seawater. Limnology and Oceanography: Methods.

⁸Elsbury K.E, Paytan, A., Ostrom N., Kendall C., Young M. McLaughlin K., Rollog M., and S. Watson (2009) Using oxygen isotopes of phosphate and cycling in Lake Erie. Environ. Sci. Technology 2009, 43, 3108-3114.

⁹Neill, C. and Cornwell, J.C., 1992, Stable carbon, nitrogen, and sulfur isotopes in a prairie marsh food web. Wetlands. 12: pp. 217-224.

¹⁰Wayland, M. and Hobson, K.A., 2001, Stable carbon, nitrogen, and sulfur isotope ratios in riparian food webs on rivers receiving sewage and pulp-mill effluents, Canadian Journal of Zoology, 79: pp. 5-15.

¹¹Coats, R. N., Leonard, R. L., & Goldman, C. R., 1976, Nitrogen uptake and release in a forested watershed, Lake Tahoe Basin, California. Ecology, 995-1004.

¹²Loeb, S.L., 1986. Algal biofouling of oligotrophic Lake Tahoe: Causal factors affecting production. In: Evans, L.V., Hoagland, K.D. (Eds.), Algal Biofouling. Elsevier Science Publishers, B.V., Amsterdam, The Netherlands, pp. 159–173.

¹³Coats, R.N., and Goldman, C.R., 2001, Patterns of nitrogen transport in streams of the Lake Tahoe Basin, California‐Nevada. Water Resources Research, 37(2), 405-415.

¹⁴Leonard, R. L., Kaplan, L. A., Elder, J. F., Coats, R. N., & Goldman, C. R., 1979, Nutrient transport in surface runoff from a subalpine watershed, Lake Tahoe Basin, California. Ecological Monographs, pp. 281-310.