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

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High concentrations of phosphorus (P) and nitrogen (N) are responsible for excessive, or nuisance algal blooms in many ecosystems world-wide, and climate change is predicted to exacerbate the problem1,2. Excessive nutrients supplied to the nearshore zone of Lake Tahoe may have significant consequences to ecological communities, water clarity, and water quality. The nearshore zone represents the interface between the surrounding watershed and into the lake to about a depth of 30 meters3. Recent changes in periphyton biomass in this zone may indicate changes in nutrient supply from human sources. Therefore, management actions that serve to limit external contributions of nutrients to the watershed will become even more important to Lake Tahoe in the future.

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. 

Periphyton and nearshore groundwater sampling locations.

Periphyton and nearshore groundwater sampling locations.

(Public domain.)

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 Δ17O5, 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 ( δ18Op) contamination7, 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.

Example Value Ranges
δ15N δ18O P Sources Identified Through
δ18O Water Sampling
Atmospheric Nitrate: −15‰ – +15‰ Atmospheric NO3: +25‰ – +80‰ Fertilizers: 15‰ – 22‰
Synthetic Fertilizer: −4‰ – +4‰ Nitrate-Containing Fertilizer: +18‰ – +22‰ Waste Water: 8‰ – 14‰
Soil Organic N: +2‰ – +5‰ Microbial Nitrification: −10‰ – +10‰ Animal Waste: dog 15.7‰; goose 18.3‰
Manure/Sewage Effluent: +5‰ – +25‰   Soil and Vegetation Leachate: 16.9‰

Ramon C Naranjo, USGS Nevada Water Science Center

(Public domain.)

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.

Periphyton and nearshore groundwater sampling locations.

Drone view of periphyton sampling along the nearshore of Lake Tahoe

(Credit: Naranjo, Ramon C. Public domain.)

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.

Task 1 Task 2 Task 3 Task 4
Periphyton sampling and stable isotope analyses. Groundwater and surface water sampling for nutrient and isotope analyses. Data quality assurance and control (QA/QC) and data entry into the USGS National Water Inventory System (NWIS) database. Correlation and spatiotemporal analyses of groundwater, surface water, and periphyton.

 

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.