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Improving estimation of phytoplankton isotopic values from bulk POM samples in rivers
Citation:
Phillips, Donald L, C. Kendall, M. Young, AND S. Peek. Improving estimation of phytoplankton isotopic values from bulk POM samples in rivers. Ecological Society of America, Portland, OR, August 06 - 11, 2017.
Impact/Purpose:
Responses of phytoplankton to excessive nutrients in rivers can cause ecological problems including harmful algal blooms and hypoxia. Remediation requires identifying the types and sources of the problem nutrients. Stable isotope analysis is a useful tool for determining the isotopic signatures of nutrients and correlating them with phytoplankton signatures to determine which nutrients are driving phytoplankton production. Particulate organic matter (POM) samples from rivers contain both phytoplankton and terrestrial organic matter. Physical separation of phytoplankton is often difficult, creating the need for calculating their isotopic signatures from isotopic and chemical data on POM and its terrestrial component. Existing methods have untenable assumptions about constant carbon:nitrogen ratios and/or improper mathematical formulations. We devised an improved method to estimate carbon and nitrogen isotopic signatures in phytoplankton from isotopic signatures of POM and terrestrial organic matter along with carbon:nitrogen ratio data. Our method is not subject to the shortcomings of previously existing methods. This method will be useful in studies of nutrient pollution in rivers to help pinpoint the nutrient causes of harmful algal blooms and hypoxia.
Description:
Background/Questions/MethodsResponses of phytoplankton to excessive nutrients in rivers cause many ecological problems, including harmful algal blooms, hypoxia and even food web collapse, posing serious risks to fish and human health. Successful remediation requires identification of the types and sources of the problem nutrients. Multi-isotopic approaches are powerful tools for solving such problems because water samples can be easily collected from water quality monitoring programs and analyzed for nutrient isotopes and organic matter (OM) isotopes, as well as various chemical measurements. Nutrient and phytoplankton isotopic compositions can be compared to determine seasonal and spatial changes in the dominant source of nutrient for phytoplankton production. Particulate OM (POM) samples are easily collected, which contain both phytoplankton and terrestrial OM. However, physical isolation of phytoplankton is often challenging. Hence, methods are needed to calculate the isotopic values of bulk phytoplankton, particularly δ13C and δ15N, from the isotopic values of bulk POM and other associated chemical data.Results/ConclusionsWe reviewed several recent proposed methods to calculate phytoplankton δ13C and δ15N based on isotopic and chemical data for POM and its terrestrial OM component, where POM is treated as a two-source mixture of terrestrial OM and phytoplankton. We found problems with these methods that include: (a) the untenable assumption that C, N, and H all partition the same way even though C:N may vary widely between the two components; (b) the need to incorporate an arbitrary adjustment factor to account for this; or (c) the inappropriate use of non-linear C:N ratios in linear mixing models. We derived a set of equations for phytoplankton δ13C and δ15N that avoids these issues. First, for POM and each of its two components, the C:N data are linearized by computing the C and N fractions of the total C+N mass. The fractions of C and N in POM that come from terrestrial OM are then calculated separately. Separate estimations avoid the assumption that C and N assort the same between the components of POM, which is not true when their C:N ratios are not equal. These separate calculations also forgo the need for an additional adjustment factor to account for differences in C:N ratios. The δ13C and δ15N values for phytoplankton can then be calculated by a two-source mixing model. Finally, this method was applied to multi-year POM sampling data from the San Joaquin River and its tributaries.