Simulating the advection and degradation of the environmental DNA

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Environmental Measurements Methods

Simulating the advection and degradation of the environmental DNA of common carp along a river Kei Nukazawa, Yuki Hamasuna, and Yoshihiro Suzuki Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02293 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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Environmental Science & Technology

Abstract Art: Conceptual diagram and result of eDNA advection and decay model 61x30mm (300 x 300 DPI)

ACS Paragon Plus Environment


Simulating the advection and degradation of the environmental DNA of common carp along a river Kei Nukazawa1*, Yuki Hamasuna1, Yoshihiro Suzuki1

Corresponding author and proof checker: Kei Nukazawa 1

Department of Civil and Environmental Engineering, Faculty of Engineering, University of

Miyazaki,1-1 Gakuen Kibanadai-Nishi, Miyazaki 889-2192, Japan

*Corresponding author E-mail: [email protected] (KN)

Conflicts of interest: The authors declare no competing ﬁnancial interest.

Short title: Advection and degradation of stream eDNA


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Abstract 1

The environmental DNA (eDNA) method is a novel technique for precise and efficient

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biological surveillance. While eDNA has been widely used to monitor various freshwater

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organisms, eDNA dynamics in streams remain poorly understood. In this study, we investigated

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the eDNA dynamics of common carp (Cyprinus carpio) in a forested headwater stream affected

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by the effluent from a carp farm. We evaluated the longitudinal variation in carp eDNA along a

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river downstream from the farm and performed a temporal eDNA decay experiment using

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digital polymerase chain reaction. Based on the resulting decay constants, we built a model to

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simulate the advection and degradation of eDNA along the studied river. The observed eDNA

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flux (concentration multiplied by flow rate) decreased exponentially with distance downstream

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from the farm, and eDNA was detected 3 km downstream of the farm. Although the water

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temperatures were similar, the eDNA decay constant was lower in autumn than in summer. The

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simulated eDNA concentration was markedly larger (>10 times) than the observed

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concentration, suggesting that eDNA removal is accelerated in the stream environment

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compared to in conventional experimental settings.

15 16



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Keywords:

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Advection, eDNA, decay, digital PCR, non-uniform flow

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Introduction The environmental DNA (eDNA) technique shows great potential for the

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biomonitoring of aquatic species as it reduces the effort and cost required for field

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surveillance1–3. eDNA method also allows the accurate identification of target species based on

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molecular barcoding4,5 and even has a potential to determine individual numbers or biomass

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based on the eDNA copy number in lentic and lotic water bodies although these are still under

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investigation6–8. Researchers have applied eDNA method to various organisms including

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amphibians9,10, freshwater fishes6,11,12, marine species13–15, reptiles16,17, mollusks18,

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invertebrates19–21, and terrestrial mammals22,23. Despite the extensive use of this method, eDNA

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detection remains challenging in lotic systems compared to that in lentic environments as

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stream eDNA dynamics (e.g., transportation and decay) are not well understood. Consequently,

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the distribution of source individuals remains unclear in lotic systems even when the molecular

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markers for the target species are detected.

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eDNA dynamics in streams and rivers have been investigated for a variety of

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organisms. In these studies, a “point source” of target species eDNA is generally created or

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assumed to exist at the uppermost part of the studied river section. Pilliod et al. (2014)24

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reported that the eDNA released from introduced salamanders was not detected 50 m



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downstream from the source. In contrast, an investigation of invertebrate eDNA in a river fed

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by a Swiss lake25 revealed that eDNA could be detected 10 km downstream of the lake, even

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though the invertebrate species were absent in the river. Jane et al. (2015)26 quantified

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variations in longitudinal eDNA released from trout at low biomass introduced into the fishless

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headwaters under different flow rates; eDNA was detected 250 m downstream of the source.

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The authors provided useful information on longitudinal DNA patterns in natural moving

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waters using real-time polymerase chain reaction (qPCR), whereas the observed eDNA

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concentrations were generally low. Because the previous study27 demonstrated that qPCR

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generated more variable results of eDNA quantification than droplet digital PCR under lower

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eDNA concentration, further studies are required to clarify the decreasing trend of stream

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eDNA. Comparison of the travel distance between the above studies is difficult because of

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differences in scale and hydrological system (e.g., discharge variability). To address this issue,

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eDNA flux (i.e., concentration multiplying flow rate) is useful to derive eDNA removal rate

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along a river and further enables intercomparion of the results.

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Degradation of eDNA is typically evaluated by monitoring a water body that contains

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the eDNA of a target species but not individuals of that species, over time. This approach has

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produced a broad range of eDNA decay patterns in different experimental settings14,28. Various


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factors affecting eDNA decay have been investigated, including water chemistry28, flow and

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sediment conditions29 , water temperature30, and microbial abundance30. These studies

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suggested that ambient abiotic and biotic factors markedly affect eDNA degradation,

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suggesting that eDNA decay patterns play important roles in stream eDNA dynamics. However,

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to the best of our knowledge, no study has assessed and compared eDNA decay ratios and in

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situ longitudinal eDNA dynamics.

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A few studies have attempted to model eDNA dynamics in rivers. Shogren et al.

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(2017)31 developed a simple conceptual model for estimating eDNA transport distances over

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short length scales (approximately 50 m). Sanson and Sassoubre (2017)32 modeled eDNA

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transport for freshwater mussel species in a creek based on eDNA decay and shedding

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experiments. Sanson and Sassoubre (2017)32 built a novel eDNA transport model based on a

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plug-flow reactor and the resulting eDNA decay constant; however, they did not consider the

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effects of the physical characteristics of the river (i.e., topography, flow rate, and flow velocity).

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Because the decrease in eDNA depended on the decay constant, spatial changes in hydraulic

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properties (e.g., velocity) can greatly affect the fate of eDNA released into the river. In addition,

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the influencing factors (e.g., water temperature) on stream eDNA degradation were not argued.

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Assuming stream eDNA dynamics are characterized by transportation, deposition, and



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resuspension along with biotic and abiotic degradation, the use of a hydraulic model should

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provide a better understanding of eDNA dynamics by evaluating how and to what extent a

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model can mimic in situ stream eDNA patterns.

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In this study, we investigated longitudinal stream eDNA patterns using in situ

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surveillance, in vitro eDNA decay experiments, and in silico numerical simulations of eDNA

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dynamics along a river. We studied a forested headwater stream subject to effluent from a

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common carp (Cyprinus carpio) farm but without carp individuals in the stream. This

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experimental setting allows the assessment of how the eDNA of common carp originating from

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the farm as a point source decreases/changes as it moves downstream. In combination with the

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in situ investigation, we sampled stream water at the outlet of the effluent for eDNA decay

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experiments. Finally, using the decay constants derived from the in vitro experiments, we

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simulated the dynamics of stream eDNA using a one-dimensional advection–decay model

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based on the non-uniform flow equation.

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Materials and Methods

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Study river

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We studied an upstream river section of the Kaeda River in southwest Japan, which


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has a catchment of approximately 53.8 km2 and a length of 17.5 km [Fig 1]. The upstream

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catchment is mostly forested without any residential area. A common carp farm is located at the

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uppermost stream; this farm intakes river water and drains effluent from aquaculture ponds into

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the river. At the observation dates of this study, the farm accommodated approximately 100

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adult individuals in the two ponds. The sampling sites were located at approximately 40 m

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(hereafter, outlet site), 150 m, 900 m, 1.9 km, and 3.1 km downstream of the location of farm

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effluent discharge [Fig 1]. We also sampled river water ca. 100 m upstream of the effluent

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discharge location and a major tributary that converges with the main flow approximately 2.3

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km downstream of the effluent discharge. A survey conducted by a local fishery organization in

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2015 indicated that no common carp individuals are found in the studied stretch of river

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(http://www.miyazaki-ngr.jp/contents/archives/3268). Visual observations confirmed that no

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carp individuals were present near each site on the dates of the observation.

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Water sampling and environmental measurements Water samples were collected in triplicate using plastic bottles (1, 2, and 10 L) in

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August and November of 2017. The samples were transported on ice in cooler boxes and were

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used for subsequent filtration within 3 h after sampling. The bottles and cooler boxes were



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sterilized with 10% bleach for at least 30 min prior to the sampling. According to preliminary

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surveys, we sampled different water volumes based on the different eDNA concentration at the

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studied sites. We collected 1 and 10 L of water at the outlet site and the site 3.1 km downstream

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of the effluent, where high and low eDNA concentrations were expected, respectively. The

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sample volume was 2 L at all other sites. For the eDNA decay experiment, we collected 20 L of

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water at the outlet site, where discharged effluent is mixed with river water. We placed 2-L

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bottles filled with sterilized distilled water in the cooler together with the samples (i.e., cooler

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blank) to check for contamination during sampling33.

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Water depth and current velocity were measured using a velocity meter (VR-301,

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KENEK) to determine the flow rate at each site. A full description of the environmental

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measurements including the collection of basic water quality data can be found in Table S1.

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eDNA decay experiments The water samples collected at the outlet site were moved to a sterilized container and

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were stirred using a magnetic stirrer (ca. 900 rpm) throughout the experimental period to

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imitate an environment of flowing water. The decay experiments were performed in the dark to

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eliminate the effects of ultraviolet radiation. Three 1-L aliquots of water were sampled from the


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container after 1, 3, 6, 10, 24, 48 h and were used for subsequent filtration. The water

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temperature was measured using a pendant temperature logger (UA-001-08, onset) every 30

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min throughout the experiment. The temperatures for the experiments in August and November

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were 22.11°C ± 1.05°C and 21.35°C ± 2.59°C (mean ± sd.), respectively.

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Filtration, DNA extraction, and measurement of double-strand DNA concentration The water samples were filtered using a glass fiber filter with a pore size of 0.7 µm

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(GE Healthcare Japan, Tokyo). Two filters were used for each sample, which was obtained from

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the site 3 km downstream of the effluent to avoid clogging. Filter funnels, bases, clamps, and

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tweezers were sterilized by soaking in 10% bleach for 10 min prior to each filtration process.

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Filters were stored in a freezer at −20°C until subsequent DNA extraction. DNA was extracted

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following the protocol proposed in the previous study34 using DNeasy PowerSoil Kit (Qiagen,

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Hilden, Germany). Eichmiller et al. (2016b)34 reported that PowerSoil Kit exhibited low

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variation in eDNA quantification for common carp and no detectable inhibition. In brief,

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tweezers and scissors that had been sterilized by soaking in 10% bleach were used to cut the

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filters into sizes of 1×3 mm. DNA was then extracted from the small fragments of the filters

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following the protocol of the kit manufacturer. The concentration of double-strand DNA



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(dsDNA) in the extracted DNA was measured using a fluorometer (Quantus Fluorometer,

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Promega, WI). The extracted template DNA solutions were stored in a freezer at −20°C until

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the subsequent PCR step.

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Quantification of common carp eDNA using digital PCR To quantify the eDNA of common carp, we used a digital PCR (dPCR) system

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(QuantStudio 3D digital PCR system, Applied Biosystems, CA) with previously designed

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primer and probe sets specific to common carp and targeting mitochondrial cytochrome b12.

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The specificity of the assay was validated for the study area (Supporting Information). Unlike

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conventionally used qPCR, which provides a relative DNA concentration, dPCR allows the

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absolute quantification of target DNA concentration. In past studies, compared to qPCR, dPCR

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resulted in lower variation in quantified eDNA in goby fishes35 and more stable quantification

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results under low eDNA concentrations for common carp27. In this study, the reaction mixture

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for eDNA quantification contained 1× QuantStudio 3D Digital PCR Master Mix (Applied

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Biosystems, CA), 2 µL DNA temperate solution, 900 nM of each primer (the forward and

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reverse primers), and 125 nM of Taqman probe. The mixture was dispensed to independent

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wells of a QuantStudio 3D Digital PCR 20K Chip using a QuantStudio 3D Digital PCR Chip


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Loader (Applied Biosystems, CA). The endpoint PCR reaction was performed using a thermal

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cycler (ProFlex, Applied Biosystems, CA). The PCR reactions followed the default protocol of

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dPCR that comprised polymerase activation at 96°C for 10 min followed by 40 cycles of

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annealing and extension at 60°C for 2 min, denaturation at 98°C for 30 s, and final extension at

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60°C for 2 min. The target eDNA concentration was then quantified based on the number of

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wells determined to be ‘positive’ (i.e., the well has a higher fluorescence intensity than the

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background) using a QuantStudio 3D digital PCR system and QuantStudio 3D Analysis Suite

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software (Applied Biosystems, CA). We adopted the software’s default threshold of

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fluorescence intensity to discriminate positive and negative wells. Note that if the number of

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positive well is less than 4 and the intensity of the positive wells was as low as negative wells,

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the positive wells were considered as false. The dPCR procedure was performed in a separate

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room from the filtration and DNA extraction processes, and none of the instruments were

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transferred between the rooms.

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Data analysis We calculated the flux of common carp eDNA by multiplying the eDNA concentration with flow rate for each site. Subsequently, the flux was used to calculate the



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removal rate via the following formula (F0 – Fi)/ F0, where F0 is the flux at the outlet site and Fi

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is the flux at the site i. This allowed the evaluation of eDNA abundance loss with distance from

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the carp farm. Following Thomson et al. (2012a)14, we fit eDNA concentration with elapsed

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time using the formula dC/dt = −βC, where C is the eDNA concentration (copies/L), β is the

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decay constant (/h), and t is the elapsed time (h). This non-linear regression model was built

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using the nls package in R version 3.4.3 (R core team, 2017). We used Pearson’s correlation

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coefficient to assess the association between the eDNA and dsDNA concentrations.

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Advection and decay model for simulating stream eDNA dynamics Using the decay constants derived from the eDNA decay experiments in August and

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November of 2017, we simulated the one-dimensional advection in eDNA with degradation

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along the studied river section. We first assumed a simple rectangular flume and computed the

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water depth h throughout the studied river section using the following formula for

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one-dimensional non-uniform flow:

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,

(1)

where H is water level (m), Q is flow rate (m3/s), B is river width (m), g is acceleration due to


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gravity (m/s2), and n is roughness coefficient (n = 0.06). We acquired digital elevation model at

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a spatial resolution of 5 m from the Geospatial Information Authority of Japan and defined the

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grids of the studied river using the river data from the Ministry of Land, Infrastructure,

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Transport, and Tourism (MLIT). This physical-based model was chosen to better simulate

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stream eDNA dynamics because it could reflect complex characteristics of the river (i.e.,

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topography and flow velocity) and be potentially useful for integrating spatially heterogeneous

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biotic and abiotic processes (e.g., retention and resuspension). We partitioned the studied river

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section into two parts: before the convergence of the tributary (0 to 2.3 km downstream of the

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outlet site) and after the convergence of the tributary (2.3 to 3.1 km downstream of the effluent).

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A constant flow rate and river width was assigned to each section based on the observed data.

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The current velocity u (m/s) for each grid was derived from the continuity equation.

200 201

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To simulate eDNA dynamics load into river, the decay constant of eDNA was integrated to the advection equation as follows:

,

(2)

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where t is time (sec), and x is spatial resolution (m). We computed the advection and

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degradation independently for August and November using the mean eDNA concentration at

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the outlet site as constant input to the uppermost grid (i.e., the grid of the outlet site). First, we



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used the decay constants derived from the eDNA decay experiments (i.e., β = 0.086 for August

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and β = 0.017 for November). In subsequent simulations, we set different decay constants (β =

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0.5, 1, 2, 4 and 10) to understand how variation in the decay constant affects the simulation

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results. The model accuracy for each decay constant was evaluated based on the root mean

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square error (RMSE). The evaluations were based on the simulated eDNA concentration 16 h

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after start of the experiment; the eDNA concentration showed a plateau at this time at the site

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3.1 km downstream of the effluent (Fig S1).

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We derived decay constants so that the simulated eDNA concentration agrees with

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the concentration at the site 1.9 km downstream of the effluent and predicted a detectable

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distance of the eDNA downstream in response to different initial eDNA concentration and

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filtration volume. For this purpose, we defined the thresholds of eDNA concentration which

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determine success or failure of detection as 20, 40, 100, 200, 1,000, and 4,000 copies/L for the

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filtration volume of 10,000, 5,000, 2,000, 1,000, 200, and 50 mL, respectively. These threshold

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concentrations approximately correspond to two positive wells in the dPCR system because a

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negative sample (i.e., without target eDNA) could result in this extent of the positive well

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number. The eDNA concentration at the 3.1 km downstream site was not used for calibration

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because the sampling volume at this site was different from those at the other sites.


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Results and Discussion

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Water temperature was 22.0–23.0°C in August and 13.6–14.4°C in November (Table

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S1). The measured pH and Electric conductivity (EC) values indicated only small differences in

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water quality among the study sites (Table S1). The discharge observed in August (0.063 to

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0.283 m3/s) was slightly lower than that observed in November (0.115~0.409 m3/s; Table S1).

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No common carp eDNA was detected in the negative control samples in August or

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November, indicating that no contamination occurred during the field surveys, filtration, DNA

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extraction, and dPCR process. Carp eDNA was also undetected in all samples taken from

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upstream of the carp farm and the tributary in both August and November. This indicates that

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eDNA inputs other than the effluent from the carp farm were negligible in the present study.

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Longitudinal profile of stream environmental DNA The concentration and flux of carp eDNA decreased exponentially with distance

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downstream from the carp farm in both August and November (Fig. 2). Notably, the removal

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rate of eDNA increased dramatically with the distance from the farm until 900 m downstream

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(removal rate = 0.73 both in August and November; Table S2); as distance downstream from



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the farm increased beyond 900 m, the removal rate increased more gradually. Earlier works

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investigated longitudinal eDNA concentration to understand the fate of eDNA released into

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rivers26. However, river water containing DNA may be diluted by water from tributaries and

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subsurface flow, complicating the interpretation of trends in eDNA along the river.

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Furthermore, because of differences in discharge, eDNA concentration probably varies between

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rivers even when species abundance is equal; this can make it difficult to determine the

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individual number or biomass of the target species. In contrast, our study successfully excluded

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the effect of eDNA dilution using the flow rate observed at each sampling site and evaluated the

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increasing trend of eDNA removal rate.

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In this study, eDNA was detected at the site 3.1 km downstream of the carp farm in all

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cases (mean ± se. = 110.11 ± 55.17 copies/L in August; 131.77 ± 17.42 copies/L in November),

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suggesting that eDNA originating from the carp farm is transported over 3 km distance along

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the river. In earlier studies, eDNA of target salamander and trout species was not detected or

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was found in very small concentrations 50–250 m downstream the source24,26. However, these

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studies were based on the introduction of exotic species, resulting in smaller individual number

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and biomass compared with that in this study. Deiner et al. (2014)25 detected the eDNA of

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invertebrate species 10 km downstream from the source (a natural lake that harbors the target


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species). Taken together, whereas the scale of hydrological system differed between these

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studies, transport distance of eDNA could depend on the initial quantity of eDNA loading,

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which could be dependent on population size or biomass of the species source.

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The mean eDNA flux at the outlet site was significantly lower in November (mean ±

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se. = 412,474 ± 78,491 copies/s) than in August (653,309 ± 40,648 copies/s) (t-test, P < 0.05).

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However, the eDNA removal with the downstream distance from the source was more

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moderate in November compared to in August (Table S2). Fukumoto et al. (2015)36 found that

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among seasons, amphibian eDNA in a temperate climate was most detectable in the winter.

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They attributed this finding to the lower water temperature in the winter. Thus, water

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temperature might have contributed to the different patterns in eDNA concentration observed in

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August and November.

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eDNA decay experiments In both August and November, the concentration of common carp eDNA decreased

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over time (Fig. 3). While the initial eDNA concentration at the outlet site was significantly

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lower in November than in August (as mentioned above), the eDNA concentration after 48 h

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was lower in August (mean ± sd. = 834.04 ± 202.24 copies/L) than in November (1,351.26 ±



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378.21 copies/L). Based on non-linear regression, we derived the following decay constants: β

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= 0.086 in August (P < 0.01) and β = 0.017 in November (P < 0.01). These parameters indicate

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that eDNA degraded faster in August than in November, even though the water temperatures

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were similar. Earlier studies found that ambient biotic factors (microbial activity and

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extracellular enzyme) and abiotic factors (water temperature, ultraviolet radiation, and water

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quality) play important roles in eDNA decay32,37. Previous eDNA decay experiments conducted

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in similar settings as this study found that the temporal eDNA degradation for common carp

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suppressed as water temperature decreased28,30,38. In these studies, the river/lake water samples

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were incubated at different temperatures. Thus, the inconsistent results between these past

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studies and the present study may be explained by seasonal differences in the driving factors of

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eDNA degradation in stream water (e.g., the effects of the microbial community).

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Many studies involving eDNA decay experiments speculated on the effects of

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microbial activity on eDNA degradation34,39. Tsuji et al. (2017)30 found an insignificant

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negative correlation between fish eDNA and microbial abundance in the similar eDNA decay

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experiment. Lance et al. (2017)40 reported that microbial abundance strongly affected eDNA

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degradation. Earlier studies demonstrated that dsDNA is a good substitute for bacterial density

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in natural water samples, including humic water with high background fluorescence41. In this


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study, the dsDNA concentration clearly increased over time during the experiments compared

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with the concentration in the river samples (

Simulating the advection and degradation of the environmental DNA

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