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Article

Observational Monitoring Records Downstream Impacts of Beaver Dams on Water Quality and Quantity in Temperate Mixed-Land-Use Watersheds

by
Erin E. Novobilsky
1,
Jack R. Navin
1,
Deon H. Knights
2 and
P. Zion Klos
3,*
1
Undergraduate Research Program, Marist University, Poughkeepsie, NY 12601, USA
2
Department of Earth Science and Geography, Vassar College, Poughkeepsie, NY 12604, USA
3
Department of Environmental Science and Policy, Marist University, Poughkeepsie, NY 12601, USA
*
Author to whom correspondence should be addressed.
Data 2025, 10(4), 51; https://doi.org/10.3390/data10040051
Submission received: 20 February 2025 / Revised: 1 April 2025 / Accepted: 3 April 2025 / Published: 7 April 2025
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Abstract

:
Beaver populations in the U.S. northeast are rising, increasing the number of beaver dams and ponds in suburban watersheds. These new beaver ponds may impact the way that harmful algal blooms occur by changing biogeochemical cycling and sediment characteristics. In this study, piezometers, installed upstream and downstream of multiple dam structures were used to evaluate changes in nitrate and orthophosphate concentrations in surface and hyporheic water. Data were also collected with seepage meters, discharge measurements, lab and field-based analytical tests, and sediment samples. These were collected from beaver dams and paired non-beaver dams upstream of unimpounded reaches to look at the potential for dormant sediment-based cyanobacteria to bloom and produce toxins under ideal light and nutrient levels. Results indicate a significant increase in orthophosphate from upstream to downstream of beaver dams. Results also demonstrate that toxin potential did not increase between cyanobacteria in beaver pond sediment and the paired unimpounded sample; however, under ideal light and nutrient levels, sediment from a beaver dam led to faster cyanobacterial growth. These findings highlight that while beaver dams and impoundments function as nutrient sinks within the tributary watersheds, there are potential risks from downstream transport of bloom-inducing sediment following a dam collapse.

1. Introduction

Eutrophication and harmful algal blooms (HABs) threaten the health of aquatic ecosystems and have become more variable due to a changing climate and other ecological influences. HABs threaten the availability and sustainability of freshwater resources, which are already vulnerable as their biodiversity is decreasing faster than terrestrial habitats [1,2]. HABs are typically caused by cyanobacteria blooms that deplete oxygen levels, limit light levels, and alter the relationships between organisms in the food web [3]. Along with this, 25 to 75% of cyanobacterial blooms contain cyanotoxins, with the most important and numerous being microcystins [3,4]. Cyanotoxins are mainly produced intracellularly during the exponential growth phase and include hepatotoxins, neurotoxins, and dermatoxins, affecting the liver, nervous system, and skin, respectively [5,6]. They are in the most recent Contaminant Candidate List (CCL), released by the Environmental Protection Agency, which has the purpose of naming potential risks to public water systems in the United States. Exposure can come from ingestion of drinking water or contaminated food, direct contact, or inhalation.
Although high nutrient levels and temperature are associated with increased cyanobacteria growth [7], when nutrient levels become too high, and especially when they coincide with limited light, cyanobacteria growth can decrease [8]; however, the ability for cyanobacteria to persist and have successful growth rates even in these unfavorable conditions is in part due to the ability of specific species to form dormant forms, such as akinetes [9,10]. These akinetes, or resting cells, are common in eutrophic lakes as organic-rich sediments are the most important banks of akinete-forming cyanobacteria [11,12]. Akinetes are found in sediment unless they experience favorable conditions, inducing germination, where they use gas vesicles to migrate from the sediment into the water column and continue to grow [12,13]. Dormant forms of the bacteria, such as akinetes, play an important role in HAB production, as akinete genomes can multiply 15 times as much as vegetative cells [14].
Climate change is contributing to an increase in HABs in the Hudson Valley by providing more favorable conditions for cyanobacteria to thrive, such as increased water temperatures and atmospheric CO2, along with changes to salinity, rainfall, and pH [8,15]. More frequent and intense precipitation events produce greater amounts of runoff into waterways, which can include septic waste and excess fertilizers that contain high levels of synthetic nitrate and phosphate in suburban watersheds. This runoff contributes to excess nutrients in surface and groundwater, which can lead to HABs, eutrophication, and a loss of biodiversity [8,16,17,18].
The distribution of HABs, and the frequency, intensity, and duration of bloom episodes are all influenced by climate change [2]. A variety of chemical, biological, and physical techniques can be used for the management of environments that have an increased risk of HABs due to a changing climate [19]. Among these techniques is hyporheic zone stream remediation [20]—a technique of stream restoration that utilizes the hyporheic zone. The hyporheic zone, a zone beneath and extending outward laterally from a stream bed, where surface and groundwater interact, is critically important to biogeochemical cycling [20,21,22,23,24]. Within these zones, dissolved oxygen gradients create strong redox environments that promote the reduction of nitrate to nitrogen gas by denitrifying bacteria and ammonification, which can occur regardless of bacterial presence [25,26]. The hyporheic zone also entraps dissolved phosphorus forms in sediments, preventing excess phosphate from reentering surface water [27].
Hyporheic biogeochemical cycling can be enhanced through the construction of in-stream structures that increase hydraulic gradient; this results in a greater force and volume of downwelling stream water, which enters the hyporheic zone and greater hyporheic flow rates [22]. The increase in hyporheic flux results in greater rates of nutrient cycling, which enables low-headed manmade dams to serve as restoration structures in streams due to their ability to create nutrient sinks [28]. Other dam impacts on hyporheic biogeochemical cycling include increased residence time and the establishment of anaerobic conditions and redox environments, which promote nitrate reduction and ammonification [23,24,26,29].
Beaver recolonization of the northeast has changed landscapes and geochemical processes as the dams they create impound the flow of water [30,31]. Beaver dams create sizeable wetlands capable of capturing large amounts of nutrients in hyporheic sediments due to increased hyporheic path lengths and strong hyporheic redox conditions [24,26]. Beaver dams are effective nitrate sinks and enable bacterial reduction of nitrate to nitrogen [24,26]. Beaver dams also enable ammonification, and ammonium produced in the hyporheic zone of beaver dam systems is exported downstream of the dam, where it may be oxidized back to nitrate [26]. Denitrification within beaver dam systems displays some seasonality as studies suggest that as summer progresses to fall, beaver dams shift from functioning as a nitrate sink to a nitrate source due to the degradation of anoxic conditions within the hyporheic zone [32]. Beaver dams also impact the phosphorus cycle, although there is no consensus on the mechanisms responsible for phosphorus removal in beaver dam systems [26]. It has been postulated that hydraulic gradients play the most significant role in determining phosphorus removal in beaver systems, but the form of phosphate measured causes significant variation in results across studies [26].
Beaver dams also accumulate sediment at high rates and can build up billions of cubic meters of sediment [26,33,34,35]. Beaver dam failures can cause the displacement of sediment, organic material, and dam-accumulated nutrients downstream, which can be catastrophic to downstream portions of dam-impacted tributaries [34,35,36]. With increasing storm frequency and intensity, dam breaches are becoming more common and causing sizable water and sediment flows that can wash out roadways, homes, and enter other waterways [37]. There could potentially be an increase in the amount of dormant cyanobacteria in the sediment, as organic-rich sediment may contain akinete-forming cyanobacteria [11,12]. Dormant cyanobacteria could build up in the sediment in beaver ponds before being flushed downstream and into other watersheds from dam breaches, beaver activity, or dam removal. Sediment from these processes could, therefore, be used to evaluate the survival capabilities and resistance of cyanobacteria from beaver dams as well as their growth rates after introduction into favorable conditions to mimic the transportation of this sediment downstream.
Beaver dams have not been studied significantly within the Hudson Valley, and knowledge of their impacts on biogeochemical cycling and cyanobacteria dynamics is an important part of developing management strategies as beavers continue to populate in the northeast. Establishing well-thought-out beaver management strategies will help maintain the integrity of Hudson Valley ecosystems and enable water resource protection groups to be a step ahead of HABs that may come from cyanobacteria being flushed out from beaver ponds. Figure 1 provides a systems model that summarizes the possible link between beaver dams and HABs.
The purpose of this study is to quantify hyporheic nutrient removal from three dams in the Hudson Valley and identify the possible threat of dam sediments and breaches on HABs downstream through a combination of field and lab methods. The study aims to determine factors that impact biogeochemical cycling and evaluate the possible threat of increased cyanobacteria growth and cyanotoxins downstream of a dam from dormant cyanobacteria build-up in the beaver dam sediment. The study presents necessary data to evaluate the potential impacts of the beaver dam presence, a changing climate, and high anthropogenic nutrient loading on the health of the Hudson River and similar watersheds in temperate climates globally, which is important for human health and future watershed management and species conservation. By collecting datasets to evaluate the impact of beaver dams on biogeochemical cycling and sediment characteristics, we hope to further answer the complex question of whether the recent expansion of beaver dam impoundments into suburban northeast U.S. tributaries puts watersheds at an even greater risk from HABs through compounding ecological and hydrologic impacts.

2. Methods

2.1. Site Descriptions

This study was performed across six dams in Dutchess County, New York, consisting of five beaver dams and one manmade dam (Figure 2). The beaver dams include Old Bullshead, The Preserve at Vassar College, Stissing Mountain, Wetland Trust, and Cary Institute of Ecosystem Studies. The manmade dam, Fern Tor, is included to assess its functionality in controlling water quality and quantity compared to beaver dams. Across two summers, the dams were evaluated to determine their impacts on nutrient cycling and possible secondary consequences downstream due to dam breaches.

2.2. Field Methods

In order to determine the impacts of beaver and manmade dams on the nitrogen and phosphorus cycles in Hudson River Tributaries, three of the six selected dams were studied. The study sites selected were Old Bullshead, The Preserve at Vassar College, and Fern Tor, the former two of which are beaver dams and the latter of which is a man-made earthen dam (Figure 2).
At each of the three dams, a Lee-type seepage meter [43] was installed in the pond immediately upstream of the dam to monitor seepage rates. A piezometer was installed adjacent to the seepage meter. A second piezometer was placed in the thalweg of the stream downstream of the dam. Both piezometers were installed at a depth of 0.5 m into the hyporheic zone. The piezometers were used to determine the hydraulic gradient across each dam site and allow for hyporheic water sampling, which was performed using an Ace Hardware 1 gph Thermoplastic Hand Pump. Grab samples were taken adjacent to both the upstream and downstream piezometers in duplicate to monitor surface water chemistry and algal populations. The downstream piezometer was used as a datum and served as a point in a line transect used to determine stream discharge measured using a Flowatch Flowmeter. A conceptual figure of each dam site set up is shown in Figure 3.
To evaluate possible secondary consequences of dams downstream due to changes in organic matter, nutrient storage, and cyanobacteria population changes in sediment, the five beaver dams were used; the man-made dam was not included. All beaver dam sites were confirmed to be active beaver dams due to visible signs of beaver activity in the area, including chew marks and paw prints. This included sample collection at Old Bullshead, the Preserve at Vassar College, Stissing Mountain, Wetland Trust, and the Cary Institute of Ecosystem Studies (Figure 2). At each of these five beaver dam sites, sampling took place in the beaver pond and at an upstream location to the dam, to represent the environment in the sediment before the introduction of the beaver.
At each site, a soil auger was used to take sediment samples at both the dam location and the upstream location. For the beaver pond location, the sample was taken directly upstream of the dam from a wedge formed by sediment accumulation. The sample at the upstream location was taken at a spot with a similar slope to that near the dam structure. This was performed in order to have paired samples of sediment that represented conditions before and after the creation of a beaver dam. Each sediment sample was refrigerated before being analyzed in the laboratory.

2.3. Lab Methods

The surface and hyporheic water samples were monitored for water quality parameters including algal population, nitrate concentration, and orthophosphate concentration to evaluate the impact of each dam on biogeochemical cycling and downstream HAB risk. The water quality parameters and instruments used are outlined in Table 1.
To expand upon the methodology presented in Table 1, nitrate levels were analyzed using a Hach DR300 Nitrate Colorimeter (Hach, Loveland, CO, USA). This colorimeter measures nitrate concentration using the Cadmium reduction method (APHA Method 8039). The colorimeter is accurate with samples ranging from 0.4 mg/L to 30.0 mg/L. Orthophosphate was monitored using a Hach DR300 Phosphate Colorimeter. This phosphate colorimeter is compatible with two different APHA methodologies. This study used Method 8048 for reactive orthophosphate. The colorimeter is accurate with samples ranging from 0.02 to 3.00 mg/L. Samples that exceeded the upper concentration of the colorimeter were diluted using a 10-fold dilution with deionized water.
The sediment collected at upstream and downstream locations of beaver dam sites was used to create growth chambers to evaluate differences in dormant cyanobacteria populations. Glass beakers were used and placed under a heat and light source for the growth period. In each beaker, 50 cc of sediment was compacted into the bottom, and sterilized Hudson River water was added. This ensured all future measured cyanobacteria concentrations from water column samples in the chamber were from the sediments. Miracle Grow was used as a nutrient medium to increase phosphorus levels to between 2 and 3 ppm to promote cyanobacterial growth. Miracle Grow is a readily available, over-the-counter fertilizer, commonly used by homeowners on their lawns and gardens and can run into nearby waterways.
Growth chambers were made for each of the five beaver dam sites. Growth chambers from each site were created using upstream and downstream sediment samples in triplicate. A control was also created containing only the sterilized Hudson River water with the nutrient medium and no sediment. Every day, a 25 mL water sample from each growth chamber was used to measure cyanobacteria. This was performed using a Fluoroprobe (BBE Moldaenke FluoroProbe), a sensitive instrument which can determine algae class and concentration of algae using fluorescence characteristics through chlorophyll-a analysis using in vivo fluorescence. The BBE Moldaenke FluoroProbe manual states that it uses turbidity correction and an integrated CDOM correction factor to eliminate interferences from yellow substances. Once samples surpassed 25 microg/L of cyanobacteria, the NYSDEC level for a confirmed bloom, which also uses chlorophyll-a for analysis, they were removed from the light source, and final samples from the water column were collected and refrigerated.

2.4. Analysis Approaches

Water quality metrics from surface and hyporheic water samples were compared for upstream and downstream. Vertical hydraulic gradients were also evaluated to determine downwelling and upwelling.
For analysis of the final water column samples from the sediment growth chambers, an ABRAXIS® Microcystins-ADDA-Elisa Microtiter Plate test was used to evaluate the microcystin concentration in each sample. This included dividing each absorbance by the blank absorbance and using standards to create a four-parameter logistic curve for absorption at 450 nm and concentration in parts per billion (ppb), shown in Figure 4. The absorption of the samples after completing the test was determined using a plate reader, and the corresponding mean concentration was found. A growth curve was also created for the concentration of cyanobacteria over time. The growth rates for the beaver dam location and the upstream location at each site were compared. A paired t-test was used for both comparisons of microcystin concentrations and cyanobacteria growth rates to determine significant differences in the sediment before and after dam formation.

3. Results

3.1. Vertical Hydraulic Gradients (Piezometers)

Vertical hydraulic gradients were calculated at each beaver dam using piezometers upstream and downstream of the three dams they were installed at to determine the magnitude of upwelling or downwelling water at each. Figure 5 displays a boxplot of all vertical hydraulic gradients recorded throughout the study at each piezometer location.
Vertical hydraulic gradients recorded at the upstream piezometer coincide with more negative gradients. Gradients recorded at the downstream piezometer are approximately 0 cm throughout the study.

3.2. Water Quality Parameters

Nitrate concentrations were recorded in ppm throughout a four-week study period, seven times in the upstream piezometer, upstream surface water, downstream piezometer, and downstream surface water. Nitrate data across study sites are shown in Figure 6.
Nitrate levels were consistently found to be the highest in the upstream and downstream hyporheic water. The highest recorded nitrate concentration across all sites was recorded at Old Bullshead Beaver dam, measuring 22.6 ppm on 17 July. Nitrate levels at both beaver dams reached their highest points on that same day, which followed a major rain event, while nitrate peaked on 24 July at the eastern Fern Tor dam with a reading of 11.8 ppm. Orthophosphate levels were also recorded at each site throughout the study and are displayed in Figure 7.
Orthophosphate levels were consistently the highest in the downstream piezometers, and like nitrate, orthophosphate levels peaked on the same day at both beaver dams on 24 July. On 24 July, Old Bullshead Beaver dam saw an orthophosphate reading of 7.1 ppm at the downstream piezometer and Vassar Beaver dam recorded a 15.9 ppm orthophosphate level at the downstream piezometer. The Fern Tor dam saw a peak orthophosphate reading on 20 July of 6.8 ppm in the downstream piezometer. Total Dissolved Solids (TDS) data were recorded across all research sites as a mechanism of comparison between orthophosphate levels and TDS to provide insight into differences in NaCl and orthophosphate transport within each dam system. Figure 8 displays the TDS readings recorded at each study site between 11 July 2023 and 10 August 2023.
Total dissolved solids measurements display major outliers on 21 July at both beaver dams, although in different sample types. In general, TDS remained within about 300 ppm across all subsites at each research dam throughout the study, aside from two outliers at the two beaver dams on 17 July. Upstream surface water measurements tended to have the highest TDS levels across all study sites. In contrast, downstream surface water tended to have the lowest TDS readings across the two beaver dams, while upstream hyporheic water carried the lowest TDS readings at Fern Tor dam.
Precipitation data are included in Figure 7, Figure 8 and Figure 9 to allow for a visualization of how changes between wet and dry periods corresponded with differences in nitrogen and orthophosphate levels at each site. Overall, the study was conducted during an abnormally wet summer outside of an extended dry period from the end of July 2023 into August 2023. There were several intense thunderstorms during this time.

3.3. ADDA-Elisa Test (Growth Chamber Samples)

Table 2 provides the mean microcystin concentration determined using the ADDA-Elisa test. The value in the table comes from the calculated mean concentration of the three replicates for each location. In compliance with the test instructions, all concentrations less than 0.150 ppb are reported as <0.150 ppb. The control and Hudson River water concentrations were <0.150 ppb. There is no significant difference in the concentration between the beaver pond location and the upstream location at each site (p > 0.05).
The maximum microcystin concentration was from the beaver dam at The Preserve at Vassar College (0.251 ppb). Three out of the five sites had higher microcystin concentrations from the beaver dam samples, but the difference was not significant (p > 0.05). It is evident that a majority of microcystin concentrations from the sites were increased compared to the control concentrations.

3.4. Cyanobacteria Growth (Growth Chamber Samples)

Throughout the time the growth chambers were under ideal conditions, cyanobacteria experienced exponential growth. The exponential growth curves represented in Figure 9 come from a best-fit mathematical model, given in Table 3.

4. Discussion

4.1. Data Availability and Use

All data from this project are provided in the Supplemental Materials, and can be freely used by others to continue researching the possible local and further downstream impacts of beaver impoundments. The impacts studied are relevant at global scales as beaver populations continue to grow. As beaver dams increase in number and man-made dams are constructed to possibly enhance hyporheic biogeochemical cycling [22], it is necessary to quantify hyporheic nutrient removal. In addition, cyanobacteria dynamics in the sediment of beaver dams can be used to predict downstream HAB threats and evaluate growth patterns of dormant sediment-based cyanobacteria if the buildup of sediment in beaver ponds is transported downstream to nutrient- and light-rich waterways.
Harmful algal blooms threaten freshwater resources [1,2], and the strong impact of nutrients on their growth [7] makes knowledge of changes to nutrient cycling and buildup from beaver dams important for remediation consideration. To manage the possible risks associated with these potentially toxic blooms, beaver populations and their ecological impacts as they repopulate many areas in North America should continue to be researched, and these data can help supplement future studies.

4.2. Biogeochemical Cycling in the Hyporheic Zone

Higher nitrate concentration in hyporheic water compared to surface water was observed across all three research dams, which is consistent with previous studies. In the presence of oxygen, nitrification takes place, and near the dam structure, there are reducing conditions due to limited dissolved oxygen from reduced flow rates [24]. We postulate that ammonium increases further downstream because of increased dissolved oxygen presence [24,26]; however, more research is needed.
The nitrate readings taken in August 2023 indicate a potential impact of seasonality on dam biogeochemical cycling. The nitrate readings converge in August across all subsites (Figure 6). This may be due to a degraded redox environment, inhibiting typical hyporheic uptake of nitrate, which is consistent with other studies [24,26,32]. Additionally, August 2023 coincided with a dry period, which seems to coincide with convergence in nitrate and orthophosphate data across all three dam sites and their respective subsites.
Orthophosphate and TDS data suggest some interesting implications for solute transport in each dam system. TDS across all three dam sites remains relatively consistent across all sub-sampling locations, which is noteworthy because previous studies have used salt injections as a means of comparison to ascertain whether phosphate is undergoing transformations in the hyporheic zone [27]. Studies have demonstrated a greater decrease in phosphorus concentration than NaCl (a conservative tracer) concentration as the two solutes move through the hyporheic zone, suggesting entrapment of phosphorus within the hyporheic zone [27]. Our study seems to corroborate these findings, particularly due to the highest orthophosphate levels consistently being found in the downstream piezometer, indicating that orthophosphate levels are the highest in the thalweg of the stream deep in the hyporheic zone, while there is a minimal change in TDS across the same flow path. This spike in downstream phosphate suggests that there is entrapment of orthophosphate in the hyporheic zone underneath each dam. Previous studies have suggested this may be due to high flow rates, which may prevent absorption of phosphorus, resulting in increased phosphorus residence time and phosphorus moving into deep layers of the stream bed before being absorbed in sediment [27]. This spike in hyporheic orthophosphate may also be due to interactions with metal ions in sediment, although metal ions were not measured in the study.
The general trends across all dam sites indicate the dams function as nutrient sinks and decrease both nitrate and orthophosphate concentration in the downstream stream water.
Rainfall plays a significant role in the convergence and divergence of nitrate and orthophosphate levels observed in the study. Convergence in nitrate and orthophosphate values corresponded across all dam sites with dry periods. This may be due to a decrease in vertical hydraulic gradient and an increase in surface water flow rate relative to hyporheic flow rate.
Nitrate and phosphate levels collected from streamwater are consistent with those found in other watersheds within the region, particularly the Fall Kill and Landsman Kill. An unpublished study on the Fall Kill and Landsman Kill performed in November of 2024, analyzing nitrate and enterococcus levels within the Landsman Kill and Fall Kill streams, found nitrate levels in the two streams ranging from 7.66 mg/L to 20.02 mg/L in the Fall Kill and 6.21 mg/L to 22.88 mg/L in the Landsman Kill. Both the Fall Kill and Landsman Kill are within Dutchess County, which contains land cover predominantly composed of developed land near the downstream portion of the watershed and agricultural land at the headwaters. These watersheds see significant nutrient loading from agricultural runoff, combined sewer overflows, and septic tank discharge, resulting in eutrophic stream water. These factors also play a role in the sites chosen for inclusion in this study. The Fern Tor Creek, as well as Casper Creek, have inflows from the City of Poughkeepsie, who have a combined sewer system that may explain the large spikes in nitrate following heavy precipitation events. The Old Bullshead Beaver dam is located directly downstream from a large farm and a lake that consistently has HABs. The highly eutrophic lake and agricultural runoff likely explain the high nutrient concentration in these bodies of water, much like eutrophic streams in agricultural portions of the US [44].
The New York State Department of Environmental Conservation (NYS DEC) does not have established numerical criteria for nutrients within surface water bodies like streams. Instream, NYS DEC provides a narrative standard delineating the level of nitrogen and phosphorus permitted in the state’s surface water bodies. This narrative standard declares phosphorus and nitrogen should not be found in quantities that result in the growth of algae, weeds, and slimes that will impair the waters for their best uses. The NYS DEC also provides guidance on limiting the amount of phosphorus in surface water bodies to 0.020 mg/L [45]. The orthophosphate values recorded in this study exceeded this guidance value in all samples collected across all study sites, indicating that the water bodies are likely nutrient-impaired and susceptible to HABs.

4.3. Implications of Cyanobacterial Findings on the Threat of HABs Downstream

Cyanotoxin concentration after the growth period was not significantly different between upstream and downstream samples. It can be assumed that the beaver dam cyanobacteria populations share similar species and toxin potential to those that would have been present before the dam was built, as gene pools influence toxin biosynthesis [5]. Due to the limited space in the water column in both the beaver pond and upstream replicates, it is possible that toxin production was not significantly different between them because of competition for nutrients and space, as the production of cyanotoxins is impacted by nutrient availability [46].
It took less time in ideal conditions for the cyanobacteria concentrations in the water column to reach HAB levels in samples containing beaver pond sediment. This may be in part due to larger dormant cyanobacteria populations in the sediment from increased organic matter and nutrient storage, as organic-rich sediments are banks for akinetes [11,12,36]. This could potentially increase the risk of downstream HABs in areas where beaver populations are expanding, as beaver dam breaches and failures can transport this cyanobacteria-rich sediment downstream to nutrient-rich, urban waterways with high temperatures and light levels, similar to the conditions mimicked in the growth chambers [34].
It is also relevant that the ideal conditions necessary for a HAB can fluctuate even hour to hour, depending on weather, runoff, and flow patterns [47]. If the characteristics of sediment from beaver ponds lead to a decreased time needed in ideal conditions to remain in one place for cyanobacteria growth to reach HAB levels, it can prove worrisome, especially as climate change is making weather and these conditions more intense and unpredictable [40]. Climate change is also increasing the risk of dam breaches and sediment deposition downstream, as very intense storms with extreme rainfall can overflow a dam structure or break it down completely [34,41]. With beaver populations increasing rapidly in the Hudson Valley, it is also possible that a dam may be broken legally to prevent damage to nearby properties or roads [48]. These factors can all increase the possibility of sediment transfer downstream of a beaver dam, increasing the potential for HABs in waterways that are relied on as a water source and for recreation.

4.4. Future Directions

The study did not investigate the reoxidation of ammonium, so the exact magnitude of ammonification in the hyporheic zone across each dam site should be investigated further to ensure that the trends determined in previous studies are matched at each study site. Future studies should also explore lateral hyporheic flow at each dam site, given the importance of geomorphology on denitrification rates, which have been found in a variety of studies to differ based on surface flow rates and stream positioning [23,32]. Adding lateral hyporheic flow and groundwater data would paint a more accurate picture of the nature of the transformations occurring in the hyporheic zone at each dam site.
The convergence in the dry and divergence in the wet in nitrate and orthophosphate data should be examined further to determine the rainfall impact on biogeochemical cycling and if rainfall is responsible for differences in nitrate and orthophosphate levels in surface and hyporheic water during wet periods. Future studies on similar dam systems should attempt to determine the mechanisms responsible for the large spike in hyporheic orthophosphate found at each dam site, which in dry periods could potentially be from beaver digging behavior.
To further explore the implications of a dam rupture downstream, the same experiment could be performed at a larger scale, using greater amounts of sediment for each replicate and more room in the water column. This could provide important insights into what these patterns and results would translate to in larger bodies of water. Additionally, information could be collected on the frequency of beaver dam repair at the sites studied. This could influence the resiliency of a beaver dam and whether a rupture would be destructive to the surrounding areas. GIS could also prove beneficial in future projects to predict possible paths that overflow from a dam rupture would take and evaluate if those areas have experienced a harmful algal bloom after a large storm previously or may be at risk for one in the future. The analysis of future risk could come from water characteristics, including nutrient content, and sunlight levels of the waterway through nearby shading. If a harmful algal bloom did occur downstream, genomic sequencing could confirm if the cyanobacteria originated at a beaver dam location that is upstream.

5. Conclusions

This dataset highlights the complexity of beaver dams’ impacts on the risk of harmful algal blooms downstream. All dam sites studied using piezometers functioned as nutrient sinks with decreased nitrate and orthophosphate concentrations in downstream water. Dry periods were seen to influence the convergence and divergence of nitrate and orthophosphate concentrations, which should be studied further. A potential threat downstream from beaver dams, which act as nutrient sinks, is the accumulation of organic material and nutrients in the sediment. We saw faster growth of dormant cyanobacteria from sediment in a beaver pond compared to sediment at a paired, unimpounded, upstream location when placed under ideal conditions. As climate change has created more frequent and intense precipitation events, the risk of dam breaches increases along with the possibility of beaver dam sediment transfer downstream. The sediment could reach ideal light and nutrient conditions from anthropogenic eutrophication, and the dormant cyanobacteria populations could grow into a harmful algal bloom in a short time period. This possibility could be further explored through GIS mapping of dam breaches and analyzing the places downstream from a dam with ideal conditions.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/data10040051/s1.

Author Contributions

Conceptualization, E.E.N., J.R.N. and P.Z.K.; methodology, E.E.N., J.R.N., D.H.K. and P.Z.K.; formal analysis, E.E.N., J.R.N., D.H.K. and P.Z.K.; investigation, E.E.N., J.R.N., D.H.K. and P.Z.K.; resources, D.H.K. and P.Z.K.; data curation, E.E.N., J.R.N., D.H.K. and P.Z.K.; writing—original draft preparation, E.E.N. and J.R.N.; writing—review and editing, E.E.N., J.R.N., D.H.K. and P.Z.K.; visualization, E.E.N., J.R.N., D.H.K. and P.Z.K.; supervision, P.Z.K.; project administration, P.Z.K.; funding acquisition, E.E.N., J.R.N., D.H.K. and P.Z.K. All authors have read and agreed to the published version of the manuscript.

Funding

Funding was provided from the Hudson River Foundation, through the Tibor T. Polgar Fellowship Program.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

See the Supplementary Materials.

Acknowledgments

Thank you to the Hudson River Foundation for providing funding. Thank you to Ray Kepner for providing cyanobacteria expertise and equipment help, and Kyra Hoffman for support at the Vassar Beaver Dam site and assisting with some lab samples. Thank you to the Cary Institute of Ecosystem Studies, Wetland Trust, Vincent Teahan, and Johanna Triegel for also approving site access for sample collection at beaver dams on their properties. Thank you to the Klos Hydrology group, in particular, Shannon Hickey, and Caitlyn Maas of Lake Superior State University for help throughout the summers in the field and lab. A final thanks to Marist University and Vassar Preserve for providing support, ordering supplies, and providing site access throughout the project.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Predictive systems model, linking beaver dam expansion and the presence of harmful algal blooms, where (+) represents a positive relationship, (—) represents a negative relationship, and (?) represents mixed findings [16,18,24,26,30,34,36,38,39,40,41,42]. The bonded lines highlight the primary discussion of this study.
Figure 1. Predictive systems model, linking beaver dam expansion and the presence of harmful algal blooms, where (+) represents a positive relationship, (—) represents a negative relationship, and (?) represents mixed findings [16,18,24,26,30,34,36,38,39,40,41,42]. The bonded lines highlight the primary discussion of this study.
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Figure 2. Sampling locations in Dutchess County, New York.
Figure 2. Sampling locations in Dutchess County, New York.
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Figure 3. Conceptual cross-sectional view of research site piezometer and seepage meter layout. Piezometers are shown as grey cylinders, and the seepage meter is shown as a white cylinder attached to a white cube.
Figure 3. Conceptual cross-sectional view of research site piezometer and seepage meter layout. Piezometers are shown as grey cylinders, and the seepage meter is shown as a white cylinder attached to a white cube.
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Figure 4. Four-parameter logistic curve used to convert sample absorbance to microcystin concentration in ppb. Y = 0.1205 + (1.0004 − 0.1205)/(1 + (x/0.68020)1.0062).
Figure 4. Four-parameter logistic curve used to convert sample absorbance to microcystin concentration in ppb. Y = 0.1205 + (1.0004 − 0.1205)/(1 + (x/0.68020)1.0062).
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Figure 5. Hydraulic gradient in centimeters recorded at each piezometer throughout a four-week study period. FT indicates Fern Tor dam, OBH indicates Old Bullshead Beaver dam, and VC indicates Vassar Beaver dam. USP denotes upstream piezometer and DSP denotes downstream piezometer. Mean vertical hydraulic gradients are indicated with an X and correspond to the head gradient between surface water and hyporheic water at piezometer depth to show the relative intensity of directional flow, from FT USP to VC DSP: −42.9 cm, 0.0561 cm, −89.5 cm, −4.05 cm, −58.7 cm, and 2.88 cm. Negative values indicate downwelling water and positive values indicate upwelling water.
Figure 5. Hydraulic gradient in centimeters recorded at each piezometer throughout a four-week study period. FT indicates Fern Tor dam, OBH indicates Old Bullshead Beaver dam, and VC indicates Vassar Beaver dam. USP denotes upstream piezometer and DSP denotes downstream piezometer. Mean vertical hydraulic gradients are indicated with an X and correspond to the head gradient between surface water and hyporheic water at piezometer depth to show the relative intensity of directional flow, from FT USP to VC DSP: −42.9 cm, 0.0561 cm, −89.5 cm, −4.05 cm, −58.7 cm, and 2.88 cm. Negative values indicate downwelling water and positive values indicate upwelling water.
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Figure 6. Nitrate readings across three sites recorded between 9 July 2023, and 11 August 2023 at Site A: Fern Tor dam, Site B: Old Bullshead Beaver dam, and Site C: The Preserve at Vassar College Beaver dam. In the figure, these are labeled as (A), (B), and (C), respectively. USP, USSW, DSP, and DSSW correspond to upstream piezometer, upstream surface water, downstream piezometer, and downstream surface water sample locations. The blue bars indicate the precipitation data, given on the right axis.
Figure 6. Nitrate readings across three sites recorded between 9 July 2023, and 11 August 2023 at Site A: Fern Tor dam, Site B: Old Bullshead Beaver dam, and Site C: The Preserve at Vassar College Beaver dam. In the figure, these are labeled as (A), (B), and (C), respectively. USP, USSW, DSP, and DSSW correspond to upstream piezometer, upstream surface water, downstream piezometer, and downstream surface water sample locations. The blue bars indicate the precipitation data, given on the right axis.
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Figure 7. Orthophosphate readings across three sites recorded between 9 July 2023, and 11 August 2023 at Site A: Fern Tor dam, Site B: Old Bullshead Beaver dam, and Site C: The Preserve at Vassar College Beaver dam. In the figure, these are labeled as (A), (B), and (C), respectively. USP, USSW, DSP, and DSSW correspond to upstream piezometer, upstream surface water, downstream piezometer, and downstream surface water sample locations. The blue bars indicate the precipitation data, given on the right axis.
Figure 7. Orthophosphate readings across three sites recorded between 9 July 2023, and 11 August 2023 at Site A: Fern Tor dam, Site B: Old Bullshead Beaver dam, and Site C: The Preserve at Vassar College Beaver dam. In the figure, these are labeled as (A), (B), and (C), respectively. USP, USSW, DSP, and DSSW correspond to upstream piezometer, upstream surface water, downstream piezometer, and downstream surface water sample locations. The blue bars indicate the precipitation data, given on the right axis.
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Figure 8. Total Dissolved Solids readings recorded between 9 July 2023, and 11 August 2023 at Site A: Fern Tor dam, Site B: Old Bullshead Beaver dam, and Site C: The Preserve at Vassar College Beaver dam. In the figure, these are labeled as (A), (B), and (C), respectively. USP, USSW, DSP, and DSSW correspond to upstream piezometer, upstream surface water, downstream piezometer, and downstream surface water sample locations. The blue bars indicate the precipitation data, given on the right axis.
Figure 8. Total Dissolved Solids readings recorded between 9 July 2023, and 11 August 2023 at Site A: Fern Tor dam, Site B: Old Bullshead Beaver dam, and Site C: The Preserve at Vassar College Beaver dam. In the figure, these are labeled as (A), (B), and (C), respectively. USP, USSW, DSP, and DSSW correspond to upstream piezometer, upstream surface water, downstream piezometer, and downstream surface water sample locations. The blue bars indicate the precipitation data, given on the right axis.
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Figure 9. Exponential growth curves determined from the cyanobacteria concentrations over time. OBH indicates Old Bullshead, VC indicates The Preserve at Vassar College, SM indicates Stissing Mountain, WT indicates Wetland Trust, and CARY indicates Cary Institute of Ecosystem Studies.
Figure 9. Exponential growth curves determined from the cyanobacteria concentrations over time. OBH indicates Old Bullshead, VC indicates The Preserve at Vassar College, SM indicates Stissing Mountain, WT indicates Wetland Trust, and CARY indicates Cary Institute of Ecosystem Studies.
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Table 1. Water quality parameters monitored, and the equipment used to evaluate each parameter.
Table 1. Water quality parameters monitored, and the equipment used to evaluate each parameter.
ParameterMeasurement Tool
pHYSI Professional Plus Multiprobe
Specific Conductivity (Converted to TDS)YSI Professional Plus Multiprobe
TemperatureSolinst WLT Meter Model 201
Dissolved OxygenYSI Professional Plus Multiprobe
NitrateHach DR-300 Colorimeter Cadmium Reduction Method
Orthophosphate Hach DR-300 Colorimeter Ascorbic Acid Method
Green AlgaeBBE Moldaenke FluoroProbe
Bluegreen AlgaeBBE Moldaenke FluoroProbe
DiatomsBBE Moldaenke FluoroProbe
TurbidityLaMotte 2020we Turbidity Meter
Table 2. Microcystin concentration (ppb).
Table 2. Microcystin concentration (ppb).
SiteBeaver PondUpstream
Old Bullshead0.2140.225
The Preserve at Vassar College0.2510.214
Stissing Mountain0.1760.159
Wetland Trust< 0.1500.234
Cary Institute of Ecosystem Studies 0.178< 0.150
Table 3. Summary of exponential growth curves and R2 for each location.
Table 3. Summary of exponential growth curves and R2 for each location.
SiteBeaver PondUpstream
Old BullsheadY = 0.0243e0.072x
R2 = 0.987		Y = 0.218e0.043x
R2 = 0.963
The Preserve at Vassar CollegeY = 0.0142e0.0672x
R2 = 0.984		Y = 0.0592e0.0551x
R2 = 0.996
Stissing MountainY = 0.127e0.036x
R2 = 0.988		Y = 1.03e0.0211x
R2 = 0.854
Wetland TrustY = 0.0582e0.0569x
R2 = 0.990		Y = 0.658e0.0193x
R2 = 0.635
Cary Institute of Ecosystem StudiesY = 0.0674e0.0797x
R2 = 0.999		Y = 0.205e0.0531x
R2 = 0.907
Following Y = AeBx, a paired t-test was used to compare the B values, which affect horizontal stretch. There was a significant difference between beaver pond and upstream growth rates (p < 0.004). Cyanobacteria growth was consistently faster in the growth chambers when the sediment was from the beaver pond location in relation to the corresponding upstream location for each site. “Faster” refers to reaching higher cyanobacteria concentrations in less time.
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Novobilsky, E.E.; Navin, J.R.; Knights, D.H.; Klos, P.Z. Observational Monitoring Records Downstream Impacts of Beaver Dams on Water Quality and Quantity in Temperate Mixed-Land-Use Watersheds. Data 2025, 10, 51. https://doi.org/10.3390/data10040051

AMA Style

Novobilsky EE, Navin JR, Knights DH, Klos PZ. Observational Monitoring Records Downstream Impacts of Beaver Dams on Water Quality and Quantity in Temperate Mixed-Land-Use Watersheds. Data. 2025; 10(4):51. https://doi.org/10.3390/data10040051

Chicago/Turabian Style

Novobilsky, Erin E., Jack R. Navin, Deon H. Knights, and P. Zion Klos. 2025. "Observational Monitoring Records Downstream Impacts of Beaver Dams on Water Quality and Quantity in Temperate Mixed-Land-Use Watersheds" Data 10, no. 4: 51. https://doi.org/10.3390/data10040051

APA Style

Novobilsky, E. E., Navin, J. R., Knights, D. H., & Klos, P. Z. (2025). Observational Monitoring Records Downstream Impacts of Beaver Dams on Water Quality and Quantity in Temperate Mixed-Land-Use Watersheds. Data, 10(4), 51. https://doi.org/10.3390/data10040051

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