Phosphorus loading and ecological impacts from agricultural tile drains in a west Michigan watershed

JGLR-01158; No. of pages: 9; 4C: 2 Journal of Great Lakes Research xxx (2016) xxx–xxx

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Phosphorus loading and ecological impacts from agricultural tile drains in a west Michigan watershed Delilah R. Clement, Alan D. Steinman ⁎ Annis Water Resources Institute, Grand Valley State University, 740 West Shoreline Drive, Muskegon, MI 49441, USA

a r t i c l e

i n f o

Article history: Received 12 July 2016 Accepted 28 October 2016 Available online xxxx Keywords: Tile drain effluent Phosphorus Eutrophication Algal blooms Bioassay

a b s t r a c t Phosphorus (P) loading from nonpoint sources is often implicated as a contributing factor to the proliferation of algal blooms in freshwater ecosystems. However, the influence of subsurface tile drains as a source of P, especially in agricultural areas, has received limited attention. We examined the importance of tile drain effluent in the Macatawa Watershed; this watershed is dominated by row crop agriculture and drains into hypereutrophic Lake Macatawa, which connects to Lake Michigan. Our objectives were twofold: 1) assess the importance of tile drain effluent as a source of P in the Macatawa Watershed by measuring tile drain P concentrations spatially and temporally over a one-year period; and 2) assess the ability of tile drain effluent to stimulate algal blooms using bioassays with natural phytoplankton communities. During March 2015–February 2016, P concentrations varied significantly among sample sites (SRP: b 0.005 to 0.447 mg/L; TP: 0.010 to 0.560 mg/L), and the highest P loads occurred during the non-growing season. Annual SRP yields from the tile drain sample sites ranged from 0.002 kg/ha to 0.248 kg/ha, and annual TP yields ranged from 0.003 kg/ha to 0.322 kg/ha. SRP, on average, accounted for 60% of TP, and the SRP:TP ratio measured at the tile drain outlets was positively correlated with area drained by the tile system. Algal bioassays failed to find a positive relationship between chlorophyll a and tile drain SRP; algal community structure was dominated by diatoms, not by cyanobacteria, as expected. © 2016 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.

Introduction Among growing public concern for surface water quality, the federal Clean Water Act was enacted in 1972 to regulate pollution sources. Regulation of discharge permits under the National Pollutant Discharge Elimination System (NPDES) has effectively reduced pollution from point sources (USEPA, 2016). However, inputs from agriculture are still a major nonpoint source of nutrient pollution to surface waters (Daniel et al., 1998). An often-limiting nutrient in freshwater ecosystems, excess phosphorus (P) is frequently implicated as a contributing factor to algal blooms (Elser et al., 2007; Paerl and Otten, 2013; Schindler, 1977), although limitation or co-limitation by nitrogen has received recent attention (Conley et al., 2009). Harmful algal blooms can impair aquatic ecosystems because they can: 1) be toxic (Carmichael, 2001); 2) decrease dissolved oxygen concentrations upon decomposition (Scavia et al., 2014); 3) disrupt food webs (Paerl et al., 2016); and 4) threaten public health (Brooks et al., 2016). An estimated 18 to 28 million hectares of cropland in the Midwest region of the U.S. are managed with the use of tile drains (King et al., 2015a). When tile drains are installed, they change the hydrology of a

⁎ Corresponding author. E-mail address: [email protected] (A.D. Steinman).

field to increase infiltration of water and reduce the amount of overland runoff (Reid et al., 2012). While tile drains can reduce surface runoff, and thereby the loss of P associated with topsoil erosion, they also represent a direct conduit from the subsurface to stream outlets and bordering ditches. Hence, nutrients can reach the surface drainage system from an area extending beyond drain field itself (Reid et al., 2012; Smith et al., 2015). In addition, once in the tiles, nutrients drain to the surface waters without the opportunity for assimilation or adsorption through the soil profile. Overland flow is generally the dominant transport mechanism for P, but there are situations when significant P transport has occurred through agricultural tile drainage (King et al., 2015a). Factors influencing P transport to tile drains include soil type, precipitation, time of year, and land-management practices such as tillage or crop regime (King et al., 2015a). Soil type influences P transport to tile drains primarily by its tendency to form macropores or to promote matrix flow. Soil matrix flow is a relatively slow pathway by which solutes have time to interact with soil particles, minerals, and organic materials (Reid et al., 2012; Sharpley et al., 2001). Alternatively, P transport through soil macropores is a relatively fast, more direct pathway via earthworm burrows, shrinkage fractures, or root channels (Laubel et al., 1999). Preferential transport through macropores is an important process during precipitation and snowmelt, as both events cause increased flow through the soil column (Macrae et al., 2007; Smith et al., 2015). Numerous studies have demonstrated that periods of high

http://dx.doi.org/10.1016/j.jglr.2016.10.016 0380-1330/© 2016 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.

Please cite this article as: Clement, D.R., Steinman, A.D., Phosphorus loading and ecological impacts from agricultural tile drains in a west Michigan watershed, J. Great Lakes Res. (2016), http://dx.doi.org/10.1016/j.jglr.2016.10.016

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D.R. Clement, A.D. Steinman / Journal of Great Lakes Research xxx (2016) xxx–xxx

flow result in increased P loss through tile drainage (Algoazany et al., 2007; Ball Coelho et al., 2012; Gentry et al., 2007; Morrison et al., 2013). Furthermore, several studies found that the majority of nutrient leaching occurs during the winter months with increased flow from tile drains (Kladivko et al., 2004; Laubel et al., 1999; Royer et al., 2006). Field-specific management practices, including type of fertilizer, tillage, and crop regime, also can affect P transport to tile drains (Kinley et al., 2007; Laubel et al., 1999; North, 2013; Sims et al., 1998). For instance, no-tillage management decreases runoff and erosion but promotes macropore formation. Conversely, tillage disturbs macropores in the topsoil, which reduces the connectivity of the surface to tile drains (Geohring et al., 2001; King et al., 2015a). Thus, the loss of P through tile drains in agricultural fields is both temporally and spatially variable and depends heavily on local factors. Because tile drains are a potentially important source of nutrients from the field to downstream receiving water bodies, this study was designed to investigate their role as a source of bioavailable P contributing to eutrophication. We examined the spatial and temporal variability of P in tile drain effluent and its ecological impacts in an agriculturally dominated watershed located in west Michigan. Excess sediment and phosphorus from the watershed flow into Lake Macatawa, an important commercial and recreational water body in the region, and negatively impact the region's economy and cultural identity. The objectives of this study were to: 1) assess the importance of tile drain effluent as a source of P in the Macatawa Watershed by measuring tile drain P concentrations spatially and temporally over a 1-year period; and 2) assess the ability of tile drain effluent to stimulate algal blooms using bioassays with natural phytoplankton communities.

Methods Site description The Macatawa Watershed encompasses 450 km2 in southwestern Michigan (Fig. 1). The outer periphery of the watershed is dominated by row-crop agriculture and has been shown to be the source for much of the watershed's excess nutrients (MWP, 2012). Agriculture accounts for ~ 45% of the watershed's land use/land cover, followed by urban/residential/industrial (33%) and natural forest/shrub/grassland (20%; Fig. 1). The watershed drains into Lake Macatawa, where average monthly surface TP concentrations have exceeded 0.125 mg/L in recent years (Holden, 2014). The lake and its tributaries are on Michigan's 303(d) list for not attaining water quality standards for a warm water fishery and other aquatic life, and the lake is currently under a total maximum daily load (TMDL) calling for a 72% reduction in TP concentration (Walterhouse, 1999). P loading to Lake Macatawa is heavily influenced by precipitation events, and lake concentrations of TP decline after long periods of baseflow (Holden, 2014). A current 10-year restoration project aims to reduce P loads and meet the TMDL target through remediation, implementation of best management practices, and education (Project Clarity, http://www.macatawaclarity.org/, 12, July 2016). Field and laboratory methods: Phosphorus runoff Nine tile drain sampling sites were selected based on landowner cooperation (Fig 1). Because the property owners wished to remain

Fig. 1. Lake Macatawa watershed boundary showing land-use distribution, location of the tile drain sampling sites (circles; 1–9), as well as Lake Macatawa sample site (10). Upper right inset: location of Macatawa watershed within lower peninsula of Michigan. Lower right inset: location of watershed in west Michigan and location of the Lake Michigan sample site (11).

Please cite this article as: Clement, D.R., Steinman, A.D., Phosphorus loading and ecological impacts from agricultural tile drains in a west Michigan watershed, J. Great Lakes Res. (2016), http://dx.doi.org/10.1016/j.jglr.2016.10.016

D.R. Clement, A.D. Steinman / Journal of Great Lakes Research xxx (2016) xxx–xxx

anonymous, the exact coordinates of each site are not given. Eight of the sites were PVC outlets draining to a ditch, and at site 9, plastic tubing (acid-washed) was used to hand-pump water from a depth of ~3 m to the surface from a tile drain vent. TP and SRP were measured monthly for one year (March 2015–February 2016) from each site, except for site 9, which was added to the study in June 2015. Samples were taken as soon after storm events as possible (usually b 1 day). Autosamplers are not practical in these agricultural ditches, as tile outlets often become submerged after major storm events (see Discussion). Grab samples were obtained using acid-washed 250-mL plastic bottles. A 20-mL subsample was filtered through a 0.45-μm nylon syringe filter (ThermoFisher Scientific, Waltham, MA) immediately after sampling for SRP analysis. All samples were transported to the laboratory on ice. TP samples were stored at 4 °C, and SRP samples were frozen at − 18 °C before analysis with a SEAL Autoanalyzer (SEAL Analytical, Mequon, Wisconsin). The ratio of SRP (mg P L−1) to TP (mg P L−1) (SRP:TP) for each tile drain was calculated and is referred to as %SRP. Discharge at each outlet was estimated using a 500-mL cup and stopwatch during sample collection. Some tile outlets were partially submerged by ditch water on a few sample dates, but active flow through the outlet allowed for grab samples and P concentration analysis. In those cases, discharge could not be measured from partially submerged outlets, so flow was estimated based on measurements taken on other (non-submerged) visually similar sampling dates. SRP and TP load were calculated by multiplying concentration values by the corresponding discharge rate. Finally, the land and management factors of each tile-drained field were recorded. Factors included hectares drained, summer and winter crop regime, fertilizer application, tillage equipment, and dominant hydrologic soil type (USDA SSURGO). Field and laboratory methods: Algal bioassays Algal bioassays were used to evaluate the bioavailability of P in tile drain water. Three bioassays were conducted to assess seasonal effects: spring (April 23), summer (August 5), and fall (October 26) of 2015. Acid-washed 10-L carboys were used to collect water from each site for the experiments. Carboys of water also were collected from Lake Macatawa and from Lake Michigan (Fig. 1) to serve as controls as explained below. Carboys were transported to the lab on ice and stored at 4 °C. Within 24 h of collection, water from all sites was filtered through 0.1-μm Graver QMC™ Series Filter Cartridges using a peristaltic pump to remove any particulate matter that might confound the algal bioassays. Filtering by this method did not affect SRP concentrations of the tile or lake water. Prior to bioassay setup, water from each carboy was filtered into a 20-mL vial using a 0.45-μm glass membrane for SRP analysis. Bioassays used phytoplankton collected from the surface (0–0.5 m depth) of Lake Macatawa with a 23-μm plankton net (Fig. 1). The Lake Macatawa phytoplankton sample was transported in an opaque plastic bottle and filtered through a 200-μm sieve to prevent zooplankton grazing before use in the bioassay. For each seasonal bioassay, 90 mL of filtered water from each sampling site was placed in an acid-washed, autoclaved 125-mL Erlenmeyer flask, in triplicate, and inoculated with 10 mL of Lake Macatawa phytoplankton. At the start of the incubation period, three initial subsamples from each bioassay were saved for chlorophyll a (Chl a) analysis as described below. Twenty mL of the Lake Macatawa phytoplankton subsamples were preserved in 1% Lugol's for initial determination of the phytoplankton community. Three flasks of Lake Michigan water were included in this bioassay as a low-SRP control. Bioassays were run for 7 days in each season in a Powers Scientific Growth Chamber. No attempt was made to keep the conditions axenic. Ambient conditions of Lake Macatawa were measured at the time of water sampling, and the temperature and light irradiance of the growth chamber were set to mimic lake conditions depending on the season. During the spring, summer, and fall, the chamber temperature was set

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to 11.0 °C, 26.6 °C, and 13.9 °C, respectively. The light:dark cycle was set to 13.5:10.5 in the spring, 14.5:8.5 in the summer, and 10.5:13.5 in the fall. Irradiance measured as photosynthetically active radiation (PAR) was set at 190 in the spring, 355 in the summer, and 303 μmol/m2/s in the fall, which was based on an average of measurements in Lake Macatawa at 0 m, 0.5 m, and 1.0 m. All irradiance measurements were made with a LiCor Li-193SA spherical quantum sensor. Flasks were rearranged to random positions on the shaker tables each day to minimize variability in irradiance within the chamber. At the end of the incubation period, flasks were subsampled for Chl a, SRP, and taxonomic structure. An aliquot from each flask was filtered using a GF/F filter (Whatman®) and frozen at −18 °C. Each filter was ground and then steeped in 90% buffered acetone in the dark for 24 h. After each sample was centrifuged, the Chl a concentration of the supernatant was analyzed using a Shimadzu UV-1601 spectrophotometer (Steinman et al., 2006). A second aliquot was preserved in 1% Lugol's in an opaque, plastic bottle for taxonomic analysis. At least 300 phytoplankton units from each sample were identified using a Nikon H550L inverted microscope (Utermöhl, 1958) to the division level, and whenever possible, to genus and species. Phytoplankton biovolumes were calculated based on the shape and appropriate measurements of 10 units of each taxon (Hillebrand et al., 1999). Statistical analyses For all the phosphorus data (both field sampling and bioassays), the Shapiro-Wilk test was used to test for normality of all data. The SRP and TP concentrations were non-normally distributed. Therefore, a nonparametric Kruskal-Wallis test was used for comparisons when P concentrations and loads were grouped by sampling location or date. A Bonferroni post-hoc adjustment revealed individual significant differences. For the field P data, coefficients of variation (CV) were calculated for P concentrations grouped by sampling location or date. Linear regression following a squared-root transformation was used to relate SRP, TP, and %SRP concentrations per sampling site to hectares drained by the tile system. For the bioassay data, relationships between Chl a concentrations and bioassay SRP concentrations were assessed using linear regression. The spring Lake Macatawa phytoplankton bioassay Chl a concentrations were non-normally distributed. Therefore, their relationship to SRP was based on Spearman's correlation analysis. All statistical analyses were conducted with R version 3.1.1 (R Core Team, 2016), and an alpha value less than or equal to 0.05 was considered a significant result. Results Tile drain SRP, TP, and %SRP varied spatially in the Macatawa watershed. SRP concentrations ranged from below detection (0.005 mg/L) to 0.447 mg/L, whereas TP concentrations ranged from 0.010 to 0.560 mg/L (Table 1). Averaged across all sites and sampling months, SRP composed a relatively large portion of the TP concentration, as indicated by both the mean (60 ± 3%) and median %SRP (69%) (Table 1). There was no significant effect of time on any of the P concentration parameters (Table 2). However, there was a highly significant effect of site location on all three P concentration parameters (Table 2). Sites 8 and 9 had the highest mean SRP and TP concentrations (Figs. 2a and Table 1 Summary of lowest, highest, mean, and median concentrations of soluble reactive phosphorus (SRP), total phosphorus (TP), and percent SRP for nine tile drain sampling sites during March 2015–February 2016. Measurement

Mean ± 1SE

Median

High

Low

SRP (mg/L) TP (mg/L) %SRP

0.093 ± 0.011 0.136 ± 0.013 60 ± 3

0.064 0.102 69

0.447 0.560 89

b0.005† 0.010

†

†

SRP concentration is below the detection limit.

Please cite this article as: Clement, D.R., Steinman, A.D., Phosphorus loading and ecological impacts from agricultural tile drains in a west Michigan watershed, J. Great Lakes Res. (2016), http://dx.doi.org/10.1016/j.jglr.2016.10.016

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D.R. Clement, A.D. Steinman / Journal of Great Lakes Research xxx (2016) xxx–xxx

Table 2 Results of Kruskal-Wallis tests comparing tile drain water soluble reactive phosphorus (SRP), total phosphorus (TP), and percent SRP concentrations when grouped by site location or time of sampling. Phosphorus fraction

Grouping

Chi-square

df

p-value

SRP

Location Time Location Time Location Time

59.834 3.4993 54.53 4.737 43.552 8.5723

8 11 8 11 8 11

b0.001⁎⁎⁎ 0.9823 b0.001⁎⁎⁎ 0.9432 b0.001⁎⁎⁎

TP %SRP

0.6613

⁎⁎⁎ Significant at the 0.001 probability level.

3a, respectively). However, discharge from the tile drain systems varied by sample site location and season; there was a highly significant effect of site location on SRP load (X2 = 42.889, p b 0.001) and TP load (X2 = 38.726, p b 0.001). Hence, site 4 with the highest mean discharge rate (1.348 L/s) resulted in the highest mean SRP (0.207 mg/s) and TP loads (0.269 mg/s; Figs. 2b and 3b, respectively). Conversely, site 2 with the lowest discharge rate (0.020 L/s), also had the lowest mean SRP (0.002 mg/s) and TP loads (0.003 mg/s; Figs. 2b and 3b, respectively). The P concentrations varied the most at site 9 (SRP CV = 0.94; TP CV = 0.77) and the least at site 2 (SRP CV = 0.21; TP CV = 0.21). When normalized by area drained by the tile system, site 4 had the highest mean TP yield (0.010 mg/s ha− 1) and site 2 had the lowest mean TP yield (b0.001 mg/s ha−1). Annual SRP loads from the tile drain sample sites ranged from 0.002 kg/ha to 0.248 kg/ha, and annual TP loads ranged from 0.003 kg/ha to 0.322 kg/ha. While sample date had no significant effect on concentration, time did have a highly significant effect on both SRP load (X2 = 32.151, p b 0.001) and TP load (X2 = 37.480, p b 0.001). All sampling sites (n = 9) were actively flowing during March–June 2015, whereas the lowest number of sites (n = 5) were flowing in October 2015. Similarly, SRP and TP loads were lowest during October 2015 (0.003 mg/s SRP; 0.004 mg/s TP) and highest during February 2016 (0.117 mg/s SRP; 0.159 mg/s TP) (Fig. 4). Overall, the highest P loads occurred during and post-snowmelt in both 2015 and 2016 (Fig. 4). The area drained by each sampled tile system ranged from 2.8 to 32.4 ha and varied in their land management practices (Table 3) based on information from the landowners. Six sampling sites drained fields containing corn, whereas two contained at least some soybeans. In addition, inorganic P fertilizer was generally applied during planting at the beginning of the growing season with the exception of sites 1 and 7. The smallest field, site 1, drains only 2.8 ha and is unique because it is

a community farm growing a wide variety of crops with the use of fish emulsion fertilizer. Most fields drained by the sampled tile drain sites did not use winter cover crops, with the exception of site 9. The majority of sites were classified as hydrologic type C soil, which has low-moderate infiltration rates (USDA SSURGO). Percent SRP and hectares drained by the tile system were significantly and positively correlated (Fig. 5). Site SRP and TP concentrations also were positively related with hectares drained, but were not statistically significant. No other relationship between tile drain P concentrations and a specific land management factor was apparent. Due to lack of flow from some tile outlets, not all sampling sites were used in each bioassay. Bioassays conducted in the spring used water from sites 1–8; those in the summer used sites 3, 4, 5, 7, 8, and 9; and bioassays in the fall used sites 1, 3, 4, 8, and 9. Contrary to expectations, none of the bioassays revealed a positive relationship between tile water SRP concentration and mean change in Chl a concentration (Table 4). The dominant algal taxa changed over the 7-day incubation during all three seasonal Lake Macatawa phytoplankton bioassays (Fig. 6). The spring bioassay inoculum was dominated by two diatom (Bacillariophyta) genera: Asterionella and Aulocoseira. Asterionella increased its dominance after incubation in tile drain water but not to the same extent in the Lake Macatawa control flasks (Fig. 6). During the summer bioassay, filamentous Oscillatoria (Cyanobacteria) was the dominant genus in the inoculum. However, after incubation in tile drain water dominance shifted to Synedra (Bacillariophyta). Oscillatoria remained dominant in the Lake Macatawa water controls along with larger populations of Synedra and Pediastrum (Chlorophyta) (Fig. 6). Similarly, the initial community of the fall bioassay primarily consisted of Oscillatoria, and the filamentous cyanobacteria remained dominant after incubation in tile drain water with an increase in Aulocoseira. There was little change in the fall bioassay community when incubated in Lake Macatawa water (Fig. 6). With the exception of Oscillatoria in the fall bioassay, there was little response from potential producers of cyanotoxins such as Microcystis or Anabaena. Microcystis was present in some initial community samples, but its proportion in the community did not increase under any treatment (data not shown). Discussion Our survey results indicate that tile drains can discharge effluent with very high concentrations of bioavailable P to the drainage ditches in the Macatawa Watershed. Percent SRP measurements frequently exceeded 50% at the majority of sampling sites, with SRP concentrations 1–2 orders of magnitude greater than what is measured in Lake

Fig. 2. (a) Mean soluble reactive phosphorus (SRP ± SE) concentration (X2 = 59.834, p b 0.001); and (b) mean SRP load (±SE) at each sample site (X2 = 42.889, p b 0.001).

Please cite this article as: Clement, D.R., Steinman, A.D., Phosphorus loading and ecological impacts from agricultural tile drains in a west Michigan watershed, J. Great Lakes Res. (2016), http://dx.doi.org/10.1016/j.jglr.2016.10.016

D.R. Clement, A.D. Steinman / Journal of Great Lakes Research xxx (2016) xxx–xxx

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Fig. 3. (a) Mean total phosphorus (TP ± SE) concentration (X2 = 54.53, p b 0.001); and (b) TP load (±SE) at each sample site (X2 = 38.726, p b 0.001).

Macatawa (Holden, 2014). Percent SRP measured in other tile drain effluent studies is highly variable. Macrae et al. (2007) found percent SRP b20% during the summer, but met or exceeded 50% during the winter and spring thaw. Similar to %SRP in the Macatawa Watershed, Gentry et al. (2007) found flow-weighted dissolved reactive phosphorus (DRP) concentrations ranging from 50% to 73% of corresponding TP concentrations, with the exception of one year with relatively high precipitation. Precipitation also influenced concentrations measured by Lam et al. (in press), who observed DRP:TP of about 0.8 during baseflow but lower ratios during high flow events, presumably due to dilution by rainwater. Percent SRP did not vary significantly over time in the Macatawa watershed, although absolute amounts did vary with time. This suggests that at least some bioavailable P was being transported from the tile drains to drainage ditches and likely further downstream throughout the year. Despite its year-round availability, it is unclear how much of this bioavailable P reaches Lake Macatawa given the opportunity for

assimilation and/or adsorption. Retention in streams can occur via a number of mechanisms, including uptake by algae, sorption onto Fe or Al hydroxides, assimilation by microbes, and sedimentation of particulate matter on flood plains (Gelbrecht et al., 2005; Reddy et al., 1999). In addition, deposition of particle-bound P on floodplains is an important aspect of ditch management; two-stage ditches represent a possible best management practice (BMP) to increase P retention in agricultural channels (Davis et al., 2015). It is also possible that, if sufficient P is retained in the sediment, then internal loading can occur (cf. Van Nguyen et al., 2016), similar to what occurs in lakes (Søndergaard et al., 2003). Studies are needed in the Macatawa watershed, and elsewhere, on P retention and uptake lengths in reaches with and without BMPs. We used the data from our nine sampling sites to estimate the contribution of P from tile drains in the entire Macatawa watershed. While these nine sample sites compose a relatively small representation of the watershed, it provides a first order estimate of P contribution. Using an

Fig. 4. Mean soluble reactive phosphorus (SRP ± SE) load and mean total phosphorus (TP ± SE) load per sample date (SRP: X2 = 32.151, p b 0.001; TP: X2 = 37.480, p b 0.001). Daily rain accumulation is also shown (National Climate Data Center – Tulip City Airport).

Please cite this article as: Clement, D.R., Steinman, A.D., Phosphorus loading and ecological impacts from agricultural tile drains in a west Michigan watershed, J. Great Lakes Res. (2016), http://dx.doi.org/10.1016/j.jglr.2016.10.016

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D.R. Clement, A.D. Steinman / Journal of Great Lakes Research xxx (2016) xxx–xxx

Table 3 Land and management factors per sample site. Site

Acres drained

Crops

Winter cover crops

Fertilizer

Tillage

Dominant soil hydrologic type††

1 2

7 65

Variety Primarily corn with some soybeans

No No

No-tillage No-tillage

A/D C

3

80

Corn

No

65

Corn

No

5

22

Corn

No

6

36

Corn

No

Disc-tilled at planting Disc-tilled at planting Disc-tilled at planting No-tillage

C

4

7

30

Soybeans

No

Fish emulsion for some crops Inorganic fertilizer at spring planting Inorganic fertilizer at spring planting; manure in the fall Inorganic fertilizer at spring planting; manure in the fall Inorganic fertilizer at spring planting; manure late summer Inorganic fertilizer at spring planting No P-containing fertilizers

B

8

50

Corn

No

Inorganic fertilizer at spring planting, manure in the fall

9

39

Corn

Yes: radish, oats, clover

Inorganic fertilizer at spring planting

Vertical tillage before planting Disc-tilled at planting & fall tillage sub-soiler Vertical till twice during planting

††

A/D, B C C

C

C

USDA SSURGO.

estimate of 25–35% of watershed land area with functional tile drain (Macatawa Area Coordinating Council, personal communication, 2016), we examined the drains with the lowest (Drain 2) and highest (Drain 4) mean TP loads (cf. Fig. 3b) to bracket the full range of P load from tile drains in the watershed. We extrapolated the mean of the instantaneous load measurements to an annual basis, converted to yield based on hectares drained by each sample site, and finally multiplied the low and high loads by the low (25%) and high (35%) estimated areas of tile drainage. At the low end, the entire tile drain system would contribute 39–90 kg/yr (85–199 lb./yr) to the watershed. In contrast, at the high end, the entire tile drain system would contribute 3737–5231 kg/yr (8238–11,533/lb. yr). Given that the TMDL for Lake Macatawa set a goal of reducing nonpoint TP sources to 15,876 kg (35,000 lb) annually (Walterhouse, 1999), tile drains could potentially contribute anywhere from 0.25% to almost 33% of the allotted nonpoint source TP load, assuming none of it was retained before reaching Lake Macatawa. Although these are coarse estimates based on instantaneous measurements, the additional positive relationships found between SRP

and TP concentrations and area drained suggest it would be prudent to: 1) map and quantify the entire tile drain system in the watershed to better estimate and manage the contribution of P by these systems; and 2) identify the optimal locations to implement tile drain management in the watershed (see below). In contrast to %SRP, the discharge rates and corresponding P loads differed by month. Tile drain discharge rates were highest during the winter and spring, resulting in higher SRP and TP loads during those seasons. These results correspond with other studies that measured tile drain P loading. For instance, Lam et al. (in press) observed the majority of P loss through tile drains in Ontario between October and May during snowmelt. In addition, the review by King et al. (2015a) lists several studies correlating tile drain P export to elevated flow. Other studies have reported similar load values from tile drains. For instance, Algoazany et al. (2007) observed median Soluble P loads of 0.10 kg P/ha in tile drain water. Gentry et al. (2007) found soluble phosphorus loads ranging from 0.2 to 1.3 kg/ha and TP loads ranging from 0.05 to 1.0 kg/ha from tile drains in Illinois. Although slightly higher than loads measured in the Macatawa Watershed, research by King et al. (2015b) found mean annual DRP and TP loads in tile drainage to be 0.39 and 0.48 kg/ha, respectively. The 2015–2016 study period in the Macatawa Watershed included a dry summer with few precipitation events. The few rain storms in the summer or fall were relatively small and did not induce flow through the consistently dry tile drain sample sites. Future summer rain events of greater intensity in this region, as predicted by climate models (Hayhoe et al., 2010), may result in greater P loads from tile drains and merit further investigation in this and other agricultural watersheds. There is a pulse of P export from an agricultural watershed during a high flow event, and tile drains can be a contributing source to this pulse (Lam et al., in press; King et al., 2015a). We acknowledge that given our monthly sampling regime, we may have missed some high P discharges immediately following storm events; hence, our results may underestimate the total contribution of tile drains to watershed P loading. However, time to outflow discharge was variable throughout

Table 4 Results of linear regression or Spearman correlation testing the relationship between soluble reactive phosphorus (SRP) concentration and mean (n = 3) change in Chl a for all seasonal bioassays. Asterisk (*) indicates statistical significance. Season

Fig. 5. Linear regression comparing mean percent soluble reactive phosphorus (%SRP ± SE) at each tile drain outlet to hectares drained by the tile system (p = 0.020).

Statistical test

Regression equation Test statistic

Spring Spearman correlation Summer Linear regression y = 132x + 9 Fall Linear regression y = 46x − 2

p-value

rho = −0.267 0.4933 R2adj = −0.145 0.6446 R2adj = −0.202 0.7104

Please cite this article as: Clement, D.R., Steinman, A.D., Phosphorus loading and ecological impacts from agricultural tile drains in a west Michigan watershed, J. Great Lakes Res. (2016), http://dx.doi.org/10.1016/j.jglr.2016.10.016

D.R. Clement, A.D. Steinman / Journal of Great Lakes Research xxx (2016) xxx–xxx

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Fig. 6. Lake Macatawa phytoplankton community based on calculated biovolume before and after the spring (top), summer (middle) and fall (bottom) bioassays. A) Initial community at the start of incubation; B) mean biovolumes of phytoplankton incubated in all tile drain water flasks; C) community after incubation in Lake Macatawa water (control flasks).

the watershed, and tile discharge points were susceptible to short-term inundation after storm events, making it logistically difficult to sample first flush events, even with autosamplers. Management to address high P loads from tile drains during the nongrowing season deserves more attention in the Macatawa Watershed. In other agricultural watersheds, constructed wetlands (Kynkäänniemi et al., 2013) or drainage control structures have shown some success in limiting P transport (Frey et al., 2013). In contrast to passively flowing drains, controlled tile systems can limit nutrient loads by restricting flow when field drainage is not crucial. Indeed, Nash et al. (2015) found that soluble P concentrations were lower in controlled tile drains than in passively flowing drains, regardless of discharge rate. Moreover, time of sample collection in the Macatawa Watershed was not compared to stream hydrographs. P concentrations have been found to peak early during a precipitation event and slightly before hydrograph peaks (Lam et al., in

press; Smith et al., 2015; Tomer et al., 2010). If this applies to the Macatawa Watershed, as we expect, a top priority should be mitigating the “first flush” of P from tile drains at the onset of a precipitation event. The seasonal bioassays allowed us to investigate the link between tile drain SRP and algal growth. A positive relationship was expected between SRP concentration and change in Chl a in the bioassays given that P is often viewed as the limiting resource in freshwater systems (Dillon and Rigler, 1974; Schindler, 1977), although this paradigm has received challenges of late (Conley et al., 2009; Harpole et al., 2011). However, the Lake Macatawa phytoplankton biomass did not respond significantly to tile drain SRP. It is possible the phytoplankton quickly assimilated the added P and became limited by other resources. Alternatively, they may have been P-saturated at the start and therefore would not respond positively to additional SRP inputs, although P measurements at the end

Please cite this article as: Clement, D.R., Steinman, A.D., Phosphorus loading and ecological impacts from agricultural tile drains in a west Michigan watershed, J. Great Lakes Res. (2016), http://dx.doi.org/10.1016/j.jglr.2016.10.016

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D.R. Clement, A.D. Steinman / Journal of Great Lakes Research xxx (2016) xxx–xxx

of the incubation (day 7) revealed P concentrations below the detection limit in most flasks; it is unclear if this uptake is associated with luxury consumption or new growth. Xu et al. (2010) experienced a similar result with bioassays in hypereutrophic Lake Taihu, China—in that case, excess available P stimulated phytoplankton growth only when N also was in excess, making P the secondarily limiting nutrient. Despite our unexpected results, the mean biovolume per bioassay flask did increase during incubation in tile drain and Lake Macatawa water, suggesting the algae were responding positively to something in the ambient water, possibly a micronutrient (cf. Drerup and Vadeboncoeur, 2016). The most notable response during the Lake Macatawa bioassay algal community was by diatoms (Bacillariophyta). Because Lake Macatawa has a history of algal blooms, we anticipated potentially toxin-producing cyanobacteria, such as Microcystis, to grow in response to the tile effluent water, especially in the summer. Microcystis thrives on high SRP concentrations and warmer water temperatures in comparison to other phytoplankton taxa (Michalak et al., 2013). It is possible that filtering the phytoplankton sample through 200 μm to remove zooplankton may have reduced the abundance of colonial phytoplankton forms such as Microcystis. Also, because the bioassays were conducted on shaker tables, the constant movement may have disrupted colony formation and growth (Cymbola et al., 2008), as calm conditions with limited water column mixing are known to facilitate Microcystis dominance in a phytoplankton bloom (Chen et al., 2003; Michalak et al., 2013). While the results of this study do not rule out the possibility that SRP originating from agricultural tile drains helps fuel algal blooms in Lake Macatawa, it does suggest that other factors may also be involved. Several caveats apply to our results. First, the sampled tile drains were not evenly distributed throughout the watershed. Although the sampled tile drains were located in priority sub-basins based on high P and turbidity values, their clustered distribution may bias our results. Second, the calculation of percent P generated from tile drains was based on the assumption that our monthly load and P concentration measurements were representative of the entire year, which is unlikely. We address this uncertainty by calculating low and high estimates of P loads from the drains to provide a range of values. Third, bioassays do not accurately reflect all processes of a lake ecosystem (Schindler et al., 2008) given their enclosed nature. Future research should focus on sampling tile drains from different areas of the watershed, the fate and transport of tile drain effluent after reaching the surface drainage system, the potential effects of nitrogen (N) given their prevalence in both inorganic and organic fertilizers, and the role of internal P loading from the lake sediments to better assess the contributing factors to potential algal blooms in Lake Macatawa. Conclusions Our field survey results indicated that P concentrations from the tile drain effluent did not vary by time but did vary by site, with variable discharge accounting for most of the differences in P loads. SRP and TP loads were highest during the non-growing season, corresponding to periods of active and post-snowmelt runoff. In addition, the SRP:TP ratios measured at the tile drain outlets were positively correlated with area drained by the tile system. The bioassay results did not show a positive relationship between tile drain SRP and algal Chl a, which was contrary to our expectations. Our findings have management implications; given that SRP and TP loads were highest during the non-growing season, P retention on agricultural fields during the winter and spring is crucial to reducing nutrient loss through tile drains. In addition, it is important to determine the reason why P associated with tile drains did not stimulate algal blooms; as best management practices are implemented in this watershed over the next decade, reductions in P concentration may result in a greater likelihood of P limitation by algae. Identifying what resource(s) control their growth will be an important management concern.

Acknowledgements Many thanks go to Mark Luttenton, Rick Rediske, Graham Peaslee, Maggie Oudsema, Brian Scull, Kurt Thompson, Mary Ogdahl, Emily Kindervater, Mike Hassett, and Brittany Jacobs for their help and input. Funding for this project was provided by the Michigan Chapter of the North American Lake Management Society, Michigan Lake and Stream Associations Research Grant, and the Grand Valley State University Presidential Research Grant. We also thank the producers who allowed us access to sample sites on their fields, as well as the Macatawa Area Coordinating Council for helping establish relationships with stakeholders in the watershed. Constructive comments and edits by Dave Allan, Joe Makarewicz, and an anonymous reviewer substantively improved the manuscript. References Algoazany, A.S., Kalita, P.K., Czapar, G.F., Mitchell, J.K., 2007. Phosphorus transport through subsurface drainage and surface runoff from a flat watershed in East Central Illinois, USA. J. Environ. 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Please cite this article as: Clement, D.R., Steinman, A.D., Phosphorus loading and ecological impacts from agricultural tile drains in a west Michigan watershed, J. Great Lakes Res. (2016), http://dx.doi.org/10.1016/j.jglr.2016.10.016