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METHODS

CURRENT ASSESSMENT AND MONITORING APPROACH

138 Iowa lakes were sampled three times over the 2016 monitoring season. Three sets of samples in a year have been shown to offer adequate precision for the estimation of annual mean water quality characteristics (Downing et al., 2006a). Sampling times were arranged to represent spring-early summer, mid-summer, and late summer-early fall. Monitoring samples were taken at one site in each lake basin representing the open-water zone, because this has been shown to be an appropriate sampling method for most lakes (Bachmann et al., 1980; Bachmann et al., 1994; Downing et al., 2006b). Two field teams traveled throughout the state collecting water samples representing the surface mixed layer of each lake. Sampling was performed using a 2 m integrated water column sampler because this method is more efficient and has been shown to be comparable to full-column upper mixed zone sampling. This sampling scheme has been designed to ensure comparability with previous surveys (Bachmann et al., 1980; Bachmann et al., 1994; Downing et al., 2006a).

FIELD SAMPLE COLLECTION

Field technicians sampled at the deepest point in each lake basin, as determined by sonar and existing bathymetric maps, and recorded the spatial locations of sampling points using a Global Positioning System (GPS). In-field deployment of YSI Multi- parameter Data Sondes provided profile data for temperature, dissolved oxygen, specific conductivity, pH, dissolved solids, and turbidity.

YSI profile data were used to determine the presence and associated depth (when present) of the thermocline. Thermocline depth was defined as the region of maximum rate of temperature decrease (>1 ºC per meter) with respect to depth (Kalff 2002). If a thermocline was present between 1 and 2 meters, the water sample was taken from the surface to the depth of the thermocline. If a thermocline was absent or was deeper than 2 meters, the water column was sampled from the surface to a depth of 2 meters. A >1°C per meter temperature change less than 1 meter depth was considered an ephemeral thermocline based on daily heating and was not reported.

Once these measurements were made and before collecting a water sample, the sampling equipment was triple rinsed with lake water on the opposite side of the boat from where the sample would be collected to avoid sample contamination (Wedepohl et al., 1990). A mixed column sample was then collected using an integrated water column sampler from the appropriate sample depth (as determined by thermocline depth). The water was emptied from the integrated water column sampler into a pre-rinsed holding container and additional column samples were collected and combined until enough water was collected to fill all required sample bottles.

Once a sufficient volume of water had been obtained, the holding container was agitated to mix the sample before transferring to the bottles. The water was then poured from the holding container into pre-cleaned sampling bottles making sure that the holding container and sample bottles do not touch. This method was used to collect water samples for nutrient, chlorophyll, suspended solid, and phytoplankton analyses. Immediately after returning to shore, phytoplankton samples were preserved with 0.5 mL Lugol’s solution per 100 mL of sample collected (American Public Health Association, 1998).

Zooplankton samples were collected by vertically towing a Wisconsin net (63 µm mesh size) from the depth of the thermocline to the surface with a maximum depth of 9 m. If a thermocline did not exist, a sample was taken from approximately 0.5 m off the bottom of the lake to the surface or from a depth of 9 m to the surface. To collect the sample, the Wisconsin net was lowered to the determined depth and then raised vertically at an even speed of approximately 0.5 m/s (American Public Health Association, 1998). The sample was rinsed down into the filter cup using a squirt bottle containing deionized water. The filter cup was removed and the sample was rinsed into a 125 mL bottle and filled approximately half full (about 60 mL). Immediately after returning to shore, zooplankton samples were preserved with approximately 60 ml of 10% Formalin solution containing sucrose (Haney and Hall, 1973) to create a 5% Formalin solution.

All sample bottles were labeled with sample location, unique site ID, sampler’s name(s), date and time of collection, and depth of column sampled (or vertical tow depth for zooplankton). All samples were kept cold until delivered to the laboratory for analysis.

LABORATORY METHODS

WATER CHEMISTRY

Soluble Reactive Phosphorus as P

Soluble reactive phosphorus samples were analyzed with a Seal Analytical AQ2 Discrete Analyzer using EPA method 365.1 v2 (USEPA, 1993). Analysis involves filtering the samples through a 0.45 µm syringe-tip filter to remove particulate matter, creating standard curves (forced through origin) daily, and running known standards and spikes with each group of samples.

Total Phosphorus

Total phosphorus samples were analyzed with a Seal Analytical AQ2 Discrete Analyzer using EPA method 365.1 v2 (USEPA, 1993). Analysis involves digesting the samples with persulfate, creating standard curves (forced through origin) daily, and running known standards and spikes with each group of samples.

Ammonia+Ammonium as N

Ammonia + ammonium (NHx) samples were analyzed with a Seal Analytical AQ2 Discrete Analyzer using EPA method 350.1 v2 (USEPA, 1993). Analysis involves filtering samples through a 0.45 µm syringe-tip filter to remove particulate matter, generating standard curves (forced through origin) daily and running known standards and spikes with each group of samples.

Nitrate+Nitrite as N

Nitrate + nitrite (NOx) samples were analyzed with a Seal Analytical AQ2 Discrete Analyzer using EPA method 353.2 v2 (USEPA, 1993). Analysis involves filtering samples through a 0.45 µm syringe-tip filter to remove particulate matter, passing samples through an open tubular cadmium coil treated with copper sulfate, generating standard curves (forced through origin) daily and running known standards and spikes with each group of samples.

Total Kjeldahl Nitrogen

Total Kjeldahl Nitrogen (TKN) samples were analyzed with a Seal Analytical AQ2 Discrete Analyzer using EPA method 351.2 v2 (EPA, 1993). Analysis involves digesting samples in a Technicon BD-40 block digestor, creating standard curves (forced through origin) daily, and running known standards and spikes with each group of samples.

Dissolved Organic Carbon

Dissolved organic carbon samples were analyzed with a Shimadzu TOC-V Analyzer using the high-temperature combustion method in Standard Methods (American Public Health Association, 1998). Analysis involves filtering samples through a 0.45 µm syringe-tip filter, acidifying and purging with CO2 to remove inorganic carbon, creating standard curves (forced through origin) daily, and running known standards with each group of samples.

Suspended Solids (Total, Volatile, & Inorganic)

Suspended solids samples were analyzed using the non-filterable residue method in Standard Methods (American Public Health Association, 1998) and the volatile residue EPA method 160.4 (Kopp and McKee, 1983). Analysis involves filtering samples through a pre-rinsed and pre-weighed GF/C filter, drying at 105°C, weighing to determine TSS, combusting at 550°C, and weighing to determine VSS and ISS fractions. Known standards are run with each group of samples.

Chlorophyll a (corrected for pheophytin)

Chlorophyll a samples were analyzed with a TD-700 Fluorometer using the non- acidified fluorometry EPA method 445.0 (Arar & Collins, 1997). Analysis involves filtering samples through a GF/C filter, extracting in 100% acetone using a probe sonicator or sonic bath and steeping overnight (Jeffrey et al., 1997), creating standard curves (forced through origin) annually, and running known standards with each group of samples.

Total Alkalinity as CaCO3

Total alkalinity samples were analyzed with a Thermo Orion 950 Analytical Titrator using the titration method in Standard Methods (American Public Health Association, 1998). Analysis involves a temperature-compensated pH measurement and automated titration to pH = 4.5.

BIOLOGY ANALYSIS

Phytoplankton

Phytoplankton samples were concentrated, sub-sampled, and examined using an Olympus BH-2 compound microscope at 400× power (American Public Health Association, 1998). Samples were analyzed using a nanoplankton counting cell (similar to a Palmer-Maloney counting cell) using methods similar to those described in Standard Methods (American Public Health Association, 1998) and protocols for the analysis of algal samples collected for the U.S. Geological Survey National Water- Quality Assessment Program (Charles et al., 2002). Phytoplankton were identified to genus, counted, and measured. Simple geometric model formulae were used to calculate the biovolume per liter of plankton divisions and the total phytoplankton community (Findenegg, 1974; Hillebrand et al., 1999). Biomass can be estimated from biovolume assuming a density of 1.1 g cm-3 (Holmes et al., 1969). For ease of conversion, we calculated wet biomass from biovolume using a density of 1 g cm-3.

Zooplankton

Zooplankton samples were concentrated, sub-sampled, and examined using a Nikon SMZ 1500 Stereoscopic Zoom Microscope and photographed using a Qimaging Retiga 2000R Digital Still Camera at 96x power (American Public Health Association, 1998). Zooplankton were identified to family, counted, and measured and length-weight regressions were used to calculate the dry mass per liter of the Cladocera and Copepoda (Dumont, et al., 1975). Rotifera dry mass per liter was determined by first calculating the biovolume per liter (Ruttner-Kolisko, 1977) then converting this measurement to dry mass per liter (Doohan, 1973 for all rotifers except Asplancha; Dumont et al., 1975 for Asplancha; McCauley, 1984).

       

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