Sample Collection:

Grab samples are collected monthly at the Parker and Ipswich dams. Water is filtered and/or partitioned for analysis for suspended solids, particulate organic nitrogen (PON), total dissolved nitrogen (TDN), ammonium (NH4), nitrate (NO3), dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), particulate phosphorus (PPO4), total dissolved phosphorus (TDP), phosphate (PO4), particulate organic carbon (POC), dissolved organic carbon (DOC). Analyses are conducted according to PIE-LTER protocols (http://ecosystems.mbl.edu/pie/data/method.htm).

Discharge:

Discharge data is mined from the USGS website (http://waterdata.usgs.gov/nwis) for the Parker River gauge at Byfield, MA and the Ipswich River gauge at Ipswich, MA. Since the gauges are NOT located directly at the dams where nutrient sampling occurs, the USGS discharge data are multiplied by a correction factor (1.1812 for the Parker data, and 1.1964 for the Ipswich data) to obtain discharge over the respective dams.

Loading/Yield Calculations:

Monthly nutrient concentrations are multiplied by total monthly discharge over the dams to estimate nutrient loading on a monthly timescale. Monthly loading numbers are added together to obtain annual loading per water year. Water years are determined according to USGS data updates, and generally start in September. The numerical year given to a water year is the new year reached in January during the September-September cycle (i.e. Sept 2003 - Sept 2004 is water year 2004). Yield calculations are derived from the loading calculations by dividing the nutrient loading by watershed area upstream of the dam (at Parker this is 65.2km2, and Ipswich this is 387.4km2).  Total N and P loads and yields reflect organic and inorganic forms of nitrogen. Total C loads and yields reflect only organic forms of carbon, ie dissolved inorganic carbon was not measured and is not included.

Sample yield calculation:

1 tonne = 1000kg

1km^2 = 100 hectares

Ipswich Dam, Nitrogen Yield, 1994

126 tonnes x (1000 kg/tonne) = 126,000 kg

387.4 km^2 x (100 hectares/km^2) = 38,740 hectares

126,000 kg / 38,740 hectares = 3.25 kg / hectare / year

Acknowledgements:

We thank the many research assistants and students that have been involved in sample collection and analysis for this long term dataset:  Sam Kelsey, Chris Whitney, Erin Vanderjeugdt, Nat Morse, Andrew Robison, Tyler Messerschmidt, Samantha Bond, Colin Millar, Rebecca Prosser, Emily Gaines, Catherine Caruso, Aaron Strong, Christina Maki, Mac Lee, Corey Lawrence, John Logan, Josh Goldstein, Greg Peterson, Nat Weston, Ishi Buffam, Hap Garritt.