ELE-00371-2016 Si-DIN data set for Ecology Letters.csv

Columns in the CSV file are (in order from left to right) lake number in the Iowa state lake surevey, lake name, county, the UTM zone of geographical coordinates of each lake, the easting in meters, the northing in meters, the dissolved silica concentration in micro-molar units (as Si), the dissolved inorganic nitrogen concentration in micro-molar units (as N), the ratio of the two preceding concentrations, the average biomass of diatoms as a percentage of total phytoplankton, the lake area as a percentage of the total watershed area, and agricultural land as a percentage of the watershed.
Surface water sampling
was performed three times for each lake during the period of greatest flow
(May-August 2001-2005). Nutrient and algal community data were determined as
summer averages for the upper mixed zone at the deepest point in each lake. Si
was determined as dissolved silicate, and DIN was determined as the sum of
nitrite (NO2-), nitrate (NO3-),
ammonia (NH3) and ammonium (NH4+) using
standard methods (Eaton et al. 1998) and
second derivative spectroscopy (Crumpton et al.
1992) for NO2- and  NO3-. We estimated the
fraction of the phytoplankton composed of diatoms in each lake by sampling on
the same days as water chemistry samples were extracted, then settling,
identifying, and measuring biomasses of plankton in subsamples by standard methods
(Hillebrand et al. 1999). In lakes we have studied year-round,
we found no spring bloom of diatoms, and our samples were taken during the time
of year when maximal phytoplankton biomass is expected.  Therefore, our diatom analyses reflect the
composition during the period when diatom sedimentation is most likely.

Geographical analysis.
Watershed boundaries and areas under agricultural land uses were analyzed using
GIS of the temporally closest available digital coverages (2002), 30 meter
resolution, Landsat 5 TM imagery. Land cover types were according the the National Land Cover Database
(2001; Homer et al. 2007). The total
agricultural land in the watershed was considered to be the sum of maize and
soybean land use, the land area in grazed and ungrazed grasslands, “other
agriculture”, alfalfa, winter wheat, lush grass, conservation reserve lands,
and barren lands, since all of these lands are usually used for cropping
or pastureland in this region. The area of the watersheds and the lakes were
measured from the same GIS layers in order to estimate the potential for
hydraulic flushing of the lake basins.