The following methods are taken directly from the associated manuscript which is under review at Limnology and Oceanography Letters.
2018 Field Survey
We collected surface grab samples (~0.5 m below the surface) in lakes in summer 2018. In most lakes, we collected samples in the pelagic zone, except for two lakes, which were sampled by wading in to the near shore area due to inclement weather. Three lakes in Pennsylvania were sampled twice and chemical parameters were averaged. We collected wetland samples as surface grab samples near the shore of each wetland.
Water samples were typically subsampled and filtered (when applicable) within 48 hours of collection, though some samples were not subsampled until 72 hours after collection. Samples were then frozen and kept in a dark environment until analysis could occur.
We analyzed phosphorus and nitrogen species via spectrophotometry and DOC concentrations using a Shimazdu total organic carbon L analyzer, and DOC specific absorbance using a Shimazdu spectrophotometer. We converted blank corrected absorbance values at 440 nm to the Naperian absorption coefficient (Green and Blough 1994; Jane and Rose 2018). We estimated water color of our samples based on the absorbance at 440 nm (Cuthbert and Del Giorgio, 1992).
To determine the amount of P bound to DOM, we estimated the dissolved organic phosphorus (DOP) as the difference between total dissolved phosphorus (TDP) and dissolved molybdate-reactive phosphorus (DRP) (e.g. Thompson and Cotner, 2018). We estimated dissolved organic nitrogen (DON) in a similar manner as DOP. We subtracted dissolved nitrate (NO3-) and total ammonium (NH4+) from total dissolved nitrogen (TDN).
PAR irradiance estimates
We estimated the amount of total radiation based on latitude using the 'solaR' R package (Perpinan, 2012) during June 21st at noon of each year (i.e. the summer solstice) for each lake in the long-term Adirondack lake data set. We made the assumption that only 48% of light reaches the surface of the Earth, and of that 45% is PAR (Kirk, 1994). From here, we estimated albedo at noon using methods described in Briegleb et al. (1986) equation 8. Given PAR at the lake surface and Kd, we estimated light availability throughout the water column in 0.1 meter depth intervals, as described in Kirk (1994); E_d (z)= E_d (0) e^(〖-K〗_d z). In this equation, Ed(z) is the irradiance at depth (z) and Kd is the light attenuation coefficient at depth (z) throughout the water column, and e is Euler's number (2.718). We next estimated the euphotic zone as the depth where 1% of the surface light remains and then quantified the proportion of the water column in the euphotic zone. We used a generic photosynthesis irradiance (PI) curve (Kirk 1994; Belay 1981) to estimate whole-lake primary productivity based on light availability in each layer of the water column. Finally, we quantified trends in whole-lake average photosynthetic potential using the ‘rLakeAnlayzer' V1.11.4 R package (Read et al. 2011; Winslow et al. 2019) by assuming a simple cone bathymetry in each lake and using the layer.temperature function (except inputting photosynthetic rate at each depth instead of temperature).
References
Belay, A. (1981). An experimental investigation of inhibition of phytoplankton photosynthesis at lake surfaces. New Phytologist, 89(1), 61-74.
Briegleb, B. P., Minnis, P., Ramanathan, V., and Harrison, E. (1986). Comparison of regional clear-sky albedos inferred from satellite observations and model computations. Journal of Climate and Applied Meteorology, 25(2), 214-226.
Cuthbert, I. D., and Giorgio, P. del. (1992). Toward a standard method of measuring color in freshwater. Limnology and Oceanography, 37(6), 1319-1326. https://doi.org/10.4319/lo.1992.37.6.1319.
Green, S. A., and Blough, N. V. (1994). Optical absorption and fluorescence properties of chromophoric dissolved organic matter in natural waters. Limnology and Oceanography, 39(8), 1903-1916. https://doi.org/10.4319/lo.1994.39.8.1903.
Jane, S. F., and Rose, K. C. (2018). Carbon quality regulates the temperature dependence of aquatic ecosystem respiration. Freshwater Biology, 63(11), 1407-1419. https://doi.org/10.1111/fwb.13168.
Kirk, J. T. (1994). Light and photosynthesis in aquatic ecosystems. Cambridge university press.
Perpiñán, O. (2012). solaR: Solar Radiation and Photovoltaic Systems with R, Journal of Statistical Software, 50(9), 1-32. URL http://www.jstatsoft.org/v50/i09/.
Read, J. S., Hamilton, D. P., Jones, I. D., Muraoka, K., Winslow, L. A., Kroiss, R., ... and Gaiser, E. (2011). Derivation of lake mixing and stratification indices from high-resolution lake buoy data. Environmental Modelling and Software, 26(11), 1325-1336.
Thompson, S. K., and Cotner, J. B. (2018). Bioavailability of Dissolved Organic Phosphorus in Temperate Lakes. Frontiers in Environmental Science, 6. https://doi.org/10.3389/fenvs.2018.00062.
Winslow, L., Read, J., Woolway, R., Brentrup, J., Leach, T., Zwart, J., Albers, S., and
Collinge, D., (2019). rLakeAnalyzer: Lake Physics Tools. R package version 1.11.4.1.
https://CRAN.R-project.org/package=rLakeAnalyzer.
