Literature search
We searched the literature extensively to compile studies that represented a wide range of aquatic (from pelagic to sediment flats to submerged macrophyte beds) and terrestrial (from marshlands to grasslands to forests) systems. Some systems have been studied more than others and were better represented in the literature, but even the values obtained for the less studied systems, albeit not numerous, covered a wide spread. Thus the data set reflects the availability of information in the scientific record.