Plant Productivity of Inland Waters

Inland water habitats have probably existed for as long as continental crust has been exposed to the atmosphere (3.5 Gy), although individual inland water bodies last for days to tens of millions of years. Inland water habitats are more physicochemically diverse than marine or terrestrial habitats for photolithotrophs in terms of spectral distribution of downwelling radiation and the range of concentrations of individual solute species. Now, and over the last 500–1000 My, there have been significant chemical inputs to inland waters from biologically enhanced weathering by, and productivity of, terrestrial biota. Photosynthetic organisms in inland waters are, at the Phylum (Division) and Kingdom level, much more diverse than those on land, but rather less diverse than those in the sea. All of the planktonic organisms involved in inland water photosynthesis originated directly from marine habitats. While benthic algae (rhizophytic, haptophytic) also came directly from marine ancestors, pleustophytic, rhizophytic and haptophytic embryophytes (bryophytes, tracheophytes) are all derived from terrestrial plants. Estimates of primary production in inland waters have come from measurements of changes in biomass, measurements of net gas exchange, tracer (mainly inorganic C) uptake, and chlorophyll fluorescence measurements, the latter being the main theme of this book. The methods show a wide range of values as a result of varied light supply (mixing depth, light attenuation) and nutrient availability. Eutrophication (more N, P) increases primary productivity, often at the expense of biodiversity, and may reduce C0₂ outgassing from inland waters. Among the contributions that fluorescence techniques can make to the study of primary productivity in inland waters the most important are remote sensing of chlorophyll a fluorescence, continuous in situ measurement of algal fluorescence as a proxy for biomass and further development of fluorescence methods for estimating primary productivity.