Nutrient cycling in the palaeorecord: Fluxes from terrestrial to aquatic ecosystems

Transfer of nutrients from terrestrial to aquatic ecosystems is a natural process with climatic, biotic, and geologic controls. Recently, increasing concern
about human manipulation of global nutrient cycles has required a long-term approach to assessing the nutrient status of aquatic systems. Data available
in palaeorecords can assess current trophic status, baseline conditions, and long-term processes controlling nutrient fluxes on decadal to millennial
timescales. Here, we review three palaeolimnological methods used to reconstruct nutrient cycling: (1) chemical compounds preserved in lacustrine
sediment, (2) aquatic biotic indicators (often using a quantitative transfer function), and (3) quantitative empirical sediment flux estimates. The millennialscale
regulation of nutrient cycling by climate and catchment geochemistry leads to a gradual trajectory of dystrophication over the Holocene in many
temperate lakes. In many systems, the magnitude of recent anthropogenic changes to nutrient cycling is large compared with natural fluctuations, but
this perspective could also be due to the selection of study sites that are currently experiencing eutrophication. Increased nutrient loading to aquatic
systems is not always accompanied by decreased ecosystem function. The powerful temporal perspective from palaeolimnology can be complemented
with modern mechanistic approaches to lead to increased understanding of the rates, patterns, and mechanisms of nutrient fluxes.