Evidence of a Biological Response to the Nitrogen Cascade in UK Upland Lakes?

Recent research has begun to undermine the prevailing paradigm that biological productivity in remote, oligotrophic lakes is limited by the availability of phosphorus (P). Many nutrient poor fresh waters are nitrogen (N) limited and, in areas that have received high levels of anthropogenic N deposition relative to baselines, lakes have switched from N to P limitation. Palaeoecological data from sensitive arctic and alpine lakes suggests that algal communities have responded to the increase in available nutrients. Direct evidence of a response to enhanced N deposition from less sensitive, oligotrophic lakes is less compelling. Is the biological response limited to ultraoligotrophic freshwaters or are the effects of enhanced N deposition more widespread? Palaeoecological data from 18 oligotrophic lakes, from upland regions located throughout the UK are presented and summarised. These data comprise geochemical and stable isotope measurements, and diatom and algal pigment proxy records. {δ ^15}N and carbon-nitrogen ratio (C:N) records document a widespread and consistent geochemical perturbation of the in-lake N-cycle and the stimulation of autochthonous production dating from early to mid 1900s. Regional bioassay surveys across a range of UK upland lakes indicates that N-limitation of phytoplankton primary production is common. At sites receiving high N deposition inputs P-limitation is prevalent suggestive of a switch from limitation by N to P. Biological palaeoproxies document significant changes in algal composition, but direct interpretation as a nutrient N response is hampered at many sites by the dual acidifying role of N. We discuss the palaeoecological evidence for a biological response in light of results from intensive studies using natural abundance isotope tracing of {δ ^{15}N_NO3 and {δ 18}O_NO3, seasonal assessments of nutrient status of UK upland lakes, and biological monitoring records. In addition we evaluate the degree to which climate, N deposition and other drivers of change (e.g. DOC increase) are influencing the biology of UK upland freshwaters. Combined, these data suggest a complex interplay of drivers is responsible for biological change in UK upland lakes and that enhanced N deposition plays a key role in this change.}