Atmospheric nitrogen‐deposition may intensify phosphorus limitation of shallow epilithic periphyton in unproductive lakes

Summary 1. We tested whether increasing atmospheric nitrogen (N) deposition along a north–south gradient intensifies epilithic phosphorus (P) limitation in oligotrophic Swedish lakes from the north to the south. We examined the epilithic community at a shallow depth from seven northern and six southern Swedish lakes, and also compared the results with a lake located geographically between the two groups. We determined lake nutrient state, epilithic nutrient ratios and epilithic algal composition, as well as grazer N : P ratios, grazer‐epilithon N : P imbalance, and N : P cycling ratios. 2. Epilithic communities appear to be generally more N‐limited in the northern lakes and more P‐limited in the southern lakes. Lake water total N (Tot‐N) and epilithic N : P ratios were lower in northern than in southern lakes and the proportion of N2‐fixing cyanobacteria was higher in northern than in southern lakes. 3. Gastropod grazers had lower N : P imbalances and cycled less N relative to P in northern than in southern lakes. 4. Atmospheric N‐deposition showed a strong positive correlation with lake water Tot‐N and a much weaker positive correlation with epilithon N : P ratios. Atmospheric N‐deposition also correlated negatively with the proportion of N2‐fixing cyanobacteria. 5. There are indications that increased atmospheric N‐deposition towards the south might intensify P‐limitation of epilithic algae and invertebrate grazers, although more studies are needed to show the strength and generality of our findings.