Concordance of chemically inferred and assayed nutrient limitation of phytoplankton along a depth gradient of alpine lakes in the Canadian Rockies

Deposition of nutrients over mountainous regions in many parts of the world is increasing due to human activities. To assess the level of agreement between chemically-inferred and experimental evidence of the potential sensitivity of alpine lake ecosystems along the Eastern Front range of the Canadian Rockies to increased deposition of nitrogen (N) and phosphorus (P), nutrient limitation was investigated in 29 sites using water chemistry and nutrient amendment bioassays of phytoplankton. The two lines of evidence agreed in 55% of all cases with discrepancies mainly involving instances of co-limitation. Dissolved inorganic N:total P (DIN:TP) mass ratios frequently suggested P limitation while the corresponding bioassays supported meta-analytical evidence of the prevalence of co-limitation of freshwater primary producers. Based on previous evidence of low DIN:TP mass ratios and experimental N amendments stimulating algal communities in shallower alpine sites, we hypothesized that nutrient limitation of phytoplankton shifts from being P- to N-driven with decreasing lake depth. However, we found little evidence of N limitation from bioassays of phytoplankton from the shallower (< 2-m deep) lakes. Most phytoplankton were co-limited by P plus N amendments (42%), while 38% responded to only P, 10% to only N, and 10% to neither nutrient. Overall, DIN:TP mass ratios and bioassay results showed that chromophytes and chlorophytes drove the greater responsiveness of whole communities to P relative to N. Since the short-term bioassay experiments often demonstrated the alpine phytoplankton to be NP-co-limited, our findings highlight a need for further research into potential synergistic fertilizing effects of anthropogenic N emissions and P deposition from forest fires on lake ecosystems at high elevations.