Nitrogen deposition making lakes more regulated by phosphorus

Nitrogen deposition is increased the extent to which lake algal populations are regulated by phosphorus, shifting lake food webs.  Because, the patterns of human amplification of nitrogen and phosphorus trasport are different this should drive different patterns in lakes in different regions.

James Elser and other write in Science Shifts in Lake N:P Stoichiometry and Nutrient Limitation Driven by Atmospheric Nitrogen Deposition (2009 326 (5954):835).  From the abstract:

Human activities have more than doubled the amount of nitrogen (N) circulating in the biosphere. One major pathway of this anthropogenic N input into ecosystems has been increased regional deposition from the atmosphere. Here we show that atmospheric N deposition increased the stoichiometric ratio of N and phosphorus (P) in lakes in Norway, Sweden, and Colorado, United States, and, as a result, patterns of ecological nutrient limitation were shifted. Under low N deposition, phytoplankton growth is generally N-limited; however, in high–N deposition lakes, phytoplankton growth is consistently P-limited.

They conclude:

Our findings show that, despite the potential of watershed vegetation uptake and sediment denitrification to buffer lakes against elevated N loading, increased inputs of anthropogenic N have accumulated in receiving waters. As a result, shifts in lake N:P stoichiometry have altered ecological nutrient limitation of phytoplankton growth. Phytoplankton in lakes that are less influenced by anthropogenic inputs experience relatively balanced or N-deficient nutrient supplies, but enhanced N inputs from the atmosphere during the past several decades of human industrialization and population expansion appear to have produced regional phytoplankton P limitation.

Producer diversity is likely to be low when resource supply ratios are skewed in favor of one particular nutrient relative to others (11, 18). Thus, increased N loading from the atmosphere may reduce lake phytoplankton biodiversity, similar to anticipated effects of N deposition on plant diversity in terrestrial ecosystems (19, 20), by possibly favoring those relatively few species that are best able to compete for the limiting P.

… Thus, sustained N deposition that generates stoichiometric imbalance between P-limited, low-P phytoplankton and their P-rich zooplankton consumers (12) may result in reduced production of higher trophic levels, such as fish. Projected increases in global atmospheric N transport during the coming decades (24) are likely to substantially influence the ecology of lake food webs, even in lakes far from direct human disturbance.

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