Water Depth Underpins the Relative Roles and Fates of Nitrogen and Phosphorus in Lakes

Eutrophication
mitigation is an ongoing priority for aquatic ecosystems.
However, the current eutrophication control strategies (phosphorus
(P) and/or nitrogen (N)) are guided mainly by nutrient addition experiments
in small waters without encompassing all in-lake biogeochemical processes
that are associated largely with lake morphological characteristics.
Here, we use a global lake data set (573 lakes) to show that the relative
roles of N vs P in affecting eutrophication are underpinned by water
depth. Mean depth and maximum depth relative to mixing depth were
used to distinguish shallow (mixing depth > maximum depth), deep
(mixing
depth < mean depth), and transitional (mean depth ≤ mixing
depth ≤ maximum depth) lakes in this study. TN/TP ratio (by
mass) was used as an indicator of potential lake nutrient limitation,
i.e., N only limitation if N/P < 9, N + P colimitation if 9 ≤
N/P < 22.6, and P only limitation if N/P ≥ 22.6. The results
show that eutrophication is favored in shallow lakes, frequently (66.2%)
with N limitation while P limitation predominated (94.4%) in most
lakes but especially in deep ones. The importance of N limitation
increases but P limitation decreases with lake trophic status while
N and P colimitation occurs primarily (59.4%) in eutrophic lakes.
These results demonstrate that phosphorus reduction can mitigate eutrophication
in most large lakes but a dual N and P reduction may be needed in
eutrophic lakes, especially in shallow ones (or bays). Our analysis
helps clarify the long debate over whether N, P, or both control primary
production. While these results imply that more resources be invested
in nitrogen management, given the high costs of nitrogen pollution
reduction, more comprehensive results from carefully designed experiments
at different scales are needed to further verify this modification
of the existing eutrophication mitigation paradigm.