Table 1.
Morphometric data, geographic coordinates, mean concentration of TN, TP and chlorophyll a for the lakes from the end of March to September 2011.
Figure 1.
Experimental design of the bioassays.
Treatments: Ctrl = control (no nutrient addition); +N = 250 µg N L−1 each of NaNO3 and (NH4)2SO4; +P = 200 µg P L−1 of KH2PO4; +NP = combined N+P addition; Standard light = 100 µmol photons m−2 s−1; In situ light = predicted Imix.
Figure 2.
Example chlorophyll a response patterns.
These patterns correspond to a subset of the nutrient limitation categories defined by Harpole et[37]. A) Single N limitation: a response to only one of the single treatments, in this example +N and the response to the +NP treatment is no different. B) Serial P limitation: a response to only one of the single nutrient treatments, in this example +P and a larger response to the +NP treatment. C) Independent co-limitation (primary P): a response to both single nutrient treatments with a larger response to +P and an even larger response to the +NP treatment; D) Simultaneous co-limitation: a response only to the +NP treatment.
Figure 3.
Possible light limitation patterns.
A) Light limitation: a lower response to the Ctrl and +NP treatments when incubated under in situ light conditions and no difference between the Ctrl and +NP treatment when incubated with in situ light. B) Co light-nutrient limitation: the +NP treatment response is greater than the Ctrl for both standard and in situ light, but responses to Ctrl and +NP treatments are lower under in situ light than standard light. C) No light limitation: no difference between in the in situ and standard light incubation for either the Ctrl or +NP treatment.
Figure 4.
Seasonal variation of limitation types and response ratio determined by a series of bioassays (2011). A–D)
Single and serial P limitation (green), independent (primary P or N) and simultaneous co-limitation (yellow), serial and single N limitation (blue), light limitation (black), co-limitation between light and nutrients (black circle around the colored point) and no response (grey). E–H) Response ratio (RR) indicating the relative strength of N versus P limitation. Negative values indicate N and positive values indicate P as the primary limiting nutrient. The colors are the same as those in A–D. RR for experiments showing no response or pure light limitation are not shown.
Table 2.
Number of observations of the different limitation types in the four lakes.
Figure 5.
Seasonal pattern of nutrient concentration, Imix and phytoplankton biovolume measured in SCH and LAN (2011).
The colored bands above the graphs indicate the limitation type identified by the bioassays: where No is no limitation; Co is simultaneous or independent co-limitation; P is serial or single P limitation; N is serial or single N limitation; Light is light or co-limitation between light and nutrients. A–D) TN, DIN, TP and SRP; the horizontal lines mark the DIN and SRP concentrations below which N or P limitation are possible according to Maberly et al. [41]. E and F) Imix, the horizontal line marks the light intensity below which in situ light treatments were conducted in the bioassays. G and H) Phytoplankton biovolume estimated according to Utermöhl [33].
Figure 6.
Seasonal pattern of nutrient concentration, Imix and phytoplankton biovolume measured in MUEG and UH (2011).
The colored bands above the graphs indicate the limitation type identified by the bioassays: where No is no limitation; Co is simultaneous or independent co-limitation; P is serial or single P limitation; N is serial or single N limitation; Light is light or co-limitation between light and nutrients. A–D) TN, DIN, TP and SRP; the horizontal lines mark the DIN and SRP concentrations below which N or P limitation are possible according to Maberly et al. [41]. E and F) Imix, the horizontal line marks the light intensity below which in situ light treatments were conducted in the bioassays. G and H) Phytoplankton biovolume estimated according to Utermöhl [33].
Figure 7.
Relationships between the ambient DIN and SRP concentrations and the limitation categories.
The vertical line marks the DIN concentration and the horizontal line marks the SRP concentration below which N or P limitation are possible according to Maberly et[41]. This plot shows that the results of the bioassays agree with the values given by Maberly et al. as SRP was predominantly below 10 µg L−1 when P limitation was observed and DIN was predominantly below 100 µg L−1 when N limitation was observed. Both dissolved nutrients were usually above these thresholds when light limitation or no response was observed.
Figure 8.
Relationships between the ambient N:P ratios and the response ratio observed in the bioassays.
A) DIN:SRP, B) TN:TP, C) DIN:TP and D) TN:SRP. A positive response ratio (RR) indicates P limitation and a negative N limitation. The point at which the fitted line crosses RR = 0 identifies the ratio at which phytoplankton switch from being N to P limited. MR: Misclassification rate, R2: Coefficient of determination of the linear regression. Experiments showing no nutrient limitation were excluded.
Figure 9.
Seasonal pattern of the N:P mass ratios measured in SCH and LAN (2011).
A and B) DIN:SRP, C and D) TN:TP, E and F) DIN:TP mass ratios. The colored bands above the graphs indicate the limitation type identified by the bioassays; where No is no limitation; Co is simultaneous or independent co-limitation; P is serial or single P limitation; N is serial or single N limitation; Light is light or co-limitation between light and nutrients. The horizontal lines mark the N:P ratio at which phytoplankton switched from being N to P limited based on an analysis of all four lakes combined (see Figure 8).
Figure 10.
Seasonal pattern of the N:P mass ratios measured in MUEG and UH (2011).
The colored bands above the graphs indicate the limitation type identified by the bioassays, where No is no limitation; Co is simultaneous or independent co-limitation; P is serial or single P limitation; N is serial or single N limitation Light is light or co-limitation between light and nutrients. The horizontal lines mark the N:P ratio at which phytoplankton switched from being N to P limited based on an analysis of all four lakes combined (see Figure 8).
Table 3.
Thresholds from this study and from the literature of TN:TP and DIN:TP mass ratios that separate N and P limitation.