Nutrient Limitation of Phytoplankton in Portage Lake, Michigan
Joshua R. Papacek & Amy M. Marcarelli
Department of Biological Sciences, Michigan Technological University
Bioassay Methods
Background
Research objective: To assess the nutrient
limitation status of phytoplankton in Portage Lake
Results
• 750 mL of water was filtered into flasks and amended
with either N, P, N+P or no nutrient (DI water as control),
with three flasks per treatment.
• Phytoplankton form the base trophic level in lake
ecosystems, and nitrogen (N) and/or phosphorus (P)
availability can limit phytoplankton growth
• Phytoplankton were P limited (p < 0.05) during the May
2011 bioassay (Figure 1)
• Phytoplankton were N limited (p < 0.05) during the July
2011 and December 2012 bioassays
• Target concentrations of (70 μg/L NH4-N) or P (10 μg/L
PO4-P) were achieved by adding 100 µL of N and P
working solutions (75 mg/L P as PO4-P or 535 mg/L N as
NH4-N).
• Little is known about the nutrient limitation status of
phytoplankton in Portage Lake
• There was no significant limitation observed in the October
2012 and November 2012 bioassays
• Flasks were incubated in growth chambers for 7 days at
temperatures similar to lake temperatures recorded
during collection and a photoperiod that varied by season.
Field Collection
Conclusions
• Limitation status of phytoplankton in Portage Lake changes
seasonally between N and P and no nutrient limitation
• Both nutrients may be important for controlling growth
rates in this lake
• Water was collected at Chassel City Park in May and July
2011 and at the GLRC docks October-December 2012
Acknowledgments
Table 1: Lake conditions on the date of water collection for each bioassay
• Near-shore surface water samples were collected using a
throw bucket
Collection
date
16 May 2011
Water
Conductivity Turbidity
temp (°C) (mS/cm)
(NTU)
13.34
0.079
1.8
Secchi
depth (m)
nm
Initial
Initial NH4- Initial SRP
chlorophyll N (ug / L)
(ug / L)
a (mg / L)
0.8
3.4
8.7
18 Jul 2011
26.30
0.119
nm
nm
7.6
10 Oct 2012
10.41
0.105
3.1
2.1
2.0
1 Nov 2012
7.87
0.118
0.0
2.3
10.3
6 Dec 2012
1.58
0.117
4.9
2.3
0.0
nm = not measured on that date; nd = samples collected by not yet analyzed
• Water temperature, air temp, conductivity, pH, and
Secchi depth were also measured (Table 1)
8.7
31.0
30.1
17.2
A special thanks to Rianne Chittenden and Tim Veverica
for data collection and analysis during the May and July
2011 experiments. Also thanks to Jamey Anderson for
assistance with water collection at the GLRC docks.
3.4
nd
nd
nd
Figure 1: Nutrient limitation for each date was determined using two-way ANOVA for N and P and following the criteria of Tank and Dodds, FW Biology, 2003: A significant N with or without a NxP-effect was interpreted as N-limitation, P effect with or
without a NxP-effect as a P limitation. If both N and P, or NxP was significant, biofilms were deemed co-limited. If the overall ANOVA was not significant, the interpretation was no limitation.
4
3
2
1
0
Control
N
N+P
P
25
N
P
N+P
p = 0.006
p = 0.0998
p = 0.0015
20
15
10
5
0
Control
N
N+P
/
P
1.6
N
P
N+P
1.4
1.2
p = 0.0595
p = 0.5688
p = 0.5987
1
0.8
0.6
0.4
0.2
0
Control
N
P
N+P
Mean Chlorophyll a (μg/L) ± 1
SE
p = 0.4575
p = 0.0009
p = 0.0471
30
November 2012 = No Limitation
Mean Chlorophyll a (μg/L) ± 1 SE
5
N
P
N+P
October 2012 = No Limitation
Mean Chlorophyll a (μg/L) ± 1 SE
6
July 2011 = N Limited
Mean Chlorophyll a (μg/L) ± 1 SE
Mean Chlorophyll a (μg/L) ± 1 SE
May 2011 = P Limited
9
8
7
December 2012 = N Limited
2
N
P
N+P
p = 0.6290
p = 0.0482
p = 0.1056
N
P
N+P
1.8
1.6
1.4
6
p = 0.0198
p = 0.8953
p = 0.0209
1.2
5
1
4
0.8
3
0.6
2
0.4
1
0.2
0
0
Control
N
P
N+P
Control
N
P
N+P