Transcript Zooplankton response to redox-induced changes in nutrient stoichiometry: Mesocosm experiments in tropical upwelling areas Helena Hauss, Jasmin Franz & Ulrich Sommer Leibniz Institute of.

Zooplankton response to redox-induced changes in
nutrient stoichiometry:
Mesocosm experiments in tropical upwelling areas
Helena Hauss, Jasmin Franz & Ulrich Sommer
Leibniz Institute of Marine Sciences (IFM-GEOMAR), Düsternbrooker Weg 20, 24105 Kiel, Germany, [email protected]
Collaborative Research Centre
Climate-Biogeochemistry Interactions in the Tropical Ocean
Introduction
Results
12x10 4
total phytoplankton
10x10 4
Cells ml-1
Recently, oxygen minimum zones beneath tropical upwelling areas are expanding.
The redox state of the water column influences inorganic macronutrient inventories
which are influencing the functional type and elemental composition of
phytoplankton. Specifically, anoxia enhances the loss of dissolved inorganic nitrogen
(N) via denitrification and anammox, while phosphate (P) is released from the
sediments from metal oxides under reducing conditions, leading to inorganic N:P
markedly below the canonical Redfield ratio (16:1). This, in turn, may affect the
nutritional value of phytoplankton for higher trophic levels.
We conducted two on-board mesocosm experiments during a cruise in the tropical
Atlantic in December 2009 in which the nutrient load was manipulated so that N:P
ranged from 5:1 to 20:1, while the addition of Silicate to all treatments ruled out
potential Si limitations.
The phytoplankton response was monitored in terms of community structure,
biomass production and fatty acid composition. Simultaneously, copepods were
supplied with phytoplankton originating from the mesocosms and their nutritional
condition (RNA/DNA ratio) measured.
Results of the second mesocosm trial are presented here.
20:1
15:1
10:1
5:1
8x10 4
6x10 4
Copepod
incubation
4x10 4
2x10 4
0
0
1
2
3
4
5
6
Time (d)
7
8
9
10
Figure
3.
Phytoplankton
abundance development over
time in the mesocosms as
determined
by
flow
cytometry.
Values
are
treatment means (± standard
deviation). Red box indicates
period
of
concomitant
copepod feeding trial. No
significant treatment effect on
the growth rates within the
exponential phase (day 2 to
day 7) could be detected;
however,
maximum
abundance on day 8 was
significantly higher in “20:1”
(red) than in “15:1” (orange)
as detected by ANOVA and
Tukey’s HSD (p<0.01).
3.0
120
100
a
a
a
80
a
a
60
a
40
RNA:DNA
The experimental setup comprised twelve 150L mesocosm bags afloat in four flowthrough cooling water baths (<2°C warmer than SST). The water baths were
gimbals-mounted to prevent spilling. Mesocosms were filled at 07°41.4’N,
024°13.5’W (Fig. 1). from approximately 5m depth using a peristaltic pump and
inoculated with 20L from the Chl-a maximum at 47m depth. As an experimental
treatment, N and P levels were manipulated by initial fertilization using Ammonium
Nitrate and Monopotassium Phosphate to the target ratios (20, 15, 10 and 5,
respectively), while the Si concentration was adjusted to equal, unlimiting levels
(10µmol) in all mesocosms using Sodium Metasilicate Penta-Hydrate.
After six days of growth, water from the mesocosms was used to feed copepods
(Neocalanus sp.) that were sorted from vertical 150µm WP-2 tows and incubated in
groups of three in 500ml kautex bottles (six replicates per treatment) for the four
fertilization treatments and a filtered seawater control. Every day, fecal pellets were
counted and the water replaced. After four days, copepods were frozen at -80°C and
transported to Kiel, where their dry weight and RNA:DNA ratio were determined.
On board, dissolved organic nutrients and phytoplankton development were
monitored on a daily basis (Fig. 2 & 3). Nanoplankton and bacterial abundance were
assessed using a FACScalibur flow cytometer. Cells were distinguished by size and
fluorescence of chlorophyll a, phycoerythrin, and allophycocyanin.
For analysis of the fatty acid profile of bulk seston, 0.5 to 2L of water, depending on
the concentration of phytoplankton, were filtered on pre-combusted 0.2µm GF/F
filters and frozen at -80°C and are currently analyzed in Kiel.
Furthermore, samples were collected for cell counts of microplankton, particulate
matter (POC, PON, POP, Pigment analysis (HPLC), Bsi and TEP) that are being
analyzed in Kiel.
Fecal pellet production (no. ind-1 d-1)
Methodology
Shipboard mesocosms (4x3)
Cape Verde
ab
2.0
ab
b
1.0
20
0
5
0.0
10 15 20
5
nominal N:P
10 15 20
nominal N:P
Figure 4. Mean individual fecal pellet
production
during
four
days
of
incubation. No significant differences
could be detected among feeding
treatments (ANOVA on log-transformed
data).
Figure 1. Experimental Setup.
a
Figure 5. Nutritional condition
(RNA:DNA) of copepods before
(“initial”) and after four days of
incubation with mesocosm algae
(N:P= 5:0 to 20:0) or filtered
seawater (“no prey”). Different
letters denote significant differences
as detected by ANOVA and Tukey’s
HSD (p<0.01).
Conclusions
initial station
•Depletion of macronutrients within five to six days (Fig. 2).
Bottle incubation (5x6)
•Rapid biomass increase (after a two-day perdiod of stress recovery) in all mesocosms
(Fig. 3).
•No consistent effect of the different fertilization treatments on biomass development as
indicated by flow cytometry, and no difference in copepod ingestion based upon fecal
pellet production (Fig. 3 & 4).
20:1
10:1
15:1
5:1
8
Figure 2.
Macronutrient
concentrations during
the mesocosm
experiment. Left:
total dissolved
inorganic nitrogen
(NO2, NO3 and NH4).
Right: inorganic
phosphorus (PO4).
Values are treatment
means (± standard
deviation).
1.6
PO4(µmol L-1)
Total DIN (µmol L-1)
10
1.2
6
0.8
4
0.4
2
0
0.0
2
4
6
8
This is likely due to changes in the quality of available food items in terms of either
community structure, nutritional quality or elemental composition of the phytoplankton
as suggested by results from an earlier mesocosm experiment off Peru (presented by J.
Franz). Our aim is to disentangle these processes by further analysis of samples taken
during this experiment and at field stations of the same cruise.
Since calanoid copepods are key players in productive marine food webs and constitute
the main prey for small pelagic fishes, negative effects of decreasing N:P in upwelling
areas may lead to declining trophic efficiency of the pelagic system.
Acknowledgements
PO4
DIN
•Nevertheless, the nutritional condition of copepods after four days of feeding on the
respective algae was significantly influenced by the experimental treatments, with
considerably lower RNA:DNA values in the two lower N:P treatments (Fig. 5).
10
Time (d)
2
4
6
8
10
The authors would like to thank Thomas Hansen, Kerstin Nachtigall and the Crew of RV
Meteor for their support during the M80-2 cruise. This work is a contribution of the
DFG-supported project SFB754.