Report on the EUROBASIN/HYDRALAB IVmesocosm experiment conducted in 8/12
1. Objectives of the work
The goal of this work has been to examine the influence of upper ocean food web structure
and functioning on both the natural and artificially enhanced sequestration of carbon within
the ocean. Data obtained in the primary mesocosm experiment run in the Bay of Hopavågen
in August 2012 will be used to assess the extent to which organic matter produced within four
different food webs is retained in the upper ocean food web versus remineralized back to
carbon dioxide and inorganic nutrients (ammonium, dissolved silicon, phosphate) versus
exported from the system in the form of rapidly sinking particles. Data from a second, related
experiment run concurrently will be used to assess the extent to which mesozooplanktoncontaining food webs enable the dissolution of calcium carbonate particles in the surface
ocean, increasing upper ocean alkalinity and the ocean’s ability to serve as a sink for carbon
dioxide. It will also be used to discern the impact of calcium carbonate addition on food web
structure and functioning and on fluxes and sinking velocities of particulate organic matter.
2. Experimental Set-up
2.1 Field site- The site chosen to conduct this experiment was the Bay of Hopavågen, Norway
(Fig. 1). Hopavågen is a semi-enclosed lagoon, 120 km west of the city of Trondheim and 20
km west of the outlet of the Trondheimsfjord. It is sheltered from wind and waves, has an
area of 27 ha, is 32 m deep at its deepest point, and remains oxic due to 14% of its water
being exchanged with
the ocean per day,
through a narrow inlet
(Fig. 1), due to tidal
forcing.
The other feature of the
Bay of Hopavågen that
made it an attractive
place
to
conduct
mesocosm experiments,
is the Sletvik Field
Station, located within a
few hundred meters of
the bay.
This field
station, run by the
Norwegian University of
Science and Technology
Fig. 1 Location of the Bay of Hopavågen and the Sletvik Field Station. Note
(NTNU), has a 250 m2 of
the mesocosm raft structure deployed in the deepest portion of the bay.
laboratory
space
(including a wet lab, a dry lab, several smaller multipurpose laboratories, a classroom, and a
fume hood). It has a small landing for access to the bay, a seawater intake system supplying
lagoon water to the laboratory, and, through the Trondhjem Biological Field Station at
NTNU, a small boat is available for use. The Sletvik Field Station also has dormitories that
may accommodate up to 50 people, and it has a kitchen and dining facilities. In addition,
access to the Sletvik Field Station was available through the HYDRALAB IV Integrating
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Activity of the European FP7. Access was applied for by De La Rocha and then granted from
HYDRALAB IV (as project HyIV-NTNU-01) for 36 days in July-August 2012. On-site
participants in the experiment were largely drawn from EUROBASIN partner institutes
(CNRS at Brest, NOC Southampton, University of Hamburg, DTU-Aqua, and MARUM at
the University of Bremen). There were also personnel, associated with NTNU and the
University of Munich, who provided critical basic support for the work.
2.2 Experimental setupThe primary experiment focusing on food webs,
organic matter cycling, and the formation and sinking
of large, rapidly sinking aggregates (a type of “marine
snow”) was carried out in a set of 12 mesocosms
covering, in triplicate, 2 different phytoplankton
communities (diatom versus non-diatom) exposed to 2
different zooplankton communities (–copepod and
+copepod) (Fig. 2). These starting conditions were
established by first filling the bags, roughly
simultaneously, with seawater from the Bay of
Hopavågen. Mesozooplankton were then removed to
the most complete extent possible immediately
removed from half of the mesocosms through repeated
vertical hauls of a plankton net (200 µm mesh). Fig. 2 The distribution of mesocosms on
mesocosm raft.
Each larger
Nitrate and phosphate was added to half mesocosms the
mesocosm
raft
ring
held
four
daily to promote the growth of non-siliceous mesocosms. Three of the larger rings
phytoplankton
(e.g.
dinoflagellates
or were used for the primary experiment,
coccolithophores).
To the other half of the and three of the rings were used for the
mesocosms, nitrate, phosphate, and silicate were calcium carbonate experiment.
added to promote the growth of diatoms. Material
was allowed to settle and the two distinct phytoplankton populations were allowed to develop
for 4 days, after which copepods collected from the Bay of Hopavågen were added back to the
half of the N+P mesocosms and to the half of the N+P+Si mesocosms from which
mesozooplankton had not been removed at the beginning. This yielded a set of four initial
starting
conditions
(N+P–copepods,
N+P+copepods,
N+P+Si–copepods,
and
N+P+Si+copepods) (Fig. 2).
The calcium carbonate mesocosms of the secondary experiment received daily additions of
N+P+Si equivalent to those added to the primary experiment mesocosms. In addition, a
suspension of calcium carbonate was added every evening so that there were 3 controls (no
added calcium carbonate) plus 3 replicates of 3 different daily additions of calcium carbonate.
The zooplankton population was left undisturbed for the first week, then supplemented with
addition copepods during the second half of the experiment.
The mesocosms used in the experiment were approximately 9000 L in volume, with a
diameter of 1.08 m and an average depth of 9.6 m. The bags were made of white plastic and
were deployed from the raft structure which was anchored in the deepest part of the Bay of
Hopavågen (Figs. 1 & 3).
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2.3 Sampling
The primary mesocosms were sampled
using a 1.5-m, 5-L integrated water
sampler lowered by hand to sample the
3-4.5 m depth interval (Fig. 3).
Repeated deployments of the integrated
sampler allowed for samples of up to
20-L of water to be collected in LDPE
carboys, allowing for measurements of
dissolved and particulate components
(e.g., particulate organic carbon,
biogenic silica, dissolved organic
carbon, nutrients, pigments, etc) to be
made on subsamples of the same water.
Water was collected before breakfast
every morning and brought back to the
lab. Subsamples were removed in the
lab for filtration after gentle mixing of
the water in each carboy.
Samples from the calcium carbonate
mesocosms were taken using a
submersible pump, to avoid aerating the
samples (which would interfere with the
accurate determination of carbonate
system parameters such as pH, total
dissolved inorganic carbon (DIC), and
total alkalinity (TA).
Fig. 3 Mesocosms being sampled using the integrated
water sampler. When full, the sampler is emptied into a
bucket and then the water is transferred into a 20-L carboy
and the process was repeated until the required sample
volume was reached. Generally, ~15 L of water was
collected for each mesocosm at each sampling time.
Over the course of the experiment, the
primary mesocosm bags were sampled
6 times, on days 1, 4, 6 10, 13, and 16
(Table 1), while the calcium carbonate mesocosms were generally sampled every second day.
In addition, sediment traps were deployed in a subset of the mesocosms in both experiments
and allowed to collect material for 4-6 days before removal (Table 1). Three sediment trap
deployments were made and the material was used for the determination of mass fluxes of
particulate organic carbon, transparent exopolymer particles, biogenic silica, and etc. In
addition to the normal sediment traps, gel traps were also deployed (on a similar but not
perfectly overlapping schedule) in order to obtain a visual record of the material settling
through the water column in the mesocosms.
The vertical density structure of the water column, the vertical distribution of chlorophyll,
integrated chlorophyll content, and the light climate in the mesocosms was monitored using
casts of a CTD and a light meter. CTD casts were carried out three times for the primary
mesocosms and several additional times for the calcium carbonate mesocosms.
2.4 Sample handling
In the primary mesocosms, samples for a set of core parameters were taken every time the
mesocosms were sampled (Table 2). Samples for particulates (PIC, BSi, POC, PON, TPP,
pigments, and TEP, as defined in Table 2) were collected on GF/F or 0.4 µm polycarbonate
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Table 1: Work Done July 31 - August 29 2012
Sun
Mon
Tue
July 30
First set of
researchers arrive at
Sletvik Field Station.
5
Wed
July 31
Mesocosm bags
made.
Mesocosm rings and
raft built.
6
7
Thu
Fri
1
More mesocosm
bags made.
Raft set in place.
Weights for
mesocosm bags
made.
Agg Bag T0 at 7:00
2
Agg Bags filled with
water.
First nutrients added
to Agg Bags.
8
Sat
3
Calc Bags filled.
Zooplankton removed
from 6 Agg Bags.
Sediment traps
tested.
First nutrients added
to Calc Bags.
9
10
Day for rest, tests,
and setup of labs.
Copepods added to 6
Agg Bags.
Testing, lab setup
continues.
Zooplankton
enumeration in Agg
Bags.
Agg Bags 1st
sampling, Incl. size fr
samples
First Calcite additions
to Calc Bags
1st Aggregation
Experiment started.
Calc Bags sampling.
Sed traps deployed
(10pm).
Agg Bags 2nd
sampling
Copepods collected
for 1st Zoop Exp (Agg
Bags).
12
Sampling of 1st Agg
Exp (cont).
1st Zoop Exp Ends.
13
Agg Bags 3rd
sampling
Sampling of 1st Agg
exp (cont)
Zoop collection for 2nd
zoop exp (Calc Bags)
Calc sampling.
14
Sampling of 1st Agg
Exp (cont)
Sed traps removed
3pm.
Agg Bag T168 at 7:00 15
CTD + light profiles
Zoop enumeration
Sed traps deployed
Start of 2nd Zoop Exp
(Calc Bags)
Calc sampling
16
End 2nd Zoop Exp
(Calc Bags)
More zoops added to
Calc Bags
Agg Bags 4th
sampling
Calc sampling.
19
Sed traps removed
Calc sampling
Sampling 2nd Agg
Exp.
Start 3rd Zoop Exp
(Agg Bags)
26
Calc Bags removed
Sampling 3rd Agg Exp
(cont)
Mesocosm raft
retrieved and
dismantled.
20
Agg Bags 5th
sampling
Sed traps deployed
Sampling 2nd Agg
Exp (cont)
End 3rd Zoop Exp
(Agg Bags)
27
Dismantling of
mesocosm raft
finished
Boxes packed.
21
11
Sampling of 1st Agg
Exp.
1st Zoop Exp started.
Calc Bags sampling.
17
18
Zoops collected for
3rd Zoop Exp (Agg
Bags)
2nd Agg Exp started
(Calc Bags)
25
Calc sampling
Sampling 3rd Agg Exp
Agg Bags removed.
Samples taken of
bags.
22
Calc sampling.
Sampling 2nd Agg
Exp (cont)
Size fractionated
samples taken for
trophic flow
3rd Agg Exp (Agg
Bags) started
Agg Bag T360 at 7:00 23
Agg Bags 6th
sampling
Calc sampling
Sampling 3rd Agg Exp
24
Zoop enumeration
Agg Bags and Calc
Bags.
Sed traps removed.
CTD + light
28
Packing completed.
Boxes picked up.
Lab cleaned up
29
Frozen good shipped.
Final group of
researchers departed
Sletvik Field Station.
30
31
4
4
Copepods collected
and put in lab before
addition to
mesocosms.
Jellies removed from
Agg Bags for the first
time.
filters and then dried or frozen, as
appropriate for later measurement. Table 2: Core parameters measured in the primary
Samples
for
counts
of mesocosm experiment
nutrients
particulate organic carbon (POC)
phytoplankton,
zooplankton,
dissolved silicon
particulate organic nitrogen (PON)
bacteria, and viruses were
nitrate
total particulate phosphorus (TPP)
preserved for later determination.
nitrite
biogenic silica (BSi)
ammonium
particulate inorganic carbon (PIC)
Samples for dissolved constituents
phosphate
transparent exopolymer particles (TEP)
(nutrients, DOC, DON, DOP)
pigments
dissolved organic carbon (DOC)
were filtered to removed
bacteria
dissolved organic nitrogen (DON)
particles then frozen for later
viruses
dissolved organic phophorus (DOP)
analysis (except for dissolved phytoplankton (Lugol’s) phytoplankton (flow cytometer)
silicon and nitrate concentrations,
which were measured on site in Norway as the experiment progressed). In addition to these
samples, on several dates, size-fractionated samples were taken for POC and PON.
Samples from the calcium carbonate experiments were handled in a similar fashion.
Material from sediment traps was allowed to settle for 4 hours, the concentrated by siphoning
off excess water from the sediment trap. The collected material was then split into 8
equivalent portions and the portions used for determinations of POC, PON, BSi; PIC, TEP,
TPP, and phytoplankton (and microzooplankton). The material collected in gel traps was
photographed for later image analysis.
3. Side experiments
Three key side experiments were performed in parallel with the mesocosms. The first looked
at the vital statistics of zooplankton incubated in water from different mesocosm treatments.
The second looked at the formation of aggregates and the character and sinking velocity of the
aggregates formed in water from different mesocosm treatments (when incubated in roller
tanks specialized for this purpose). The third investigated the feeding of zooplankton on
aggregates from the different types of mesocosms.
4. Preliminary results
Different standing stocks of biomass and sinking fluxes were quickly established between the
different treatments in the primary mesocosm experiment. After one week, the +Si
mesocosms, representing a diatom-dominated phytoplankton community, contained 4 times
more chlorophyll, 10 times more net nitrate utilization, and notably greater sinking fluxes than
the mesocosms that lacked silicon and therefore were dominated by smaller, non-siliceous
phytoplankton (Table 3; Fig. 4)
Table 3: Depth-integrated chlorophyll (mg) in
the primary mesocosm experiment
Treatment
Day 8
Day 17
N+P+Si
6 ±3
17 ± 14
N+P+Si+copepods
21 ± 7
48 ± 16
N+P
2 ± 0.3
12 ± 5
N+P+copepods
4 ±2
20 ± 11
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The total amount of chlorophyll
suspended in the water column in the
mesocosms increased throughout the
experiment, along with a marked decrease
in the amount of flux as diatomdominated communities made way for
non-siliceous phytoplankton in all
mesocosms. During the final 10 days,
daily net nitrate utilization was similar
4.0
M1
3.5
M2
M3
3.0
Nitrate (µM)
M4
2.5
M5
2.0
M6
M7
1.5
M8
1.0
M9
0.5
M10
0.0
8/7/2012 8/11/20128/15/20128/19/20128/23/2012
Date
M11
M12
Fig. 4 Evolution of the nitrate concentration in the different primary mesocosms.
Mesocosms 1-3 are N+P+Si, 4-6 are N+P+Si+copepod, 7-9 are N+P, and 10-12 are
N+P+copepod. The black line represents the expected increase in nitrate concentration due
to the daily nitrate addition (i.e. it represents the case of no net nitrate uptake).
among all treatments and approximately equal to the daily nitrate addition. Net silicate uptake
ceased by day 9 in the +Si mesocosms, indicating a temporary collapse in the diatom
community, but resumed by day 12. Sinking fluxes during the last week of the experiment
were minor in all mesocosms (although they were still markedly highest in the two +Si
treatments, despite the temporary collapse of the diatom community).
The different zooplankton communities significantly affected the composition of sinking
fluxes (phytoplankton aggregates, fecal pellets, appendicularian houses) but had less influence
on the quantity of material sinking.
Although there are many more results to come in as we work through our sample set, it is
clear that we have succeeded with the main objectives of the experiment, which were to
establish a matrix of different phytoplankton and zooplankton communities to see if this
would result in different levels of phytoplankton growth, nutrient cycling, and sinking carbon
flux. At this time, we can already say that these things all appear to be true.
5. Remaining work
Through the end of 2012 and into January 2013, we have been analyzing the samples that
have been brought back from the experiment. Most of this work will have been carried out in
Brest, France. The carbonate system measurements and the measurements of DOC and DON,
however, will be measured by the team in Southampton (at the National Oceanography
Centre), and the counts of mesozooplankton abundance and related work for the vital statistics
will be done at the University of Hamburg.
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As data are acquired and cleared through a process of quality control, they will be archived in
the PANGAEA database. For the first two years, we would prefer them to be password
protected (i.e. available only to experiment participants) and after that, open to public access.
A series of“post-mesocosm” data meetings will be held for participants via teleconferencin
beginning in March 2013 to facilitate data analysis and the planning and writing of papers. It
is possible that we may try to group the resulting papers together for publication as a special
issue of a journal. It is, however, not yet clear whether or not this is the best course of action.
The first presentation based on results from the experiment will be given by De La Rocha et al
as a talk at the Aquatic Sciences Meeting in New Orleans in February 2013 in a session
focusing on food web interactions and sinking fluxes of carbon via the biological pump.
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