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Methods applied in the large littoral mesocosms study of nutrient enrichment in rocky shore ecosystems—EULIT
Continental Shelf Research 21 (2001) 1925–1936
Review article
Methods applied in the large littoral mesocosms study of nutrient enrichment in rocky shore ecosystemsFEULIT T.L. Bokna,*, E.E. Hoellb, K. Kerstingc, F.E. Moya, K. S rensena a
Norwegian Institute for Water Research (NIVA), P.O. Box 173 Kjelsaas, N-0411 Oslo, Norway Norsk Hydro, Research Centre Porsgrunn (HRE), P.O. Box 2560, N-3901 Porsgrunn, Norway c IBN-DLO, P.O. Box 167, 1790 AD Den Burg, The Netherlands
b
Received 8 December 1999; accepted 15 January 2001
Abstract Eight concrete land-based mesocosms have been set up for a study of the effect of nutrient enrichment on littoral hard bottom ecosystems. The construction of the mesocosms and the community establishment were initiated 2 yr ahead of the MAST-III project. The littoral communities were established by transplanting rocks with attached macroalgae and associated animals from the Oslofjord, followed by a period of natural community development which has resulted in a highly diverse flora and fauna. During this 2-yr pilot project, the efficiency of relevant techniques and statistical design for the EULIT experiments in the mesocosms have been tested, as well as the performance of the mesocosms ahead of the nutrient manipulation. The pilot project has thus extended the total experimental period. The mesocosms are fed with water from 1 m depth in the OslofjordFresidence time about 2 hFwhich also acts as a source for spores, zygotes and larvae. A tidal regime with a 36 cm amplitude is maintained and waves are regularly generated. Oxygen concentration, temperature and salinity are measured continuously by probes. Nutrient addition started in May 1998. Nutrient and carbon input and outflow are measured on an average weekly basis. Community analyses, species compositions, biomass, growth and grazing of the different components of the mesocosms are studied during 3-week periods (spring, summer and autumn) each year. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: methods
Phytobenthos; Zoobenthos; Marine mesocosms; Nutrient experiments; Eutrophication; Standard
*Corresponding author. Tel.: +47-22-185173; fax:+47-22-185200. E-mail address: [email protected] (T.L. Bokn). 0278-4343/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 8 - 4 3 4 3 ( 0 1 ) 0 0 0 3 5 - 8
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1. Introduction The Marine Research Station, Solbergstrand’s facilities, situated 30 km south of Oslo by the Oslofjord, Norway, is the most comprehensive system reported dealing with marine rocky shore mesocosms (Bakke, 1990). Manipulation with marine ecosystems in mesocosms was started at the end of the 1970s with four 60 m3 big tanks constructed for studies of rocky shore, intertidal communities (Bokn and Kirkerud, 1981). During 1982–1985, these communities were used as a target for diesel oil effect studies on contract for British Petroleum (Bokn, 1984; Bakke, 1986; Gray, 1987; Bokn et al., 1993). The Baltic Sea Laboratory in Karlskrona, Sweden, has been running smaller mesocosms with estuarine ecosystems from the early 1970s (Notini et al., 1977). A recent bibliographical search has not revealed any other experiments with rocky shore mesocosms. The overall objectives for the EULIT project is to predict the response of rocky shore ecosystems to an increased nutrient availability. By use of the mesocosms the aim is: *
*
*
*
*
*
*
*
To detect quantitative and qualitative changes in the plant- and associated animal-community with increased nutrient availability. To test whether increased nutrient loading may affect biodiversity characteristics in the rocky shore ecosystem either directly or indirectly through changes of key species. To analyse the relationship between eutrophication induced changes at the level of individual species performance (biomass, cover, growth rates, production) with changes at the community level (species composition, species interactions, dominance-diversity patterns) and at the ecosystem level (overall production and respiration, flows of matter and energy). To detect if the functional changes such as primary production and consumption occur before structural changes are observed. To follow the changes in the processes at different organisational scales and compare the turnover rates of the communities and the sub-communities. To explain changes in plant community composition and production from alterations in growth rates and grazing losses of the specific components (species or functional groups). To develop a model of rocky shore responses to an increased nutrient load, based on the processes measured and make simulations of large-scale events and predict effects of nutrient addition. To support the European ELOISE initiative with the increased knowledge on nutrient cycles in the coastal environment, relation between excess nutrient load and eutrophication effects, pressure in the coastal shore line, and ecological management of coastal zone resources
The present EULIT project was preceeded by a pilot project dealing with the construction of the mesocosms, community establishment and applicable technics for nutrient manipulation. The idea was to mimic natural rocky shore marine environment within the controlled experimental facilities. A disadvantage with these experiments lies in the conflict between the need for the statistical replication and the need for representative gradients with sufficient data. As a compromise, we chose to set up the experiments with two controls and the gradient of six nutrient additions without parallels. Bakke (1990) raised several questions regarding community structure and function, which experiments can be preferably performed in situ and which projects will be improved through
T.L. Bokn et al. / Continental Shelf Research 21 (2001) 1925–1936
1927
manipulating the environmental regime in mesocosms. Hard-bottom model ecosystems have proven to be valuable experimental tools in pollution research and they are the only alternatives to laboratory experiments if chronic discharge into coastal water is to be imitated. The present project EULIT, dealing with nutrient (N and P) effects on intertidal, rocky shore communities using mesocosms meets many of Bakke’s criteria for testing hypotheses concerning structure and function (Bakke, 1986). The many subtasks performed in the EULIT basins assume appropriate design and reliable standardized methods for the current mesocosm project. Of great importance are the concrete basins, the water system, nutrient manipulation, light, temperature, oxygen and salinity, and community establishment in the basins. The aim of this paper is to describe in detail the standard methods used in the mesocosms of the EU project MAS3-CT97-0153 (EULIT).
2. The concrete basins The mesocosm facility consists of two rows of four mesocosms each. Each mesocosm is a concrete basin, 4.75 m long, 3.65 m wide and 1.35 m deep (about 23 m3) oriented with the long side in west-east direction. The western part of the basin is a flat bottom of 2.70 m 3.65 m. In the eastern part a series of five steps of 0.4 m width, slightly slanting down from south to north is constructed. The eastern-most step (step 0) extends from 1.0 m above the bottom at the south wall to 0.86 m above the bottom at the north wall. Each next step is 0.18 m lower than the previous step. The last step (step 4) extends from 0.28 m at the south wall to 0.14 m at the north wall. Fig. 1 gives an overview of the eight basins and Table 1 presents some technical data.
3. The water system Water from the Oslofjord is taken at 1 m depth and supplied to each mesocosm (Fig. 2) at a rate of 5 m3/h. A tidal regime with an amplitude of 0.36 m and a period of 12 h 20 min is applied. The high tide level is set at 0.93 m (half way step 0) and the low tide level at 0.57 m (half way step 2). The tidal level was regulated by raising and lowering the outflow. During the pilot project, the water left the basin through a vertical pipe (10 cm Ø) 10 cm below the low tide water-level. Each outflow pipe was connected to a flexible hose outside the basins. The flexible hoses of all basins were simultaneously raised and lowered to create the tidal regime. This system was replaced at the start of the EULIT project (April 1998) by export through a slanting pipe inside the basin that is raised and lowered by an activator (ELERO, type Econom 0 G/A) in each basin (Fig. 2). This latter construction is more accurate and flexible. The former tidal system used in the pilot project functions as a backup system for the current EULIT project. The original outflow took its water from 10 to 46 cm below the water surface while the reconstructed outflow skims the water surface. This might influence the export of material from the basins. In June 1998, the programme controlling the tidal activators was changed to achieve a sinusoidal tidal curve. The form of the tidal curve is shown in Fig. 3.
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Fig. 1. Eight mesocosms used in the EULIT project.
Table 1 Technical data of the eight mesocosms Volume in each tank (4.75 3.65 1.35 m3) Volume of seawaterFlow tide/high tide Wave generator Water intake Flow rate Water residence time (mean) Tide level mimic Nominal amplitude (similar to the fjord outside) Steps imitating the littoral levels
E23 m3 6.25/11.75 m3 17 strokes/min 1 m depth E5 m3/h E2 h 12 h 20 min 36 cm 4 of each tank
Waves are created by a horizontal girder (wave bar; 20 20 335 cm3) situated half-way to the flat bottom parallel to the steps. The girder was fixed at both sides at a distance of 1 m from a horizontal axis swinging over a 401 angle with a frequency of 17 r.p.m. This frequency corresponded to the resonance frequency of the water in the basins and resulted in moderate wave energy compared with the environment in the inner Oslofjord from where the mesocosm communities were transplanted. The waves created by the generators are extremely regular, and
T.L. Bokn et al. / Continental Shelf Research 21 (2001) 1925–1936
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Fig. 2. Diagram of one mesocosm with four steps mimicking the intertidal zone, a wave generator and a tidal regulator. Water input is through a pipe at the northern-east corner and the outlet is in the south end of the basin.
Fig. 3. Tidal curves of (a) the old tidal regime and (b) the new one.
the currents make well-mixed water in the basins. The water turbulence has been measured at different places inside the basins. Based on the geometry of the basin, the water volume is calculated for each water depth. In the volume calculations, the volume of objects in the basins have to be subtracted. The total volume of permanent objects (wave bar, supporting constructions, artificial substrates) is estimated to be less than 0.1 m3. The total volume of the
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vegetation and fauna varies in time, but does not exceed 0.2 m3, including the boulders on which the macro-algae are attached. In the volume calculation, a correction of 0.2 m3 is assumed. At low tide (0.57 m), the water volume is 6.24 m3 and at high tide (0.93 m) it is 11.65 m3. The volume varies with the depth according to the equation: Volume ¼ 0:0902Depth þ 0:0004Depth2 0:2 ðvolume in m3 ; depth in cmÞ: Depending on the water level, and thus the water volume, the nominal residence time varies between 1 h 15 min and 2 h 19 min.
4. Nutrient manipulation Nutrient concentrations in the Oslofjord at Solbergstrand varies in the range of 1–25 mmol N/l and 0.1–0.8 mmol P/l. Background concentration and annual variation in input concentration to the system are presented in Fig. 4 for the first experimental year (June 1998–June 1999). High winter values appear in late October and decrease in March during the spring bloom in the Oslofjord. There is a tendency of increased concentrations during the period of low salinity in input waters (May and June 1999) related to the nutrient content of freshwater runoff to the fjord. Extreme runoff during May and June 1999 resulted in high values of nutrients during the summer 1999 compared with the preceding summer. It was decided to superimpose these concentrations with a gradient from 1 to 32 mmol N/l and with an N/P ratio of 16 : 1 (Redfield ratio). The gradient was set up according to Fig. 5 in a randomised treatment design.
Fig. 4. Background concentration of nutrients and salinity in the intake waters to the EULIT mesocosms during one year, June 1998–June 1999.
T.L. Bokn et al. / Continental Shelf Research 21 (2001) 1925–1936
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Fig. 5. Applied nutrient concentrations in the EULIT mesocosms (basins 1–8).
4.1. Nutrient addition and regularity Nutrient addition started in May 1998. The nutrients are added as phosphoric acid (H3PO4) and ammonium nitrate (NH4NO3) of technical grade quality. The amount of fertilisers used totally in all the basins per year sums up to 8 kg H3PO4 and 55 kg NH4NO3 at a continuous operation for 365 days/yr. These two fertiliser compounds are mixed in a stock solution to be diluted further (B6–200 times dilution) into the dosing tanks. The dosing tanks are refilled weekly to ensure a continuous dosing of the mesocosms and actual amounts dosed are logged. The system is inspected daily for dosing irregularities and leakage, etc. The operation has been maintained continuously, with only a few minor disruptions. The dosing pump (Watson Marlow, type 505 S) was set to a feeding velocity of 1 ml/min and the nutrient solutions were mixed with approximately 200 ml freshwater ahead of the input to the basins to smooth the transport through the tubings. 4.2. Measurements Nutrient analyses are performed on weekly, pooled samples (5 samples/week) and analysed for PO4-P, NO3-N, NH4-N, total nitrogen, and total phosphorus. The samples are taken close to the outflow from the basins. Sample preservation and analysis are according to the standard methods at NIVA’s laboratory. (Norwegian Standard (NS): phosphate, modified NS 4724; nitrate, modified NS 4745; ammonium, modified NS 4746; total phosphorus, modified NS 4725 and total nitrogen, modified NS 4743). Examples of measured nutrient concentrations from two mesocosms are shown in Fig. 6. Table 2 shows the annual average background levels, dosed amounts, and total calculated input of N and P compared with the measured concentrations in the basins. 4.3. Internal mixing and homogeneity of nutrient concentrations The mixing and homogeneity of dosed nutrients were controlled during one day in July 1999 in some of the basins. The sampling positions were equally distributed within the basins with 9 samples from the surface and 6 at the bottom. The results are shown in Table 3. The control basins and low nutrient basins have the highest variation, and the homogeneity increases with
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Fig. 6. Example of measured nutrient concentrations (outflow) in the two manipulated mesocosms B2 (high) and B5 (low) during first EULIT year June 1998–June 1999. Included background concentrations.
Table 2 Annual mean concentration of background, dosed, total input and measured nutrients in the basins during one EULIT year June 1998–June 1999 (mmol/l) Basin
1 8 4 5 6 3 7 2
Background
Dosed
Input
Measured
P
N
P
N
P
N
P
N
0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33
10.62 10.62 10.62 10.62 10.62 10.62 10.62 10.62
0 0 0.063 0.125 0.25 0.5 1 2
0 0 1 2 4 8 16 32
0.33 0.33 0.393 0.455 0.58 0.83 1.33 2.33
10.62 10.62 11.62 12.62 14.62 18.62 26.62 42.62
0.33 0.32 0.37 0.44 0.56 0.79 1.20 2.11
10.09 9.73 10.56 11.99 13.74 18.01 24.43 38.96
higher dosing. The mixing seems to be effective and the nutrients added are evenly distributed within the basins.
5. Light, temperature, oxygen and salinity ABB model 9408 submersible oxygen sensors provided with model 8012-170 sensor capsules are installed in all mesocosms and in the inflowing water. The sensors are lowered in a 1.2 m long tube
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with an inner diameter of 5 cm fitted to the axis of the wave machine down to the wave bar proper. The sensors extend about 5 cm outside the tube to make sure that they experience the oxygen concentration of the mesocosm. The movement of the wave machine assures that the sensors experience a water flow above 0.3 m/s, which is necessary for a reliable oxygen signal. One electrode is placed in the inlet tube in basin 7, which measures the oxygen concentrations of the inflowing water. The water flow in the inlet tube is also higher than 0.3 m/s. The electrodes are connected to ABB model 4640-800 transmitters set to read percentage saturation. The transmitters are provided with an electrical output between 4 and 20 mA for oxygen and temperature. In April 1999, a WTW model Trioxmatic 600-DU oxygen sensor coupled to a WTW model Trioxmatic 161 T oxygen meter was installed in the inflow of basin 8. The instrument was set to read percentage saturation and the output of oxygen concentration and temperature was set to 4–20 mA. In basin 6, a WTW model LF161 conductivity meter measures the salinity. In basins 2 and 8, Dynamic Logic model SH3102 depth sensors are installed. On the roof of a nearby
Table 3 Example of the variation (CV%) of nutrients in the control basin and some of the manipulated basins Basin
Mean P (mM)
SD P (mM)
CV P (%)
Mean N (mM)
SD N (mM)
CV N (%)
1 4 5 6 2
0.08 0.08 0.14 0.34 1.40
0.02 0.02 0.02 0.06 0.03
26.5 28.9 14.2 18.0 2.1
3.18 3.25 4.71 5.46 27.0
0.35 0.20 0.25 0.22 0.31
11.0 6.0 5.4 4.1 1.1
Fig. 7. Example of the diurnal variation of the oxygen saturation values of the different basins related to the amount of light.
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building a LiCor model Li-190SA light meter measuring PAR is installed. Barometric pressure is measured with a Vaisala model PTA 427 Pressure Transmitter. All output signals are fed into a Campbell datalogger system based on the model 10CX datalogger. The datalogger system is placed in the main building and the connections are electrically separated to avoid problems of strong static electrical fields (lightning, thunderstorms, etc.). Every 30 s readings are taken by the data logger system and 15 min averages are stored. The stored data are retrieved once a day by PCs at Solbergstrand, and at the IBN Texel (Netherlands). A typical example of data output is shown in Fig. 7. The oxygen saturation values of the different basins show a clear diurnal variation related to the amount of light. The inflow showed considerable and rapid variation in the oxygen saturation. Similar variations are observed in the
Fig. 8. Abundance (per cent cover, July 1996) of fucoids transplanted to the four steps exemplified by four of the mesocosms (B1–B4).
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Table 4 Total number of taxa registered and abundant species within the monitoring area on the littoral steps in the basins at the start of the eutrophication experiment (May 1998) (d, dominant; c, common; r, rare) Basin
B1
B2
B3
B4
B5
B6
B7
B8
Number of taxa
37
41
41
38
32
38
42
40
Cyanophyceae
c
c
c
c
c
c
c
c
Phaeophyceae Ascophyllum nodosum Fucus vesiculosus Fucus serratus Fucus evanescens Fucus juvenile Elachista fucicola Ectocarpales Sphacelaria sp.
c d d r c c c r
c d d F c c c F
c d d r c c c F
c d d r r c c F
c d d F c c c r
c d d F r c c r
c d d F c c c r
c d d F c c c F
Rhodophyceae Ceramium rubrum Chondrus crispus Dumontia contorta Hildenbrandia rubra Phymatolithon lenormandii Polysiphonia elongata Polysiphonia fucoides Polysiphonia stricta Polysiphonia fibrillosa Rhodomela confervoides
c r F c r r c r r r
c r F c r r c r r r
c r r c r F c r r r
c r r c r r c r r r
c r F c r F c r r r
c r r c r r c r r r
c r r c r r c r r r
c r r c r r c r r r
Chlorophyceae Cladophora sp. Cladophora rupestris Enteromorpha sp. Ulothrix/Urospora Ulva lactuca
F r r c r
F r r c r
r r r c c
F r r c r
F r r c r
F r r c c
r r r c r
r r r c r
Animals Amphipods Asterias rubens Athanas sp. Balanus sp. Bittium sp. Carcinus maenas Idotea sp. Jaera sp. Littorina littorea Littorina mariae/obtusata Metridium senile Mytilus edulis Nereis sp. Platyhelminthes sp. Polynoida sp. Rissoa sp.
c r r c r r r c c r F r F r F c
c r r r r r r c c r F r r F r c
c r r c r r r c c F r r r F F r
r r r c r r r c c F F r F F r c
c r r c r
r r r r r r r r c F r r r r F r
c r r c r r r r c r F r F r r r
r r r c r r r r c r r c F r r r
r c c F F r F F F r
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temperature. These rapid variations can be explained as an effect of vertical stratification at the depth of the water intake. Internal waves lead to water intake from different strata. 6. Community establishment in the basins The communities were established in June 1996, by transplantation of rocks covered by algae and animals from the littoral and upper part of the sublittoral zone at the adjacent shoreline. Rocks were positioned on four steps in all the basins covering all levels of the littoral and upper sublittoral zones. Four species of fucoids (Ascophyllum nodosum (L.) Le Jol., Fucus vesiculosus L., F. evanescens C. Agardh and F. serratus L.) represent the key species, and one species dominates each of the steps (Fig. 8). The blue mussel Mytilus edulis L. and periwinkle Littorina littorea L. are regarded as the animal key species. During the succeeding months, the establishment continued by self-propagation and from larvae, zygotes and spores entering the basins with the seawater. Natural community development has been allowed for about 2 yr ahead of the EULIT experiment with the nutrient manipulation. In the beginning of May 1998, the communities on the steps consisted of approx. 40 species. The most abundant species are shown in Table 4. During this pilot project even the efficiency of relevant monitoring techniques and statistical design were tested. The pilot project has thus provided an appropriate length of the EULIT experimental period. Acknowledgements This work is a contribution to the ELOISE Programme (publication No. 235) in the framework of the EULIT project carried out under Contract MAS3-CT97-0153. The project is also partly funded by the MARICULT/Norsk Hydro Programme. We would like to thank Torgeir Bakke and Lars Kirkerud for their valuable advise and Einar Johannessen for assistance with the description of the methods. This paper represents also Contribution No. 33 from the Marine Research Station, Solbergstrand, Norway. References Bakke, T., 1986. Experimental long term oil pollution in a boreal rocky shore environment. Proceedings of the Ninth Annual AMOP Technical Seminar, June 1986. Beauregard Press, Ltd., Ottawa, pp. 167–178. Bakke, T., 1990. Benthic mesocosms: II. basic research in hard-bottom benthic mesocosms. In: Lalli, C.M. (Ed.), Coastal and Estuarine Studies. Springer, New York, pp. 122–135. Bokn, T., 1984. Effects of diesel oil on recolonization of benthic algae. Hydrobiologia 116/117, 383–388. Bokn, T., Kirkerud, L., 1981. Oil monitoring experiments on marine littoral communities kept in basins. Water and Science Technology 13, 625–629. Bokn, T.L., Moy, F.E., Murray, S.N., 1993. Long-term effects of the water-accomodated fraction (WAF) of diesel oil on rocky shore populations maintained in experimental mesocosms. Botanica Marina 36, 313–319. Gray, J.S., 1987. Oil pollution studies of the Solbergstrand mesocosms. Philosophical Transactions of Royal Society London B 316, 641–654. . Notini, M., Nagell, B., Hagstrom, A., Grahn, O., 1977. An outdoor model simulating a Baltic Sea littoral ecosystem. Oikos 28, 2–9.
Review article
Methods applied in the large littoral mesocosms study of nutrient enrichment in rocky shore ecosystemsFEULIT T.L. Bokna,*, E.E. Hoellb, K. Kerstingc, F.E. Moya, K. S rensena a
Norwegian Institute for Water Research (NIVA), P.O. Box 173 Kjelsaas, N-0411 Oslo, Norway Norsk Hydro, Research Centre Porsgrunn (HRE), P.O. Box 2560, N-3901 Porsgrunn, Norway c IBN-DLO, P.O. Box 167, 1790 AD Den Burg, The Netherlands
b
Received 8 December 1999; accepted 15 January 2001
Abstract Eight concrete land-based mesocosms have been set up for a study of the effect of nutrient enrichment on littoral hard bottom ecosystems. The construction of the mesocosms and the community establishment were initiated 2 yr ahead of the MAST-III project. The littoral communities were established by transplanting rocks with attached macroalgae and associated animals from the Oslofjord, followed by a period of natural community development which has resulted in a highly diverse flora and fauna. During this 2-yr pilot project, the efficiency of relevant techniques and statistical design for the EULIT experiments in the mesocosms have been tested, as well as the performance of the mesocosms ahead of the nutrient manipulation. The pilot project has thus extended the total experimental period. The mesocosms are fed with water from 1 m depth in the OslofjordFresidence time about 2 hFwhich also acts as a source for spores, zygotes and larvae. A tidal regime with a 36 cm amplitude is maintained and waves are regularly generated. Oxygen concentration, temperature and salinity are measured continuously by probes. Nutrient addition started in May 1998. Nutrient and carbon input and outflow are measured on an average weekly basis. Community analyses, species compositions, biomass, growth and grazing of the different components of the mesocosms are studied during 3-week periods (spring, summer and autumn) each year. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: methods
Phytobenthos; Zoobenthos; Marine mesocosms; Nutrient experiments; Eutrophication; Standard
*Corresponding author. Tel.: +47-22-185173; fax:+47-22-185200. E-mail address: [email protected] (T.L. Bokn). 0278-4343/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 8 - 4 3 4 3 ( 0 1 ) 0 0 0 3 5 - 8
1926
T.L. Bokn et al. / Continental Shelf Research 21 (2001) 1925–1936
1. Introduction The Marine Research Station, Solbergstrand’s facilities, situated 30 km south of Oslo by the Oslofjord, Norway, is the most comprehensive system reported dealing with marine rocky shore mesocosms (Bakke, 1990). Manipulation with marine ecosystems in mesocosms was started at the end of the 1970s with four 60 m3 big tanks constructed for studies of rocky shore, intertidal communities (Bokn and Kirkerud, 1981). During 1982–1985, these communities were used as a target for diesel oil effect studies on contract for British Petroleum (Bokn, 1984; Bakke, 1986; Gray, 1987; Bokn et al., 1993). The Baltic Sea Laboratory in Karlskrona, Sweden, has been running smaller mesocosms with estuarine ecosystems from the early 1970s (Notini et al., 1977). A recent bibliographical search has not revealed any other experiments with rocky shore mesocosms. The overall objectives for the EULIT project is to predict the response of rocky shore ecosystems to an increased nutrient availability. By use of the mesocosms the aim is: *
*
*
*
*
*
*
*
To detect quantitative and qualitative changes in the plant- and associated animal-community with increased nutrient availability. To test whether increased nutrient loading may affect biodiversity characteristics in the rocky shore ecosystem either directly or indirectly through changes of key species. To analyse the relationship between eutrophication induced changes at the level of individual species performance (biomass, cover, growth rates, production) with changes at the community level (species composition, species interactions, dominance-diversity patterns) and at the ecosystem level (overall production and respiration, flows of matter and energy). To detect if the functional changes such as primary production and consumption occur before structural changes are observed. To follow the changes in the processes at different organisational scales and compare the turnover rates of the communities and the sub-communities. To explain changes in plant community composition and production from alterations in growth rates and grazing losses of the specific components (species or functional groups). To develop a model of rocky shore responses to an increased nutrient load, based on the processes measured and make simulations of large-scale events and predict effects of nutrient addition. To support the European ELOISE initiative with the increased knowledge on nutrient cycles in the coastal environment, relation between excess nutrient load and eutrophication effects, pressure in the coastal shore line, and ecological management of coastal zone resources
The present EULIT project was preceeded by a pilot project dealing with the construction of the mesocosms, community establishment and applicable technics for nutrient manipulation. The idea was to mimic natural rocky shore marine environment within the controlled experimental facilities. A disadvantage with these experiments lies in the conflict between the need for the statistical replication and the need for representative gradients with sufficient data. As a compromise, we chose to set up the experiments with two controls and the gradient of six nutrient additions without parallels. Bakke (1990) raised several questions regarding community structure and function, which experiments can be preferably performed in situ and which projects will be improved through
T.L. Bokn et al. / Continental Shelf Research 21 (2001) 1925–1936
1927
manipulating the environmental regime in mesocosms. Hard-bottom model ecosystems have proven to be valuable experimental tools in pollution research and they are the only alternatives to laboratory experiments if chronic discharge into coastal water is to be imitated. The present project EULIT, dealing with nutrient (N and P) effects on intertidal, rocky shore communities using mesocosms meets many of Bakke’s criteria for testing hypotheses concerning structure and function (Bakke, 1986). The many subtasks performed in the EULIT basins assume appropriate design and reliable standardized methods for the current mesocosm project. Of great importance are the concrete basins, the water system, nutrient manipulation, light, temperature, oxygen and salinity, and community establishment in the basins. The aim of this paper is to describe in detail the standard methods used in the mesocosms of the EU project MAS3-CT97-0153 (EULIT).
2. The concrete basins The mesocosm facility consists of two rows of four mesocosms each. Each mesocosm is a concrete basin, 4.75 m long, 3.65 m wide and 1.35 m deep (about 23 m3) oriented with the long side in west-east direction. The western part of the basin is a flat bottom of 2.70 m 3.65 m. In the eastern part a series of five steps of 0.4 m width, slightly slanting down from south to north is constructed. The eastern-most step (step 0) extends from 1.0 m above the bottom at the south wall to 0.86 m above the bottom at the north wall. Each next step is 0.18 m lower than the previous step. The last step (step 4) extends from 0.28 m at the south wall to 0.14 m at the north wall. Fig. 1 gives an overview of the eight basins and Table 1 presents some technical data.
3. The water system Water from the Oslofjord is taken at 1 m depth and supplied to each mesocosm (Fig. 2) at a rate of 5 m3/h. A tidal regime with an amplitude of 0.36 m and a period of 12 h 20 min is applied. The high tide level is set at 0.93 m (half way step 0) and the low tide level at 0.57 m (half way step 2). The tidal level was regulated by raising and lowering the outflow. During the pilot project, the water left the basin through a vertical pipe (10 cm Ø) 10 cm below the low tide water-level. Each outflow pipe was connected to a flexible hose outside the basins. The flexible hoses of all basins were simultaneously raised and lowered to create the tidal regime. This system was replaced at the start of the EULIT project (April 1998) by export through a slanting pipe inside the basin that is raised and lowered by an activator (ELERO, type Econom 0 G/A) in each basin (Fig. 2). This latter construction is more accurate and flexible. The former tidal system used in the pilot project functions as a backup system for the current EULIT project. The original outflow took its water from 10 to 46 cm below the water surface while the reconstructed outflow skims the water surface. This might influence the export of material from the basins. In June 1998, the programme controlling the tidal activators was changed to achieve a sinusoidal tidal curve. The form of the tidal curve is shown in Fig. 3.
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Fig. 1. Eight mesocosms used in the EULIT project.
Table 1 Technical data of the eight mesocosms Volume in each tank (4.75 3.65 1.35 m3) Volume of seawaterFlow tide/high tide Wave generator Water intake Flow rate Water residence time (mean) Tide level mimic Nominal amplitude (similar to the fjord outside) Steps imitating the littoral levels
E23 m3 6.25/11.75 m3 17 strokes/min 1 m depth E5 m3/h E2 h 12 h 20 min 36 cm 4 of each tank
Waves are created by a horizontal girder (wave bar; 20 20 335 cm3) situated half-way to the flat bottom parallel to the steps. The girder was fixed at both sides at a distance of 1 m from a horizontal axis swinging over a 401 angle with a frequency of 17 r.p.m. This frequency corresponded to the resonance frequency of the water in the basins and resulted in moderate wave energy compared with the environment in the inner Oslofjord from where the mesocosm communities were transplanted. The waves created by the generators are extremely regular, and
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Fig. 2. Diagram of one mesocosm with four steps mimicking the intertidal zone, a wave generator and a tidal regulator. Water input is through a pipe at the northern-east corner and the outlet is in the south end of the basin.
Fig. 3. Tidal curves of (a) the old tidal regime and (b) the new one.
the currents make well-mixed water in the basins. The water turbulence has been measured at different places inside the basins. Based on the geometry of the basin, the water volume is calculated for each water depth. In the volume calculations, the volume of objects in the basins have to be subtracted. The total volume of permanent objects (wave bar, supporting constructions, artificial substrates) is estimated to be less than 0.1 m3. The total volume of the
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vegetation and fauna varies in time, but does not exceed 0.2 m3, including the boulders on which the macro-algae are attached. In the volume calculation, a correction of 0.2 m3 is assumed. At low tide (0.57 m), the water volume is 6.24 m3 and at high tide (0.93 m) it is 11.65 m3. The volume varies with the depth according to the equation: Volume ¼ 0:0902Depth þ 0:0004Depth2 0:2 ðvolume in m3 ; depth in cmÞ: Depending on the water level, and thus the water volume, the nominal residence time varies between 1 h 15 min and 2 h 19 min.
4. Nutrient manipulation Nutrient concentrations in the Oslofjord at Solbergstrand varies in the range of 1–25 mmol N/l and 0.1–0.8 mmol P/l. Background concentration and annual variation in input concentration to the system are presented in Fig. 4 for the first experimental year (June 1998–June 1999). High winter values appear in late October and decrease in March during the spring bloom in the Oslofjord. There is a tendency of increased concentrations during the period of low salinity in input waters (May and June 1999) related to the nutrient content of freshwater runoff to the fjord. Extreme runoff during May and June 1999 resulted in high values of nutrients during the summer 1999 compared with the preceding summer. It was decided to superimpose these concentrations with a gradient from 1 to 32 mmol N/l and with an N/P ratio of 16 : 1 (Redfield ratio). The gradient was set up according to Fig. 5 in a randomised treatment design.
Fig. 4. Background concentration of nutrients and salinity in the intake waters to the EULIT mesocosms during one year, June 1998–June 1999.
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Fig. 5. Applied nutrient concentrations in the EULIT mesocosms (basins 1–8).
4.1. Nutrient addition and regularity Nutrient addition started in May 1998. The nutrients are added as phosphoric acid (H3PO4) and ammonium nitrate (NH4NO3) of technical grade quality. The amount of fertilisers used totally in all the basins per year sums up to 8 kg H3PO4 and 55 kg NH4NO3 at a continuous operation for 365 days/yr. These two fertiliser compounds are mixed in a stock solution to be diluted further (B6–200 times dilution) into the dosing tanks. The dosing tanks are refilled weekly to ensure a continuous dosing of the mesocosms and actual amounts dosed are logged. The system is inspected daily for dosing irregularities and leakage, etc. The operation has been maintained continuously, with only a few minor disruptions. The dosing pump (Watson Marlow, type 505 S) was set to a feeding velocity of 1 ml/min and the nutrient solutions were mixed with approximately 200 ml freshwater ahead of the input to the basins to smooth the transport through the tubings. 4.2. Measurements Nutrient analyses are performed on weekly, pooled samples (5 samples/week) and analysed for PO4-P, NO3-N, NH4-N, total nitrogen, and total phosphorus. The samples are taken close to the outflow from the basins. Sample preservation and analysis are according to the standard methods at NIVA’s laboratory. (Norwegian Standard (NS): phosphate, modified NS 4724; nitrate, modified NS 4745; ammonium, modified NS 4746; total phosphorus, modified NS 4725 and total nitrogen, modified NS 4743). Examples of measured nutrient concentrations from two mesocosms are shown in Fig. 6. Table 2 shows the annual average background levels, dosed amounts, and total calculated input of N and P compared with the measured concentrations in the basins. 4.3. Internal mixing and homogeneity of nutrient concentrations The mixing and homogeneity of dosed nutrients were controlled during one day in July 1999 in some of the basins. The sampling positions were equally distributed within the basins with 9 samples from the surface and 6 at the bottom. The results are shown in Table 3. The control basins and low nutrient basins have the highest variation, and the homogeneity increases with
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Fig. 6. Example of measured nutrient concentrations (outflow) in the two manipulated mesocosms B2 (high) and B5 (low) during first EULIT year June 1998–June 1999. Included background concentrations.
Table 2 Annual mean concentration of background, dosed, total input and measured nutrients in the basins during one EULIT year June 1998–June 1999 (mmol/l) Basin
1 8 4 5 6 3 7 2
Background
Dosed
Input
Measured
P
N
P
N
P
N
P
N
0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33
10.62 10.62 10.62 10.62 10.62 10.62 10.62 10.62
0 0 0.063 0.125 0.25 0.5 1 2
0 0 1 2 4 8 16 32
0.33 0.33 0.393 0.455 0.58 0.83 1.33 2.33
10.62 10.62 11.62 12.62 14.62 18.62 26.62 42.62
0.33 0.32 0.37 0.44 0.56 0.79 1.20 2.11
10.09 9.73 10.56 11.99 13.74 18.01 24.43 38.96
higher dosing. The mixing seems to be effective and the nutrients added are evenly distributed within the basins.
5. Light, temperature, oxygen and salinity ABB model 9408 submersible oxygen sensors provided with model 8012-170 sensor capsules are installed in all mesocosms and in the inflowing water. The sensors are lowered in a 1.2 m long tube
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with an inner diameter of 5 cm fitted to the axis of the wave machine down to the wave bar proper. The sensors extend about 5 cm outside the tube to make sure that they experience the oxygen concentration of the mesocosm. The movement of the wave machine assures that the sensors experience a water flow above 0.3 m/s, which is necessary for a reliable oxygen signal. One electrode is placed in the inlet tube in basin 7, which measures the oxygen concentrations of the inflowing water. The water flow in the inlet tube is also higher than 0.3 m/s. The electrodes are connected to ABB model 4640-800 transmitters set to read percentage saturation. The transmitters are provided with an electrical output between 4 and 20 mA for oxygen and temperature. In April 1999, a WTW model Trioxmatic 600-DU oxygen sensor coupled to a WTW model Trioxmatic 161 T oxygen meter was installed in the inflow of basin 8. The instrument was set to read percentage saturation and the output of oxygen concentration and temperature was set to 4–20 mA. In basin 6, a WTW model LF161 conductivity meter measures the salinity. In basins 2 and 8, Dynamic Logic model SH3102 depth sensors are installed. On the roof of a nearby
Table 3 Example of the variation (CV%) of nutrients in the control basin and some of the manipulated basins Basin
Mean P (mM)
SD P (mM)
CV P (%)
Mean N (mM)
SD N (mM)
CV N (%)
1 4 5 6 2
0.08 0.08 0.14 0.34 1.40
0.02 0.02 0.02 0.06 0.03
26.5 28.9 14.2 18.0 2.1
3.18 3.25 4.71 5.46 27.0
0.35 0.20 0.25 0.22 0.31
11.0 6.0 5.4 4.1 1.1
Fig. 7. Example of the diurnal variation of the oxygen saturation values of the different basins related to the amount of light.
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building a LiCor model Li-190SA light meter measuring PAR is installed. Barometric pressure is measured with a Vaisala model PTA 427 Pressure Transmitter. All output signals are fed into a Campbell datalogger system based on the model 10CX datalogger. The datalogger system is placed in the main building and the connections are electrically separated to avoid problems of strong static electrical fields (lightning, thunderstorms, etc.). Every 30 s readings are taken by the data logger system and 15 min averages are stored. The stored data are retrieved once a day by PCs at Solbergstrand, and at the IBN Texel (Netherlands). A typical example of data output is shown in Fig. 7. The oxygen saturation values of the different basins show a clear diurnal variation related to the amount of light. The inflow showed considerable and rapid variation in the oxygen saturation. Similar variations are observed in the
Fig. 8. Abundance (per cent cover, July 1996) of fucoids transplanted to the four steps exemplified by four of the mesocosms (B1–B4).
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Table 4 Total number of taxa registered and abundant species within the monitoring area on the littoral steps in the basins at the start of the eutrophication experiment (May 1998) (d, dominant; c, common; r, rare) Basin
B1
B2
B3
B4
B5
B6
B7
B8
Number of taxa
37
41
41
38
32
38
42
40
Cyanophyceae
c
c
c
c
c
c
c
c
Phaeophyceae Ascophyllum nodosum Fucus vesiculosus Fucus serratus Fucus evanescens Fucus juvenile Elachista fucicola Ectocarpales Sphacelaria sp.
c d d r c c c r
c d d F c c c F
c d d r c c c F
c d d r r c c F
c d d F c c c r
c d d F r c c r
c d d F c c c r
c d d F c c c F
Rhodophyceae Ceramium rubrum Chondrus crispus Dumontia contorta Hildenbrandia rubra Phymatolithon lenormandii Polysiphonia elongata Polysiphonia fucoides Polysiphonia stricta Polysiphonia fibrillosa Rhodomela confervoides
c r F c r r c r r r
c r F c r r c r r r
c r r c r F c r r r
c r r c r r c r r r
c r F c r F c r r r
c r r c r r c r r r
c r r c r r c r r r
c r r c r r c r r r
Chlorophyceae Cladophora sp. Cladophora rupestris Enteromorpha sp. Ulothrix/Urospora Ulva lactuca
F r r c r
F r r c r
r r r c c
F r r c r
F r r c r
F r r c c
r r r c r
r r r c r
Animals Amphipods Asterias rubens Athanas sp. Balanus sp. Bittium sp. Carcinus maenas Idotea sp. Jaera sp. Littorina littorea Littorina mariae/obtusata Metridium senile Mytilus edulis Nereis sp. Platyhelminthes sp. Polynoida sp. Rissoa sp.
c r r c r r r c c r F r F r F c
c r r r r r r c c r F r r F r c
c r r c r r r c c F r r r F F r
r r r c r r r c c F F r F F r c
c r r c r
r r r r r r r r c F r r r r F r
c r r c r r r r c r F r F r r r
r r r c r r r r c r r c F r r r
r c c F F r F F F r
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temperature. These rapid variations can be explained as an effect of vertical stratification at the depth of the water intake. Internal waves lead to water intake from different strata. 6. Community establishment in the basins The communities were established in June 1996, by transplantation of rocks covered by algae and animals from the littoral and upper part of the sublittoral zone at the adjacent shoreline. Rocks were positioned on four steps in all the basins covering all levels of the littoral and upper sublittoral zones. Four species of fucoids (Ascophyllum nodosum (L.) Le Jol., Fucus vesiculosus L., F. evanescens C. Agardh and F. serratus L.) represent the key species, and one species dominates each of the steps (Fig. 8). The blue mussel Mytilus edulis L. and periwinkle Littorina littorea L. are regarded as the animal key species. During the succeeding months, the establishment continued by self-propagation and from larvae, zygotes and spores entering the basins with the seawater. Natural community development has been allowed for about 2 yr ahead of the EULIT experiment with the nutrient manipulation. In the beginning of May 1998, the communities on the steps consisted of approx. 40 species. The most abundant species are shown in Table 4. During this pilot project even the efficiency of relevant monitoring techniques and statistical design were tested. The pilot project has thus provided an appropriate length of the EULIT experimental period. Acknowledgements This work is a contribution to the ELOISE Programme (publication No. 235) in the framework of the EULIT project carried out under Contract MAS3-CT97-0153. The project is also partly funded by the MARICULT/Norsk Hydro Programme. We would like to thank Torgeir Bakke and Lars Kirkerud for their valuable advise and Einar Johannessen for assistance with the description of the methods. This paper represents also Contribution No. 33 from the Marine Research Station, Solbergstrand, Norway. References Bakke, T., 1986. Experimental long term oil pollution in a boreal rocky shore environment. Proceedings of the Ninth Annual AMOP Technical Seminar, June 1986. Beauregard Press, Ltd., Ottawa, pp. 167–178. Bakke, T., 1990. Benthic mesocosms: II. basic research in hard-bottom benthic mesocosms. In: Lalli, C.M. (Ed.), Coastal and Estuarine Studies. Springer, New York, pp. 122–135. Bokn, T., 1984. Effects of diesel oil on recolonization of benthic algae. Hydrobiologia 116/117, 383–388. Bokn, T., Kirkerud, L., 1981. Oil monitoring experiments on marine littoral communities kept in basins. Water and Science Technology 13, 625–629. Bokn, T.L., Moy, F.E., Murray, S.N., 1993. Long-term effects of the water-accomodated fraction (WAF) of diesel oil on rocky shore populations maintained in experimental mesocosms. Botanica Marina 36, 313–319. Gray, J.S., 1987. Oil pollution studies of the Solbergstrand mesocosms. Philosophical Transactions of Royal Society London B 316, 641–654. . Notini, M., Nagell, B., Hagstrom, A., Grahn, O., 1977. An outdoor model simulating a Baltic Sea littoral ecosystem. Oikos 28, 2–9.