BELLA workshop
Productivity of large lakes
1
FISHERIES CHANGES IN LAKE TANGANYIKA IN RELATION WITH CLIMATE AND
LIMNOLOGICAL VARIABILITY
Plisnier, P.-D.1, Y.Cornet 2, J. Naithani3, S.Horion2, E. Deleersnijder3,5, Makasa6, C. ,
F.Zulu6, J.Chimanga6, A. Chande7, I. Kimirei7, H. Mgana7, & J.-P. Descy8
1
Royal Museum for Central Africa, Leuvensesteenweg, 13 B - 3080 Tervuren, Belgium
Unité de Géomatique, Dépt. de Géographie - Fac. des Sciences- ULg
Laboratoire SURFACES Allée du 6 Août, 17, Bât B5, B-4000 Sart-Tilman (Liège), Belgium
3
Institute of Astronomy and Geophysics G. Lemaître, Catholic University of Louvain
Chemin du Cyclotron, 2, B - 1348 Louvain-la-Neuve, Belgium
4
Dipt. di Scienze Chimiche e dei Biosistemi, Università degli Studi di Siena, Via Aldo Moro
2, 53100 Siena, Italy
5
Centre for Systems Engineering and Applied Mechanics (CESAME), 4 Avenue G. Lemaître,
Catholic University of Louvain, B - 1348 Louvain-la-Neuve, Belgium
6
Department of Fisheries, BP 55 Mpulungu, Zambia
7
Tanzanian Fisheries Research Institute, BP 90 Kigoma, Tanzania
8
Laboratory of Freshwater Ecology, URBO, University of Namur, Rue de Bruxelles 61, B 5000 Namur, Belgium
2
Currently, three main species of pelagic fishes are caught in Lake Tanganyika: Lates
stappersii, Stolothrissa tanganicae and Limnothrissa miodon. Their catches per unit of effort
(boat night) display considerable changes at different time scales (monthly, seasonal,
interannual, trend..). It is observed that fish catches per unit of effort (CPUE) are largely
correlated with environmental variability in lake Tanganyka.
Internal waves and climate condition (air temperature, wind speed..) have a major impact on
mixing and physico-chemistry of the epilimnion waters as well as on planktonic abundance,
and fishes (absence/presence, reproduction, fat content...) in diverses areas of the lake. Their
impact on the biotic upper layers lead to varying specific optimal limnological conditions that
are fluctuating spatially at all mentioned time scales.
Longer time series indicate that decreases in CPUE of Stolothrissa tanganicae and
Limnothrissa miodon happened simultaneously at both lake ends, and corresponded with an
increased rate of air temperature warming. At the same moment, Lates stappersii CPUE
increased considerably in the south of the lake in relation with limnological changes. The
increased fishing effort effect shows either less or even inconsistent relations with fish CPUE,
contrarily to climate and limnological variability. Strong links were found between CPUE of
pelagic species with local, regional and global scale climate or proxy climatic indices
including ENSO and the Pacific Decadal Oscillation. Some of those could be used for
forecasting fisheries catches.
Living organisms integrate many environmental parameters. In Lake Tanganyika, the
fluctuating abundance of pelagic fishes may be considered as a signal of major environmental
changes that modified the hydrodynamics and spatial repartition of the pelagic fishes in the
lake, particularly in the last 30 years.
BELLA workshop
Productivity of large lakes
2
HYDRODYNAMIC MODELLING OF LAKE TANGANYIKA BY MEANS OF A FINITE
ELEMENT MODEL: WIND-INDUCED THERMOCLINE OSCILLATIONS AND
EPILIMNION WATER RENEWAL
Gourgue, O.1 & Deleersnijder, E.2,1
1
Centre for Systems Engineering and Applied Mechanics (CESAME), Université catholique
de Louvain, Louvain-la-Neuve, Belgium.
2
Georges Lemaître Institute of Astronomy and Geophysics (ASTR), Université catholique de
Louvain, Louvain-la-Neuve, Belgium.
In Lake Tanganyika, the primary production is mainly influenced by the water exchange
between the epilimnion and the hypolimnion (vast reservoir of nutrients), and by the
thermocline displacements. This is why these processes have to be simulated by any
hydrodynamic model of the lake that is to be coupled to ecological models.
Within the framework of SLIM (Second-generation Louvain-la-Neuve Ice-ocean Model –
www.cliamte.be/SLIM), we have built a finite element reduced-gravity model of Lake
Tanganyika, in order to study the wind-induced oscillations of the thermocline. The wind
stress may be decomposed into a seasonal component and an intraseasonal one, respectively
responsible for the free and forced oscillations of the thermocline. It is found that the forced
oscillations are much larger than the free ones.
We also defined a method to study the water renewal in any semi-enclosed domain. With the
above-mentioned reduced-gravity model, we applied this method to the epilimnion of Lake
Tanganyika to evaluate the age of the different water types present in it (water from
hypolimnion, from precipitation and from rivers) and the residence time of the water inside
the epilimnion. It appears that the only significant renewing water process is the one between
the epilimnion and the hypolimnion. The water exchange from precipitation, evaporation or
the rivers is negligible compared with the huge fluxes crossing the thermocline.
BELLA workshop
Productivity of large lakes
ESTIMATING PHYTOPLANKTON BIOMASS
TANGANYIKA FROM SATELLITE IMAGES
AND
3
PRODUCTION
IN
LAKE
Bergamino, N.1, S. Horion2, Y. Cornet2, S. Stenuite3 & J.-P. Descy3
1
2
Dipartimento di Scienze e Tecnologie Chimiche e dei Biosistemi, University of Siena, Italy
Unit of Geomatics, Dept. Geography, University of Liège, Belgium
3
Laboratory of Freshwater Ecology, URBO, Dept. Biology, University of Namur, Belgium
The conservation of biodiversity and sustainable management of the Lake Tanganyika
requires ecosystem monitoring, in particular information on primary production, nutrient
dynamics and community structure. Given the size of the lake and its spatial and temporal
heterogeneity, an improved understanding of the mechanisms controlling the lake productivity
needs to be carried out at the scale of the whole ecosystem. Satellite based measurements can
be of high interest to estimate photosynthetic pigments and primary production. A calibrated
MODIS Aqua time series of daily bio-optical parameters including chlorophyll a
concentrations and the attenuation coefficient at 490 nm were processed at a spatial resolution
of 1 km² from July 2002 to November 2005 within the SeaDAS 4.6 environment.. Missing
data resulting from cloudiness or aberrant values were refilled following a reconstruction
method based on empirical orthogonal functions. With this database, we determined a
regionalisation of the lake, based on the temporal covariance of chlorophyll a concentrations.
By combining these spatially explicit data with a phytoplankton community specific model, it
was possible to explore spatial differences in the daily primary productivity. The average
primary production in each region, as well as an average primary production for the whole
Lake Tanganyika, were calculated.
BELLA workshop
Productivity of large lakes
4
BIOMASS AND TROPHIC FATE OF PICOPLANKTON IN LAKE TANGANYIKA
Stenuite, S.1, A.-L. Tarbe1, H. Sarmento1, F. Unrein2, A. Wilmotte3, J.M. Gasol2 & J.-P.
Descy1
1
2
3
Laboratory of Freshwater Ecology, URBO, Dept. Biology, University of Namur, Belgium
Departament de Biologia Marina i Oceanografia, Institut de Ciencies del Mar, CSIC,
Barcelona, Catalunya, Spain
Center for Protein Engineering, Institute of Chemistry, Sart Tilman, University of Liège,
Belgium
Recent studies combining flow cytometry and epifluorescence microscopy have shown that
picoplankton, comprising photosynthetic picoplankton (PPP) and heterotrophic bacteria (HB),
may be the largest plankton compartment in Lake Tanganyika. In this study, conducted in (the
different seasons of 2004-2006, PPP biomass in the upper 100 m ranged between 2.5 and 3.4
g C m-2 on average, while HB biomass ranged between 0.52 and 1.32 g C m-2, with highest
values observed during the dry season. Biomass ratios between HB and PPP showed clear
seasonal patterns, with dominance of picocyanobacterial biomass in the upper layers
throughout the year, but strengthened dominance of HB below the euphotic zone. Using clone
libraries and DGGE based on the 16S rRNA gene, we found 5 different Synechococcus
sequence types in 2 stations of Lake Tanganyika. Fractionated
14
C incubations indicated that
PPP accounted for a major part of particulate primary production. Grazing experiments with
different techniques were used to evaluate grazing of picoplankton by microzooplankton.
Heterotrophic nanoflagellates grazing impact on picoplankton was higher than that of ciliates,
and the grazing impact of protozoans was much higher on PPP than on HB. Our data confirm
the hypothesis of a major role of picoplankton, especially picocyanobacteria, in the pelagic
food web of Lake Tanganyika.
BELLA workshop
Productivity of large lakes
5
PAST ENVIRONMENTAL CHANGES AT LAKE LAJA (CENTRAL-CHILE) DURING
THE LAST 1000 YR
Fabiola Cruces1, 2, Alberto Araneda2, Laura Torres1, 2, Fernando Torrejón2 & Roberto Urrutia2
1: Departamento de Botánica, Universidad de Concepción
2: GEP (Grupo de Estudios Paleolimnológicos), Unidad de Sistemas Acuáticos, Centro
EULA-Chile, Universidad de Concepción
Environmental changes in an Andean lake were investigated by the analysis of different
proxies preserved into a sediment core retrieved in Lake Laja (36º54’S; 71º05’W). Using the
core −that extended to the last ~1000-years−, physical and chemical parameters (textural
characteristics, organic matter, biogenic silica, carbonates, phosphorous, iron, manganese,
sodium and calcium) and diatom, pollen and chironomid assemblages were analyzed. The
diatom assemblages showed a decrease of Fragilaria construens and Fragilaria pinnata to
the most recent period, while the taxa Asterionella formosa, Aulacoseira distans and the
genus Cyclotella were increased; at the same time the diatom concentration was decreased
into the volcanic layers. Chironomids also show changes in these layers, diminishing the
abundances of Parachironomus, Paratanytarsus and Macropelopia and the diversity index,
which increases gradually to the superficial layers of the sedimentary column. Pollen
evidenced humidity fluctuations which were revealed by changes in Nothofagus type
dombeyi, Poaceae and Ephedra. On the other hand, the palynological evidence also suggests
an important anthropic impact during the last 100 years, inferred from the appearance of
Plantago and the increase of Poaceae and Asteraceae subf. Cichorioidae. Finally, it is possible
to indicate that the changes observed in the sedimentary records of Lake Laja would be
associated to volcanism, and the changes which occurred in recent time would be related to
anthropic activity. Moreover, the changes observed principally in the biological records
between A.D 1600 and A.D 1800 would suggest cooler conditions in the area than currently
registered.
BELLA workshop
Productivity of large lakes
6
PLANKTONIC ARCHAEA IN LAKE KIVU
Llirós, Marc1, F. Darchambeau.2, A. Plasencia1, F. Gich1, B. Leporcq2, A.V. Borges3, B.
Delille3, P. Isumbisho4, E.O. Casamayor5, C.M. Borrego1 & J.-P. Descy2
1
, Institute of Aquatic Ecology, University of Girona, Campus Montilivi, 17071 Girona,
Spain.
2
Laboratory of Freshwater Ecology-URBO, University of Namur, B-5000 Namur, Belgium.
3
Chemical Oceanography Unit, Institut of Physique, University of Liège, B-4000 Liège,
Belgium. 4
Unité d’Enseignement et de Recherche en Hydrobiologie Appliquée, Institut Supérieur
Pédagogique, BP 854-Bukavu, DR Congo
5
Group of Limnology, Centre d’Estudis Avançats de Blanes-CSIC, Accés Cala Sant Francesc
14, 17300 Blanes, Spain.
The use of culture independent techniques to aquatic microbial ecology has shown that
Archaea are ubiquitous and abundant in prokaryotic planktonic assemblages from both
freshwater and marine environments. In some freshwater lakes mesophilic Crenarchaeota
have been found to constitute between 1 and 10% of the total planktonic community.
Recently, the discovery that some mesophilic Crenarchaeota can act as chemolithoautotrophs
has broadened our view of archaeal metabolism and has posed questions on their impact on
biogeochemical cycles.
Lake Kivu (2ºS, 29ºE) is a deep meromictic and oligotrophic high altitude (1,463m a.s.l.)
tropical lake. It has a permanent thermal- and haline stratification that produces a wide oxic
mixolimnion and a deep anoxic monimolimnion (from 60 to 489 m depth) which is very rich
in both CO2 and CH4. Although most of this CH4 comes from the reduction of magmatic CO2,
degradation of organic matter in anoxic conditions by active methanogenic Archaea counts
for one third of CH4 production.
Accordingly, we studied the presence of Archaea, both Crenarchaeota and Euryarchaeota, in
the planktonic prokaryotic assemblage of Lake Kivu during a sampling campaign in March
2007. The study has been performed at different depths in four sampling stations to check for
differences along the vertical depth profile and between sampling sites. Most abundant
retrieved phylotypes were assigned to methanogenic Euryarchaeota - as expected by the
physico-chemistry of the lake - and to mesophilic Crenarchaeota. Phylotypes assigned to this
latter group were closely similar to clones retrieved from different anoxic environments.
BELLA workshop
Productivity of large lakes
7
LAKE KIVU - STEERING TOWARDS A HUMAN-INDUCED LIMNIC ERUPTION?
Martin Schmid, Natacha Pasche, Bernhard Wehrli & Alfred Wüest
Surface Waters – Research and Management (Surf), Eawag: Swiss Federal Institute of
Aquatic Science and Technology, Seestrasse 79, CH - 6047 Kastanienbaum, Switzerland
Lake Kivu is one of the large East African Rift Lakes with a surface area of 2400 km2 and a
maximal depth of 485 m. The deep waters of Lake Kivu contain enormous amounts of
dissolved carbon dioxide (250 km3 STP) and methane (60 km3 STP). The release of only a
fraction of these gases could have catastrophic consequences for the densely populated region.
The gas eruptions from the two Cameroonian crater lakes Nyos and Monoun in the 1980’s,
which caused the death of altogether about 1800 people, showed that the sudden release of
gases accumulated over long time scales in the deep water of a lake is possible. In the case of
Lake Kivu, the vertical mixing is extremely weak, allowing the gases to accumulate for
almost 1000 years in the deep water below 250 m depth. Because of its lower solubility, the
partial pressure of methane is much higher than that of CO2, despite its lower concentrations.
A gas eruption in Lake Kivu would therefore be triggered mainly by methane, but the erupted
gas mixture would nevertheless mainly contain CO2. Presently, however, the gas
concentrations in the lake are below 60% saturation throughout the water column, and an
extraordinary event would be needed to trigger a gas eruption.
Recent measurements indicate that the methane concentrations in the deep water of Lake Kivu
increased by 15-20% during the last 30 years. Because of the increased gas concentrations,
the heat input needed to trigger a devastating gas release by a magma intrusion into the deep
water has significantly decreased. With the estimated recent methane production, the gas
concentrations could approach a critical level within this century. Simultaneously to this
increase in methane concentrations changes occurred as well in the vertical distribution of
nutrients within the lake. As the methane is mainly of biogenic origin, it can be concluded that
the increase in methane concentrations was most probably due to changes in the food-web
within the lake. Two main hypotheses were set up to explain the enhanced nutrient export
from the surface layers: higher nutrient inputs due to the growing human population or
changes in the lake-internal nutrient pathways caused by the introduction of the fish species
Limnothrissa miodon. These hypotheses are currently studied in an ongoing research project.
BELLA workshop
Productivity of large lakes
8
DIFFERENTIAL RESPONSE OF PHYTOPLANKTON TO NITROGEN, PHOSPHORUS
AND IRON ADDITIONS IN LAKE TANGANYIKA
De Wever, A.1, Muylaert, K.2, Langlet, D.3, Alleman, L.3,5, Descy, J.-P.4, André, L.3, Cocquyt,
C.1,6 & W. Vyverman1
1
Laboratory for Protistology and Aquatic Ecology, Department Biology, Ghent University,
Ghent, Belgium
2
K.U.Leuven-Campus Kortrijk, Kortrijk, Belgium
3
Musée Royal de l'Afrique Centrale, Section de Pétrographie-Minéralogie-Géochimie,
Tervuren, Belgium
4
Laboratory of Freshwater Ecology, URBO, Dept. Biology, University of Namur, Belgium
5
Département Chimie et Environnement, Ecole des Mines de Douai, Douai, France
6
National Botanic Garden of Belgium, Meise, Belgium
Multiple nutrient enrichment bioassays using addition of iron (Fe) and combined addition of
nitrogen and phosphorus (NP) were carried out in the north and the south of Lake Tanganyika
during the rainy and dry seasons in 2003 and 2004 in order to compare the response of
different phytoplankton groups to the nutrient additions. Nutrient additions resulted in an
increase in phytoplankton growth rate relative to control treatments in all experiments. HPLC
pigment data and epifluorescence microscopy counts indicated differential stimulation of the
dominant
phytoplankton
groups.
Iron
additions
mainly
stimulated
prokaryotic
picophytoplankton, while enrichments of nitrogen and phosphorus stimulated green algae and
in some cases diatoms. Extended incubation (3 days) indicated co-limitation of Fe and NP, in
particular for picocyanobacteria. Fe limitation in Lake Tanganyika may be related to low
availability of Fe (due to high pH and alkalinity) rather than low total concentrations of Fe.
BELLA workshop
Productivity of large lakes
9
NITROGEN DYNAMICS IN NORTHERN LAKE TANGANYIKA: INPUTS BY RIVERS
AND USE BY PHYTOPLANKTON.
Natacha Brion1, David Nahimana1, 2, Evariste Nzeyimana2, Leo Goeyens1,
3
and Willy
Baeyens1
1
ANCH-WE, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
2
Département de Chimie, Faculté des Sciences, Université du Burundi (U.B.), B.P. 2700
Bujumbura, Burundi
3
Department of Pharmacology-Bromatology, Scientific Institute of Public Health, J.
Wytsmanstraat 14, B-1050 Brussels, Belgium
Northern Lake Tanganyika is characterized by an almost permanently stratified water column
which causes severe nutrient depletion in surface waters. Any external N source to surface
waters, therefore, is of importance in sustaining primary production. This study attempted to
quantify riverine input of dissolved inorganic nitrogen (DIN) to the extreme northern end of
Lake Tanganyika (surface = 900 km²) as well as the DIN uptake by surface phytoplankton.
Results showed that riverine DIN inputs (1930 tons of N/year) were of similar importance to
atmospheric deposition (1520 to 1720 tons of N/year) and were maximal during the dry
season. Moreover, seasonal DIN variations in river and lake waters showed maximum
concentrations during part of the dry season (May to July 1999) probably due to high
atmospheric inputs. Phytoplanktonic nitrate and ammonium uptake rates were measured
during 9 cruises and varied from 0.01 to 19.3 nM/h. These values suggest that uptake by
phytoplankton in the surface waters could represent a DIN sink of about 14400 tons of
N/year, thereby utilizing all available DIN coming in from external sources. External DIN
sources represent approximately 25 % of the annual phytoplankton N requirements, showing
the major importance of unquantified N sources in sustaining primary production in the
northern basin of Lake Tanganyika. These sources could include organic N present in the
external sources, and internal N supply.
BELLA workshop
Productivity of large lakes
10
MODELLING PRIMARY PRODUCTIVITY IN LAKE TANGANYIKA
Jaya Naithani.1and Eric Deleersnijder2
1
Université catholique de Louvain, Institut d'astronomie et de géophysique Georges Lemaître
(ASTR),
Chemin
du
Cyclotron
2,
B-1348
Louvain-La-Neuve,
Belgium.
naithani@astr.ucl.ac.be
2
Université catholique de Louvain, Institut d'astronomie et de géophysique Georges Lemaître
(ASTR) and Centre for systems engineering and applied mechanics (CESAME), 4 Avenue
Georges Lemaître, B-1348 Louvain-La-Neuve, Belgium
The ecosystem response of Lake Tanganyika is studied using a four-component, nutrientphytoplankton-zooplankton-detritus (NPZD), phosphorus-based ecosystem model coupled to
a non-linear reduced-gravity circulation model. The ecosystem model, an improved version of
the earlier ECOH model developed for Lake Tanganyika, is used to estimate the annual
primary production of Lake Tanganyika and its spatial and temporal variability.
The
simulations are driven with the National Centres for Environmental Protection (NCEP) winds
and solar radiation forcing. The simulated annual cycles of the four ecosystem variables and
the daily net primary production are compared with the observations. The comparison shows
that simulations reproduce realistically the general features of the annual cycles of epilimnion,
phosphate, net primary production and plankton dynamics. The climatic simulations for the
years 1970 through 2006 yield a daily averaged integrated upper layer net production ranging
from 0.11 – 1.78 g C m-2 d-1 and the daily averaged chlorophyll a from 0.16 – 4.3 mg m-3.
Although the nutrient levels in the epilimnion during the strong wind years are high, the net
production is low which is partly because of the greater vertical mixing, produced by strong
winds, exposing the phytoplankton to low light conditions at deeper waters. The simulated
annual net production and chlorophyll a agrees quite well with observed production available
in the literature. The model sensitivity was tested for various upper layer temperatures and
reference depths.
BELLA workshop
Productivity of large lakes
11
BACTERIAL PRODUCTION AND ITS FATE IN LAKE TANGANYIKA
Pirlot, S.1, J.-P. Descy1 & P. Servais2
1
2
Laboratory of Freshwater Ecology, URBO, Dept. Biology, University of Namur, Belgium
Ecology of Aquatic Systems (ESA), Université Libre de Bruxelles, B-1000 Brussels,
Belgium
Bacterial production (BP) and its fate were estimated between 2002 and 2004 in both north
and south basins of Lake Tanganyika using 3H-thymidine radiotracers and measurements of
ingestion rate of bacteria by protozoa using Fluorescent Micro-Particles (FMPs). BP was high
(41 – 559 mg C m-2 d-1) in comparison with primary production (PP) data. In the upper mixed
layer, BP corresponded to up to 28% of PP. BP was substantial below the mixed layer, so that
total production of heterotrophic bacteria amounted to 99 g C m-2 y-1 (51 % of PP). No direct
correlation was found between bacterial and phytoplankton production; a time lag was usually
observed between maximum of PP and of BP. The measurements of microzooplankton
grazing showed that heterotrophic nano-flagellates were responsible for most of the grazing
pressure on the bacterial community of the pelagic zone (92 to 99%). Bacterial cell lysis was
the second process involved in bacterial mortality, ranking before ciliate grazing. Overall,
bacterial mortality was balanced with bacterial production. These results allow a first
assessment of the role of heterotrophic bacteria in carbon transfer through the microbial food
web of this large tropical lake.
BELLA workshop
Productivity of large lakes
12
LIGHT ENVIRONMENT AND PHYTOPLANKTON BIOMASS RELATIONSHIPS IN
THE COASTAL AREAS OF LAKE VICTORIA
S.A. Loiselle1, N. Azza2, A. Cózar3, N. Bergamino1, C. Rossi1
1
Department of Chemical and Biosystems Sciences, CSGI, University of Siena, via Aldo
Moro 1, 53100 Siena, Italy
2
Department of Water Resources Management, Entebbe, Uganda
3
Área de Ecología. Facultad de Ciencias del Mar y Ambientales, Universidad de Cádiz,
Campus Río San Pedro.
The availability of solar radiation, together with nutrients and grazing usually control changes
in the biomass of pelagic primary producers. The availability and spectral quality of
photosynthetically active radiation is affected by the absorption and scattering properties of
the optically active components present in the medium. These are mainly phytoplankton,
chromophoric dissolved organic matter (CDOM), suspended particulate matter and the water
itself. A modification in any one component will affect the spectral distribution and
penetration depth of the solar irradiance necessary for primary production. The relationship
between phytoplankton biomass and the major optical components present on the Ugandan
and Kenyan coasts of Lake Victoria was used to examine the controlling factors of light
limitation. It was found that chromophoric dissolved organic matter is often the dominating
optical component and that dilution and degradation of terrestrial sources of organic matter is
the main source, rather than in situ production. Attenuation of solar irradiance was also
correlated to phytoplankton biomass, increasing in importance towards the open lake area.
However, optical properties were not found to covary with Chla, indicating that the coastal
areas of Lake Victoria cannot be considered as Case I waters. Light limitation appears to
occur throughout most bays and some coastal areas receiving catchment waters. This spatial
information, geographically referenced to bathymetric and catchment conditions was utilised
to understand the importance of the environmental conditions on limiting phytoplankton
biomass.
BELLA workshop
Productivity of large lakes
13
CARBON DIOXIDE FLUXES IN LAKE KIVU
Borges, A.V.1, B. Delille1, J.-P. Descy2, F. Darchambeau2, B. Leporcq2, P. Servais3 & S.
Bouillon4,5
1
Chemical Oceanography Unit, University of Liège
Laboratory of Freshwater Ecology, URBO, Dept. Biology, University of Namur, Belgium
3
Ecologie des systèmes aquatiques, Université Libre de Bruxelles
4
Department of Analytical and Environmental Chemistry, Vrije Universiteit Brussel,
Brussels, Belgium.
5
Centre for Estuarine and Marine Ecology, Netherlands Institute of Ecology (NIOO-KNAW),
Yerseke, Netherlands
2
Lakes are significant sources of CO2 to the atmosphere ranging between 0.14 à 0.17 PgC yr-1
globally. This emission of CO2 is comparable to the one from rivers of 0.34 PgC yr-1 and from
estuaries of 0.32 PgC yr-1. Africans lakes are characterized by partial pressures of CO2 (p
CO2) twice higher than the global average (2300 ppm versus 1060 ppm). Also, African lakes
represent about 10% of the total lake surface area (225,000 km2 versus 2426,000 km2). The
emission of CO2 is attributed to the net heterotrophy of these systems sustained by the organic
carbon inputs from the watershed. However, several unknowns remain on the CO2 dynamics
in lakes, in particular African ones : (1) few simultaneous and integrated studies of CO 2
dynamics and metabolic performance are available; (2) African lakes are under-sampled in
relation to temperate and boreal lakes, (3) most pCO2 estimates in lakes are based on pH and
alkalinity measurements with unkown quality, (4) seasonal and diurnal pCO2 variations in
lakes are significant but not well constrained, and (5) spatial variability of pCO2 in lakes is
strong but not well documented. Here we present preliminary results on CO2 dynamics in
surface waters of lake Kivu that was sampled in March 2007, in the frame of the Carbon and
Nutrient cycles in lake Kivu (CAKI) project (http://www.co2.ulg.ac.be/kivu.htm).
BELLA workshop
Productivity of large lakes
14
STRUCTURE AND PRODUCTION OF MESOZOOPLANKTON IN LAKE KIVU (EAST
AFRICA).
Isumbisho, M.1, H. Sarmento2, F. Darchambeau2 & J.-P. Descy2
1
UERHA, ISP/Bukavu, B.P. 854-BUKAVU, Democratic Republic of Congo,
2
Laboratory of Freshwater Ecology, URBO, Dept. Biology, University of Namur, Belgium
During three years (2002-2005), we assessed the status of the mesozooplankton community in
Lake Kivu, several decades after the introduction into this lake of Limnothrissa miodon, a
planktivorous sardine. The results indicate that present Lake Kivu mesozooplankton has low
diversity poor and is dominated by cyclopoid copepods. Total crustacean biomass increased to
a distinct seasonal maximum following a rise of phytoplankton production associated with a
deep epilimnetic mixing in the dry season. This suggests that mesozooplankton dynamics in
Lake Kivu are mainly bottom-up controlled. However, measurements of body size indicate
that the sardine predation affects the cladoceran Diaphanosoma excisum Sars while copepods
may efficiently escape predation by diel vertical migration.
Total copepod biomass in Lake Kivu (mean=0.3 g C m-2) is lower than in lakes Tanganyika
and Malawi and mean annual total production (8.3 g C m-2 y-1) is about three times as low as
in the two others large lakes.
The ratio between phytoplankton primary production and zooplankton secondary production
is also low (about 1.6 % against 5-8 % in lakes Tanganyika and Malawi), suggesting a
relatively very low transfer of phytoplankton production to consumers, which can be
attributed to the disappearance of the key grazer, Daphnia curvirostris Eylmann, following
the planktivorous fish introduction.
BELLA workshop
Productivity of large lakes
15
MONSOON-RELATED EVENTS RECORDED IN LAKE TANGANYIKA MUSSELS
Langlet D., P.-D. Plisnier & L. André
Section de Pétrographie-Minéralogie-Géochimie, Musée Royal de l'Afrique Centrale,
Leuvensesteenweg 13, B-3080 Tervuren, Belgium
As bivalves grow, they sequentially deposit new layers of shell, the composition of which
may reflect the environmental conditions at the time they formed. The life span of some
freshwater bivalves was reported to be more than 50 years, then potentially allowing records
to be extended into the past. We investigated the shell of freshwater mussels (Pleiodon spekii)
in Lake Tanganyika as a geochemical archive of the periodic hydrological changes related to
the monsoon, which affect the biogenic productivity of the Lake. A 3-year-long limnological
and geochemical survey was performed on the dissolved and particulate fractions of the
coastal waters. The results were compared to high-resolution chemical profiles (Mn/Ca,
Sr/Ca, δ18O) in the aragonitic mussel shells obtained by laser ablation ICP-MS and
micromilling methods coupled with ICP-MS analysis.
Variations of δ18O were correlated to Sr/Ca ratio and exhibited an annual periodicity with
high δ18Oshell values corresponding to low water temperature (i.e. upwelling period). In this
temporal isotope-based reference, the main peak of Mn/Ca observed in three similar profiles
matched the increase of Mn and chlorophyll a in surface waters during the 2002 upwelling,
while a shell collected during the 2003 dry season detected both 2002 and 2003 upwelling
events. Larger shells showing an extremely reduced growth displayed more than 8 Mn/Ca
peaks, which suggest at least an 8-year-record of seasonal changes in water composition. We
postulate that Mn/Ca in P. spekii shells records the conjunction of an increase in the
biological activity with supply of dissolved Mn and nutriments in coastal waters resulting in
an enhanced assimilation of biogenic Mn-rich particles.
Therefore, Pleiodon shell should provide a record of short-term environmental changes linked
with recent or past mixing events in Lake Tanganyika. This approach would be useful to
reconstruct at a seasonal time scale the variations of ENSO events over the past century from
historical collections.
BELLA workshop
Productivity of large lakes
16
RECONSTITUTION OF HOLOCENE EL NIÑO CYCLES IN SOUTHERN AMERICA
FROM HIGH-RESOLUTION LAKE SEDIMENTS.
X. Boës1, M.F. Loutre2, M. De Batist3, N. Fagel4
1
Royal Observatory, Ringlaan 3, 1180 Brussels, Belgium
Institut d'astronomie et de géophysique G. Lemaître, Université catholique de Louvain,
Louvain-la-Neuve, Belgique
3
Ghent University, Department of Geology, Renard Centre for Marine Geology Krijgslaan
281-S8, 9000 Gent.
4
University of Liège, Geology Department, 1000 Liege,Belgium.
2
The aim of this communication is to show how laminated lake sediments driven by winds can
be utilized for age modeling and for tracking El Niño Cycles. In South America, the lake
systems located between 40° to 55° S are particularly sensitive to atmospheric air-humidity
changes driven by the Southern Westerlies, El Niño, and the polar front. For example, Lago
Puyehue (40°S) is stratified in summer and mixed by the stronger austral winter winds (i.e.,
monomictic temperate lake). The autumn/winter atmospheric conditions involve water
circulation and nutrients (phosphorus and nitrogen) increase in the lake (Campos, 1989). The
seasonal thermal lake cycle and nutrients increase lead to annual blooms of phytoplankton
(diatoms). A varved micro-structure (varve couplets) is formed every year at the floor of the
lake. The recurrence of the distinct light layers produced by the mass flocculation of diatoms
blooms after winter nutrient turn-over therefore provides an annual temporal resolution (by
varve counting).
We present chronologies obtain over the “Little Ice Age” (LIA) and the Last
Deglaciation. The comparison between radionuclide (210Pb,137Cs) and 14C sedimentation rates
estimate validate the annual character of the laminations over thousand years, making this
lake one of the few resolved varved record in southern hemisphere. The evolutive MTM and
wavelet analyses of the varve thickness allow the detection of some dominant climate
periodicities. For the AD 1400-2000 time window, our results shows that the 15, 9, 4.4, 3.2
and 2.4 years represent the most robust periodicities, and that the typical El Niño 4.4
periodicity is dominant during the Little Ice Age in South America.