Evidence for solar forcing on the ecological variability of Lake Malawi during the last 700 years
by
Gasse, Françoise
CEREGE, BP80, 13545, Aix-en-Provence Cedex 04, France.

The large East African lakes are of crucial economical importance for riparian countries, but their variability at time scales relevant to societies is poorly known. These ecosystems are extremely sensitive to climate change. Their water balance is primarily controlled by precipitation on, and evaporation from the lake surface. Rainfall and lake-level experience strong interannual variability, which reflects teleconnections to large scale circulation patterns including ENSO. Primary production is controlled by nutrient supply linked to surface runoff, and to a large degree to the water mass dynamics. The deepest Lakes Tanganyika and Malawi (Fig. 1) are permanently stratified. Biogenic material and nutrients accumulate in deep water. Primary production is controlled by dissolved nutrient flux from the deep to the surface layer, and thus by vertical exchange within the lake. Therefore, any fluctuations in environmental factors acting on mixing processes (temperature, solute content, wind intensity and direction) have significant biological impact. Over the last century, climate warming on Lake Tanganyika led to reduce primary production, through increased density gradients, slower vertical mixing, and lower nutrient loading to surface water (1,2). Instrumental records are, however, too short to represent the full range of natural variability at the century to decadal time-scales.

Diatoms are one of the most important components of the primary production of Lake Malawi and are excellent monitors of environmental conditions. Here, the variability of this ecosystem is analysed using a 700 yrs diatom record from the northern basin (Fig. 1).

Lake Malawi is a large Rift Valley lake (650 km by 40 km, 29000 km2, >700 m deep) extending from 10° to 15°S in tropical southern East Africa (Fig. 1). It has a strongly seasonal rainfall regime, with one rainy season during the austral summer when the Intertropical Convergence Zone (ITCZ) is centered around 5-15°S. The dry season (May to October) is characterized by southerly winds, which generate intense upwelling in the southern basin. Presently, diatoms reach their greatest abundance in the southern basin during the dry winter season.

Sediment cores were recovered from the northern basin of Lake Malawi by the Large Lakes Observatory of the University of Minnesota-Duluth (Fig. 1) in 1998. The study of a long piston core (24,000 yrs) show that diatoms are a major sedimentary component (3). The upper sedimentary unit (≤116 cm) was collected by several short cores, which had provided a detailed varve chronology confirmed by 210Pb-ages measured on the uppermost sediments (4). This varve chronology is used here. The present study refines a biogenic silica record which has suggested higher diatom productivity during the Little Ice Age (LIA)(4). It is based on 86 samples, 1 cm thick, from two of these short cores, 10MCB and 2PG, collected at ≈363 m depth (Fig.1). They together span the interval ≈AD 1995-1300 (Fig. 2).

Diatoms were analysed for: (i) their assemblage composition (percentage of individual taxa) which informs on the ecosystem through the ecological requirements of dominant taxa; (ii) the absolute diatom concentration of dry sediment (valves.g-1) which is an index of surface water diatom production, although variable influx of other materials, sediment focussing and early diagenesis may modulate the signal; (iii) the biovolume of sedimented diatoms (mm3.g-1), based on (i) and (ii) and on the cell volume of individual taxa. Biovolume provides a first order approximation of the diatom biomass that lived in surface water at time of sedimentation. Same technical procedures as for the long core(3) were used.

Only 6 taxa among the 125 encountered taxa reach percentages ≥5% in at least one level (Fig. 2). They are the centric planktonic taxa Aulacoseira nyassensis, Stephanodiscus mülleri, S. nyassae, S. af. medius, Cyclostephanos malawensis, and the pennate Nitzschia epiphytica. In the modern lake, the abundance of A. nyassensis is associated with deep mixing; while the Stephanodiscus-Cyclostephanos dominated sectors are thought to be less well mixed and to receive fewer nutrients from the deep waters (3). N. epiphytica usually lives on large planktonic forms, but may also develop in shallow environments. Other taxa are highly diversified but occur in low number. Most of them are periphytic or facultative planktonic forms (Fragilaria) from shallow water.

In order to analyse the diatom response to climate change, some samples have been eliminated from the diatom record. They are: (i) the 3 samples overlying an ash layer (A1, 48-51 cm, Fig. 2) where very high diatom concentration is likely is attributed to volcanic silica which favoured blooms and preservation of some small during 5-10 years; (ii) 3 homogenite layers (8-9cm, 21-23 cm, and 41-43 cm; Fig. 2) which are likely turbidites (4).

Spectral analyses of the time series were performed using the classical Blackman-Tukey method of individual taxa percentages and biovolumes, Absolute Diatom Content (ADC) and Total Diatom Biovolume (TBV), normalized by their standard deviation. In addition, Singular Spectrum Analysis (SSA) was applied to these series to enhance the signal/noise ratio, after having equally spaced the data every 10 yr. Time series reconstructed with the 4 first SSA eigenvalues provide the major features of the diatom signals (Fig. 3).

Two types of diatom time series are evident from Fig. 3, and were confirmed by periodograms. Type (1) clearly shows a periodicity of ≈70-76 yr. It includes the percentage series of A. nyassensis (1a) and of the group of small centric forms (1b: S. af. medius, S. nyassae and C. malawensis), but (1a) and (1b) are in antiphase. All other time series belong to Type (2) with periodicities of ≈200-215, ≈100-130, and ≈60-65 yr. These periodicities are observed for the percentage series of N. epiphytica (2a) and of S. damasii and the shallow water forms (2b), but (2a) and (2b) are in antiphase. N. epiphytica series (percentage and biovolume) are closely correlated with ADC (r=0.92) and TBV. Because the cell volume varies considerably between the largest and the smallest abundant taxa (A. nyassensis: 18,000µm3; S. nyassae: 25 µm3), TBV is almost similar to the biovolume of A. nyassensis (r=0.98). Despite the 70 yr periodicity observed in its percentage, these coarse, resistant taxa alone can be regarded as a good representative of the total diatom biomass expressed in terms of volume.

Figure 3 shows two reconstructions of Total Solar Irradiance (TSI) derived from 10Be and 14C measured in Antarctic ice-cores (5) and from sun-spot numbers (6), respectively, and suggests relationships between the diatom assemblage composition and diatom productivity in Lake Malawi and solar activity. Periodicities of ≈200-215, ≈100-130 and ≈60 yr are known from solar cycles. In order to test this hypothesis, cross-correlation spectral analyses (Blackman-Tukey method) were performed between the longest TSI record (6) and the diatom series. Raw data have been equally spaced every 10 yr and normalized by their standard deviation. The coherence between TSI and diatom time series is extremely significant at a 95% confidence level (Fig. 4).

The maximum coherence is centred on a period of 135 yr for A. nyassensis biovolume (87%), with an average response time (lag between TSI and A. nyassensis peaks) of ≈28±3 yr. The N. epiphytica coherence curve culminates (87%) around the period of 128 yr, with a response time to TSI of 16±3 yr (biovolume) and 18±3 yr (percentages). The coherence with ADC is slightly lower, although significant at a 90% confidence level around periods of 128 yr, 77 yr, and 65 yr, with a time lag of about 27±4.5 for the 128 yr period.

This record shows that the diatom production has been generally high during the LIA period, but also illustrates the complexity of the ecosystem variability at the decadal to century time-scales. The potential mechanisms that may link primary production and diatom assemblage composition are then discussed. They include: direct influence of radiation on photosynthesis, minor changes in air and tropical sea-surface temperatures, changes in the amplitude of the seasonal migration of the ITCZ, and subsequent changes in the intensity, direction, and seasonality of predominant winds. The apparent periodicity of about 120-140 yr in primary production can be regarded as important for authorities in charge of the lake management.

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Date received: January 27, 2004

Copyright © 2004 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # camu-25.