Spatio-temporal dynamics of phytoplankton and primary production in Lake Tanganyika using a MODIS based bio-optical time series

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Abstract

Lake Tanganyika, the second largest freshwater ecosystem in Africa, is characterised by a significant heterogeneity in phytoplankton concentration linked to its particular hydrodynamics. To gather a proper understanding of primary production, it is necessary to consider spatial and temporal dynamics throughout the lake. In the present work, daily MODIS-AQUA satellite measurements were used to estimate chlorophyll-a concentrations and the diffuse attenuation coefficient (K490) for surface waters. The spatial regionalisation of Lake Tanganyika, based on Empirical Orthogonal Functions of the chlorophyll-a dataset (July 2002–November 2005), allowed for the separation of the lake in 11 spatially coherent and co-varying regions, with 2 delocalised coastal regions. Temporal patterns of chlorophyll-a showed significant differences between regions. Estimation of the daily primary production in each region indicates that the dry season is more productive than the wet season in all regions with few exceptions. Whole-lake daily primary productivity calculated on an annual basis (2003) was 646 ± 142 mg C m 2 day 1. Comparing our estimation to previous studies, photosynthetic production in Lake Tanganyika appears to be presently lower (about 15%), which is consistent with other studies which used phytoplankton biovolume and changes of δ13C in the lake sediments. The decrease in lake productivity in recent decades may be associated to changes in climate conditions.

Introduction

Knowledge of the spatial variations of primary production, nutrient concentration and community structure is fundamental to the understanding of ecosystem dynamics (Bootsma and Hecky, 1993). Lake Tanganyika is characterised by extensive patchiness in its water chemistry, plankton concentration and fish distribution (Plisnier et al., 1999, Salonen et al., 1999). Due to variations in incident light, mixing depth and nutrient availability, algal production is expected to be particularly variable in time and space.

The lack of extensive data on seasonal and local variations in large tropical lakes, such as Lake Tanganyika, limits the possibility to make reliable estimates of lake wide primary production. Indeed, estimates of phytoplankton biomass and primary production for Lake Tanganyika have been based on measurements made in a small number of offshore sites, often with different sampling design and estimation methods. This has led to significantly different estimates of primary production (Hecky & Fee, 1981, Sarvala et al., 1999, Descy et al., 2005, Stenuite et al., 2007). Accordingly, Descy et al. (2005) have stressed that apparent historical changes in primary production should be interpreted with care, as different methods of phytoplankton analysis could lead to different results.

To improve the assessment of primary production and its dynamics in Lake Tanganyika, it is necessary to extend information gained from spatially and temporally localised measurements to the entire lake. On the other hand, the synoptic perspective provided by satellite data provides valuable information regarding the surface spatio-temporal characteristics of extensive ecosystems. The use of backscattered solar radiation to model global distributions of phytoplankton primary production has provided valuable insights into ocean dynamics and global biogeochemistry (Bricaud et al., 2002, Tilstone et al., 2009). However, little has been done to develop approaches appropriate for lake ecosystems, in particular in the southern hemisphere.

In the present study, we used remotely sensed estimates of phytoplankton biomass (chlorophyll-a) and optical properties (K490) to perform a regionalisation of Lake Tanganyika according to geographic areas with similar temporal patterns. From these estimates, regional patterns of daily primary production were calculated, using photosynthesis parameters derived from in situ 14C incubations. Finally, we calculated the overall lake phytoplankton productivity and compared it with other studies.

Section snippets

Data and methods

Level 1B MODIS-AQUA images (1 km2 resolution) were selected from July, 2002 to November, 2005. The parameters of interest, namely chlorophyll-a concentration (chl-a) and the diffuse attenuation coefficient at 490 nm (K490), were derived and optimized on the basis of in situ data and considering local atmospheric conditions (see Horion et al., 2010-this volume). The optimization utilised data from the CLIMLAKE cruises sampling series (Descy et al., 2006).

The missing values (pixels) in the daily

Spatial analysis

The EOF analysis of the extensive satellite dataset showed a variance dominated by the first 11 modes for the chlorophyll-a and by the first 6 for the light attenuation coefficient. The proportion of the spatio-temporal variance explained by these EOFs is larger than 94% (Table 1).

The spatial regionalisation of the Lake Tanganyika, based on EOFs modes, showed a similar spatial structure for both chl-a and K490, with a higher variability present in the time series of chl-a images. In particular,

Discussion

An extensive dataset of satellite images provides an opportunity to track dynamics in ecosystems with high spatial and temporal variabilities (Alvera-Azcárate et al., 2005). For Lake Tanganyika, a filled spatio-temporal matrix for chlorophyll-a and K490 was built at a spatial resolution of 1 km2 (lake area = 31,745 pixels) for an average of 394 days, extending from 5/7/2002 to 30/11/2005. The analysis of this extensive dataset confirmed the patterns observed in the field studies and allowed us to

Conclusions

Large lakes may be a good metric for global processes: large enough to damp out localised phenomena, yet small enough (compared to oceans) to generate observable responses. The African Great Lakes are sensitive to climate change and therefore serve as important long-term indicators. Previous research has established that Lake Tanganyika may be affected by global change through a decrease in primary production. The present study demonstrates that satellite based measurements can provide valuable

Acknowledgements

The CLIMLAKE and CLIMFISH projects were financed by the BELSPO (Belgian Science Policy Office), in partnerships with TAFIRI (Tanzanian Fisheries and Research Institute) and the Department of Fisheries (Zambia). We thank the Belgian Federal Science Policy Office and the Belgian Technical Cooperation for co-financing CLIMLAKE through a frame agreement with the Royal Museum for Central Africa (Tervuren, Belgium). Nadia Bergamino had a fellowship from the University of Siena PAR funds for

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