Reconstructing environmental change in the Lake Alaotra region, Madagascar

The extent to which the central highlands of Madagascar were once covered by forests and whether these were closed or more open forest types is still a matter of intense debate. Reconstructing past environments and disentangling the underlying drivers, e.g. natural climatic variations versus human impact, is a particularly complex task. Different types of archives exist which record information from the past (e.g. lake sediments, tree cores, fossil remains, soil profiles, cave deposits, …), this information must be extracted by using a combination of proxies, which all have inherent assumptions and uncertainties. The catchment of Lake Alaotra, a large shallow lake in the Malagasy central highlands, is a region where the landscape is dotted by major gullies called “lavaka”, which are expected to lead to very high erosion rates. Sedimentary archives in lakes such as Lake Alaotra could be of great help to resolve our central questions about the natural versus anthropogenic influences on the changing landscape, yet it is imperative that we understand carbon sources and carbon cycling within the lake, as well as its connection with the surrounding landscape through the input of material via inflowing rivers.

Soil profiles (2 m depth) have been analysed from pristine forests and grasslands in the Lake Alaotra region. Along grassland hillslopes, the soil organic carbon (SOC) content was low, and decreased rapidly from 0.4–1.8 % in the top layer to ca. 0.2 % below 100 cm depth. The current vegetation predominantly consists of C4 grasses (δ13C ~-13 ‰), yet topsoil δ13C-OC ranged from -23.0 ‰ to -15.8 ‰, and most profiles show a decrease in δ13C-OC with depth. In contrast, the C3-dominated forest profiles showed a typical profile whereby δ13C values increased slightly with depth. The SOC stock of grasslands was ~55.6 % lower than along the forested hillslopes for the upper 0-30 cm layer. δ13C values in grassland and forest profiles converge to similar values (within 2.0 ± 1.8 ‰) at depths below ~80 cm, consistent with the grasslands in the Lake Alaotra region having developed on soils formerly covered by C3 plant-dominated landscape. Moreover, the percent of modern carbon of the bulk OC in the top, middle and lower middle positions along the hillslope gradient in the grasslands was unusually low – less than 85 % near the surface. This could reflect a combination of (i) the long residence time of remaining forest OC in the soil, (ii) the slow replacement rate of grassland-derived OC, and (iii) the substantial erosion of the top positions towards the valley position of grasslands, thereby exposing deep soil layers to surface positions.

To provide a first comprehensive survey of the biogeochemistry of the Lake Alaotra system, we investigated the seasonal variability of the concentrations and stable isotope composition of inorganic and organic carbon pools, as well as a range of other relevant proxies, including dissolved greenhouse gas concentrations (pCO2, CH4, N2O), total alkalinity, physico-chemical parameters, and Chl-a (chlorophyll a) from spatially distributed sampling and seasonal monitoring of several rivers. Inflowing rivers were found to carry high total suspended matter (TSM) loads with a low particulate organic carbon (POC) content, while the lake itself and its outflow were characterised by much lower TSM values and high %POC. The POC concentration of the outflow (13.0 ± 7.7 mg L-1) was substantially higher than in the inflowing rivers (1.9 ± 2.1 mg L-1), and δ13C values were also distinct between inflowing rivers (-24.6 ± 1.8 ‰) and the lake (-26.5 ± 2.1 ‰) or its outflow (-25.2 ± 1.4 ‰). Similarly, the lake outflow was surprisingly rich in dissolved organic carbon (DOC) (9.5 ± 1.4 mg L-1) compared to inflowing rivers (2.6 ± 1.1 mg L-1). This indicates that the lake and its surrounding wetlands act as a substantial source of new organic carbon which is exported downstream. δ13C data were consistent with marsh vegetation being the main source of DOC inputs, while phytoplankton is expected to be an important source of POC in the lacustrine waters: lake suspended matter is characterized by relatively low POC/Chl-a ratios (143–564), high %POC (10.1 % to 28.9 %), and δ13C values around 20 ‰ lower than the dissolved inorganic carbon (DIC) pool (-26.5 ± 2.1 ‰ versus -6.7 ± 1.6 ‰). Water stable isotope data from inflowing rivers and lake outflow differed substantially. The outflow data showed a strong enrichment in 18O and 2H, increasing during the course of the dry season and suggesting important water losses via evaporation. Despite the importance of phytoplankton production to the lake POC pool, the lake acted as a net source of CO2 to the atmosphere, likely due to the high OC inputs from the surrounding marshes, and sediment respiration considering the shallow water depth. The biogeochemical functioning of Lake Alaotra differs substantially from the majority of East African (sub)tropical lakes studied so far, likely due to a combination of its large surface area and shallow water depth, and the large extent of surrounding wetlands and floodplains. Lake Alaotra acts as an abrupt element in the land-ocean gradient of the catchment, whereby the biogeochemical characteristics of the outlet river are strongly affected by processes taking place in the lake and its wetlands, rather than being reflective of characteristics and processes higher up in the catchment.

To understand how sediment and carbon are mobilized and transported through the Lake Alaotra catchment, we examined OC content and δ13C-OC, OC/TN (total nitrogen) ratios of soil and sediment profiles in the depositional zones. Sediment cores from different parts of the lake showed a high OC content (5 to 18 %) and contain only minor amounts of sand, the dominant grain size class on the hillslopes. The high OC content of the lake sediments, in combination with data on OC/TN ratios and δ13C, indicate that the OC in the lake sediments is mainly derived from the surrounding marshes and, to a lesser extent in situ primary production, rather than from terrestrial C eroded from the catchment. Floodplains, however, are likely to form the main sink for soil-derived sediments: similar to grassland hillslope soils, sediment profiles in the floodplains show a low %OC and relatively high δ13C values ranging from -21 to -14 ‰. Most of the sediments and carbon mobilised on the hillslopes through erosion are thus not thought to reach Lake Alaotra, even though erosion rates in the landscape are extremely high. This is of key importance for a correct interpretation of the sedimentary record of Lake Alaotra. Thus, our study highlights that lake sediment archives, even in erosive catchments, should be carefully considered in terms of their connection to the catchment landscape and the spatial footprint represented by the accumulating sediments. Bulk proxies such as stable isotope ratio in organic matter will relate more to past changes within the lake and its wetlands, rather than to changes in the upper catchment.

The bulk OC and TN data, as well as their stable isotope composition from eight sediments cores collected in Lake Alaotra were compared to pollen and charcoal data from a selection of these sediment cores. Pollen data suggests an opening in the landscape between 2050 and 1700 Cal y BP, during which there was a transition from a wooded grassland or woodland/grassland mosaic to an open grassland – a period also characterized by a peak abundance in charcoal. Human activities are likely directly or indirectly an important factor causing these changes in vegetation. Both the OC content and δ13C-OC exhibited a clear change in the same period as the shift found in pollen and charcoal composition. At ~1911 Cal y BP, a sudden shift in %OC from 5 % to 18 % was observed, while δ13C-OC decreased from -17 to -20 ‰ over this interval. The shift in δ13C is thus opposite of what is expected based on the pollen data: a shift towards a more open landscape, dominated by C4 grasses. This suggests that the relatively sudden change in the catchment vegetation cover has led to local changes in the marshlands surrounding Lake Alaotra. Moreover, exploratory δ13C analyses on microcharcoal from one of our sediment cores reveal that fires throughout the time frame covered by our core were to a large extent fuelled by C4 vegetation (50 – 100 %), even before human settlement – further confirming a partially open forest system. Thus, our study highlights the different spatial footprint of bulk organic matter and specific proxies (pollen and charcoal) in sediment archives: these proxies have different delivery pathways and provide complementary information at different scales.