Biogeochemistry of the Tana estuary and delta (northern Kenya)

Abstract

The estuarine mixing zone of the Tana River (northern Kenya) and an extensive deltaic area just south of the estuary were sampled in April 2004 with the aim of identifying the distribution, sources, and processing of particulate and dissolved organic carbon (POC, DOC) and inorganic carbon (DIC). C4 inputs from the catchment contributed ,50% to the POC pool in the Tana River and estuary, and in the mangrove creek water column and intertidal sediments. The d 13 C values of DOC, however, were typically much more negative than that of POC, indicating a substantially higher contribution by C3 and/or mangrove-derived carbon in the DOC pool. The undersaturation of O 2 , high pCO 2 , and the nonconservative nature of DIC and d 13 C DIC suggest a strongly heterotrophic water column, particularly in the freshwater part of the Tana and in the tidal creeks in the delta, where high additional inputs of organic matter were observed. However, some of these sites showed d 18 O DO signatures lower than the atmospheric equilibrium (i.e., +24.2%) indicative of significant O 2 production by photosynthesis. Therefore, the heterotrophic signature in the water column is likely the result of a strong interaction with the large intertidal areas, whereby respiratory activity in sediments and in the overlying water column during tidal inundation leave a marked signature on the water column. This is confirmed by the covariation between salinity-normalized total alkalinity and DIC, whose slope indicates an important role for anaerobic diagenetic processes. If our data are representative for other large river systems in the region, current estimates are likely to underestimate suspended matter and both inorganic and organic C fluxes to the Indian Ocean from tropical east Africa.

Importance of terrestrial subsidies for estuarine food webs in contrasting East African catchments

“…For example, if catchment vegetation was not considered in the mixing models, high consumer d 13 C at the Tana would be interpreted as resulting from a greater incorporation of planktonic algae. This is however not the case as POC in this system is mostly detrital (up to 90%; Bouillon et al 2009), and due to very high sediment loads aquatic primary production is limited (Robertson and Blaber 1992, Bouillon et al 2007, Bouillon et al 2009). Although MPB (characterised by high d 13 C (Clementz and Koch 2001)) can be the most important aquatic producers in turbid estuaries (MacIntyre et al 1996, Underwood andKromkamp 1999), high consumer d 13 C in the Tana, as in the Betsiboka, is also not likely to be a result of a reliance on these algae as the area available for benthic production is limited due to high turbidity, meaning that MPB can only photosynthesise at low tide, but for most of this period they are subjected to photosynthesis-limiting factors such as variations in light, temperature, salinity and to desiccation (Admiraal 1984).…”

“…Also, in these two highly turbid estuaries, %POC values were low (1.2-2.1%), indicating that most suspended material was refractory. Previous chlorophyll a data indeed indicate that the contribution of phytoplankton to POC in these estuaries is low (Bouillon et al 2007, Ralison et al 2008, implying that d 13 C POC mostly reflects d 13 C of terrestrial producers. In the less turbid sites of Rianila and Ambila, %POC was relatively high (22.2-40.5%), consistent with previous largescale studies showing an inverse relationship between TSM and %POC (e.g., Meybeck 1982).…”

“…13 C POC was higher in the Betsiboka and Tana, indicating that the C4 grasslands from the catchment contribute significantly to the POC pool (Bouillon et al 2007, Ralison et al 2008, Bouillon et al 2009), and even though there are large areas of C3 mangroves in the Tana estuary, d…”

“…Because of a lack of substrate suitable for attachment, macroalgae are very rare. While the waters in Rianila and Ambila are clear, due to the high turbidity levels in the Betsiboka, Zambezi and Tana estuaries, aquatic productivity is expected to be low (Bouillon et al 2007, Ralison et al 2008, Tamooh et al 2012) and almost no seagrass occurs in these estuaries. Seagrass was only observed in the Rianila where it was abundant in the pre-wet season, but rare in the post-wet season.…”

Changes in Organic, Inorganic contents, Carbon Nitrogen ratio in decomposingAvicennia marinaand Rhizophora mucronataleaves on tidal mudflats in Hajambro creek, Indus delta, Pakistan

“…Mangrove swamps usually constitute a shallow ecosystem. The shallow coastal sedi ments are generally considered as important sites for the remineralization of organic matter [8][9][10][23][24][25][26][27].…”