Mutia, Damaris, Carpenter, Stephen, Jacobs, Zoe, Jebri, Fatma, Kamau, Joseph, Kelly, Stephen J., Kimeli, Amon Kibiwot, Langat, Philip Kibet, Makori, Amina, Nencioli, Francesco, Painter, Stuart C., Popova, Ekaterina, Raitsos, Dionysios and Roberts, Michael (2021) Productivity driven by Tana river discharge is spatially limited in Kenyan coastal waters. Ocean & Coastal Management, 211 . p. 105713. DOI https://doi.org/10.1016/j.ocecoaman.2021.105713.

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Abstract

The Tana River is the longest river system in Kenya (~1000 km) and contributes ~ 50% of the total river discharge to Kenyan coastal waters. The river discharges significant amounts of nutrients and sediments, reaching ~24,000 tons per day during the rainy season (March–April), into Ungwana Bay (North Kenya Banks). The bay is an important habitat for high-value Panaeid prawn species which sustain important small-scale fisheries, semi-industrial bottom trawl prawn fisheries, and is the livelihood mainstay in the surrounding counties. In this study we analysed >20 years of satellite-derived chlorophyll-a observations (Chl-a, an index of phytoplankton biomass), along with in situ river discharge and rainfall data, to investigate if the Tana River discharge is a major driver of local phytoplankton biomass in Ungwana Bay and for the neighbouring Kenyan shelf. We find that during the rainy inter-monsoon (March–April), a significant positive relationship (r = 0.63, p < 0.0001) exists between river discharge and phytoplankton biomass. There is a clear time-lag between rainfall, river discharge (1-month lag) and local chlorophyll biomass (2-months lag after discharge). Unlike offshore waters which exhibit bi-annual chl-a peaks (0.22 mg m−3 in February, and 0.223 mg m−3 in August/September), Ungwana Bay displays a single peak per annum in July (2.51 mg m−3), with indications that river discharge sustains phytoplankton biomass for several months. Satellite-derived observations and Lagrangian tracking simulations indicate that higher Chl-a concentrations remain locally within the bay, rather than influencing the broader open waters of the North Kenya Banks that are mainly impacted by the wider oceanic circulation.

Document Type:	Article
Programme Area:	PA3
Research affiliation:	Biogeochemistry and Geology > Geoecology & Carbonate Sedimentology
Refereed:	Yes
Open Access Journal?:	No
DOI:	https://doi.org/10.1016/j.ocecoaman.2021.105713
ISSN:	09645691
Date Deposited:	04 Jul 2022 18:00
Last Modified:	26 Mar 2024 13:31
URI:	http://cris.leibniz-zmt.de/id/eprint/4970

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