On the dynamics of the <scp>Z</scp>anzibar <scp>C</scp>hannel

Zavala‐Garay, Javier; Theiss, J.; Moulton, Melissa; Walsh, Cameron; Woesik, Robert van; Mayorga‐Adame, C. G.; García‐Reyes, Marisol; Mukaka, D. S.; Whilden, Kerri A.; Shaghude, Yohanna W.

doi:10.1002/2015jc010879

Cited by 14 publications

(13 citation statements)

References 42 publications

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“…Interestingly, a clear genetic cluster co-occurs with a local system of eddies between the mainland of Tanzania and Zanzibar. Gene flow was directional and strongest from Madagascar toward the coasts of Kenya and western Zanzibar, spatially coinciding with the NEMC and the "continuing" EACC (Zavala-Garay et al, 2015). Our findings indicate that the genetic structure of T. hemprichii in the WIO is influenced by large oceanic currents (SEC, NEMC, and EACC), as well as by local hydrodynamic patterns (Zanzibar channel).…”

Section: Discussionmentioning

confidence: 62%

“…Gene flow was directional and strongest from Madagascar toward the coasts of Kenya and western Zanzibar, spatially coinciding with the NEMC and the "continuing" EACC (Zavala-Garay et al, 2015). The study contributes to the knowledge of population genetic patterns of important species in the region, but also where in fact little is known on population genetic patterns in general.…”

Section: Discussionmentioning

confidence: 85%

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Population genetic structure and connectivity of the seagrass Thalassia hemprichii in the Western Indian Ocean is influenced by predominant ocean currents

Jahnke

Gullström

Larsson

et al. 2019

Ecology and Evolution

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This study is the first large‐scale genetic population study of a widespread climax species of seagrass, Thalassia hemprichii, in the Western Indian Ocean (WIO). The aim was to understand genetic population structure and connectivity of T. hemprichii in relation to hydrodynamic features. We genotyped 205 individual seagrass shoots from 11 sites across the WIO, spanning over a distance of ~2,700 km, with twelve microsatellite markers. Seagrass shoots were sampled in Kenya, Tanzania (mainland and Zanzibar), Mozambique, and Madagascar: 4–26°S and 33–48°E. We assessed clonality and visualized genetic diversity and genetic population differentiation. We used Bayesian clustering approaches (TESS) to trace spatial ancestry of populations and used directional migration rates (DivMigrate) to identify sources of gene flow. We identified four genetically differentiated groups: (a) samples from the Zanzibar channel; (b) Mozambique; (c) Madagascar; and (d) the east coast of Zanzibar and Kenya. Significant pairwise population genetic differentiation was found among many sites. Isolation by distance was detected for the estimated magnitude of divergence (DEST), but the three predominant ocean current systems (i.e., East African Coastal Current, North East Madagascar Current, and the South Equatorial Current) also determine genetic connectivity and genetic structure. Directional migration rates indicate that Madagascar acts as an important source population. Overall, clonality was moderate to high with large differences among sampling sites, indicating relatively low, but spatially variable sexual reproduction rates. The strongest genetic break was identified for three sites in the Zanzibar channel. Although isolation by distance is present, this study suggests that the three regionally predominant ocean current systems (i.e., East African Coastal Current, North East Madagascar Current, and the South Equatorial Current) rather than distance determine genetic connectivity and structure of T. hemprichii in the WIO. If the goal is to maintain genetic connectivity of T. hemprichii within the WIO, conservation planning and implementation of marine protection should be considered at the regional scale—across national borders.

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Section: Discussionmentioning

confidence: 62%

Section: Discussionmentioning

confidence: 85%

Population genetic structure and connectivity of the seagrass Thalassia hemprichii in the Western Indian Ocean is influenced by predominant ocean currents

Jahnke

Gullström

Larsson

et al. 2019

Ecology and Evolution

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“…The greatest speeds are observed during the SEM, exceeding 1 ms −1 from 4 to 5°S, and the lowest during the NEM, less than 0.3 ms −1 (Leetmaa et al, 1982;Swallow et al, 1991). In the shallow Zanzibar Channel, the EACC actually reverses during the NEM (Nyandwi, 2013), which is also seen in a high-resolution ocean model (Zavala-Garay et al, 2015).…”

Section: Introductionmentioning

confidence: 86%

Shelf‐Break Upwelling and Productivity Over the North Kenya Banks: The Importance of Large‐Scale Ocean Dynamics

et al. 2020

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The North Kenya Banks (NKBs) have recently emerged as a new frontier for food security and could become an economically important fishery for Kenya with improved resources providing better accessibility. Little research has been done on the mechanisms supporting high fish productivity over the NKBs with information on annual and interannual environmental variability lacking. Here we use a high-resolution, global, biogeochemical ocean model with remote sensing observations to demonstrate that the ocean circulation exerts an important control on the productivity over the NKBs. During the Northeast Monsoon, which occurs from December to February, upwelling occurs along the Kenyan coast, which is topographically enhanced over the NKBs. Additionally, enhanced upwelling events, associated with widespread cool temperatures, elevated chlorophyll, nutrients, primary production, and phytoplankton biomass, can occur over this region. Eight such modeled events, characterized by primary production exceeding 1.3 g C/m −2 /day, were found to occur during January or February from 1993-2015. Even though the upwelling is always rooted to the NKBs, the position, spatial extent, and intensity of the upwelling exhibit considerable interannual variability. The confluence zone between the Somali Current and East African Coastal Current (referred to as the Somali-Zanzibar Confluence Zone) forms during the Northeast Monsoon and is highly variable. We present evidence that when the Somali-Zanzibar Confluence Zone is positioned further south, it acts to enhance shelf-edge upwelling and productivity over the NKBs. These findings provide the first indication of the environmental controls that need to be considered when developing plans for the sustainable exploitation of the NKB fishery. Plain Language SummaryThe North Kenya Banks (NKBs) have recently emerged as a region capable of sustaining a rich fishery, which would boost Kenya's economy. Little research has been conducted on the environmental controls that affect these fisheries and whether there is annual variability over multiple years. Here an ocean model is used, with satellite remote sensing data, to investigate how the ocean influences this region. Specifically, the convergence of two currents during the Northeast Monsoon, the Somali Current and the East African Coastal Current, which meet near the NKBs and flow away from the coast. This induces the upwelling of cold, nutrient-rich waters, which are brought up from depth to the surface, resulting in the NKBs being a highly productive area. The confluence of these two currents forms from December to February and its position along the Kenyan coast varies from year-to-year. This leads to considerable variability in the intensity, position, and spatial extent of productive waters. Productivity is enhanced over the NKBs when the confluence of the two currents is generally positioned further south. These findings provide the first indication of the environmental controls that need to be taken into account when developing plans for sust...

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“…The climate is warm and moist, with temperatures between 27 and 35°C, with an annual rainfall of 1600 mm that is strongly influenced by two opposing monsoon seasons driven by the southeast monsoonal circulation of the central WIO (Mahongo and Shaghude, 2014;McClanahan, 1988). The regional hydrodynamics are complex and primarily influenced by 20 ebb-flood tidal phases but are also influenced by the East African Coastal Current (EACC) and monsoon winds (Mahongo and Shaghude, 2014;Shaghude et al, 2002;Zavala-Garay et al, 2015). The tidal cycles are semi-diurnal ranging from mesotidal during neap tide (~1 meter amplitude) to macrotidal (from 3 to 4 meters in amplitude) during spring tide (Shaghude et al, 2002;Zavala-Garay et al, 2015).…”

Section: Description Of Study Sitesmentioning

confidence: 99%

“…The regional hydrodynamics are complex and primarily influenced by 20 ebb-flood tidal phases but are also influenced by the East African Coastal Current (EACC) and monsoon winds (Mahongo and Shaghude, 2014;Shaghude et al, 2002;Zavala-Garay et al, 2015). The tidal cycles are semi-diurnal ranging from mesotidal during neap tide (~1 meter amplitude) to macrotidal (from 3 to 4 meters in amplitude) during spring tide (Shaghude et al, 2002;Zavala-Garay et al, 2015). Strong tidal currents can reach velocities that range from 0.25 to 2 m/s (Shaghude et al, 2002).…”

Section: Description Of Study Sitesmentioning

confidence: 99%

Seagrass community-level controls over organic carbon storage are constrained by geophysical attributes within meadows of Zanzibar, Tanzania

Belshe¹,

Hoeijmakers²,

Herran³

et al. 2017

Preprint

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Abstract. The aim of this work was to explore the feasibility of using seagrass functional traits to predict differences in sediment carbon storage. At 19 sites within highly diverse seagrass meadows of Zanzibar, Tanzania, species cover was 10 estimated along with three community traits hypothesized to influence sediment carbon storage (amount of above and belowground biomass, seagrass tissue nitrogen content, and shoot density). We identified five distinct seagrass communities that had notable variations in key plant traits but these differences did not translate into differences is sediment organic carbon (OC) storage. Across all communities, sediment OC was very low (ranging from 0.15% to 0.75%) and there were no differences in OC storage among communities, which was considerably lower (33.9±7.7 Mg C ha -1 ) than the global average 15 (194.2±20.2 Mg C ha -1 ) reported for other seagrass ecosystems. In spite of high seagrass diversity and clear zonation among plant communities, sediments in all communities were shallow (ranging from 19 to 78 cm) and composed of medium-coarse grained carbonate sand on top of carbonate rock. We propose that geophysical conditions of the sediment were not conducive to OC stabilization, and outweighed any variation in the quantity or quality of plant litter inputs, ultimately leading to low OC storage within all seagrass communities. This highlights the complexity of OC cycling in seagrass 20 ecosystems and cautions against the use of plant traits as a proxy for OC storage across all seagrass ecosystems.

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On the dynamics of the Zanzibar Channel

Cited by 14 publications

References 42 publications

Population genetic structure and connectivity of the seagrass Thalassia hemprichii in the Western Indian Ocean is influenced by predominant ocean currents

Population genetic structure and connectivity of the seagrass Thalassia hemprichii in the Western Indian Ocean is influenced by predominant ocean currents

Shelf‐Break Upwelling and Productivity Over the North Kenya Banks: The Importance of Large‐Scale Ocean Dynamics

Seagrass community-level controls over organic carbon storage are constrained by geophysical attributes within meadows of Zanzibar, Tanzania

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