Browsing by Author "Noyon, Margaux"

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    Evidence of localised upwelling in Pemba Channel (Tanzania) during the southeast monsoon
    (Elsevier Ltd, 2021) Painter, Stuart C.; Sekadende, Baraka; Michael, Angelina; Noyon, Margaux; Shayo, Salome; Godfrey, Brian; Mwadini, Mtumwa; Kyewalyanga, Margareth
    Oceanographic and biogeochemical observations collected in Pemba Channel, a deep-water (800 m) channel separating Pemba Island from mainland Tanzania, during the South East monsoon indicate the presence of active upwelling along the western edge of Pemba Island. Surface salinity values, nutrient concentrations and the presence of coccolithophore species previously reported from the mid to lower euphotic zone all suggest upwelling from at least 80–100 m depth. The surface waters of the channel were characterised with low NO3−:PO43− (0.68:1) and NO3−:Si (0.04:1) ratios far below the Brzezinski-Redfield ratio indicating the presence of N-limitation and the possibility that these waters may be susceptible to anthropogenic N inputs. Surface NO3− concentrations averaged 0.09 ± 0.10 μmol L−1 but increased to 0.5 μmol L−1 in the centre of upwelling where coincidentally both integrated nutrient concentrations and surface POC/PON pools were approximately 2-fold higher than the channel average. Despite its significance for local productivity upwelling is tentatively estimated, via stoichiometric assumptions, to enhance local productivity by only 20%. The modest productivity response to upwelling may be explained by picoplankton (0.2–2 μm) dominance of the phytoplankton community with this size-class representing ~80% of total chlorophyll-a. Nevertheless, important spatial variability was identified in larger size fractions and supported by taxonomic analyses with indications that the distribution of Chaetoceros spp. alone may be particularly relevant for understanding the variability in larger (>20 μm) chlorophyll-a size fractions. The location of upwelling has previously been shown to host large concentrations of small pelagic fish thus management of this regionally important resource would benefit from additional investigation of the underlying physical mechanism driving upwelling and subsequently how trophic interactions and ecosystem productivity are influenced.
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    Spatial variation in the phytoplankton community of the Pemba Channel, Tanzania, during the south-east monsoon
    (Elservier Ltd, 2021) Sekadende, Baraka C.; Michael, Angelina; Painter, Stuart C.; Shayo, Salome; Noyon, Margaux; Kyewalyanga, Margareth S.
    The spatiotemporal distribution of phytoplankton within Tanzanian coastal waters is considered to be poorly known. Improved knowledge of the phytoplankton community, including the spatiotemporal distribution, is desirable for understanding certain aspects of trophic linkages to small pelagic fish, a key marine resource in Tanzania, and for understanding how anthropogenic activities such as terrestrial run-off and effluent discharge may impact the phytoplankton community of these waters. In this study the abundance, diversity and distribution of microphytoplankton was determined throughout the Pemba Channel during the Southeast monsoon and assessed in relation to local environmental parameters. A total of 88 species were identified during this study. Generally, diatoms dominated the microphytoplankton community of the Pemba Channel with 55 identified species (62.5% of total), followed by 31 dinoflagellate species (35.2%) and 2 cyanobacteria species (2.27%). Abundances were low, generally less than 100 cell L−1. Despite a clear dominance by diatoms within the samples, no obvious spatial pattern in their distribution was identified within the channel. Cyanobacteria meanwhile displayed a significant latitudinal gradient being more abundant in the northern half of the channel. Several harmful or potentially harmful phytoplankton species were also identified in this study including Pseudo-nitzschia sp. and Dinophysis sp., although their abundances were below the thresholds of harmful algal bloom warning systems. Nevertheless, to understand whether these species are increasing with time in response to anthropogenic activities, regular phytoplankton monitoring programs are recommended.