Browsing by Subject "Diatoms"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Open Access
    Spatio-temporal variation in effects of upwelling on the fatty acid composition of benthic filter feeders in the Southern Benguela ecosystem: not all upwelling is equal
    (Public Library of Science, 2016) Puccinelli, Eleonora; McQuaid, Christopher David; Noyon, Margaux
    Variability in mesoscale nearshore oceanographic conditions plays an important role in the distribution of primary production and food availability for intertidal consumers. Advection of nutrient rich waters by upwelling usually allows the proliferation of diatoms, later replaced by dinoflagellates. We examined upwelling effects on the fatty acid (FA) signature of a benthic intertidal filter feeder to identify its response to pulsed variability in food availability. The study took place in two contrasting seasons and at two upwelling and two non-upwelling sites interspersed within the southern Benguela upwelling system of South Africa. We investigated the FA composition of the adductor muscles and gonads of the mussel Mytilus galloprovincialis to assess how FA are apportioned to the different tissues and whether this changes between upwelling and non-upwelling conditions. In situ temperature loggers used to identify upwelling conditions at the four sites indicated that such events occurred only at the upwelling centres and only in summer. Tissues differed strongly, with gonads presenting a higher proportion of essential FAs. This could reflect the faster turnover rate of gonad tissue or preferential retention of specific FA for reproductive purposes. FA composition did not vary as a direct function of upwelling, but there were strong dissimilarities among sites. Upwelling influenced mussel diets at one upwelling site while at the other, the expected signature of upwelling was displaced downstream of the core of upwelling. Condition Index (CI) and Gonad Index (GI) differed among sites and were not influenced by upwelling, with GI being comparable among sites. In addition, FA proportions were consistent among sites, indicating similar food quality and quantity over time and under upwelling and non-upwelling conditions. This suggests that the influence of upwelling on the west coast of South Africa is pervasive and diffuse, rather than discrete; while nearshore retention or advection of upwelled water is critical and site-specific so that the effects of upwelling differ even among sites categorised as upwelling centres.
  • Thumbnail Image
    Item
    Open Access
    The diatom record: Reconstructing historically recent environmental change in the Knysna estuary
    (2022) Antonopoulos, Helen Grace; Meadows, Michael Edward; Kirsten, Kelly L
    Estuaries are highly productive systems responsible for many vital ecosystem goods and services. Therefore, it is not surprising that the diversity and abundance of exploitable estuarine resources have attracted human settlers for centuries. Increased anthropogenic pressures have placed much stress on estuarine and coastal ecosystems. This is the case of the Knysna Estuary, which is South Africa's highestrated estuary in terms of biodiversity and conservation importance. This study represents the first highresolution diatom record from the Knysna Estuary encompassing the last ∼680 years. Diatom analysis was employed to reconstruct salinity, nutrients, and saprobity variables in addition to inferring community structure and trophic status over time by creating an age-depth model based on a combination of ²¹⁰Pb and radiocarbon dates. The record is divided into three distinctive phases, namely the Pre-Colonial (∼610 to ∼200 cal BP), Colonial (∼200 cal BP to ∼1900 CE), and Anthropogenically Impacted Lagoon Phase (∼1900 CE to Present). These phases correspond with the Little Ice Age (LIA), the arrival of colonialists, and anthropogenic impacts linked to rapid population growth and land use change. More specifically, the dominance of marine species illustrate that the first phase of the LIA is associated with drier conditions, whereas a growing dilute and eutrophic assemblage reveals a wetter second phase of the LIA coinciding with the arrival of colonialists in the 1700s. Consequently, it is challenging to disentangle natural climate change with the effects of deforestation and agriculture during the Colonial Phase. A shift towards an increasingly fresh, hypertrophic, and polysaprobic diatom assemblage is indicative of the intensification of agricultural practices in the catchment from ∼1900 CE to present, stormwater inflow, the inefficiency of the Knysna Waste Water Treatment Works (WWTW), and sewage entering the estuary via streams during the final phase of the estuary's development. This high-resolution record is of vital importance, as it is one of a few palaeoestuarine studies in the Southern Hemisphere to illuminate the effects of natural climate change and human-induced impacts on an estuarine system. Despite the limitations, this study illustrates that diatoms are a useful tool for tracking environmental change in estuaries.