Abstract
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.
Highlights
Temporal and spatial variation in mesoscale nearshore oceanographic conditions plays an important role in the distribution of primary production [1,2], resulting in differences in the availability of resources for intertidal consumers [3,4]
Using fatty acid (FA) techniques, we aimed to assess the effects of upwelling on the dietary signatures of benthic filter feeders in the Benguela upwelling region, by comparing sites that are considered
The suspended organic matter (SPM) collected on the same day as the mussels does not necessary represent what the organisms were feeding on a few days/weeks before
Summary
Temporal and spatial variation in mesoscale nearshore oceanographic conditions plays an important role in the distribution of primary production [1,2], resulting in differences in the availability of resources for intertidal consumers [3,4]. These differences in food availability can strongly influence the distribution and metabolism of these organisms [5,6,7]. One feature that can influence food availability is represented by upwelling events, which bring deep, nutrient-rich waters into coastal ecosystems, promoting nearshore phytoplankton production [8,9,10]. The reproductive peak of some invertebrates has been shown to coincide with the upwelling season [15,16], enabling planktotrophic larvae to benefit from the phytoplankton rich water
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