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Can δ15N and δ13C stable isotopes and fatty acid signatures indicate changes in phytobenthos composition on an artificial substrate?

Temporal changes in fatty acid composition and δ15N, δ13C stable isotope values of the phytobenthos growing on artificial clay substrates under natural conditions over a 28-day period at an upstream and a downstream site in the Kowie River near Grahamstown were investigated in 2012. High concentrations of diatom markers 16:1ω7 and 20:5ω3 fatty acids were recorded, especially at the downstream site, reflecting the importance of diatoms in contributing to the phytobenthos communities at that station. After day 7 at the downstream site the average δ15N value of the phytobenthos was lighter, gradually increasing by ∼2‰ and ∼5‰ overall to heavier values on day 28. At the upstream site there were no significant changes (<1‰ increase) in δ15N values of the phytobenthos over time. Stable nitrogen (δ15N) and carbon (δ13C) signatures in the phytobenthos communities were significantly different between sites (one-way ANOVA; p < 0.001). The stable isotope values and fatty acid concentrations of phytobenthos at the downstream site were different to those of the phytobenthos at the upstream site, and they changed concurrently with changes in the phytobenthos community structure. At the downstream site there was a strong correlation of the δ15N of phytobenthos with nitrates (R = 0.56) and time (weeks; R = 0.81). However, the fatty acids were not specific enough to characterise the composition of phytobenthos communities. Other biomarker methods, such as stable isotopes and microscopic examination of the communities, were found to be useful. The results from this relatively small-scale tile experiment indicate the complexity of changes in fatty acid composition and δ15N, δ13C stable isotope values of a phytobenthos community. Stable isotope and fatty acid composition can be successfully used to map changes in phytobenthos composition and carbon and nitrogen flow patterns along a river continuum.