PhD ThesisDespite the increasing use of stable isotope data as diet and trophic position
descriptors in food-web ecology, their generic value relies on basic assumptions of
constant trophic step enrichment, steady state conditions and accuracy of isotopic
analyses for the estimation of trophic level (TL). This thesis explores the
implications of these assumptions for understanding reef community
trophodynamics in Oman, where upwelling events produce seasonal patterns among
potential food sources.
Nitrogen isotopes (δ15N) revealed 3-4 TL’s (6.16 to 17.8‰) and the wide
range of carbon isotope (δ13C) values (-21.92 to -6.43‰) indicated that there were
both benthic and pelagic sources of production. Primary producer and primary
consumer δ15N fluctuated seasonally by 2.24‰ (9.02-11.26‰) however, this
variability was not consistently observed at higher TLs, and therefore the δ15N of
high-TL consumers may not accurately reflect their trophic position. Long-lived
marine bivalves had no temporal isotopic variability, allowing the trophic position
of higher consumers to be estimated using their δ15N as a baseline. Baseline
organisms also allowed spatial comparison of part of the trophic structure of two
reef communities; Barr al Hickman had δ15N values 2.7‰ enriched compared to a
similar community at Bandar Kayran 360km to the north. Across the Western
Indian Ocean, macroalgae δ15N values correlated well with differences in
underlying nutrient regimes of surrounding waters. Trophic-step fractionation in
herbivorous fish was 4.69-5.25‰, higher than the generally accepted value of 3.4‰
and was explained by a dynamic model incorporating feeding rate, diet quality and
excretion rate, which are inherently different between herbivorous and carnivorous
fish. δ15N was strongly correlated with body size in some fish species but across the
entire community body size was a poor descriptor of trophic position. Use of dietspecific
trophic-step fractionation values and sulphur isotopes (δ34S) greatly
improved the resolution of food-web models.NERC CASE Newcastle University
Centre for Environment Fisheries and Aquaculture Sciences (CEFAS