δ15N and δ13C elucidation of size-structured food webs in a Western Arabian Sea demersal trawl assemblage

Abstract

In aquatic systems, predators are usually larger in body size than their prey, and within a species larger individuals tend to occupy higher trophic levels. This has led to the expectation that communities are strongly size-structured. In the present study, we tested whether size structuring was present in a demersal trawl assemblage from the Western Arabian Sea. Nitrogen and carbon stable isotopes were used to describe trophic level and basal resource utilisation and were assessed against body size (log 2 size class) at both the community (species independent) and species levels. Nitrogen and bulk carbon isotopic values were significantly and positively related to body mass at the community level. Despite sampling over 15 log 2 mass classes (0.001 to 32.8 kg), the trawl assemblage spanned approximately 1 trophic level. The community relationship between δ 13 C and log 2 body mass was associated with higher lipid content in smaller size classes. In contrast, comparisons of the δ 15 N and δ 13 C values of species at a similar stage of their life history (55 to 80% of maximum length) were independent of body size, suggesting reduced niche overlap at different sizes. While some small species feed at high trophic levels and vice versa, they contributed comparatively little to the sampled community biomass, and overall, the community was size-structured in terms of trophic level.

The paradox of inverted biomass pyramids in kelp forest fish communities

“…Part of this variability stems from the assumptions made in different studies. The only other marine fish community of which we are aware for which PPMR has been estimated is the Western Arabian Sea, where the estimate was 2327 [21].…”

“…PPMR can be calculated from the slope of the empirical relationship between trophic position and body size using stable isotope data [20]. Available community-wide PPMR estimates predominantly come from fishes (and in one case, invertebrates) in pelagic and soft-sediment demersal systems, where values have fallen within the expected range of hundreds to thousands [5,7,21]. These PPMRs, combined with TEs of approximately 10%, lead to flat biomass spectra and biomass columns.…”

Preliminary assessment of the trophic structure of demersal fish community in the Sea of Oman

“…The overall δ 15 N range values and variance were also calculated to infer the diversity of trophic levels potentially represented in the demersal fish community at the two sampling periods, assuming that different positions in the food web would result in a difference of at least 3.4 ‰ between two trophic levels (Post, 2002;Post et al, 2007). Additionally, the relationship between community-level fish size (all species combined) and δ 13 C, δ 15 N and δ 34 S were examined to test for a possible size structuration of trophic relationships within the demersal fish community, such as investigated in Al-Habsi et al (2008). Those relationships were tested using linear regression models, and considered significant if P < 0.05.…”

“…Only few published studies used stable isotopes to investigate wild fish diets in coastal waters of the Sultanate of Oman (Al-Reasi et al, 2007;Al-Habsi et al, 2008;Mill, 2007;Mill et al, 2007). For example, Al-Habsi et al (2008) used carbon and nitrogen isotopic ratios to investigate a potential size-structuration of demersal food webs in the coastal area of the Arabian Sea (located on the south of the country).…”

“…For example, Al-Habsi et al (2008) used carbon and nitrogen isotopic ratios to investigate a potential size-structuration of demersal food webs in the coastal area of the Arabian Sea (located on the south of the country). The same stable isotopes were used to analyze the trophodynamics of coral reef communities from the Arabian Sea and the Sea of Oman (located on the northwest of the country), the two sites being characterized by contrasted influences of regional upwelling (Mill, 2007).…”