Assessing cetaceans’ intra and interspecific trophic interactions and niche width using δ13C and δ15N from basal sources to top predators

Description

Cetaceans have an important role in the structure and function of their ecosystems. Understanding the trophic relationships among them is essential in order to assess how co-existing species shape their community structure and functioning. Information on the level of niche overlap or partitioning among these predators provides a better understanding regarding their coexistence. Isotopic niche can be used as an indicator of predators' ecological niches, helping to describe their trophic positions, feeding areas, and the level of segregation or overlap in their resource use. Nevertheless, variations in isotopic compositions at the base of the food web, driven by environmental physical and chemical gradients, are reflected in the isotopic compositions of these top predators. These gradients in isotopic patterns could result in erroneous interpretation regarding top predator’s trophic levels and feeding behaviors, if not taken into account. Therefore, incorporating environmental isotopic information can be used to understand patterns in isotopic variability in top predators and how shifts in basal isotopic composition are propagated into them. We analyzed carbon (δ13C) and nitrogen (δ15N) stable isotope composition (SIC) in skin tissues of seven odontocete species along the outer continental shelf and slope off Brazil: Stenella attenuata, Stenella frontalis, Stenella longirostris, Tursiops truncatus, Delphinus delphis and Steno bredanensis (Delphinidae) and Physeter macrocephalus (Physeteridae). SIC at the base of the food web (basal) was obtained by measuring Particulate Organic Matter (POM) and zooplankton. The sampling area was divided into four sub-areas with distinct oceanographic characteristics: south-shelfbreak (S-SB, ≈34°S – 28°S), south-offshore (S-off), southeast-shelfbreak (SE-SB, ≈28°S – 24°S) and southeast-offshore (SE-off). We calculated the isotopic niche width (Standard Ellipse Area, SEA) for POM, zooplankton and cetaceans for each area using Stable Isotope Ellipses in R (SIBER). There was no statistical difference in basal δ13C among the different areas (Mean ± SD -21.5±1.5‰). However, basal δ15N values were significantly higher in S-SB (POM: 6.2±2.4‰; zooplankton: 5.8±2.8‰) than in the other areas. In general, models estimated larger basal’ SEA for S-SB and SE-SB. Both areas are subject to a greater influence of continental waters or upwelling events. Highest mean δ15N values were observed in S. bredanensis (17.9±0.5‰) and D. delphis (15.3±1.5‰). These two species were sighted only in the southern area, which had higher basal δ15N values. Moreover, cetaceans sampled along the southern area had larger values for SEA, in comparison to those sampled in the southeastern region. Regarding those species registered in both areas (T. truncatus, S. frontalis and S. longirostris), all individuals sampled in the south had higher average δ15N values than their southeastern counterparts. At the present study we observed that top predators’ higher δ15N values might result from shifted basal δ15N values which are being propagated up the food web, rather than from a higher trophic position of the southern dolphin groups. Further analysis will aid to the understanding on the trophic dynamics of the cetaceans and how they interact with the surrounding environment and organisms.

Periode	19 jun 2017 → 23 jun 2017
Gehouden op	Conference Ecology & Behaviour 2017, France
Mate van erkenning	National