Comparative study of trophic and elemental characteristics of zooplankton in deep (500–3500 m) and shallow (0–200 m) layers

The deep sea has limited food resources and is considered one of the most rigorous environments on Earth. In contrast, zooplankton and their food supply are abundant in shallow sea layers. In this study, the trophic and elemental characteristics of zooplankton taxa in the deep and shallow layers were explored with respect to trophic position, trophic niche overlap among taxa, and growth performance. Our results showed that higher values of δ15N were detected in most taxa or taxa groups in the deep layer compared to similar taxa in the shallow layer, indicating that depth-dependent δ15N values existed in most zooplankton taxa. A lower isotopic niche overlap was detected among taxa in the shallow layer, since the overlap of SEAc (standard ellipse area corrected for small sample sizes) between pairs of taxa was lower in the shallow layer than in the deep layer. In the deep layer, SEAc values of taxa were higher than those in the shallow layer respectively, showing higher isotopic niche widths. This could be due to the high trophic diversity of each taxa in the deep layer and the area sampled actually including at least two zones (mesopelagic and bathypelagic). From an ecological stoichiometry perspective, most taxonomic groups in the deep layer exhibited higher C:P and N:P ratios than similar taxa in the shallow layer, suggesting lower maximal growth rates in the deep layer. Lower growth rates potentially reduce the consumption of limited resources in the deep layer, and higher growth rates in the shallow layer facilitate the rapid exploitation of abundant resources and allow zooplankton to flourish.