Protistan microzooplankton and the trophic position of tuna: quantifying the trophic link between micro- and mesozooplankton in marine foodwebs

The importance of the trophic link between <200-µm protistan microzooplankton and mesozooplankton is a cornerstone of modern understanding of foodweb structure of marine pelagic ecosystems that is well demonstrated in experimentally measured contributions of protistan consumers to mesozooplankton diets, in constrained global budgets, and in regional studies that show the inadequacies of meeting zooplankton metabolic requirements by herbivory alone. Nonetheless, protistan trophic steps are poorly reflected in stable isotope analyses by standard methods and systematically neglected in fisheries-related trophic research, which focuses instead on interactions that can be measured in stomach contents. Here, we apply recent advances in compound-specific isotope analysis of amino acids (CSIA-AA) based on alanine as a trophic indicator of protistan foodweb steps to evaluate the implications of lower foodweb structure on trophic position (TP) estimates of tunas. CSIA-AA results for mesozooplankton of the subtropical North Pacific suggest that tuna TPs are underestimated by 0.9, which give rise to new TP estimates of 4.7–5.4 for the three main tuna species of that region. These, in turn, are used to compute a trophic enrichment factor (TEF = 4.45) based on alanine that includes 15N enrichments for protistan consumers and can be applied more broadly in the region. Accounting for the magnitude and variability of protistan trophic steps in the foodwebs supporting pelagic fisheries has important implications for understanding regional variability in energy flows and foodweb structure and their temporal responses to climate change.

An evaluation of stable nitrogen isotopes and polychlorinated biphenyls as bioenergetic tracers in aquatic systems

This study investigated the relationship between stable nitrogen isotopes (δ15N) and polychlorinated biphenyl (PCB) concentrations in rock bass (Ambloplites rupestris) and bluegill sunfish (Lepomis macrochirus) with respect to age, size, and diet. δ15N signatures in both species exhibited enrichment with increasing size and approached steady state with respect to dietary δ15N values by the second year of growth. Young-of-the-year fish, however, exhibited lower nitrogen isotope enrichment over the diet, indicating that the commonly held trophic enrichment factor of 3.4‰ is more suitable for older, slow-growing individuals. PCB accumulation in both species progressed from being dominated by uptake from water in small (<100 g) fish to dietary uptake in larger individuals as a function of bioenergetic constraints such as food energy conversion and contaminant assimilation efficiencies. Significant increases in PCB accumulation were attributed to decreased specific growth rates such that ≤60% of body mass was gained on an annual basis. This effect was most noted in bluegills where higher PCB biomagnification factors were a consequence of increased foraging costs associated with an invertebrate diet. It is concluded that growth-related changes in species bioenergetics regulate both contaminant accumulation and δ15N dynamics.