Zooplankton trophic niches respond to different water types of the western Tasman Sea: A stable isotope analysis

The trophic relationships of 21 species from an oceanic zooplankton community were studied using stable isotopes of carbon and nitrogen. Zooplankton and suspended particulate organic matter (POM) were sampled in three different water types in the western Tasman Sea: inner shelf (IS), a cold core eddy (CCE) and a warm core eddy (WCE). delta N-15 values ranged from 3.9 parts per thousand for the parasitic copepod Sapphirina augusta to 10.2 parts per thousand for the euphausiid, Euphausia spinifera. delta C-13 varied from -22.6 to -19.4 parts per thousand as a result of the copepod Euchirella curticauda and E. spimfera. The isotopic composition of POM varied significantly among water types; as did the trophic enrichment of zooplankton over POM, with the lowest enrichment in the recently upwelled IS water type (0.5 parts per thousand) compared to the warm core eddy (1.6 parts per thousand) and cold core eddy (2.7 parts per thousand). The WCE was an oligotrophic environment and was associated with an increased trophic level for omnivorous zooplankton (copepods and euphausiids) to a similar level as carnivorous zooplankton (chaetognaths). Therefore carnivory in zooplankton can increase in response to lower abundance and reduced diversity in their phytoplankton and protozoan prey. Trophic niche width comparisons across three zooplankton species: the salp Thalia democratica, the copepod Eucalanus elongatus and the euphausiid Thysanoessa gregaria, indicated that both niche partitioning and competition can occur within the zooplankton community. We have shown that trophic relationships among the zooplankton are dynamic and respond to different water types. The changes to the zooplankton isotopic niche, however, were still highly variable as result of oceanographic variation within water types. (C) 2015 Elsevier Ltd. All rights reserved.