A multi-method approach for identifying meiofaunal trophic connections

Abstract

Meiofauna can play an important role in the energetics of benthic communities, but determining their diet remains problematic due to their small size. In this study, the contribution of primary producers to the diet of meiofauna and to bulk and fine (<125 µm) sediment organic matter (SOM) was investigated in Papanui Inlet, southern New Zealand, using stable isotopes (δ 13 C and δ 15 N) and fatty acid analyses in the field and in a microcosm experiment. Seston did not contribute significantly to SOM of vegetated (seagrass Zostera muelleri) and unvegetated habitats, suggesting limited bentho-pelagic coupling. The contribution of different benthic primary producers to SOM and meiofaunal diet could not be determined accurately based on isotopic signatures alone due to over- lapping isotopic signatures. The high content of highly unsaturated fatty acids (HUFAs) of harpacti- coid copepods from vegetated and unvegetated sites suggested that their main food item was micro- phytobenthos (MPB), although bacterial biomarkers were twice as abundant in copepods sampled from the seagrass bed than in copepods sampled from unvegetated sediments. Isotopic and fatty acid analyses showed that the uptake of 13 C-labeled macroalgal (Enteromorpha sp.) detritus by harpacti- coid copepods in the microcosm experiment was minimal. Nematode diet could not be assessed with certainty, but seagrass detritus and MPB are likely to represent significant food sources at the vege- tated and unvegetated sites, respectively, based on the fatty-acid profile of fine SOM. This combined approach (i.e. isotopic and fatty acid analyses, field sampling and microcosms), and analysis of bulk SOM, fine SOM, and aged seagrass detritus helped circumvent limitations associated with the indi- vidual methods. The combination of approaches was still insufficient to quantify the contribution of different primary producers to meiofaunal diet, however, highlighting the difficulties associated with the study of meiofaunal trophic connections in coastal systems.