Species interactions and energy transfer in aquatic food webs

Abstract: Food webs are structured by intricate nodes of species interactions which govern the flow of organic matter in natural systems. Despite being long recognized as a key component in ecology, estimation of food web dynamics is still challenging due to the difficulty in accurately assessing species interactions within a food web. Suitable empirical tracing methods that estimate species diet uptake and trophic position are essential for measuring food web dynamics.I studied the use of compound specific stable isotopes for assessing predator-prey interactions, and applied molecular and isotope tracer methods to categorize energy flow in natural aquatic ecosystems, with a particular focus on the species links between phytoplankton and zooplankton.I evaluated the use of δ15N amino acid values to predict organism trophic position through a meta-analysis of available literature which included measurements from 359 marine species (article I).Through a controlled feeding study I further assessed species isotope incorporation in aquatic organisms, across both plant-animal and animal-animal species linkages (article II).In both studies I found that δ15N amino acid ratios were useful tools for categorizing organism trophic position. I also found that organism feeding ecology influences nitrogen trophic discrimination (difference in isotope ratio between consumer and diet), with higher discrimination in herbivores compared to omnivores and carnivores (article I). Furthermore nitrogen isotope trophic discrimination also varied among feeding treatments in our laboratory study (article II). The combined findings from articles I & II suggest that researchers should consider using group specific nitrogen trophic discrimination values to improve accuracy in species trophic position predictions. A key finding in the controlled laboratory study was consistently low C isotope discrimination in essential amino acids across all species linkages, confirming that these compounds are reliable dietary tracers.The δ13C ratios of essential amino acids were applied to study seasonal dynamics in zooplankton resource use in the Baltic Sea (article III). Data from this study indicated that zooplankton assimilate variable resources throughout the growing season. Selective resource use was also apparent when using molecular diet analysis (article IV) as all zooplankton genera ingested a range of both autotrophic and heterotrophic resources. Evidence from both article III & IV revealed that zooplankton feed on a relatively broad range of diet items but not opportunistically on all available food sources. Mesozooplankton feeding patterns suggested that energy and nutritional flows were channelled through an omnivorous zooplankton food chain which includes microzooplankton prey items.In this thesis I showed how energy and nutrient transfer among species can be accurately evaluated using isotope analysis (articles I & II) and applied isotopes and other tracer techniques to study species interactions and diet usage in natural systems (article III & IV).