Abstract of PhD Thesis

Functional signatures of model crustaceans in lake littoral zones in response to nutrient status and local habitat

Saliha Yeşim Tunali, Trinity College Dublin (2015)

The littoral zones of lakes provide a critical interface between terrestrial and water habitats. Traditionally monitoring of lakes has focussed on the open-water, although for many lakes this may not be the zone of greatest ecological production or species diversity. Furthermore, what monitoring that has been initiated within lake littoral zones has tended to assess the structure of communities, rather than their role in ecological functioning.  This study employed three approaches to quantify macroinvertebrate ecological function: carbon and nitrogen stable isotope ratios, isotopic niche metrics and C:N:P stoichiometry. Using common “model” organisms, the effects of two distinct types of shoreline habitat, was tested for effects on the isotopic signatures of Asellus aquaticus, Gammarus duebeni celticus and Crangonyx pseudogracilis across eight Irish lakes that showed a nutrient enrichment gradient. Results from linear mixed-effects models indicated the importance of water colour for invertebrate isotopic signatures and significant effects of shoreline habitat type on both the δ13C and δ15N signatures of A. aquaticus and G. d. celticus, and on the δ15N signatures of C. pseudogracilis. Distinct responses across species, suggest both specific dietary needs and the importance of species-specific traits, such as body size, in explaining species’ isotopic variation within lakes. A second component of the study demonstrated competitive overlap between the native amphipod G. d. celticus and two invasive species, G. pulex and C. Pseudogracilis. Using metrics of isotopic niche space, the study highlighted the high trophic position of C. pseudogracilis compared with Gammarus spp., contradicting widely held views, and there was no evidence that Gammarus spp. prayed on C. pseudogracilis. In the final component of the thesis, the taxonomic and spatial variation of the C:N:P stoichiometry of littoral macroinvertebrates within and across three lakes confirmed that taxonomy, rather than spatial differences, are more likely to be the primary driver of among and within lake variation of C:N:P stoichiometry. Significant stoichiometric differences between Gammarus spp. and C. pseudogracilis were demonstrated for the first time.

The study demonstrated a novel approach for testing changes in species trophic interactions in spatially distinct habitats, and highlighted that the chemical signatures of species distribution in lakes can be complementary to sampling for taxonomic identifications. The project contributes to a developing understanding of habitat quality in lakes, and the role of invasive species in that, and supports ongoing attempts for the classification of European lakes under the EU Water Framework Directive.