Trophic transfer of nearshore basal resources: interpreting fatty acid and stable isotope biomarkers

Abstract

Benthic heterotrophs living in aphotic aquatic habitats rely upon subsidies of detrital energy, in the form of complex organic molecules, which are synthesized by macrophytes and phytoplankton in the photic zone. Identifying the relative importance of different basal energy resources to consumers is critical for understanding ecosystem function and trophic connectivity in aquatic habitats. Partitioning the contribution of differing detrital resources to deep nearshore subtidal consumers is impractical using solely observational techniques such as surveys of macrophyte biomass or gut content analyses. Biochemical markers such as fatty acids (FA) and multiple stable isotopes (MSI; collectively FAMSI) can help resolve which sources are actually important in consumers, but several key uncertainties in the assumptions of the approach limit its utility. In this dissertation, I used a combination of observational and experimental approaches to determine what heterotrophs are eating using a FAMSI signature analyses. Chapter 1 is an introduction and summary. In Chapter 2, I tested whether different marine macrophyte taxa have distinct FA signatures. I found that algal FA signatures are closely linked to phylogeny; regardless of collection region, families, orders and phyla differ strongly from one another. In Chapter 3, I compared FAMSI signatures of a diverse group of conspecific consumers across photic and aphotic depths to evaluate the hypothesis that consumer biomarkers would reflect evidence of basal resource subsidies from the shallow photic zone. A diverse assemblage of organisms had FAMSI signatures that differed among depths. In an algal aging experiment, I found support for a microbe-induced diagenesis hypothesis to explain the biochemical differences found in the field. In Chapter 4 I used a fast-growing herbivorous isopod in a laboratory feeding trial to test the hypothesis that the FAMSI signatures of diets are assimilated predictably into consumer tissue. This experimental work took advantage of the earlier discovered patterns from Chapter 2 and evaluated whether the fractionation of FAMSI signatures between diets and consumers was diet-specific. Each of these three chapters represents an important step in advancing the FAMSI biomarker field beyond qualitative observational studies that are based largely on untested assumptions.