Spatial and temporal variance in fatty acid and stable isotope signatures across trophic levels in large river systems

Abstract

Fatty acid and stable isotope signatures allow researchers, managers, and policy makers to better understand food webs, food sources, and trophic relationships.Research in marine and lentic systems has indicated that the variance of these biomarkers can exhibit substantial differences across spatial and temporal scales, but this type of analysis has not been completed for large river systems. Our objectives were (a) to evaluate variance structures for fatty acids and stable isotopes (i.e., δ 13 C and δ 15 N) of seston, threeridge mussels, hydropsychid caddisflies, gizzard shad, and bluegills across spatial scales (10s-100s of km) in large rivers of the Upper Mississippi River Basin, USA, that were sampled annually for 2 years and (b) to evaluate the implications of this variance on the design and interpretation of trophic studies. The greatest variance for both stable isotopes was present at the largest spatial scale for all taxa (except seston δ 15 N), indicating that these isotopic signatures are responding to factors at a larger geographic level rather than being influenced by local-scale alterations. Conversely, the greatest variance for fatty acids was present at the smallest spatial scale (i.e., among individuals) for all taxa except caddisflies, indicating that the physiological and metabolic processes that influence fatty acid profiles can differ substantially between individuals at a given site. Our results highlight the need to consider the spatial partitioning of variance during sample design and analysis, as some taxa may not be suitable for use in assessing ecological questions at larger spatial scales. KEYWORDS carbon stable isotope, fatty acid, food webs, lotic, multivariate variance, nitrogen stable isotope

Identification of food sources in tropical seagrass bed food web using triple stable isotopes and fatty acid signatures

“…For example, rooted marine vascular plants obtain sulfur most from anaerobic sediments, whereas other food sources, such as algae, obtain sulfur mostly from seawater sulfate. These factors result in a clear dissimilation in their d 34 S values (Fritts et al, 2018). Similarly, the d 34 S values of rooted seagrass may be also clear dissimilation from that of macroalgae or epiphytes in seagrass beds.…”

“…For example, in tropical seagrass ecosystems, the isotopic values of seagrass are usually close to those of epiphytes (Mittermayr et al, 2014;Cui et al, 2021a). In addition, the temporal variances in environmental factors may also result in similar stable isotopic values between seagrass and other food sources (Fritts et al, 2018). Therefore, it is essential to improve this approach.…”

Low redundancy among caddisflies sharing a mid‐mountain stream

“…In other cases, whole insect orders are grouped under a common isotopic value (e.g. Trichoptera in Fritts et al., 2018) disregarding the fact that this could represent an over‐simplification of their role within stream food webs, hampering the correct interpretation of overall feeding relationships.…”