Fatty acid composition at the base of aquatic food webs is influenced by habitat type and watershed land use

Abstract

Spatial variation in food resources strongly influences many aspects of aquatic consumer ecology. Although large-scale controls over spatial variation in many aspects of food resources are well known, others have received little study. Here we investigated variation in the fatty acid (FA) composition of seston and primary consumers within (i.e., among habitats) and among tributary systems of Lake Michigan, USA. FA composition of food is important because all metazoans require certain FAs for proper growth and development that cannot be produced de novo, including many polyunsaturated fatty acids (PUFAs). Here we sampled three habitat types (river, rivermouth and nearshore zone) in 11 tributaries of Lake Michigan to assess the amount of FA in seston and primary consumers of seston. We hypothesize that among-system and among-habitat variation in FAs at the base of food webs would be related to algal production, which in turn is influenced by three land cover characteristics: 1) combined agriculture and urban lands (an indication of anthropogenic nutrient inputs that fuel algal production), 2) the proportion of surface waters (an indication of water residence times that allow algal producers to accumulate) and 3) the extent of riparian forested buffers (an indication of stream shading that reduces algal production). Of these three land cover characteristics, only intense land use appeared to strongly related to seston and consumer FA and this effect was only strong in rivermouth and nearshore lake sites. River seston and consumer FA composition was highly variable, but that variation does not appear to be driven by the watershed land cover characteristics investigated here. Whether the spatial variation in FA content at the base of these food webs significantly influences the production of economically important species higher in the food web should be a focus of future research.

Figure 1. Conceptual figure showing predictions and study design in the context of a hypothetical tributary system.

Our underlying prediction is that watershed land cover significantly influences fatty acid (FA) content of seston and consumers via effects on algal production and composition. The land cover properties investigated here are highlighted in white font (surface waters, intense land uses, and forest buffers) with their presumed mechanistic relationship to algal dynamics in yellow boxes or arrows (algal export, nitrogen [N] and phosphorus [P] loading and stream shading). Habitat types sampled in this study are also shown (river, rivermouth and nearshore lake sites).

Figure 2. Location of rivermouth systems sampled during this study.

More detailed geographic information is available in Appendix S1.

Figure 3. Total fatty acid (∑FA) quantity and as a proportion of the total seston, total suspended solids (TSS) and chlorophyll a (chl a) in rivers (R), rivermouths (RM) and the nearshore of Lake Michigan (L).

Raw data (points) and averages (with 95% credible intervals) are shown here. Although not presented, the trends in ∑FA in seston largely mirror trends in other seston FAs measured here (see Table S3 and Appendix S2). Statistical significance is inferred by the lack of overlap between credible intervals (no statistically significant differences occur here).

Figure 4. Total fatty acid (∑FA), EPA and ARA in river (R) caddisflies, and dreissenid mussels collected in rivermouths (RM) and the nearshore of Lake Michigan (L).

Raw data (points) and averages (with 95% credible intervals) are shown here. Although not presented, the trends in ∑FA in seston largely mirror trends in PUFAs, MUFAs, DHA, ALA and LIN (see Table S3 and Appendix S2). Statistical significance is inferred by the lack of overlap between credible intervals (no statistically significant differences occur here). To highlight differences in taxa, the bar representing caddisflies has been shaded darker.

Figure 5. Standardized slopes (β) between land cover characteristics and fatty acid (FA) properties.

A) Relationships between land cover and seston TSS, chlorophyll a and FA properties at river (R), rivermouth (RM) and nearshore lake Michigan sites (L). B) Relationships between land cover and consumer FA properties at R, RM and L sites. Other measured FA metrics not presented here vary in the same manner as total fatty acids (∑FA; Table S3). Slopes where 95% credible intervals overlapped zero are colored light grey and indicate non-significant (ns) relationships. Statistically significant positive (+) relationships are shaded dark grey, while statistically significant negative (−) relationships are shaded black.

Figure 6. Relationships between selected FA properties and land cover characteristics.

A) The percent of the watershed that is open water versus the total fatty acids (∑FA) in seston. Solid and dashed lines show the model at rivermouth (RM) and nearshore lake (L) sites, respectively. This relationship is driven by a single outlier, casting doubt on its usefulness, so no credible interval has been estimated. The proportion of the watershed that is intense land use and the B) seston ω3:ω6 at river (R) sites, C) dreissenid mussel (DM) EPA at rivermouth (RM) and nearshore zone (L) sites and D) DM ω3:ω6 in L sites. For statistically significant relationships in B-D, the solid lines show the model and dashed lines of the same color indicate 95% credible intervals of the estimated model.