Biogeography of Macrophyte Elemental Composition: Spatiotemporal Modification of Species-Level Traits

Authors: Sarah L. Close, Sally D. Hacker, Bruce A. Menge, Francis Chan & Karina J. Nielsen

The elemental composition of primary producers represents a potentially important pathway for linking ecosystem scale, climate-driven changes in nutrient supply to ecological processes at the population and community scales. However, such cross-scale linkages may also be dampened by conservation of species-specific differences in tissue stoichiometry. We investigated biogeographic patterns of elemental composition (%N, %C, and C:N ratio) of four ecologically dominant and functionally diverse marine intertidal macrophyte species (the brown algae Fucus distichus and Hedophyllum sessile, the red alga Mazzaella splendens, and the surfgrass Phyllospadix scouleri) along 900 km of coastline of the California Current System over a 10-year period. We used a nested hierarchical design to identify the dominant spatial and temporal scales of elemental composition variability and to test the sensitivity of macrophyte stoichiometry to changing ocean conditions in upwelling, the El Niño-Southern Oscillation (ENSO), the North Pacific Gyre Oscillation (NPGO), and the Pacific Decadal Oscillation (PDO). Elemental composition was highly conserved at the species level but was also sensitive to the environment, displaying marked species-dependent responses to spatial and temporal variation in ocean conditions. Moreover, the effects of local and seasonal processes were strong and with conserved species-specific differences, likely limited coherent cross-species sensitivity to climate variability from ENSO, NPGO and PDO. Unanticipated long-term steady increases in %C and bimodal increases in %N in brown algae and similar decreases in elemental composition of the red alga generated trimodal changes in C:N for these species. These changes may reflect responses of macrophyte communities to continuing changes in carbonate chemistry from ocean acidification.

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