Nutrient content of rocky intertidal macrophytes and its relation to environmental variability across a large biogeographic region

Background/Question/Methods

In the California Current Large Marine Ecosystem (CCLME), diverse rocky intertidal macrophyte assemblages play key roles in organizing community dynamics and ecosystem processes. At the same time, macrophyte growth and abundance can reflect local ecological interactions and oceanographic processes that modulate the availabilities of limiting resources. Across the CCLME where the supply of nutrients can be highly dynamic in space and time, the nitrogen content of macrophytes can provide an integrated metric of the nutrient environment encountered by the organism. Resolving the patterns and drivers of macrophyte nutrient content can thus help guide our understanding of the coupling of local ecological and regional oceanographic processes. However, no investigations have examined temporal and spatial variation in macrophyte nutrient content at the scales appropriate to assess the relationship to environmental forcing in upwelling-influenced habitats.   

To investigate these relationships, we analyzed the tissue carbon and nitrogen content of four common intertidal macrophytes (Fucus distichus, Mazzaella splendens, Saccharina sessilis, and Phyllospadix scouleri) biweekly from March-August in 2006 and 2008-2010. Samples were collected from 16 sites spanning 900 km in Oregon and northern California. We examined the relationships between carbon and nitrogen content and environmental factors including: nutrient availability, temperature, upwelling indices, and climate indices.

Results/Conclusions

Across the latitudinal gradient in this study, macrophyte C:N appears to cohere most strongly with environmental variables at the largest spatial scale (region; northern vs. southern sites), coinciding with the sharpest differences in regional oceanographic patterns. All species differed in the strength with which environmental drivers predict patterns of macrophyte carbon and nitrogen content across the spatial and temporal extent of this study. Additionally, the relative contribution of each environmental variable to explaining carbon and nitrogen content differed between species, based on the independent contribution of environmental factors obtained from hierarchical partitioning analysis, a regression method correcting for correlation between explanatory factors. We also examined the relationship between nutrient content and growth rates of S. sessilis and P. scouleri. Macrophyte C:N was positively related to growth, and growth itself differed between sites. The variation in species responses to environmental forcing seen here reflects the dynamic interactions between ecosystem and community processes and can help us to understand the multiple ways in which these are linked in upwelling-influenced rocky intertidal habitats.