Biogeography of macrophyte productivity: Effects of oceanic and climatic regimes across spatiotemporal scales

Abstract

AbstractSpatiotemporal variability in primary producer growth rates is a fundamental aspect of community structure. Understanding drivers of these patterns and their response to climate variability and change are ongoing challenges. Nutrient and light limitations often are invoked as proximate drivers of these patterns, but many other environmental and biological factors vary across spatial and temporal scales. In temperate rocky intertidal habitats, macrophytes are major space occupiers and the base of the food web, and thus their patterns of primary production relate directly to their functions and services in these communities. We investigated spatiotemporal patterns of the primary production of two species of macrophytes, the kelp Hedophyllum sessile and the surfgrass Phyllospadix scouleri, across 908 km of Oregon and California coastline. Spatiotemporal variability in macrophyte growth rates and their relationships to regional or local‐scale environmental variables (upwelling, nutrients, temperature, light, phytoplankton blooms) and climate regimes were explored. Paradoxically, we found that both warmer water temperature (e.g., warm phases of climate patterns, weaker upwelling) and increased nutrients (e.g., with stronger upwelling) increased macrophyte productivity. Kelp growth decreased with dense phytoplankton blooms, while surfgrass growth decreased with increasing air temperature. Growth rates reflected tissue elemental content in surfgrass but only weakly in kelp. Hence, as climate warms and/or if upwelling intensifies, productivity of these and perhaps other macrophytes should increase, at least until thermal conditions, particularly low tide air temperature, become too stressful.