- Understanding and predicting how regional to global scale processes affect macroalgal populations and communities requires elucidating the mechanisms underlying observed patterns. This dissertation identifies some of the underlying mechanisms that produce complex multi-scale responses of macroalgae across space and time by delineating the role of key local environmental drivers and species identity in modifying the effects of large scale processes (i.e., El Niño) on macroalgae.
On an organismal scale, survival of early life history stages is critical to the successful establishment of benthic populations. Although light availability and herbivory are likely to influence passage of marine macroalgae through a “recruitment bottleneck” at the sporeling stage, the interactive effect of these factors on subsequent community patterns of macroalgae is not well studied. In Chapter 2, I investigated local responses of an intertidal kelp Hedophyllum sessile to upwelling-mediated light availability and herbivory. Studies were conducted at two sites along the Oregon coast: Strawberry Hill (an intermittent upwelling region) and Cape Blanco North (a persistent upwelling region). My study found that persistent coastal upwelling in southern Oregon strengthened the controls of herbivory and light availability on the kelp. However, the direction of these controls depended on the demographic trait and life history stage of the kelp. Herbivores negatively affected the survival of juvenile kelps and positively affected the growth of juvenile and adult kelps while shading kelps from light negatively and positively affected the growth of juvenile and adult kelps, respectively.
In Chapter 3, I expanded the single-species experiment on the Oregon coast to a multi-species experiment on the South Island of New Zealand to assess the ecological responses of entire intertidal macroalgal communities to herbivory and light availability. In this experiment, I also added a third element, pre-emptive competition. I found that, by distinguishing between algal functional groups and including different starting conditions in the experimental design, the community structure in upwelling and downwelling regions on the South Island of New Zealand was strongly context-dependent. The upwelling-dominated region was driven by a complex array of species interactions, including grazing, predation, strong pre-emptive competition and interference competition, and high colonization rates, and these effects were modulated by light availability and the oceanic environment. In contrast, the downwelling region showed stronger effects of grazing and relatively weak effects of other interactions, low colonization rates of invertebrates, and light effects that were limited to crustose algae.
With a better understanding of these lesser-known interactive effects among light availability, herbivory, coastal upwelling, and macroalgal species identity and life history stages, the arrival of the historically third-most severe El Niño event in 2015-2016 presented me with an opportunity to scale up my experiments beyond local processes to incorporate mesoscale processes such as El Niño. In Chapter 4, I analyzed the spatiotemporal variations in the effects of the 2015-16 El Niño on three intertidal kelps (Hedophyllum sessile, Egregia menziesii, and Postelsia palmaeformis) at 7 sites across 300 km of the Oregon coast and over three years post El Niño. I found that the responses of intertidal kelps to the 2015-16 El Niño varied among species within the order Laminariales through space (i.e., among sites [local scales] and capes [mesoscale] along the coastline), and time (i.e., days, weeks, months and years). More specifically, kelp population dynamics changed from year to year, and were strongly governed by local and regional environmental processes and species identity and demographic traits.
El Niño is predicted to increase in frequency under greenhouse warming. However, long term studies comparing multiple El Niño events and their ecological consequences on rocky intertidal macroalgal communities across large spatial scales are rare. The few studies assessing El Niño impacts on intertidal kelps were spatially limited and short-term (i.e., spanning over a single El Niño event). Using short-term studies to assess the ecological consequences may be problematic considering ENSO occurs over a large geographic area and may have multi-scale effects on intertidal kelp populations.
In Chapter 5, I addressed the above issues by comparing the responses of three common intertidal kelp species (Hedophyllum sessile, Egregia menziesii, and Postelsia palmaeformis) to the 1997-98 El Niño and 2015-16 El Niño events at several locations along the Oregon coast. As expected, I found that the magnitude of intertidal kelp responses along the Oregon coast was proportional to the intensity of El Niño events. Furthermore, each kelp species generally followed a latitudinal trend with the negative effects of El Niño being the strongest in lower latitudes (i.e. central and southern Oregon). However, the strength and direction of the latitudinal trend were modified by kelp species identity and spatial variation (i.e., upwelling currents, coastal geomorphology, and bottom topography).
Given the uncertainty associated with El Niño and climate change, especially in the California Current Upwelling System, and its biological implications, potential changes to nearshore oceanographic processes could create cascading effects that dramatically alter the community dynamics and structure of coastal environments. The findings from my studies reiterate the importance of acquiring better insight into how context-specific underlying conditions modify ecosystem processes. More specifically, understanding how each demographic trait and life history stage of macroalgae change with biological interactions and environmental forcing over temporal and spatial scales is crucial to anticipating future climate change ramifications.