Article: Burnaford, J. L., et al. (2014). “Celestial mechanics affects emersion time and cover patterns of an ecosystem engineer, the intertidal kelp Saccharina sessilis.” Marine Ecology Progress Series 509: 127-136.
The earth experiences several climatic cycles such as El Nino, the Pacific Decadal Oscillation, and the North Atlantic Oscillation. These events happen in a somewhat predictable pattern and have well documented climatic impacts. However, the earth is also influenced by celestial cycles. The moon undergoes a cycle in which it oscillates its declination, or angle in the celestial sphere (Fig 1). Imagine the moon sitting level on the same axis as the earth’s equator, the moon’s oscillation results in the movement north and south or the equator, changing the angle from the original axis and changing its proximity to certain latitudes on earth. For the moon to make one full oscillation (moving from its largest distance south of the equator to its largest distance north of the equator) it takes 18.6 years. Over this time span the size and range of tides are impacted. Intertidal ecosystems are already considered “extreme” systems because organisms that inhabit them have to deal with spending roughly half their lives either submerged or exposed, and now adding tidal fluctuations via celestial cycles means sessile organisms, like seaweeds and barnacles, have to withstand changing patterns on a longer time scale.
At the base of intertidal ecosystems are seaweeds. Many species of seaweeds are considered ecosystem engineers based on their ability to create complex habitats used for sheltering other species, providing refuge, and creating damp, shady spaces. The abundance and cover of intertidal seaweeds is partly dictated by how often they are submerged and exposed, which would change with celestial cycling. It has been shown that this process can double the annual emersion time (or time spent out of water) in a given location. Increased emersion times also lead to increases in exposure to high temperatures and radiation. Entire intertidal communities are likely to be impacted if seaweed abundances drastically change. Researchers on the Pacific coast of the US decided to put together a long-term study looking at how celestial cycles and the resulting tidal changes impacted the cover patterns and abundance of Saccharina sessilis (Fig 2), an intertidal kelp.
Starting in 1998, researchers set out permanent plots in the intertidal zones of San Juan Island, WA. These plots were established to track the changes in cover by S. sessilis as well as track the abundance of a common intertidal herbivore, Katharina tunicata (Fig 3). Herbivore abundance was tracked as a way to quantify how changes in algal cover would impact other organisms in the community. Percent canopy cover of S. sessilis and abundance of K. tunicata were recorded every summer from 1998-2000 and again from 2007 to 2012. Emersion time was calculated based on tidal heights and ranges for each year of the study.
In effort to understand changes in algal cover, researchers also implemented a laboratory study investigating the impacts of emersion time on the biomass and photosynthetic activity in their kelp species. Individual specimens of S. sessilis were collected in the field were brought back to the lab and then exposed to one of three environmental treatments for 3 days: fully submerged, low tide + high stress, and low tide + low stress. High stress involved increased light and wind, while low stress received low light and no wind.
Overall, researchers found that during summers of longer exposure (more extreme tides) the algal cover was low (around 20%) (Fig 4 + 5). During summers of shorter exposure, algal cover was much higher (around 80%) (Fig 4). It was shown that invertebrate abundance was directly correlated to algal cover. In summers of low algal cover, invertebrate abundance was also low and in summers of high algal cover, invertebrate abundance was also high (Fig 6). The more exposed the intertidal zone, the less kelp cover there was. Inter-annual climate variability was investigated in order to determine whether kelp cover was influenced by sources outside of celestial cycles and the resulting changes in tides.
From the laboratory portion of this experiment, researchers found that the kelps exposed to low tide and high stress lost biomass quickly and were less photosynthetically active (Fig 7). This helped provide context as to why algal cover was decreasing in years of higher exposure.
This research shows that celestial cycles play a role in ecosystem structure and function. By changing tidal regimes, celestial cycles were able to significantly change the abundance of important intertidal species. Changes in algal cover were directly linked to changes in invertebrate abundance, thus showing the rippling impact within the community. Kelps are invaluable to intertidal ecosystems, and changes to their health and abundance could be detrimental to the entire ecosystem. This is especially important as our global ecosystems are being threatened by a changing climate. As the climate warms, intertidal species like K. tunicata are going to be in need of damp, shady refuge provided by seaweed. If seaweed cover is lower, there is less refuge for species looking to escape the heat. These organisms are already exposed to extreme conditions, and it seems that taking into account climate change, climate cycles, and now celestial cycles may make things more difficult. This study shows the importance of long-term studies and the importance of taking into account large cycles and the changes they bring about.