Where the red alga grows : summertime growth patterns of Neorhodomela oregona in response to climate change simulations and in relation to ecosystem functioning

As oceans rapidly warm and acidify, anthropogenically induced climate change threatens to disrupt the baseline patterns and functioning of intertidal ecosystems. The experiment presented here manipulated temperature and CO₂, independently and simultaneously, in natural tide pools on a rocky beach in Sitka, Alaska, USA. This was one of the first in situ, multi-factor, multi-level simulations and quantifications of climate change in the intertidal. Of particular focus in this analysis were the summertime growth patterns of the abundant red alga Neorhodomela oregona. Synthesizing data from the summers of 2018 and 2019, N. oregona was examined as a population responding to simulated climate change and in relation to ecosystem functioning. Average N. oregona cover declined significantly in unmanipulated tide pools between June and September of 2018, representing the first quantitative account of this species’ ambient seasonality. Mechanisms of this seasonal decline are proposed, including invertebrate herbivory and environment-triggered senescence. Over the course of summer 2019, neither climate-manipulated nor control tide pools experienced any significant changes in N. oregona cover. Interpretation of whether added heat and CO₂ additions affected N. oregona cover trends thus depends on whether 2019 manipulated pools are compared to 2018 or 2019 baselines. Between-year variations may be partially attributable to larger-scale climatic processes such as extreme heat waves and El Niño events. N. oregona cover was not correlated with ecosystem functioning (net community productivity or community respiration). The statistical methods chosen for this analysis may not have been ideal for small sample sizes in 2019, but many opportunities exist for future research that continues to investigate and link N. oregona, intertidal ecosystems, and climate change.