Changes in Species Interactions Among Coralline Algae Suggest Ecological Response to Ocean Acidification

Our changing climate has entrained a host of known and unknown changes to the ocean environment. Among these, coastal water chemistry is changing at a greater rate than ever before, and will drive coastal pH lower than has been experienced by any modern organism. Although much assessment of ocean acidification is focused on single-species responses, ocean acidification is likely to change species interactions. It is therefore important to assess the potential for biological response on both the species and community levels to such changes, particularly where coastal records document rapid pH decline. We have examined the ecological response of a guild of crustose coralline algae from the northeastern Pacific through field studies at Tatoosh Island, Washington over the last 30 years. Documented competitive networks among crustose coralline algae and the important role that their grazers play in mediating competitive interactions are based on different growth strategies of coralline algae. Because changes in ocean carbon chemistry will affect calcium carbonate skeletal production in both coralline algae and their grazers, future species membership and diversity in the coastal community will be a function of changing interaction strength. Thus, changes in observed species interaction strengths among coralline species and between corallines and their grazers are used as indicators of change in ecosystem function. Experiments replicating those previously done by R.T Paine at Tatoosh Island, Washington (1981-1999) indicate marked changes in frequency and intensity of interactions within this competitive network over the last 30 years. These results are discussed within the context of ocean acidification and seawater chemistry trends from Tatoosh Island, Washington.