Ocean Acidification Favors the Domin-ance of Turf Algae over Kelp Forests

Research into habitat effects of ocean acidification<!–[if supportFields]> XE “ocean acidification” <![endif]–><!–[if supportFields]><![endif]–> are primarily concerned with calcifying coral<!–[if supportFields]> XE “coral” <![endif]–><!–[if supportFields]><![endif]–> reefs. While reefs are critical ecosystem builders in tropical areas, these are not the only habitats that could potentially be harmed by acidification<!–[if supportFields]> XE “acidification” <![endif]–><!–[if supportFields]><![endif]–>. A recent study has examined the effects of increased temperature and ocean acidification associated with global warming on temperate habitats dominated by kelp forests (Connell and Russell 2010). The early stage of temperate algal communities is dominated by algae that form a turf over large rocky surfaces. Following a period of relative stability, kelp (also a type of alga) overtakes the turf algae and creates a forest<!–[if supportFields]> XE “forest” <![endif]–><!–[if supportFields]><![endif]–> habitat. Connell and Russell hypothesized that removal of turf algae would result in a greater recruitment of kelp and that turf algae would increase in abundance in high temperature and pH conditions. They found that these hypotheses were true, further solidifying the perception that ocean acidification will have far-reaching effects even in communities not dominated by calcifying organisms. —Emily Putnam
Connell, S.D., Russell, B.D., 2010. The direct effects of increasing CO2 and temperature on non-calcifying organisms: increasing the potential for phase shifts in kelp forests. Proceedings of the Royal Society B: Biological Sciences 277, 1409–1415.

Connell and Russell from the Southern Seas Ecology Laboratories at the University of Adelaide first tested the ability of turf algae to inhibit kelp recruitment. The authors removed turf algae from square meter plots that were found less than 5 meters from existing kelp canopies. These sites were chosen for their location along a metropolitan coastline. After one year, the number of kelp recruits was recorded and compared to control plots where no turf algae were removed. Mesocosm experiments were set up in large 40-liter tanks where turf algae specimens were exposed to one of four combinations of current summer maximal ocean temperature and pH and predicted temperature and pH for the year 2050. After 14 weeks, visual estimations of algal growth on previously uninhabited substrate were made. The algae from this substrate was then scraped from the substrate, dried for two days and weighed. Chlorophyll fluorescence was measured to determine the photosynthetic, or quantum, yield of the algae. Quantum yield is calculated comparing the relative fluorescence following exposure of the algae to a weak beam of red light and then to a strong beam of light.
Kelp recruitment was greater for all sites where turf algae had been removed. Increased temperature led to an increase in turf algae cover, but increased CO2 had no effect. Increased temperature and CO2 combined led to an even greater increase in turf coverage than increased temperature alone. Quantum yield increased slightly with CO2 increases but decreased with temperature increases. The synergistic effects of increased temperature and acidity on turf coverage suggests that the dual threat of acidification<!–[if supportFields]>XE “acidification” <![endif]–><!–[if supportFields]><![endif]–> and global warming would further favor the dominance of turf algae at sites that were once home to large kelp forests. While action can be taken to reduce nutrient levels as a means of reducing turf algae on a local scale, any mitigation will eventually be overcome by these global stresses. The gradual dominance of turf algae will also likely result in a further weakening of existing kelp forests due to reduced resilience. While the results from this study apply directly to coastlines with local human populations, even remote habitats will probably be affected by changes in seawater chemistry and temperature. As habitats shift and change, local herbivorous species distributions will adjust accordingly. Thus, temperate marine communities face as much risk as tropical communities in the face of global warming and ocean acidification<!–[if supportFields]> XE “ocean acidification” <![endif]–><!–[if supportFields]><![endif]–>.

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