Global Change Impacts on Ecological Networks

Impacts of global change on networks of interacting ecological species, particularly documenting declining ocean pH (ocean acidification) and its ecological effects.

Predicting the effects of environmental impacts in complex natural ecosystems characterized by networks of ecological interactions presents one of the biggest challenges for ecology. We are investigating this problem in the context of global change brought about by increasing the concentration of atmospheric carbon dioxide, which arises from human activities such as burning fossil fuels and land clearing. Most people are only aware of the potential effects of atmospheric CO2 as a greenhouse gas which traps heat, leading to global warming. However, CO2 can have other equally important effects on ecosystems, particularly aquatic ecosystems where it dissolves in, and then readily reacts with, water to form carbonic acid and its ionic products (the same process that causes the zip in soda pop):

CO2 + H2O <β€”-> H2CO3 (carbonic acid) <β€”-> H+ + HCO3–

Variation in ocean pH through time at Tatoosh Island. Top:daily variation, Bottom: variation across years.

Marine scientists are now realizing that this reaction, which causes water pH (the negative log concentration of H+ ions) to decline (ocean acidification), might pose serious problems for ocean ecosystems because pH is a key control on the rates of numerous chemical reactions in plants and animals, and because many marine organisms, such as corals, sponges, mollusks, sea urchins, phytoplankton, and crustaceans, have skeletons of calcium carbonate, which dissolves when it reacts with acids. Hence, predictions have been made that ocean pH will decline with increasing atmospheric CO2 emissions, and that this decline will be sufficient to disrupt major physiological processes such as calcification. While the physics of this reaction are well known, there are surprisingly few published data of measurements of pH change in the ocean through time. Furthermore, although laboratory studies demonstrate that many calcifying organisms perform poorly in acidified water, extrapolating these results to predict the response in complex ecosystems is difficult.

Top: Correlation of annual transitions among species with average annual ocean pH at Tatoosh Island. Bottom: responses of calcified (white) and non-calcified (black) species show calcifiers perform more poorly than non-calcifiers as pH declines.

Since 2000, we have been monitoring physical ocean conditions, including ocean pH, at our main study site in the northeastern Pacific Ocean: Tatoosh Island, Washington, USA. We use a submersible data logger to record water conditions at 30 minute intervals, yielding a data set of very high temporal resolution (>65,000 data points total and growing) to explore changes in pH through time.

In contrast to the widely-held notion that the ocean is well buffered, our pH data exhibit a surprising degree of systematic variability through time. Even over the course of a day, pH typically varies by 0.24 units, a consequence of the uptake and production of CO2 through photosynthesis and respiration (Wootton et al. 2008). Hence biological processes, which are often left out of models of ocean pH, can have strong effects. Over the entire span of the data, ocean pH is clearly declining as atmospheric CO2 increases, but at a rate an order of magnitude faster than predicted by current physical models (Wootton et al. 2008, Pfister et al. 2011, Wootton and Pfister 2012). Hence, declining ocean pH may be a more acute issue, at least in some areas, than is currently appreciated. Over 70% of the variability in pH we observed can be related to changes in a small set of factors with known mechansitic links to pH: atmospheric CO2, water temperature, the daily photosynthesis-respiration cycle, phytoplankton abundance, upwelling of high CO2 subsurface water, alkalinity, salinity, and the Pacific Decadal Oscillation. We have begun collaborating with Andrew Dickson, an ocean chemist at Scripps Institute of Oceanography, to expand the parameters we measure and interpret our results in greater detail (Wootton and Pfister 2012).

Projected composition of the intertidal community under different pH conditions.

Beyond documenting how ocean pH changes through time and exploring drivers of this change, we are addressing the challenge of predicting and understanding how coastal ocean ecosystems will respond to this change. We have drawn upon our extensive work investigating how environmental impacts ramify through webs of interacting species. One approach has been to apply our experimentally tested transition based models, by exploring how patterns of transitions among species change as pH declines, and using these changes to predict shifts in community structure. This analysis reveals reduced performance of dominant calcifying intertidal species (mussels and goose barnacles) as pH declines, but increases in both sub-dominant calcifying species (acorn barnacles) and non-calcifying algae (Wootton et al. 2008). These changes likely arise as a result of the well-documented web of competitive and consumer-resource interactions in this system. Sub-dominant species are released from competition, and prey species may be also be released from consumers with highly calcified shells, such as predatory snails, and grazing limpets, chitons and sea urchins. The long-term consequences of these changes will likely be the loss of the dominant intertidal habitat on rocky shores of western North America, mussel beds, and replacement with a short-statured algal canopy.

 

We have just started to study this issue, and many questions remained to be explored. Why is pH declining so rapidly at this site? How do patterns of pH at larger spatial scales, such as over the extent of the Olympic Coast National Marine Sanctuary, correspond to the detailed measures we have made? Are there systematic patterns of change from nearshore to offshore areas? What role do biological processes play in affecting pH and the buffering capacity of the water (total alkalinity), particularly in areas with high biological activity? By what mechanisms does pH affect dominant calcifier species: resistance to disturbance, reduced physiological function, changes in consumer pressures…? How do mobile species respond to changes in pH? How are species interactions altered with changing pH? Can we find historical data that contain signals of calcification changes in important organisms such as mollusks and coralline algae (Pfister et al. 2011)? We plan to address these issues in future research, and many of these would make excellent projects for graduate students and post-doctoral fellows.