Oecologia DOI 10.1007/s00442-013-2683-y
GLOBAL CHANGE ECOLOGY - ORIGINAL RESEARCH
Direct and indirect effects of ocean acidification and warming on a marine plant–herbivore interaction Alistair G. B. Poore • Alexia Graba-Landry • Margaux Favret • Hannah Sheppard Brennand Maria Byrne • Symon A. Dworjanyn
•
Received: 5 November 2012 / Accepted: 1 May 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract The impacts of climatic change on organisms depend on the interaction of multiple stressors and how these may affect the interactions among species. Consumer–prey relationships may be altered by changes to the abundance of either species, or by changes to the per capita interaction strength among species. To examine the effects of multiple stressors on a species interaction, we test the direct, interactive effects of ocean warming and lowered pH on an abundant marine herbivore (the amphipod Peramphithoe parmerong), and whether this herbivore is affected indirectly by these stressors altering the palatability of its algal food (Sargassum linearifolium). Both increased temperature and lowered pH independently reduced amphipod survival and growth, with the impacts of temperature outweighing those associated with reduced Communicated by Steve Swearer.
Electronic supplementary material The online version of this article (doi:10.1007/s00442-013-2683-y) contains supplementary material, which is available to authorized users. A. G. B. Poore (&) Evolution and Ecology Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia e-mail:
[email protected] A. Graba-Landry H. Sheppard Brennand S. A. Dworjanyn National Marine Science Centre, Southern Cross University, PO Box 4321, Coffs Harbour, NSW 2450, Australia M. Favret Agro Campus Ouest, Poˆle Halieutique, 65 rue de St-Brieuc, CS 84 215, 35042 Rennes cedex, France M. Byrne Schools of Medical and Biological Sciences, University of Sydney, Sydney, NSW 2006, Australia
pH. Amphipods were further affected indirectly by changes to the palatability of their food source. The temperature and pH conditions in which algae were grown interacted to affect algal palatability, with acidified conditions only affecting feeding rates when algae were also grown at elevated temperatures. Feeding rates were largely unaffected by the conditions faced by the herbivore while feeding. These results indicate that, in addition to the direct effects on herbivore abundance, climatic stressors will affect the strength of plant–herbivore interactions by changes to the susceptibility of plant tissues to herbivory. Keywords Acidification Warming Herbivory Multiple stressors Macroalgae
Introduction Predicting the impacts of a changing climate on organisms depends not only on measuring the impact of single stressors on single species but also on the interactions among stressors and how these affect the interactions among species. Temperature is a fundamentally important determinant of biological processes (Angilletta 2009), but its effects on organisms can vary widely due to complex relationships with other abiotic variables (Darling and Coˆte´ 2008). Similarly, the effects of elevated atmospheric CO2 frequently interact with other abiotic variables (Robinson et al. 2012). The responses of organisms to climatic change are further complicated by possible indirect effects through impacts on other organisms with which they interact. Climate change may affect predator–prey (Englund et al. 2011), plant–herbivore (Lindroth 2012) and host–pathogen (Harvell et al. 2002) interactions by population-level changes in the abundance of either of the interacting
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species, or by affecting the per capita interaction strength (Kordas et al. 2011). By affecting species interactions, indirect effects can reverse the direct negative effects of climate change (Suttle et al. 2007) and greatly complicate predictions of impacts based on single abiotic variables (Connell et al. 2011). Marine organisms are faced with the combined stress of increasing ocean temperatures and ocean acidification (Doney et al. 2012). The direct effects of warming or acidification on a wide variety of marine organisms is increasingly well known (impacts reviewed by Byrne 2011; Kroeker et al. 2010), but far fewer studies have examined the interactive effects of warming and acidification, or the effects of both of these stressors on the interaction between species (Harley et al. 2012; Wernberg et al. 2012a). This is despite the expectation that environmental stressors are unlikely to act independently (Crain et al. 2008) and the known importance of indirect effects in structuring marine communities (Menge 1995). This emerging research has shown that predicted changes in temperature and pH of the oceans may alter the relationships among predators and prey (e.g. Ferrari et al. 2011), herbivores and primary producers (e.g. O’Connor 2009), competitors (e.g. Diaz-Pulido et al. 2011), and pathogens and parasites and their hosts (e.g. Campbell et al. 2011; Macnab and Barber 2012). For predator–prey and plant–herbivore interactions, metabolic theory predicts increased consumption rates per capita with increased temperature (O’Connor 2009), but predicting the outcome of these interactions is complicated by possible changes to anti-predator behaviour and predator selectivity (e.g. Ferrari et al. 2011), and prey or plant palatability (Lindroth 2012). Competitive interactions may be affected by changes to traits that affect the outcome of the interaction (e.g. the production of allelopathic secondary metabolites; DiazPulido et al. 2011). Host–pathogen interactions may be affected by increased virulence of the pathogen or lowered resistance of the host (e.g. Campbell et al. 2011). Given the need to examine how multiple stressors interact to determine the impact of climate change, and how they may combine to affect relationships between species, we test the interactive effects of ocean warming and acidification on a marine plant–herbivore interaction. Marine herbivores have a profound effect on the abundance of primary producers (seagrasses, macroalgae, microalgae), with grazing resulting in a 68 % reduction, on average, of benthic producer abundance (Poore et al. 2012). Global variation in temperature is a poor predictor of the impact of herbivores in the field (Poore et al. 2012), but per capita consumption rates (e.g. O’Connor 2009; Yee and Murray 2004) and feeding preferences (Sotka and Giddens 2009) can vary with temperature. Many ecologically important marine herbivores (urchins, gastropods) and macroalgae
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are calcifying species and are negatively affected by predicted changes in the pCO2 of seawater due to hypercapnic suppression of growth, reduced rates of calcification and/or calcium carbonate dissolution (Byrne 2011; Koch et al. 2013; Kroeker et al. 2010). The effects of ocean acidification on animals with a lower dependence on carbonate skeletons (e.g. crustaceans) and non-calcifying macroalgae and seagrasses are, however, not well understood (Connell and Russell 2010; Wernberg et al. 2012a; Whiteley 2011). Indirect effects of increased temperature or pCO2 on marine herbivores may arise if these stressors alter the composition of algal or seagrass tissues and thus their quality as food. Temperature is an important determinant of nutrient uptake in marine primary producers (Raven and Geider 1988) and can alter the nitrogen content in algal tissues. For example, the C:N ratio of the kelp Ecklonia radiata increases by *15 %, and thus nutritional value decreases, with each 1 °C rise in temperature (Staehr and Wernberg 2009). Similarly, the value of producer tissues for herbivores may be altered by temperature-induced changes in the concentration of defensive secondary metabolites (e.g. Sudatti et al. 2011). Arnold et al. (2012) recently demonstrated that lowered pH is associated with lower concentrations of defensive phenolics in the seagrass Cymodocea nodosa and the estuarine macrophytes Ruppia maritima and Potamegeton perfoliatus. These findings are consistent with observations of higher grazing on these species at acidified sites (pH 7.3). An increase in the quality of plants or algae for herbivores when grown at lower pH may also arise due to increased carbohydrate content (e.g. Thalassia hemprichii at pH \ 7.75; Jiang et al. 2010), increased nitrogen uptake (e.g. Gracilaria lemaneiformis at pH 7.85; Xu et al. 2010) or decreased calcification (e.g. Padina pavonica at pH 7.49–8.06; Johnson et al. 2012). In contrast, lower pH (7.17–7.99) increased the concentration of defensive phenolics (phlorotannins) in two species of brown macroalgae (Swanson and Fox 2007). With a limited number of studies, we lack any clear prediction of whether lower pH will result in primary producers being more or less palatable to marine herbivores. In this study, we test the direct, interactive effects of warming and lowered pH on the survival and growth of the abundant herbivorous amphipod Peramphithoe parmerong, and test whether this crustacean herbivore is affected indirectly by changes to the palatability of its food source, the brown macroalga Sargassum linearifolium, also grown under experimental conditions. Peramphithoe parmerong is abundant in the macroalgal beds of south-eastern Australia (Poore et al. 2008), a region which is predicted to be a hotspot for climate change, having already experienced an increase in sea surface temperatures (SSTs) of 1.4 °C over the last 60 years—a rate approximately four times that
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of the global average (Hobday and Lough 2011; Ridgway 2007; Wernberg et al. 2011). Predicting community change will require an understanding of how marine invertebrates, and their interactions with other species, respond to these changes in mean temperatures, and to the predicted increases in the frequency and magnitude of extreme weather events (Trenberth 2012) (as evidenced by a recent marine heat wave in Australia with anomalies up to 5 °C that altered ecosystem structure; Wernberg et al. 2012b). We asked the following specific questions: (1) are the survival and growth of P. parmerong affected by increases in temperature (?3 and ?6 °C over ambient) and lowered pH (7.6 and 7.8)? (2) Do the feeding rates of P. parmerong on S. linearifolium vary with (a) the temperature and pH conditions in which the algae were held prior to a feeding experiment, and (b) the temperature and pH conditions that amphipods experience during the feeding experiment? (3) Can differences in feeding rates be explained by variation in tissue traits (carbon, nitrogen and phlorotannin content and frond density) of S. linearifolium?
Materials and methods Study species and site Peramphithoe parmerong Poore and Lowry 1997 (Crustacea: Amphipoda: Ampithoidae) is an abundant, herbivorous amphipod in shallow coastal waters of eastern Australia (southern Queensland to Tasmania). They inhabit and consume brown macroalgae, especially species in the genus Sargassum, where they live in open-ended nests built with a silken secretion (Poore et al. 2008). They are readily maintained in culture, reproductively active all year, and mature in approximately 30 days (Poore and Steinberg 1999). We collected P. parmerong from beds of Sargassum linearifolium (Turner) C. Agardh at 1–3 m depth at Charlesworth Bay in Coffs Harbour, NSW, Australia (30°180 S, 153°80 E). Once collected, the amphipods were acclimatised in flowing seawater in the aquarium system at the National Marine Science Centre at ambient temperatures for at least 24 h. Seawater chemistry and experimental conditions The effects of altered temperature, pH and their interaction on P. parmerong and its algal diet were quantified by rearing each in combinations of temperature (ambient, ?3 and ?6 °C) and pH (ambient, 7.8 and 7.6) in flowing seawater. The average pH and salinity of un-manipulated in-flow water into the aquarium system was pH 8.12 and 34.6 ppt. The average SST for the Coffs Harbour area during the experimental period was 22.97 °C (Navy Metoc
2011: http://www.metoc.gov.au/products/data/aussst.php). The combination of 23 °C and pH 8.12 was thus used as ambient conditions with 29–30 °C and pH 7.6 representing the upper threshold for near-future (2,070–2,100) warming of SST and acidification in south-eastern Australia, an ocean warming hot spot (IPCC 2007; Caldeira and Wicket 2005; Doney et al. 2009; Hobday and Lough 2011). In addition to these predicted changes in mean temperature, our ?6 °C treatment tests responses to the predicted increases in the frequency and magnitude of extreme events for the world’s coasts (Lima and Wethey 2012), and also reflects similar increases due to a marine heat wave in temperate Western Australia (?5 °C at 10 depth; Wernberg et al. 2012b). The experiments were conducted in a purpose-built flow-through seawater system (*180 L h-1 from Charlesworth Bay) with UV sterilised and filtered (1 lm) water delivered independently into each individual rearing container using irrigation dripper valves. The experimental pH was regulated by injection of pure CO2 into the seawater as it passed through reservoirs in the system at *60 L h-1 using an automatic CO2 injection system, mixed using a vortex mixer (Red Sea) and continuously bubbled with air to aid mixing and to maintain dissolved oxygen[90 %. The pH in sections of system was regulated according to water chemistry conditions in the rearing containers with two pH controllers (Tunze), set at pH 7.6 and pH 7.8, with a third section allowed to track ambient pH. This water was fed into subsequent reservoirs (flow rate *20 L h-1) where it was warmed to the required temperatures, ?3 and ?6 °C, using aquarium heaters (200 W; Jager) or un-manipulated for the ambient control. Temperature was automatically regulated using temperature sensors in the rearing containers and a temperature controller (Tunze) connected to the heaters. Water from each sub-header tank was continually circulated using 20-W pumps to maintain even temperatures within each treatment. Temperature, pH and salinity were measured daily from within the rearing containers in all treatments using a Hach Hqd Portable Multiprobe, calibrated with high precision buffers (Oakton). Water samples (100 ml) were also collected daily during the experiment, filtered through a 0.45lm syringe filter, and fixed with 10 ll of saturated HgCl. These water samples were then used to determine total alkalinity by potentiometric titration using a Metrohm 888 Titrando using certified reference standards (Dickson and Millero 1987). pCO2, and XCalcite were calculated using CO2SYS (Pierrot et al. 2006, using the dissociation constants of Mehrbach et al. (1973) as refitted by Dickson and Millero 1987) from measures of salinity, temperature, pHNIST and total alkalinity (TA) (Online resource 1). pHT
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was calculated using a Tris buffer (Riebesell et al. 2010). The measured temperature and pH were contrasted among treatments by analysis of variance (ANOVA) with nominal pH or temperature treatment as fixed factors and day as a random factor. Effects of pH and temperature on growth and survival To test the effects of increased temperature, lowered pH and their interaction on the survival and growth of P. parmerong, amphipods were raised from hatching in the nine combinations of temperature (ambient, ?3 and ?6 °C) and pH (ambient, 7.8, 7.6). Female amphipods that were brooding eggs were collected from the field in November 2009 and placed individually into 50-mL containers supplied with seawater in the experimental flowthrough seawater system (flow rate ca. 0.47 L h-1). Females were provided with fresh algal tissue in excess for consumption and nest-building. When the females moulted and thus released their offspring, all families (i.e. all offspring from a single female) were allocated to one of the nine combinations of temperature and pH. In total, the offspring from 62 brooding females were used, with at least six replicate families in each combination of treatments. Maternal effects, in which the female’s environment alters offspring performance, are considered unlikely in this species, as previous experiments have found no effects of varying maternal temperature (ambient vs. ?3 °C; Graham et al., in preparation) or diet (Poore and Steinberg 1999) on offspring growth and survival. Each container of amphipods was examined daily, provided with fresh food if needed, and their containers were cleaned of faeces and detritus. After 7 and 14 days (approximately half the time to sexual maturity in this species), the number of surviving offspring per family was recorded, and two juveniles from each family removed from the container, photographed and quickly replaced. Amphipod size was measured from a digital image using the length of the curved line along the dorsal surface from where the head meets the first antenna to the tip of the telson. At 7 days, the mother of each family was removed and measured. There were no significant differences in the size of females allocated to each treatment (pH: F2,37 = 0.47, P = 0.63; temperature: F2,37 = 2.69, P = 0.08). Survival at days 7 and 14 was contrasted among treatments by analysis of variance (ANOVA) with the number surviving per family as the dependent variable, and pH and temperature as fixed, factorial independent variables. The size of surviving amphipods was contrasted among treatments using ANOVA with temperature and pH as fixed factors and amphipod family as a random factor nested within each combination of pH and temperature. The size
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analyses did not involve the highest temperature (?6 °C) treatments due to poor survival. Effects of pH and temperature on feeding rates To test the hypotheses that reduced survival of P. parmerong at higher temperatures and lower pH could arise indirectly through changes to their food affecting feeding rates, or directly through changes in amphipod behaviour, we cultured growing fronds of S. linearifolium in two pH (ambient and 7.6) and temperature (ambient and ?3 °C) treatments and then fed fronds from each combination of conditions to amphipods held in each of the same four combinations of treatments. Thus, the experimental design of this feeding assay was a 2 9 2 9 2 9 2 design, involving the 16 combinations of pH for the algal growth period, temperature for the algal growth period, pH for the feeding assay and temperature for the feeding assay. We used only ?3 °C as an elevated temperature due to high mortality of amphipods at ?6 °C (see ‘‘Results’’). Apical fronds of S. linearifolium with actively growing meristems were maintained in each of the four combinations of pH and temperature for 2 weeks in the experimental flow-through seawater system (as described above). The rearing containers were irradiated with 30 lmol m-2 s-1 of light (photosynthetically active radiation) from fluorescent ‘cool white’ tubes. The light level was determined by the average of 10 light (PAR) meter (Skye) readings taken at midday at approximately 2 m depth at Charlesworth Bay immediately prior to the experiment. After 2 weeks, the fronds were weighed (mean ± SE = 16.5 ± 0.07 mg) and randomly allocated to each of the four combinations of pH and temperature and an individual of P. parmerong added to each container. Over 14 days, the meristems increased in mass by 196 % on average, and, thus, almost half the tissue was formed under experimental conditions. There were ten replicate amphipods, each held individually in 50-mL containers that were supplied individually with flowing seawater, for every combination of treatments. A further 40 replicate containers (10 for each of the four combinations of pH and temperature) had no amphipod added to obtain estimates of the likely changes in algal mass that were not due to herbivory during the feeding assay (e.g. growth). Amphipods were allowed to feed for 24 h after which the frond was reweighed and the amphipod measured. Temperature, pH and salinity were measured daily in all treatments (as described above). Mass loss per frond (mg) was contrasted among treatments using ANOVA with algal pH, algal temperature, amphipod pH, and amphipod temperature as fixed, factorial factors. Mass loss was adjusted for possible changes in the absence of herbivory by subtracting the loss in one of the
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controls lacking herbivores from each replicate measure (randomly pairing control and test replicates, and thus incorporating both the mean and variance of these autogenic controls (Peterson and Renaud 1989). Given the variation in amphipod size, the mass loss was standardised by amphipod length (mm). The strength of the association between each factor and the dependent variable (mass loss) was measured using x2 (Hays 1994). Tissue traits of algae in experimental conditions To test whether change in tissue traits of S. linearifolium was associated with the observed variation in herbivore performance or feeding rates, we analysed a nutritional trait (the tissue content of nitrogen, often a limiting resource for herbivores), a measure of potential chemical defence (the concentration of phlorotannins, the most abundant secondary metabolites in Sargassum), and a measure of physical defence (a proxy for toughness) of fronds grown in the temperature and pH treatments. The methods and results from these experiments are fully detailed in Appendix 1 of the supplementary material (Online Resource 1). Statistical analyses Univariate analyses of variance were carried out using SYSTAT 13 and the PERMANOVA routine of Primer V6. The assumptions of normality and heterogeneity of variance were checked using frequency histograms of residuals and plots of residuals versus means, respectively. The significance level was taken as P \ 0.05.
Results Effects of pH and temperature on growth and survival Decreased pH and increased temperature both resulted in reduced survival of juvenile P. parmerong (Table 1; Fig. 1). After both 7 and 14 days, the number of P. parmerong per brood that survived was highest at ambient pH (differing significantly from the more acidic conditions of pH 7.8, but not pH 7.6, in Tukey’s post hoc tests). The number of individuals surviving per brood was highest at ambient temperature, and reduced, on average, by 55 % at day 7 and 76 % at day 14 at ?3 °C (Fig. 1a). In the highest temperature (?6 °C), there were almost no survivors after 7 days, and none survived to day 14 (Fig. 1a; ambient [ ?3 °C [ ?6 °C in Tukey’s post hoc tests). There were no significant interactions between pH and temperature (Table 1). The size of juveniles was reduced, on average, by 17 %, at day 7 and 19 % at day 14, at ?3 °C (Fig. 1b). There was no significant effect of decreased pH on amphipod size, nor any interaction between pH and temperature (Table 1). The mean (±SE) temperatures in the ambient, ?3 and ?6 °C treatments during the experiment were 24.3 ± 0.06, 27.3 ± 0.06 and 30.0 ± 0.07 °C, respectively (n = 189). Each of these three temperatures differed significantly (F2,40 = 567.7, P \ 0.001, ambient °C \ ?3 °C \ ?6 °C in post hoc pairwise tests). The means (±SE) pHNIST of the treatments during the experiment were 8.2 ± 0.002, 7.8 ± 0.003 and 7.6 ± 0.003 (n = 189) with each of the three pH treatments differing significantly (F2,40 = 1,624.7, P \ 0.001, ambient [ 7.8 [ 7.6 in post hoc pairwise tests). Experimental pHT, pCO2, salinity, and
Table 1 Analysis of variance contrasting survival and growth of P. parmerong among temperatures (ambient, ?3 and ?6 °C) and levels of pH (ambient, 7.8 and 7.6) Source
Day 7 df
Day 14 MS
F
P
df
MS
F
P
Survival pH
2
78.5
6.24
0.004
2
39.5
4.85
0.01
Temperature
2
862.8
68.53
