Scale-dependent effects of marine subsidies on the island biogeographic patterns of plants.

Although species richness can be determined by different mechanisms at different spatial scales, the role of scale in the effects of marine inputs on island biogeography has not been studied explicitly. Here, we evaluated the potential influence of island characteristics and marine inputs (seaweed wrack biomass and marine-derived nitrogen in the soil) on plant species richness at both a local (plot) and regional (island) scale on 92 islands in British Columbia, Canada. We found that the effects of subsidies on species richness depend strongly on spatial scale. Despite detecting no effects of marine subsidies at the island scale, we found that as plot level subsidies increased, species richness decreased; plots with more marine-derived nitrogen in the soil hosted fewer plant species. We found no effect of seaweed wrack at either scale. To identify potential mechanisms underlying the decrease in diversity, we fit a spatially explicit joint species distribution model to evaluate species level responses to marine subsidies and effects of biotic interactions among species. We found mixed evidence for competition for both light and nutrients, and cannot rule out an alternative mechanism; the observed decrease in species richness may be due to disturbances associated with animal-mediated nutrient deposits, particularly those from North American river otters (Lontra canadensis). By evaluating the scale-dependent effects of marine subsidies on island biogeographic patterns of plants and revealing likely mechanisms that act on community composition, we provide novel insights on the scale dependence of a fundamental ecological theory, and on the rarely examined links between marine and terrestrial ecosystems often bridged by animal vectors.

Whole-organism responses to constant temperatures do not predict responses to variable temperatures in the ecosystem engineer Mytilus trossulus.

Understanding and predicting responses of ectothermic animals to temperature are essential for decision-making and management. The thermal performance curve (TPC), which quantifies the thermal sensitivity of traits such as metabolism, growth and feeding rates in laboratory conditions, is often used to predict responses of wild populations. However, central assumptions of this approach are that TPCs are relatively static between populations and that curves measured under stable temperature conditions can predict performance under variable conditions. We test these assumptions using two latitudinally matched populations of the ecosystem engineer Mytilus trossulus that differ in their experienced temperature variability regime. We acclimated each population in a range of constant or fluctuating temperatures for six weeks and measured a series of both short term (feeding rate, byssal thread production) and long-term (growth, survival) metrics to test the hypothesis that performance in fluctuating temperatures can be predicted from constant temperatures. We find that this was not true for any metric, and that there were important interactions with the population of origin. Our results emphasize that responses to fluctuating conditions are still poorly understood and suggest caution must be taken in the use of TPCs generated under constant temperature conditions for the prediction of wild population responses.

Energetic context determines species and community responses to ocean acidification.

Physiological responses to ocean acidification are thought to be related to energetic trade-offs. Although a number of studies have proposed that negative responses to low pH could be minimized in situations where food resources are more readily available, evidence for such effects on individuals remain mixed, and the consequences of such effects at the community level remain untested. We explored the potential for food availability and diet quality to modify the effects of acidification on developing marine fouling communities in field-deployed mesocosms by supplementing natural food supply with one of two species of phytoplankton, differing in concentration of fatty acids. After 12 weeks, no species demonstrated the interactive effects generally predicted in the literature, where a positive overall effect of diet mitigated the negative overall effects of acidification. Rather, for some species, additional food supply appeared to bring out or exacerbate the negative effects of low pH. Community richness and structure were only altered by acidification, while space occupation and evenness reflected patterns of the most dominant species. Importantly, we find that acidification stress can increase the relative abundance of invasive species, even under resource conditions that otherwise prevented invasive species establishment. Overall, the proposed hypothesis regarding the ability for food addition to mitigate the negative effects of acidification is thus far not widely supported at species or community levels. It is clear that acidification is a strong driving force in these communities but understanding underlying energetic and competitive context is essential to developing mechanistic predictions for climate change responses.

Author Correction: Increased food supply mitigates ocean acidification effects on calcification but exacerbates effects on growth.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Increased food supply mitigates ocean acidification effects on calcification but exacerbates effects on growth.

Because many of the negative effects of ocean acidification on marine life may result from underlying energetic short-falls associated with increased metabolic demands, several studies have hypothesized that negative responses to high CO2 could be reduced by energy input. Although this hypothesis was supported by a recent meta-analysis, we believe that the meta-analytic calculation used was not appropriate to test the stated hypothesis. Here, we first clarify the hypothesis put forward, the crux being that the effects of increased food supply and CO2 interact statistically. We then test this hypothesis by examining the available data in a more appropriate analytical framework. Using factorial meta-analysis, we confirm that food addition has a positive effect and CO2 has a negative effect on both growth and calcification. For calcification, food addition did indeed reduce CO2 impacts. Surprisingly, however, we found that food addition actually exacerbated the effects of acidification on growth, perhaps due to increased scope upon which CO2 effects can act in food-replete situations. These interactive effects were undetectable using a multilevel meta-analytic approach. Ongoing changes in food supply and carbonate chemistry, coupled with under-described, poorly understood, and potentially surprising interactive outcomes for these two variables, suggest that the role of food should remain a priority in ocean acidification research.Arising from: L. Ramajo et al., Sci. Rep. 6: 19374 (2016).

Natural acidification changes the timing and rate of succession, alters community structure, and increases homogeneity in marine biofouling communities.

Ocean acidification may have far-reaching consequences for marine community and ecosystem dynamics, but its full impacts remain poorly understood due to the difficulty of manipulating pCO2 at the ecosystem level to mimic realistic fluctuations that occur on a number of different timescales. It is especially unclear how quickly communities at various stages of development respond to intermediate-scale pCO2 change and, if high pCO2 is relieved mid-succession, whether past acidification effects persist, are reversed by alleviation of pCO2 stress, or are worsened by departures from prior high pCO2 conditions to which organisms had acclimatized. Here, we used reciprocal transplant experiments along a shallow water volcanic pCO2 gradient to assess the importance of the timing and duration of high pCO2 exposure (i.e., discrete events at different stages of successional development vs. continuous exposure) on patterns of colonization and succession in a benthic fouling community. We show that succession at the acidified site was initially delayed (less community change by 8 weeks) but then caught up over the next 4 weeks. These changes in succession led to homogenization of communities maintained in or transplanted to acidified conditions, and altered community structure in ways that reflected both short- and longer-term acidification history. These community shifts are likely a result of interspecific variability in response to increased pCO2 and changes in species interactions. High pCO2 altered biofilm development, allowing serpulids to do best at the acidified site by the end of the experiment, although early (pretransplant) negative effects of pCO2 on recruitment of these worms were still detectable. The ascidians Diplosoma sp. and Botryllus sp. settled later and were more tolerant to acidification. Overall, transient and persistent acidification-driven changes in the biofouling community, via both past and more recent exposure, could have important implications for ecosystem function and food web dynamics.

Field-based experimental acidification alters fouling community structure and reduces diversity.

Increasing levels of CO2 in the atmosphere are affecting ocean chemistry, leading to increased acidification (i.e. decreased pH) and reductions in calcium carbonate saturation state. Many species are likely to respond to acidification, but the direction and magnitude of these responses will be based on interspecific and ontogenetic variation in physiology and the relative importance of calcification. Differential responses to ocean acidification (OA) among species will likely result in important changes in community structure and diversity. To characterize the potential impacts of OA on community composition and structure, we examined the response of a marine fouling community to experimental CO2 enrichment in field-deployed flow-through mesocosm systems. Acidification significantly altered the community structure by altering the relative abundance of species and reduced community variability, resulting in more homogenous biofouling communities from one experimental tile to the next both among and within the acidified mesocosms. Mussel (Mytilus trossulus) recruitment was reduced by over 30% in the elevated CO2 treatment compared to the ambient treatment by the end of the experiment. Strong differences in mussel cover (up to 40% lower in acidified conditions) developed over the second half of the 10-week experiment. Acidification did not appear to affect the mussel growth, as average mussel sizes were similar between treatments at the end of the experiment. Hydroid (Obelia dichotoma) cover was significantly reduced in the elevated CO2 treatment after 8 weeks. Conversely, the percentage cover of bryozoan colonies (Mebranipora membranacea) was higher under acidified conditions with differences becoming apparent after 6 weeks. Neither recruitment nor final size of barnacles (Balanus crenatus) was affected by acidification. By the end of the experiment, diversity was 41% lower in the acidified treatment relative to ambient conditions. Overall, our findings support the general expectation that OA will simplify marine communities by acting on important ecological processes that ultimately determine the community structure and diversity.

Ocean acidification through the lens of ecological theory.

Ocean acidification, chemical changes to the carbonate system of seawater, is emerging as a key environmental challenge accompanying global warming and other human-induced perturbations. Considerable research seeks to define the scope and character of potential outcomes from this phenomenon, but a crucial impediment persists. Ecological theory, despite its power and utility, has been only peripherally applied to the problem. Here we sketch in broad strokes several areas where fundamental principles of ecology have the capacity to generate insight into ocean acidification's consequences. We focus on conceptual models that, when considered in the context of acidification, yield explicit predictions regarding a spectrum of population- and community-level effects, from narrowing of species ranges and shifts in patterns of demographic connectivity, to modified consumer-resource relationships, to ascendance of weedy taxa and loss of species diversity. Although our coverage represents only a small fraction of the breadth of possible insights achievable from the application of theory, our hope is that this initial foray will spur expanded efforts to blend experiments with theoretical approaches. The result promises to be a deeper and more nuanced understanding of ocean acidification'and the ecological changes it portends.

Tricuspid annuloplasty significantly reduces early tricuspid regurgitation after biatrial heart transplantation.

BACKGROUND: The incidence of tricuspid annuloplasty (TR) observed early after cardiac biatrial implantation is unpredictable and in our experience not infrequently problematic. Although the bicaval method of implant may reduce the incidence of TR, its benefit has not been conclusively documented. METHODS: In an attempt to reduce the incidence of TR observed early after cardiac transplantation, 25 consecutive patients undergoing cardiac transplantation received donor heart tricuspid annuloplasty (TA) with either a DeVega or Ring technique. Early transthoracic echocardiograms were analyzed and compared with an immediately prior and consecutive cohort of 25 patients undergoing transplantation without TA. The biatrial technique of cardiac transplantation with a Cabrol modification was used for donor heart implant in both groups. Echocardiograms obtained 5 days after cardiac transplantation were reviewed in blinded fashion. TR was scored 0 = none, 1 = mild, 2 = moderate, and 3 = severe. RESULTS: Donor and recipient characteristics were not different between groups. No hospital deaths occurred in either group. Patients undergoing transplantation without TA had a higher TR score, 1.3 (range 0-3), than did patients with TA, 0.7 (range 0-1.5, p = 0.002). Moderate or severe TR was present in 8 of 25 patients without TA compared with 0 of 25 patients with TA (p = 0.004). No patients required permanent pacemaker. CONCLUSIONS: TA can significantly reduce the incidence of early postoperative TR after biatrial cardiac transplant without adding to the complexity of operation.