FORECASTED CO2 MODIFIES THE INFLUENCE OF LIGHT IN SHAPING SUBTIDAL HABITAT1

Some abiotic conditions are well known to play disproportionately large roles in shaping contemporary assemblages, yet their roles may not continue to have similar magnitudes of effect into the future. We tested whether forecasted levels of CO₂ could alter the strength of influence of an abiotic factor (i.e., light intensity) well known for its strength of influence on the subtidal ecology of photosynthetic organisms. We investigated these dynamics in two subtidal algal species that form contrasting associations with kelp forests, one negatively associated with kelp canopies (turf‐forming brown algae, Feldmannia spp.) and the other positively associated with kelp as understory (calcifying red crustose algae, Lithophyllum sp.). Using an experimental approach, we assessed the independent and combined effects of [CO₂] (control and elevated) and light (shade, low ultraviolet B [UVB], full light) on growth, recruitment, and relative electron transport rate (rETR). Under control [CO₂], the effects of light corresponded to the relative light environments of the two groups of algae. The influence of light on the percentage cover and biomass of understory crusts was substantially reduced under elevated [CO₂], which caused crusts to grow less. While elevated [CO₂] had the opposite effect of positively influencing turf cover and biomass, it had the same effect of reducing the structuring effects of light and UVB. Hence, if we are to predict the ecological consequences of future CO₂ conditions, the role of contemporary processes cannot be assumed to produce similar effects relative to other processes, which will change with a changing climate.     

Synergistic effects of climate change and local stressors: CO2 and nutrient-driven change in subtidal rocky habitats

Climate-driven change represents the cumulative effect of global through local-scale conditions, and understanding their manifestation at local scales can empower local management. Change in the dominance of habitats is often the product of local nutrient pollution that occurs at relatively local scales (i.e. catchment scale), a critical scale of management at which global impacts will manifest. We tested whether forecasted global-scale change [elevated carbon dioxide (CO₂) and subsequent ocean acidification] and local stressors (elevated nutrients) can combine to accelerate the expansion of filamentous turfs at the expense of calcifying algae (kelp understorey). Our results not only support this model of future change, but also highlight the synergistic effects of future CO₂ and nutrient concentrations on the abundance of turfs. These results suggest that global and local stressors need to be assessed in meaningful combinations so that the anticipated effects of climate change do not create the false impression that, however complex, climate change will produce smaller effects than reality. These findings empower local managers because they show that policies of reducing local stressors (e.g. nutrient pollution) can reduce the effects of global stressors not under their governance (e.g. ocean acidification). The connection between research and government policy provides an example whereby knowledge (and decision making) across local through global scales provides solutions to some of the most vexing challenges for attaining social goals of sustainability, biological conservation and economic development.     

Combined effects of ocean acidification and solar UV radiation on photosynthesis,growth, pigmentation and calcification of the coralline alga Corallina sessilis (Rhodophyta)

Previous studies have shown that increasing atmospheric CO₂ concentrations affect calcification in some planktonic and macroalgal calcifiers due to the changed carbonate chemistry of seawater. However, little is known regarding how calcifying algae respond to solar UV radiation (UVR, UVA+UVB, 280–400 nm). UVR may act synergistically, antagonistically or independently with ocean acidification (high CO₂/low pH of seawater) to affect their calcification processes. We cultured the articulated coralline alga Corallina sessilis Yendo at 380 ppmv (low) and 1000 ppmv (high) CO₂ levels while exposing the alga to solar radiation treatments with or without UVR. The presence of UVR inhibited the growth, photosynthetic O₂ evolution and calcification rates by13%, 6% and 3% in the low and by 47%, 20% and 8% in the high CO₂ concentrations, respectively, reflecting a synergistic effect of CO₂ enrichment with UVR. UVR induced significant decline of pH in the CO₂‐enriched cultures. The contents of key photosynthetic pigments, chlorophyll a and phycobiliproteins decreased, while UV‐absorptivity increased under the high pCO₂/low pH condition. Nevertheless, UV‐induced inhibition of photosynthesis increased when the ratio of particulate inorganic carbon/particulate organic carbon decreased under the influence of CO₂‐acidified seawater, suggesting that the calcified layer played a UV‐protective role. Both UVA and UVB negatively impacted photosynthesis and calcification, but the inhibition caused by UVB was about 2.5–2.6 times that caused by UVA. The results imply that coralline algae suffer from more damage caused by UVB as they calcify less and less with progressing ocean acidification.     

QUANTIFYING GROWTH AND CALCIUM CARBONATE DEPOSITION OF CALLIARTHRON CHEILOSPORIOIDES (CORALLINALES,RHODOPHYTA) IN THE FIELD USING A PERSISTENT VITAL STAIN1

Growth and calcium carbonate deposition rates of the coralline alga Calliarthron cheilosporioides Manza were quantified by monitoring fronds in the intertidal zone that had been chemically labeled with the nontoxic fluorescent brightener Calcofluor white. This vital stain effectively labeled apical meristems of coralline thalli in the field: fronds exposed for only 5 min had detectable chemical marks at least 1.5 years later. By distinguishing portions of thalli that developed before and after exposure, this methodology permitted accurate measurement of growth and calcium carbonate deposition at each meristem. In Calliarthron, meristematic activity declined with increasing frond size. However, because growing fronds dichotomize, the total number of meristems and the deposition rate of new calcified tissue both increased with frond size. Growth rates reported here suggest that large fronds may not be as old as previously estimated. The Calcofluor white method may improve demographic studies of corallines by resolving growth and age of fronds in the field and may facilitate studies of climate change on calcium carbonate deposition in these ecologically important, calcifying algae.     

Carbon burial over the last four millennia is regulated by both climatic and land use change

Carbon sequestration by sediments and vegetated marine systems contributes to atmospheric carbon drawdown, but little empirical evidence is available to help separate the effects of climate change and other anthropogenic activities on carbon burial over centennial timescales. We used marine sediment organic carbon to determine the role of historic climate variability and human habitation in carbon burial over the past 5,071 years. There was centennial‐scale sensitivity of carbon supply and burial to climatic variability, with Little Ice Age cooling causing an abrupt ecosystem shift and an increase in marine carbon contributions compared to terrestrial carbon. Although land use changes during the late 1800s did not cause marked alteration in average carbon burial, they did lead to marked increases in the spatial variability of carbon burial. Thus, while carbon burial by vegetated systems is expected to increase with projected climate warming over the coming century, ecosystem restructuring caused by abrupt climate change may produce unexpected change in carbon burial whose variability is also modulated by land use change.     

Metabolically induced pH fluctuations by some coastal calcifiers exceed projected 22nd century ocean acidification: a mechanism for differential susceptibility?

Anthropogenically mediated decreases in pH, termed ocean acidification (OA), may be a major threat to marine organisms and communities. Research has focussed mainly on tropical coral reefs, but temperate reefs play a no less important ecological role in colder waters, where OA effects may first be manifest. Herein, we report that trends in pH at the surface of three ecologically important cold‐water calcifiers (a primary producer and herbivores), under a range of fluid flows, differ substantially from one another, and for two of the three calcifiers, the pH, during darkness, is lower than the mean projected pH due to OA for the surface waters of the global ocean beyond the year 2100. Using micro‐electrodes, we show that each calcifier had a different pH gradient between its surface and mainstream seawater, i.e. within the diffusion boundary layer (DBL) that appears to act as an environmental buffer to mainstream pH. Abalone encountered only mainstream seawater pH, whereas pH at the sea urchins’ surface was reduced by ~0.35 units. For coralline algae, pH was ~0.5 units higher in the light and ~0.35 units lower under darkness than in ambient mainstream seawater. This wide range of pH within the DBL of some calcifiers will probably affect their performance under projected future reductions in pH due to OA. Differing exposure to a range of surface pH may result in differential susceptibility of calcifiers to OA. Such fluctuations are no doubt regulated by the interplay of water movement, morphology and metabolic rates (e.g. respiration, calcification and/or photosynthesis). Our study, by considering physics (flow regime), chemistry (pH gradients vs. OA future projections) and biology (trophic level, physiology and morphology), reveals that predicting species‐specific responses and subsequent ecosystem restructuring to OA is complex and requires a holistic, eco‐mechanical, approach.     

INTERACTIONS BETWEEN OCEAN ACIDIFICATION AND WARMING ON THE MORTALITY AND DISSOLUTION OF CORALLINE ALGAE1

Coralline algae are among the most sensitive calcifying organisms to ocean acidification as a result of increased atmospheric carbon dioxide (pCO₂). Little is known, however, about the combined impacts of increased pCO₂, ocean acidification, and sea surface temperature on tissue mortality and skeletal dissolution of coralline algae. To address this issue, we conducted factorial manipulative experiments of elevated CO₂ and temperature and examined the consequences on tissue survival and skeletal dissolution of the crustose coralline alga (CCA) Porolithon (=Hydrolithon) onkodes (Heydr.) Foslie (Corallinaceae, Rhodophyta) on the southern Great Barrier Reef (GBR), Australia. We observed that warming amplified the negative effects of high pCO₂ on the health of the algae: rates of advanced partial mortality of CCA increased from <1% to 9% under high CO₂ (from 400 to 1,100 ppm) and exacerbated to 15% under warming conditions (from 26°C to 29°C). Furthermore, the effect of pCO₂ on skeletal dissolution strongly depended on temperature. Dissolution of P. onkodes only occurred in the high‐pCO₂ treatment and was greater in the warm treatment. Enhanced skeletal dissolution was also associated with a significant increase in the abundance of endolithic algae. Our results demonstrate that P. onkodes is particularly sensitive to ocean acidification under warm conditions, suggesting that previous experiments focused on ocean acidification alone have underestimated the impact of future conditions on coralline algae. Given the central role that coralline algae play within coral reefs, these conclusions have serious ramifications for the integrity of coral‐reef ecosystems.     

Ocean acidification weakens the structural integrity of coralline algae

The uptake of anthropogenic emission of carbon dioxide is resulting in a lowering of the carbonate saturation state and a drop in ocean pH. Understanding how marine calcifying organisms such as coralline algae may acclimatize to ocean acidification is important to understand their survival over the coming century. We present the first long‐term perturbation experiment on the cold‐water coralline algae, which are important marine calcifiers in the benthic ecosystems particularly at the higher latitudes. Lithothamnion glaciale, after three months incubation, continued to calcify even in undersaturated conditions with a significant trend towards lower growth rates with increasing pCO₂. However, the major changes in the ultra‐structure occur by 589 μatm (i.e. in saturated waters). Finite element models of the algae grown at these heightened levels show an increase in the total strain energy of nearly an order of magnitude and an uneven distribution of the stress inside the skeleton when subjected to similar loads as algae grown at ambient levels. This weakening of the structure is likely to reduce the ability of the alga to resist boring by predators and wave energy with severe consequences to the benthic community structure in the immediate future (50 years).     

Multiple phases of mg‐calcite in crustose coralline algae suggest caution for temperature proxy and ocean acidification assessment: lessons from the ultrastructure and biomineralization in Phymatolithon (Rhodophyta,Corallinales)1

Magnesium content, strongly correlated with temperature, has been developed as a climate archive for the late Holocene without considering anatomical controls on Mg content. In this paper, we explore the ultrastructure and cellular scale Mg‐content variations within four species of North Atlantic crust‐forming Phymatolithon. The cell wall has radial grains of Mg‐calcite, whereas the interfilament (middle lamella) has grains aligned parallel to the filament axis. The proportion of interfilament and cell wall carbonate varies by tissue and species. Three distinct primary phases of Mg‐calcite were identified: interfilament Mg‐calcite (mean 8.9 mol% MgCO₃), perithallial cell walls Mg‐calcite (mean 13.4 mol% MgCO₃), and hypothallium Mg‐calcite (mean 17.1 mol% MgCO₃). Magnesium content for the bulk crust, an average of all phases present, showed a strongly correlated (R² = 0.975) increase of 0.31 mol% MgCO₃ per °C. Of concern for climate reconstructions is the potential for false warming signals from undetected postgrazing wound repair carbonate that is substantially enriched in Mg, unrelated to temperature. Within a single crust, Mg‐content of component carbonates ranged from 8 to 20 mol% MgCO₃, representing theoretical thermodynamic stabilities from aragonite‐equivalent to unstable higher‐Mg‐calcite. It is unlikely that existing current predictions of ocean acidification impact on coralline algae, based on saturation states calculated using average Mg contents, provide an environmentally relevant estimate.