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.
Rising ocean temperature is a major driver of kelp forest decline worldwide and one that threatens to intensify over the coming decades. What is not particularly well understood are the mechanisms ...that drive loss and how they operate at differing life stages. This study aimed to establish an understanding of the effects of increasing temperature on the early developmental stages of the giant kelp, Macrocystis pyrifera. Sporulation was carried out across 10 temperature treatments from 9.5 to 26.2°C ± 0.2°C at approximately 2°C intervals. Spores were then incubated at these temperatures under a 20.3±1.7 μmol photons m-2 s-1, 16L:8D photoperiod for 5 days. Results indicate that spore release was positively correlated with increasing temperature, whereas an inverse trend was observed between temperature and the growth of germ-tube. The thermal threshold for spore and germling development was determined to be between 21.7°C and 23.8°C. Spore settlement was the most drastically effected developmental phase by increasing temperature. This study highlights the vulnerability of early life stages of M. pyrifera development to rising ocean temperature and has implications for modelling future distribution of this valuable ecosystem engineer in a changing ocean.
Ocean acidification (OA) is a reduction in oceanic pH due to increased absorption of anthropogenically produced CO2 . This change alters the seawater concentrations of inorganic carbon species that ...are utilized by macroalgae for photosynthesis and calcification: CO2 and HCO3 (-) increase; CO3 (2-) decreases. Two common methods of experimentally reducing seawater pH differentially alter other aspects of carbonate chemistry: the addition of CO2 gas mimics changes predicted due to OA, while the addition of HCl results in a comparatively lower HCO3 (-) . We measured the short-term photosynthetic responses of five macroalgal species with various carbon-use strategies in one of three seawater pH treatments: pH 7.5 lowered by bubbling CO2 gas, pH 7.5 lowered by HCl, and ambient pH 7.9. There was no difference in photosynthetic rates between the CO2 , HCl, or pH 7.9 treatments for any of the species examined. However, the ability of macroalgae to raise the pH of the surrounding seawater through carbon uptake was greatest in the pH 7.5 treatments. Modeling of pH change due to carbon assimilation indicated that macroalgal species that could utilize HCO3 (-) increased their use of CO2 in the pH 7.5 treatments compared to pH 7.9 treatments. Species only capable of using CO2 did so exclusively in all treatments. Although CO2 is not likely to be limiting for photosynthesis for the macroalgal species examined, the diffusive uptake of CO2 is less energetically expensive than active HCO3 (-) uptake, and so HCO3 (-) -using macroalgae may benefit in future seawater with elevated CO2 .
Coralline algae play foundational roles in coastal ecosystems and are globally significant components of benthic habitats down to the limits of the photic zone. Despite their vulnerability to ocean ...acidification (OA) and importance in low light environments, there is a limited understanding of how the interplay between irradiance and OA influences coralline reproduction and recruitment. To better understand this interaction, a 212 d experiment was run exposing coralline communities to 2 pH levels (present-day pH 8.03, OA pH 7.65) and a gradient of daily light dose (0.35, 0.17 and 0.1 mol m
-2
d
-1
), based on
in situ
measurements. In the highest light dose treatment, lowered seawater pH projected for 2100 (pH
(T)
7.65) reduced recruitment by 56%. This OA-driven reduction in recruitment was amplified under reduced light, with recruitment near zero in the lowest light treatment. This study shows, for the first time, the increased vulnerability of coralline community recruitment to OA under low light. Coralline algae are known to be the deepest growing macroalgae and thus, in these low light zones, OA may have the potential to reduce coralline depth distribution.
Understorey macroalgae can alter pH at their surface via metabolic activity within the concentration boundary layer (CBL), but it is unknown to what degree the presence of larger macroalgal canopies ...can modify the pH micro-environment of understorey species. We examined whether flow reduction by a canopy-forming macroalga could alter the thickness of the CBL at the surface of understorey crustose coralline macroalgae (CCA). This could lead to a greater metabolic influence of macroalgae on the pH and oxygen environment at the coralline’s surface. Three experimental treatments were examined in a re-circulating flume: (1) a full canopy (consisting of Carpophyllum maschalocarpum) and understorey (Corallina officinalis and CCA), (2) a mimic (plastic/silk) canopy plus understorey, and (3) an understorey only. Profiles of seawater velocity and pH/O₂ concentration gradients were measured at 3 bulk seawater velocities (2, 4 and 8 cm s−1) above the CCA in both the light and dark. Canopy macroalgae altered the pH and O₂ environment encountered by understorey coralline algae via their physical presence rather than by directly altering bulk seawater chemistry through their metabolism. Reduced seawater velocities beneath Carpophyllum and mimic canopies resulted in increased CBL thicknesses, higher pH (up to 8.9) and O₂ concentrations in the light, and lower pH (down to 7.74) and O₂ concentrations in the dark. The ability of canopies to facilitate greater metabolic changes in pH at the surface of understorey species highlights a previously unrecorded species interaction that could play an important role in influencing the physiology and ecology of understorey assemblages.
The giant kelp
Macrocystis pyrifera
is in global decline as a result of numerous stressors operating on both local and global scales. It is a species that holds significant value in terms of the ...ecosystem services that it provides and its application in aquaculture. In order to safeguard, restore and utilize this species, it is essential that a sound understanding of genetic structure and diversity is established at scales relevant to local management. Seven microsatellite markers were used to analyze 389 individuals from sites across eight geographical regions in New Zealand. While samples of
M. pyrifera
from the west coast of the South Island (Fiordland), were genetically isolated, the biogeographic separation of sites along the east coast of New Zealand, between Wellington and Stewart Island, remained unclear due to low genetic differentiation between regions. The greatest genetic diversity was seen in the southeast sites, whereas the northeast had the lowest diversity. This pattern is likely driven by the effects of stressors such as high sea surface temperature in these areas as well as oceanic circulation patterns. A key finding from this work was the significant genetic isolation, and therefore vulnerability of
M. pyrifera
in the Fiordland population, an area that is being subjected to more intense and longer lasting heatwave events.
Nitrogen is essential for algal productivity but often reaches limiting concentrations in temperate ecosystems. Increased water motion enhances nitrogen uptake by decreasing the thickness of the ...diffusion boundary layer surrounding algal surface tissue, allowing for increased nitrogen mass-transfer across this boundary. Macrocystis pyrifera forms large beds that span the water column and can alter the surrounding physical environment by creating bed-wide boundaries that may reduce current and wave propagation to the bed interior; reduced water motion may decrease mass-transfer rates and therefore alter nitrogen uptake. We investigated whether a water mass-transfer gradient across M. pyrifera beds exists by identifying 3 bed types likely to experience different water motion intensities (open, shoreline exterior and shoreline interior) and whether this gradient influenced heterogeneity in M. pyrifera growth and tissue status during low nitrogen (summer) and high nitrogen (winter) conditions. Gypsum dissolution suggested that mass-transfer significantly increased across beds; open bed dissolution rates were approximately 6% higher than the shoreline exterior, which exhibited mean dissolution rates 17% higher than the shoreline interior. Summer kelp growth, pigmentation, tissue %N and C:N paralleled mass-transfer, where exterior kelp exhibited higher values than interior kelp. The same trends did not exist during the winter, when ambient nitrogen concentrations were high, suggesting that mass-transfer is an important mechanism for nitrogen acquisition during limitation events. This study highlights mass-transfer variability across relatively small macroalgal beds and the corresponding effects on kelp growth and nitrogen status, which previously might have been assumed as uniform due to the general wave exposure.
Benthic primary producers in coastal ecosystems provide important habitat for marine organisms through the provision of complex 3D habitat. Primary producers produce organic matter, while ...simultaneously producing reactive oxygen species, including hydrogen peroxide (H₂O₂), a driver of oxidative stress. Through their high biomass, productivity and effect on local hydrodynamics, benthic primary producers can potentially increase H₂O₂ concentrations surrounding the biogenic structures they form. The aim of this study was to identify the potential role of H₂O₂ produced by benthic primary producers as an external stressor in coastal ecosystems. This was achieved by measuring H₂O₂ concentrations within sea lettuce blooms (Ulva sp.), giant kelp forests (Macrocystis pyrifera), and seagrass meadows (Zostera muelleri); quantifying H₂O₂ production rates of these species; and testing heterotrophic bacterial response to relevant H₂O₂ concentrations. Ulva sp. produced five times more H₂O₂ than other species. At in situ concentrations, H₂O₂ inhibited bacterial production and carbon flow through the microbial loop by 75%. This study reveals H₂O₂ as an additional stressor in bloom-forming Ulva sp. with higher H₂O₂ production compared to the ecosystem engineers M. pyrifera and Z. muelleri. H₂O₂ production by benthic primary producers can affect carbon flow through the microbial loop, with the potential to propagate a stress signal up the food web.
Macrophytes vary in their ability to utilize carbon in the form of HCO
3
−
and/or CO
2
for photosynthesis. Some functional groups that solely use CO
2
for photosynthesis could receive competitive ...advantages from the predicted increase in CO
2
compared to groups with efficient carbon acquisition strategies of HCO
3
−
. The aim of this study was to identify carbon use strategies in the common macrophytes (macroalgae, charophytes, seagrass, and other angiosperms) that represent a broad range of functional traits to CO
2
concentrations in the northeastern Baltic Sea. Mechanistic assessment of the carbon physiology of macrophytes was used to predict productivity and competitive interactions between different functional groups under future climate. Carbon use strategies in macrophytes were determined by analysing the carbon isotopes (
δ
13
C), pH drift experiments, and photosynthesis versus dissolved inorganic carbon. In addition, habitat mapping data was used to interpret the potential implications of the elevated CO
2
to this coastal ecosystem. The results suggested that the primary productivity of macrophytes is often limited by carbon availability, and the increasing CO
2
concentrations in the brackish Baltic Sea are expected to enhance photosynthetic production. While all species tested showed evidence of carbon concentrating mechanisms (CCMs), differential levels of CCM activity indicate varying levels of competitive fitness in a future high-CO
2
environment. Overall, macrophytes which inhabit the shallowest and deepest parts of the vegetated zone are expected to experience physiological benefits under future CO
2
conditions, while intermediate communities dominated by the perennial brown alga
Fucus vesiculosus
may experience loss of fitness. These fitness differences have implications for competitive interaction and species range under future climate.
Ocean acidification describes changes in the carbonate chemistry of the ocean due to the increased absorption of anthropogenically released CO₂. Experiments to elucidate the biological effects of ...ocean acidification on algae are not straightforward because when pH is altered, the carbon speciation in seawater is altered, which has implications for photosynthesis and, for calcifying algae, calcification. Furthermore, photosynthesis, respiration, and calcification will themselves alter the pH of the seawater medium. In this review, algal physiologists and seawater carbonate chemists combine their knowledge to provide the fundamental information on carbon physiology and seawater carbonate chemistry required to comprehend the complexities of how ocean acidification might affect algae metabolism. A wide range in responses of algae to ocean acidification has been observed, which may be explained by differences in algal physiology, timescales of the responses measured, study duration, and the method employed to alter pH. Two methods have been widely used in a range of experimental systems: CO₂ bubbling and HCl/NaOH additions. These methods affect the speciation of carbonate ions in the culture medium differently; we discuss how this could influence the biological responses of algae and suggest a third method based on HCl/NaHCO₃ additions. We then discuss eight key points that should be considered prior to setting up experiments, including which method of manipulating pH to choose, monitoring during experiments, techniques for adding acidified seawater, biological side effects, and other environmental factors. Finally, we consider incubation timescales and prior conditioning of algae in terms of regulation, acclimation, and adaptation to ocean acidification.
