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Studies that use CO2 vents as natural laboratories to investigate the impacts of ocean acidification (OA) typically employ control-impact designs or local-scale gradients in pH or pCO2, where impacted sites are compared to reference sites. While these strategies can accurately represent well-defined and stable vent systems in relatively homogenous environments, it may not adequately encompass the natural variability of heterogeneous coastal environments where many CO2 vents exist. Here, we assess the influence of spatial heterogeneity on the perceived impacts of OA at a vent system well established in the OA literature. Specifically, we use a multi-scale approach to investigate and map the spatial variability in seawater pH and benthic communities surrounding vents at Whakaari-White Island, New Zealand to better understand the scale and complexity of ecological impacts of an acidified environment. We found a network of vents embedded in complex topography throughout the study area, and spatially variable pH and pCO2 levels. The distribution of habitats (i.e. macroalgal forests and turfing algae) was most strongly related to substratum type and sea urchin densities, rather than pH. Epifaunal communities within turfing algae differed with sampling distance from vents, but this pattern was driven by higher abundances of a number of taxa immediately adjacent to vents, where pH and temperature gradients are steep and white bacterial mats are prevalent. Our results contrast with previous studies at White Island that have used a control-impact design and suggested significant impacts of elevated CO2 on benthic communities. Instead, we demonstrate a highly heterogeneous reef where it is difficult to separate effects of reduced pH from spatial variation in reef communities. We urge that future research carefully considers and quantifies the biological and physical complexity of venting environments, and suggest that in dynamic systems, such as White Island, the use of control-impact designs can oversimplify and potentially overestimate the future impacts of OA.

Macroalgal-dominated habitats are rapidly gaining recognition as important contributors to marine carbon cycles and sequestration. Despite this recognition, relatively little is known about the production and fate of carbon originating from these highly productive ecosystems, or how anthropogenic- and climate-related stressors affect the role of macroalgae in marine carbon cycles. Here, we examine the impact of increasing turbidity on carbon storage, fixation and loss in southern hemisphere kelp forests. We quantified net primary production (NPP) and biomass accumulation (BA), and estimated carbon release via detritus and dissolved organic carbon (DOC) across a large-scale turbidity gradient. We show that increased turbidity, resulting in a 63% reduction in light, can result in a 95% reduction in kelp productivity. When averaged annually, estimates of NPP and BA per plant at high-light sites were nearly six and two times greater than those at low-light sites, respectively. Furthermore, the quantity of carbon fixed annually by kelp forests was up to 4.7 times greater than that stored as average annual standing stock. At low-light sites, the majority of C goes directly into tissue growth and is subsequently eroded. In contrast, excess production at high-light sites accounts for up to 39% of the total carbon fixed and is likely released as DOC. Turbidity is expected to increase in response to climate change and our results suggest this will have significant impacts on the capacity of kelp forests to contribute to carbon sequestration pathways. In addition to demonstrating that turbidity significantly reduces the quantity of carbon fixed by kelp forests, and subsequently released as detritus, our results highlight the negative impacts of turbidity on a large source of previously unaccounted for carbon.

Underwater light is essential for fuelling coastal productivity. However, elevated turbidity, resulting from land-based activities and climate change, is often overlooked as a threat to coastal ecosystems. Understanding how low light, and specifically the temporal delivery of light, affects the productivity of forest-forming species is necessary to predict how ecosystems and species will respond to future increases in turbidity. Outdoor mesocosm experiments were used to compare the low-light tolerance of 2 forest-forming macrophytes that vary in their distribution in relation to turbidity, and investigate how the temporal delivery of light, i.e. press vs. pulse low-light disturbance, affects net primary productivity (NPP). We showed that the kelpEcklonia radiata, which dominates reefs with low turbidity, is more productive per unit biomass under high-light conditions than the fucoidCarpophyllum flexuosum, which typifies more turbid waters. Under low light,E. radiatasuffered greater tissue loss and had lower NPP thanC. flexuosum. Under both press and pulse treatments,E. radiatashowed significant losses of lamina biomass and reduced NPP, whileC. flexuosumshowed net growth under press disturbance, and only lost tissue and had reduced NPP under pulse disturbance. The greater tolerance ofC. flexuosumto decreased light, and differential responses ofE. radiataandC. flexuosumto press and pulse low-light conditions, provide mechanistic support forC. flexuosumbeing better suited to turbid low-light environments thanE. radiata. These results suggest future increases in turbidity may facilitate a shift from kelp-dominated forests to alternate states, resulting in reduced primary productivity.

Understanding the photoacclimation response of macroalgae across broad spatial and temporal scales is necessary for predicting their vulnerability to environmental changes and quantifying their contribution to coastal primary production. This study investigated how the photosynthesis–irradiance response and photosynthetic pigment content of the kelp Ecklonia radiata varies both spatially and seasonally among seven sites located across a turbidity gradient in the Hauraki Gulf, north-eastern New Zealand. Photosynthesis–irradiance curves were derived under laboratory conditions for whole adult E. radiata using photorespirometry chambers. Lab-derived photosynthesis–irradiance curves in summer were also compared with in situ measurements made on kelp at each of the seven study sites. Photosynthetic parameters and pigments showed clear seasonal patterns across all sites as demonstrated by higher photosynthetic pigment levels and photosynthetic efficiency occurring in autumn and winter, and higher maximum rates of photosynthesis and respiration occurring in summer. Lamina biomass was similar across sites, yet thalli exhibited a clear photokinetic response to increasing turbidity. At turbid sites photosynthetic pigment levels and photosynthetic efficiency was higher, and respiration and saturation and compensation irradiances lower, compared to high-light sites. The results presented here further our understanding of low-light acclimation strategies in kelp and highlight the degree of seasonality in photosynthetic parameters. Though E. radiata demonstrates a clear capacity to photoacclimate to a degrading light environment, further research is needed to investigate the extent to which the observed acclimation can offset the likely negative effects of increasing turbidity on kelp forest primary production.

We used three laboratory experiments as well as measurements, throughout an entire growth season, of the length and survival of sporophytes, sea temperature, and wave height at two subtidal sites in Newfoundland (Canada), to investigate causal relationships between environmental variability, intracellular acidity, growth, and mortality in the highly acidic (H2SO4), annual brown seaweed Desmarestia viridis (O.F. Muller) J.V. Lamouroux. Light, grazing, and epibionts did not affect acidity. In the absence of waves, acid loss was threefold lower in cold (6.5 °C) than warm (11 °C) water, and threefold higher in the absence than presence of waves in warm (11 to 13 °C) water. There were three phases of change in frond length at both sites: (1) increase [March to late June], (2) no change [July to mid-August], and (3) decrease [mid-August to late October]. Mortality rates and sea temperature in Phase 1 were low, whereas the onset of increasing mortality at the end of Phase 2 coincided well with the end of a 2-week period during which temperature increased rapidly, from 6 to 11 °C. Results strongly suggested the more severe wave climate at the most wave-exposed site in September, when temperature was ~ 10 °C, facilitated the dispersal of the acid being released, which delayed mortality by ~ 10 days compared to the other site. All sporophytes had disappeared at both sites in late October. This is the first integrated analysis of seasonal variation in growth and mortality of D. viridis sporophytes. The strong connections between laboratory and field data attest to the critical role that thermal and hydrodynamic environments play in the life history of this unique seaweed.

Seaweeds exhibit a number of adaptations to cope with strong selective pressures imposed by shallow marine environments. The exceptional ability of the annual, brown seaweed Desmarestia viridis, to produce and store high concentrations of sulfuric acid (H2SO4) in intracellular vacuoles, makes it a particularly compelling model for studies of causes and consequences of acid production in seaweeds. We used two laboratory experiments, as well as measurements of intracellular pH of sporophytes and sea salinity and temperature over an entire growth season at one site in Newfoundland and Labrador (Canada), to test hypotheses about effects of salinity and temperature on acid production and die-off in D. viridis. We show that the acid is continuously and irreversibly accumulated (resulting in an intracellular pH as low as 0.53) as the seaweed grows from recruit to adult (March–June) and that this build-up inevitably culminates into dramatic mass releases of acid and die-offs. Progressive, synchronous death among individuals is under the predominant control of sea temperature (death systematically occurred around 12 °C in both laboratory and field), which suggests the evolution of a life-history strategy in which death occurs at a time when individuals reach a size that may correspond to reproductive maturity. The seaweed exhibits a low tolerance to changes in salinity (death was inevitable below 29 psu), which likely imposes severe limitations on its distribution range throughout its existence as a sporophyte. Our findings suggest that major phenological events and survival in D. viridis are intimately linked to synergistic effects of oceanographic controls, which either impair the ability of the seaweed to retain the acid in its tissues (seasonal sea warming), or contribute to dissipating the acid that is released to the environment (wave and current action). They also provide novel insights into the ecophysiological and evolutionary constraints within which marine organisms adapt.