Forecasted Co2 Modifies the Influence of Light in Shaping Subtidal Habitat1

Abstract

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.

Macroalgal responses to ocean acidification depend on nutrient and light levels

“…Work in other regions has also shown that ocean acidification can directly benefit some macroalgae, such as Gracilaria lemaneiformis in China (Zou and Gao, 2009) and mat-forming Feldmannia spp. in Australia (Russell et al, 2011), as well as canopy-forming phaeophytes such as Nereocystis luetkeana and Macrocystis pyrifera (Swanson and Fox, 2007;Roleda et al, 2012). We found that the benefits of increased DIC were even more pronounced when combined with increased nutrients.…”

Physiological responses of the calcifying rhodophyte, Corallina officinalis (L.), to future CO2 levels

“…However, even at low irradiances, the rETR and O 2 evolution data for the two low pH treatments did not complement each other-they were only similar at the ambient pH treatment (8.3). Russell et al (2009) found that elevated CO 2 had a negative eVect on the eVective quantum yield of coralline crusts, while Russell et al 2011 found no CO 2 eVect on their relative electron transport rate. The relative electron transport rates may therefore not be a good indication of macroalgal health under elevated CO 2 conditions.…”

“…Moreover, as UV radiation as a single stress factor was shown to strongly aVect competition between non-calcifying rhodophytes (Bischof et al 2000(Bischof et al , 2006, the combination of these two stressors might have strong inXuences on community structure. Furthermore, several studies have shown that biotic and abiotic stressors such as increased grazing pressure and high CO 2 levels lower recruitment of crustose calcifying algae (Belliveau and Paul 2002, Martin et al 2008, while turf macroalgal cover increases (Russell et al 2009;Connell and Russell 2010;Russell et al 2011). If increased CO 2 levels weaken the skeletal structure of C. oYcinalis, the potential combination of increased grazing pressure, slower growth, higher UV stress, and less recruitment will likely cause a phase shift in C. oYcinalis communities toward more Xeshy, non-calcifying macroalgae, and could even amplify changes in competition between non-calcifying algae.…”

Long-term effects of warming and ocean acidification are modified by seasonal variation in species responses and environmental conditions

“…Establishing an overview, however, has been complicated by the variability in documented responses, not least because of differences in timing (life stage, season) and context (habitat condition, exposure history) between investigations, but the influence of these aspects has not been considered. Without a more holistic consideration of the variety and timing of responses to climate change [25,26], summarizing the most likely net response across multiple habitats will remain challenging [27] and subject to a great deal of uncertainty [12], and could lead to misleading conclusions.Provision of more accurate projections of the ecological consequences of warming and ocean acidification requires an improved understanding of longer term processes that moderate the susceptibility of species to a changing environment [28,29]. Much of the currently available evidence stems from short-term experimental manipulations, typically performed over days to weeks [30], that do not allow the development of diachronic response mechanisms, such as acclimation, adaptation [29,31,32], physiological or behavioural compensatory mechanisms linked to biodiversity-environmental context and/or seasonal timing [16,33,34].…”