Posts Tagged 'mesocosms'

Future ocean climate homogenizes communities across habitats through diversity loss and rise of generalist species

Predictions of the effects of global change on ecological communities are largely based on single habitats. Yet in nature, habitats are interconnected through the exchange of energy and organisms, and the responses of local communities may not extend to emerging community networks (i.e. metacommunities). Using large mesocosms and meiofauna communities as a model system, we investigated the interactive effects of ocean warming and acidification on the structure of marine metacommunities from three shallow‐water habitats: sandy soft‐bottoms, marine vegetation and rocky reef substrates. Primary producers and detritus – key food sources for meiofauna – increased in biomass under the combined effect of temperature and acidification. The enhanced bottom‐up forcing boosted nematode densities but impoverished the functional and trophic diversity of nematode metacommunities. The combined climate stressors further homogenized meiofauna communities across habitats. Under present‐day conditions metacommunities were structured by habitat type, but under future conditions they showed an unstructured random pattern with fast‐growing generalist species dominating the communities of all habitats. Homogenization was likely driven by local species extinctions, reducing interspecific competition that otherwise could have prevented single species from dominating multiple niches. Our findings reveal that climate change may simplify metacommunity structure and prompt biodiversity loss, which may affect the biological organization and resilience of marine communities.

Continue reading ‘Future ocean climate homogenizes communities across habitats through diversity loss and rise of generalist species’

A future 1.2 °C increase in ocean temperature alters the quality of mangrove habitats for marine plants and animals

Highlights
• Mangrove habitats are more resilient to climate change than other habitats.

• Climate change might have positive effects on mangrove-root species communities.

• Using mesocosms we show that an increase of 1.2 °C leads to community homogenisation.

• Warming also led to diversity loss and flattening of mangrove root epibiont communities.

• Juvenile fish altered their use of mangrove habitats under warming and acidification.

Abstract
Global climate stressors, like ocean warming and acidification, contribute to the erosion of structural complexity in marine foundation habitats by promoting the growth of low-relief turf, increasing grazing pressure on structurally complex marine vegetation, and by directly affecting the growth and survival of foundation species. Because mangrove roots are woody and their epibionts are used to ever-changing conditions in highly variable environments, mangrove habitats may be more resilient to global change stressors than other marine foundation species. Using a large-scale mesocosm experiment, we examined how ocean warming and acidification, under a reduced carbon emission scenario, affect the composition and structural complexity of mangrove epibiont communities and the use of mangrove habitat by juvenile fishes. We demonstrate that even a modest increase in seawater temperature of 1.2 °C leads to the homogenisation and flattening of mangrove root epibiont communities. Warming led to a 24% increase in the overall cover of algal epibionts on roots but the diversity of the epibiont species decreased by 33%. Epibiont structural complexity decreased owing to the shorter stature of weedy algal turfs which prospered under elevated temperature. Juvenile fishes showed alterations in mangrove habitat use with ocean warming and acidification, but these were independent of changes to the root epibiont community. We reveal that the quality of apparently resilient mangrove habitats and their perceived value as habitat for associated fauna are still vulnerable under a globally reduced carbon emission scenario.

Continue reading ‘A future 1.2 °C increase in ocean temperature alters the quality of mangrove habitats for marine plants and animals’

Species-specific calcification response of Caribbean corals after 2-year transplantation to a low aragonite saturation submarine spring

Coral calcification is expected to decline as atmospheric carbon dioxide concentration increases. We assessed the potential of Porites astreoides, Siderastrea siderea and Porites porites to survive and calcify under acidified conditions in a 2-year field transplant experiment around low pH, low aragonite saturation (Ωarag) submarine springs. Slow-growing S. siderea had the highest post-transplantation survival and showed increases in concentrations of Symbiodiniaceae, chlorophyll a and protein at the low Ωarag site. Nubbins of P. astreoides had 20% lower survival and higher chlorophyll a concentration at the low Ωarag site. Only 33% of P. porites nubbins survived at low Ωarag and their linear extension and calcification rates were reduced. The density of skeletons deposited after transplantation at the low Ωarag spring was 15–30% lower for all species. These results suggest that corals with slow calcification rates and high Symbiodiniaceae, chlorophyll a and protein concentrations may be less susceptible to ocean acidification, albeit with reduced skeletal density. We postulate that corals in the springs are responding to greater energy demands for overcoming larger differences in carbonate chemistry between the calcifying medium and the external environment. The differential mortality, growth rates and physiological changes may impact future coral species assemblages and the reef framework robustness.

Continue reading ‘Species-specific calcification response of Caribbean corals after 2-year transplantation to a low aragonite saturation submarine spring’

Nutrient enrichment promotes eutrophication in the form of macroalgal blooms causing cascading effects in two anthropogenically disturbed coastal ecosystems

Humans are impacting almost every major ecological process that structures communities and ecosystems. Examples of how human activity can directly control key processes in ecosystems include destruction of habitat changing trophic structure, nutrient pollution altering competitive outcomes, overharvesting of consumers reducing top down control, and now climate change impacting virtually every global biogeochemical cycle. These human impacts may have an independent effect on the ecosystem, but they also have the potential to cause cascading effects and promote subsequent stressors. Also, these impacts are not limited to a particular system or geographic location making research on their overall effects vital for management practices. For example, tropical reefs have been transitioning from coral to mixed communities dominated by macroalgae, motivating research on how macroalgae respond to anthropogenic stressors and interact with each other during these stressful events. Further, while eutrophication of coastal estuaries due to increased anthropogenic supplies of nutrients has been of critical global concern for decades, the potential for eutrophication to drive new stressors is a growing concern. To address these knowledge gaps, I investigated how human stressors impact two different and major coastal ecosystems known to be vulnerable to anthropogenic disturbances.

In chapter 1, I demonstrate that anthropogenic stressors in the form of increased nutrients in the water and sediments have strong impacts on interspecific interactions of coral reef macroalgae. Abiotic stressors such as nutrients have been linked to phase-shifts from coral to algal domination on tropical reefs. However, few studies have considered how these stressors impact changes in the biotic and abiotic constituents of dominant species of calcifying macroalgae, and how this may be mediated by species-species interactions. I conducted 4 mesocosm experiments to examine whether different nutrient sources (water column vs. terrestrial sediment) as well as species interactions (alone vs. mixed species) affected total mass (biomass + calcium carbonate (CaCO3)) of two common calcifying macroalgae (Padina boryana and Galaxaura fasciculata). P. boryana gained total mass with increased water column nutrients but declined with increased nutrients supplied by the sediment. Conversely, G. fasciculata gained total mass with increased nutrients in the sediment but declined with increased water column nutrients. In both interactions, the “winner” (i.e., G. fasciculata in the sediment experiment) also had a greater % of thallus mass comprised of CaCO3, potentially due to the subsequent decomposition of the “loser” as this result was not found in the alone treatments. These findings ultimately suggest that nutrient stressors can cause cascading effects, such as promoting calcification and biomass growth or loss in these macroalgal communities, and the potential for domination or decline is based on the nutrient source and community composition.

In chapter 2, I demonstrate that decomposition of macroalgal blooms cause a sequence of biogeochemical processes that can drive acidification in shallow coastal estuaries, and that these processes are mediated by a dynamic microbial community. Eutrophication and ocean acidification are both widely acknowledged as major human-induced stressors in marine environments. While the link between eutrophication and acidification has been established for phytoplankton, it is unclear whether eutrophication in the form of macroalgal blooms can cause cascading effects like acidification in shallow eutrophic estuaries. I conducted seasonal field surveys and assessed microbial communities and functional genes to evaluate changes in biotic and abiotic characteristics between seasons that may be associated with acidification in Upper Newport Bay, CA, USA. Acidification, measured as a drop in pH of 0.7, occurred in summer at the site with the most macroalgal cover. Microbial community composition and functional gene expression provide evidence that decomposition processes contributed to acidification, and also suggest that other biogeochemical processes like nitrification and degradation of polyphosphate also contributed to acidification. To my knowledge, my findings represent the first field evidence that eutrophication of shallow coastal estuaries dominated by green macroalgal blooms can cascade to acidification.

In chapter 3, I demonstrate that macroalgal blooms in shallow estuaries are strong drivers of key microbially-mediated biogeochemical processes that can cause cascading effects, such as acidification and nutrient fluxing, regardless of simulated tidal flushing. Estuaries are productive and diverse ecosystems and are vulnerable to eutrophication from increased anthropogenic nutrients. While it is known that enhanced tidal flushing can reduce adverse effects of anthropogenic disturbances in larger, deeper estuarine ecosystems, this is unexplored for eutrophication in shallow coastal estuaries where macroalgae usually dominate. I simulated eutrophication as a macroalgal bloom in a mesocosm experiment, varied tidal flushing (flushed daily vs unflushed), and assessed the effects on water column and sediment biogeochemical processes and the sediment microbial community. While flushing did not ameliorate the negative effects of the macroalgal bloom, it caused transient differences in the rate of change in biogeochemical processes and promoted increased fluxes of nutrients from the sediment. In the beginning, the macroalgal bloom induced basification and increased total alkalinity, but during decomposition, acidification and the accumulation of nutrients in the sediment and water column occurred. The findings from this chapter ultimately suggest that macroalgal blooms have the potential to be the cause of, yet may also offer a partial solution to, global ecological changes to biogeochemical processes.

Overall, my results indicate that anthropogenic disturbances, particularly in the form of increased nutrients, can cause cascading effects like macroalgal blooms that in turn cause acidification, basification, increased interspecific interactions, nutrient depletion, and nutrient fluxing in multiple ecosystems. These data advance our current understanding of the ecological consequences of eutrophication in the form of macroalgal blooms in different ecosystems. It also provides mechanistic links to microbial communities and biogeochemical processes not previously identified for shallow coastal estuaries. As human population and subsequent nutrient pollution increases in watersheds globally, ecological phenomenon such as eutrophication will only be intensified, and macroalgal communities will continue to dominate. Consequently, this dominance, especially during decomposition as shown here, can drive a multitude of subsequent stressors that can impact the entire ecosystem.

Continue reading ‘Nutrient enrichment promotes eutrophication in the form of macroalgal blooms causing cascading effects in two anthropogenically disturbed coastal ecosystems’

Quantification of the effects of ocean acidification on benthic foraminifera

The global ocean has experienced an alteration of its seawater chemistry due to the continuing uptake of anthropogenic carbon dioxide (CO₂) from the atmosphere. This ongoing process called Ocean acidification (OA) has reduced seawater pH levels, carbonate ion concentrations (CO₃⁻²) and carbonate saturation state (Ω) with implications for the diversity and functioning of marine life, particularly for marine calcifiers such as foraminifera. The vulnerability of this ubiquitous calcifying group to future high 𝘱CO₂ /low pH scenarios has been assessed naturally and experimentally in the last decades. However, little is known about how benthic foraminifera from coastal environments such as intertidal environments will respond to the effects of OA projected by the end of the century. This research aimed to quantify the effects of OA on a series of biological parameters measured on the benthic foraminifera 𝘌𝘭𝘱𝘩𝘪𝘥𝘪𝘶𝘮 𝘸𝘪𝘭𝘭𝘪𝘢𝘮𝘴𝘰𝘯𝘪 and 𝘏𝘢𝘺𝘯𝘦𝘴𝘪𝘯𝘢 𝘨𝘦𝘳𝘮𝘢𝘯𝘪𝘤𝘢 through a laboratory-based experimental approach where future scenarios of a high CO₂ atmosphere and low seawater pH were explored.
Experimental evidence revealed that survival rates, test weight and size-normalized weight (SNW) of 𝘌. 𝘸𝘪𝘭𝘭𝘪𝘢𝘮𝘴𝘰𝘯𝘪 were negatively affected by OA. Whereas 𝘏. 𝘨𝘦𝘳𝘮𝘢𝘯𝘪𝘤𝘢 was positively affected (i.e. enhanced growth rates) showing a species-specific response to OA at 13°C. However, the combined effect of OA and temperature (15°C) reduced survival and growth rates for 𝘌𝘭𝘱𝘩𝘪𝘥𝘪𝘶𝘮 𝘸𝘪𝘭𝘭𝘪𝘢𝘮𝘴𝘰𝘯𝘪 and 𝘏𝘢𝘺𝘯𝘦𝘴𝘪𝘯𝘢 𝘨𝘦𝘳𝘮𝘢𝘯𝘪𝘤𝘢. Test morphology (i.e. test surface and feeding ornamentation) of live 𝘌. 𝘸𝘪𝘭𝘭𝘪𝘢𝘮𝘴𝘰𝘯𝘪 and 𝘏. 𝘨𝘦𝘳𝘮𝘢𝘯𝘪𝘤𝘢 were severely affected after 6 weeks by OA, negatively influencing the uptake of 13C-labelled diatoms of 𝘕𝘢𝘷𝘪𝘤𝘶𝘭𝘢 𝘴𝘱., notably for 𝘌. 𝘸𝘪𝘭𝘭𝘪𝘢𝘮𝘴𝘰𝘯𝘪. Test dissolution rates were enhanced by OA and negatively affected foraminiferal morphology of recently dead assemblages with implications for net accumulation and preservation. These results imply that the long-term storage of inorganic carbon and cycling of carbon in coastal benthic ecosystems will be considerably altered by future OA.

Continue reading ‘Quantification of the effects of ocean acidification on benthic foraminifera’

Impact de l’acidification et du réchauffement sur les communautés planctoniques de l’estuaire du Saint-Laurent et la production de diméthylsulfure (in French)

Anthropogenic carbon dioxide (CO2) emissions have increased since the industrial revolution, leading to modifications in atmospheric CO2 content and an increase in oceanic CO2 partial pressures (pCO2). The uptake of CO2 by the oceans has resulted in a lowering of surface water pH, corresponding to an increase in the acidity of the oceans by ~30 % compared with pre-industrial times. Furthermore, climate change resulting from the accumulation of anthropogenic CO2 in the atmosphere is responsible for the observed warming of sea surface temperatures since the mid 20th century. The fate of planktonic communities in the face of these changes in the marine environment over the next century remains uncertain. Even less understood are the possible interactions of acidification and warming on the production of dimethylsulfide (DMS), a sulfur-containing gas produced by planktonic communities and involved in climate regulation. The aim of this thesis is to determine the impact of heightened pCO2 on the development of the phytoplanktonic blooms in the Lower St. Lawrence Estuary (LSLE), and their production of DMS, as well as to evaluate how concomitant warming could modulate the effects of acidification. Two intricate experiments were carried out during this study. First, a microcosm experiment (~20 L) was conducted in the summer of 2013 to assess the effects of pCO2 on the development of the LSLE spring diatom bloom, paying special attention to the microbial processes governing the production of DMS. Second, a multifactorial mesocosm experiment (~2600 L) was carried out in the fall of 2014 to investigate the combined effects of pCO2 and temperature on the development of the fall bloom in the LSLE and the production of DMS. Results from our microcosm experiment show that the blooming phytoplankton community of the LSLE during spring is resistant to pCO2 increases superior to the expected values for 2100. This resistance likely reflects its adaptation to the estuarine setting, an environment known for rapid and intense fluctuations of pCO2. This first experiment has also highlighted a reduction of the average concentrations of DMS by 15 and 40 % in planktonic assemblages respectively subjected to pCO2 of ~1850 μatm and ~2700 μatm compared to the control (~775 μatm). Parallel incubations have shown, using 35S-DMSPd, that the negative effect of acidification on DMS mostly stemmed from a decrease in the conversion efficiency of DMSP to DMS by bacteria. The second experiment has also highlighted a strong resistance of the diatom Skeletonema costatum to a wide range of pH (~8.0–7.2), and corresponding pCO2 (~90–3000 μatm). In this study, a warming of 5 °C accelerated the development and decline of the bloom, but did not affect the integrated primary production over the duration of the experiment. As in the first experiment, heightened pCO2 resulted in a decrease of average concentrations of DMS of ~66 % in the most acidified mesocosms compared to the least acidified mesocosms at in situ temperature (10 °C). However, the negative effect of an increase in pCO2 on the net production of DMS could be mitigated by a warming of surface waters. Indeed, my results reveal that the net production of DMS was higher at 15 °C compared to 10 °C over the whole pCO2 gradient in our mesocosm study. These novel results suggest that warming of surface waters could mitigate, at least partly, the negative effect of acidification on DMS net production in the LSLE and perhaps in the world’s oceans.

Continue reading ‘Impact de l’acidification et du réchauffement sur les communautés planctoniques de l’estuaire du Saint-Laurent et la production de diméthylsulfure (in French)’

Grazers increase the sensitivity of coralline algae to ocean acidification and warming

Highlights

  • Stimulation of the primary production and calcification of corallines by grazing
  • Different response of maerl between winter and summer conditions
  • High vulnerability of corallines to ocean acidification in the presence of grazers

Abstract

Coralline algae are expected to be adversely impacted by ocean acidification and warming. Most research on these algae has involved experiments on isolated species, without considering species interactions, such as grazing. This myopic view is challenging because the impact of climate change on coralline algae will depend on the direct impacts on individual coralline species and the indirect effects of altered interactions with other species. Here, we tested the influence of grazing on the response of the coralline alga Lithothamnion corallioides to near-future ocean acidification and warming. Two three-month experiments were performed in the winter and summer seasons in mesocosms under crossed conditions of pCO2 (ambient and high pCO2) and temperature (ambient and +3 °C) in the presence and absence of grazers. In the winter, L. corallioides photosynthesis decreased with rising temperature in the presence of grazers, while calcification increased. It is likely that increased calcification may act as a structural protection to prevent damage from grazing. However, increasing calcification rates in the presence of grazers may be detrimental to other physiological processes, such as photosynthesis. In the summer, L. corallioides primary production, respiration, and calcification were higher in the presence of grazers than in their absence. Light calcification rates were reduced under high pCO2 in the presence of grazers only. Moreover, dark calcification rates were more adversely affected by pCO2 increase in the presence of grazers. Through their feeding activity, grazers may alter the structural integrity of thalli and increase the sensitivity of coralline algae to ocean acidification. Our results indicate that both season and grazing play a key role in the response of L. corallioides to acidification and warming. Seasonal variations and species interactions are thus critical to consider to make ecologically relevant predictions of the effects of future environmental changes.

Continue reading ‘Grazers increase the sensitivity of coralline algae to ocean acidification and warming’


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Ocean acidification in the IPCC AR5 WG II

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