Posts Tagged 'algae'



The role of herbivores in a near future ocean: positive and negative effects of climate change on herbivore ecological function

Earth’s climate is characterised by abrupt change through its history, yet human induced climate change is warming and acidifying our oceans at unprecedented rates. Such alterations in the seawater’s chemical and physical properties are anticipated to disrupt a multitude of ecological processes leading to potential reductions in productivity and biodiversity of marine systems. Functional groups such as marine herbivores are renowned for meditating competition between benthic organisms, affecting the physical structure and primary production in marine systems, countervailing the deleterious effects of global and local disturbances. Within this context, it is important to not only understand how herbivorous species respond to climate change, but also how their overall functional role are affected and how this might have cascading effects on other species. In this thesis, I reveal that whilst populations of many species are forecast to collapse due to the effects of future climate, some herbivorous species may capitalize on environmental change and boost their densities by increasing the carrying capacity of the environment by actively modifying the habitat under an otherwise stressful condition. I also show that the modifications performed by herbivorous species trough the strengthening of positive interaction under ocean acidification can assist other species to densify, stimulating species coexistence and ecosystem function, and perhaps mitigate the deleterious effect of CO2 enrichment expected at population and community level. Therefore, under ocean warming the functional role of herbivores is eroded releasing opportunistic algae from trophic control which can potentially lead marine systems to undergo structural modification. I show that loss of this functional role, reduces the capacity of the system to control the expansion of opportunistic algae. The identification of the circumstances as to whether herbivores functional role in marine systems will strengthen or decrease provides insights into the impacts of ocean warming and acidification at local scale and their potential management.

Continue reading ‘The role of herbivores in a near future ocean: positive and negative effects of climate change on herbivore ecological function’

Functional loss in herbivores drives runaway expansion of weedy algae in a near-future ocean

Highlights

• Elevated CO2 and warming increased productivity of turf algae.

• Elevated CO2 increased per capita feeding rates of gastropods.

• Ocean warming reduced grazer diversity, density, and biomass.

• As a result, ocean warming drove a fourfold expansion of weedy algal species.

Abstract

The ability of a community to absorb environmental change without undergoing structural modification is a hallmark of ecological resistance. The recognition that species interactions can stabilize community processes has led to the idea that the effects of climate change may be less than what most considerations currently allow. We tested whether herbivory can compensate for the expansion of weedy algae triggered by CO2 enrichment and warming. Using a six-month mesocosm experiment, we show that increasing per capita herbivory by gastropods absorbs the boosted effects of CO2 enrichment on algal production in temperate systems of weak to moderate herbivory. However, under the combined effects of acidification and warming this compensatory effect was eroded by reducing the diversity, density and biomass of herbivores. This loss of functionality combined with boosted primary productivity drove a fourfold expansion of weedy algal species. Our results demonstrate capacity to buffer ecosystems against CO2 enrichment, but loss of this capacity through ocean warming either in isolation or combined with CO2, driving significant algal turf expansion. Identifying compensatory processes and the circumstances under which they prevail could potentially help manage the impacts of ocean warming and acidification, which are further amplified by local disturbances such as habitat loss and herbivore over-exploitation.

Continue reading ‘Functional loss in herbivores drives runaway expansion of weedy algae in a near-future ocean’

Differential photosynthetic response of a green tide alga Ulva linza to ultraviolet radiation, under short‐ and long‐term ocean acidification regimes

Both ocean acidification (OA) and solar ultraviolet (UV) radiation can bring about changes in macroalgal physiological performance. However, macroalgal responses to UV radiation when acclimatized to OA under different time scales are rare. Here, we investigate the response of Ulva linza, a green tide alga, to UV radiation in the form of photosynthetically active radiation (PAR) or PAB (PAR+UVA+UVB) radiation. Radiation exposures were assessed following long‐term (from spore to adult stage, 1 month) and short‐term (adult stage, 1 week) OA treatments. Results showed that increased CO2 decreased the damage rate (k) and repair rate (r) of thalli grown under short‐term OA conditions with PAB treatment, the ratio of r:k was not altered. Following long‐term OA conditions, r was not affected, although k was increased in thalli following PAB treatment, resulting in a reduced ratio of r:k. The relative level of UV inhibition increased and UV‐absorbing compounds decreased when algae were cultured under long‐term OA conditions. The recovery rate of thalli was enhanced when grown under long‐term OA after UV radiation treatment. These results show that blooming algae may be more sensitive to UV radiation in marine environments, but it can develop effective mechanisms to offset the negative effects, reflecting acclimation to long‐term OA conditions.

Continue reading ‘Differential photosynthetic response of a green tide alga Ulva linza to ultraviolet radiation, under short‐ and long‐term ocean acidification regimes’

Shifts in seawater chemistry disrupt trophic links within a simple shoreline food web

Marine intertidal systems have long served as focal environments for ecological research, yet these environments are changing due to the entry of human-produced carbon dioxide into seawater, which causes ‘ocean acidification’ (OA). One component of OA is a decline in seawater pH, an alteration known to disrupt organism behaviors underlying predator–prey interactions. To date, however, studies examining OA’s effects on feeding relationships consider predominantly simple direct interactions between consumers and their food sources. Here, we extended these established approaches to test how decreased seawater pH might alter cascading effects that span tiered linkages in trophic networks. We employed a model shoreline food web incorporating a sea star predator (Leptasterias hexactis), an herbivorous snail prey (Tegula funebralis), and a common macroalgal resource for the prey (Mazzaella flaccida). Results demonstrate direct negative effects of low pH on anti-predator behavior of snails, but also weakened indirect interactions, driven by increased snail consumption of macroalgae even as sea stars ate more snails. This latter outcome arose because low pH induced ‘foolhardy’ behaviors in snails, whereby their flight responses were supplanted by other activities that allowed for foraging. These findings highlight the potential for human-induced changes in seawater chemistry to perturb prey behaviors and trophic dynamics with accompanying community-level consequences.

Continue reading ‘Shifts in seawater chemistry disrupt trophic links within a simple shoreline food web’

Effects of increased CO2 and temperature on the physiological characteristics of the golden tide blooming macroalgae Sargassum horneri in the Yellow Sea, China

Highlights

• Sargassum horneri was benefit from moderately elevated temperature.

• S. horneri showed insensitive to CO2 enrichment.

• Elevated temperature rather than CO2 enrichment will enhance S. horneri bloom.

Abstract

The golden tide, caused by the brown algae Sargassum horneri, exerts severe influences on the Pyropia aquaculture of Jiangsu coast, China. To study the outbreak of the golden tide in response to increasing greenhouse gas emissions, S. horneri was cultured under four conditions: ambient condition (10 °C, 400 μatm), elevated temperature condition (14 °C, 400 μatm), elevated CO2 level (10 °C, 1000 μatm), and potential greenhouse condition (14 °C, 1000 μatm). The growth, photosynthetic performances, and inorganic carbon affinity of S. horneri were studied. The results showed that elevated temperature exerted a more pronounced positive influence on S. horneri growth, photosynthesis, and carbon assimilation than CO2 enrichment. The growth of S. horneri was significantly improved by moderately elevated temperatures, especially under concurrently elevated CO2 levels. This suggests that the greenhouse effect will benefit growth and carbon sequestration of S. horneri, which may enhance the frequency and scale of golden tides.

Continue reading ‘Effects of increased CO2 and temperature on the physiological characteristics of the golden tide blooming macroalgae Sargassum horneri in the Yellow Sea, China’

Future CO2-induced ocean acidification enhances resilience of a green tide alga to low-salinity stress

To understand how Ulva species might respond to salinity stress during future ocean acidification we cultured a green tide alga Ulva linza at various salinities (control salinity, 30 PSU; medium salinity, 20 PSU; low salinity, 10 PSU) and CO2 concentrations (400 and 1000 ppmv) for over 30 days. The results showed that, under the low salinity conditions, the thalli could not complete its whole life cycle. The specific growth rate (SGR) of juvenile thalli decreased significantly with reduced salinity but increased with a rise in CO2. Compared to the control, medium salinity also decreased the SGR of adult thalli at low CO2 but did not affect it at high CO2. Similar patterns were also found in relative electron transport rate (rETR), non-photochemical quenching, saturating irradiance, and Chl b content. Although medium salinity reduced net photosynthetic rate and maximum rETR at each CO2 level, these negative effects were significantly alleviated at high CO2 levels. In addition, nitrate reductase activity was reduced by medium salinity but enhanced by high CO2. These findings indicate that future ocean acidification would enhance U. linza’s tolerance to low salinity stress and may thus facilitate the occurrence of green tides dominated by U. linza.

Continue reading ‘Future CO2-induced ocean acidification enhances resilience of a green tide alga to low-salinity stress’

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’


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OA-ICC HIGHLIGHTS

Ocean acidification in the IPCC AR5 WG II

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