Posts Tagged 'phytoplankton'

Clam feeding plasticity reduces herbivore vulnerability to ocean warming and acidification

Ocean warming and acidification affect species populations, but how interactions within communities are affected and how this translates into ecosystem functioning and resilience remain poorly understood. Here we demonstrate that experimental ocean warming and acidification significantly alters the interaction network among porewater nutrients, primary producers, herbivores and burrowing invertebrates in a seafloor sediment community, and is linked to behavioural plasticity in the clam Scrobicularia plana. Warming and acidification induced a shift in the clam’s feeding mode from predominantly suspension feeding under ambient conditions to deposit feeding with cascading effects on nutrient supply to primary producers. Surface-dwelling invertebrates were more tolerant to warming and acidification in the presence of S. plana, most probably due to the stimulatory effect of the clam on their microalgal food resources. This study demonstrates that predictions of population resilience to climate change require consideration of non-lethal effects such as behavioural changes of key species.

Continue reading ‘Clam feeding plasticity reduces herbivore vulnerability to ocean warming and acidification’

Climate change and harmful benthic microalgae

Highlights

• Global SST increases of 0.4–1.4 °C by 2055 will promote growth rates of many BHABs.

• Steep declines in growth are expected in areas where temperatures exceed 31 °C.

• Migration to deeper, cooler habitats may provide protection from high temperatures.

• Latitudinal range extensions to both the north and the south are expected.

• Changes in salinity, pH, light are secondary to temperature in regulating BHABs.

• Sentinel sites recommended for long-term monitoring to detect range extensions.

Abstract

Sea surface temperatures in the world’s oceans are projected to warm by 0.4–1.4 °C by mid twenty-first century causing many tropical and sub-tropical harmful dinoflagellate genera like Gambierdiscus, Fukuyoa and Ostreopsis (benthic harmful algal bloom species, BHABs) to exhibit higher growth rates over much of their current geographic range, resulting in higher population densities. The primary exception to this trend will be in the tropics where temperatures exceed species-specific upper thermal tolerances (30–31 °C) beyond which growth slows significantly. As surface waters warm, migration to deeper habitats is expected to provide refuge. Range extensions of several degrees of latitude also are anticipated, but only where species-specific habitat requirements can be met (e.g., temperature, suitable substrate, low turbulence, light, salinity, pH). The current understanding of habitat requirements that determine species distributions are reviewed to provide fuller understanding of how individual species will respond to climate change from the present to 2055 while addressing the paucity of information on environmental factors controlling small-scale distribution in localized habitats. Based on the available information, we hypothesized how complex environmental interactions can influence abundance and potential range extensions of BHAB species in different biogeographic regions and identify sentinel sites appropriate for long-term monitoring programs to detect range extensions and reduce human health risks.

Continue reading ‘Climate change and harmful benthic microalgae’

Algal density mediates the photosynthetic responses of a marine macroalga Ulva conglobata (Chlorophyta) to temperature and pH changes

Highlights

• Increased algal densities reduce photosynthesis and respiration of Ulva conglobata.

• Algal density mediates the interactive effect of increased temperature and lowered pH.

• Altered temperature and pH oppositely affect photosynthetic rate and saturation light.

Abstract

Growing of macroalgae increases their biomass densities in natural habitats. To explore how the altered algal density impacts their photosynthetic responses to changes of environmental factors, we compared the photosynthesis versus irradiance characteristics of a marine green macroalga Ulva conglobata under low [2.0 g fresh weight (FW) L−1], medium (6.0 g FW L−1) and high biomass densities (12.0 g FW L−1), and under a matrix of temperatures (20, 25, 30 and 35 °C) and pH levels (7.8, 8.2 and 8.6). Increased algal densities decreased the photosynthetic O2 evolution rate among all combined temperature and pH treatments, in parallel with the decrease of light-utilizing efficiency (α, the initial slope) and maximum photosynthetic rate (Pmax) and the increase of light saturation point (EK). Rising temperature interacted with lowered pH to increase the α under low but not under high algal densities. Rising temperature increased the Pmax and decreased the EK under low algal density, but not under high density. Lowered pH promoted the Pmax and EK under all three algal densities. The increased temperature enhanced the dark respiration (Rd) and light compensation point (EC), while the altered pH showed a limited effect. Moreover, the increased algal density reduced the Rd, and had a limited effect on the EC. In addition, our results indicate that changing algal densities caused the complex photophysiological changes in responses to the temperature and pH changes, and these complex responses resolved into a close relation between Rd and Pmax across the matrix of temperatures and pH levels.

Continue reading ‘Algal density mediates the photosynthetic responses of a marine macroalga Ulva conglobata (Chlorophyta) to temperature and pH changes’

Combined effects of CO2 level, light intensity, and nutrient availability on the coccolithophore Emiliania huxleyi

Continuous accumulation of fossil CO2 in the atmosphere and increasingly dissolved CO2 in seawater leads to ocean acidification (OA), which is known to affect phytoplankton physiology directly and/or indirectly. Since increasing attention has been paid to the effects of OA under the influences of multiple drivers, in this study, we investigated effects of elevated CO2 concentration under different levels of light and nutrients on growth rate, particulate organic (POC) and inorganic (PIC) carbon quotas of the coccolithophorid Emiliania huxleyi. We found that OA treatment (pH 7.84, CO2 = 920 μatm) reduced the maximum growth rate at all levels of the nutrients tested, and exacerbated photo-inhibition of growth rate under reduced availability of phosphate (from 10.5 to 0.4 μmol l−1). Low nutrient levels, especially lower nitrate concentration (8.8 μmol l−1 compared with 101 μmol l−1), decreased maximum growth rates. Nevertheless, the reduced levels of nutrients increased the maximum PIC production rate. Decreased availability of nutrients influenced growth, POC and PIC quotas more than changes in CO2 concentrations. Our results suggest that reduced nutrient availability due to reduced upward advective supply because of ocean warming may partially counteract the negative effects of OA on calcification of the coccolithophorid.

Continue reading ‘Combined effects of CO2 level, light intensity, and nutrient availability on the coccolithophore Emiliania huxleyi’

Coccolith morphological and assemblage responses to dissolution in the recent sediments of the East China Sea

Highlights

• Gephyrocapsa coccoliths (>3 μm) got thinner and lighter in response to dissolution in the sediments of East China Sea.

• The acidification experiments showed selective dissolution of the four dominant coccolith species/genera.

• Coccolith morphological parameters can be used as indices to evaluate coccolith dissolution in sediments.

Abstract

Evaluating carbonate dissolution in deep sea sediments is of key importance in understanding the variation of the carbonate compensation depth and the ocean carbon cycle in the geological past. Since coccoliths are one of the main contributors to oceanic CaCO3, their dissolution and preservation degrees in sediments can be a useful indicator for deep sea carbonate chemistry. Varying coccolith preservation conditions have been found due to dissolution caused by organic matter degradation in the recent surface sediments of the East China Sea, which provides a good basis for the study of coccolith morphological and assemblage responses to dissolution. We measured the coccolith weight, thickness, and length of Gephyrocapsa spp. (>3 μm) using a circularly polarized light microscope. It has been found that Gephyrocapsa spp. (>3 μm) coccoliths become thinner and lighter in response to dissolution, and coccolith assemblages are also altered in poorly preserved sediments. This phenomenon was confirmed by an acidification experiment on a sediment sample, which also showed that coccoliths became thinner and lighter under increasingly acidified conditions. There is selective dissolution, i.e., Emiliania huxleyi coccoliths are most dissolution-prone, followed by Gephyrocapsa spp. (3 μm), and Helicosphaera spp.. Coccolith morphological parameters can be used to quantitatively evaluate coccolith preservation and dissolution in sediment samples. We suggest that using size-normalized weight, a mean coccolith weight loss of ~30–50% can be assigned to moderate-poor preservation for coccoliths, as reflected by the measured coccolith morphological changes in the surface sediments and in the acidification experiment.

Continue reading ‘Coccolith morphological and assemblage responses to dissolution in the recent sediments of the East China Sea’

Harmful algal traits and bloom dynamics under climate change

Anthropogenic activities have caused a rapid increase in greenhouse gas emissions, such as CO2, which also led to a rise in global mean temperatures. Next to changes in climate conditions, anthropogenic perturbations also include the release of vast amounts of nutrients into coastal waters. These global alterations in environmental conditions have severe consequences for marine phytoplankton species, which are responsible for half of all primary production on Earth. Some phytoplankton species, however, can become a nuisance for the environment through the formation of dense harmful algal blooms (HABs). Especially HABs caused by dinoflagellates can have far-reaching consequences, as they are known to produce potent toxins that may be detrimental for the environmental and human health. The aims of this thesis were to assess the environmental drivers behind HAB development, determine the extent of intraspecific trait variation in HAB populations, and to evaluate the impact of climate change on HAB proliferations. Although nutrient dynamics play a substantial role in the emergence of HABs, other biotic and abiotic factors may strongly modulate the magnitude and duration of bloom events. For instance, low salinities due to excessive rainfall and increased wind speeds led to significant reductions in Alexandrium ostenfeldii bloom densities in the Netherlands, while highest population densities generally corresponded to high temperatures, low N:P ratios and low grazer densities. This demonstrates the important role of the combination of physical, chemical and biological factors in the development of HABs. An important factor that may contribute to the success of HAB species is the considerable intraspecific trait variation that can be found within populations. Strains derived from two A. ostenfeldii populations expressed substantial phenotypic variation in functional traits, such as growth rate, cell size, toxin production, allelopathic potency, elemental stoichiometry, and nitrogen uptake kinetics. This observed high trait variation may facilitate development and resilience of HABs, especially under changing environmental conditions. Different environmental variables may also influence phenotypic trait expression. For instance, elevated CO2 concentrations caused an increase in growth rates in three A. ostenfeldii strains derived from the same population. Phenotypic plasticity in trait responses, and variation therein, towards environmental stressors may be important for species adaptation, especially on the short term. Although substantial variation in trait responses of HAB species towards climate change variables can be found, I did identify clear trends when combining data from a multitude of culture experiments. Specifically, HAB growth rates showed an overall increase in response to elevated pCO2. This may represent a competitive advantage for HAB species in future waters, particularly since a similar trend was not found for other phytoplankton species. In addition, elevated temperatures also led to an increase in growth rates, but only for HAB species isolated at higher latitudes. Since the success of HAB species ultimately depends on growth rates, these findings warn for a greater potential of HAB development in future oceans, particularly in temperate regions. Overall, the results of this thesis contribute to a better understanding of dinoflagellate HAB dynamics and the potential impacts of climate change on HAB proliferations. show less

Continue reading ‘Harmful algal traits and bloom dynamics under climate change’

Climate change and harmful algal blooms: insights and perspective

Highlights

• Climate change is transforming aquatic ecosystems.

• Coastal waters have experienced progressive warming, acidification, and deoxygenation.

• The impacts of harmful algal blooms (HABs) on coastal systems have increased in recent decades.

• HABs display an expansion in range and frequency in response to climatic and non-climatic drivers.

• This Special Issue considers linkages between climate change and HABs.

Abstract

Climate change is transforming aquatic ecosystems. Coastal waters have experienced progressive warming, acidification, and deoxygenation that will intensify this century. At the same time, there is a scientific consensus that the public health, recreation, tourism, fishery, aquaculture, and ecosystem impacts from harmful algal blooms (HABs) have all increased over the past several decades. The extent to which climate change is intensifying these HABs is not fully clear, but there has been a wealth of research on this topic this century alone. Indeed, the United Nations’ Intergovernmental Panel on Climate Change’s (IPCC) Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) approved in September 2019 was the first IPCC report to directly link HABs to climate change. In the Summary for Policy Makers, the report made the following declarations with “high confidence”:

• Harmful algal blooms display range expansion and increased frequency in coastal areas since the 1980s in response to both climatic and non-climatic drivers such as increased riverine nutrients run-off.

• The observed trends in harmful algal blooms are attributed partly to the effects of ocean warming, marine heatwaves, oxygen loss, eutrophication and pollution.

• Harmful algal blooms have had negative impacts on food security, tourism, local economy, and human health.

In addition, the report specifically outlines a series of linkages between heat waves and HABs. These statements about HABs and climate change and the high levels of confidence ascribed to them provides clear evidence that the field of HABs and climate change has matured and has, perhaps, reached a first plateau of certainty. While there are well-documented global trends in HABs being promoted by human activity, including climate change, individual events are driven by local, regional, and global drivers, making it critical to carefully evaluate the conditions and responses at appropriate scales. It is within this context that the first Special Issue on Climate Change and Harmful Algal Blooms is published in Harmful Algae.

Continue reading ‘Climate change and harmful algal blooms: insights and perspective’


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