#112: Episode 3—Diatoms' dance with Climate Change
From the Mediterranean to the Arctic, a 300-Year Tale of Resilience and Adaptation
Welcome back, ocean explorers! In our previous episodes, we journeyed through the microscopic world of diatoms, marveling at their intricate glass houses and discovering their outsized role in shaping our planet's climate and ecosystems. We've seen how these tiny algae are the unsung heroes of our oceans, responsible for every fifth breath we take and playing a crucial part in the global carbon cycle.
But as we delve deeper into the world of diatoms, we find that their story is far from simple. These microscopic marvels are facing a myriad of challenges in our rapidly changing world. From warming waters and acidifying seas to altered nutrient dynamics and extreme weather events, diatoms are on the front lines of global environmental change.
In today's episode, we're going to explore how these tiny titans are coping with monumental challenges. We'll journey from the depths of ancient lakes to the vast expanses of the open ocean, uncovering the complex and sometimes surprising ways diatoms are responding to our changing planet.
Prepare to be amazed by the resilience of these microscopic organisms, concerned by the threats they face, and inspired by what their story tells us about the future of our blue planet. So grab your imaginary scuba gear, and let's dive into the turbulent waters of diatoms under threat!
Watch out for a bonus section, if you make it to the end.
Section 1: Diatoms in a Changing World
Our journey begins in the Mediterranean region, a hotspot for climate change impacts. Here, in the Pyrenean mountain range, lies Lake Montcortès—a natural time capsule that has been recording the story of diatoms for centuries. This lake, with its annually layered sediments, offers us a window into the past, present, and potential future of diatom communities.
But Lake Montcortès is just one piece of a global puzzle. As we explore the challenges facing diatoms, we'll venture far beyond this mountain lake. We'll traverse the vast expanses of the North Atlantic, where shifting ocean currents and rising temperatures are reshaping diatom communities. We'll dive into the depths of the Pacific, where increasing carbon dioxide levels are altering the very chemistry of the water diatoms call home.
Along the way, we'll grapple with some profound questions: How do tiny organisms cope with global-scale changes? Can diatoms adapt quickly enough to keep pace with our rapidly changing climate? And what do the struggles of these microscopic algae mean for the health of our oceans and the future of our planet?
The challenges facing diatoms are as diverse as they are daunting. In the Mediterranean, including our case study site of Lake Montcortès, climate projections paint a stark picture. By the end of the 21st century, temperatures could increase by up to 5°C, and seasonal rainfall may decrease by 15–25% (Barrera, 2011; Calbó et al., 2010; IPCC, 2021). For diatoms, these aren't just numbers—they represent a fundamental reshaping of their watery world.
But it's not just about warming and drying. The very chemistry of our oceans is changing. Since the Industrial Revolution, our oceans have absorbed about 30% of the CO₂ we've emitted into the atmosphere, leading to a decrease in surface ocean pH by approximately 0.1 units—equivalent to a 30% increase in acidity (Gattuso et al., 2015). This process of ocean acidification is like changing the recipe of the seawater soup that diatoms inhabit.
And let's not forget about extreme events. From heatwaves to storms, the weather patterns that influence diatom communities are becoming more erratic and intense. In Lake Montcortès, for instance, extreme rainfall events have left their mark in the lake's sediments, coinciding with dramatic shifts in diatom communities.
As we embark on this exploration of diatoms under threat, we'll see that their story is not a simple tale of decline. It's a complex narrative of resilience, adaptation, and unexpected responses. Some species struggle while others thrive. Communities shift and reorganize. And through it all, these microscopic algae continue to play their crucial role in Earth's ecosystems and climate.
So join us as we dive into the challenges facing the invisible forests of our seas and lakes. It's a story of resilience, adaptation, and the delicate balance of life in a changing world. And who knows? By the end, you might find yourself rooting for these microscopic underdogs as much as we do.
Section 2: Climate Change Impacts
2.1 Thermal Stress and Warming Trends
The warming of Earth's waters stands at the forefront of challenges facing diatom communities. To grasp the magnitude of this change, consider that ocean temperatures have risen by approximately 0.13°C per decade since 1900 (Trombetta et al., 2019). While this increment might appear modest to us land-dwellers, for diatoms and other marine microorganisms, it represents a significant shift in their environmental conditions.
The impacts of this warming trend on diatoms are multifaceted:
Metabolic alterations: Higher temperatures generally accelerate metabolic rates in microorganisms. For diatoms, this can potentially increase growth rates but also elevate resource demands, potentially leading to faster nutrient depletion in their immediate environment.
Enhanced stratification: Warming enhances water column stratification, fundamentally altering nutrient distribution patterns. Behrenfeld et al. (2006) demonstrated that increased stratification in the North Atlantic led to a 7% decline in marine net primary production between 1999 and 2004. This reduction in vertical mixing can significantly impact diatom communities that rely on nutrient upwelling.
Geographic redistribution: Many diatom species are exhibiting poleward migration, seeking cooler waters that more closely match their thermal preferences. A study by Barton et al. (2016) revealed that phytoplankton communities in the North Atlantic have shifted northward by up to 23° of latitude over the past 50 years - a dramatic redistribution of these crucial primary producers.
Community composition shifts: Warming tends to favor smaller phytoplankton species, potentially altering the structure of marine food webs. This shift towards smaller species could have cascading effects throughout the ecosystem, as different diatom species have varying nutritional values for the organisms that consume them.
The high-resolution study of Lake Montcortès in the central Pyrenees provides a fascinating window into these changes. From the 18th century until the 1970s, the planktonic diatom community showed remarkable resilience, with the species Cyclotella cyclopuncta maintaining dominance despite significant environmental stressors. However, the 20th century brought notable shifts. As global temperatures increased, other centric diatom species gained prominence in the community. This coincided with a period of natural reoligotrophication following the cessation of human activities like hemp retting.
These changes in diatom communities ripple through marine and freshwater ecosystems. Fish populations that rely on diatoms for sustenance are adjusting their distributions, creating a cascading effect up the food chain. Moreover, shifts in diatom communities influence the ocean's capacity for carbon sequestration, potentially feeding back into the climate system.
The implications of these changes extend beyond marine ecology. Diatoms play a crucial role in global biogeochemical cycles, particularly the carbon and silicon cycles. Alterations in diatom populations could have far-reaching consequences for atmospheric CO2 levels and, by extension, global climate regulation.
The impacts of ocean warming on diatoms are not uniform across species and sizes. A study on three Thalassiosira species of different sizes revealed that:
The optimum growth temperature (Topt), maximum growth rates (μmax), and thermal niche width (w) decreased by 16%, 48%, and 29% per order of magnitude in cell size increment, respectively.
Cell volumes diminished significantly and linearly with increased temperatures, by 22% in T. punctigera, 17% in T. weissflogii, and 11% in T. pseudonana.
Cellular biogenic silica (BSi) contents increased with rising temperature until the optimum growth temperature.
These findings suggest that warming oceans may favor smaller diatom species, potentially altering the structure of marine food webs and biogeochemical cycles.
2.2 Changes in Precipitation Patterns
The impact of climate change on diatom communities extends beyond rising temperatures. Shifts in precipitation patterns, particularly in regions like the Mediterranean, are emerging as a significant force shaping aquatic ecosystems.
In the Mediterranean region, climate projections paint a picture of increasing aridity. By the end of the 21st century, seasonal rainfall is expected to decrease by 15-25% (Barrera, 2011; Calbó et al., 2010; IPCC, 2021). This trend towards drier conditions is not uniform across seasons or locations, adding complexity to the challenges faced by aquatic ecosystems.
The effects of changing precipitation patterns on diatom communities are multifaceted:
Altered Nutrient Influx: Rainfall is a crucial mechanism for delivering nutrients from terrestrial environments to aquatic systems. Changes in precipitation can significantly alter the timing and quantity of nutrient inputs. In periods of reduced rainfall, nutrient limitation may become more pronounced, potentially favoring diatom species that are more efficient at nutrient uptake or those with lower nutrient requirements.
Fluctuating Water Levels: In lakes and other enclosed water bodies, reduced precipitation can lead to lower water levels. This can result in concentration of nutrients and other dissolved substances, potentially altering the competitive dynamics among diatom species.
Changes in Mixing Regimes: Precipitation events can influence the mixing of water columns, particularly in stratified lakes. A reduction in the frequency of these events could lead to more stable stratification, potentially benefiting species adapted to specific layers of the water column.
Increased Importance of Extreme Events: While overall precipitation is projected to decrease, the frequency and intensity of extreme rainfall events are expected to increase in many regions. In the Mediterranean sector of the Iberian Peninsula, an increase in the frequency of high and very high precipitation events has been detected (Serrano-Notivoli et al., 2018). These pulse disturbances can have significant impacts on diatom communities.
The Lake Montcortès study provides a compelling example of how changes in precipitation patterns, particularly extreme events, can influence diatom communities.
From the 1970s onward, the planktonic diatom community experienced notable shifts, with Cyclotella ocellata beginning to compete with C. cyclopuncta for dominance. Intriguingly, these shifts coincided with several extreme rainfall events.
Particularly noteworthy were the events in November 1982 with >200 mm over 3 days and in August 1996 with >220 mm over 125 hours (García-Ruiz et al., 1996). These extreme precipitation events were visible in the lake's sediment record and appeared to trigger significant changes in the diatom community structure. The abrupt changes in the planktonic diatom community were likely related to alterations in key ecological factors such as water temperature, mixing depth, light penetration, and nutrient input.
These findings highlight the complex interplay between gradual climate change and extreme events in shaping aquatic ecosystems. While diatom communities have shown remarkable resilience to long-term environmental changes, pulse disturbances in the form of extreme precipitation events can potentially push these communities towards alternative stable states.
As we look to the future, the projected intensification of heavy rainfall events in the Mediterranean region by 10 to 20% in all seasons except summer (Lange, 2020; United Nations Environment Programme/Mediterranean Action Plan and Plan Bleu, 2020) suggests that these pulse disturbances may become increasingly important in shaping diatom communities and, by extension, aquatic ecosystems as a whole.
Understanding these precipitation-driven dynamics is crucial for predicting and managing the future of our aquatic ecosystems in a changing climate. The story of diatoms and changing precipitation patterns serves as a powerful reminder of the intricate connections between climate, water, and life at the microscopic scale.
2.3: Extreme Weather Events.
While gradual changes in temperature and precipitation patterns pose significant challenges to diatom communities, the increasing frequency and intensity of extreme weather events add another layer of complexity to their environmental pressures. These episodic, high-intensity disturbances can have profound and sometimes unexpected impacts on aquatic ecosystems.
In the context of climate change, extreme weather events relevant to aquatic ecosystems include:
Intense Rainfall Events: As we've seen in the Mediterranean region, the frequency and intensity of heavy rainfall are projected to increase. These events can cause rapid changes in water chemistry, nutrient influx, and physical disturbance of aquatic habitats.
Prolonged Droughts: Extended periods without rainfall can lead to reduced water levels, increased concentration of dissolved substances, and altered mixing regimes in water bodies.
Heat Waves: Periods of exceptionally high temperatures can cause rapid warming of surface waters, potentially leading to thermal stress for diatom species and enhancing stratification.
Storms and High Winds: These can cause mixing of stratified waters and resuspension of sediments, altering light availability and nutrient distribution.
The Lake Montcortès study provides a compelling case study of how extreme weather events, particularly intense rainfall, can impact diatom communities. The study revealed that samples showing Cyclotella ocellata's prevalence coincided with extraordinary rainfall events, particularly in 1992 CE (23.2% higher precipitation than the reference period) and the extreme events of 1982 and 1996 (Corella et al., 2014).
These extreme rainfall events appeared to trigger significant shifts in the diatom community structure. The mechanism behind these shifts likely involves a complex interplay of factors:
Altered Light Conditions: Intense rainfall can increase turbidity through runoff and sediment resuspension, potentially favoring species better adapted to low-light conditions.
Nutrient Pulses: Extreme rainfall events can lead to sudden influxes of nutrients from the catchment area, potentially benefiting opportunistic species.
Physical Disturbance: The physical force of extreme rainfall and associated water movements can disrupt existing community structures, creating opportunities for rapid colonizers.
Changes in Mixing Depth: Intense rainfall can alter the mixing depth of lakes, affecting nutrient distribution and potentially breaking down or reinforcing stratification.
The study suggests that C. ocellata might have been able to outcompete C. cyclopuncta during these events due to its ability to thrive under conditions of increased nutrient availability and reduced light (Malik and Saros, 2016). This highlights how extreme events can create windows of opportunity for certain species, potentially leading to long-lasting changes in community composition.
Importantly, the impact of these extreme events seems to be amplified by the backdrop of gradual warming. The study hypothesizes that the gradual impact of global warming on environmental conditions may have created the necessary antecedent conditions to destabilize the diatom community, making it more susceptible to shifts triggered by extreme events.
These findings align with broader ecological theory suggesting that extreme events can serve as "ecosystem reset buttons," potentially pushing systems towards alternative stable states (Scheffer et al., 2001). In the case of diatom communities, these events may be creating opportunities for species turnover and reshaping competitive dynamics.
As climate change progresses, understanding the impacts of extreme weather events on diatom communities becomes increasingly crucial. These events may serve as important drivers of ecological change, potentially accelerating shifts that might occur more gradually under steadier environmental change. Moreover, the potential for these events to trigger long-lasting changes in community composition underscores the need for long-term monitoring and high-resolution paleoecological studies to fully capture their impacts.
For ecosystem managers and policymakers, these findings highlight the importance of considering not just gradual climate trends, but also the increasing frequency and intensity of extreme events in strategies for aquatic ecosystem conservation and management.
2.4 Case Study: Lake Montcortès
The high-resolution study of Lake Montcortès in the central Pyrenees provides an invaluable window into the long-term responses of diatom communities to climate change and extreme events. This karstic lake, with its annually laminated sediments, offers a unique opportunity to examine diatom community dynamics at a subdecadal scale over the past 300 years.
Key features of the Lake Montcortès study include:
Temporal Scope: The study spans from ca. 1716 until 2013 CE, encompassing the end of the Little Ice Age, the transition to the industrial and postindustrial eras, and the recent period of accelerated global warming.
High Resolution: The varved nature of the sediments allowed for subdecadal analysis, providing a detailed chronology of changes in diatom communities.
Multi-proxy Approach: The study incorporated various environmental proxies, including pollen, fungal spores, charcoal, and elemental composition, allowing for a comprehensive understanding of environmental changes.
Key findings from the Lake Montcortès study reveal a complex narrative of diatom community responses to environmental change:
Long-term Resilience (1716-1971 CE): For over two centuries, the diatom community, dominated by Cyclotella cyclopuncta, showed remarkable stability despite significant environmental stressors. This period included strong cooling episodes, droughts, and intense human activities such as hemp retting.
20th Century Shifts: As global temperatures increased, other centric diatom species began to gain prominence. This shift coincided with a period of natural reoligotrophication following the cessation of hemp retting activities.
Impact of Extreme Rainfall Events (post-1970s): From the 1970s onward, a notable change occurred in the planktonic diatom community. Cyclotella ocellata began to compete with C. cyclopuncta for dominance. This shift coincided with several extreme rainfall events, particularly in 1982 and 1996.
Differential Responses: Interestingly, while the planktonic diatom community showed clear responses to these climatic shifts, the benthic diatom community did not reflect comparable changes under the same climatic and environmental variables.
Alternative Stable States: The study suggests the existence of an attraction domain with three alternative states in the planktonic diatom community: a) C. cyclopuncta as the only significant planktonic and dominant species b) Dominance of C. ocellata in coexistence with C. cyclopuncta c) Dominance of C. cyclopuncta in coexistence with C. ocellata
The Lake Montcortès study provides several crucial insights:
Resilience and Tipping Points: The long-term stability followed by abrupt shifts suggests that diatom communities can be highly resilient to gradual changes but may have tipping points when faced with extreme events.
Importance of Extreme Events: The coincidence of community shifts with extreme rainfall events highlights the potential for these pulse disturbances to trigger long-lasting changes in ecosystem structure.
Context Dependency: The study suggests that the impacts of extreme events may be amplified by the context of gradual warming, underscoring the complex interplay between long-term trends and short-term disturbances.
Differential Vulnerability: The contrasting responses of planktonic and benthic communities highlight the varying vulnerabilities of different components of the aquatic ecosystem.
The Lake Montcortès study serves as a powerful example of the value of high-resolution, long-term paleoecological studies in understanding ecosystem responses to climate change. It underscores the need to consider both gradual changes and extreme events when predicting future ecological trajectories and developing management strategies for aquatic ecosystems.
As we face an uncertain climatic future, studies like this provide crucial insights into the potential responses of aquatic ecosystems to ongoing environmental change, offering valuable lessons for conservation and management in a changing world.
Section 3: Ocean Acidification
3.1 Mechanisms and Trends
Ocean acidification, often called the "other CO2 problem," represents a significant threat to marine ecosystems, including diatom communities. As atmospheric CO2 levels rise due to human activities, the oceans absorb a substantial portion of this excess CO2, leading to changes in seawater chemistry.
The basic mechanism of ocean acidification is as follows:
CO2 dissolves in seawater
It reacts with water to form carbonic acid (H2CO3)
Carbonic acid dissociates, releasing hydrogen ions (H+)
This increase in H+ lowers the pH of the water, making it more acidic
Since the Industrial Revolution, our oceans have absorbed about 30% of the CO2 we've pumped into the atmosphere. This has resulted in a decrease in surface ocean pH by approximately 0.1 units, equivalent to a 30% increase in acidity. If current trends continue, projections suggest that by 2100, ocean pH could decrease by an additional 0.3-0.4 units (Orr et al., 2005).
For diatoms, these changes in ocean chemistry pose several challenges:
3.2 Impact on Diatom Shell Formation
Diatoms construct their intricate glass-like shells, or frustules, from silica. While silica formation is not directly impacted by pH in the same way as calcium carbonate structures (like those of corals or coccolithophores), ocean acidification can still affect diatom shell formation in several ways:
Altered Silica Dissolution Rates: In more acidic conditions, the dissolution rate of biogenic silica may increase (Petrou et al., 2019). This could potentially lead to thinner or weaker frustules.
Changes in Trace Metal Availability: Ocean acidification can alter the availability of trace metals that diatoms need for various physiological processes, including frustule formation.
Energy Allocation: In more acidic conditions, diatoms may need to expend more energy maintaining their internal pH, potentially leaving less energy available for shell formation.
3.3 Species-Specific Responses and Adaptation
The impacts of ocean acidification on diatoms are not uniform across species. Some diatoms show remarkable resilience, while others are more vulnerable. This variability in response could lead to significant shifts in diatom community composition as ocean acidification progresses.
For instance, a study by Petrou et al. (2019) in Nature Communications found that under increased ocean acidity, some diatom species produced thinner, weaker shells. They observed a 40% reduction in silica content in some cases. This is akin to trying to build a fortress out of paper instead of stone.
However, it's not all doom and gloom. Some diatom species seem to be adapting to these new conditions. A fascinating study by Wolf et al. (2018) in Nature Climate Change found that certain diatoms could actually increase their growth rates in more acidic conditions. This adaptability showcases the potential resilience of some diatom species in the face of changing ocean chemistry.
The Lake Montcortès study, while focused on a freshwater system, provides insights into how diatom communities might respond to changing environmental conditions over long time scales. The observed shifts in species dominance and the emergence of alternative stable states in response to environmental changes could have parallels in marine systems facing acidification.
These varied responses to ocean acidification could lead to significant shifts in diatom community composition in the coming decades. Such changes could have far-reaching consequences for marine food webs and biogeochemical cycles, given the crucial role diatoms play in these systems.
Recent studies have shown that the effects of ocean acidification on diatoms can be complex and species-specific. For instance:
The large chain-forming diatom Biddulphia biddulphiana greatly increases in abundance as pCO2 increases along natural seawater CO2 gradients in the north Pacific Ocean.
This increase in abundance can lead to significant changes in benthic communities, potentially simplifying and homogenizing coastal food-web structures.
However, the effects of acidification can vary depending on nutrient availability. Under N-limited conditions, the carbon to nitrogen ratios in Phaeodactylum tricornutum, Thalassiosira pseudonana, and Thalassiosira weissflogii were raised by acidification treatment.
Section 4: Human Activities and Catchment Alteration.
4.1 Eutrophication and Nutrient Loading
Human activities have profoundly altered the nutrient dynamics of aquatic ecosystems, with significant consequences for diatom communities. Eutrophication, the excessive enrichment of water bodies with nutrients, particularly nitrogen and phosphorus, is a widespread issue in both freshwater and marine environments.
In the context of Lake Montcortès, we see a clear example of how human activities can impact nutrient levels and, consequently, diatom communities. The study revealed that during the 18th and 19th centuries, intense hemp retting activities in the lake led to nutrient enrichment and enhanced turbidity (Rull et al., 2022; Trapote et al., 2018a). This period of increased nutrient input coincided with shifts in the diatom community, particularly favoring species like Fragilaria tenera, known to be an indicator of enriched nutrient conditions in mountain lakes (Sheibley et al., 2014).
Key impacts of eutrophication on diatom communities include:
Shifts in Species Composition: Nutrient enrichment often favors fast-growing, opportunistic species that can quickly capitalize on abundant resources.
Altered Competitive Dynamics: Changes in nutrient ratios (e.g., N:P:Si) can shift competitive advantages among different diatom species.
Increased Biomass: In some cases, eutrophication can lead to diatom blooms, which can have cascading effects on the ecosystem.
Reduced Water Clarity: Increased algal growth can reduce light penetration, potentially disadvantaging some diatom species.
4.2 Land Use Changes
Changes in land use within a water body's catchment area can significantly impact aquatic ecosystems. The Lake Montcortès study provides insights into how land use changes can affect diatom communities over long time scales.
Key findings from the Lake Montcortès study related to land use changes include:
Agricultural Impacts: The study tracked changes in pollen from crops like Secale (rye) and other cereals, providing a record of agricultural activity in the catchment area (Trapote, 2019). These changes in land use likely affected nutrient inputs to the lake.
Livestock Impacts: The presence of coprophilous fungal spores (e.g., Sporormiella) in the sediment record indicated changes in livestock presence in the catchment area (Trapote, 2019). This would have affected nutrient inputs and potentially erosion patterns.
Erosion and Runoff: Changes in land use, as indicated by variations in elemental composition (e.g., titanium levels) in the sediment, affected erosion and runoff patterns (Vegas-Vilarrúbia et al., 2018). This, in turn, influenced nutrient and sediment inputs to the lake.
4.3 Case Study: Hemp Retting in Lake Montcortès
The practice of hemp retting in Lake Montcortès provides a fascinating case study of how specific human activities can impact diatom communities. Hemp retting, the process of submerging hemp stalks in water to separate fibers, was a significant activity in the lake during the 18th and 19th centuries.
Key impacts of hemp retting on the lake ecosystem included:
Nutrient Enrichment: The decomposition of hemp stalks released nutrients into the water, leading to eutrophic conditions (Trapote et al., 2018a).
Increased Turbidity: The retting process likely increased water turbidity, affecting light penetration.
Diatom Community Shifts: During periods of intense hemp retting, there were notable shifts in diatom community composition, with species like Fragilaria tenera gaining prominence (Rull et al., 2022).
Legacy Effects: Even after hemp retting activities ceased, the impacts on the lake ecosystem persisted, influencing diatom community dynamics into the 20th century.
This case study highlights how specific human activities can have long-lasting impacts on aquatic ecosystems and diatom communities. It also underscores the value of paleolimnological studies in understanding the long-term consequences of human activities on aquatic ecosystems.
The insights from Lake Montcortès and other studies emphasize the need to consider both current and historical human activities when assessing and managing aquatic ecosystems. They also highlight the complex interplay between human activities, climate change, and ecological responses in shaping the future of these vital ecosystems.
Section 5: Diatom Responses and Adaptation.
5.1 Community Shifts and Species Competition
The Lake Montcortès study provides a detailed look at how diatom communities respond to environmental changes over long time scales. One of the most striking findings is the shift in community composition, particularly in the planktonic diatom community.
Key observations include:
Long-term Dominance: From the 18th century until the 1970s, the planktonic diatom community was dominated by Cyclotella cyclopuncta (Rull et al., 2022). This species showed remarkable resilience to various environmental stressors, including climate fluctuations and human activities like hemp retting.
20th Century Shifts: During the 20th century, other centric diatom species began to gain prominence in the community (Rull et al., 2022). This coincided with gradual warming and changes in nutrient dynamics following the cessation of hemp retting activities.
Competition Dynamics: From the 1970s onward, Cyclotella ocellata began to compete with C. cyclopuncta for dominance (Rull et al., 2022). This shift appeared to be linked to extreme rainfall events and ongoing climate change.
These community shifts demonstrate the complex ways in which diatoms respond to environmental changes. They highlight the potential for both gradual changes and abrupt shifts in community composition in response to environmental drivers.
5.2 Planktonic vs. Benthic Diatom Responses
An intriguing aspect of the Lake Montcortès study is the differential response observed between planktonic and benthic diatom communities:
Planktonic Community: Showed clear responses to climatic shifts and extreme events, with notable changes in species composition and dominance patterns.
Benthic Community: Did not reflect comparable changes under the same climatic and environmental variables (Rull et al., 2022).
This difference in response highlights the varying sensitivities of different diatom communities to environmental changes. It suggests that planktonic diatoms may be more responsive to water column changes, while benthic communities might be somewhat buffered by their association with substrates.
5.3 Long-term Resilience and Tipping Points
The Lake Montcortès study reveals a fascinating pattern of long-term resilience punctuated by apparent tipping points:
Resilience: For over two centuries (ca. 1716-1971 CE), the diatom community showed high structural persistence at the subdecadal scale (Rull et al., 2022). This demonstrates the ability of diatom communities to maintain their structure despite various environmental pressures.
Tipping Points: After the 1970s, the community experienced abrupt shifts, particularly in response to extreme rainfall events (Rull et al., 2022). These shifts suggest the existence of ecological thresholds beyond which the community structure changes dramatically.
Alternative Stable States: The study suggests the existence of an attraction domain with three alternative states in the planktonic diatom community (Rull et al., 2022): a) C. cyclopuncta as the only significant planktonic and dominant species b) Dominance of C. ocellata in coexistence with C. cyclopuncta c) Dominance of C. cyclopuncta in coexistence with C. ocellata
These observations align with ecological theories about resilience, alternative stable states, and regime shifts. They suggest that while diatom communities can be highly resilient to gradual changes, they may have tipping points when faced with extreme events or when gradual changes accumulate over time.
The adaptability of diatoms to changing conditions is also evident in their responses to ocean acidification. As mentioned earlier, some diatom species have shown the ability to increase their growth rates in more acidic conditions (Wolf et al., 2018). This adaptability could be crucial for the survival and continued ecological importance of diatoms in a changing world.
Understanding these patterns of resilience, tipping points, and adaptation is crucial for predicting how diatom communities - and the ecosystems they support - might respond to ongoing environmental changes. It underscores the importance of considering both long-term trends and short-term extreme events in ecological studies and conservation efforts.
Long-term studies have revealed complex patterns in diatom responses to climate change. A significant study by Edwards et al. (2023) titled "Climate variability and multi-decadal diatom abundance in the Northeast Atlantic" provides crucial insights into these patterns:
In the NE Atlantic, a prevalent multidecadal trend has been established where climate warming is increasing diatom populations in more northerly regions (e.g., Icelandic Basin), but decreasing populations in more southerly regions (e.g., Bay of Biscay).
The Atlantic Multidecadal Oscillation (AMO) has been shown to have a strong influence on diatom abundance in the NE Atlantic at the multidecadal scale. During a negative phase of the AMO, total diatom abundance is reduced despite decreases in temperature and increases in wind. The opposite effect is observed during a warm phase of the AMO.
The study found that since the positive phase shift in the AMO beginning in the mid-1990s, there has been a substantially tightened relationship between diatom abundance and climate warming (as measured by AMO and Northern Hemisphere Temperature).
In well-mixed areas like the North Sea, diatom growth may be enhanced by temperature increases, as these regions are not inhibited by stratification and hence nutrient availability.
These findings from Edwards et al. (2023) highlight the complexity of diatom responses to climate variability and the importance of considering local and regional factors in addition to global trends. They illustrate that mechanisms will differ between different regional areas of the world with varying hydro-climatic regimes, and that regional nuances may not always be well-represented by homogenized global models
Section 6: Cascading Effects on Aquatic Ecosystems.
6.1 Impact on Food Webs
Diatoms form the base of many aquatic food webs, and changes in diatom communities can have far-reaching effects throughout these ecosystems. The shifts observed in diatom communities in response to climate change and human activities can lead to significant alterations in food web dynamics:
Changes in Energy Flow: Diatoms account for approximately 20% of global primary production (Field et al., 1998). Shifts in diatom community composition or abundance can alter the amount and quality of energy available to higher trophic levels.
Nutritional Quality: Different diatom species have varying nutritional profiles. The Lake Montcortès study showed shifts between Cyclotella cyclopuncta and C. ocellata dominance (Rull et al., 2022). Such changes could affect the nutritional value available to grazers, potentially impacting their growth and reproduction.
Timing Mismatches: Climate change can alter the timing of diatom blooms. In the North Sea, Edwards and Richardson (2004) found that spring blooms have advanced by up to 6 weeks since the 1970s. This can lead to trophic mismatches if the life cycles of grazers don't shift in tandem.
Ripple Effects: Changes at the diatom level can propagate up the food chain. For instance, Beaugrand et al. (2003) linked changes in plankton communities in the North Sea to northward shifts in cod populations.
6.2 Changes in Carbon Cycling and Sequestration
Diatoms play a crucial role in the global carbon cycle, particularly through the biological carbon pump. Changes in diatom communities can significantly impact carbon sequestration:
Carbon Export: Tréguer et al. (2018) estimated that diatoms are responsible for up to 40% of marine primary production and up to 40% of the particulate organic carbon exported to the ocean interior. Changes in diatom abundance or community composition could alter this carbon export.
Silica Pump: Diatoms are also key players in the oceanic silica cycle. Changes in diatom communities can affect silica sequestration, which is linked to carbon sequestration through the biological pump.
Feedback Loops: If shifts in diatom communities lead to less carbon being exported to the deep ocean, more CO2 could remain in the atmosphere, potentially accelerating climate change.
6.3 Implications for Ecosystem Services
The cascading effects of changes in diatom communities can impact various ecosystem services:
Fisheries: Given their role at the base of many aquatic food webs, changes in diatom communities can affect fish populations and, consequently, fisheries. This has economic implications for communities relying on these resources.
Water Quality: Diatoms can influence water clarity and quality. Shifts in diatom communities, especially in freshwater systems like Lake Montcortès, can affect water clarity and the aesthetic value of water bodies.
Climate Regulation: Through their role in carbon sequestration, diatoms contribute to climate regulation. Changes in this service could have global implications.
Biodiversity Support: Diatoms support biodiversity across trophic levels. Changes in diatom communities can ripple through ecosystems, affecting overall biodiversity.
Biogeochemical Cycling: Beyond carbon, diatoms play crucial roles in the cycling of other elements like silicon, nitrogen, and phosphorus. Alterations in diatom communities can affect these biogeochemical cycles.
The Lake Montcortès study provides a vivid example of how changes in diatom communities can persist over long time scales and potentially lead to alternative stable states in ecosystems. The study suggests that extreme events, in the context of gradual warming, can push diatom communities towards new states (Rull et al., 2022). These shifts, if replicated in other systems, could have significant implications for the structure and function of aquatic ecosystems globally.
Understanding these cascading effects is crucial for predicting and managing the impacts of climate change and human activities on aquatic ecosystems.
Section 7: Monitoring and Research Challenges
As our understanding of diatom responses to environmental change grows, so do the challenges in monitoring and researching these crucial organisms:
Multifactor Experiments: There is an increasing need for studies that investigate the combined effects of multiple environmental factors (e.g., CO2, temperature, and nutrient availability) on diatom communities.
Long-term Monitoring: Given the importance of multidecadal variability in shaping diatom communities, long-term monitoring programs are crucial for understanding and predicting future changes.
Regional Variability: The complex and sometimes contradictory responses of diatoms to climate change in different regions highlight the need for spatially comprehensive monitoring and research efforts.
Size-specific Responses: As different-sized diatoms show varying responses to environmental changes, size-structured monitoring and modeling approaches may be necessary to fully capture community dynamics.
Integration of Methods: Combining traditional microscopy with newer molecular and remote sensing techniques presents both opportunities and challenges for comprehensive diatom research.
Data Management and Analysis: As the volume and complexity of diatom data increase, developing robust methods for data management, integration, and analysis becomes increasingly important.
Predictive Modeling: Developing models that can accurately predict diatom responses to future environmental conditions, incorporating both global trends and regional variability, remains a significant challenge.
Addressing these challenges will be crucial for advancing our understanding of diatom ecology and their responses to environmental change. The insights gained from such research will be invaluable for predicting future changes in aquatic ecosystems and informing conservation and management strategies in the face of ongoing climate change and human impacts
Conclusion: The Future of Diatoms in a Changing World
As we've explored throughout this episode, diatoms are facing a complex array of challenges in our rapidly changing aquatic environments. From warming waters to ocean acidification, from altered nutrient dynamics to extreme weather events, these microscopic powerhouses of primary production are on the front lines of global environmental change.
Key takeaways from our exploration include:
Variability in Responses: Diatom responses to environmental changes are not uniform. Tatters et al. (2013) demonstrated that growth rates of several diatom species can be enhanced, depressed, or unaltered by elevated temperature and CO2, with temperature often showing more prominent effects than CO2.
Multifactorial Impacts: The interplay between different environmental factors is crucial. Li et al. (2012b) and Hong et al. (2017) showed that the effects of acidification on carbon to nitrogen ratios in diatoms differed under nutrient-limited compared to replete conditions.
Size-dependent Responses: A study on three Thalassiosira species revealed that optimum growth temperature, maximum growth rates, and thermal niche width decreased with increasing cell size, suggesting that warming oceans may favor smaller diatom species (Gao et al., 2018).
Regional Variability: Edwards et al. (2023) demonstrated that in the NE Atlantic, climate warming is increasing diatom populations in more northerly regions but decreasing them in more southerly regions, highlighting the importance of regional factors.
Ecosystem-wide Implications: Agusti et al. (2015) found that diatoms play a critical role in the biological CO2 pump, with live cells found even in the deep ocean (down to 4000 m), underscoring their importance in carbon sequestration.
Acidification Effects: A study by Harvey et al. (2013) showed that the large chain-forming diatom Biddulphia biddulphiana greatly increases in abundance as pCO2 increases along natural seawater CO2 gradients, potentially leading to simplified and homogenized coastal food-web structures.
Looking to the future, several key questions emerge:
How will the complex interplay of warming, acidification, and changing nutrient dynamics shape diatom communities in different regions?
What are the long-term implications of shifts in diatom community structure for marine and freshwater ecosystems?
How can we best incorporate our understanding of diatom responses into ecosystem management and conservation strategies?
Boyd et al. (2015) highlighted that the negative effects of ocean warming on net primary production and phytoplankton biomass of low- and mid-latitude oceans could be further exacerbated under future ocean conditions. This underscores the urgency of continued research and monitoring efforts.
As we grapple with these questions, the importance of continued research cannot be overstated. Taucher et al. (2015) emphasized the need for studies investigating combined effects among elevated CO2, temperature, and nutrient limitation, given the complexity of interactive effects.
Moreover, the story of diatoms under environmental change is not just a tale of ecological interest – it has profound implications for global biogeochemical cycles, climate regulation, and food security. As Falkowski et al. (2004) pointed out, diatoms are responsible for about 40% of oceanic primary production and account for over 50% of organic carbon burial in marine sediments.
As we conclude this exploration, we're left with a deep appreciation for these microscopic marvels and the complex ways in which they interact with their changing environment. The challenges facing diatoms are significant, but so too is their potential for adaptation and resilience. By continuing to study and understand these crucial organisms, we gain invaluable insights into the health of our aquatic ecosystems and the future of our blue planet.
In the grand symphony of Earth's ecosystems, diatoms may be small players, but their voice is mighty. As we face an uncertain climatic future, listening to what these tiny glass-housed wonders have to tell us may be more important than ever.
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TL;DR: Diatoms Under Threat - A Global Perspective
Diatoms, microscopic algae responsible for 20% of global primary production and 40% of marine carbon export, face multiple challenges due to climate change.
Key threats include:
Ocean warming (projected temperature increase up to 5°C by 2100 in Mediterranean regions)
Ocean acidification (pH decreased by 0.1 units, 30% more acidic since Industrial Revolution)
Changes in nutrient availability and extreme weather events (e.g., intense rainfall events in Lake Montcortès)
Responses vary by species and region:
Some diatoms show resilience, others struggle
Smaller species may be favored (e.g., Thalassiosira pseudonana cell volume decreased 11% with increased temperature)
Regional differences observed (e.g., increasing populations in Icelandic Basin, decreasing in Bay of Biscay)
Certain species (e.g., Cyclotella ocellata) thrive during extreme rainfall events
Long-term studies reveal complex patterns:
Lake Montcortès (Pyrenees): 300-year record shows shifts in dominant species
North Atlantic: Multi-decadal trends linked to Atlantic Multidecadal Oscillation (AMO)
Ecosystem-wide implications:
Alterations in marine food webs (e.g., northward shift of cod populations in North Sea)
Changes in carbon sequestration (diatoms responsible for up to 40% of particulate organic carbon export)
Potential impacts on fisheries and global biogeochemical cycles
Research challenges:
Need for multifactor experiments (e.g., combined effects of CO2, temperature, and nutrients)
Long-term monitoring (e.g., Continuous Plankton Recorder survey in North Atlantic)
Integration of new technologies (e.g., DNA metabarcoding, imaging flow cytometry)
Understanding diatom responses is crucial for predicting future climate scenarios and ecosystem health, given their role in oxygen production and carbon sequestration.
That’s it for today, I know it was difficult to go through it. So, if you were able to read the article in entirety, you are amazing.
Thank you.