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Phytoplankton, microscopic photosynthetic organisms, form the foundation of aquatic food webs. Their diversity is crucial for ecosystem health, influencing productivity, water quality, and supporting complex food chains in lakes and oceans.

Various factors shape phytoplankton communities, including nutrients, temperature, light, and grazing pressure. Understanding these influences helps predict how aquatic ecosystems respond to environmental changes and informs effective management strategies for maintaining diverse phytoplankton populations.

Types of phytoplankton

  • Phytoplankton are the foundation of aquatic food webs and play a crucial role in the ecology and productivity of lakes and other freshwater ecosystems
  • Phytoplankton are microscopic, photosynthetic organisms that inhabit the upper sunlit layer of nearly all oceans and bodies of fresh water
  • There is a wide diversity of phytoplankton species, each with unique characteristics and ecological roles

Cyanobacteria vs eukaryotic algae

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  • Cyanobacteria, also known as blue-green algae, are prokaryotic organisms that lack membrane-bound organelles
  • Eukaryotic algae, such as green algae and diatoms, have a nucleus and other membrane-bound organelles
  • Cyanobacteria are often associated with harmful algal blooms (Microcystis) while eukaryotic algae are generally considered beneficial primary producers (diatoms)

Major phytoplankton groups

  • Diatoms are a major group of eukaryotic phytoplankton characterized by their silica cell walls (frustules)
  • Dinoflagellates are another important group of eukaryotic phytoplankton, many of which are motile and have a unique cell structure (theca)
  • Green algae, such as Chlorella and Chlamydomonas, are common in freshwater environments and play a significant role in primary production
  • Coccolithophores are marine phytoplankton that produce calcium carbonate plates (coccoliths) and can form large blooms (Emiliania huxleyi)

Rare phytoplankton groups

  • Chrysophytes, also known as golden algae, are a diverse group of freshwater phytoplankton that include both motile and non-motile forms (Dinobryon)
  • Cryptophytes are small, flagellated algae that often have a reddish color due to the presence of phycobiliproteins (Cryptomonas)
  • Euglenoids are a group of freshwater flagellates that can be both photosynthetic and heterotrophic (Euglena)
  • Raphidophytes are a small group of marine phytoplankton known for their ability to produce toxins (Chattonella)

Factors influencing phytoplankton diversity

  • Phytoplankton diversity in lakes is influenced by a complex interplay of physical, chemical, and biological factors
  • Understanding the key drivers of phytoplankton diversity is essential for predicting how aquatic ecosystems will respond to environmental changes

Role of nutrient availability

  • Nutrient availability, particularly nitrogen and phosphorus, plays a critical role in shaping phytoplankton communities
  • Eutrophic lakes with high nutrient levels often have lower phytoplankton diversity and are dominated by a few species adapted to high nutrient conditions (cyanobacteria)
  • Oligotrophic lakes with low nutrient levels typically have higher phytoplankton diversity and a more even distribution of species

Impact of water temperature

  • Water temperature influences phytoplankton growth rates, metabolic processes, and species composition
  • Warm water temperatures can favor the growth of cyanobacteria and other heat-tolerant species, while cold water temperatures may support the growth of diatoms and other cold-adapted species
  • Seasonal changes in water temperature can drive shifts in phytoplankton community structure

Influence of light intensity

  • Light intensity determines the amount of energy available for photosynthesis and influences the vertical distribution of phytoplankton in the water column
  • High light intensity can inhibit the growth of some phytoplankton species (photoinhibition) while low light intensity may limit the growth of others
  • The depth of the euphotic zone, where there is sufficient light for photosynthesis, can affect phytoplankton diversity

Effects of water column mixing

  • Water column mixing influences the distribution of nutrients, light, and phytoplankton in the water column
  • Deep mixing can bring nutrients from the bottom of the lake to the surface, promoting phytoplankton growth
  • Stratification can lead to nutrient depletion in the surface layer and limit phytoplankton growth
  • The intensity and frequency of mixing events can shape phytoplankton community structure

Grazing pressure from zooplankton

  • Zooplankton grazing can have a significant impact on phytoplankton diversity and abundance
  • Selective grazing by zooplankton can alter phytoplankton community composition by removing certain species or size classes
  • High grazing pressure can reduce phytoplankton biomass and shift the community towards smaller, less edible species
  • Seasonal changes in zooplankton abundance can influence phytoplankton diversity

Spatial patterns in phytoplankton diversity

  • Phytoplankton diversity varies spatially within and between lakes, reflecting differences in environmental conditions and ecological processes
  • Understanding spatial patterns in phytoplankton diversity is important for assessing the health and functioning of aquatic ecosystems

Vertical distribution in water column

  • Phytoplankton diversity often varies with depth in the water column, reflecting gradients in light, nutrients, and temperature
  • The euphotic zone typically has the highest phytoplankton diversity, as it receives sufficient light for photosynthesis
  • The deep chlorophyll maximum (DCM) is a layer of high phytoplankton biomass and diversity that occurs at the bottom of the euphotic zone in some lakes
  • The hypolimnion, the deep, cold layer of a stratified lake, may have lower phytoplankton diversity due to limited light and nutrients

Horizontal distribution across lake

  • Phytoplankton diversity can vary horizontally across a lake, reflecting differences in water chemistry, mixing, and other factors
  • Nearshore areas may have higher phytoplankton diversity due to increased nutrient input from terrestrial sources and greater habitat heterogeneity
  • Offshore areas may have lower phytoplankton diversity due to more stable conditions and lower nutrient availability
  • Bays and other sheltered areas can have distinct phytoplankton communities compared to open water areas

Diversity differences between lakes

  • Phytoplankton diversity can vary significantly between lakes, reflecting differences in lake morphometry, watershed characteristics, and other factors
  • Lakes with complex shorelines and diverse habitats may support higher phytoplankton diversity than lakes with simple, homogeneous basins
  • Lakes with different trophic states (oligotrophic, mesotrophic, eutrophic) often have distinct phytoplankton communities adapted to the prevailing nutrient conditions
  • Geographic location and climate can influence phytoplankton diversity, with lakes in different regions having unique species assemblages

Temporal patterns in phytoplankton diversity

  • Phytoplankton diversity exhibits temporal variability on seasonal, annual, and long-term scales
  • Understanding temporal patterns in phytoplankton diversity is crucial for predicting how aquatic ecosystems will respond to environmental changes and for developing effective management strategies

Seasonal succession of phytoplankton

  • Phytoplankton communities undergo predictable changes in composition and diversity over the course of a year, known as seasonal succession
  • In temperate lakes, spring blooms are often dominated by diatoms, which thrive in cold, nutrient-rich water following winter mixing
  • Summer communities are typically more diverse, with a mix of green algae, cyanobacteria, and other groups adapted to warm, stratified conditions
  • Fall blooms may occur as temperatures cool and mixing resumes, with a resurgence of diatoms and other groups

Annual variability in diversity

  • Phytoplankton diversity can vary from year to year, reflecting differences in weather patterns, nutrient loading, and other factors
  • Wet years with high runoff may lead to increased nutrient input and higher phytoplankton diversity, while dry years may result in lower diversity
  • Extreme events, such as storms or heat waves, can have significant impacts on phytoplankton diversity in a given year
  • Inter-annual variability in phytoplankton diversity can affect the stability and resilience of aquatic food webs

Long-term changes in diversity

  • Phytoplankton diversity can undergo long-term changes in response to climate change, land use change, and other anthropogenic stressors
  • Warming water temperatures may favor the growth of cyanobacteria and other heat-tolerant species, leading to a decline in overall diversity
  • Eutrophication can cause a shift towards dominance by a few nutrient-loving species, reducing diversity
  • Invasive species can disrupt native phytoplankton communities and alter diversity patterns over time
  • Long-term monitoring of phytoplankton diversity is essential for detecting and understanding these changes

Methods for assessing phytoplankton diversity

  • Assessing phytoplankton diversity requires a combination of microscopic, pigment-based, and molecular techniques
  • Each method provides unique insights into the composition and structure of phytoplankton communities

Microscopic identification techniques

  • Light microscopy is the traditional method for identifying and enumerating phytoplankton species based on their morphological characteristics
  • Samples are concentrated and examined under a microscope, with species identified using taxonomic keys and reference materials
  • Microscopic identification allows for the detection of rare or delicate species that may be missed by other methods
  • Challenges include the time-consuming nature of the process and the need for taxonomic expertise

Pigment analysis using chromatography

  • Pigment analysis using high-performance liquid chromatography (HPLC) can provide a rapid assessment of phytoplankton community composition
  • Different phytoplankton groups have unique pigment signatures, allowing for their identification and quantification based on pigment concentrations
  • Pigment analysis can reveal the relative abundance of major phytoplankton groups (diatoms, dinoflagellates, cyanobacteria) without the need for microscopic identification
  • Limitations include the inability to identify species-level diversity and the potential for pigment degradation in preserved samples

Molecular approaches for diversity

  • Molecular techniques, such as DNA sequencing and fingerprinting, can provide a high-resolution assessment of phytoplankton diversity
  • Next-generation sequencing (NGS) of environmental DNA (eDNA) can reveal the presence of rare or cryptic species that may be missed by traditional methods
  • Quantitative PCR (qPCR) can be used to target specific species or groups of interest and quantify their abundance
  • Molecular approaches can be used to study the diversity of phytoplankton communities in the past by analyzing sediment cores
  • Challenges include the high cost of sequencing and the need for bioinformatic expertise to analyze large datasets

Ecological significance of phytoplankton diversity

  • Phytoplankton diversity is essential for the functioning and stability of aquatic ecosystems
  • A diverse phytoplankton community can provide a range of ecological services and support complex food webs

Importance for primary productivity

  • Phytoplankton are the primary producers in aquatic ecosystems, converting sunlight and nutrients into organic matter through photosynthesis
  • A diverse phytoplankton community can optimize the use of available resources and maximize primary productivity
  • Different phytoplankton species have different light and nutrient requirements, allowing for the efficient use of resources across a range of conditions
  • High phytoplankton diversity can enhance the stability of primary production in the face of environmental fluctuations

Role in aquatic food webs

  • Phytoplankton form the base of aquatic food webs, supporting the growth and reproduction of zooplankton, fish, and other consumers
  • A diverse phytoplankton community can provide a variety of food sources for different types of consumers, promoting the stability and resilience of food webs
  • The seasonal succession of phytoplankton can drive changes in the abundance and composition of higher trophic levels
  • Phytoplankton diversity can influence the transfer of energy and nutrients through food webs, with implications for ecosystem functioning

Influence on water quality

  • Phytoplankton diversity can have significant impacts on water quality in lakes and other aquatic ecosystems
  • A diverse phytoplankton community can help maintain clear water conditions by competing with nuisance algae and cyanobacteria for resources
  • Some phytoplankton species (diatoms) can help regulate water clarity by removing suspended particles through their silica shells
  • Harmful algal blooms (HABs) are often associated with low phytoplankton diversity and can have negative impacts on water quality, human health, and aquatic life
  • Maintaining phytoplankton diversity is important for the provision of clean water and other ecosystem services

Anthropogenic impacts on phytoplankton diversity

  • Human activities can have significant impacts on phytoplankton diversity in lakes and other aquatic ecosystems
  • Understanding the effects of anthropogenic stressors is crucial for developing strategies to protect and restore phytoplankton diversity

Effects of eutrophication

  • Eutrophication, the excessive enrichment of water bodies with nutrients, can lead to significant changes in phytoplankton diversity
  • High nutrient levels can favor the growth of a few fast-growing, nutrient-loving species (cyanobacteria), leading to a decline in overall diversity
  • Eutrophication can cause harmful algal blooms (HABs) that can produce toxins, reduce water clarity, and lead to oxygen depletion
  • The loss of phytoplankton diversity due to eutrophication can have cascading effects on aquatic food webs and ecosystem functioning

Consequences of climate change

  • Climate change can have complex and far-reaching impacts on phytoplankton diversity in lakes
  • Warming water temperatures may favor the growth of cyanobacteria and other heat-tolerant species, leading to a decline in overall diversity
  • Changes in precipitation patterns can alter nutrient loading and water column mixing, with consequences for phytoplankton diversity
  • Shifts in the timing and duration of ice cover can affect the seasonal succession of phytoplankton and alter diversity patterns
  • Ocean acidification, a consequence of rising atmospheric CO2 levels, can impact the growth and survival of some phytoplankton species, particularly those with calcified shells (coccolithophores)

Introduction of invasive species

  • The introduction of invasive phytoplankton species can have significant impacts on native phytoplankton diversity
  • Invasive species can outcompete native species for resources, leading to a decline in diversity and changes in community composition
  • The establishment of invasive species can be facilitated by human activities, such as ballast water discharge from ships or the release of aquarium specimens
  • Once established, invasive phytoplankton species can be difficult to eradicate and can have long-lasting impacts on aquatic ecosystems
  • Monitoring and early detection of invasive species is important for preventing their spread and minimizing their impacts on phytoplankton diversity


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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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