💧Limnology Unit 6 – Zooplankton ecology

Key Concepts and Definitions

• Zooplankton are small, free-floating animals that inhabit aquatic environments and play a crucial role in the food web
• Consist of a diverse group of organisms, including crustaceans (copepods, cladocerans), rotifers, and larval stages of larger animals (fish, mollusks)
• Occupy various trophic levels, acting as primary consumers, secondary consumers, and prey for larger organisms
• Exhibit diel vertical migration (DVM), moving up and down the water column in response to light and predation risk
• Contribute to nutrient cycling by grazing on phytoplankton and excreting nutrients back into the water
• Serve as bioindicators of water quality and ecosystem health due to their sensitivity to environmental changes
• Key terms:
  • Holoplankton: organisms that spend their entire life cycle as plankton (copepods, cladocerans)
  • Meroplankton: organisms that spend only a part of their life cycle as plankton (larval stages of fish, crabs)

Types and Classification of Zooplankton

• Copepods are small crustaceans that dominate marine and freshwater zooplankton communities
  • Divided into three main orders: Calanoida, Cyclopoida, and Harpacticoida
  • Calanoid copepods are typically herbivorous and are important grazers of phytoplankton
  • Cyclopoid copepods are omnivorous or carnivorous and prey on smaller zooplankton
• Cladocerans, also known as water fleas, are small crustaceans found primarily in freshwater habitats
  • Include genera such as Daphnia, Bosmina, and Ceriodaphnia
  • Daphnia is a well-studied genus known for its parthenogenetic reproduction and sensitivity to environmental stressors
• Rotifers are microscopic, multicellular animals characterized by a crown of cilia (corona) used for locomotion and feeding
  • Divided into three main classes: Monogononta, Bdelloidea, and Seisonidea
  • Monogonont rotifers (Brachionus, Keratella) are common in freshwater habitats and exhibit sexual reproduction
• Larval stages of larger animals, such as fish (ichthyoplankton), mollusks (veliger larvae), and crustaceans (zoea larvae), are temporary members of the zooplankton community
• Classification is based on morphological characteristics, such as body shape, appendages, and feeding structures

Zooplankton Life Cycles and Reproduction

• Zooplankton exhibit diverse life cycles and reproductive strategies adapted to their aquatic environments
• Holoplankton have life cycles entirely within the plankton community
  • Copepods undergo a series of larval stages (nauplius, copepodite) before reaching adulthood
  • Cladocerans, such as Daphnia, reproduce primarily through parthenogenesis, producing genetically identical offspring
• Meroplankton have a planktonic larval stage followed by a benthic or nektonic adult stage
  • Fish larvae (ichthyoplankton) undergo metamorphosis and develop into juvenile and adult stages
  • Mollusks (bivalves, gastropods) have a planktonic veliger larval stage before settling on the substrate
• Rotifers exhibit both sexual and asexual reproduction
  • Monogonont rotifers have a cyclical parthenogenesis, alternating between asexual and sexual reproduction depending on environmental conditions
  • Bdelloid rotifers reproduce exclusively through parthenogenesis and are known for their ability to survive desiccation
• Reproductive strategies, such as resting egg production (diapause), allow zooplankton to survive unfavorable conditions and maintain population persistence

Feeding Strategies and Trophic Interactions

• Zooplankton employ various feeding strategies depending on their morphology and prey availability
• Suspension feeders, such as calanoid copepods and many rotifers, use their appendages to generate currents and filter small particles (phytoplankton, detritus) from the water
  • Calanoid copepods use their mouthparts to create a feeding current and capture food particles
  • Brachionus rotifers use their corona to generate a vortex and direct food towards their mouth
• Raptorial feeders, such as cyclopoid copepods and some cladocerans (Leptodora), actively capture and consume larger prey (other zooplankton, small fish larvae)
  • Cyclopoid copepods use their antennae and mouthparts to grasp and manipulate prey
  • Leptodora kindtii is a predatory cladoceran that uses its large, grasping appendages to capture prey
• Omnivorous zooplankton, such as many cladocerans (Daphnia), can switch between herbivory and carnivory depending on food availability and quality
• Zooplankton are important links in aquatic food webs, transferring energy from primary producers to higher trophic levels
  • Grazing on phytoplankton regulates primary production and influences algal community composition
  • Serve as prey for planktivorous fish, invertebrates, and other larger predators
• Trophic cascades can occur when changes in zooplankton populations (due to predation or environmental factors) indirectly affect phytoplankton abundance and water clarity

Distribution and Migration Patterns

• Zooplankton distribution is influenced by a combination of physical, chemical, and biological factors
• Vertical distribution is often characterized by diel vertical migration (DVM), where zooplankton move up and down the water column on a daily cycle
  • DVM is a strategy to avoid visual predators during the day and exploit food resources at night
  • Factors triggering DVM include light intensity, predation risk, and food availability
• Horizontal distribution is affected by water currents, wind-driven circulation, and the presence of physical barriers (thermoclines, oxyclines)
  • Zooplankton can form patches or aggregations in response to food availability or physical gradients
  • Langmuir circulation can concentrate zooplankton in surface convergence zones
• Seasonal succession of zooplankton communities is common in temperate lakes and coastal areas
  • Changes in temperature, light, and food resources drive shifts in species composition and abundance
  • Cladocerans (Daphnia) often dominate during summer, while copepods (Cyclops) may be more abundant in spring and fall
• Zooplankton migration can also occur on seasonal or ontogenetic scales
  • Some species undergo seasonal vertical migration, overwintering in deep waters and migrating to the surface during spring and summer
  • Ontogenetic vertical migration involves different life stages occupying different depths (copepod nauplii in surface waters, adults in deeper layers)

Environmental Factors Affecting Zooplankton

• Temperature is a key factor influencing zooplankton growth, development, and reproduction
  • Warmer temperatures generally accelerate metabolic rates and shorten generation times
  • Thermal stratification can create distinct habitats with different zooplankton communities
• Light intensity and photoperiod affect zooplankton behavior, vertical migration, and visual predation risk
  • High light levels can increase predation risk from visual predators, triggering downward migration
  • Changes in photoperiod can cue seasonal transitions and influence reproductive strategies
• Nutrient availability, particularly nitrogen and phosphorus, indirectly affects zooplankton by regulating phytoplankton growth and community composition
  • Eutrophic conditions can lead to high phytoplankton biomass and alter zooplankton community structure
  • Nutrient limitation can reduce food quality and quantity for herbivorous zooplankton
• Oxygen concentration can limit zooplankton distribution and survival, especially in the hypolimnion of stratified lakes
  • Hypoxic or anoxic conditions can restrict zooplankton vertical migration and force them to remain in oxygenated surface waters
• pH and water chemistry can influence zooplankton community composition and diversity
  • Acidification can reduce zooplankton species richness and favor acid-tolerant species (Bosmina, Holopedium)
  • Calcium concentration is important for the growth and reproduction of many crustacean zooplankton (Daphnia)
• Predation pressure from fish and invertebrate predators can shape zooplankton size structure and community composition
  • Size-selective predation can lead to dominance of smaller zooplankton species or individuals
  • Predator kairomones can induce morphological and behavioral defenses in zooplankton prey (helmet development in Daphnia)

Zooplankton's Role in Aquatic Ecosystems

• Zooplankton are key components of aquatic food webs, linking primary producers to higher trophic levels
  • Grazing on phytoplankton regulates primary production and influences algal community composition
  • Transfer energy and nutrients to planktivorous fish, invertebrates, and other predators
• Play a crucial role in the biological pump, contributing to the vertical transport of carbon and nutrients in the water column
  • Fecal pellets and molts of zooplankton sink and provide a source of organic matter for benthic communities
  • Diel vertical migration can enhance the active transport of carbon and nutrients to deeper waters
• Zooplankton excretion recycles nutrients (nitrogen, phosphorus) back into the water column, supporting primary production
• Grazing by zooplankton can influence water clarity and the occurrence of algal blooms
  • High zooplankton grazing pressure can reduce phytoplankton biomass and improve water transparency
  • Selective feeding can favor certain algal species and shape phytoplankton community composition
• Serve as bioindicators of water quality and ecosystem health
  • Changes in zooplankton community structure, diversity, and abundance can reflect environmental stressors (eutrophication, pollution, climate change)
  • Presence or absence of certain indicator species (Daphnia, Bosmina) can provide insights into the trophic state and ecological condition of aquatic systems
• Zooplankton biodiversity contributes to the resilience and stability of aquatic ecosystems
  • High species diversity can buffer against environmental perturbations and maintain ecosystem functioning
  • Functional diversity (different feeding strategies, life histories) enhances resource use efficiency and nutrient cycling

Research Methods and Sampling Techniques

• Zooplankton sampling involves the use of various nets and traps to collect organisms from the water column
  • Plankton nets (Wisconsin net, Tucker trawl) are towed vertically or horizontally to capture zooplankton
  • Mesh size of the net determines the size range of zooplankton retained (typical sizes: 63 μm, 100 μm, 200 μm)
• Discrete depth sampling using closing nets (Nansen net, Multinet) allows for the collection of zooplankton from specific depth intervals
  • Useful for studying vertical distribution and migration patterns
  • Samples can be stratified by depth to assess community structure and abundance in different layers
• Pump sampling is an alternative method that can provide more quantitative estimates of zooplankton abundance and biomass
  • Water is pumped from specific depths and filtered through a mesh to concentrate zooplankton
  • Allows for the collection of delicate or fragile organisms that may be damaged by net tows
• In situ imaging systems (Video Plankton Recorder, Underwater Vision Profiler) enable non-invasive, high-resolution sampling of zooplankton
  • Provide information on the size, shape, and spatial distribution of zooplankton in their natural environment
  • Allow for the study of fine-scale patterns and interactions that may be missed by traditional sampling methods
• Acoustic methods (echosounders, acoustic Doppler current profilers) can be used to estimate zooplankton biomass and distribution
  • Sound scattering by zooplankton is related to their size, shape, and abundance
  • Provides continuous, high-resolution data on zooplankton distribution and migration patterns
• Sample preservation and analysis
  • Zooplankton samples are typically preserved in formalin or ethanol for later identification and enumeration
  • Subsampling techniques (Folsom plankton splitter, Stempel pipette) are used to obtain representative aliquots for counting and measuring
  • Microscopic examination is used to identify zooplankton to the species or genus level based on morphological characteristics
  • Biomass estimates can be obtained by measuring the length or dry weight of individuals
• Experimental approaches, such as mesocosm studies and in situ enclosures, allow for the manipulation of environmental factors and the study of zooplankton responses
  • Used to investigate the effects of temperature, nutrients, predation, or other stressors on zooplankton communities
  • Provide insights into the mechanisms underlying zooplankton population dynamics and trophic interactions