Zooplankton 2

Freshwater Biology - Zooplankton in Open Water Communities

Overview

• Study on the role of zooplankton within freshwater ecosystems, specifically focusing on their interactions and relationships with other aquatic organisms, such as phytoplankton.

Factors Affecting Zooplankton Communities

Distribution

• Zooplankton communities are influenced by various factors affecting their distribution in both space and time.

Zooplankton Succession

Seasonal Patterns

• Spring: Diatoms

• Summer: Green algae, B-g algae

• Autumn: Diatoms, Rotifers & Cladocera, copepods

  • The changes in phytoplankton influence zooplankton succession, raising questions about causality (Cause or Effect?).

Bottom-up Control

Interactions with Phytoplankton

• Abundance Impact: High abundance of food leads to increased zooplankton populations.

• Herbivory: Zooplankton herbivory reduces algal abundance.

• Nutrient Recycling: Phosphorous recycling boosts algal growth.

• Algal Susceptibility: Varies by size, shape (single-celled, colonial, filamentous) & presence of toxins.

• Selective Feeding: Zooplankton can avoid feeding on protected individuals.

Phytoplankton Dynamics

Biomass Control

• Influenced by various physical factors and nutrients.

• Phytoplankton populations cycle seasonally, with variance due to grazing interactions.

Impact of Mesozooplankton

• Studies show that in enclosures without predators, mesozooplankton (0.2-20 mm) presence can reduce phytoplankton abundance, indicating a significant predatory impact.

• Effect size varies, being stronger for larger phytoplankton (those >30 μm).

Evidence for Bottom-up Control

• Increased zooplankton populations correlate with improved phytoplankton growth conditions (nutrient addition).

• Observed declines in zooplankton follow reductions in available phytoplankton, hinting at strong food availability impacts in predator-free environments.

Top-down Control: Planktivores Influence

Planktivores Description

• Types:

  • Invertebrate: Copepoda, insect larvae

  • Vertebrate: white fish, fish fry, Amphibia

• Population growth can be modeled using Nt = N0ert where r = b - d, based on population size changes.

Predation Patterns in Zooplankton

Predator Impact Assessment

• Example Predators:

  • Leptodora kindtii: A predatory water flea.

  • Daphnia schødleri: Grows up to 18 mm length.

Environmental Effects on Biomass

Light Impact Study

• Observations made under varying light conditions in different lakes (clear vs turbid) documented differences in overall zooplankton biomass.

• Increased turbidity linked to lower light levels which translated to less zooplankton growth.

Behavioral Responses to Predation

Diurnal Vertical Migration (DVM)

• Behavior induced by visual predation from fish.

• Hypotheses include physiological reasons (e.g. UV avoidance) and adaptive responses related to food availability.

Morphological Responses in Zooplankton

Cyclomorphosis

• Many multivoltine zooplankton species exhibit changes in morphology in response to predation threats, aiding in survival.

  • Example Species: Daphnia pulex, Keratella sp.

Effect of Historical Predation

• Introduction of species like Alosa may alter community dynamics, reducing the size of dominant zooplankton species as part of ecological interactions.

Size-Efficiency Hypothesis

• Larger grazers tend to outcompete smaller ones, impacting food particle size, metabolic efficiency, and predator detection rates.

• Key comparison in detection between Copepods (5-100 µm) and Cladocera (1-50 µm).

Community Control Factors

Bottom-up vs Top-down Effects

• Bottom-up:

  • Dependent on nutrient and light availability leading to algal productivity.

• Top-down:

  • Involves planktivores controlling herbivore populations through trophic cascades.

Alternative Stable States in Lakes

Shallow Eutrophic Lakes

• Clear vs Turbid waters are defined by factors such as nutrient levels and macrophyte presence, impacting light penetration and overall aquatic health.

• Example shift observed in Lake Tåkern where high precipitation altered phytoplankton biomass which in turn affected water clarity.

Implications of State Changes

Impact on Zooplankton Communities

• Clear water stability favors macrophytes improving nutrient uptake and providing habitats for grazers.

• Turbid states lead to shading of macrophyte shoots and a decline in herbivore refuges, prompting shifts in zooplankton community compositions.