💧Limnology Unit 8 – Freshwater invertebrates

Key Concepts and Definitions

• Limnology studies the biological, chemical, and physical features of freshwater ecosystems (lakes, rivers, wetlands)
• Freshwater invertebrates are animals lacking a backbone that inhabit freshwater environments
  • Includes insects, crustaceans, mollusks, and other groups
• Biodiversity refers to the variety of life forms within an ecosystem
• Trophic levels describe the feeding positions in a food web (primary producers, primary consumers, secondary consumers)
• Benthic zone is the lowest level of a body of water, including the sediment surface and subsurface layers
• Littoral zone is the nearshore area where sunlight penetrates to the bottom, allowing aquatic plants to grow
• Lentic ecosystems are still water habitats (lakes, ponds) while lotic ecosystems are flowing water habitats (rivers, streams)

Freshwater Habitats and Ecosystems

• Lakes are large, deep, standing bodies of water formed by natural processes (glacial activity, tectonic movements)
  • Stratify into distinct layers based on temperature and density
• Ponds are smaller, shallower standing water bodies often formed by human activities (dam construction, excavation)
• Rivers are large, natural streams of water flowing from higher to lower elevations
  • Characterized by unidirectional flow, erosion, and sediment transport
• Streams are smaller, shallower flowing water bodies that feed into rivers
• Wetlands are areas where water covers the soil or is present at or near the surface (marshes, swamps, bogs)
  • Support unique plant and animal communities adapted to hydric soils
• Groundwater is water held underground in soil pore spaces and rock fractures
  • Provides base flow to surface water systems and supports subterranean ecosystems

Major Groups of Freshwater Invertebrates

• Insects are the most diverse group of freshwater invertebrates
  • Includes orders such as Ephemeroptera (mayflies), Plecoptera (stoneflies), Trichoptera (caddisflies), and Diptera (true flies)
• Crustaceans are arthropods with hard exoskeletons and jointed appendages
  • Includes subclasses like Copepoda (copepods), Ostracoda (seed shrimp), and Malacostraca (crayfish, shrimp)
• Mollusks are soft-bodied animals, often with a calcified shell
  • Includes classes such as Bivalvia (clams, mussels) and Gastropoda (snails, limpets)
• Annelids are segmented worms with a closed circulatory system
  • Includes classes like Oligochaeta (earthworms) and Hirudinea (leeches)
• Nematodes are unsegmented roundworms found in sediments and aquatic vegetation
• Platyhelminthes are flatworms with a simple, flattened body structure (planarians)
• Rotifers are microscopic, wheel-like animals that feed on suspended particles

Adaptations and Life Cycles

• Aquatic insects have evolved various adaptations for living in water
  • Gills for extracting dissolved oxygen, streamlined bodies for swimming, and hooks or suckers for attachment to substrates
• Many aquatic insects have complex life cycles with distinct larval and adult stages
  • Larvae develop in water, feeding and growing before emerging as flying adults (hemimetabolous development)
  • Some undergo complete metamorphosis with a pupal stage between larva and adult (holometabolous development)
• Crustaceans have specialized appendages for feeding, locomotion, and reproduction
  • Some have direct development from eggs to juveniles while others have larval stages (nauplii, zoea)
• Mollusks have muscular feet for movement and feeding, and some have siphons for filtering water
  • Bivalves have a planktonic larval stage (veliger) before settling and metamorphosing into adults
• Annelids have segmented bodies with chaetae (bristles) for locomotion and feeding
  • Reproduce sexually or asexually through fragmentation and regeneration

Ecological Roles and Interactions

• Freshwater invertebrates occupy various trophic levels and play crucial roles in ecosystem functioning
  • Primary consumers feed on algae, detritus, and other organic matter (grazers, collectors, shredders)
  • Secondary consumers prey on other invertebrates and small vertebrates (predators, parasites)
• Invertebrates contribute to nutrient cycling by breaking down organic matter and releasing nutrients back into the system
• Some invertebrates engineer their habitats by modifying the physical environment
  • Caddisfly larvae construct cases from sediment and plant material, stabilizing substrates and providing shelter for other organisms
• Invertebrates serve as important food sources for fish, amphibians, reptiles, birds, and mammals
  • Insect emergence provides a critical energy subsidy to terrestrial ecosystems
• Interactions among invertebrates include competition for resources, predation, and symbiosis
  • Mussels have a parasitic larval stage (glochidium) that attaches to fish gills before developing into juveniles

Sampling and Identification Techniques

• Invertebrate sampling methods depend on the habitat type and research questions
  • Nets (kick nets, D-nets) are used to collect organisms from the water column and substrates
  • Grab samplers (Ekman, Ponar) are used to collect sediment samples from the benthic zone
  • Artificial substrates (Hester-Dendy plates) are used to assess colonization and community composition
• Preserved samples are sorted and identified in the laboratory using taxonomic keys and reference collections
  • Identification is based on morphological characteristics such as body shape, mouthparts, and genitalia
• Molecular techniques (DNA barcoding) are increasingly used for species identification and phylogenetic analyses
• Quantitative methods (density, biomass, diversity indices) are used to analyze invertebrate community structure and function
  • Functional feeding groups (FFGs) classify invertebrates based on their feeding modes and resource utilization

Environmental Impacts and Conservation

• Freshwater invertebrates are sensitive to environmental stressors and are used as bioindicators of ecosystem health
  • Changes in community composition and diversity can indicate water quality impairment
• Pollution (nutrients, sediments, toxins) can alter invertebrate communities and disrupt ecosystem processes
  • Eutrophication can lead to oxygen depletion and shifts in species dominance
• Habitat degradation (channelization, flow regulation, riparian removal) can reduce invertebrate diversity and abundance
  • Dams and diversions can alter flow regimes and disrupt life cycles of aquatic insects
• Invasive species can compete with or prey upon native invertebrates, altering community structure
  • Zebra mussels (Dreissena polymorpha) have invaded many North American waterways, outcompeting native bivalves and altering food webs
• Conservation efforts aim to protect and restore freshwater habitats and their associated invertebrate communities
  • Riparian buffers, flow management, and pollution control are used to mitigate human impacts
  • Reintroduction and captive breeding programs are used to support populations of rare and endangered species

Applications in Limnology Research

• Freshwater invertebrates are used as model organisms for studying ecological and evolutionary processes
  • Daphnia (water fleas) are used to investigate population dynamics, predator-prey interactions, and eco-evolutionary feedbacks
• Invertebrate assemblages are used as surrogates for assessing biodiversity and ecosystem integrity
  • Macroinvertebrate-based indices (EPT, BMWP) are used to evaluate water quality and inform management decisions
• Stable isotope analysis of invertebrate tissues is used to trace energy flow and trophic relationships in food webs
  • Carbon and nitrogen isotopes can reveal the sources and pathways of organic matter in aquatic systems
• Experimental manipulations of invertebrate communities are used to test ecological hypotheses and predict responses to environmental change
  • Mesocosm experiments can simulate the effects of nutrient enrichment, predation, and species invasions on invertebrate assemblages
• Long-term monitoring of invertebrate populations is used to detect trends and shifts in community structure over time
  • Citizen science programs engage the public in collecting data on invertebrate diversity and distribution