Chapter 8: Trophic Relationships

The complexity of food webs in river ecosystems comprises both consumers and resources.
Energy supplies in these systems originate from various sources, including:
- Living resources: algae and higher plants.
- Nonliving resources: particulate organic matter (POM) and dissolved organic matter (DOM).
Microorganisms play a vital role in mediating the availability of organic matter, serving as resources for various consumers.
Energy subsidies come from external sources, such as:
- Falling terrestrial arthropods.
- Eggs and carcasses from migrating fish.
Not all energy generated in a stream is consumed there; it can be exported downstream, impacting distant ecosystems.

Trophic organization is often complex, with many consumers being polyphagous (eating multiple food types).
Diet overlap among consumers complicates classification, as most invertebrates are generally categorized under the broad term of herbivore-detritivore.
Most fishes in temperate waters primarily consume invertebrates.
Trophic roles categorized by feeding behavior (how food is obtained) rather than food type provide clearer distinctions.
The concept of guilds facilitates understanding when species share similar dietary resources and capture methods:
- Example: Fish capturing invertebrates from the bottom vs. from the water column belong to different guilds.
Functional Feeding Groups (FFGs) categorize invertebrates by both what they eat and their feeding strategies (Cummins 1973).
Distinctions in feeding modes enhance the classification system's robustness, especially for polyphagous species.

Studies now prioritize microbial roles in trophic dynamics.
Biofilms composed of algae and microorganisms are crucial for energy capture and linking autotrophic and heterotrophic pathways.
Microbial populations consist mainly of:
- Bacteria.
- Fungi, which use organic matter as carbon sources, leading to significant carbon remineralization.
The microbial food web not only recycles nutrients but may also be crucial in energy flow to higher trophic levels.
Microbial production is consumed directly through ingestion of individual cells or larger particles like FPOM (fine particulate organic matter) and CPOM (coarse particulate organic matter).
Evidence shows that digestion can bypass many trophic levels, leading to effective transfer to macroinvertebrates and fish.
Meiofauna (small metazoans) are significant in microbial food webs, capable of consuming bacteria and contributing to stream diversity.

Functional Feeding Groups (FFGs) reflect four critical food resources in streams:
- Periphyton, CPOM, FPOM, and animal prey.
Categories of FFGs include:
1. Shredders:
  - Consume nonwoody CPOM, primarily leaves.
  - Feeding mechanism: chewing and mining.
  - Key taxa: Trichoptera, Plecoptera, Crustacea.
2. Collectors:
  - Feed on FPOM from the water column or streambed.
  - Feeding mechanisms include nets and specialized filtering apparatus.
3. Predators:
  - Consume animal prey through biting and piercing.
4. Grazers:
  - Consume periphyton using scraping adaptations.
Feeding efficiency varies based on leaf conditions, and shredders prefer ‘conditioned’ leaves enriched with microbial life.

Leaf shredders significantly impact the breakdown of CPOM.
Studies identify key shredders:
- Crustaceans (i.e., amphipods, isopods) and insect larvae (i.e., caddisflies).
Microorganisms enhance leaf nutritional quality through direct and indirect contributions:
1. Microbial Production:
  - Involves nutrients from microbial biomass being incorporated into the detrital food web.
2. Microbial Catalysis:
  - Involves processes that make substrates more digestible for consumers.
Shredders show varied feeding behaviors, selectively consuming conditioned versus unconditioned leaves.

Collectors and filter feeders capture FPOM differently:
- Collector-gatherers feed on particulate matter from deposits and sediment.
- Filter feeders use specialized structures such as nets or setae to capture particles in suspension.
Advanced feeding adaptations include:
- Caddisfly nets designed for specific particle sizes, influencing food efficiency.
- Larval black flies with cephalic fans that filter out food particles from water.

Grazers, including snails and certain caddisflies, consume living periphyton, affecting algal community dynamics.
Variability among grazing invertebrates affects their dietary preferences and assimilation efficiencies based on environmental conditions.

Predators play critical roles in controlling invertebrate populations and exert top-down effects on food webs.
The mode of predation influences dietary breadth; common strategies include ambushing and active searching.
Predators often alter prey behavior and distribution, demonstrating interconnectedness in ecological food webs.

Vertebrates in riverine systems are primarily represented by fish, with various guilds based on feeding habits.
Diverse trophic roles include:
1. Piscivores:
  - Feed mainly on fish and larger invertebrates (16% of fish species).
2. Benthic Invertebrate Feeders:
  - Primarily feed on benthic invertebrates (33%).
3. Generalized Invertebrate Feeders:
  - Feed at all depths (11%).
4. Planktivores:
  - Feed on phytoplankton and zooplankton (3%).
5. Herbivorous-Detritivores:
  - Ingest periphyton and detritus (7%).
6. Omnivores:
  - Consume a wide range of foods (6%).
7. Parasites:
  - Ectoparasitic feeders, including lampreys (3%).
Fish guild proportions vary, with herbivory being less common in temperate regions compared to tropical ecosystems, where detritivory plays a significant role.

Secondary production in fluvial ecosystems consists mainly of new biomass from meiofauna, macroinvertebrates, and fish.
Measured in mass per area over time, turnover rates can indicate ecosystem productivity.
High production correlates with rapid growth rates and short life cycles, significantly influenced by available resources, temperature, and ecological dynamics.
A comparison of secondary production across studies indicates considerable variation, highlighting the diversity in stream ecosystems.

Trophic relationships in rivers are complex and interconnected, involving various factors from primary producers to top predators.
Understanding these relationships requires careful classification of feeding roles, environmental influences, and direct observations of interactions within the food web.