Lotic vs Lentic environments

  • Lotic habitats = flowing waters; e.g. streams and rivers
  • Lentic habitats = ponds and lakes
  • Streams: generally cool, shallow, stony bottoms; rivers: muddier, larger and deeper
  • Habitat features: riffles and pools; runs
  • Riparian vegetation
  • Discharge = water flow as volume per unit time; current = distance travelled per unit time

Hard vs Soft water systems

  • Hard waters:
    • Rich in calcium, magnesium, carbonate and sulphate
    • Slightly alkaline pH
    • Often nutrient rich
    • On limestone
    • Vegetation often deciduous woodlands
  • Soft waters:
    • Low in ions
    • Low pH
    • Mostly nutrient-poor
    • On sandstone and metamorphosed rock
    • Vegetation often coniferous forest; also Sphagnum

Blackwater vs Whitewater rivers

  • Blackwater rivers:
    • Acidic
    • Heavily stained with humic substances
    • Drain infertile leached soils
    • Low productivity due to low pH and low nutrient concentrations
    • Example: Rio Negro
  • Whitewater rivers:
    • Large rivers stained "white" by suspended silts
    • Often relatively high nutrient concentrations
    • Productivity low due to light limitation
    • Often high productivity at junctions between black and white water
  • Rio Negro meets Rio Solimoes at Manaus (Brazil)

River order nomenclature

  • River order helps classify size/scale of rivers
  • Note: the Mississippi is a 12th order river

Vannote et al. 1980: The River Continuum Concept (RCC)

  • The river is a continuous but changing habitat
  • The change in food supply is an important regulator:
    • From coarse-grained to fine-grained allochthonous organic matter (CPOM to FPOM)
    • And from allochthonous to autochthonous organic matter (cf. changes in P/R)
  • The relative importance of major groups of macroinvertebrates (shredders, grazers, collectors, predators) gradually changes downstream

The River Continuum Concept (detailed)

  • Across stream/river orders, the community composition shifts:
    • Shredders and grazers dominate upstream; collectors and predators become more important downstream
  • Visual sequence (examples):
    • Trout: P/R < 1
    • Bass: P/R > 1
  • Relative channel width increases downstream
  • Organisms and resources change from course to fine particulate matter; from microbe-dominated to phytoplankton, periphyton
  • Key terms: periphyton, coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), dissolved organic matter (DOM)

The River Continuum Concept (illustrative layout)

  • Stream/River order progression shows shifts in:
    • Shredders, Grazers, Microbes, Predators, Collectors
    • Periphyton, Coarse Particulate Matter (CPOM), Fine Particulate Matter (FPOM)
    • Microbes, Phytoplankton, Collectors, Predators
  • Depth, Turbidity, Submerged plant biomass, Emergent plant biomass, Phytoplankton biomass change along the continuum

Benthic invertebrates and habitat indicators

  • Common taxa/functional groups: mask (likely a typo; refer to crustaceans such as Opsum shrimp), Crayfish, Waterlouse, Freshwater shrimp, Dragonfly, Damselfly, Mayfly, Alderfly, Caddisfly, Ephemeroptera, Plecoptera, Case types
  • Benthic invertebrates as indicators of water quality
  • Many aquatic insects have flying adults (dragonflies, stoneflies, mayflies, midges, caddisflies) causing seasonal abundance changes
  • Drift: major food source for fish; can be accidental or nocturnal
  • Adaptations to avoid being swept away: firmly attached (e.g. black flies, Simulium); very flattened; hiding under stones

Interstitial fauna and the hyporheic zone

  • Interstitial fauna: organisms living between stones and pebbles
  • Hyporheic zone: slow-water movements through sediments; an important habitat for many organisms

The food web in a river (conceptual view)

  • Key components:
    • Bacteria; Fungi, Protozoans
    • Large allochthonous particulate organics; Shredders
    • Allochthonous dissolved organics and inorganics; Scrapers and grazers
    • Micro-meso-macrophyte primary autochthonous producers; DETRITUS
    • Dissolved organic and tiny particulate organics; Predators
    • Periphyton; Zooplankton; Filters and collectors; Predators
  • Energy pathways:
    • Allochthonous inputs (CPOM/FPOM, DOM)
    • Autochthonous production (periphyton, phytoplankton)
    • Nutrient cycling through bacteria, fungi, and meiofauna

The food web (continued) – details

  • Interactions: light availability drives primary production; detrital inputs drive heterotrophy
  • Flow of energy: from coarse particulate to fine particulate and dissolved organic matter; from microbes to macroinvertebrates to fish
  • Role of epilithic algae and larger plants in supporting the food web

Trophic structure: lakes vs rivers

  • Lake and river trophic structure differences:
    • Plankton community dynamics; respiration dynamics; decomposer pathways
    • In streams, consumers and decomposer pathways differ from lentic (lake) systems; NPP, DOM, and NFP (net primary production) dynamics vary
  • Conceptual schematic: vertical flow of energy and matter in riverine vs lacustrine systems

Applicability of RCC to African rivers

  • RCC treated as an uninterrupted continuum in streams and rivers
  • Nutrient recycling is unidirectional and biologically mediated across the continuum
  • Consider regional variations and hydrology specificities when applying RCC to African rivers

Plankton and RCC: annual and diel cycles

  • Plankton dynamics studied on annual and diel timescales
  • Factors: substrate bottom, size, P/R ratio, CPOM/FPOM ratio
  • Dams and impoundments alter RCC via the Serial Discontinuity Concept

Serial Discontinuity Concept (Ward & Stanford, 1983)

  • Dams create a break in the longitudinal continuity of the river ecosystem
  • Effects include changes in:
    • Biotic integrity, nutrient dynamics, hydrology, and flow regime along the river continuum
    • Substrate characteristics and habitat connectivity across order
  • Graphical depiction shows dam-induced shifts in biotic reliability, nutrient spiraling, and environmental gradients

Environmental stressors associated with dams

  • Flow stressors: altered hydrographs, peak flows, and flood timing
  • Habitat alteration: channel modification, fragmentation of habitats
  • Water quality changes: temperature, dissolved oxygen, sediment load, nutrient concentrations
  • Alien/introgressive invasive species potential
  • Impacts on timing, duration, and quantity of water flow; consequences for species composition and distribution; resource usage

Big rivers and their floodplains

  • Big rivers defined as > 2000extkm2000 ext{ km} long
  • Always turbid due to suspended sediments (Secchi depth < 1 m)
  • Differences in the ratio of discharge to drainage basin area (varies by climate region; higher in tropical rivers, lower in desert rivers)

Role of megaherbivores

  • Megaherbivores (e.g., hippos) contribute to nutrient transfer between terrestrial and aquatic systems
  • Mechanisms include grazing, egestion, litter fall, and resuspension; both aquatic and terrestrial pathways contribute to biogeochemical fluxes

Hippos and terrestrial-aquatic nutrient transfer (quantitative fluxes)

  • Upstream influxes and downstream effluxes of silica-based nutrients (BSi and DSi) and biogenic silica fractions:
    • Upstream influx: 1.3exttBSiday−11.3 ext{ t BSi day}^{-1}; BSI 830Si = -0.23 ext{%}
    • Influx: 8.5exttDSiday−18.5 ext{ t DSi day}^{-1}; DSI 830Si = 1.03 ext{%}
  • Litter fall and hippo consumption:
    • Egestion in water: 0.4exttBSiday−10.4 ext{ t BSi day}^{-1}; BSi 830Si = -0.52 ext{%}
    • Egestion on land: 0.4exttBSiday−10.4 ext{ t BSi day}^{-1}; BSi 830Si = -0.52 ext{%}
  • Indirect flux and resuspension:
    • Resuspension: 0.7exttBSiday−10.7 ext{ t BSi day}^{-1}
  • Grazing in water and sediment interactions:
    • Grazing: 0.8exttBSiday−10.8 ext{ t BSi day}^{-1}; BSi 830Si = -0.42 ext{%}
    • Sediment BSi: 0.98 ext{%}
    • Porewater: DSi 830Si = 1.84 ext{%}
  • Biogeochemical transformations:
    • $BSi$ dissolution: 5.2exttBSiday−15.2 ext{ t BSi day}^{-1}
    • Clay mineral formation: 5.8exttBSiday−15.8 ext{ t BSi day}^{-1}
  • Downstream fluxes:
    • Efflux: 2.4exttBSiday−12.4 ext{ t BSi day}^{-1}; BSI 830Si = -0.21 ext{%}
    • Efflux: 7.9exttDSiday−17.9 ext{ t DSi day}^{-1}; DSI 830Si = 1.20 ext{%}

Flood pulse concept

  • Flood pulses are crucial for big rivers:
    • (1) Productivity for the river and its floodplain
    • (2) Trigger for seasonality (migrations, spawning, etc.)
  • Lateral and longitudinal migrations and spawning behaviors are adaptations to flooding
  • Flood pulses create habitat connectivity and resource pulses across the floodplain

Fish zones and synthesis: RCC and Flood Pulse concepts

  • Fish zone framework aligns with RCC and Flood Pulse concepts, describing spatially explicit productivity and predator-prey linkages across different flood regimes
  • FPZs (Flood Pulse Zones) outline spatially structured productivity patterns across the floodplain and riverine ecosystem

Landscape and floodplain habitats

  • High forest environments and river-floodplain mosaics include:
    • Main river channel, Floodplain, Várzea lake, Gallery forest, Fluvial island
    • Recently deposited alluvium; Tertiary sediments
  • Varzea lakes and related aquatic habitats support diverse communities

Lake and varzea interconnections

  • Varzea lakes and edges interact with river channels; include epilithic and endobenthos communities; ichthyonektontos
  • Plankton, zooplankton, phytoplankton, intertidal and littoral zones contribute to energy flow

Intermittent rivers and habitat mosaics

  • Intermittent rivers exhibit shifting aquatic-terrestrial habitat mosaics (Datry et al. 2014)
  • Spatial mosaic creates variable refugia and recolonization dynamics

Hydrology and biogeochemistry: precipitation-driven responses

  • Scenarios showing isolated pools, connected surface flow, and dry streambeds
  • Magnitude of biogeochemical response linked to precipitation events, streambed surface, surface water level changes, and fluxes of TSS, NO3-, CO2, CH4
  • Seasonal and event-driven pulses shape chemistry and nutrient dynamics

Headwater sites: flow permanence and riparian structure

  • Flow permanence categories: Perennial vs Temporary
  • Riparian vegetation density varies (sparse vs dense)
  • Landscape features influence taxa richness, particularly dispersal ability
  • Taxa richness of weak dispersers vs strong dispersers differs with landscape context
  • Landscape effects on status estimation: potential biases in assessments if dispersal is not accounted for

Connections and key themes

  • RCC emphasizes continuity of habitat and shifts in energy pathways along the river continuum
  • Flood Pulse Concept highlights the centrality of floods in productivity and ecosystem processes
  • Damming introduces the Serial Discontinuity Concept, disrupting continuity and altering biogeochemical cycles and community structure
  • Differences between hard vs soft waters influence nutrient availability, productivity, and vegetation communities
  • Blackwater vs whitewater dynamics illustrate how water chemistry, light, and nutrients interact to shape productivity
  • Megaherbivore activity (e.g., hippos) creates substantial fluxes of nutrients and silica between terrestrial and aquatic domains
  • Intermittent rivers and hyporheic zones provide refugia and ecological opportunities in variable hydrology
  • Food webs in rivers are tightly linked to both allochthonous inputs (CPOM/FPOM, DOM) and autochthonous production (periphyton, phytoplankton), with energy transfer modulated by hydrology
  • Trophic structure and NPP pathways differ between lakes and rivers, reflecting distinct physical and biological constraints