Aquatic Ecosystem Dynamics: Lakes and Estuaries

Posted on Jun 23, 2026 in Geology

Q1. Vertical Stratification: Lakes vs. Estuaries

Key Point: Lakes vary slowly (solar-driven), while estuaries vary rapidly (physical energy-driven). Both can develop density gradients that suppress oxygen and nutrient exchange.

Q2. Organic Matter Flux Across Ecosystems

• Allochthonous: Imported from outside; Autochthonous: Produced within.
• Terrestrial to Freshwater: Leaf litter fuels shredder food webs (River Continuum Concept).
• Freshwater to Terrestrial: Emergent insects (e.g., mayflies) subsidize riparian predators.
• Marine to Freshwater: Anadromous fish (e.g., salmon) transport marine-derived N and P.
• Freshwater to Estuarine: River-borne nutrients fuel estuarine productivity.
• Concept: Ecosystems act as semi-permeable membranes with cross-boundary subsidies that buffer consumers.

Q3. Vertebrates and Ecosystem Functioning

• Fish: Planktivores suppress zooplankton (releasing phytoplankton); benthic feeders bioturbate sediment.
• Birds: Waterfowl control macrophyte biomass; guano concentrates nutrients; waders structure invertebrate communities.
• Mammals: Top predators regulate fish; ecosystem engineers (e.g., beavers) transform hydrology.
• Mechanisms: Top-down trophic control, bioturbation, nutrient vectoring, and physical habitat modification.

Q4. Surface Area to Volume (SA:V) Ratio Effects

General Rule: Smaller objects have higher SA:V ratios.

• Nutrient Uptake: High SA:V (small cells) allows efficient acquisition in oligotrophic waters.
• Sinking: Larger particles (low SA:V) have less drag relative to weight and sink faster.
• Desiccation: High SA:V (small bodies) leads to faster water loss in intertidal organisms.
• Decomposition: Smaller fragments (high SA:V) decompose faster due to increased microbial access.

Q5. Recolonization After Disturbance

• Dispersal: Planktonic larvae vs. direct development.
• Supply: Depends on hydrodynamics and proximity to source populations.
• Disturbance: Patch size, severity, and timing relative to breeding seasons.
• Succession: Opportunistic colonizers (polychaetes) arrive first, followed by competitive dominants (bivalves).

Q6. The Estuarine Paradox

High Productivity: Nutrient inputs from river/sea, shallow light penetration, and nutrient trapping via estuarine circulation.

Low Diversity: High physiological stress from rapid salinity and temperature fluctuations. Only euryhaline species survive (Remane’s species minimum). The few tolerant species face little competition, leading to high biomass.

Q7. NE New Zealand Estuaries

• Origin: Drowned river valleys formed by postglacial sea-level rise.
• Morphology: Shallow, dendritic, with extensive intertidal flats.
• Hydrology: Tide-dominated; generally well-mixed.
• Human Impacts: Sedimentation from land-use change, eutrophication, and mangrove expansion.

Q8. Nutrient Limitation in NZ Lakes

While phosphorus (P) is often cited as the limiting nutrient in freshwater, NZ lakes are variable. Limitation is context-specific, driven by catchment geology (e.g., volcanic/geothermal) and land use. N-fixers (cyanobacteria) often indicate N-limitation, but P-control remains effective in many systems.

Q9. GPP and ER in Forest Streams

• GPP (Gross Primary Production): Light-limited; peaks in spring before canopy leaf-out.
• ER (Ecosystem Respiration): Driven by leaf litter and temperature; peaks in autumn.
• Net Pattern: Most streams are net heterotrophic (ER > GPP), relying on allochthonous carbon.

Q10. Trophic Cascades

A top-down effect where predators suppress intermediate levels, indirectly benefiting the base of the food web. Example: Piscivorous bass → planktivorous fish ↓ → zooplankton ↑ → phytoplankton ↓. Inconsistency in results often stems from nutrient recycling, omnivory, or shifts to grazing-resistant phytoplankton.