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Unusual: chlorophyll a increases with depth rather than declining toward the photosynthetic compensation depth, which is opposite normal summer patterns. Hypothesis 1: surface nutrient competition causes nutrients to accumulate near the thermocline so phytoplankton concentrate there. Hypothesis 2: intense surface mixing (wind) makes the surface environment hostile, so phytoplankton survive better at lower depths. Tests: (a) recreate lake nutrient profile in lab with chlorella and no predators to see vertical positioning; (b) run two mesocosms with identical thermoclines but different surface-mixing regimes (mixed vs calm) and compare vertical chl a profiles.
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Unusual: chlorophyll a increases with depth rather than declining toward the photosynthetic compensation depth, which is opposite normal summer patterns. Hypothesis 1: surface nutrient competition causes nutrients to accumulate near the thermocline so phytoplankton concentrate there. Hypothesis 2: intense surface mixing (wind) makes the surface environment hostile, so phytoplankton survive better at lower depths. Tests: (a) recreate lake nutrient profile in lab with chlorella and no predators to see vertical positioning; (b) run two mesocosms with identical thermoclines but different surface-mixing regimes (mixed vs calm) and compare vertical chl a profiles.
Nutrient model: Tilman/Monod — growth µ = µmax [S]/(Ks+[S]); species with low R* or Ks win at low nutrients. Light model: Huisman — defines a critical depth and critical turbulence; turbulence and light regime control bloom formation and vertical position. Diatoms: high µmax, thrive in turbulent, nutrient-rich mixing (spring/fall overturn). Cyanobacteria: *low nutrient requirement (low R)**, favor warm, stable, stratified, low-turbulence waters. Dinoflagellates: motile heterotrophs/mixotrophs, can graze others and position for light in stratified, low-nutrient conditions.
Lake 227 (N+P): long-term additions transformed the lake to eutrophic with cyanobacterial blooms; showed P controls algal biomass because cyanobacteria can fix N and dominate. Lake 226 (split): C+N side remained oligotrophic while C+N+P side developed dense cyanobacterial blooms → P is limiting. Lake 304 (P only): P additions alone produced strong, N-fixing cyanobacterial blooms; stopping P led to rapid recovery → P alone can trigger eutrophication. General significance: phosphorus is the primary controller of algal biomass and eutrophication.
(a) Residence time = average time water spends in the lake (lake volume ÷ inflow or outflow rate). (b) Two characteristics: nutrient cycling/availability and stratification/mixing/retention of pollutants. (c) Long residence time allows nutrients and pollutants to accumulate and supports development of stratification and biological uptake; short residence time flushes nutrients and reduces potential for accumulation and prolonged stratification.
Acidification shifts the carbonate system toward higher dissolved CO₂ as bicarbonate and carbonate convert to CO₂ at lower pH, increasing freely available CO₂. Elevated CO₂ can favor filamentous algae because their morphology/physiology may allow more efficient CO₂ uptake or growth under higher CO₂, letting them outcompete other periphyton forms.
(a) Cell division drives exponential increase until resources (nutrients/light) limit growth; division rate sets how fast biomass increases in the patch. (b) Eddy diffusion controls how quickly water (and phytoplankton) are mixed spatially; low eddy diffusivity → limited spread, high eddy diffusivity → wider daily mixing. (c) If cells divide once per day and nutrients are not limiting, patches can double in biomass daily; spatial extent depends on advection/diffusion rates, so patch size grows as organisms reproduce and are transported by eddy diffusion.
Factors: nutrient gradients, temperature gradients, light penetration, density, and oxygen gradients. Seasonal mixing redistributes nutrients and oxygen, reducing sharp gradients and allowing microbial communities to become more homogenized vertically; stratification isolates layers so specific communities concentrate where conditions (e.g., low O₂ or high organic matter) suit them.
Water-balance shows required basin:lake ratio becomes negative (AB/AL ≈ -0.33) when plugging EL-P and P-ET, which is physically impossible. Interpretation: because precipitation exceeds lake evaporation, the lake has net water surplus and will overflow rather than concentrate salts; thus with these inputs the lake cannot become saline under steady-state assumptions (all runoff enters lake, no extra losses). Key assumptions: steady state, P-ET runoff enters lake, no other gains/losses.
(a) The stagnant-film model treats a thin boundary layer of thickness z where molecular diffusion dominates; flux F = D * ΔC / z (D = molecular diffusivity). (b) Using F = DΔC/z, one estimates gas flux per unit area; multiply by lake surface area to get whole-epilimnion exchange. The model links boundary-layer thickness and concentration gradients to gas exchange rates.
Sensible heat transfer's importance depends on lake thermal buffering and surface-area-to-volume effects. (a) Small deep lakes have smaller heat capacity relative to surface forcing, so sensible heat exchanges (air–water conduction/convection) can cause larger temperature changes and be more important. (b) Large deep lakes have greater thermal inertia and buffer temperature change, making sensible heat less important relative to other terms (e.g., solar input). Wind and temperature gradients also modulate sensible flux importance.
Oxygen-change method is better for estimating net ecosystem O2 production over incubation intervals and when respiration is of interest. The 14C method is better for measuring gross or instantaneous carbon fixation (C uptake). Using both is useful to compare carbon assimilation with O2 production to detect processes like photorespiration, dark respiration, or to cross-validate productivity estimates.
(a) Processes: light attenuation (absorption + scattering), vertical mixing, self-shading, DOM and particle attenuation, and diel light variation. (b) Yes — bottle incubations can exaggerate surface photo-inhibition because they remove natural vertical mixing (constant high light exposure), alter turbulence and light spectra, and prevent phytoplankton from experiencing fluctuating light, producing artifactual depression near the surface.
(a) Reynolds number Re = U·L/ν (U = velocity, L = characteristic length, ν = kinematic viscosity). It quantifies inertial vs viscous force dominance. (b) Large organisms (Lake Trout) operate at high Re → inertia dominates, turbulent flow; small organisms (copepod) operate at low Re → viscous forces dominate and motion is “sticky.” (c) High Re lets fish use flow cues and detect prey over longer distances; low Re limits copepods to short-range chemical/mechanical cues and makes prey detection and feeding governed by viscous-dominated flows.
(a) If compensation depth is shallower than the thermocline, autotrophs will be concentrated above the compensation depth (mostly in the epilimnion) because below that they have negative net photosynthesis; few persist below the thermocline. (b) If compensation depth is deeper than the thermocline, light penetrates well below the thermocline so autotrophs can persist across and below the thermocline where light still supports positive net photosynthesis.
Water is densest at ~4°C. In Arctic lakes, surface waters can cool below 4°C and remain less dense than ~4°C bottom water, producing inverse density gradients (cold near surface, ~4°C deeper) unlike warm lakes. Under ice, warming/cooling and density differences (near-4°C water below colder surface) plus wind-driven and convective processes can produce under-ice circulation and internal movement despite ice cover.
(a) Factors: inputs (runoff, atmospheric deposition), zooplankton excretion, decomposition, release from sediments (under low O₂), stratification and mixing, and biological uptake. (b) Phytoplankton exploit frequent small pulses/patches of phosphate and turbulent mixing that create transient nutrient-rich microzones, plus biochemical strategies (internal storage, alkaline phosphatases to use organic P) that let them acquire P despite bacteria’s lower affinity.
Range of variability: the natural fluctuation in streamflow magnitude and timing (low flows, high flows, floods) creating spatial and temporal heterogeneity. This variability creates diverse habitats (pools, riffles, floodplain connections) and periodic disturbances that maintain species diversity by providing different niches and renewing habitats; it also expands habitat extent during high flows by accessing floodplains and connecting habitats.
Effective N = 0.36 + (1×0.36) = 0.72 mol N m⁻² yr⁻¹. Effective P = 0.018 + (3×0.018) = 0.072 mol P m⁻² yr⁻¹. Effective N:P ≈ 0.72/0.072 = 10:1. Lowering the supply ratio below Redfield (16:1) makes the system relatively N-limited, favoring N-fixing cyanobacteria which can fix atmospheric N and exploit elevated P.
(a) Small particles (silt, clay, organic matter) stay suspended longer and are transported far by turbulence; large particles (gravel, cobble) settle quickly and move only during strong flows (bedload). (b) Coarse substrates provide stable attachment habitat for grazers/scrapers (mayfly, caddisfly larvae), whereas fine sediments favor burrowers and deposit feeders (oligochaetes, chironomids) and can reduce grazer success by clogging feeding structures.
(a) A lake district is a region with many nearby lakes sharing similar landscape, climate, geology, and hydrology. (b) Spatial–temporal coherence means lakes show parallel changes over time (e.g., temperature, nutrient trends) driven by common external drivers. (c) Lake districts often promote coherence because shared drivers cause similar responses, but coherence is not guaranteed—local factors (depth, land use, groundwater) can produce divergent behaviors.
(a) DOM = dissolved organic molecules from decaying plants, soils, algae, and microbes (ranges from simple compounds to humic substances). (b) As bacterial fuel, labile DOM supports heterotrophic bacterial growth and the microbial loop. (c) As a UV attenuator, colored DOM absorbs/scatters UV, reducing UV stress and limiting light penetration. (d) Interactions: high DOM can boost bacterial production (shifting energy to the microbial pathway) while shading reduces phytoplankton production, thereby altering food-web structure and shifting energy away from phytoplankton-based chains toward bacteria-mediated pathways.
Nutrient supply sets the long-term ceiling for primary production (especially phosphorus). Food-web effects (grazing, trophic cascades) modulate how close phytoplankton come to that ceiling. For example, abundant large grazers (Daphnia) can suppress phytoplankton biomass; if fish reduce grazer abundance, grazing weakens and phytoplankton increase. Thus food webs alter realized production within the nutrient-imposed limit.
Molecular diffusion = slow movement of molecules down concentration gradients (microscale). Eddy diffusion = turbulent mixing moving packets of water (much faster at large scales). Highest vertical eddy diffusion: epilimnion (wind-driven turbulence). Lowest: thermocline (strong density gradient suppresses mixing). Hypolimnion usually has low–intermediate mixing but can increase during storms or turnover. Rates change with turbulence energy and density stability: weak stratification → higher eddy diffusion; strong stratification (thermocline) → low eddy diffusion.
Temperature profiles reveal mixed layers (uniform T) vs thermocline (sharp T change). Sample strategy: take few samples evenly spaced in a mixed column; take more, closely spaced samples across the thermocline where chemical/biological gradients are steep. Example—mixed 10 m: few samples (surface, mid, bottom). Stratified 10 m: sample within epilimnion (0–4 m), densely across thermocline (4–6 m), and in hypolimnion (6–10 m). This targets zones of expected change in nutrients and chlorophyll.
