Planktonic Communities: Algae and Cyanobacteria

Comparative Characteristics of Phytoplankton among Different Ecosystems

  • Phytoplankton Diversity

    • Rivers: Very low in low-order streams due to rapid flow, shallow depth, and often high turbidity limiting light penetration; diversity generally increases in larger, high-order rivers where flow is reduced and water depth allows for more stable light conditions and longer residence times.

    • Reservoirs: Low in riverine zones due to continuous inflow, high turbidity, and short residence times; diversity significantly increases in lacustrine (lake-like) zones which are deeper, have longer water residence times, and more stable limnological conditions, allowing a broader range of species to establish.

    • Natural Lakes: High in oligotrophic (nutrient-poor) lakes, which typically support specialized species adapted to low nutrient conditions and clear waters; diversity generally decreases in eutrophic (nutrient-rich) lakes, where a few highly competitive species (often cyanobacteria) can dominate due to high nutrient concentrations, self-shading, and reduced light availability for other species.

  • Phytoplankton Biomass

    • Rivers: Very low in low-order streams primarily due to severe light limitation caused by high turbidity and rapid flushing rates; biomass increases in large rivers, but light often remains a primary limiting factor, exacerbated by advective losses and reduced light penetration through the water column.

    • Reservoirs: Moderately high to high in riverine sections, often influenced by continuous nutrient supply from upstream; biomass can fluctuate significantly, with high nutrient loads sometimes leading to blooms, particularly during periods of reduced flow or thermal stratification.

    • Natural Lakes: Highly variable in temperate lakes, exhibiting strong seasonal patterns (e.g., spring/fall blooms); relatively stable in tropical lakes, where more consistent light and temperature conditions lead to less pronounced seasonal fluctuations and often continuous, but lower, productivity.

  • Phytoplankton Productivity

    • Rivers: Generally low productivity, often severely light-limited due to high turbidity from suspended sediments, and continually flushed out by advective flows. The short residence time prevents significant biomass accumulation and primary production rates.

    • Reservoirs: Highest productivity often occurs in transitional zones between the riverine and lacustrine sections, where a balance of nutrient supply from the river and increased water stability allows for optimal growth; productivity is reduced in purely riverine sections due to light and flow limitations.

    • Natural Lakes: Productivity is generally low in comparison to the littoral zone (shallow, nearshore areas) which often supports macrophytes; productivity increases with moderate nutrient loading (mesotrophic conditions) but declines at very high nutrient loading (hypereutrophic conditions) due to severe light limitation from self-shading and surface algal scums.

Factors Affecting Phytoplankton

  • Light and Nutrient Availability

    • Light energy-absorbing pigments are diverse and critical for photosynthesis, including primary pigments like Chlorophyll a (found in all photosynthetic phytoplankton), and accessory pigments such as Chlorophyll b (green algae), Chlorophyll c (diatoms, dinoflagellates), and various carotenoids (which broaden the spectrum of light absorbed and offer photoprotection). These pigments allow phytoplankton to utilize different wavelengths of light.

    • For phytoplankton cells below 20 \mu m, the transfer of essential materials (nutrients in, waste out) is mainly by molecular diffusion, which is efficient over short distances characteristic of small cell sizes. Larger cells depend more on turbulent mixing for nutrient encounter.

  • Size Classes

    • Ultraplankton (0.2-20 \mu m): Consist of picoplankton (0.2-2 \mu m) and nanoplankton (2-20 \mu m). These small cells often form self-contained communities within the water column, characterized by very rapid nutrient recycling due to their high surface-to-volume ratios, which facilitates efficient nutrient uptake even at low concentrations. They are often the dominant primary producers in oligotrophic oceans and lakes.

    • Microplankton (20-200 \mu m): Include larger single-celled algae and colonial forms. They are generally less efficient in nutrient uptake compared to ultraplankton due to lower surface-to-volume ratios; consequently, they depend more on water movements (advection and turbulence) to encounter new nutrient patches for transport across their cell membranes.

Movement and Structural Adaptations

  • Most phytoplankton species lack active locomotion and are passively dispersed throughout the water column by water movements, making them entirely dependent on prevailing currents and turbulence. Their position in the water column is crucial for light exposure and nutrient acquisition.

  • Sinking is a critical challenge as it disrupts advantageous nutrient gradients and, more importantly, can lead to loss of light, ultimately leading to mortality. Adaptations to reduce sinking include:

    • Increased surface-to-volume ratios: Achieved through flattened shapes, spines, or colony formation, which increases drag and slows descent.

    • Reduced density: By storing lighter compounds (lipids) instead of heavier carbohydrates, forming gas vacuoles (especially in cyanobacteria), or producing mucilage/gelatinous sheaths which have lower density than cell protoplasm.

    • Specific ion regulation: Some species can regulate intracellular ion concentrations to alter buoyancy.

  • Certain motile algae, such as dinoflagellates and some cryptophytes, possess flagella that allow them to swim vertically. This active movement is crucial for optimizing their position in the water column to acquire limiting nutrients from deeper waters or move towards optimal light intensities near the surface, enhancing both nutrient and light acquisition.

Coexistence and Competition

  • Diverse phytoplankton communities typically feature a range of species, with a few dominant types and many rarer species coexisting, which is often explained by the paradox of the plankton. Mechanisms facilitating this coexistence include:

    • Different efficiencies of resource utilization: Species vary in their optimal nutrient uptake kinetics (e.g., high affinity for phosphorus at low concentrations versus rapid uptake at high concentrations), allowing them to thrive under different nutrient regimes.

    • Temporal and spatial niche partitioning: Species exploit different environmental conditions at different times or locations within the water body.

    • Commensalism and selective herbivory: Commensal relationships (where one species benefits without affecting the other) or grazing by zooplankton can reduce the competitive pressure of dominant species, allowing rarer species to persist. For instance, selective grazing on fast-growing species can prevent competitive exclusion.

    • Seasonal and spatial environment changes: Fluctuations in light, temperature, and nutrient availability over seasons or across different parts of a lake create dynamic conditions that prevent any single species from continuously dominating, favoring species succession and maintaining diversity.

Environmental Factors for Growth

  • Photosynthesis: The rate of photosynthesis is critically affected by light intensity and spectral quality, water temperature, and the availability of essential inorganic nutrients.

  • Nutrient limitation is a dynamic process and can shift among primary macronutrients like phosphorus (P), nitrogen (N), and silica (Si) based on their seasonal availability and the specific requirements of different algal groups:

    • Phosphorus: Often limiting in freshwater systems during summer stratification.

    • Nitrogen: Can limit marine systems and some freshwater systems, particularly during intense blooms.

    • Silica: Essential for diatom growth and can become limiting after significant diatom blooms, potentially leading to shifts in community composition towards non-siliceous algae.

  • Certain species, particularly cyanobacteria and some flagellates, require specific organic micronutrients, such as vitamins (B_{12}, thiamine, biotin), for optimal growth, which they cannot synthesize themselves and must obtain from the environment or from other organisms.

Seasonal Patterns and Biomass Dynamics

  • Seasonal biomass maxima typically occur in spring and fall for temperate regions, following predictable patterns known as the seasonal succession of phytoplankton populations.

REDOX: Iron, Sulfur, & Silica

  • Redox Potential Definition

    • Redox potential, denoted as pE, is expressed using the formula: pE = - \text{log}(\text{free electrons})

    • When corrected to pH 7, where hydrogen ions (H^{+}) equal hydroxide ions (OH^{-}), it is referred to as Eh.

  • Eh Values and Environments

    • Positive Eh values indicate an oxidizing environment.

    • Negative Eh values signify a reducing environment.

  • Redox vs. pH Graphical Representation

    • A diagram likely displayed the relationship between redox potential and pH.

  • Redox Potentials of Various Environments and Water Sources

    • Redox potentials measured in mV:

      • 1200 mV: Environment created by electrolyzed strong acid aqueous solution.

      • 800 to 400 mV: Microbial survival zones.

      • Negative values: Represent deeper, often more anoxic regions.

    • Redox Potentials of Common Water Sources (A table of redox characteristics of various common waters was provided):

      • Hydrogen-Rich Water: -300 mV

      • Tap Water: 0 mV

      • Evian: +200 mV

      • Volvic: +260 mV

      • Vittel: +273 mV

      • Coca-cola: +230 mV

      • Morino mizu: +230 mV

      • Asahi Tennensui: +250 mV

      • Kirin Arukarionsui: +174 mV

      • Cold Boiled Water: +170 mV

    • Reductive and Oxidative Distinctions:

      • Reduced Water: 0 mV

      • Oxidized Water: +650 mV

  • Example of Redox Data

    • Redox potential values were quantified from Florida springs caves, showing varying redox levels.

    • Measured values ranged, with highest observed around 400 mV and lower levels near 360 mV.

  • pH and ORP in Lake Skienni

    • Recorded pH levels varied from 5.8 to 7.0.

    • A vertical distribution depicting temperature, oxygen, pH, total iron, and redox potential was presented from June 1967 (After Kjensmo, 1970).

  • Photosynthesis and Redox Reactions

    • Pigment Involved: P680

    • Reaction Mechanism: Excitation leads to transformation: P680^* \rightarrow P680^{++} + e^{-}

    • Subsequent carbon transformations: CO2 + H2 \rightarrow CH_2O

    • Various states of nitrogen reduction with related products specified.

    • Examples of biochemical transformations for iron and sulfur respiration, emphasizing oxidation and reduction processes in bacteria which either store energy (reduction) or release energy (oxidation).

  • Redox and Metal Cycling

    • Eh (volt) Summary: Approximate distributions of iron and manganese were shown with respect to Eh and pH levels.

    • Different oxidation states of Fe and Mn were indicated at various pH levels, showing species dissolution and precipitation transitions.

  • Redox and Sulfur Compounds

    • As pH increases, the stability of sulfur oxidation states shifts, indicating a complexity in sulfur dynamics within aquatic systems.

    • Sulfate and hydrogen sulfide distributions in contrasting oligotrophic (low productivity) and eutrophic (high productivity) environments.

  • Bacterial Transformations of Minerals

    • The role of various sulfur and iron bacteria were defined, exemplifying reactions and metabolic pathways, such as Desulfovibrio and Acidithiobacillus residues.

    • Reactions mediating mineral transformations during microbial processes were systematically listed.

  • Transport of Iron in a Lake

    • Schematic processes detailing iron transport in the water column: Inflows, oxidation to Fe(III), precipitation states including FeS, turbulence effects, and dissolution dynamics.

    • Emphasized the importance of both lateral mixing and vertical eddy diffusion in iron cycling through anoxic and oxic zones.

  • Diagrammatic History of Iron and Manganese

    • Historical data charting the concentrations of Fe and Mn across depth profiles of mesotrophic and eutrophic systems were detailed, showing fluctuation peaks correlating with environmental conditions over time.