Aquatic Ecosystem Overview:

• We need to understand the physical (e.g. hydrodynamics) and chemical environment that ultimately control the productivity, interactions, and evolution of life in aquatic ecosystems.

• Organisms reside in habitats characterized by their physicochemical setting, perform a particular niche or function, and represent a particular trophic level.

• Individual organisms are members of a population, which interacts with other populations within a community.

• Ecosystems may possess multiple community types based on groupings of common habitat and/or niche characteristics.– Littoral (fringe with light) - Benthic (bottom)– Plankton (drifting) - Nekton (swimming) – Detritus (dead organics) - Groundwater (subsurface; hyporheic)

The Lake Example:

Temperate lake formed by geological faulting, depicted during the summer season.

Density differences due to temperature result in stable layers, or strata. This process is called stratification.

Know your lake regions for benthic and open water habitats.

Why the difference in O2 and CO2?

(Cole, 1994; Fig 2.1)

Biological Productivity (Rawson Diagram):

Volumetric units: mg C/m3/d

Areal units: mg C/m2/d

(Cole, 1994; Fig 1.1)

Productivity and Trophic Status• Ecosystem productivity is generally classified according to its trophic

state for convenience: – Oligotrophic (low)– Mesotrophic (medium)– Eutrophic (high)

• These states represent ranges within a continuum of potential productivity within a system.

• The impetus for identifying trophic state came from observations that certain biota were often associated with specific nutrient levels.

• Being able to classify systems is convenient for comparison of different areas as well as for defining management goals and reference points.

• Trophic state is generally defined as a function of phytoplankton biomass / chlorophyll concentrations, nutrient concentrations, and/or water clarity.

Probability Distribution

(Dodds, 2002; Fig. 17.2)

Fixed Boundary Classifications

(Dodds, 2002)

OECD = Organization for Economic Cooperation and Development

Total P = phosphorous in dissolved and solid forms; inorganic and organic.

Mean Chl = annual average of chlorophyll a.

Mean Secchi = average depth to which a black and white disc can no longer be seen.

Continuous Value Scale

(Dodds, 2002; Fig. 17.2)

Stream Ecosystems

• Unless flow is slow enough, stream systems usually do not have a truly planktonic biomass of primary producers. Trophic state is measured by:– Attached benthic algae (periphyton) biomass– Nutrient (N & P) concentrations

• Due to dependence of streams on terrestrial (allocthonous) organic matter, primary producers are not always a good indicator of system productivity.

Properties of Water• Polarity & H-bonds• Water Density • Water as a Solvent• Heat

(Sensible vs Latent)• Surface Tension• Reynolds Number

(Inertia to Viscosity)• Molecular Diffusion• Flow Type

(Laminar vs Turbulent)• Light Attenuation

Water Density

Stratification by Temperature or Dissolved Ions (salts):

Water as a SolventGeologic Weathering

Solute solubility increases with increasing temperature due to increases frequency of molecular interaction and less H-bonding between water molecules and more hydration of solutes by water molecules.

Gas solubility decreases with increasing temperature due to increased vibration and expansion of molecules plus the reduced partial pressure favors a gas equilibrium shift from water to air, something solutes don’t experience because water binds solutes electrostatically.

Sensible vs Latent HeatHigh Heat Capacity

Water is our natural thermal buffer!

Surface Tension•At water-air or water-solid interfaces molecules of water are nearly completely H-bonded together, like a molecule thick layer of ice.

• This is added tension force is utilized by many aquatic & semi-aquatic arthropods

• It is also what explains waters ability to be drawn against gravity when in narrow spaces; capillary action.

Water strider (right)

Fishing Spider (left)

Reynolds Number (Re)• Viscosity: resistance of a liquid to change of form.• Inertia: resistance of an object (body) to a change in its state

of motion.• These two forces together influences an organism’s ability to

move in water and how water flows.• The ratio of inertial force to viscous force is Re.

• U = velocity of object relative to the fluid; l = object length; S = surface area; µ = dynamic viscosity; p = fluid density

lU

lSUSU

F

F

v

i

2

Plankton Nekton

Viscosity Inertia

• High Re values are associated with dense, fast and long (streamline) objects.

• Low Re values are associate with small and slow objects, particularly those with a high surface to volume (S:V) ratio.

• The Y-axis below could be switched from velocity to body size (length) a similar trend would be seen.

Molecular Diffusion: 21

21

xx

CCDJ

• Water molecules move constantly in a vibrating fashion, called Brownian Motion.

• Browning Motion is increased by increased temperature.

• Solutes in water also experience Brownian Motion.

• Molecular diffusion it that solely due to solute and solvent Brownian Motion (expressed as the diffusion coefficient at a given temperature; D) and the solute concentration gradient between two locations.

• The rate of molecular diffusion (J) is expressed by Fick’s Law.

• Small organisms at low Re are dependent on molecular diffusion.

Laminar versus Turbulent Flow• Laminar flow is unidirectional, turbulent flow is more chaotic.

• At larger spatial scales flow is often turbulent. Eddies may form. Solute diffusion becomes dominated by eddy transport for water parcels, not solely molecular diffusion.

• At smaller spatial scales flow is more often laminar, due to the viscous force dampening turbulence.

• Flow across a surface experience a viscous (frictional) force and becomes more laminar.

• The transition between turbulent and laminar flow is called the flow boundary layer.

Laminar Flow Turbulent Flow

Flow Boundary Layer Thickness:

• It ↓ with ↑ velocity.

• It ↓ with ↓ surface roughness.

• It ↓ with ↓ object size.

• It ↓ with ↓ distance from the upstream edge.

Relationships between Re and flow type.

• For any given object; lower Re will translate to more laminar flow.

• Re is lower when velocity decreases.

• Flow is more laminar at higher Re when the object is streamline (long).

Colonial Diatom

Note refraction

Light Penetration & Attenuation

Amount reflected depends on wave conditions and ice cover.

Light that penetrates the surface may then be absorbed, reflected of transmitted.

Light intensity decreases logarithmically with depth; a process called attenuation

1% Io

(Io)

Euphotic zone

• Different wavelengths of light absorb differently in pure water. Blues and greens penetrate deepest and reds the least. Why the “deep blue sea”?

• Particles and colored dissolved organic matter will further influence how quickly different colors of light are absorbed in natural waters.

Attenuation (or extinction) coefficient (k is defined as:

k = (ln Io – lnIz) / z

Light intensity at a given depth can be calculated as:

Iz = Ioe-kz

Secchi Depth and Trophic Status

(k)

Secchi depth (ZSD) can be used to estimate the attenuation coefficient (k).

k = 1.7 / ZSD ; This translates into the 1% light level being 2.7 times ZSD, or

that 16% of surface irradiance remains at the ZSD.