Plankton and Factors Affecting Abundance
Plankton and Factors Affecting Abundance
Definitions
Pelagic: Refers to the open sea.
Nekton: Organisms capable of active swimming against ocean currents.
Plankton: Organisms that are either floating or drifting, unable to move against currents. Examples include net plankton and smaller forms.
Phytoplankton: Photosynthetic plankton; essentially marine plants.
Zooplankton: Heterotrophic plankton; marine animals.
Mixoplankton (= Mixotrophic): Plankton that can switch between autotrophic (photosynthetic) and heterotrophic (consuming other organisms) nutrition, like Ochromonas sp. flagellates that consume bacteria cells.
Holoplankton: Organisms that spend their entire life cycle as plankton.
Meroplankton: Organisms that spend only part of their life cycle as plankton (e.g., larval stages of benthic organisms).
Neuston: Plankton associated with the surface slick of the water.
Pleuston: Organisms that stick up above the water surface (e.g., Portuguese Man-o'-War).
Pelagic Food Web
Components: The pelagic food web involves a complex interaction of various organisms and processes within the water column.
Euphotic Zone (approx. 20 - 150 m): The upper layer where sufficient light penetrates for photosynthesis.
Phytoplankton: Primary producers, converting CO2 and H2O into organic matter using light energy and nutrients (C, N, P, S, Fe).
Zooplankton: Grazers of phytoplankton.
Fish: Predators of zooplankton.
Protozoa: Microscopic grazers.
Bacteria, Virus: Decomposers and components of the microbial loop.
Mesopelagic Zone (approx. 150 - 6,000 m): Deeper zone with less light.
Benthos (incl. bacteria): Organisms living on or in the seafloor, fed by sinking organic matter.
**Major Food Chains:
Grazing Food Chain:** Direct consumption of phytoplankton by zooplankton, which are then eaten by fish.
Microbial Loop: Flow of dissolved organic matter (DOM) back into the food web via bacteria and protozoa, which are then consumed by larger zooplankton.
Processes:
Sinking Flux: Organic matter sinking from the euphotic zone to deeper waters and the benthos.
Aggregation/Packaging: Formation of larger particles from smaller ones, accelerating sinking.
Advection: Horizontal transport of substances.
Gases: Exchange of O2, CO2, methane, and DMS with the atmosphere.
Other Inputs: Pollutants, radionuclides, pathogens, sewage also enter the system.
Examples in a Herring Food Web:
Phytoplankton are consumed by smaller herbivorous copepods.
Small herbivorous copepods are eaten by carnivorous copepods, water fleas, amphipods, mollusc larvae, and comb jellies.
Large herbivorous copepods and Krill are also primary grazers.
Larvaceans, arrow worms, jellyfishes, comb jellies are also zooplankton predators.
Young herring (7-12 mm) feed on similar prey as carnivorous copepods.
Sand eels (12-42 mm) consume smaller zooplankton.
Adult herring (42-130 mm) are apex predators in this simplified food web.
Phytoplankton Groups
General Characteristics: Most are microscopic bacteria and eukarya.
Net Plankton: Dominated by two unicellular eukaryotic groups:
### Diatoms (Phylum Bacillariophyta)
Structure: Occur singly or form chains. Range from nano- to microplankton. Encased in a silica shell (SiO_2) called a frustule, consisting of two valves. Usually radially symmetrical.
Reproduction: Primarily asexual by modified fission (doubling once or twice per day). Can also undergo sexual reproduction.
Productivity: Estimated to contribute to 45 ext{%} of ocean primary productivity.
Distribution: Dominant in temperate to polar regions, can be planktonic or benthic.
Toxins: A few species produce Domoic Acid, which causes amnesic shellfish poisoning.
Examples: Thalassiosira, Chaetoceros, Asterionella japonica.
### Dinoflagellates (Phylum Dinoflagellata)
Locomotion: Swim using two flagella.
Structure: Possess a silica skeleton; some have a cellulose theca.
Reproduction: Both sexual and asexual reproduction.
Diversity: A very diverse group.
Unique Features:
Some bioluminesce.
Many produce toxins (causing red tides).
Can be heterotrophic (in some stages).
Form mutualistic relationships (e.g., zooxanthellae in corals).
Life History: Often have multiple life history stages, including benthic cysts.
Distribution: Often abundant in tropics and mid-latitudes during summer.
Red Tides: Some species, like Cochlodinium polykrikoides, cause harmful algal blooms known as red tides.
Other Significant Groups:
### Coccolithophores
Characteristics: Eukaryotic, unicellular, nanoplankton. Spherical and covered with calcium carbonate plates called coccoliths.
Significance: Especially important in tropical regions. Blooms can be seen from space.
### Silicoflagellates
Characteristics: Unicellular, eukaryotic, biflagellate. Possess an internal skeleton of silica scales.
Distribution: Often found in the Antarctic and open ocean.
### Cyanobacteria (Blue-green algae)
Classification: Prokaryote, Domain Bacteria.
Key Process: Capable of nitrogen fixation (converting N_2 to ammonia or ammonium).
Distribution: Abundant in open ocean and nearshore environments.
Productivity: Contribute an estimated 1/3 to 4/5 of ocean productivity.
Ecological Role: Very important in nutrient cycling. Some live symbiotically (e.g., in sponges).
Crucial Points about Phytoplankton:
High diversity among groups.
Varying nutrient needs and limitations (e.g., Fe, Si, Ca, P, N).
Different densities and abilities to swim.
Photosynthesis and Primary Productivity
Photosynthesis Equation: 6 CO2 + 6 H2O \xrightarrow{\text{Light, Nutrients, Chlorophyll}} C6H{12}O6 + 6 O2
Carbon dioxide + Water $\xrightarrow{Energy}$ Sugar + Oxygen.
Global Significance: Approximately 50 ext{%} of global photosynthesis occurs in the oceans.
Marine Dominance: Marine photosynthesis is primarily carried out by phytoplankton.
Ecosystem Foundation: The sugar generated by photosynthesis serves as food for the entire marine ecosystem.
Measuring Standing Crop: Satellite images of chlorophyll a concentrations are used to estimate the