Lecture 19+20 Plankton Notes

Plankton

• Definition: Organisms living in the water column that are unable to maintain their position against water currents.

Objectives

• Define plankton and explain categorizations.

• Describe major types of plankton and their life cycles.

• Describe characteristics of planktonic organisms and their functional significance.

• Describe the ecological significance of plankton in marine systems.

Collection Methods

• Plankton nets are used to collect plankton.

Taxonomic Composition

• All major groups are represented: prokaryotes, fungi, protists, plants, and animals (also viruses).

• Underlined groups are represented by many taxa and tend to be common.

Categories of Plankton

1. General Trophic Role

   • Phytoplankton: Photosynthetic plankton.

   • Zooplankton: Animal and non-photosynthetic protists (protozoans).

2. Size

   • Femtoplankton: < 0.2 µm (virus)

   • Picoplankton: 0.2 - 2 µm (bacteria)

   • Nanoplankton: 2 - 20 µm (phytoplankton)

   • Microplankton: 20 - 200 µm (phytoplankton & zooplankton)

   • Macroplankton: 200 - 2,000 µm (zooplankton)

   • Megaplankton: > 2,000 µm (zooplankton)

Size Considerations

• Generally small, enormous size range.

• Different types tend to be different sizes which influences sampling accuracy.

Phytoplankton

• Definition: Photosynthetic plankton (includes prokaryotic & eukaryotic algae).

Taxonomic Composition

• Approximately 4,000 described eukaryotic species, plus many prokaryote species.

• Many types: diatoms, dinoflagellates, coccolithophores, and prochlorophytes.

Diatoms

• Class Bacillariophyceae.

• Approximately 1,800 species in marine plankton (a recent DNA study found 4,748 OTUs).

• Usually range between 10 and 200 µm (mainly microplankton but some are nanoplankton).

• Occur everywhere – most species at low latitudes, main primary producers at high latitudes.

• Diatoms are protists with a cell wall in two halves and impregnated with silica.

Dinoflagellates

• Subphylum/superclass Dinoflagellata.

• Approximately 1,800 species in marine plankton (only ~600 are photosynthetic).

• Size range ~2 to ~200 µm (mainly microplankton but some are nanoplankton).

• Occur everywhere - maximum species diversity and abundance at low latitudes.

• Dinoflagellates are protists with two flagella orientated perpendicular to each other.

Coccolithophores

• Part of the Phylum Haptophyta.

• Approximately 500 species (almost all are marine and planktonic).

• Size range ~2 to ~20 µm (nanoplankton).

• Collectively occur at all latitudes, most species in subtropical areas.

• Important primary producers in nutrient-poor areas.

• Coccolithophores are protists that possess calcified scales called coccoliths.

Prochlorophytes

• A type of cyanobacteria (prokaryote).

• Tiny cells (0.6 – 0.8 µm).

• Presence of prochlorophytes in marine plankton was first noted as recently as 1988.

• Occur everywhere - very important in nutrient-poor waters/open ocean.

Contribution to Phytoplankton Primary Productivity (Example)

• This table shows the percentage contribution to phytoplankton primary productivity in different regions.

Vertical Distribution

• Phytoplankton are restricted to the euphotic zone.

• Euphotic zone = part of the marine environment where there is sufficient light for photosynthetic organisms to grow.

Morphological Characteristics

• Phytoplankton are typically unicells with an irregular shape.

• Typically minute organisms with a high surface area to volume ratio.

Forces Influencing Movement in Seawater

1. Viscous Forces: 'Stickiness' of the water molecules; impede movement.

2. Inertial Forces: Tendency of a moving object to continue moving; facilitate movement (if the object is already moving).

• The relative influence of viscous versus inertial forces varies according to the size and speed of the object.

• Small ('microscopic') objects are relatively strongly influenced by viscous forces, and vice-versa.

Implications of Small Size

• Movement of small objects is strongly influenced by viscosity of water, which means that small organisms sink relatively slowly.

• SA:VOL ratio is high compared to larger objects.

Implications of Irregular Shape

• For objects of the same size, the rate of sinking also depends on the shape of the object.

• Objects with a high SA:VOL ratio have relatively more contact with the seawater.

  • Experience relatively more drag.

  • Sink relatively slowly.

• High SA:VOL ratio may also facilitate nutrient assimilation.

• Irregular shapes are also associated with non-linear sinking patterns.

Advantages of Small Size and Irregular Shape

1. Retard rate of sinking, less energy to maintain a position in the euphotic zone.

2. Aid in the assimilation of nutrients (nutrient availability may limit primary productivity in marine systems).

Ecological Significance of Phytoplankton

• > 90% total marine primary productivity (~50% of global total).

• Basis of oceanic food webs.

• Significant contribution to neritic ones.

Zooplankton

• Definition: Animal & non-photosynthetic protists (= protozoans) component of the plankton.

• Essentially the consumers in the plankton but generally not heterotrophic bacteria (heterotrophic bacteria are usually included in the bacterioplankton).

Taxonomic Composition

• High diversity and many types, includes:

  1. Species that spend their entire life cycle in plankton (called holoplankton).

  2. Species that spend only a part of their life cycle in plankton (called meroplankton).

Meroplankton

• Early life stages (ELS) are typically planktonic, and adults/juveniles benthic or nektonic.

• High diversity and abundance.

• Most nektonic and benthic invertebrates and teleosts have planktonic ELS, e.g., ~70% of benthic invertebrates in coastal environments.

• Duration of the planktonic phase is highly variable (minutes to months).

  • Many ascidians: hours

  • Black-lip abalone: 4 - 10 days

  • Blue swimmer crab: ~21 days

  • Western rock lobster: ~9 months

Holoplankton

• Approximately 5,000 animal species (and probably at least as many protozoan species).

• Animal component is dominated by crustaceans, especially:

  1. Copepods

  2. Euphausiids (krill)

• Also range of other taxa, e.g., chaetognaths.

Subclass Copepoda

• More than 2,500 described species in marine plankton.

• Occur throughout the pelagic environment, with 1,000s individuals per m^3 in surface waters – one of the most numerous animals on the planet.

• Most ~1 mm (macroplankton).

• Many herbivores (but some are omnivores or carnivores).

Phylum Chaetognatha

• ~120 species, all are marine & most are planktonic.

• Most are ~3 - 10 cm long (megaplankton).

• Carnivores, active predators of smaller zooplankton, including copepods.

• Can consume 37% of their body weight per day.

Vertical Distribution

• Ranges from 0 m to deepest ocean (collectively).

• However, peak diversity & abundance is in shallower waters (to 1,000 m).

Ecological Significance of Zooplankton

1. Dispersal: Often the main dispersive phase in the life cycle of meroplanktonic species.

2. Vital Link in Marine Food Webs: e.g., Antarctic Krill, Euphausia superba.

Order Euphausiacea (krill)

• ~100 species, exclusively marine & pelagic.

• Huge biomass (amongst the highest for animals).

• Range in length from ~2 to 6 cm (megaplankton).

• Mainly omnivores.

Euphausia superba

• Type of krill.

• Dominant zooplankter in Antarctic waters, ca. biomass 125 - 725 million tonnes.

• Omnivorous, mainly grazes phytoplankton, especially during spring and summer.

• Vital component of Antarctic food webs, virtually all larger consumers rely directly or indirectly on krill.

Krill Consumption

Keystone Species

• Euphausia superba is said to be a keystone species in Antarctic waters.

• Keystone species = species that impacts on its community disproportionately to its abundance and in such a way that its removal would dramatically alter the community.

• More likely in a relatively 'simple' community.

• Antarctic communities may be relatively 'simple', e.g., food web is shorter than 'typical' pelagic food webs.

Features Allowing Euphausia superba to Play an Important Role

1. Relatively large compared to the phytoplankton it consumes (> 10^3 longer; > 10^6 heavier).

   • Fewer levels in food web (energetically efficient).

2. Swarming behaviour.

   • Efficiently harvested.

Details of Some Large Krill Swarms

Conclusions

• Marine plankton comprises a very diverse range of organisms.

• Play many vital roles in marine ecosystems.