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Ecology of Zooplankton

1. Pleuston and Neuston

  • Pleuston = organisms whose bodies project at least partly into the air (Physalia physalis = Portuguese-Man-of-War; Velella velella; the water bug Halobates sp., the only oceanic insect)
  • Neuston = organisms that live underneath the water surface film (e.g. Janthia sp. = purple bubble raft snail; Glaucus sp. [nudibranch])
  • Color transparent or blue-violet; predator defense 
  • Harsh environment: high temperature variation, high UV irradiation, high light level inhibit photosynthesis, exposed to wave action, marine and aerial predators
  • Sampling: neuston net
  • Sargassum: special community on and within aggregates of the floating seaweed Sargassum sp.; inhibited by mostly benthic species; some endemic species, ressemble Sargassum in color and shape
2. Vertical Zonation of Zooplankton
  • Epipelagic: upper 200-300 m water column; high diversity, mostly small and transparent organisms; many herbivores
  • Mesopelagic = 300 1000 m; larger than epipelagic relatives; large forms of gelatinous zooplankton (jellyfish, appendicularians) due to lack of wave action; some larger species (krill) partly herbivorous with nightly migration into epipelagic regimes; many species with black or red color and big eyes with maximum sensitivity to blue-green light (why?); 
  • Oxygen Minimum Zone: 400 800 m depth, accumulation of fecal material due to density gradient, attract high bacterial growth, which in turn attracts many bacterial and larger grazers; strong respiration reduces O2 content from 4-6 mg l-1 to < 2 mg l-1
  • Bathypelagic: 1000 3000 m depth, many dark red colored, smaller eyes
  • Abyssopelagic: > 3000 m depth, low diversity and low abundance
  • Demersal or epibenthic: live near or temporarily on the seafloor; mostly crustaceans (shrimp and mysids) and fish
3. Bioluminescence
  • Definition: Light produced and emitted by organisms themselves (sometimes symbiontic bacteria)
  • Depth: Occurs in surface waters, but most important > 1000 m depth; 90% of species in bathypelagial are bioluminescent
  • Orgamisms: Only one species in freshwater, no bioluminescent amphibia, reptiles, birds, mammals; occurs in various marine invertebrates, fish, and protozoa (dinoflagellates)
  • Mechanism: luciferin(s) is oxidized by enzyme luciferase; the resulting energy is released as light; can be red, blue, green)
  • Regulation: special cells = photocytes, or complex organs = photophores; some photophores have lids to regulate light emission/flashig
  • Reason: Communication, prey attraction, counter-shading
4. Vertical Migration
  • Definition: Migration pattern over 24 hrs, typically upwards at night and downwards during the day; known since Challenger-expedition (1872) but still poorly understood, several hypotheses:
    • Avoid visual predators during daylight at greater depths and return to shallow zones with abundant food during night
    • Save energy during non-feeding daylight time in deeper, colder water
    • Exploit different currents at different depths to remain in general area or to ascent to fresh, ungrazed food resources the next day
  • Range: up to 200 m (copepods) to 800 m (krill); speed 10 200 m h-1
  • Migration patterns
    • Nocturnal migration: single daily ascent (sunset) and descent (sunrise); most common pattern
    • Twilight migration: two ascents and two descents every 24 hrs; sunset rise to minimum midnight depth followed by midnight sink; at sunrise, animals ascent again, followed by sink to daytime depth
    • Reverse migration: surface rise during the day, descent at night; seldom
  • Consequences:
    • Increased and expedited vertical transport of organic matter: animals capture prey at shallower depths and transport it downwards either as their body mass or fecal products; both are faster than sedimentation
    • Not all individuals migrate the same range at the same time; population will loose some and gain others, enhances genetic mixing
    • Samples from same depths taken during day and night will differ in species composition and total biomass
  • Deep Scattering Layers: False echosound signals by larger zooplankton (krill, shrimp) and fish, but sometimes also copepods; track migration patterns

  • 5. Seasonal Vertical Migration
    • Neocalanus plumchrus, North Pacific:
      • Adults overwinter at ~400 m and lay eggs
      • Eggs float upwards, nauplii hatch and move further towards the surface in spring
      • Copepodites are present in surface March June, when primary production is highest
      • Copepodite C-V descent late summer, contain large amounts of lipids from phytoplankton; eventually they mature into adults at ~400 m, where they lay eggs and do not feed
    • Calanus helgolandicus and Calanus finmarchicus, Celtic Sea: C-V and C-VI distributed uniformly in winter; in spring, both species concen- trate near the surface and show diel migration; in summer, C. helgolandicus inhabits mixed layer, C. finmarchicus lives below thermocline; 
    6. Patchiness
    • Origins of Patchiness:
      • Physical processes that concentrate or disperse plankton (upwelling, eddies, gyres, Langmuir); scale 100 m 1000 km 
      • Exclusion theory of Bainbridge (1953): high food concentration attracts zooplankton that will diminish food recources; outside the zooplankton patch, phytoplankton can grow faster than zooplankton generation times to form a new patch of food; eventually zoo-plankton will move to new food patch, etc. 

    • Problems of Patchiness: 
      • Difficult to sample because plankton nets are towed over long distances to collect sufficient material for analysis 
      • Variation in abundance causes inaccuracies in numbering/budgeting whole communities in a representative manner
    7. Zoogeography of Zooplankton
    • Oceans have less physical barriers for species distribution than terrestrial ecosystems; physical barriers (continents) for longitudinal distribution but not for latitudinal (north-south) distribution
    • North-south distribution mainly set by temperature tolerance of species
    • Most zooplankton (50%) from tropical to temperate regions; only 1/3 is restricted to warm water; few species restricted to polar regions, among them some show bipolar distribution 
    • Number of epipelagic species decreases from low to high latitudes, but the number of individuals (biomass) increases from low to high latitudes
    • Bathypelagic species diversity and abundance relatively constant
    • Antarctica: circumglobal distribution due to circular current pattern around the Antarctic continent; Antarctic deep water can transport cold-water species far north into the Pacific Ocean
    • Artic Sea: little water exchange between Pacific and Atlantic Oceans, species in North Pacific and North Atlantic differ largely
    • Human activity (e.g. new connections such as Suez Channel, transport of organisms with ship ballast water) can alter global species distribution