Chapter 17: The Open Sea
Feb 24, 2016
Chapter 17:The Open Sea
300 million cubic milesLimited amounts of nutrients
Restricts numbers of primary producers
Supports few large animals
Regions of the Open SeaBased on physical characteristics of the
water and life forms in themVertical zonation
Depends on depth that light penetrates to support photosynthesis
Photic zoneReceives sunlightCorresponds to the epipelagic zone
Location of pelagic animals in upper 200m of oceanAphotic zone
Extends to ocean bottomLight disappears (total darkness)
Life in the Open Ocean2 groups: plankton and nektonClassification of plankton
Taxonomic groupsSeston: particles suspended in the
seaConsists of tripton (mineral particles, dead, organisms, decaying organic matter) and plankton
PhytoplanktonZooplanktonBacterioplankton (archaea and
eubacteria)Virioplankton (free viruses)
Functional groupsAkinetic: do not move at allKinetic: move by flagella, jet propulsion, body
undulation, or appendages
SizeMacroplankton: visible to the naked eye (>1mm)Microplankton: could be caught with plankton netsNanoplankton (centrifuge plankton)
Pass through plankton nets Concentrated best through centrifugation
Femtoplankton and picoplankton (viruses and smallest prokaryotes)
Mesoplankton and megaplankton (animal plankton larval fish)
Life historyHoloplankton
Planktonic throughout their lives Ex: microbes, invertebrates
Meroplankton Benthic as adults but plankton as larvae
Spatial distributionNeritic plankton (presence of meroplankton)Neuston
Live close to water’s surface Use surface tension of water to remain at or near surface
Pleuston Break the surface of water Can be buoyed by gas bladders or bubbles
Patchiness in the Open SeaPatches: localized aggregationsFactors that contribute to their formation:
Areas of upwellingVariations in sea-surface conditionsVertical mixing of waterDownwelling eventsMeeting waters of different densitiesGrazing by zooplankton
MicropatchinessMicrobes attach to organic matter particles to
form marine snow
Plankton migrationsZooplankton
Daily migrations from the surface down to 1 mile
Densely packed form a deep scattering layer
Can give a false sonar reading
MegaplanktonMost are animalsCnidarian zooplankton: moon jellyfish, lion’s mane, Pelagia noctiluca
Molluscan zooplankton: sea butterflies, purple sea snail, Glaucus (nudibranch)
Urochordates: salps, larvaceans
NektonActive swimmersInvertebrates: squidFish: billfish, tuna, ocean sunfish, sharks, manta rays
Birds: penguinsMammals: whales
Survival in the Open SeaAdaptations for staying afloat:
SwimmingFlagella, cilia, jet propulsionAppendages: legs, antennae, paired limbs
Body undulation: side-to-side horizontally or vertically
Reduce sinking ratesIncrease friction
Decrease volumeFlatten bodyIncrease body length
BuoyancyStorage of oils
Increase water content of bodyExchange of ionsUse gas spaces
Avoiding predationCamouflage
CountershadingDark dorsal surfaces with light ventral surfaces
Hard to see from above and belowSome species are almost transparent
Form coloniesHelps to capture prey, digest food, maintain
colony, provide floatation, or reproduce
Ecology of the Open SeaPelagic ecosystem
Inhabitants live in the water column
Few seaweeds; no vascular plantsGet nutrients from the surrounding seawater
Surface water receives large amounts of light but no nutrients from the landLow levels of nitrogen and phosphorus needed to support phytoplankton
Water above the deep-sea floor has higher levels of nutrients but not enough light to make them productiveLittle mixing of deep high-nutrient water with low-nutrient water at surface
Tropical waterLow level of nutrientsPermanent layers separated by a thermoclineWarmer, less-dense layer of water on top of colder, denser water
Prevents exchange of nutrients
Food webs in open seaBase formed by phytoplankton and heterotrophic
bacteria Provide food for herbivores Release photosynthetic products into surrounding
seawater as dissolved organic matter (DOM)Heterotrophic bacteria rapidly recycles DOM in open seaBacteria can form a bacterial loop
Returns nutrients rapidly to phytoplankton in water easily depleted of critical nutrients
Metabolize some of the DOM and return it as an inorganic form
VirioplanktonMost abundant plankton in the oceanLysis of bacterioplankton and
phytoplankton by viruses releases DOM directly Produces particulate organic matter (POM)
Disrupts bacterial loop