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