Open Oceans

• Major surface ocean currents driven by combined effect of global wind patterns and latitudinal variation in Coriolis force
• Wind currents east towards equator and poles and west in between - effect movement of ocean currents

Coriolis force - winds are deflected by rotating Earth

• If a ball is thrown on a rotating disc = ball is deflected 
• Winds rotate in different directions depending on position
• North hemisphere - rotate to right and south hemisphere - rotate to left
• Less effect in poles and more near equatotr

Wind pattern and Coriolis forces = Ekman spiral = Ekman transport (water mass in 1 direction)

• Wind creates tension on surface of ocean = currents form
• Currents deflected by Coriolis forces
• Currents at deeper layers affected more = more deflections
• Ekman transport = sum of deflection at different depths
• Northern hemisphere - currents deflected to right and southern hemisphere - left

Gyre - large system of circulating ocean currents, those involved with large wind movements

Sub-tropical gyres located w/in central sections of ocean

Atlantic Meridional Transect Programme (AMT):

• Addressed question relating to mesoscale/basin-scale ocean plankton and biogeochemistry and linked to atmosphere
• Research vessel RSS James Clark Ross
• Samples at different depths along transect
• 50' N (UK) in May to 50' South in September 
• Different times - reduce effects of seasonality

Sea surface temps:

• Equator highest temp
• Poles lowest temp
• High temps and upwelling = more precipitation = inter-tropical convergence zone

Sea surface salinity:

• Near equator = precipitation>evaporation = low salinity
• Sub-tropical areas = evaportation>precipitations = high salinity
• Enclosed oceans = high salinity

Surface nitrates:

• Oligotrophic waters - water body w/ poor plant nutrients and abundant oxygen in deep areas
• Eutrophic - water body rich in nutrients = dense plant population 
  • If closed water body = decomposition can deprive fauna of oxygen
• Eutrophic waters = polar and equatorial oceans

Surface Chlorophyll a:

• North West African upwelling - wind pushing water and dust off coast
  • Western winds carry sand from Sahara Desert = blooms
  • Enzyme responsible for N fixation requires iron = dust inputs important
• Equatorial upwelling with wind = divergence of water

Warm fast-flowing boundary current on western side of subtropical gyre by USA

North of it = Labrabor current = warm and cold core rings form

• Rings have different temperatures/nutrients to outside the ring = mesoscosms

Eddy - circular movement of water causing small whirlpool

Meso-scale eddies form in Gulf Stream = pump nutrient-rich waters from ocean depths and stimulate new primary productions

Bumps - movements from deep to surface

Depression - movements from surface to deep

New nutrients - enter euphotic zone from outside systems (wind, deep waters, rivers) = support new primary production

Regenerated nutrients - recycled within euphotic zone and support regenerated population e.g. urea

f-ratio - ratio of nitrate (NO3) uptake by phytoplankton to total inorganic (N-) and organic (urea) uptake by phytoplankton

Old view:

• Sub-tropical gyres are marine deserts = low biomass and production
• Inefficient ecosystems w/ nutrient-limited growth
• Low f-ratios ~0.1

New view:

• Evidence from AMT transects etc
• Dynamic systems w/ episodic mixing events
• Important in annual production - 65% annual production in 2 weeks
• Episodic production increases annual f-ratio
• Clearly defined vertical structure for annual production
• In deep waters - blooms during winter (mixing) and NO3 inputs
• Upper waters - blooms in late summer (N-fixing bacteria and eddy events)

N-fixation:

• Can supply 60-90% nitrogen required if no other inputs
• Carried out by N-fixing cyanobacteria

Vertical migration:

• Some phytoplankton migrate between upper and deeper waters
• Rhizosolenia (diatom) = mats which move vertically - take NO3 at depth and return to surface for photosynthesis
  • Repeat when intracellular stores depleted
• Oscillatoria spp. (cyanobacteria) migrate to mine deep inorganic phosphorus pools

Autotrophs - Prokaryotes (P.marinus) and eukaryotes flagellates

• Dominanted by picoplankton - 60-90% chlorophyll a biomass and 60-80% carbon fixation due to <2um cells
• Larger autotrophs occasionally important - provide fresh nutrient supply
• Larger species = diatom blooms

Biogeography of Phytoplankton 

• Chlorophyll maxium at different depths = vertical structure of water column and cellular response to different light levels
• Small phytoplankton - regions where nutrient levels low = regenerated nutrients
• Large phytoplankton - regions with high levels of nutrients = no regenerated nutrients

Grazers - small heterotrophs e.g. heterotrophic flagellates, planktonic ciliates, heterotrophic dinoflagellates

Oceanic zooplankton - copepods, crustaceans, gelatinous species

Both microbial loop and classical food chains

LIMITED NUTRIENTS = microbial loop dominant

NUTRIENT INPUTS = classical food-chain dominant briefly

Bacterial production involved - important

Inputs = dissolved organic matter from plankton

Small heterotrophic zooflagellates link to classic food web

Mixotrophy common

Important for remineralisation and nutrient cycling - when nutrients are limited

Generally 6 trophic levels

Small pelagic forage fish = important intermidiate level

High trophic leves = fast-swimming predators, marine mammals and surface-feeding birds

Low levels of regenerated production continously grazed by zooplankton

Allochthonous - material imported into an ecosystem from outside e.g. wind, rivers