? Flashcards
what are fronts and what do they come from
narrow, high gradient regions - cause upwellings
- when 2 different currents meet: converge or diverge
what are shelf break front
edge of continental shelf as a result in change of depth (e.g. coastal waters to very deep waters)
what are estuarine-plume fronts
formed by salinity gradient
what’s a tidal mixing front
friction between tidal flows and sea bed - create turbulent bottom boundary layer (BBL) - injects nutrients into water column + affects phyto growth
in deeper waters (outer shelf):
- winter: SML overlaps BBL (big SML due to wind)
- spring: SML + BBL = separate
in shallower waters tidal flows + stirring = stronger - water column remains mixed all year
how do tidal fronts affect phyto growth
pinger et al 1975 - investigated phyto distribution off the Ushant, summer
- near shore: well mixed (high nutrients), low chl a (also high suspended solids so low light)
- off shore: stratified (phyto already used the nutrients available), low chl a
- front: high chl a - interface between the two - plenty light + plenty nutrients
what’s the largest circulation feature of the globe
N pacific gyre - oligotrophic - low f-ratio (high recycling)
what did Karl et al, 1997 find whilst investigating Nitrogen fixation in the north Pacific
over 1st + 3rd years: trichodesmium increased significantly - calculated this was able to provide half N required to sustain PON export
- another way assessed changes in N:P ratios - found a drop in this from the Redfield ratio as phyto became increasingly limited
what’s an important source of ‘new’ nitrogen to oligotrophic waters even though N is recycled
N2 fiction by trichodesmium - leads to P (not N) controlling / limiting PP rate in the N pacific gyre
what are equatorial upwellings
oceanic upwelling - when wind drives the surface water anticlockwise in the north + clockwise in the south meet together (in the equator) it pulls the water apart, drawing nutrient rich water u from below - leads to higher chl a conc
why do we see low chlorophyll conc in the pacific equatorial upwelling
high nutrient low chlorophyl area - iron deficient
- galapogas is in the same place yet there is high chl a conc (surrounded by volcanos that fertilise water with Fe)
3 coastal upwelling examples
- Arabian sea monsoon system
- El Niño
- Benguela
what happens in the Arabian sea monsoon upwelling system
happens near Indian Ocean - increases effects from land weather
- during inter-monsoon periods, wind directions reverse - pushes warm oligotrophic surface waters onto the land - restrains upwellings
what happens in the El Niño upwelling system
- easterly (trade) winds drives warm surface waters to the west - water level ‘piles up’ on the west (La Niña) - causes upwelling of cold nutrient rich water
- but wind structures can change - potential energy in this pile can be released and flow back to east (El Niño) - effects weather patterns
what happens in the Benguela upwelling system
encountered 3 distinct water masses by age (from temp and NO3-):
- initial maturing upwelled water
- intermediate maturing upwelled water
- age maturing upwelled water
initial maturing upwelled water characteristics
- cold + high NO3-
- High chl a + PP
- classical food chain
- diatoms dominate phyto but still high bacteria
- high f-ratio
