Systems Without Blooms Flashcards
If provided with a mix of N
compounds, which would be preferentially
utilized by phytoplankton? Why?
In order to use nitrogen to make amino acids and other compounds they must first reduce it to NH4+
So they will preferentially take up NH4+
Major difference between P and N cycling
Nitrogen cycling occurs in many steps that can produce N of different types that may be unusable to plankton. This means it is often limiting in an environment
Phosphorus is readily respired back into the system and cycles through the system very quickly
Silicon cycle
This cycle only involves inorganic forms
Dissolution of biogenic Si is solubility driven
Ocean is undersaturated therefore corrosive to Si
Skeletal material dissolves at all depths following death of organism and decay of organic material
Importance of nutrient recylcing
increase productivity of food webs
enhances and extends phytoplankton productivity
influences composition of phytoplankton assemblages
a key component of the microbial loop
works with other biotic and abiotic processes to create biogeochemical cycles
Why is iron important to phytoplankton
Photosynthesis: photosystems I&II and cytochrome complex require iron
Nitrogen Assimilation: Nitrogen assimilation, used in synthesis of nitrate reductase
Synthesis of chl a
Sources of Iron to the ocean
Margin sediments
Dust from atmosphere
Hydrothermal vents
Karin Ridge
HNLC Regions
Areas with high macronutrients but very low chlorophyll
Subarctic NE Pacific
Eastern Equatorial Pacific
Southern Ocean
Development of Iron Hypothesis
Fe is not very soluble in oxygenated water and most ocean far from Fe sources
Open ocean is very low in Fe and in limiting supply compared to other nutrients
Preliminary bottle experiments showed that Fe stimulated growth of large diatoms led to nutrient drawdown
Indirect evidence from Island Effects
Glacial Fe Hypothesis
Links dust flux to the strength of the biological carbon pump
Fe rich dust enters ocean: increase PP and decrease carbon in atmosphere
Temperature decreases: decrease precipitation, increased desertification and more dust
Importance of Vostok Ice Core
Provided evidence that Fe supply may affect atmospheric CO2
What were some arguments against the iron hypothesis
Artifacts associated with bottle experiments hinder extrapolation to ocean temporal and spatial scales
Bottle experiments are not a true representation of phytoplankton communities
Possible Fe contamination or improper analytical technique could lead to faulty conclusions
Testing the iron hypotheisis
Fertilizing paths in the ocean and observing the effects
Mesoscale iron enrichment expermints showed significant increase in Chl a and drawdown of CO2
Most of the phytoplankton growth and N uptake is dominated by the large phytoplankton (diatoms)
What we know so far about HNLC and Fe
About 30% of the ocean is HNLC
Mesoscale fertilizations have shown that Fe is the limiting nutrient in these cases
Some scientists have suggested that large scale iron fertilization could reverse the effects of climate change
Kasatochi Eruption Event Effect
Caused serious phytoplankton bloom
Removed ~0.01 Pg-C, but volcano produced about the same
Kasatochi Bloom Event Impact on Sockeye Salmon
The juveniles effected by the bloom went on to have record-breaking runs in 2010
However the bloom mostly occurred where juveniles were not, and productivity in 2011 was similar to 2010
Haida Salmon Restoration
Massive Iron Fertilization event in 2012, and saw record-breaking returns in 2014
Ocean Station Papa Trends (NE Pacific)
No obvious cycle of chl, occasional peaks, mostly just noise
Subarctic NE pacific lower and more random that North Atlantic
Subtropical North pacific trends
No seasonal cycle
Slow steady production
Very low chl
What does the euphotic zone depth at Ocean Station papa tell us about light availability
It tells us nothing
Only gives an attenuation coefficient, not surface irradiance
Light Variability between OSP, North Atlantic and Subtropical pacific
OSP factor of 4 between winter and summer
North Atlantic similar difference to OSP
Subtropical pacific has very little variation
North Atlantic Mixed Layer and Crit depth
Has very deep mixed layers (300m)
mixed layer must be less than crit depth for blooms
Low light in winter and deep mixed layer, N Atlantic is very light limited in winter
Subarctic NE pacific mixed layer
has a layer of FW on its surface
The mixed layer shallow in summer, and limited to 100m in winter
similar winter light availability, but with shallower mixed layer light limitation is not much of an issue
Subtropical Pacific Mixed layer
Shallow mixed layer summer, winter limited to 100m
High light year round so never light limited
Macronutrient cycle at OSP
Half of macronutrient levels utilized year round
Lower in summer but not low
High all year round (HNLC)
Macronutrient cycle in North Atlantic
Following spring bloom nitrate drawn down to zero
Macronutrient limited all summer
Replenished in the winter by deep mixing
Macronutrient cycle in Subtropical pacific
Very low nutrients year-round
Classically macronutrient limited system
Low levels of productivity
supported by tight nutrient
recycling in the euphotic zone
Fe concentrations at OSP
Fe is low in surface waters (100m) all year round
Increase with depth below 100m
Mixed layer sometimes reaches 100m but not enough to replenish Fe in surface waters
Simulated Surface Fe concentrations
Limited at Subarctic NE pacific and antarctic regions
Subtropical Pacific probably Fe limited too, but has low nutrients so doesn’t matter
N Atlantic is lowish, but not limiting or as low as OSP
High near coasts and wind deposits
Iron Fertilization Bottle Experiments at OSP
Smallest phytoplankton dont respond to iron addition
Medium have a small response
Largest have a very strong response
Why don’t we get big blooms of small phytoplankton?
What could be limiting small phytoplankton in the subarctic NE
Pacific if not iron
Likely grazing of the small phytoplankton
Grazing Hypothesis
Not nutrient or light limited over the winter for small phytoplankton
Maintains a population of small zooplankton over winter
This allows zooplankton to respond quickly and graze down small phytoplankton
Evidence for grazing hypotheis
Phytoplankton growing 2-3x more in summer than in winter
They are growing more in summer, but their biomass is not increasing
Less zooplankton in winter and their biomass increases in summer
Grazing rate increases as phytoplankton grow more
Prevents phytoplankton bloom in summer of small phyto
Microbial Loop in NE pacific
Microzooplankton graze on small phytoplankton or heterotrophic bacteria
Zooplankton produce NH4+, to increase recycling efficiency (still very low)
Small phyto use low levels of NH4+ to grow
Larger phyto need Fe in order to utilize the abundant NO3 here
Which size
class of phytoplankton
are likely responsible
for the occasional
peaks in chlorophyll?
Occasional large blooms of
diatoms caused by Fe inputs
What provides the Fe input to allow large phytoto grow
volcanic ash
dust storms
glacial flour
coastal water transported by eddies
