9 Critical Factors in Plankton Abundance Notes for Marine Biology: Function, Biodiversity, Ecology By Jeffrey S. Levinton

Spring Phytoplankton Increase (or Spring Diatom Increase) In midlatitudes, phytoplankton increase in the spring, decline in summer, and may increase to a lesser extent in fall

Nutrients at surface Spring Diatom Increase Available sunlight Zooplankton Winter Spring Summer Fall Winter

Month Arctic Phytoplankton Herbivore zooplankton Temperate Tropical Herbivore zooplankton Phytoplankton J F M A M J J A S O N D Month

Mechanisms causing the spring phytoplankton bloom and its decline

Light and Phytoplankton - concepts Consider a phytoplankton cell held in jar at a certain depth ( = a certain light intensity: Compensation depth - that depth at which oxygen produced by a phytoplankton cell in photosynthesis equals the oxygen consumed in respiration

Light and Phytoplankton - concepts 2 Consider a phytoplankton cell held in jar at a certain depth ( = a certain light intensity: Compensation light intensity - the light intensity corresponding to the compensation depth

Before the spring phytoplankton increase: Water density similar at all depths Wind mixing homogenizes water column

Cause of the spring phytoplankton increase: Important concepts: Mixing depth - depth above which all water is thoroughly mixed, due to wind. Critical depth - depth above which total oxygen coming from primary production in the water column equals total consumption (respiration) If: Mixing depth < Critical depth: bloom If: Mixing depth > Critical depth: no bloom

Total photosynthesis, P Total respiration, R Total photosynthesis, P Mixing depth 1: area P > R Mixing depth 2: area P < R Oxygen production ( P ) or respiration ( R ) Compensation depth Mixing depth 1 Critical Depth Mixing depth 2 Depth

Cause of the spring phytoplankton increase: Important concepts 2: Key processes: 1. Water column becomes more stable in spring as sun heats water from above. 2. Surface nutrients are rich and trapped in surface waters. 3. Phytoplankton cells are no longer stirred to darker deep waters ----> BLOOM!!

Decline of the Spring Phytoplankton Increase Why do phytoplankton decline? Water column is STABLE In shallow water shelf waters: diatoms start sinking from surface water to bottom, which removes nutrients.

Decline of the Spring Phytoplankton Increase 2 Why do phytoplankton decline? Zooplankton grazing? Has some effect but often secondary to sinking

Rejuvenation of conditions for the Spring Phytoplankton Increase Why do phytoplankton increase again in Fall? In fall and winter: water cools, water column becomes isothermal with depth, wind mixing restores nutrients to surface waters until conditions are right next spring

Water column exchange in shallow waters and estuaries In very shallow estuaries, nutrient exchange or benthic-pelagic coupling, occurs between the bottom and the water column, fueling more phytoplankton growth

Water column exchange in shallow waters and estuaries 2 Beach phytoplankton blooms

Water column exchange in shallow waters and estuaries 3 In estuaries, the spring freshet combines with net water flow to the sea and mixing to determine nutrient regime: 1. Freshwater rivers create a net downstream flow 2. Tides cause mixing up and down estuary as well as vertical mixing 3. Nutients may be released to coastal zone

Water column exchange in shallow waters and estuaries 4 Important factors in nutrient exchange: Residence time - time water remains in estuary before entering ocean 2. Rate of nutrient input from watershed 3. Nutients may be released to coastal zone

Light Two components of loss in the water Column: Absorption: Molecular absorption of light energy Scattering: Light interaction with particles

Light 2 Penetration into water column varies: with wavelength Clear Open Ocean Water: Maximum penetration at 480 nm Turbid inshore water: Maximum penetration at 500-550 nm

Light 3 Ultraviolet light strongly attenuated In water column: Inshore waters: Incident light with wl of 380 nm or less is almost attenuated at depth of 1-2 m Clear open ocean water: 20 m may be required to remove 90% of surface incident light

Photosynthesis in Water Column Phytoplankton species may use Chlorophyll a, c and “accessory pigments”, which absorb energy over the light spectrum

Photosynthesis in Water Column 2 Action spectrum - utilization of different wavelengths of light by a given species for photosynthesis Chlorophyll absorbs wavelengths of mainly > 600 nm Accessory pigments absorb wavelengths < 600 nm

Photosynthesis in Water Column 3 Photosynthesis increases with increasing light intensity to a plateau, Pmax, then decreases

+ _ Pmax Photosynthetic rate Net Gross Photosynthesis photosynthesis Compensation point Respiration Light intensity (I)

Nutrients Nutrients are substances required by plants. They are resources that can be limited in supply Nutrient dependence and use: Autotrophs, auxotrophs, heterotrophs

Nutrients 2 Nitrogen - what for? Nitrates NO3 - Nitrites NO2 - Ammonium ion, NH4 - excretion product recycling from animal excretion in the water column

Nutrients 3 Nitrogen - uptake depends upon source Ammonium ion is taken up the fastest by phytoplankton cells, requires the least chemical alteration (no reduction), but Nitrate is usually the most abundant source Of nitrogen in shallow coastal water columns

Nutrients 4 Nitrogen - New vs. Regenerated Production New Production: Nutrients for primary production may Derive from circulation of nutrients from Below the surface waters (upwelling, storms That bring deeper waters to the surface) Regenerated Production: Nutrients derive from recycling in surface waters from excretion

Nutrients 5 Nitrogen - Microbial control Nitrogen added to ocean from atmospheric nitrogen by nitrogen fixing bacteria Nitrifying bacteria convert NH4 to NO2, others convert NO2 to NO3 Denitrifying bacteria convert N03 to NH4 Nitrate reducing bacteria return NO3 to atmosphere

Nitrogen Cycle Atmospheric nitrogen Denitrification Nitrogen fixation Primary production Dissolved inorganic nitrogen Organic nitrogen Respiration Advection, Mixing External nitrogen sources and sinks Nitrogen Cycle

Nutrients 6 Phosphorus - occurs dissolved in water mainly as phosphate PO4 Also can find particulate phosphorus, some dissolved P in organic molecules Phosphorus required for synthesis of ATP, source of energy of cellular reactions

Phosphorus Cycle Primary production Advection and mixing Dissolved Inorganic phosphorus Organic phosphorus Respiration Advection and mixing External phosphorus sources and sinks Phosphorus Cycle

Nutrients 7 The limiting nutrient? In ocean, nitrogen is believed to be the main element limiting phytoplankton growth, Rather than phosphorus Important question are these the only limiting nutrients or nutrient elements?

Nutrients 8 Silicon - important limiting element for diatoms, exact role in controlling phytoplankton growth not well understood

Nutrients 9 Iron - important cofactor in oxygen production step of photosynthesis Shown in lab experiment to enhance phytoplankton growth May be crucial in parts of the ocean (eastern equatorial Pacific, parts of Antarctic, north Pacific where nitrogen appears not to be limiting factor

Nutrients 10 Trace elements such as Mn, Zn, Mo, Co, Cu can be important, but poorly understood Organic trace substances such as vitamins important, especially for auxotrophic phytoplankton (e.g., many dinoflagellates)

Phytoplankton Succession Seasonal change in dominance by different phytoplankton species e.g.: diatoms in early spring followed by dinoflagellates in summer

Phytoplankton Succession 2 Mechanisms poorly understood: Shift in advantage of nutrient uptake, Favoring different cell types 2. Species later in season may depend upon substances that are not in the water column in early spring (e.g., auxotrophic species might follow autotrophic species)

Microbial Loop 1. Bacteria are abundant and take up large Amounts of nutrients from the water column 2. Bacteria are consumed by ciliates and other Heterotrophs 3. These heterotrophs are consumed by other Smaller zooplankton, which incorporates Bacterially derived nutrients into the planktonic Food web

Microbial Loop 2 Larger consumers Herbivores Microbial loop DOC & POC Phytoplankton Viruses DIOC and nutrients Bacteria Microconsumers DOC=dissolved organic carbon POC=particulate organic carbon DIOC=dissolved inorganic carbon

Nutrient uptake Nutrient uptake by phytoplankton cells varies with nutrient concentration Modelling uptake: Need to know (1) nutrient concentration C And (2) rate of uptake of nutrients, which we measure indirectly as D, cell doublings/day (3) K is concentration at which cell doubling rate is one half of maximum doubling

Nutrient uptake 2 Dmax Cell doublings/day Dmax/2 K Nutrient concentration

Nutrient uptake 3 K is nutrient concentration at which half of maximum cell doubling rate occurs - useful measure of phytoplankton Nutrient uptake Cell doubling rate K Nutrient concentration

Nutrient uptake 4 Application of model: Inshore versus open ocean phytoplankton nutrient uptake Inshore species: live in higher nutrient concentrations, should be good at uptake at high concentrations, but may be tradeoff and lower efficiency at low nutrient concentrations

Nutrient uptake 5 Application of model: Inshore versus open ocean phytoplankton nutrient uptake 2 Open ocean species: live in lower nutrient concentrations, should be better at uptake at lower concentrations but tradeoff is inability to deal with higher concentrations.

Nutrient uptake 6 Application of model: Inshore versus open ocean phytoplankton nutrient uptake Adapted to high nutrient concentration 1 Dmax-1 Adapted to low nutrient concentration 2 Dmax-2 K2 K1

Nutrient uptake 7 General results for nitrate: Environment K Inshore Offshore 0.1-0.2 M Adapted to high nutrient concentration Dmax-1 Adapted to low nutrient concentration Dmax-2 K2 K1

Zooplankton Grazing Grazing effect: Difference between grazing rate and phytoplankton growth rate Grazing quite variable, sometimes causes: 1. Strong spatial variation in phytoplankton abundance, 2. Cycles of phytoplankton abundance and decline

Zooplankton Feeding Zooplankton feeding increases with increasing phytoplankton cell density, up to a plateau Cell ingestion rate Phytoplankton cell density Copepod feeding response on a diatom

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