9 Critical Factors in Plankton Abundance

1. 9 Critical Factors in Plankton Abundance Notes for Marine Biology: Function, Biodiversity, Ecology By Jeffrey S. Levinton

2. Patchiness of the Plankton • Plankton occur in spatially discontinuous patches, sometimes concentrated at interfaces between water bodies

3. Patchiness of the Plankton 2 • Spatial changes in physical conditions (light, temperature, salinity) - behavioral responses and population growth/mortality responses • Water turbulence and current transport • Spatially discontinuous levels of grazing • Localized reproduction • Social behavior

4. Patchiness of the Plankton 3 • Phytoplankton - population density determined by interaction between turbulence and population growth • Low turbulence allows population to increase in any one place • High turbulence disperses phytoplankton and density does not increase

5. Example of spread of plankton with general turbulence (left) or in a current (right)

6. Spring Phytoplankton Bloom Spring diatom increase

9. Mechanisms causing the spring phytoplankton bloom and its decline

10. Light and Phytoplankton - Concepts IMPORTANT: LIGHT INTENSITY DECREASES WITH INCREASING DEPTH Consider a phytoplankton cell held in jar at a certain depth ( = a certain light intensity): Compensation depth: O2 produced = O2 consumed Increase of O2: from photosynthesis Decrease: from respiration

11. Light and Phytoplankton - Concepts 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

12. SO… AS YOU INCREASE DEPTH, LIGHT INTENSITY DECREASES EVENTUALLY YOU REACH A DEPTH WHERE OXYGEN CONSUMED BY A CELL = OXYGEN PRODUCED BY PHOTOSYNTHESIS (IF CELL IS KEPT AT THAT DEPTH) THAT IS THE COMPENSATION DEPTH, WHICH IS AT THE COMPENSATION LIGHT INTENSITY A CELL KEPT DEEPER THAN THAT DEPTH WOULD HAVE NET OXYGEN PRODUCED (LESS THAN, =, MORE THAN ??) OXYGEN CONSUMED?

13. SO… AS YOU INCREASE DEPTH, LIGHT INTENSITY DECREASES EVENTUALLY YOU REACH A DEPTH WHERE OXYGEN CONSUMED BY A CELL = OXYGEN PRODUCED BY PHOTOSYNTHESIS (IF CELL IS KEPT AT THAT DEPTH) THAT IS THE COMPENSATION DEPTH, WHICH IS AT THE COMPENSATION LIGHT INTENSITY A CELL KEPT BELOW THAT DEPTH WOULD HAVE OXYGEN PRODUCED < OXYGEN CONSUMED

14. Before the spring phytoplankton Increase Water density similar at all depths Wind mixing homogenizes water column

15. WINTER-->SPRING: CHANGES IN MIXING DEPTH AND LIGHT

16. 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 produced in the water column equals total consumed If: Mixing depth < Critical depth: bloom If: Mixing depth > Critical depth: no bloom

18. 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, thermocline develops. 2. Surface nutrients are rich and trapped in surface waters. 3. Phytoplankton cells are no longer stirred to darker deep waters ----> BLOOM!!

19. Gulf of Maine

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

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

22. Rejuvenation of Conditions for the Spring Phytoplankton Increase Why do phytoplankton sometimes 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

23. 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

24. Water Column Exchange in Shallow Waters and Estuaries Beach phytoplankton blooms

25. Water Column Exchange in Shallow Waters and Estuaries 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

26. Water Column Exchange in Shallow Waters and Estuaries 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, especially if flow is high (NY Harbor releases nutrients to NY Bight, for example)

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

28. Light 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

29. Light Ultraviolet light strongly attenuated In water column: Inshore waters: Incident light with wavelength 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

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

31. Photosynthesis in Water Column 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

33. Nutrients Nutrients are substances required by plants; they are resources that can be limited in supply Nutrient dependence and use: autotrophs, auxotrophs, heterotrophs

34. Nutrients Nitrogen - what is it used for? Nitrates NO3 - MOST ABUNDANT SOURCE USUALLY Nitrites NO2 Ammonium ion, NH4 - excretion product recycling from animal excretion in the water column - TAKEN UP THE FASTEST

35. Nutrients 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

36. Nutrients 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

37. Nitrogen Cycle

38. Nutrients 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

39. Phosphorus Cycle

40. Nutrients 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?

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

42. Nutrients Iron - important cofactor in oxygen production step of photosynthesis FERREDOXIN - ELECTRON DONOR, NITROGENASE IN N-FIXERS 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 (high nutrient - low productivity (HNLP) - commonly wind delivered

43. Fe, Si often enter the ocean by wind-borne particles

44. Nutrients 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)

45. 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

46. Microbial Loop DOC=dissolved organic carbon POC=particulate organic carbon DIOC=dissolved inorganic carbon

47. 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

48. Nutrient Uptake

49. Nutrient Uptake K is nutrient concentration at which half of maximum cell doubling rate occurs - useful measure of phytoplankton Nutrient uptake

50. Nutrient Uptake 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 Open ocean species: lower nutrient concentrations, should be better at uptake at lower concentrations but tradeoff is inability to deal with higher concentrations