Structure and Productivity of Aquatic Systems

1. Structure and Productivity of Aquatic Systems

2. Functional Lake Zones Pelagial

3. Living Things in Lakes • Distribution & abundance of living things in lake controlled by physical and chemical conditions in different zones

4. Organic Matter in Lakes • Living things make up only small portion of organic matter in lakes • Most is in form of non-living detritus • Both particulate and dissolved

5. Organic Matter in Lakes • In most lakes, dissolved organic matter is 10 X more abundant than particulate • Living things make up small portion of particulate • Detritus is habitat & energy resource for living things

6. Organic Matter in Lakes • Much of the organic production of photosynthesis within a system is not consumed, but becomes part of detritus reserve

7. Primary Producers in Lakes • 3 major categories of primary producers: • Phytoplankton • Photic zone throughout lake • Generally small, unicellular or colonial organisms

8. Primary Producers in Lakes • Emergent macrophytes • Shallow portions of littoral zone • Roots and lower portions in water, tops above water surface

9. Primary Producers in Lakes • Submersed macrophytes • Deeper portions of littoral zone • Completely underwater

10. Productivity Hierarchy • Emergents most productive (Carbon fixed/area/year) • More productive than terrestrial grassland, forest • Submersed much less productive • Phytoplankton least productive

11. Phytoplankton • Cyanobacteria or blue-green algae • Important nitrogen fixers • High densities in late-summer • Odor (and taste) problems

12. Phytoplankton Desmids • Green algae • Tremendous diversity • Planktonic, but can be attached, benthic (often filamentous)

13. Phytoplankton • Golden-brown algae • Low diversity, but can be important segment of phytoplankton • Dinobryon important under low P conditions

14. Phytoplankton • Diatoms • Very important group • Planktonic and attached forms • Cell walls with silica -- maximum abundance in spring when silica is most abundant

15. Phytoplankton • Cryptomonads • Extremely small • May reach high densities during cold periods with low light intensities (winter under ice)

16. Phytoplankton • Dinoflagellates • Unicellular, flagellated, with spines • Strict requirements for Ca, pH, temperature, dissolved organics

17. Phytoplankton • Some exhibit cyclomorphosis - seasonal change in size & form • Ceratium - more spines, longer spines, more divergent spines as water temperature increases • Reduce sinking rate out of photic zone in less viscous water

18. Phytoplankton • Euglenoids • Unicellular • Most abundant in areas with high ammonia, dissolved organics • Shallow farm ponds in cow pastures

19. Paradox of the Plankton • Lakes usually have a few dominant species and many rarer species • Theoretically should have only single dominant species (niche overlap leads to competitive exclusion)

20. Paradox of the Plankton • Multispecies equilibrium in open waters • 4 possible explanations:

21. Paradox of the Plankton • Environmental change too rapid for competitive exclusion to occur • Symbiotic relations among species (commensalism) • Selective grazing on competitive dominants by zooplankton (size-based) • Some species alternating between plankton and benthos • Not truly competing with pure planktonic forms

22. Phytoplankton andWater Quality • Assemblage indicates level of nutrient enrichment • Desmids and certain diatoms in nutrient-poor systems • Different diatoms, greens, and blue-greens dominate as enrichment increases

23. Phytoplankton andEnvironmental Factors • Temperature and light control type, abundance of plankton • Diatoms have lower temperature optimum, blue-greens higher optimum

24. Phytoplankton andEnvironmental Factors • Many can adapt to changing light intensities • Chlorella changes pigments per cell to maintain same rate of photosynthesis • Blue-greens regulate gas pressure in vacuoles to position themselves at depth with optimum light intensities

25. Phytoplankton andEnvironmental Factors • Some phytoplankton experience photoinhibition • High light intensities near lake surface may temporarily destroy enzymes and decrease photosynthesis • Sunny days - less photosynthesis near surface than at greater depths

26. Phytoplankton - Seasonal Succession • Changes in light, nutrients, temperature drive a shift in phytoplankton during the year

27. Phytoplankton - Seasonal Succession • Low growth in winter • Diatoms and cryptophytes dominate in spring • Greens take over in summer, joined or replaced by blue-greens as N runs low in productive lakes • Less productive lakes - few greens, blue-greens, only peaks of diatoms spring and fall (silica)

28. Phytoplankton - Seasonal Succession • Seasonal abundance varies much more in temperate (1000 X) than in tropical (5 X) lakes, but total populations are much greater in tropical lakes • Selective grazing by zooplankton can influence succession • Eating some, providing nutrients for others

29. Phytoplankton - Nutrient Enrichment • Enrichment can greatly increase productivity (per volume) up to a point • Eventually self-shading develops and thickness of photic zone reduced • Inhibits further increases • Productivity/m2 of surface remains virtually unchanged • Photosynthetic efficiency low (<1% of incident light)

30. Phytoplankton - Variation in Production • More production in littoral zones than pelagial areas • Peak production during midday (except at surface - earlier in day) • Seasonal production peaks in summer

31. Macrophytes • Restricted to the littoral zones • In small, shallow lakes with no profundal zone, macrophytes may occur basin-wide

32. Emergent Macrophytes • Rooted in water or saturated soil with aerial leaves/stems • Upper littoral - out to 1.5 m depth • Typha - cattail

33. Emergent Macrophytes • Special category occupying mid-littoral region - 0.5-3.0 m • Floating-leaved plants • Water lily

34. Submersed Macrophytes • All depths within photic zone down to ~10 m for vascular plants • Macroalgae - may occur slightly deeper • Coontail, curlyleaf pondweed, Elodea

35. Free-floating Macrophytes • Not rooted • May have well-developed submersed roots, or no roots • Lemna - duckweed

36. Aquatic vs. Terrestrial • Aquatics mostly similar to terrestrial macrophytes • One major difference - rooting tissues grow in anaerobic substratum

37. Aquatic vs. Terrestrial • Roots need O2 to respire • Only can get it by transporting it from tissues in other parts of plant • Extensive system of intercellular gas lacunae for gas transport, exchange

38. Aquatic vs. Terrestrial • Emergent macrophytes have leaf structure similar to terrestrial plants • Linear, thick leaves - no problem obtaining light, CO2 • High transpiration - lose lots of water

39. Aquatic vs. Terrestrial • Submersed macrophytes often look much different than terrestrials • >70% of volume is intercellular lacunae • Leaves very thin, divided and broadened to increase surface area to volume ratio • Better absorb sunlight, CO2

40. Aquatic vs. Terrestrial • Some submersed forms also capable of assimilating bicarbonate for use in photosynthesis • Based on relative scarcity of free CO2 in most environments

41. Nutrient Needs • Most nutrients required by macrophytes come from sediments • Free floaters get it from water

42. Nutrient Needs • Interstitial waters generally contain much higher concentration of nutrients than waters above sediments (anoxic conditions) • Most macrophytes can assimilate nutrients from water if concentrations rise (just like phytoplankton)

43. Leaky Macrophytes • Submersed macrophytes are very leaky • Lose nutrients to surrounding water during active growth • Developed on land and not adapted to water? • Compromise - improved light, CO2 uptake at cost of losing some nutrients?

44. Light Limitations • Emergent macrophytes are seldom light-limited - tremendous capacity for production • Submersed macrophytes are light-limited • Depth distribution regulated by light, in part

45. Depth Limitations • Even in systems with light penetrating to great depths (unproductive systems), macrophytes only occur down to ~10 m • Results from hydrostatic pressure - doubles atmospheric pressure by 10 m • Inhibits movement of gas through lacunae

46. Macrophytes vs. Phytoplankton • Phytoplankton productivity may be very low in littoral areas with many macrophytes - 3 reasons: • 1) Competition for nutrients • 2) Shading • 3) Release of inhibitory organic chemicals by macrophytes

47. Macrophytes vs. Algae • Productivity of some types of algae may be very high in close proximity to macrophytes • Grow attached to macrophytes and live off materials leaking out