Phytoplankton

• Announcements:– Exam: next Wednesday– Review Tuesday pm at Olin O4 (here?!)

Two nearby lakes are similar in area and productivity, but one experiences winterkills, and

the other does not. Why?

• Concentrations same, volume different

• Same productivity ~~ same decomposition (O2 demand)

• Little to no photosynthesis (why?)

• Not really strong stratification under ice (why?)

Explain why Lake Washington's watershed, morphology and flushing rate influenced recovery from nutrient loading. WHY are these

characteristics important? Under what conditions (lake characteristics) would simply reducing

P-inputs not work? Why not?

Lake Washington

Deep basin that never went anoxic so little to know internal loading (P buried in sediments; how?)

Forested and urban water (little non-point sources of P; why?)

Low WRT (after sewage was diverted, P-laden water quickly washed out)

Doesn’t work when…

Hypolimnion goes anoxic, resulting in lots of internal loading of P (how does this work?)

Non-point sources of P persist (like what?)

Organisms

• Plankton: organisms that weakly swim or go where the water takes them

• Phytoplankton

• Periphyton: benthic algae• Epiphyton: algae growing on macrophytes

Phytoplankton taxonomy• Was once based on morphology or pigments, now more

molecular. See Graham and Wilcox 2000 Algae for more information.

• Usually grouped in Divisions (VARIABLE!) • Also often grouped by

– Size– Mobility (motility)

• Flagella: movable filament that can be used to propel organism through the water

• Gas vacuoles

Phytoplankton groupings, con't

– Origin:• Periphyton (benthic)• Tychoplankton (detach from benthos)• Meroplankton (part of life on sediments)• Euplankton/holoplankton (entire life in water column)• Potomoplankton (resuspended algae in lotic systems)

Phytoplankton Taxonomy (Divisions)

• Cyanophyta - cyanobacteria • Chlorophyta - green algae• Euglenophyta - single flagella• Bacillariophyta - diatoms• Chrysophyta - golden brown algae• Cryptophyta - flagellated • Pyrophyta - dinoflagellates

Cyanobacteria ~1,350 species

• Prokaryotes: lack plastids and distinct membrane bound nucleus

• Photosynthesize functionally like plants• Chloroplasts of other algae and plants originated from

cyanobacteria through endosymbiosis

Cyanobacteria, con't

Often dominant, esp. eutrophic lakes– Some species fix N

(heterocysts)– Large cyanobacteria often

dominate due to disproportionate losses of other species

– Allelopathy (toxic or inhibitory effects on other species)

• Buoyant (gas vacuoles)Anabaena 400x

heterocysts

Cyanobacteria, con't

• Resting stages: • thick-walled resting cells (cysts)

called akinetes (Anabaena & Aphanizomenon)

• Vegetative resting stage (Mycrocystis)

• linkage between benthos and pelagic

Chlorophyta: Green algae ~2,400 species

• Eukaryotes• Includes unicellular flagellated and nonflagellated cells,

colonies and filaments and macroalgae (Chara)• Represent 40-60% species with high biomass

contribution in eutrophic and hypereutrophic lakes• Often dominate benthic algae

Volvox

Chlamydomonas 400xCla

doph

ora

40x

Spirogyra 200x

Hydrodictyon 40xChlorophyta

Chlorophyta

Scenedesmus 600x

Assorted desmids

Euglenophyta ~1,020 species

• Small to medium sized flagellated species

• Often abundant in well-mixed eutrophic ponds and littoral areas

www.mib.uga.edu/.../mibo3000/ eukaryotic/01232001.html

Euglena

bio.rutgers.edu/euglena/ mainpage.htm

Bacillariophyta - diatoms ~5,000 species

• Wide range in size: 2um - 2mm• Require silica (Si) to build frustules

• abundant during mixing when Si abundant• when lake stratifies, diatoms sink to bottom & remove Si from epilimnion

• Heavy & no flagella: sink after stratification & form resting stage on sediments: viable after 100's years

• Two groups:– pennate: bilaterally symmetrical– centric: radially symmetrical

Diatoms

www.mib.uga.edu/.../mibo3000/ eukaryotic/diatoms.jpg

www.cnas.smsu.edu/labimages/ Biology/Bio122/week1.htm

Chrysophyta ~450 species

• Small single-celled flagellates and flagellated colonies

• Common in oligotrophic clear lakes and humic lakes

• Often codominate with cryptophytes

• Diatoms are often grouped under chrysophyta

Synura, http://microbes.limnology.wisc.edu/outreach/majorgroups.php

Cryptophyta ~100 species

• Small or medium-sized flagellates

• Common in oligotrophic lakes• Single-cell cryptophytes,

chrysophytes, dinoflagellates main food of rotifers and crustacean zooplankton (next week!)

• Mixotrophic (more than one more of nutrition): eat bacteria & smallest algae

http://protist.i.hosei.ac.jp/taxonomy/Phytomastigophora/Cryptophyta/Cryptomonadaceae.html

Pyrophyta - dinoflagellates ~ 220 species

• Motile (flagellates)• Have resting cysts• Some do not have

chlorophyll• Red tide in the ocean Peridinium

Ceratium

www.cnas.smsu.edu/labimages/ Biology/Bio122/week1.htm

Sizeinfluences- growth rate- energy paths (consumption)- sinking time

Size

• Picoplankton (0.2-2 m dia)• Nanoplankton (2-30 m dia)• Microplankton (30-200 m

dia)• < 30 m = edible algae

A bacterium

E Daphnia head (e - eye) (large zooplankton)

B Cryptomonas (Cryptomonad)

D Keratella (small zooplankton)

C Scenedesmus (green)

Influences of size

• Pico- and nanoplankton: high rates of production• Large surface to volume ratio (exchange of nutrients)• Very slow sinking rates• Nanoplankton are tasty

• Microplankton• Sink faster• Grow slower• Not tasty

Extracellular release of organic compounds• Represent a significant loss of fixed C (<20%)• Multiple functions:

– modify growth & behavior– e.g., fischerellin released by cyanobacteria; inhibits photosynthesis by

algae• Release of metabolic intermediates of low molecular weight by diffusion

(glycolic acid, organic acids, organic phosphates, peptides…)• Release of metabolic end products of high molecular weight more deliberate

(?) (carbohydrates, peptides, volitile compounds, growth-promoting and growth-inhibiting compounds)

• Bacteria rapidly utilize LMW compounds

Photosynthesis

• Photosynthesis= fixing carbonnCO2 + nH2O ------> (CH2O)n + nO2 (n=# molecules)

• Change in population biomass = growth - consumption - sinking

• Growth=photosynthesis

Compensation point

• Compensation point: photosynthesis = respiration

• Maximize the amount of time spent above the compensation point (in the light)

Ways to stay in light

• Mixing• sink slow enough to stay in mixed epilimnion

• Mobility• flagella• gas vacuoles

• Change sinking rate• change shape or density

modifications

Muscilaginous cover around Staurastrum species (green)- reduce sinking (to a point)- reduce consumption (or digestion)

Effects of light & temperature on photosynthesis

Maximum photosynthesis

Light Limited(photo-chemicalrxns)

Light Saturated (enzymatic rxns limited by temp) Photo-

inhibited

Ph

oto

syn

the

sis

rate

(m

g C

)

Available light

Photosynthesis distribution=specific primary production * light

climate * algae biomass

Mesotrophic epilimnion (well mixed)

Eutrophic with surface bloom

Oligotrophic with max. biomass at metalimnion

Shallow transparent lakes with max. biomass on bottom

De

pth

Photosynthesis Biomass

Depth distribution of photosynthesis

Trophogenic zone ~euphotic zone

Note that phytoplankton on the surface of hypereutrophic lakes shade out the water column

Horizontal distribution

• Wind & currents

Langmuir spirals

Foam, buoyant algae Neg. buoyant algae

surface algae

deep algae

Lake Mendota cyanobacteria blooms

Horizontal distribution

Proximity to littoral zone often results in less phytoplankton– Must compete for nutrients

with periphytic algae and other microorganisms attached to macrophytes and sediments

– Macrophytes are refuge for herbivorous zooplankton

Factors influencing seasonal distribution

Physical• Temperature• Light

Limiting nutrients• silica• nitrogen• phosphorus

Biological • competition• resources, sinking

Biological • grazing • parasitism

Seasonal distribution in a temperate, dimictic lake

(green)(diatoms)

1. Light limited: small, often motile (but productive)2. Light increasing,still ice cover, no mixing (dynoflagellates can swim up towards

light)

3. Spring mixing: high nutrients, low grazing, increasing light, diatoms dominate

4. Initial stratification: diatoms settle & die, loss of Si to < 0.5 mg/L

5. Clearwater phase: high light availability, warm temperatures, but many herbivores and reduction of nutrients leads to population crashes

6. Mid-summer stratification: Cyanobacteria dominate (fix N, migrate between nutrient-rich lower depths & epilimnion)

7. Fall mixing: high nutrients, less light, diatoms dominate again with increases in Si

8. Late autumn decline

The plankton