ECOLOGY OF PHYTOPLANKTON: CHAPTER 6
MORTALITY AND LOSS PROCESSES IN
PHYTOPLANKTON
Kevin Kapuscinski and Katie Woodside
6.1 Introduction
Principle Loss Processes
 Hydromechanical dispersion (wash-out, downstream transport, dilution)
 Sedimentation
 Consumption by grazers
Other Loss Processes
 Parasitism
 Physiological death
 Wastage
Rate of Population Change
rN = r’ – rL (Eqn. 5.1) where rN = increase rate, r’ = replication rate, rL = instantaneous
rate of loss to all mortalities
rL = rw + rs + rg + .... where rw = loss due to wash-out, rs = loss due to sedimentation, rg =
loss due to grazing, etc
If rL > r’ than rN < 0 and the population is declining.
6.2 Wash-out and dilution
6.2.1 Expressing dilution
rw = qs / V where qs = volume of particle free water that enters the impoundment
(magnitude of hydraulic replacement rate), V= volume of the impoundment (if particles
are evenly distributed/ there is no patchiness)
6.2.2 Dilution in the population ecology of phytoplankton
 Sensitivity to flushing increases with slow growth rate
6.2.3 Phytoplankton population dynamics in rivers
 Larger rivers (3rd or 4th order) support populations of river plankton
(potamoplankton) in non-flowing water
 Non-flowing water

Boundary friction between banks and bed

Fluvial ‘deadzones’ (little ponds within the river) sensitive to changes in
discharge, fluid exchange, and turbidity
 C-strategists and CR-strategists do well in rivers
 Consumption by filter-feeding zooplankton and zoobenthos
 Macrophytes in headwaters and lateral dead-zones act as shelters and substrata
6.3 Sedimentation
6.3.1 Loss by sinking
 Turbulent entrainment slows sinking
 Non-motile organisms vulnerable to variations in mixed depth
 Small, motile, or minimization of density helps reduce sinking rates (Stokes eqn.)
6.3.2 Mixed depth and the population dynamics of diatoms
 Colony formation and siliceous exoskeletons provide increased form resistance
and entrainability
 Dependent on turbulence and absolute mixed-layer depth for dispersal and
population recruitment (intensity and extent of vertical mixing)
 Lack of nutrients = heavy sinking losses
 Onset of stable thermal stratification = higher sinking losses
 Shortening mixing depth = accelerated rate sinking rate = accelerated sinking loss
 Accelerated sinking rate could be positive mechanism used to escape near-surface
insolation, could allow for population re-establishment upon better conditions
 Large phytoplankton succumb more to sedimentation than colonial phytoplankton
 Sedimentation main loss of limnetic (open-water) phytoplankton
 Sedimentation leads to seasonal succession of other phytoplankton
6.3.3 Accumulation and resuspension of deposited material
 ‘Seed banks’ need to survive and escape back into water column
 Resting stages with independent capacity for germination, regrowth, and
reinfection
 Resting cysts and stages that depend on still-suspended or resuspended propagules
encountering tolerable conditions
 As sediment builds up, materials are compacted and lost from semifluid layer,
water and biominerals are lost upon compaction
 Filaments and chains exist in semifluid layer longer than centric unicells
 Resuspension dependent on:

Sufficient turbulent force

Depth of sediments

Borrowing of invertebrates, fish, etc
6.4 Consumption by herbivores
6.4.1 The diversity of pelagic phagotrophs and their foods
 Zooplankton – feed on live or detrital organic particles for most/all energy and
carbon
 Protistan microzooplankton

<200 m heterotrophic protistans and metazoans

Many photoautotrophs with phagotrohpic capabilities

Planktic ciliates can ingest long filaments by coiling them intracellularly

Feeding largely depends on encounter/chance
 Multicellular microzooplankton

Marine - larval crustaceans, rotifers, larvaceans, larvae of other groups
(molluscs and echinoderms)

Marine - feeding largely depends on encounter, cilia around mouth,
mandibular mouthparts

Lakes – rotifers, copepod nauplii
 Freshwater mesozooplankton

0.2 – 2 mm

Can exploit currents and turbulence

Common adaptations – transparency, ability to propel self

Copepods (cyclopoids and calanoids)

Cyclopoids – short biramous antennules, pear-shaped, thoracic legs

Calanoids – long antennules, cylindrical shape, can filter feed via
currents created by appendages or actively capture larger algae and ciliates

Branchiopods (Cladocera)

Specialized filter-feeders drawing water through carapace

Short abdomen and thorax covered by carapace, 4 to 6 pairs limbs
with setae, large biramous antennae

Daphniidae
 Marine mesozooplankton

Calanoids, cladocerans, thaliacean tunicates (salps)

Calanoids are more efficient carbon harvesters than cladocerans

Cladocerans can filter more water, harvest more food, faster metabolism
and growth than calanoids in nutrient rich environments

Tunicates – gelatinous, barrel-shaped, low body mass filter-feeders
 Planktivorous macroplankton, megaplankton and nekton

Trophic cascades with zooplanktivorous fish

Macroplankton (2 to 20 mm), Megaplankton (>20 mm) – polychaetes,
amphipods, larval decapods, larval hemipterans

Swimming nekton (fish, squid) and their juvenile hatchlings
6.4.2 Impacts of filter-feeding on phytoplankton
 Means to sieve and concentrate particles
 Filtration rates versus Feeding rates
 Food availability

Depends size of filter, leakage

Size and texture of food

Chemoreception

Kairomones (undigestable cells)

Production of toxic substances by phytoplankton

Mucilage decreases successful ingestion
 Algal removal and grazer nutrition

Without predators it depends on temperature and food availability
 Food thesholds and natural populations

Larger filter-feeders have larger resource base than smaller filter-feeders

Algal food resource supplemented by detritus and bacteria

Larger zooplankton more vulnerable to predation

Feeding pressure on zooplankton varies (fish switch between benthic and
planktic resources)
6.4.3 Selective feeding
 Filter-feeding plus active capture through chemoreception, scraping, fragmenting
food
6.4.4 Losses to grazers
 Filter-feeding more damaging than selective feeding
6.4.5 Phytoplankton-zooplankton interactions (zooplankton don’t control
phytoplankton in predictable way)
 Competitive interactions

Cladocerans select against small algae leaving large indigestible algae for
daphniids
 Feedbacks

Excretory wastes, sloppy eating (recycled nutrients)
 Bottom-up and top-down processes in oligotrophic systems (resource
restraints)
 Bottom-up and top-down processes in enriched systems

Control of system can switch from top-down to bottom-up and vice versa

Seasonal, temperature influences who has control
 Intervention in food-web interactions

Catastrophic events (fish kills, toxic substances)

Invasions of exotics
 Food-chain length

Determined by stability of key components, availability of resource base,
usable energy influx. (Overall transfer of energy through trophic levels important)

Stable isotope analyses of food webs show that size of ecosystem and
totality of resources more important determinants
6.5 Susceptibility to pathogens and parasites
6.5.1 Fungal parasites
 Difficult to distinguish except based on host
 Host cells almost always killed
 Under low light, low infection rates
6.5.2 Protozoan and other parasites
 Often wrap around algae or suck out contents through holes made in cell walls
5.5.3 Pathogenic bacteria and viruses
 More common in lakes than oceans
 Viruses may be dormant for years
6.6 Death and Decomposition


Failure of organism to maintain basic metabolic functions
Programmed cell death (apoptosis)
6.7 Aggregated impacts of loss processes on phytoplankton composition

Seasonal succession, succession of different phytoplankton species
Salps Class Thaliacea
http://www.earthlife.net/inverts/images/others/salp.jpg
http://www.amonline.net.au/fishes/fishfacts/images/salp.jpg
Tunicates
http://www.aquamarinediving.com/images/DFugitt_Bali_Tunicates.jpg
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