Shelf Seas

Inshore neritic zone (shallow zone) from coastline to shelf break

Ends at ~200m depth as break begins to form continental slope

Contributes to 8% global sea suface

Ecology strongly influenced by physical processes e.g. temperature, tides - nutrient availability

Turbulence disturbs water - draws nutrients from depths in shelf seas

= abundance of life - greatest fish population and krill (highest total biomass on planet)

All primary producers unicellular and contain chlorophyll a

Many different sizes - picoplankton or micro/mesoplankton

Contribute <75% oxygen in shelf seas

Prokaryotes - Cyanobacteria

• Highest biodiversity in littoral zones
• Symbiotic associations with sponges and ascidians
• Single long filaments or clumps 
• 2 main genera: Oscillatoria and Synechococcus

Dinoflagellates e.g. Ceratium

• Second most abundant phytoplankton group
• Autotrophic, heterotrophic, mixotrophic
• Motile - posses 2 flagella
• Thecate (cellulose plates) or naked
• Blooms - Red Tides
• Some toxic (especially to filter feeders)

Diatoms 

• All aquatic environments
• Most abundant phytoplankton group
• Main contributor to spring phytoplankton blooms
• Silica frustule (skeleton) in 2 halves - epitheca and hypotheca
• Non-motiel - move w/ currents but ionic regulation means little movement (float more or less)
• SA:V ratios
• Solitary or colonial

Coccolithophores

• Posses ~30 CaCO3 coccoliths (plates) per cell
• ~150 species - main is Emiliania hyxleyi 
• Reflect sunlight and heat back out of water = cooling of water
• Ocean acidification affectes coccolith formation

To photosynthesis and grow phytoplankton need light as energy, CO2 as carbon, O2 for respiration and nutrients to make cell structures

Growth occurs in the euphotic zone - sufficient light to support growth and reproduction

Light extinction coefficient of water depends on wavelength, dissolved material and particles

• Moonlight - low light = no phytoplankton growth
• Costal - sunlight support phytoplankton growth
• Ocean - sunlight supports phytoplankton growth at lower depths

Phytoplankton only use visible light (400-700nm) = photosynthetically active radiation

Clear oceanic waters - red absorbed strongly by water but green and blue reflected

Costal water - green reaches furthest depths

Estuarien water - contain substances produced by degredation of organic matter = reflects yellow-red part of spectrum = attenuates further

• R = respiration
• a = Photosynthetic efficiency (line upwards)
• Pmax = max photosynthetic rate
• Photoinhibition = damage to reactions caused by excess light energy
• Pn = net photosyntesis
• Pg = gross photosynthesis
• Compensation light intensity = photosynthesis rate equals respiration rate - bare minimum for survival
• Ek = light saturation intensity

• Different types of phytoplankton have different curves - different photosynthesis rates
• Sun type species - higher photosynthetic rates at higher light intensities
• Shade type species - higher photosynthetic rates at lower light intensities
• Some species reach Pmax at same intensity but one has higher rate = more efficient = out compete the other - community densities affected

Compensation Light Intensity (Ec) - light intensity required for photosynthesis to be at same rate as respiration

Compensation depth (Dc) - depth where photosynthesis rate equals respiration rate = lower boundary of euphotic zone

Above Dc = net gain (P>R) of growth

Below Dc = net loss (P<R) of growth = less primary productivity

Photoautotrophs move up and down water column - be above Dc

To study critical depth models - average light intensity must be known

Thermocline affects mixing of organisms and nutrients - changes in density

Higher depths = increased metabolic rate

Critical depth (Dcr) - no net photosynthesis occurs beyond here

Macronutrients - elements need to be supplied in large pop. in order for organism to live (C, H, O)

Proteins, lipids, nucleic acids and nucleotides - contain N, P, S = also required

Diatoms require Si for skeleton

In healthy, actively growing cells - uptake ratio of C:N:P = 106:16:1 = Redfield Ratio

Micronutrients/trace elements - required in small quantities for cellular function = Fe (enzyme activity) and K, Ca, Mg, Na (ion transport)

Elements not usually found as separate identities - compounds:

• Nitrogen - NO3, NO2, NH3, N2
• Photophorous HPO2-4, PO3-4
• Sulfur - SO24, H2S
• Carbon - CO2, HCO-3, CO2-3, H2CO3

Greatest in shelf seas and upwelling regions in warmer climates

Oligotrophic regions - low nutrient concentration

Eutrophic regions - high nutrient concentration

High nutrients = high primary production = larger food chains

• Winter:
  • Low phytoplankton biomass - low temperatures
  • High mixing = nutrients distributed in water column
• Spring:
  • Warming increases biomass
  • Light increases photosynthesis
  • Decrease in nutrients at shallow due to increased phytoplankton
  • = Spring bloom (mainly diatoms)
• Summer
  • Increase temperature/light
  • Nutrients depleted at shallow waters - phytoplankton blooms
  • Large primary productivity
• Late summer
  • Complete collapse of phytoplankton at shallow seas
  • Phytoplankton move to deeper water = more nutrients
  • Thermocline will increase in water again

Little ability to swim against major water currents

Diverse group of larval and adult forms - most animal phyla

Occupy several trophic levels

Play key role in pelagic food webs and biogeochemical cycling

Mostly heterotrophs - some mixotrophs called protozooplankton

Size between 2um and 200cm

Holoplankton - plankton that spend entire life cycle as zooplankton

Meroplankton - spend some of life in benthic region of ocean

Examples

• Copepods
• Starfish/gastropod/bivalve larvae
• Jellyfish
• Tunicates
• Copepods
• Radiolarians

Patchy distribution common - physical and biological stresses e.g. food availability

Langmuir vortices influence zooplankton distribution - accumulate particles in certain areas 

• Caused by consistent wind blowing in one direction = small upwelling and downwelling cycles

Diel vertical migration common in epipelagic and mesopelagic species - light acts as stimulus = organisms move up to epipelagic at night and down to mesopelagic in day

= Greatest migration in terms of biomass - influence particle flux on seabed

Preferred depth ranges vary w/ life stage, weather, season and latitude

3 types of migration:

• Noctural - move up at night
• Twilight - move up at twilight
• Reverse - move up at day

Seasonal vertical migration - reproductive cycles = organisms have life histories w/ different depth preferences at different stages

Example

• Non-feeding adults over winter at ~400m
• Lay eggs at depth Dec-April
• Population matures at surface - primary production high
• By June - storage lipids accumulated
• Migrate to deeper waters in autumn
• Cues - temperature and food availability

Climate variability - distribution of zooplankton

North Atlantic oscillation = position of western winds from ocean 

• Negative (low) NAO - high S. Europe winds = cooling in South
• Positive (high) NAO - high N. Europe winds = warming in South
• Affects migration directions - distribution of both cold water and warm water species

• Phase 1 -
  • Initial spring phytoplankton bloom = nutrient levels begin to decrease
• Phase 2
  • Zooplankton levels - increase = more phytoplankton available
  • This and lack of nutrients = demise of phytoplankton bloom
  • Zooplankton - increase organic matter
• Phase 3
  • Mid-summer recycling phase
  • Zooplankton levels decrease - phytoplankton levels have depleted
  • Phase 4
  • Regenerative period
  • Low abundance of plankton = nutrient stocks increase
• Differences with latitude:
  • Arctic - delayed increase in zooplankton due to phytoplankton
  • North Atlantic - 2 bloom later in year - 2nd increase in nutrients due to decomposers
  • North Pacfic - very delayed increase in zooplankton (weird)
  • Tropical - constant fluctating due to storms causing nutrient mixing

Production = Energy in ingested food - (Energy in exceretion + Energy in respiration)

Growth yield (Ygr) = Growth/Growth + Respiration

OR 

Growth yield (Ygr) = Growth/Food intake

Ygr = 10-30% depending on type of organism, level of complexity, swimming ability and life history stage

Trophic yield (Yt) = Production at trophic level t+1/Production at trophic levelt

Ecological efficiency - energy transferred between trophic levels

Simple food chains = highly productive systems (less trophic levels to loss energy)

Initial size of number of phytoplankton influences number of trophic levels