Lecture 11: Temporal Dynamics




Temporal Dynamics in the Ocean:

  • Plankton blooms and seasonal succession 

    • High latitude and temperate places have large variations in phytoplankton blooms seasonally; low in winter, high in spring, low in summer, high in fall 

      • Winter is dark thus less productivity due to lack of sunlight 

      • Spring has more light so photosynthesis can occur and productivity increases 

      • In high latitude/temperate places, blooms occur in late winter or spring because the sunlight allows for an abundance of nutrients

    • Subtropics are warm and lack nutrients, thus they have a low biomass of phytoplankton (still experience seasonal changes)

      • Occurs in lower magnitudes, relatively 

      • Blooms occur in winter due to the abundance of light in subtropics

      • Growth in these areas are limited by nutrient availability 

    • High phytoplankton biomass dominated by larger phytoplankton and zooplankton (who eat the phyto)

      • Diatoms and copepods

    • Low phytoplankton biomass dominated by smaller phytoplankton and zooplankton 

      • Picoplankton and microzooplankton 

    • Dynamics: phytoplankton are nutrient dependent 

      • When everything is at equilibrium, the quantity of phytoplankton is dictated by the amount of nutrients 

      • However, too many nutrients causes growth to saturate 

      • Nutrient types:

        • Macronutrients: cells require more of this (ex. Phosphate, nitrate, silicate) 

        • Micronutrients: cells require less (ex. iron) 

    • MIxed Layer Depth 

      • Warmer waters are less dense (ie at the surface) but the colder the water, the more dense = harder to mix (nutrients at the bottom) 

      • Pycnocline: deepest it will go unmixed with the same density 

      • Thermocline: deepest it will go with the same temperature 

        • Separates cold water and warm water 

      • Stratification: how divided something is (difference between surface and depth)

        • Dictates how hard it is to mix (if the stratification is high, it is hard to mix and vice versa) 

        • If unable to mix due to densities of the ocean, nutrients are not being circulated = no phytoplankton blooms 

      • Deep mixed layer = nutrients that are normally at the bottom are at the surface and this allows phytoplankton to grow 

      • Shallow mixed layer = phytoplankton lack nutrients at the surface 

      • Winter: surface cools = less stratification  = deeper mixed layer = more nutrients 

      • Summer: warmer conditions, less wind = more stratification = harder to mix (thus shallow mixed layer) = less nutrients (depletion also from phytoplankton consuming nutrients (not limited from light but nutrient limited)) 

    • Biological pump:

      • Phytoplankton absorb carbon dioxide from the atmosphere through photosynthesis and then sink to the bottom where they are eaten and decomposed 

    • Light limitations 

      • More light = more photosynthesis = more productivity

      • Too much light = photoinhibition = damages cells 

      • Northern and southern hemisphere are inverse of each other

        • When it is winter in the northern hemisphere, it is summer in the southern hemisphere

        • Light is always limited during winter times and abundant during summer

        • Only exception is the equator where light is always in abundance 

      • Since it gets darker as you go deeper into the ocean, photosynthesis reduces exponentially the lower you go 

        • Phytoplankton require light to grow 

        • Light at certain depth = light at surface (e)^ (-attenuation coefficient * depth) 

          • Low attenuation coefficient (k) = light penetrates deeply into ocean column; implies clearer waters, phytoplankton can go deeper since the light reaches there

          • High attenuation coefficient = light only reaches the surface; muddy/cloudy water; higher concentration of phytoplankton or organic matter causing it to be muddy; but they stay at the surface because they require the light 

        • Deepening the mixed layer will lessen the average amount of light phytoplankton can get… why? 

          • Mixed layers are cloudy due to the nutrients being distributed throughout the water 

          • The mixing occurs from vertical movement of water turbulence that also moved phytoplankton to the deeper parts of the ocean with no light 

          • Due to the lack of light, phytoplankton experience less photosynthesis and slower growth 

        • High latitude area (iceland/southern ocean) = light limited with very deep mixed layers… why?

          • Deep mixed layers inhibit blooms because no light and no photosynthesis 

          • Cooler oceans = less stratification = easier to mix 

          • In the spring/summer, waters become warmer and more stratified = harder to mix so the layers are more shallow 

          • Since it is more shallow, the phytoplankton stay at the top with a lot of light = more photosynthesis = bloom 

      • Nutrient limited areas (lower latitudes): blooms more reliant on nutrient supply; thus deepening the mixed layer would increase productivity in these areas 

    • Low light = deep mixed layers = high nutrients, low herbivory… why?

      • Deeper mixed layers have phytoplankton scattered through water, less dense and harder for zooplankton to graze on them 

      • Another factor is that since they are not exposed to light, their growth is limited, thus the biomass of phytoplankton is exponentially less 

    • More light w stratification and season = nutrients available, low herbivory… why?

      • In summer/spring, waters are warmer, causing a larger divide between the layers due to density in relation to the temperature

      • Cycle, so it follows after mixing has occurred and there are some nutrients at the surface, but over time, this supply is depleted and as a result, the phytoplankton become nutrient limited in the upper layers

      • So at the start of summer/spring there are often blooms of phytoplankton due to the abundance of light and nutrients (following seasonal mixing) 

      • But there is low herbivory since zooplankton lag behind and there are movement restrictions due to stratification (basically phytoplankton outpace zooplankton) 

    • High light = nutrients depleted = high herbivory 

      • Too much light results in over saturation, nutrients are all used up since they are the limiting factor; phytoplankton growth slows and their biomass will stabilize/decline 

      • High herbivory due to herbivores response to increased population of phytoplankton 

      • Since there is no mixing, the phytoplankton are trapped at the top and easy prey; herbivore grazing rates peak when phytoplankton become nutrient-limited, as their growth slows and becomes easier to control. 

    • Large blooms = large phytoplankton = large predators = export carbon and nutrients from surface with strong biological pump 

      • Nutrients are stripped from surface and exported downward to replenish the mixed deep layer 

    • Periods between blooms dominated by small phytoplankton = small predators = more focused on recycling carbon and nutrients within the surface 

      • After bloom, nutrients recycled locally (dies and remineralizes on the spot) = microbial loop 

    •  Supply and demand: diffusive supply/size of cell 

      • Smaller phytoplankton dominate when resources are scarce because they do not require a lot to run 

      • They are more efficient (ex. Smaller cell with radius of 1uM vs 10uM is 100x greater) 

      • When nutrients are abundant, larger cells dominate and outcompete the smaller cells because they will grow quicker 

      • Zooplankton prefer smaller cells because they are easier to catch and consume 

  • Interannual variations (SST = sea surface temperatures) ENSO

Feature

Normal Conditions

El Niño

La Niña

Trade Winds

Moderate easterlies

Weak or reversed

Stronger than normal easterlies

SST (Western Pacific)

Warm

Cooler

Warmer

SST (Tropical and Eastern Pacific)

Cool

Warmer

Cooler

Rainfall (Western)

High (wet, thunderstorms)

Low (dry, droughts)

Very high (flooding in Indonesia, Australia)

Rainfall (Eastern)

Low (dry)

High (floods)

Very low (drier than normal)

Upwelling (Eastern)

Strong (nutrient-rich waters)

Weak or absent

Enhanced (more nutrient-rich waters)

Ecosystem

Productive fisheries, healthy life

Fisheries collapse, ecological disruptions

Highly productive fisheries, marine blooms

  • El nino = warmer = more stratification = less chlorophyll because upwelling is reduced and less nutrient delivery 

    • Occurs every 2-8 years and climate impacts felt on a global scale 

  • Changes in environment and marine phytoplankton associated with ENSO (el nino southern oscillation) felt in higher trophic levels = bottom up forcing 

    • Ex: el nino = higher rainfall = more vegetation = increase in herbivores one year and predators the next 

  • Long-term, natural climate change 

    • North atlantic oscillation (NAO) 

      • Positive NAO = stronger pressure gradient and winds, wind position shifted north

      • Negative NAO = weaker pressure gradient and winds, wind position shifted south 

    • In positive or negative state for many years because climate impacts associated with NAO are long lasting (years to decades) 

    • Example : C. finmarchicus abundance tied to spring blooms of phytoplankton 

      • While NAO index increased, abundance of the copepod in Northeast atlantic decreased 

      • Negative NAO phase = weaker winds in spring strengthened the spring bloom because less mixing left all phyto at the top and there was higher light levels and cooler temp which they prefer 

      • Positive NAO phase = stronger winds in spring weakened the bloom since there is lower light and warmer temperatures which they hate 

  • Alternative stable states and regime shifts 

    • Environments can support multiple ecosystem states 

      • Example: A lake can be either clear with aquatic plants or turbid with algae blooms, depending on the balance of factors.

      • Key Concept: Small changes in conditions (e.g., nutrient levels) might not cause a shift, but a critical threshold can push the system into a completely different state.

      • Stability: Each state resists small disturbances and remains stable unless pushed past its tipping point.

    • However, feedbacks tend to keep ecosystem in current state (negative) because they stabilize it to return to equilibrium = harder to switch out

    • Regime shifts = abrupt changes in community structure and function (often involving key species) 

      • Describes big changes that are not necessarily from one stable state to another 

      • Represent a stark switch from one ecosystem alternative stable state to another and feedbacks keep these ecosystems in alternative stable states 

        • Example: Overfishing and coral bleaching can shift a coral reef ecosystem to a seaweed-dominated state.

      • Irreversibility: Once a regime shift occurs, it can be hard or impossible to return to the original state without significant intervention.