Lecture 11: Temporal Dynamics

What causes phytoplankton blooms to vary seasonally in high latitude and temperate regions?

The availability of sunlight and nutrients, leading to high productivity in spring and fall, while winter and summer have lower productivity.

Why is winter less productive for phytoplankton growth?

Winter is characterized by darkness which results in less sunlight for photosynthesis.

In which season do phytoplankton blooms commonly occur in high latitude areas?

Late winter or spring, due to increased sunlight and nutrients.

What limits phytoplankton biomass in subtropical regions?

Nutrient availability, despite experiencing seasonal changes.

What are macronutrients?

Nutrients required by cells in larger quantities, such as phosphate, nitrate, and silicate.

What is the definition of stratification in ocean dynamics?

The degree to which the ocean is layered or divided, affecting mixing and nutrient distribution.

What is the pycnocline?

The layer in the ocean where there is a rapid change in density.

How does summer affect mixed layer depth and phytoplankton growth?

Summer causes warmer conditions and more stratification, resulting in a shallower mixed layer and nutrient depletion.

What is the biological pump?

The process by which phytoplankton absorb CO2 through photosynthesis and transport it to deeper waters when they die.

What effect does too much light have on phytoplankton?

It can lead to photoinhibition, damaging the cells and reducing productivity.

How does light availability change with depth in the ocean?

Light decreases exponentially with increased depth, limiting photosynthesis.

What happens to phytoplankton biomass during periods of low light and deep mixed layers?

Phytoplankton experience limited growth, leading to lower biomass.

What occurs during El Niño events in terms of ocean conditions?

Warmer sea surface temperatures, reduced upwelling, and less nutrient delivery to the surface.

Describe the impact of La Niña on upwelling.

La Niña enhances upwelling, resulting in more nutrient-rich waters.

During what phase do copepod populations often decrease in response to spring phytoplankton blooms?

During a positive NAO phase, where stronger winds weaken blooms.

What is the effect of negative NAO phases on spring phytoplankton blooms?

It strengthens blooms due to less mixing and increased light levels.

What type of ecosystem shift can occur due to overfishing or coral bleaching?

A shift from coral-dominated reefs to seaweed-dominated states.

Define ‘regime shifts’ in marine ecosystems.

Abrupt changes in community structure and function, often caused by changes in key species.

What is meant by ‘irreversibility’ in the context of regime shifts?

Once a regime shift occurs, returning to the original state without significant intervention is often very difficult.

Why do smaller phytoplankton cells dominate in conditions of resource scarcity?

They require fewer resources to exist and are more efficient at utilizing available nutrients.

Why might larger phytoplankton outcompete smaller ones when nutrients are plentiful?

Larger cells grow quicker in nutrient-rich conditions, allowing them to dominate the ecosystem.

What happens to herbivore populations when phytoplankton biomass becomes nutrient-limited?

Herbivore grazing rates peak as phytoplankton become easier targets due to slowed growth.

What is the relationship between light availability and stratification in summer?

Higher light and stratification lead to more nutrients initially, but eventually phytoplankton become nutrient-limited.

What effect does a shallow mixed layer have on phytoplankton blooms?

It allows for more light and nutrient access, promoting phytoplankton growth.

What factors limit phytoplankton bloom dynamics during winter?

Low light and deep mixed layers that restrict nutrient access.

What happens to phytoplankton during a significant storm event?

Mixing can reinvigorate nutrient availability, allowing for blooms.

What is the significance of the attenuation coefficient in ocean light penetration?

It determines how deeply light can penetrate the ocean; low values indicate clearer waters.

What functionality do zooplankton serve in the ocean ecosystem?

They graze on phytoplankton, affecting phytoplankton biomass dynamics.

How does nutrient recycling occur post-phytoplankton bloom?

Nutrients from dead phytoplankton are locally recycled, often referred to as the microbial loop.

Describe the seasonal pattern of phytoplankton biomass dynamics in relation to light and nutrient availability.

Spring and summer see blooms due to ample light and nutrients; fall sees a decline, and winter sees low biomass.

How does the position of the equator affect light availability throughout the year?

Light is consistently abundant at the equator, unlike other regions that experience seasonal variability.

What is the relationship between the depth of the mixed layer and phytoplankton growth?

A deeper mixed layer can distribute nutrients but may reduce light availability, inhibiting phytoplankton growth.

What is the consequence of high herbivory during phytoplankton blooms?

Increased grazing pressure can lead to fast depletion of phytoplankton biomass.

Describe how interannual variability, such as ENSO, impacts ocean ecosystems.

ENSO variations cause changes in sea surface temperatures, rainfall, and nutrient dynamics, affecting productivity.

What is the result of weak upwelling during El Niño conditions?

It leads to lower nutrient levels, adversely affecting phytoplankton productivity.

Why may phytoplankton growth slow when nutrients are in excess?

Excess nutrients can lead to oversaturation, limiting growth due to other factors like light availability.

What role does temperature play in stratification during different seasons?

Warmer waters lead to more stratification in summer, while cooler waters reduce stratification in winter.

Explain the concept of multiple alternative stable states in ecosystems.

Ecosystems can exist in various stable forms depending on the balance of environmental factors.

What is the relationship between biomass of larger and smaller phytoplankton in nutrient-replete conditions?

Larger phytoplankton dominate in nutrient-rich conditions due to rapid growth rates.

What can cause abrupt transitions in ecosystem structure?

Change in environmental conditions, nutrient levels, or species dominance can trigger regime shifts.

How does light attenuation affect phytoplankton distribution in the water column?

High attenuation limits light to the surface, keeping phytoplankton growth constrained near the top.

What was a notable ecological effect of the North Atlantic Oscillation (NAO) on spring phytoplankton?

Shifts in NAO states affect nutrient mixing and light levels, impacting bloom dynamics.

What defines a 'tipping point' in marine ecosystems?

A critical threshold where small changes can lead to a significant and often irreversible shift in community structure.