Kelp Forests – Comprehensive Lecture Notes

Administrative Announcements

• Call for a Tauranga–based class representative

  • Role: liaison between students & lecturers; 2 meetings per trimester

  • Perks: free lunch, CV experience

• Follow-up email to be sent for volunteers

Defining Kelp Forests / Beds

• Taxon: large brown algae (Phaeophyceae)

• Dense assemblages = forests or beds

• Typical setting: shallow subtidal rocky shores to depths <\,30\,\text{m} (within the photic zone)

• Ecological roles

  • Habitat complexity & refuge for numerous species

  • Wave attenuation: dampens coastal energy

  • Primary production & indirect food supply to marine food webs

Lecture Goals & Core Readings

• Understand kelp life-history & reproduction

• Identify physical & biological requirements (light, nutrients, temperature)

• Examine drivers of kelp productivity

• Analyse herbivory / predation dynamics: kelp ⇌ sea-urchin alternative states

• Key references

  • Marine Biology – Brian Levitan (kelp distribution & life cycle)

  • Ludington (community-level factors)

  • Mann, Chapter 12 (productivity & biomass fate)

  • Duggins 1989 (trophic cascades)

Global & Regional Distribution Patterns

• Concentrated at high-latitude / temperate coasts; absent in tropics

• Environmental filters

  • Cool, nutrient-rich water (cold water holds more NO_3^-)

  • Hard substrate in euphotic zone

• Basin-scale asymmetry driven by major surface currents

  • Pacific

  • Southern Hemisphere: anti-clockwise gyre brings warm tropical water to New Zealand → limited kelp extent on NZ–Australian side; cool upwelled water along S-America expands kelp range toward tropics

  • Northern Hemisphere: clockwise gyre increases range along N-American coast relative to Asian coast

  • Atlantic shows analogous east–west contrasts

Major Kelp Taxa in Aotearoa-NZ

Morphology & Reproductive Cycle

• Adult sporophyte structure: holdfast + stipe + fronds with blades & sporophylls

• Alternation of generations

  1. Sporophyte (diploid) releases haploid zoospores from sporophylls

  2. Zoospores settle; develop into male & female gametophytes (microscopic)

  3. Male gametophyte produces sperm → fertilises female → embryonic sporophyte

  4. Juvenile sporophyte grows into adult

• Settlement strategy

  • Gametophyte spores are negatively buoyant (“sinkers”); \approx70\% land within 8\,\text{m} of parent—maximises chance of suitable substrate/light

• Long-distance dispersal via drift algae

  • Wave-detached fronds float thousands of km while still releasing zoospores

Environmental & Physical Drivers of Productivity

Light & Water Clarity

• Sediment plumes (e.g.
  Christchurch rivers) reduce PAR, compressing lower depth limit

• Physiological plasticity: adjust pigment quantity & composition; similar \text{P}_{max} at 1 vs 10\,\text{m} depth

• Turbidity + warming surface layers → compounded thermal stress

Nutrients (especially NO_3^-)

• Seasonal stratification in temperate summers isolates surface from deep nutrient pool

• Adaptations

  • Ecklonia stores winter excess for summer use

  • Macrocystis (California)

  • Semi-diurnal internal waves lift thermocline 2× daily; fronds access NO_3^- pulses

  • Translocation: basal tissues below thermocline absorb nutrients, move them \uparrow to photosynthesising canopy

Water Movement & Diffusive Boundary Layer (DBL)

• Sessile kelps rely on flow for solute exchange

• High velocity → thinner DBL → enhanced nutrient uptake & gas exchange

• Morphological plasticity

  • Sheltered sites: frilly, ruffled blades to self-generate turbulence

  • Exposed sites: smoother blades (no need for extra drag)

• Lab flume study (Pilditch & student)

  • Oxygen micro-profiles across blade

  • Low flow: thick DBL (O$_2$ build-up)

  • High flow: DBL compressed to blade surface

Ecosystem Services & Fate of Kelp Biomass

• Shading & temperature buffering for understory species

• Wave-energy dissipation = coastal protection

• Adds vertical & horizontal structure → refuge for cryptic fauna

• Nutrient sponge: lowers local DIN; potential blue-carbon sink

  • Burial in deep-sea sediments (e.g.
    Kaikōura canyon) sequesters carbon long-term

• Food-web integration

  • Direct herbivory only \approx10\% (mainly sea urchins)

  • Remaining \approx90\% enters detrital pool:

  • \approx30\% → DOM

  • \approx60\% → POM

  • Bacteria remineralise detritus, bind N, form aggregates → consumed by filter feeders → higher trophic levels

• Raw kelp tissue = low-quality food (structural carbon, low N)

Kelp–Urchin Dynamics & Alternative Stable States

• Cryptic phase: urchins shelter in crevices, feed on delivered drift algae

• Mobile grazing phase: urchins aggregate, mow down stipes → urchin barrens

• Transition drivers

  • Storm-induced kelp loss (↓ drift supply)

  • Warming / low-nutrient episodes

  • Predator removal (fishing, ecological shifts)

  • Disease events

Case Studies

1. California Giant-Kelp Forests

• Stable state: <7 urchins m^{-2}; benign storms; high nutrients; ample drift algae → kelp persists

• Shift to barren: severe storm + warm, nutrient-poor water → canopy loss → urchins mobilise & aggregate → overgrazing

2. Nova Scotia

a) Predation Hypothesis

• Large lobsters viewed as keystone urchin predators

• Over-fishing lobsters → ↑ urchins → ↓ kelp; subsequent urchin starvation may permit kelp rebound every 3{-}4 yrs

• Diet analyses: lobster stomachs mostly crabs—casts doubt on strength of this control

b) Disease Hypothesis

• Periodic warm Gulf\,Stream incursions introduce parasites → mass urchin die-offs → kelp recovery under cool, nutrient-rich conditions

3. Aleutian Islands, Alaska

• Pre-exploitation: abundant sea otters regulate urchins → lush kelp (biomass \times10 higher where otters present)

• 18^{\text{th}}{-}19^{\text{th}} C fur trade → otter collapse → urchin boom → kelp loss

• Recent twist: killer whales switch prey from depleted seals/sea-lions to otters; need \sim1000 otters per whale per yr → further otter decline, reinforcing barren state

4. Leigh Marine Reserve (NZ) – No-Take Protection

• Established 1975; long-term data from 1978 onward

• Recovery of snapper & large crayfish (urchin predators) inside reserve

• Observed trends

  • ↓ Urchin density

  • ↑ Ecklonia radiata canopy & other algae at multiple depths

• Demonstrates efficacy of MPAs in re-establishing trophic controls

• NZ also utilises customary tools: rāhui, taiāpure, mātaitai for local stewardship

Management, Conservation & Ethical Considerations

• Over-exploitation of keystone predators can propagate through food webs (ethical duty to manage fisheries sustainably)

• MPAs & customary closures provide spatial refuges, aiding kelp forest resilience & blue-carbon services

• Recognition of kelp forests in climate mitigation policies (carbon sequestration) presents socio-economic opportunities & responsibilities

• Restorative actions (urchin culls, predator re-introductions, sediment control on land) must weigh ecological benefits vs.
  ethical treatment of organisms & community interests

Key Take-Home Points

• Kelp forests thrive where light, cool temperature, hard substrate, & nitrate intersect

• Life cycle combines local retention (negatively-buoyant gametophytes) with long-range dispersal (drift fronds)

• Physical processes (tides, waves, currents) and physiological plasticity underpin productivity

• Vast majority of kelp production fuels detrital pathways, not direct grazing

• Urchin–kelp interactions generate alternative stable states; predator presence, disease, storms, and nutrients can tip the balance

• Long-term datasets (e.g.
  Leigh) and natural experiments (Aleutians) highlight the power of trophic cascades and the value of effective management