Introduction to Marine Science 1007ENV: Benthic Communities

The Plan

• Week 7: Plankton communities
• Week 8: Benthic communities
• Week 9: Tropical marine communities
• Week 10: Marine vertebrates
• Week 11: Environmental issues
• Week 12: Exam revision / Q&A (Chris Frid)

Source: coastalenvironments.com

Benthic Ecosystems

• Benthic = from the Greek word benthos,
  meaning "the depths [of the sea]".
• Organisms that live on, in or attached to the sea
  floor
• Adapted to a narrow range of specific pressures
• Stay within small spatial area (sessile or limited
  movement)
• Huge variety
• Communities vary based on benthic composition
  . Most found within shallow continental shelf

Neuston
(Ocean surface)
Plankton
Nekton
Benthos

Benthic Substrates

• Soft substrates
• Seagrass beds
• Seagrasses
• Sandy shores
• Animals are interstitial (live
  amongst the sand grains)
• Saltmarshes
  . Inundated during spring tides)
• Grasses, herbs
• Low diversity
• Mangroves
• Nurseries
• Carbon sequestration
• Hard substrates
• Rocky shores
• Moderate diversity
• Abundant seaweeds
• Influence of tides,
  wave action
• Coral Reefs
• Kelp Forests
• Hydrothermal vents
• Volcanically active places
• Chemosynthetic bacteria

Benthic Ecosystems Interconnected

• Planktonic
• Pelagic
• Seagrasses
• Estuaries
• Mangroves
• Salt marshes
• Rocky shores
• Sandy bottoms
• Coral Reefs
• Kelps

catchments
estuaries ~
nutrients flow downstream
seagrass
isolates
inter-reef
gardens
Halimeda
mounds
deepwater
reefs
seagrass
upwelling nutrients
deepwater
estuaries
reefs
seagrass
seagrass
isolates
Inter-reef
Halimeda
mounds
catchments
gardens
Red Emperor,
GBRMPA
5

Interactions Between Ecosystems

• Animal migrations
• Food
• Shelter
• Complete life cycles
• Physical
• Reefs create barriers
• Nutrients
• Export N, P, C, POM

LAND
MANGROVE
STAND
OFFSHORE
WATERS
SEAGRASS
CORAL REEF
TERRESTRIAL INFLUENCE
OCEANIC INFLUENCE
Figure 13-4. Schematic diagram of the tropical coastal seascape. The opposing arrows
show the buffering of land influence by shoreward ecosystems and the buffering of ocean
influence by the coral reef (from Ogden, 1987).
6

Seagrasses: Generalities

What are Seagrasses?

• Seagrass meadows are marine communities, often
  composed of co-existing seagrass species
  . Marine flowering plants
• Similar to algae, but have roots, tissue differentiation

leaf vein
-leaf
blade
cross-vein
mid-vein
-petiole
persistent
sheath
leaf scar
ņode
growing!
tip
internode
rhizome
root -
Thalassia
intramarginal-vein
leaf scale
internode
node
growing tip
Halophila
Photos: G.D-P
From Waycott et al, 2004. Tropical Seagrasses of the Indo-West Pacific.

Seagrasses: Generalities

Distribution and Diversity of Seagrasses

• What are they:
• Distributed along temperate & tropical areas
  - Play key roles in coastal ecosystems
  - Extensive meadows supporting high diversity
• Area: 0.3 million km2
• Diversity of seagrasses:
  · Low (< 60 spp;
  algae = 15,000 spp)
• Tropics: Family Hydrocharitaceae (Thalassia & Halophila)
• Temperate: Zosteraceae (Zostera); Posidonia

Thalassia
Zostera
(Eelgrasses)
Halophila
Posidonia
Photos: G.D-P

Seagrasses: Factors Affecting Survival

Light Requirements for Seagrasses

• Light
• Determines depth distribution
• High light requirements
• Thalassia, Syringodium, Halodule 15-30 % of surface irradiance
• Halophila: low light (5%-11% surface irradiance), deep &turbid waters
• Reductions of light:
  cause seagrass mortality
• Seagrasses are sensitive to light reduction > may cause mortality
  - Why the sensitivity of seagrasses to low light?
• Seagrasses colonise hypoxic-to-anoxic, sulphide-rich sediments.
• Sulphide toxicity causes \ photosynthesis, 1 respiration &
  ↓
  seagrass production
• So, seagrasses need high O2 to maintain metabolic processes for
  a large biomass of rhizomes & roots (non-photosynthetic tissue)

100 %
0 m
50 %
10/33'm
25 %
20 m/66' m
12.5 %
30 m/100' m
Light penetration of various wavelengths
Low O2

Seagrasses: Effects of Light Limitation

High light habitats
Low light habitats
Chỉ
P
R
H3C
Leaf area
12
130
Carbon uptake
Chl
?
Leaf thickness/absorption
[Ch]
Chlorophyll concentration
0000
P/R Photosynthesis/respiration
P
07
R
?
High-light/low-light photosynthesis

• Higher chlorophyll
  Physiological integration
  'High light' / 'low light' adapted species
  Fig. 3. Conceptual model showing a deep and shallow mixed meadow, where the differences in light attenuation result in physiological and
  morphological adaptation. The shallow meadow has high shoot density, with a large below-ground biomass, higher rates of photosynthesis and
  respiration, substantial self-shading with thinner leaves containing less chlorophyll pigments in comparison to the deeper meadow.
  Physiological & morphological
  adaptations (to low light):
• Î Leaf length & area
• 1 Shoot size
• relaxation of intra-specific
  competition
  12CM
  13C
• V Shoot density
• reduced leaf self-shading
• Low photosynthesis
  Ralph et al 2007. JEMBE
  10

Seagrasses: Factors Affecting Survival

Nutrients, Temperature, Salinity, Sediments

Nutrients

• Seagrass productivity is often nutrient limited
  . Increased nutrients may increase seagrass growth
• BUT ... > increased nutrients ->
  more algae
  seagrass decline

Temperature:

• Wide temperature tolerances, but absent from polar region.

Salinity

• Grow best in salinities of 35, but ranges from 4 - 65
• Some are more tolerant to fluctuations: e.g. Halophila

Sediments

• Variety of sediments, fine (e.g. Halophila) to coarse (Thalassia)
  11

Seagrasses: Factors Affecting Survival

Grazing

• Grazing
• Critical for removal of algal epiphytes
• Macrograzers (turtles; dugongs)
  n Lefcheck.
  Dugong trail
  https://www.seagrasswatch.org/
  Seagrass Restoration Project;
  /https://www.tropwater.com/
  Waycott et al. 2004
  12

Seagrasses: Adaptations

Salinity Adaptation

• Plants experience some challenges in the marine environment
  Conditions
  Mechanisms - Traits
  Salinity
• Exclude salts (Na) from leaves
  -
  13

Seagrasses: Adaptations

Water Movement Adaptation

• Plants experience some challenges in the marine environment
  Conditions
  Mechanisms - Traits
  Salinity
• Exclude salts (Na) from leaves
  Water movement,
  currents, etc
  .Ribbon-like; strap-shaped leaves: leaves oriented
  along the direction of water flow;
  .Offer low mechanical resistance, flexibility
  .Secure anchoring system
  -
  14

Seagrasses: Adaptations

Low Light Level Adaptation

• Plants experience some challenges in the marine environment
  Conditions
  Mechanisms - Traits
  Salinity
• Exclude salts (Na) from leaves
  Water movement,
  currents, etc
  .Ribbon-like; strap-shaped leaves: leaves oriented
  along the direction of water flow;
  .Offer low mechanical resistance, flexibility
  .Secure anchoring system
  Low light levels
• Thin cuticle
• Epidermis densely packed with chloroplasts
  (opposite in land plants)

Cross section of
seagrass leaf
15

Seagrasses: Adaptations

Water Environment Adaptation

• Plants experience some challenges in the marine environment
  Conditions
  Mechanisms - Traits
  Salinity
• Exclude salts (Na) from leaves
  Water movement,
  currents, etc
  .Ribbon-like; strap-shaped leaves: leaves oriented
  along the direction of water flow;
  .Offer low mechanical resistance, flexibility
  .Secure anchoring system
  Low light levels
• Thin cuticle
• Epidermis densely packed with chloroplasts
  (opposite in land plants)
  Water environment
  .No desiccation-water loss = No stomata on leaves
  ·Presence of air spaces in leaves:
  -Provide buoyancy : leaves held up
  -Reservoirs of gas exchange in leaves
  -High surface area to tissue ratio
  -> High Production/Respiration ratio
  .Hydrophilous pollination mechanism

Cross section of
seagrass leaf
16

Seagrasses: Importance

Productivity and Habitat

• Highly productive ecosystems
• Primary productivity (seagrasses & epiphytic algae) >
  food for vertebrates, invertebrates
• Fisheries production
• Complex physical structure > provide food & shelter
  enabling high productivity
• Nurseries
• For species that support offshore fisheries & adjacent
  habitats (e.g. coral reefs)
• Habitat
• For a diversity of fauna (fish,
  inverts, dugongs, turtles)
• Moreton Bay: population of
  aprox 600 dugongs !

Pelagic
Mammals
e.g. Otter
Benthic
Small fish
e.g. Stickleback,
Juvenile Cod,
Herring
Small invertebrates
e.g. Shrimp, Amphipods,
Isopods, Snail
Epiphytes
e.g. Algae,
Hydroid
Large fish
e.g. Rockfish, Eel
Large invertebrates
e.g. American lobster,
Mud-snail
Infauna
Rhizomes
Bivalves and worms
e.g. Clam, Nereis worm
Murphy et al, 2021, Facets
Photo: G.D-P

Seagrasses: Importance

Sediment Stabilization and Nutrient Recycling

• Sediment stabilization (trap sediments)
  . Reduce currents -> enhance settlement of sediments > inhibiting resuspension
• Underground root & rhizome system > important roles in binding sediments
• Protection of shorelines from erosion
• Nutrient recycling (trap nutrients) - Filters
  - Remove nutrients & contaminants (by sedimentation + N uptake ) = Filters
  - Nutrients are released slowly by decomposition & consumption > reduce problems of
  eutrophication & binding organic pollutants
  - Export organic mater [C, & nutrients (decomposition)]
  Carbon
  Nitrogen
  Phosphorus
  Photos: G.D-P
  18

Seagrasses: Importance

Global Carbon Sequestration

• Global carbon sequestration
• Remove CO2 from seawater & binds it as
  organic matter
• Turnover time of seagrass biomass is long
• Leaves & roots: 2 weeks to 5 years
• Rhizomes : > 1,000 yrs > significant role in
  oceanic carbon budget
• Long-term C burial = 83 g C / m2 /yr > global
  storage = 27 - 40 Tg C yr-1
  . Highly productive > may ameliorate impacts
  from ocean acidification ?
  Posidonia, 2 m thick; . Mateo et al, 2011
  19

Seagrasses: Global Distribution

1
4
3
2
5
6
Bioregions:
Short et al 2007. JEMBE

Temperate North Atlantic Bioregion

1
Temperate North Atlantic
Low diversity (5 spp): Ruppia, Zostera, Cymodocea nodosa, Halodule wrightii

Tropical Atlantic Bioregion

2
Tropical Atlantic
High diversity (10 spp): Halodule spp. Halophila spp., Syringodium, Thalassia

Mediterranean Bioregion

3
Mediterranean
Large meadows, Moderate div. (9 spp):C ymodocea, Posidonia, Ruppia, Zostera, Halodule

Temperate North Pacific Bioregion

4
Temperate North Pacific
High diversity (15 spp): Phyllospadix, Ruppia, Zostera, Halodule, Halophila

Tropical Indo-Pacific Bioregion

5
5
Tropical Indo-Pacific
Highest diversity (24 spp): Cymodocea, Enhalus, Halodule, Halophila, Ruppia,
Syringodium, Thalassia, Thalassodendron, Zostera

Temperate Southern Oceans Bioregion

6
Temperate Southern Oceans
Extensive meadows of low-to-high diversity (18 species):
Amphibolis, Halophila, Posidonia, Ruppia, Thalassodendron, Zostera, Halophila, Syringodium
20