Download 15_soft-sediment ecology

Ecology of Soft-Sediments
I.
Background
II.
Production
I. Background
A. Prevalence of Soft Sediments
III. Effects of Sediment Properties
B. “Goods & Services”
IV. Recruitment
V.
C. Soft-sediment Species
Species Interactions
VI. Human Impacts
A. Prevalence of Soft Sediments
•
80% of ocean floor is covered in soft sediments
estuaries (intertidal)
coastal zones
continental shelves
deep sea
•
B. Goods & Services
All regions: tropical, temperate, and polar
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Fisheries
•
Nutrient Recycling
•
Detoxification of Pollutants
•
Trophic Links
Polychaetes
Crustaceans
C. Species (0.5 - 10 million)
•
Dominant Taxa (mobile & sessile)
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Epifauna vs. Infauna
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Suspension Feeders vs. Deposit Feeders
•
Opportunistic vs. Late Successional
•
Bioturbators vs. Sediment Stabilizers
Echinoderms
Plants
Molluscs
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Dominant Taxa
Epifauna - animals that live on top of the sediments
• Polychaetes burrow or build tubes; suspension (filter) feeders,
Infauna - animals that live within the sediments
deposit (detritivores) feeders, and predators
• Crustaceans (ostracodes, isopods, amphipods, tanaids,
Infauna (Macoma)
decapods): burrow or on surface; all feeding modes including parasites
Epifauna (Cerithidea)
• Echinoderms (sea stars, brittle stars, sand dollars, sea urchins,
sea cucumbers): on or near sediment surface; most feeding modes
• Molluscs (snails, nudibranchs, octopods on surface): herbivores,
detritivors, and predators; suspension and deposit-feeding clams in
sediments, siphons extent to surface
• Algae and Vascular Plants (seagrasses) only in shallow
habitats
Suspension feeders (filter feeders) eat suspended particles
(e.g., plankton & other suspended organic material)
Deposit feeders eat organic detritus in or on the sediments
deposit feeder (Macoma)
suspension feeder (Protothaca)
Opportunistic vs. Late Successional
• Succession occurs in soft-sediment communities after
disturbance
• follows same general progression we’ve discussed
already
- opportunistic species (r-selected colonize first)
- K-selected species colonize later
- facilitation can be very important
Bioturbators vs. Sediment Stabilizers
II. Production
Bioturbators resuspend, erode, or otherwise
move sediments by burrowing or digging
-- uncovers, oxygenates, and irrigates
sediments
A. Allochthonous Input
-- e.g., burrowing bivalves
B. In situ Primary Production
Sediment Stabilizers build structures that bind
sediments and decrease sediment resuspension and increase deposition
-- e.g., tube building worms & seagrasses
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A. Allochthonous Input
A. Allochthonous Input
Allochthonous Input = input of organic carbon from outside
the system (i.e., not local primary production)
•
Density and Species Diversity are often related to
allochthonous input
• deep water no sunlight no photosynthesis no primary
production
•
example: Dayton & Oliver 1977, Antarctica
• input comes from pelagic & terrestrial systems
- high input areas: high density and species diversity
- low input areas: low density and species diversity
• low in tropics, higher in temperate and polar regions (seasonality
weak in tropics)
• pelagic input affected by magnitude of production, depth, sinking
rates, & intensity of vertical mixing
• secondary productivity in sediments strongly tied to productivity
in overlying water column
B. In-situ Primary Production
Benthic Algae (micro- and macroalgae) & plants grow in
water < 150 m deep
•
•
III. Sediment Properties & Effects
macroalgae & vascular plants
require stable substrates
A. Properties
grazed by many taxa, especially amphipods and sea urchins
B. Causes of variation in properties
fed on by deposit feeders if resuspended
microalgae (diatoms & dinoflagellates)
attach to or live between sediment grains (& get some of their
nutrients from the sediments)
grazed by crustaceans, polychaetes, echinoderms, and
gastropods
fed on directly by some deposit feeders
A. Sediment Properties
•
•
•
•
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grain size (0.05-mm mud on up)
organic content
chemical properties
stability
porosity
C. Effects of sediment variation on
distribution & abundance of
organisms
B. Causes of Variation in Sediment Properties
• sediment origin: terrigenous vs. marine
• chemical reactions (e.g., precipitation of calcium
carbonate)
• hydrodynamic conditions (flow resuspends fine
sediment)
• biological activities
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Biological Activities
Depth of habitable sediments is determined by several factors:
Bacteria break down organic matter buried in
sediment…
• quantity of organic material: more material shallower habitable depth
• near sediment surface, aerobic bacteria use O2 to beak
down organic matter
• rate of decomposition (i.e., rate of generation of toxic
chemicals)
• deeper in sediment, where oxygen is limited or absent,
anaerobic bacteria use oxidized inorganic and organic
compounds (e.g., nitrates, sulfates, and phosphates)
• rates of O2 supply and use
- produces ammonia, hydrogen sulfide, and ferrous ions,
which are toxic. (this is the black layer)
• grain size: larger grains deeper habitable depth
• flow speed: higher flow speed deeper habitable depth
• these toxic chemicals & lack O2 of limits organisms to
the upper sediments
C. Effects of sediment variation on distribution &
abundance
C. Effects of sediment variation on distribution &
abundance (cont.)
• sediment chemistry limits all but bacteria to top layer
(generally a few mm to 30-50 cm)
• burrowing animals may escape predators by living in
the toxic black sediment
•
abundance of soft-sediment organisms is generally
positively correlated with amount of organic carbon in
sediments
•
diversity of species increases with diversity of sediment
grain size & food particle size in sediment
(but must have adequate water circulation into burrow to
deliver oxygen)
abundance
species
diversity
• generally…
- suspension feeders in coarse grain sediments
- deposit feeders in fine grain sediments
IV. Recruitment
organic carbon content
grain size diversity
IV. Recruitment
• variable in space & time, as in other marine systems
• can be important in establishing patterns of abundance &
distribution
• set by same processes we’ve already covered, except…
- movements of sediments (e.g., burial, can affect
recruitment)
- high flow may wash settlers off sediment surfaces
•
nice example of factors affecting recruitment: (Highsmith
1982) sand dollar Dendraster excentricus
—
larvae are attracted to chemical cues of adults
—
adults baffle water creating eddies that allow larvae to
successfully settle near them
—
adults stabilize sediments and protect young from predators,
increasing early post-settlement survival
results in highly patchy (and predictable) distributions of sand dollar “beds”
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A. Predation
V. Species Interactions
A. Predation - important
•
usually found to have major effects on abundance
•
no effect or negative effects on species diversity (i.e., no
keystone predation)
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often restricted to certain sediment depths (shallow sediments
accessible to predators)
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often lower in certain habitats (e.g., vegetated vs. unvegetated)
B. Competition - relatively unimportant
C. Facilitation - important
D. Biotic Disturbance - important
A. Predation
A. Predation
(Meyer and Byers 2005)
Types of predators
•
Surface predators feed in surface sediments & don’t
affect deep-living organisms (e.g., birds, crabs, fish)
•
clam siphons
Non-lethal predation
•
Burrowing predators move down burrows and
attack prey (e.g., some sea stars & nemertean worms)
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Siphon nippers - some species (mainly fishes, but also
some decapods) bite off the exposed tops of clam
siphons
Digging predators dig out prey and also disturb
- doesn’t kill clams, but reduces their growth rates
sediments, causing other changes (e.g., crabs, rays,
horseshoe crabs)
- for species with short siphons, ultimately causes higher mortality
because…
-- must move closer to sediment surface to feed
-- surface predators (e.g., crabs) are more effective when prey are
closer to sediment surface
A. Predation
Trophic Cascades
•
higher level predators reduce abundance of lower level
predators, thus indirectly increasing the abundance of
the prey of the lower level predators
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example: gulls prey on large predatory polychaete
worms, which prey on smaller polychaetes
B. Competition
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tends not to result in death
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can set sediment depth distributions (e.g., Peterson 1977)
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reduced growth from crowding and food depletion (e.g.,
Peterson 1982)
- gulls indirectly increase abundance of smaller polychaetes
(Ambrose 1984)
predatory
polychaete
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C. Facilitation
D. Biotic Disturbance
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common with plants generating habitat that provides food
and protects from predators
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example: seagrasses – higher density and species diversity
of animals in seagrass beds versus adjacent unvegetated
sediments
- seagrass protects prey from predators
creates patches in different successional stages, enhancing
diversity
- e.g., bat rays (VanBlaricom 1982) and gray whales (Oliver and Slattery
1985)
•
special kind: Trophic Group Amensalism
(Rhoads and Young 1970)
negative effects of deposit feeders on suspension feeders
-- deposit feeders re-suspend sediments
- enhances food deposition by slowing water
-- suspended sediments clog feeding apparatus & bury settlers of
suspension feeders
oyster reefs, too
-- should generate non-overlapping patterns of distribution for the two
trophic groups (sometimes does, sometimes doesn’t)
VI. Effects of Humans
A. Fishing
• direct removal of animals
• damage to bottom
deposit feeder (Macoma)
suspension feeder (Protothaca)
B. Pollution
• nutrients
• toxic chemicals
A. Fishing
•
direct removal of animals - overfishing reduces abundance
and changes trophic structure and community interactions
B. Pollution
•
nutrients
nitrogen (e.g., fertilzer runoff)
- can cause eutrophication (& anoxia) in shallow waters
- increases primary productivity & changes food web
structure and species composition
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damage to bottom
- sediments are altered
- non-target animals are injured or killed
- trawls dragged over the sediment can weight 10 tons
- some areas are trawled at very high frequencies: 71% the
entire bottom of the Danish North Sea is swept at least
once a year
carbon (e.g., sewage)
- increases productivity
- but makes sediment more anaerobic
- changes community structure
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B. Pollution
Effects of pollution on sediment conditions
•
toxic chemicals (e.g., DDT, pesticides, metals)
— accumulate in sediments, especially in
shallow coastal waters
Sediment Depth
— population and community level responses
are poorly understood
— effects on individuals better known &
include…
-- mortality
-- reduced growth
Normal
Transitional
Polluted
Grossly Polluted
-- reduced reproductive output
-- disease
Summary
• soft sediments are the dominant benthic marine
habitat & are found world wide
• huge numbers of species live in this habitat and many
provide valuable “services”
• production comes from allochthonous input and in-situ
primary production
• sediment properties have strong effects on the
distribution and abundance of species
• biotic interactions are important, mainly predation,
facilitation, and biotic disturbance
• humans have a variety of impacts on soft-sediment
communities, especially pollution
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