Ecology of Soft-Sediments

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 •
Fisheries •
Nutrient Recycling •
Detoxification of Pollutants •
Trophic Links Polychaetes Crustaceans C. Species (0.5 - 10 million) •
Dominant Taxa (mobile & sessile) •
Epifauna vs. Infauna •
Suspension Feeders vs. Deposit Feeders •
Opportunistic vs. Late Successional •
Bioturbators vs. Sediment Stabilizers Echinoderms Plants Molluscs 1 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 2 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 3 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” 4 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) •
often restricted to certain sediment depths (shallow sediments accessible to predators) •
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) •
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 •
example: gulls prey on large predatory polychaete worms, which prey on smaller polychaetes B. Competition •
tends not to result in death •
can set sediment depth distributions (e.g., Peterson 1977) •
reduced growth from crowding and food depletion (e.g., Peterson 1982) - gulls indirectly increase abundance of smaller polychaetes (Ambrose 1984) predatory polychaete 5 C. Facilitation D. Biotic Disturbance •
•
common with plants generating habitat that provides food and protects from predators •
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 •
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 6 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 7