Lecture 8Soft Sediment Intertidal,

Estuaries

Soft Sediment Intertidal

Sand vs. Mud Bottom BenthosSand Mud

Interstitial microfauna present No interstitial fauna

Meiofauna dominated by nematodes, polychaetes, copepods

Meiofauna dominated by nematodes, coppepods, ostracods

Macrofauna dominated by filter feeding bivalves

Macrofauna dominated by deposit feeding polychaetes

Abundance and productivity low (thousands/m2)

Abundance and productivity high (tens to hundreds of thousnads/m2)

Soft Sediment Intertidal• Zonation not as distinct as on rocky shores• Reduced vertical desiccation and temperature stress

gradients• Organisms can burrow to avoid temperature stress

and desiccation• Variation in larval settlement not as important as it is

on hard substrates

Variation in ZonationTemperate Tropical

Supra-littoral amphipods ghost crabs

Mid-littoral isopods

Intra-littoral Moon snail, large clams, sand dollars

Biogeography in Soft-Bottom Sediments

• Widest variation in densities and highest species diversity occurs in tropics

• Temperate beaches usually have a high amount of faunal diversity, a greater amount of longer-lived species, and a greater stability of faunal composition than tropical beaches

Interspecific Competition in Soft Sediments

• Food and space for burrows is limited• Burrowing invertebrates compete for space

within sediment• Dominant species found at different levels

below sediment-water interface• Little evidence of competitive exclusion

Soft Sediments - Vertical Stratification

• Experimentally reduce density of deep-dwelling clams, remaining individuals grow faster

• Removal of shallow dwelling species of bivalves has no effect on growth of deeper-dwelling species

• Likely limiting factor = space

Saccoglossus bromophenolosus

• Some burrowing species produce Bromine poisons

• Discourages settlement of other species (possibly discourages predation also)

Soft Sediments - Competition

Food Supply in Soft-Sediment Intertidal

• Suspension Feeders – Phytoplankton suspended in water

• Deposit Feeders– Microalgae and bacteria on sediment surface– Decomposing organic matter– Input can be spatially variable

• Patchy occurrence of sea lettuce (Ulva sp.) leads to spatially patchy inputs of particulate organic matter

• Patchy POM leads to patchy distribution of small polychaetes and mud snails

Food Supply in Soft-Sediment Intertidal

Food Supply in Soft-Sediment Intertidal

• Food supply for deposit feeders is more stable than the food supply for suspension feeders – Diatoms and other microalgae that deposit

feeders eat are a renewable resource• Can have seasonal “blooms” of deposit

feeders

Movement of Organisms• Swash riders: move up and down to maintain

burrowing position in moist sand, as tide rises and falls

Predation in Soft Substrates

• Predation and physical disturbance are likely the main processes responsible for maintaining high variability in distribution of organisms

• Do not see a lot of competitive exclusion, so predation typically has little effect on species diversity

• Partial predation is significant in soft substrates

Predation – Types of Predators

• Surface predators – prey at or near surface; consume whole animals or only parts of their prey

• Burrowing predators – move down tubes and burrows of prey to attack them

• Digging predators – excavate through the sediments to obtain their prey

Differential Effects of Predators

• Quammen (1984) examined effects of birds, crabs, and fishes on tidal flat communities– Crabs had greatest impact; fishes had least

impact; effects of birds were variable and depended on habitat type

• Reise (1978) – found that smaller predators can have greater effects than larger predators

Predation – Seasonal Effects• Seasonal influxes of predators can devastate local

soft-sediment communities

• Predators focus on most abundant species

Disturbance and Habitat Heterogeneity

• Disturbance re-suspends sediments and blasts out organisms

• 1st successional species usually small polychaetes

• Physical and biological disturbance• Organisms can affect habitat heterogeneity

Spatial Scales of Disturbance

Estuaries, Salt Marshes, Seagrasses, and Mangroves

Estuaries

Estuaries

• Estuary = partially enclosed section of the coast where freshwater from rivers mixes with seawater

• Watershed = the surrounding land that provides freshwater input to the estuary

Watersheds

Tampa Bay Watershed Mobile Bay Watershed

Types of Estuaries

Estuarine Structure

• Estuarine structure is controlled by seaward flow of freshwater combined with tidal mixing

Estuarine Structure

• Salinity structure of an estuary is determined by:– Watershed topography– Slope and size of river(s) feeding into main part of

estuary– Size of main estuary channel– Tidal flow

Estuarine Structure

• Overall river discharge important to salinity transitions within estuaries

• Storm events (hurricanes, etc.) can lower salinity throughout estuary

Productivity in Estuaries

• Geologically ephemeral but biologically rich• Nutrients from freshwater sources and nutrients

recycled from seabed support high levels of primary production

Estuarine Species and Salinity

• Marine species can generally tolerate salinity fluctuations as long as salinity stays above 10-15 ppt

• Vertical salinity stratification – bottom organisms can go farther upstream than planktonic species

• Mixed estuaries – infaunal species experience less salinity fluctuation than epifaunal species b/c of buffering effect of sediment pore waters

Estuarine Species and Salinity

• Number of marine species declines with decreasing salinity, especially in so-called critical salinity range of 3-8 ppt

Two-Phase Life Cycles of Some Estuarine Inhabitants

• Some species complete their entire life cycles within an estuary

• Other species have a two-phase life cycle in the estuary and on the continental shelf

Suspension Feeders in Estuaries

• Retention time = the average number of days that a phytoplankton cell stays in an estuary

• Turnover time = the number of days that it takes for a bivalve population to completely filter the water column

• Not all water in estuary may be able to be filtered by bivalves due to:– Stratification– Spatial heterogeneity of current flow

Suspension Feeders in Estuaries

• In well-mixed estuaries, bivalves may be able to greatly reduce phytoplankton densities

Top-down and Bottom-up Effects in Estuaries

• Increased nutrient inputs (bottom-up)– High levels of phytoplankton in water column can

decrease water clarity – Ungrazed phytoplankton dies and sinks to bottom

Top-down and Bottom-up Effects in Estuaries

• Loss of top predators due to overfishing (top-down) can have cascading effects on lower trophic levels

• Example:

Threats to Estuaries

• Pollution• Shoreline habitat alteration• Biological invasions