Chapter 16 Key Concepts

Vertical zonation, the occurrence of dominant species in distinct horizontal bands, is a nearly universal feature of the intertidal zone, but many localities do not “obey” the rules.
Different assemblages occur in protected and wave- swept waters in the same area.
The intertidal zone is alternately a marine and a terrestrial habitat. At the time of low tide, the heat stress, desiccation, and shortage of oxygen increase, and opportunities for feeding and respiring decrease.
Climate change is altering the geography of temperature stress and acidification stress of intertidal communities.
Living in the upper intertidal reduces the time of access to food and usually to oxygen in nearly all species, although the specific mechanisms differ.
Mobile intertidal animals may remain fixed at one tidal level, becoming active at the time of low tide, while others migrate up and down the shore with each tidal cycle in order to maintain normal activity and to remain moist.
Wave shock is a major factor determining the distribution and morphology of intertidal organisms.
Force transducers can be used to measure the force of wave shock.
Phenotypically plastic changes in form can reduce the risk of wave shock, but there are often trade-offs with other biological functions.
Zonation results from preferential larval settlement and adult movement, differential physiological tolerance, and biological interactions such as competition and predation.
Field experiments show the importance of interspecific competition for space on rocky shores.
Climate change may tip the balance of biological interactions toward changes in intertidal community structure.
Predation (or herbivory) may ameliorate the dominance achieved by competition and may strongly affect species composition and species richness.
Spatial heterogeneity may strongly affect the pattern and intensity of predation on rocky shores.
Mobile predators can often detect newly recruited sessile benthos and may form strong aggregations and feed on new recruits.
Many intertidal organisms have behavioral and structural defenses against predators.
Strongly interacting species often cause indirect effects in rocky-shore food webs.
Physical and biological disturbance often determines the species composition of intertidal communities.
Succession of intertidal seaweeds may be irregular, but an overall spectrum of life histories begins with early successional good colonizers, which are prone to grazing, in many cases yielding in late succession to good space holders that are resistant to grazing and to competitors.
The spatial scale of disturbance often results in different outcomes. Large-scale disturbances may result in unpredictable recolonization, leading to alternative stable states in large patches on rocky shores.
Water flow and primary productivity by phytoplankton interact strongly to determine intertidal community dynamics.
Combined with flow, larval supply often is a major determinant of community interactions.
Soft intertidal sediments are also physiologically stressful at low tide, but water retention reduces desiccation and temperature stress.
Soft-sediment intertidal species compete for space and food by direct displacement and chemical secretions.
Inputs of resources may be spatially variable, which may affect interspecific interactions even at the same tide level and cause significant spatial differences of entry of detritus and local dominance and effects by benthic species.
In soft sediments, deposit feeders appear to be food limited, whereas suspension-feeding populations are more variable and are not affected as much by population density.
Feeding by deposit-feeding populations may involve overexploitation of renewable resources or seasonal decline of food in the sediment, causing severe food limitation.
Seasonal influxes of predators in the intertidal can devastate local soft-sediment communities.
Invasions of predators have had major effects on intertidal communities.
Invasions of a number of sessile suspension-feeding species, combined with sea-surface temperature warming, have caused major reorganizations of intertidal communities.
Spartina salt marshes are dominated by cordgrasses, which function as ecosystem engineers by binding fine sediment, creating and causing the buildup of meadows above low water.
Spartina plants and the root-rhizome system exists in a complex association with root-associated mycorrhizae, nitrogen-processing bacteria, and some macroinvertebrate species that interact with nitrogen processes.
Vegetational zones in salt marshes develop from the interaction of competition and physiological ability to survive salt and drowning.
Spartina-dominated zones exert larger-spatial-scale impacts on adjacent ecosystems.
Floating wrack often smothers plants and creates patches of bare sediment, which become salty and inhibit colonization for a time.
Salt marsh assemblages may exert positive and negative effects as ecosystem engineers.
Salt marsh systems include creeks and mudflats, which are often biologically diverse and abundant and are corridors between salt marshes and other habitats for many marine fish species.
Spartina salt marshes produce large amounts of particulate and dissolved organic matter, which may influence the food webs of salt marsh benthos and perhaps the food webs of coastal marine systems.
Spartina marshes in the southeastern United States appear to be controlled by top-down effects in a trophic cascade.
Grazing on salt marshes of the eastern United States increases in intensity toward the south.
Spartina species have been introduced, accidentally and purposefully, and have greatly modified shoreline environments throughout the world. Dredging and filling have destroyed many salt marshes.
Nutrient addition and near-shore development have caused measurable effects on salt marshes and associated fauna.
Salt marshes are very sensitive to sea-level fluctuations and may be affected by sea-level rise derived from anthropogenic global climate change.
Mangrove forests are intertidal and emergent plant communities dominated by trees, which are rooted in marine soft sediment.
Mangroves are adapted to the anoxic sediments by air- projecting and shallow roots. Mangroves are salt tolerant.
Mangrove species show vertical zonation, which is strongly affected by seedling dispersal and invertebrate predation on seedlings.
Mangroves and upland vegetation compete for space, which is probably determined from differences in salt tolerance and local precipitation and evaporation in the vadose layer.
Mangrove sediments have abundant and diverse invertebrate populations, and particulate organic matter is important in the economy of mangrove communities.
Mangrove shallow waters and creeks are important nursery grounds for fisheries.
Mangrove forests are very endangered throughout the tropics because of shoreline development and the dredging of mangroves for the use of shrimp farms.
Sea-level rise from global warming is a great threat to mangroves but particularly upland hammocks, which are salt intolerant.
Overview: Estuarine structure is controlled by seaward flow of fresh water combined with tidal mixing.
Estuaries range from open marine to a range of successively decreasing salinity zones, to tidal fresh water and associated creeks and marshes, to fresh water.
Estuaries are geologically ephemeral but abundant in nutrient supply and biological production.
The decreased salinity at the headwaters of estuaries can reduce the number of marine species.
Overall, estuaries and shelf environments comprise a two-phase system that corresponds to life-history stages of many fish and invertebrate species.
Some estuarine species are adapted to counteract the estuarine flow to the sea, in order to be retained within the estuary; others are broadcasted onto the shelf and return to estuaries at the time of metamorphosis, making the estuary a two-phase system in coordination with the continental shelf.
Estuarine suspension feeders may control phytoplankton of shallow parts of large estuaries or in entire well-mixed estuaries; tidal exchange may be a driving force importing suspension feeder food into smaller estuaries.
A combination of historical high nutrient inputs and removal of predators has created great ecological instability from a combination of bottom-up and top- down processes.
Large estuaries are targets for biological invasions and occasional strong ecological alteration, although the rate of invasion is quite variable.
Oyster reefs occur in estuarine environments throughout the world.
Oysters are ecosystem engineers that construct reefs by means of larval settlement, attachment, and accumulation of oyster shell.
Oyster reefs attract many sessile, mobile benthic, and demersal species.
Oyster reefs thrive in relatively shallow water and when the reef shell is in high relief on the seabed.
Oyster reefs may greatly influence estuarine ecosystem processes and perform important ecosystem services, such as reducing phytoplankton density and enhancing nitrogen processing.
Oyster reefs in eastern North America and the Gulf coast succeed in relatively low salinities, where disease and predators are excluded but salinity is still high enough to allow physiological functioning, feeding, and growth.
Strong trophic cascades may influence species abundances on oyster reefs.
Oyster reefs are in strong decline because of overexploitation, habitat disruption, disease, and pollution.