Sedimentation is a widespread and increasing process on most rocky coasts. The literature on its effects is reviewed and support is found for the general conclusion that sedimen- tation is an important ecological factor for hard bottom organisms. Sediments deeply affect the composition, structure and dynamics of rocky coast assemblages, and increased sediment load as a consequence of anthropogenic activities can be a threat to their diversity and functioning. Sediments that accumulate on rocky substrata are important agents of stress and disturbance. They can cause burial, scour and profound modifications to the characteristics of the bottom surface, and interact with other important physical and biological processes. The effects of sedi- mentation are complex, because they involve both direct outcomes on settlement, recruitment, growth or survival of individual species and indirect outcomes through mediation of competitive and/or predator-prey interactions. Not all species and assemblages are equally affected by sedi- mentation and responses vary over space and time, depending on the characteristics of the depo- sitional environment, life histories of species and the stage of development of individuals and assemblages, and in relation to variable physical factors, including hydrodynamics, light intens- ity and bottom topography. Recent studies have much improved our ability to detect and under- stand the effects of sedimentation on rocky coast assemblages. However, little is still known about the underlying mechanisms. Overall, our present ability to make generalisations and pre- dictions is limited by a paucity of quantitative and experimental research, and by the scant atten- tion devoted to measuring the regime of perturbation by sediments and responses of organisms at relevant spatial and temporal scales. Predicting the magnitude of the effects that different sed- imentation regimes have on rocky coast organisms and the critical levels above which detrimen- tal effects become manifest remains a key issue for the ecology of rocky coasts and a challenge for future studies.

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The effects of sedimentation on rocky coast assemblages

Rocky shores occur at the interface of the land and sea. Typically they are open ecosystems, with steep environmental gradients. Their accessibility to man has rendered them susceptible to a variety of impacts since prehistoric times. Access can be regulated, however, and they are more amenable to management than open ocean habitats. This review uses examples from throughout the world to demonstrate the extent to which rocky shores have been, and are currently, affected by pollution (examples used are endocrine disrupters, oil, eutrophication), over-collection of living resources, introduced alien species, modification of coastal processes (coastal defences, siltation) and global change (climate, sea level). These impacts are put into the context of natural fluctuations in time and variability in space of both the environment and the organisms. The relative magnitudes of some anthropogenic disturbances differ between the industrialized, developed world and the developing world. For example, in developed, industrialized countries pollution based impacts should diminish over the next 25 years due to improved regulation and a reduction in older ‘dirtier’ heavy industry. Conversely, in many developing countries pollution will increase as a consequence of growth in the human population and industrialization. Except for large-scale disasters such as oil spills, pollution tends mainly to influence embayed coastlines. Chronic effects such as eutrophication can have broader-scale impacts over whole coastlines and elevated nutrient levels have also been implicated in a trend of increasing frequency of catastrophic kills due to harmful algal. Direct removal of living resources has had major effects on coastlines at both local and regional scales and is likely to increase over the next 25 years, especially in developing countries where rapidly expanding human populations will put further pressure on resources. Impacts from recreational activities are likely to increase with greater leisure time in wealthier regions of the world, and cheaper travel will spread these impacts to poorer regions. Invasions by alien species have increased in frequency during the last 20 years leading to some dramatic effects on native assemblages. Problems associated with alien species, especially pathogens, will continue to increase over the next few decades. The proportion of the coastline modified by artificial structures (breakwaters, seawalls, groynes) will increase because of coastal development and defences against sea-level rise and the greater frequency of storms. This will increase connectivity between areas of rocky habitat. Siltation will continue to increase due to urbanization of catchments and estuaries, and changes in agricultural practice. This may have considerable impacts at local and regional scales, favouring sediment tolerant organisms such as turf algae and anemones. In the future, greater frequency of environmental extremes is likely, including large-scale events such as the El Nino Southern Oscillation (ENSO). Global change in temperature, sea-level rise and increases in the frequency of storms will affect rocky shores throughout the world, but this will occur over long time scales; over the next 25 years most of the responses by rocky shore communities will mostly be quite subtle. Thus rocky shores will be subject to increasing degradation over the next 25 years. They are, however, less vulnerable than many other aquatic habitats due to their hard substratum (rock), their relative lack of large biogenic structures and to their generally open nature. They are also remarkably resilient, and recovery can occur rapidly due to recruitment from unaffected areas. Their susceptibility to both terrestrial and marine disturbances does make them more vulnerable than sublittoral and offshore habitats. There are considerable gaps in knowledge, particularly of certain microhabitats such as crevices, boulders, sand-scoured areas and rock pools. These have been much less studied than more accessible assemblages on open, freely draining rock. More research is needed to establish the effects of increasing sediment loads, ultraviolet radiation and introduced species on rocky shore communities. Strategic and applied research programmes should integrate field experiments and carefully selected monitoring programmes to verify management regimes. Hindcasting from the palaeo-record would be valuable, to compare rates of predicted change with periods when change was rapid in the past. This information could, in principle, be used to help conserve rocky shores through networks of marine protected areas and a general reduction of environmental pollution.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0376892902000115

Rocky intertidal communities: past environmental changes, present status and predictions for the next 25 years

A common assumption of ecological theories addressing the causes of spatial dominance and stability in natural communities is that organisms have trade-offs between resistance to disturbance, environmental tolerance, and competitive abilities, i.e., that different species or groups of species dominate under different regimes of disturbance and stress. A multifactorial field experiment was conducted from June 1994 to October 1995 in a turf-dominated macroalgal assemblage on a rocky subtidal shore south of Livorno (western Mediterranean, Italy) to test the hypothesis that variable patterns of disturbance and stress influence the differential success of species, and to identify functional characteristics resulting in persistent monopolization of space. The experiment, which included manipulation of the depositional environment, was designed specifically to test whether patterns of recovery following abrasion or removal of the biota differed among patches of different sizes, produced at different times of the...

Roles of disturbance, sediment stress, and substratum retention on spatial dominance in algal turf

Effects of sedimentation on subtidal macroalgal assemblages: an experimental study from a mediterranean rocky shore

Mixing plots, in which a dissolved constituent is plotted against salinity or chlorinity, are commonly used to interpret conservative and non-conservative processes in estuarine systems. A bend in the resulting curve is generally interpreted as indicative of the reactive or non-conservative nature of the constituent or the presence of multiple sources or sinks within the estuary. This paper demonstrates analytically that bends in mixing curves may also result from temporal variations in end-member (river or ocean) constituent concentrations even for conservative constituents. A one-dimensional dispersion equation is used to calculate the distribution of salinity and a conservative constituent in a model estuary. Both straight and bent mixing curves are shown resulting simply from changing the variability of the river constituent concentration. For no variability the curve is straight. For variability with a period much less than the flushing time, the average curve for a general data set straight, whereas the curve for a synoptic data set is bent. For variability with a period greater than the flushing time a bent curve results. Since bent mixing curves can occur for conservative properties, the use of these curves for interpretation of estuarine processes must be undertaken with an understanding of the temporal variability of the river and ocean constituents and their relationship to the estuary mixing properties and flushing time.

The dynamics of conservative mixing in estuaries

A seasonal study of sand movement and the benthic intertidal organisms at Bound Rock, New Hampshire, USA was conducted between November, 1973 and February, 1975. The site is subjected to irregular sand fluctuations, as well as diurnal, neapspring and major summer sand intrusions. The abundance and distribution of intertidal species was interrelated with the historical sand fluctuations at the area. For example, the lower limits of Mytilus edulis, Balanus balanoides and Porphyra umbilicalis approximated the highest summer sand elevations. Highly abraded rock surfaces in the lower intertidal zone were dominated by opportunistic annuals (e.g. Enteromorpha spp.) and perennial psammophytic or sand-loving seaweeds (e.g. Ahnfeltia plicata and Sphacelaria radicans). Overall, the intertidal seaweed populations at Bound Rock showed a lower number of perennials and fewer species than adjacent rocky shores. The low species diversity of seaweeds at the study site is attributable to unstable environmental conditions and a limited number of habitats. Even so, several psammophytic microhabitats are evident at Bound Rock depending upon the extent of sand burial and abrasion. The morphological and reproductive adaptations of several psammophytic species are discussed.

The effects of sand movement on intertidal seaweeds and selected invertebrates at Bound Rock, New Hampshire, USA

The water exchange between a small (4.1 hectare) salt marsh adjoining the Great Bay Estuary system of New Hampshire, USA was sampled during 16 tidal cycles between July, 1976 and November, 1977. Tidal amplitude, temperature, salinity, nutrient concentrations (ammonia-N, nitrate-N, nitrite-N, orthophosphate-P, total-P, silicates) and suspended particulates were measured. Conspicuous tidal hydrographic patterns were observed. Mean concentrations of nitrate-N and silicates varied with season. The tidal information, combined with volume determinations, was extrapolated to determine the net flux of hydrographic parameters on monthly and yearly bases. Ammonia-N showed a pronounced seasonality of net exchange by regression analyses. Ammonia-N and suspended particulates had a statistically significant import into the marsh. However, the net fluxes of the other materials were not statistically different from O.

Nutrient fluxes within a small north temperate salt marsh

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Maureen A. Daly

Papers

The effects of sand movement on intertidal seaweeds and selected invertebrates at Bound Rock, New Hampshire, USA

Nutrient fluxes within a small north temperate salt marsh

Temperature, Salinity, Turbidity, and Light Attenuation in the Great Bay Estuary System 1974-1978

Hydrographic variation in eight tidal tributaries associated with the Great Bay Estuary System