红树林生长带与潮汐水位关系的研究

根据海南东寨港林市村,廉江车扳镇那肋村,深圳锦锈中华等３个实验点１０条横断面红树林潮滩高程水准测量,林市村潮汐水位观测,潮汐特征水位及浸淹频率分布计算,参照有关港点潮汐实测或预测报资料,确定红树林外边界为平均海面或稍上,内边界（或林内最大高程）为回归潮平均高高潮位（或大潮平均高潮位）对国内外有关报道进行了评述,指出国内许多红树林文献中使用的“高潮位”,“中潮位”“低潮位”用潮汐学术语表述应为“回归     

Variation in numbers and behaviour of waders during the tidal cycle: implications for the use of estuarine sediment flats

Estuarine sediment flats are essential feeding areas for waders, but their exploitation is constrained by the movements of tides. In this cyclic environment the exposure period of sediment flats decreases several fold from upper to lower flats, and the moving tidal waterline briefly creates particular conditions for waders and their prey. This study attempts to determine how the exposure period and the movement of the tide line influence the use of space and food resources by waders across the sediment flats. Wader counts and observations of feeding behaviour were carried out in all phases of the tidal cycle, in plots forming a transect from upper to lower flats, thus representing a gradient of exposure periods. Pecking, prey intake, and success rates varied little along the gradient. Some species actively followed the tide line while foraging, whereas others are evenly spread over the exposed flats. Black-tailed Godwit, Dunlin and Avocet were 'tide followers', whereas Grey Plover, Redshank and Bar-tailed Godwit were 'non-followers'. Densities of 'followers' near the tide line were up to five times higher than elsewhere. Species differed markedly in the way they used space on the flats, but in general the rate of biomass acquisition (in grams of ash-free dry weight per time exposed) was much higher in lower flats. However, this preference was insufficient to counter the much longer exposure of the upper flats, so the total amount of biomass consumed on the latter was greater. Therefore, it was in these upper flats that waders fulfilled most of their energetic needs. Consequently, upper flats are of particular importance for the conservation of wader assemblages, but because they are usually closer to shore they tend to suffer the highest pressure from disturbance and land reclamation.     

When to worry about the weather: role of tidal cycle in determining patterns of risk in intertidal ecosystems

Species range boundaries are determined by a variety of factors of which climate is one of the most influential. As a result, climate change is expected to have a profound effect on organisms and ecosystems. However, the impacts of weather and climate are frequently modified by multiple nonclimatic factors. Therefore, the role of these nonclimatic factors needs to be examined in order to understand and predict future change. Marine intertidal ecosystems are exposed to heat extremes during warm, sunny, midday low tides. Thus, the timing of low tide, a nonclimatic factor, determines the potential contact intertidal invertebrates and algae have with heat extremes. We developed a method that quantifies the daily risk of high temperature extremes in the marine intertidal using solar elevations and spatially continuous tidal predictions. The frequency of 'risky days' is variable over time and space along the Pacific Coast of North America. Results show that at some sites the percentage of risky days in June can vary by 30% across years. In order to do a detailed analysis, we selected San Francisco as a study site. In San Francisco, May is the month with the greatest frequency of risky days, even though September is the month with the greatest frequency of high air temperature, ≥30 °C. These results indicate that marine intertidal organisms can be protected from high temperature extremes due to the timing of tides and local weather patterns. In addition, annual fluctuations in tides influence the frequency of intertidal zone exposures to high temperature extremes. Peaks in risk for heat extremes in the intertidal zone occur every 18 years, the length of the tidal epoch. These results suggest that nonclimatic variables can complicate predictions of shifts in species ranges due to climate change, but that mechanistic approaches can be used to produce predictions that include these factors.     

Characterizing biogeochemical fluctuations in a world of extremes: A synthesis for temperate intertidal habitats in the face of global change

Coastal and intertidal habitats are at the forefront of anthropogenic influence and environmental change. The species occupying these habitats are adapted to a world of extremes, which may render them robust to the changing climate or more vulnerable if they are at their physiological limits. We characterized the diurnal, seasonal and interannual patterns of flux in biogeochemistry across an intertidal gradient on a temperate sandstone platform in eastern Australia over 6 years (2009–2015) and present a synthesis of our current understanding of this habitat in context with global change. We used rock pools as natural mesocosms to determine biogeochemistry dynamics and patterns of eco‐stress experienced by resident biota. In situ measurements and discrete water samples were collected night and day during neap low tide events to capture diurnal biogeochemistry cycles. Calculation of pH_T using total alkalinity (TA) and dissolved inorganic carbon (DIC) revealed that the mid‐intertidal habitat exhibited the greatest flux over the years (pH_T 7.52–8.87), and over a single tidal cycle (1.11 pH_T units), while the low‐intertidal (pH_T 7.82–8.30) and subtidal (pH_T 7.87–8.30) were less variable. Temperature flux was also greatest in the mid‐intertidal (8.0–34.5°C) and over a single tidal event (14°C range), as typical of temperate rocky shores. Mean TA and DIC increased at night and decreased during the day, with the most extreme conditions measured in the mid‐intertidal owing to prolonged emersion periods. Temporal sampling revealed that net ecosystem calcification and production were highest during the day and lowest at night, particularly in the mid‐intertidal. Characterization of biogeochemical fluctuations in a world of extremes demonstrates the variable conditions that intertidal biota routinely experience and highlight potential microhabitat‐specific vulnerabilities and climate change refugia.     

A refutation of critical tidal levels as determinants of the structure of intertidal communities on British shores

Early hypotheses to account for the pattern of zonation and vertical distribution of species on rocky shores in Britain invoked the concept of critical tidal levels. These levels were at heights on the shore where the upper or lower boundaries of distribution of a number of species coincided. Critical levels were correlated with heights on the shore where there were changes in the rate of change, with height, of the annual proportion of the time spent emersed. This was calculated from predicted tide tables. One implications of this was that different assemblages of intertidal species were present between particular levels on the shore. Futhermore, this hypothesis depended on patterns of vertical distribution of several species being identically controlled by physical factors associated with the rise and fall of the tide. This conflicts with more recent hypotheses, based upon experimental evidence, which include the effects of biological interactions among species on their patterns of distribution.The methods of calculation of the annual emersion curve were approximate, with a high degree of extrapolation. The hypothesis that critical levels exist has not been quantitatively tested.New, more accurate calculations of the emersion curve, from predicted tidal heights, indicate a smooth, monotonic curve against height on the shore. This eliminates the possibility of correlation between the coincident boundaries of distribution of a group of species and a height at which the curve changes in slope.The upper and lower boundaries of species were recorded in transects on five shores in different parts of Britain. If critical level exist, the upper or lower boundaries of a set of species must depart from random dispersion up and down the shore, to be clumped or aggregated at the critical levels. A test for non-random dispersion of the boundaries indicated no significant departure from random on any of the shores sampled.There is thus no evidence that critical tidal levels exist, because there is no evidence that the upper, or lower, boundaries of vertical distribution of intertidal species are in any way aggregated. There is no possibility of correlation between changes of slope of the emersion curve at particular heights on the shore and the patterns of distribution of intertidal species on British shores. The hypothesis of critical tidal levels must be abandoned and the rôle of the effects of tidal rise and fall on the vertical distribution of intertidal species must be re-examined in any new hypotheses to account for observed patterns of zonation.     

Factors limiting the intertidal distribution of the mangrove species<Emphasis Type="Italic"> Xylocarpus granatum</Emphasis>

The tree species Xylocarpus granatum is commonly described as occurring in the upper intertidal zone of mangrove forests, but mature trees are occasionally found at lower elevations. In the Utwe River basin, on the Pacific island of Kosrae, we investigated the relative importance of several biotic and abiotic factors that may control the intertidal distribution of X. granatum. Factors we evaluated included differential seed predation across the lower, mid, and upper intertidal zones and seedling responses to salinity, tidal flooding, and shade. Seed predation was 22.4% over the first 34 days and varied little among zones or in gaps versus under the forest canopy. By day 161, there were still no differences in seed mortality, but a significant difference was found in seedling establishment, with much greater establishment in the upper intertidal plots. X. granatum seedlings in a greenhouse experiment exhibited greater growth in freshwater than seedlings in 23 ppt salinity, which is typical of salinity levels found in the mid intertidal zone in our field study sites in Micronesia, where mature X. granatum trees are generally absent. Seedlings grown in 23 ppt salinity, however, exhibited few visible signs of stress associated with patterns in growth. Seedlings grown in a simulated tidal flooding treatment (with 23 ppt salinity) also showed few signs of stress. Growth declined dramatically under 80% shade cloths, but there were few interactions of shading with either 23 ppt salinity or simulated tidal flooding. Differential seed predation is not likely to be the primary factor responsible for the intertidal distribution of X. granatum on Kosrae. However, seedling tolerance of flooding or salinity may be more important, especially relative to a potential contribution to secondary stress mortality. Other factors may ultimately prove to be more critical, such as physiological effects of salinity on seed germination, effects of tides on seed dispersal and rooting, or differential herbivory on seedlings.     

Capacity of a temperate intertidal seagrass species to tolerate changing environmental conditions: Significance of light and tidal exposure

Seagrass meadows perform several important ecological roles in coastal areas. However, with multiple stressors threatening these aquatic plants, their current rate of decline is likely to increase, so understanding their ability to acclimate to a range of environments may be key to their survival. Light availability is critical for seagrass health, with loss of water clarity likely to cause decline. This study assessed the ability of the temperate intertidal species, Zostera muelleri, to compensate for variations in light in its natural environment. The impacts of light during inundation (high tide) and exposure (low tide) were examined along a vertical gradient from the high to low intertidal at two sites with differing light histories. Photo-acclimation was evaluated through morphological and physiological characteristics over two tidal cycles. Results were consistent with seagrasses having optimized their photosynthetic capacity, with physiological acclimations being site specific. Longer-term morphological changes were also noted, suggesting a dissimilar light history between sites for an extensive period. Interestingly, at the site with the greatest light range, but the lowest light penetration at depth, a significant reduction in photosynthetic activity during air exposure was observed. This suggests that Z. muelleri is capable of tolerating light stress through photo-acclimation, allowing for the more efficient harvesting of light at low levels. Overall, this study demonstrates that Z. muelleri has the ability to adjust both physiologically and morphologically to changing environmental conditions, a key aspect to survival and persistence in temperate intertidal zones.     

POTOS: a portable topographic system for measuring inaccessible muddy creek areas

Tidal flats and marshes in intertidal environments and estuaries generally cover large areas. The access to these places is usually complicated. The area suffers cycles of flooding and drying under the influence of the tide, which in conjunction with soft and muddy soils makes every movement very difficult. In particular, in tidal channels mud is softer and, many times, it is impossible to move around these areas. Furthermore, muddy tidal flat creeks are often located far away from conventional vehicle access. Then, to perform a topographic study using traditional methodology is not feasible; therefore, an alternative methodology has been developed to deal with these problems. POTOS is a light metallic frame developed to measure creek topography indirectly with good precision. The tensed frame, up to 12 m long, simply supported and weighting only 40 kg allows making relative height measurements of the surface between the supporting posts. The field methodology is described and some results obtained with the instrument developed are presented. Errors were calculated by comparing measurements obtained with the instrument and those gathered by traditional methodologies. Field results are a series of measurements obtained in Bahía Blanca Estuary tidal flats and marshes at different periods of the year.     

Environmental correlates of phenotypic variation: do variable tidal regimes influence morphology in intertidal seaweeds?

Seaweed morphology is often shaped by the hydrodynamic environment. However, exposure to air at low tide represents an additional factor potentially affecting the morphology of intertidal species. Here, we examined the relationships between the morphology of Hormosira banksii, an important intertidal habitat‐forming seaweed in southern Australia, and environmental factors across multiple spatial scales around the island of Tasmania, Australia. Tasmania is surrounded by a diverse coastline with differences in wave exposure, tidal parameters, and temperature. We sampled Hormosira from four regions (100s km apart), three sites (10s km apart) within each region, and two zones (meters apart; eulittoral and sublittoral) at each site, and measured multiple morphological variables to test for differences in morphology at those different spatial scales. Thirteen environmental variables reflecting wave exposure, tidal conditions, and temperature for each site were generated to assess the relationship between Hormosira morphology and environmental variation. Morphology varied at all spatial scales examined. Most notably, north coast individuals had a distinct morphology, generally having smaller vesicles and shorter fronds, compared to other regions. Tidal conditions were the main environmental factors separating north coast sites from other sites and tidal regime was identified as the best predictor of morphological differences between regions. In contrast to other studies, we found little evidence that wave exposure was associated with morphological variation. Overall, our study emphasizes the role of tidal conditions, associated with emersion stress during low tide, in affecting the morphology of intertidal seaweeds.     

Tower construction by the manicure crab <Emphasis Type="Italic">Cleistostoma dilatatum</Emphasis> during dry periods on an intertidal mudflat

Animals living on upper intertidal mudflats experience habitat desiccation during neap tides when water does not flood the habitat. Individuals of the manicure crab Cleistostoma dilatatum construct cone-shaped towers at the entrance of their burrows, in which they remain during neap tides. These towers are the tallest known structures compared to body size built by crabs living on intertidal flats. The frequency of tower construction followed semilunar tidal cycles with most building done prior to neap tides when few crabs were active on the mudflat surface. Bigger crabs tended to make taller and wider towers with a wider pinhole on the top. These towers may regulate the microclimate in burrows.     

Recruitment of shore crabsCarcinus maenas on tidal flats: Mussel clumps as an important refuge for juveniles

During the late summer and early fall, juvenile shore crabs (Carcinus maenas L.) occurred in high abundances in mussel clumps scattered on tidal flats of the Wadden Sea. Abundances were much lower on bare tidal flats without mussel clumps and decreased substantially from July to November, whereas numbers in mussel clumps remained high. Large crabs left the tidal flats in early fall, whereas juveniles undertook tidal migrations only in the late fall. In March very few shore crabs were found in the intertidal area. The size of juvenile shore crabs living between mussels did not increase significantly during fall. On the bare tidal flats surrounding the mussels, a size increase was observed. Mussel beds and mussel clumps serve as a spatial refuge for the early benthic phases of juvenile shore crabs. Between mussels they can hide effectively from their epibenthic predators. Juvenile shore crabs do not leave the intertidal area and the mussel habitats before their major predators have left the area. Mussel clumps scattered over the tidal flats may be a critical refuge for juvenile shore crabs settling on tidal flats. Intensified efforts in mussel culturing in the European Wadden Sea during recent decades may have caused an increased abundance of mussel clumps on tidal flats, thus enhancing habitat availability for some mussel-clump inhabitants.     

Holocene reef evolution in a macrotidal setting: Buccaneer Archipelago,Kimberley Bioregion,Northwest Australia

This study uses information derived from cores to describe the Holocene accretion history of coral reefs in the macrotidal (up to 11 m tidal range) Buccaneer Archipelago of the southern Kimberley coast, Western Australia. The internal architecture of all cored reefs is broadly similar, constituting well-preserved detrital coral fragments, predominantly branching Acropora, in a poorly sorted sandy mud matrix. However, once the reefs reach sea level, they diverge into two types: low intertidal reefs that maintain their detrital character and develop relatively narrow, horizontal or gently sloping reef flats at approximately mean low water spring, and high intertidal reefs that develop broad coralline algal-dominated reef flats at elevations between mean low water neap and mean high water neap. The high intertidal reefs develop where strong, ebb-dominated, tidal asymmetry retains seawater over the low tide and allows continued accretion. Both reef types are ultimately constrained by sea level but differ in elevation by 3–4 m.     

High-resolution analysis of metabolic cycles in the intertidal mussel Mytilus californianus

Inhabitants of the marine rocky intertidal live in an environment that alternates between aquatic and terrestrial due to the rise and fall of the tide. The tide creates a cyclical availability of oxygen with animals having access to oxygenated water during episodes of submergence, while access to oxygen is restricted during aerial emergence. Here we performed liquid chromatography and gas chromatography-mass spectrometry enabled metabolomic profiling of gill samples isolated from the California ribbed mussel, Mytilus californianus, to investigate how metabolism is orchestrated in this variable environment. We created a simulated intertidal environment in which mussels were acclimated to alternating high and low tides of 6 h duration, and samples were taken every 2 h for 72 h to capture reproducible changes in metabolite levels over six high and six low tides. We quantified 169 named metabolites of which 24 metabolites cycled significantly with a 12-h period that was linked to the tidal cycle. These data confirmed the presence of alternating phases of fermentation and aerobic metabolism and highlight a role for carnitine-conjugated metabolites during the anaerobic phase of this cycle. Mussels at low tide accumulated eight carnitine-conjugated metabolites, arising from the degradation of fatty acids, branched-chain amino acids, and mitochondrial β-oxidation end products. The data also implicate sphingosine as a potential signaling molecule during aerial emergence. These findings identify new levels of metabolic control whose role in intertidal adaptation remains to be elucidated.     

Untersuchungen über die Auswirkung kommunaler Abwässer auf das benthische Ökosystem mariner Watten

The influence of domestic sewage on intertidal sand and mud flat benthic communities has been studied at three minor outfalls around the North Sea island of Sylt (F. R. Germany) and at the main outfall of Vancouver, B. C. (Canada). During high tide, domestic waste water was found at the water surface due to its lower specific gravity. Consequently, O₂ depletion and salinity decrease occured mainly in surface waters. Oxygen deficiency prevailed in the sediment-water layer, when settled sewage particles were re-suspended by strong tidal currents. This resulted in anaerobic conditions at the sediment surface for up to 40% of the day. Substantial pH decrease and increase in seston load of the water were observed only along the sewage channels and around the outlets when large quantities of sewage were discharged. The amount of particulate organic material increased up to 10% and more in muddy sediments due to sedimentation of sewage sludge. Considerable eutrophication of the mud flats resulted from nutrient load of the sewage and from decomposition of faecal material deposited on the sediments. The vertical distribution of microalgae in the sediment was not influenced by sewage. Primary productivity of microalgae on the sediment surface was positively correlated with the degree of pollution. In heavily and mediumly polluted zones, heterotrophic bacterial production exceeded primary production rates at the sediment surface. Compared with unpolluted intertidal flats the number of macrobenthic species was not reduced in polluted mud flats. In regard to overall species abundance, the macrofauna associations of the disposal area showed three distinct zones: (1) Heavy pollution around the outfall resulted in a degradation zone either without benthic invertebrates or with a very low density of individuals; (2) medium polluted areas were characterized by a zone of maximum density; (3) with decreasing or diminishing pollution the number of individuals returned to normal values (background values). The distribution of species within these zones indicated a significant difference between the response of the channel fauna and the flat fauna to pollution: while the number of species increased continously inside the channels with decreasing pollution, the species variety of the tidal flats was positively correlated with the number of individuals. The diversity of the benthos inside the channels was negatively correlated with the degree of pollution, whereas the diversity indices for the fauna on the tidal flats increased quickly from the centre of pollution, remained at a high level in the medium polluted zone, and decreased again to the area of diminishing pollution. The number of dominating species in the sewage channel increased from the outlet towards the low-tide level, with ratios of 0.9 to 0.33; on the flats the number remained constant throughout the three zones, with maximum ratios of 0.5, demonstrating that no qualitative changes for the macrobenthos of intertidal flats result from domestic sewage disposals. In polluted intertidal flats the macrobenthos is dominated by muddy-sand species whose adaptability to varying sedimentary factors, such as grain size distribution and content of organic material (coefficients of variability: 50.7 and 45.7%, respectively), is relatively high compared with mud and sand species. A special indicator community for polluted intertidal areas does not exist. Intensified waste-water treatment prior to discharge reduced the size of the degradation zone around the outfalls while it simultaneously caused a lower density of the macrobenthos in the medium polluted zone.

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Dissertation aus dem Fachbereich Biologie der Universität Hamburg.     

In situ study of short-term variations of redox species chemistry in intertidal permeable sediments of the Arcachon lagoon

We investigated the composition of porewaters in intertidal sediments in response to the diurnal rise and fall of tides. For this reason, we deployed an in situ voltammetric system to measure vertical distribution and time-series at defined depths of O₂, Mn(II), Fe(II), and S(?II) in the porewater of permeable sediments from a protected beach in the Arcachon Bay. We also report microprofiles of O₂ and pH together with sediment properties (organic carbon, particulate reactive manganese and iron, porosity and permeability). Results shows that the oxygen dynamics in the upper sediment at low tide appeared to be mainly controlled by microphytobenthos activity, which may migrate downward just before immersion. The tidal forcing seemed to influence the oxygen dynamic in a minor way through flushing of the uppermost sediment porewater layer at the beginning and end of immersion. Vertical profiles and time-series measurements showed that the distributions of reduced species varied with tides. Although this work reveals that the upper sediment layer was subject to redox changes, the response of vertical distributions of redox species to tidal and night?Cday cycles did not have a cyclic pattern.     

UK intertidal foraminiferal distributions: implications for sea-level studies

Foraminiferal assemblages have been collected from ten intertidal study areas situated on the east, south and west coasts of the UK. The assemblages display a vertical zonation which indicates that the distribution of foraminifera in these intertidal environments is usually the direct function of altitude with the duration and frequency of intertidal exposure the most important factors. Multivariate analyses separate foraminiferal assemblages into two faunal zones: a high- and middle marsh zone consisting of differing abundances of Jadammina macrescens, Trochammina inflata and Miliammina fusca; and a low-marsh and tidal flat zone dominated by calcareous foraminiferal species, notably Elphidium williamsoni, Haynesina germanica and Quinqueloculina spp. These faunal zones are similar to those in other mid-latitude, cool temperate intertidal environments although there are spatial and temporal variations between areas. The altitudinal ranges of the faunal zones are employed to identify the vertical relationship of the local environment in which the assemblage accumulated to a reference tide level.     

Heat-shock response and antioxidant defense during air exposure in Patagonian shallow-water limpets from different climatic habitats

Climate warming involves not only a rise of air temperature means, but also more frequent heat waves in many regions on earth, and is predicted to intensify physiological stress especially in extremely changeable habitats like the intertidal. We investigated the heat-shock response (HSR) and enzymatic antioxidant defense levels of Patagonian shallow-water limpets, adapted to distinct tidal exposure conditions in the sub- and intertidal. Limpets were sampled in the temperate Northern Patagonia and the subpolar Magellan region. Expression levels of two Hsp70 genes and activities of the antioxidants superoxide dismutase (SOD) and catalase (CAT) were measured in submerged and 2- and 12-h air-exposed specimens. Air-exposed Patagonian limpets showed a tiered HSR increasing from South to North on the latitudinal gradient and from high to low shore levels on a tidal gradient. SOD activities in the Magellan region correlated with the tidal rhythm and were higher after 2 and 12 h when the tide was low at the experimental site compared to the 6 h value taken at high tide. This pattern was observed in intertidal and subtidal specimens, although subtidal individuals are little affected by tides. Our study shows that long-term thermal adaptation shapes the HSR in limpets, while the oxidative stress response is linked to the tidal rhythm. Close to the warm border of their distribution range, energy expenses to cope with stress might become overwhelming and represent one cause why the limpets are unable to colonize the shallow intertidal zone.     

Fortnightly light and temperature variability in estuarine intertidal sediments and implications for microphytobenthos primary productivity

Photosynthetically active radiation (PAR) and temperature were measured continuously at the surface of estuarine intertidal sediments in the Tagus estuary, Portugal, along two spring-neap tidal cycles. PAR and temperature were strongly conditioned by the periodic tidal inundation, with large and abrupt variations occurring during flooding and ebbing. PAR levels reaching the sediment surface decreased very rapidly to zero or very low values during most of the daytime immersion. Inundation during high tide had the general effect of attenuating the amplitude of daily temperature fluctuation, with the incoming water usually warmer than the sediment during the night or early morning and cooler during the day. The daily progression of tidal emersion resulted in a clear fortnightly variation in total daily PAR reaching the sediment surface, while both daily mean temperature and mean temperature of diurnal low tide periods failed to exhibit a well-defined fortnightly periodicity. The obtained results indicate that the estuarine intertidal environment is dominated, at sub-seasonal time scales, by fortnightly periodicity in irradiance and temperature conditions favourable for benthic photosynthesis.     

An optimal foraging and migration model for juvenile plaice

Summary A state-dependent model has been used to predict daily and tidal patterns of migration, feeding and inactivity in juvenile plaice (Pleuronectes platessa L.) in their intertidal and shallow subtidal nursery areas. Vertical position, quantity of energy reserves and fullness of the gut characterized the state of individual fishes. If feeding is visually mediated, the model predicts that, by night, plaice should move to areas of low predation risk and become inactive, whereas by day, plaice should migrate to feed in areas of high prey encounter rate. Vertical zones adopted by day and night and, hence, patterns of migration, should depend on the distributions of predators and prey. When prey are more abundant in the intertidal zone, plaice should move onshore to feed as the tide rises and when prey are more abundant offshore, plaice should move offshore to feed by day. If predators are equally abundant in all zones, the fish should behave as if no predators were present, having no effective refuge. An increase in the abundance of predators with depth results in the restriction of plaice activities to shallower vertical zones, depending on the magnitude of the predation threat. Zones adopted thus depend on the trade-off between energy intake and predation risk. Concordance between predicted behaviour and observed patterns is evident in contrasting habitats. Migration and feeding in the Wadden Sea, where prey are more abundant on intertidal flats, is dominated by the tidal component, whereas on impoverished exposed beaches of the west coast of Scotland, the diurnal component is dominant. Tidally related behaviour persists in the latter environment, not predicted by the model and may be a consequence of using endogenous rhythms to approach optimal behavioural patterns.