Heavy Wave Action Research Articles

The relative impacts of grazing by caribbean coral reef fishes and Diadema: effects of habitat and surge

The effects of grazing on fleshy algal turfs by large herbivorous fishes and by the black spiny urchin, Diadema antillarum (Philippi), were investigated in three shallow (<3 m), moderately exposed, reef flat habitats in the San Blas Islands on the Caribbean coast of Panama. Grazing by Diadema was found to be a primary determinant of the biomass of fleshy algae in this shallow reef environment, and its impact was exposure-dependent. Feeding activity of Diadema was inhibited during the protracted periods of heavy wave action that characterize the dry season when strong north winds (24–27 kph) are typical. In the shallowest, most exposed elkhorn bluff habitats urchin grazing was minimal even during calm periods due to the effects of waves generated by unpredictable, light winds. In a relatively protected backreef habitat, water movement was inadequate to discourage grazing significantly, even during the turbulent dry season. As a result, algal biomass (decalcified dry weight) was typically three-fold greater in the elkhorn bluff habitat than on the ackreef. In habitats of intermediate exposure (elkhorn fields) algal biomass was reduced during calm periods, but increased to the level characteristic of the elkhorn bluff habitat during protracted rough periods that inhibited grazing by Diadema. A caging experiment in the backreef habitat demonstrated a relatively minor effect of grazing by herbivorous fishes in comparison to that of grazing by Diadema. Observations on the distributions and abundances of herbivorous fishes also indicated a relatively minor role for these grazers as determinants of algal biomass in the more exposed habitats.

Seasonality in some British intertidal Acari

The life cycles of halacarid and non-halacarid littoral Acari are examined, and the results compared with those of other workers. All species can carry out one life cycle in approximately 12 months, with the exception of the genus Isobactrus (Halacaridae) which may have two or more generations per annum. Species exposed to heavy wave action have mechanisms to reduce juvenile mortality, including larval retention, and/or larviparity, extended periods of oviposition to optimize losses as a result of wave action, and niche protection of juveniles. However, these adaptations are preadaptations, because similar mechanisms exist among related species in less stressful environments. The sex ratios of some species follow an annual cycle with females dominant before and during oviposition, suggesting that they live longer than males and die after oviposition. Sex ratios among small groups of Hydrogamasus salinus (Laboulbene) (Cyrtolaelapidae: Mesostigmata) are variable, though these are relatively constant in larg...

The maintenance of shore-level size gradients in an intertidal snail (Littorina sitkana).

The size of many intertidal animals varies with tidal height. These size gradients could be produced by growth or survival varying with tidal height, or by animals moving to a preferred tidal level. The body size of the snail, Littorina sitkana, increases steadily with tidal height in rocky high intertidal habitats of British Columbia. To determine how size gradients were maintained in L. sitkana, I quantified how growth, survival, and snail movement varied with tidal height. I studied populations of L. sitkana found on sheltered pebble beach and exposed basaltic shelf habitats. Mark-recapture studies and experimental transplants showed that growth could not have produced the size gradients because snail growth rates in both habitats were as fast or faster at low tidal levels (where the snails were the smallest) than at high tidal levels. However, survival rates were lowest at low tidal levels. On pebble beaches, this was due to size selective predation on large snails by the pile perch, Rhacochilus vacca. On basaltic shelves, heavy wave action at low tidal levels may have caused the poor survival rates. Transplanted snails moved homeward on pebble beaches, but not on basaltic shelves. Reduced survival rates at low tidal levels cause size gradients in both habitats, and snail movement helps to maintain size gradients on pebble beaches.

The establishment and maintenance of vertical size gradients in populations of Littorina africana knysnaensis (Philippi) on an exposed rocky shore

Littorina africana knysnaensis (Philippi) exhibits a size gradient of increasing shell length up the shore on the Cape Peninsula caused by separate size dependent effects on juveniles and adults. There is no apparent correlation between mean shell length and density, and animals of separate size classes which are transferred to zones where they do not normally occur rapidly migrate to their original zones. Juvenile settlement occurs almost exclusively on the upper shore to avoid the heavy wave action of the lower Littorina zone. If tethered in the lower zones juveniles show a loss of body weight or may even be torn free of their tethers and lost. Juveniles do, however, occur in the lower zones where shelter from the effects of wave action is available in the form of narrow crevices. Adult animals have a much greater tenacity and when tethered, they develop a lower length/body weight ratio in the lower Littorina zone than in the upper where microphytic productivity decreases considerably. The greater tenacity of adults allows a gradual downshore migration of growing animals in response to gradients of food availability. The resulting size gradient of L.a. knysnaensis is the reverse of that predicted for such a supralittoral fringe species.

Upwelling in a nearshore marine ecosystem and its biological implications

The flow of water into and out of a kelp-bed on the west coast of southern Africa has been monitored over a 24-day period, spanning three upwelling cycles. Currents, measured by SCUBA divers at five stations around the study site reached 40 cm s−1 at the surface, but were slower and more variable in mid-water (4–11 m) and just above the bottom (8–22 m). Surface currents correlated with wind, but not tide, offshore winds causing offshore surface drift and onshore currents below, while onshore winds reversed these flow patterns. Water transport budgets indicate that the volume of water at the study site (3·75×106m3) turned over between three and seven times per day during upwelling. Nutrients (NO3, P and Si) peaked during active upwelling, when temperatures fell below 12 °C, and were minimal under downwelling conditions, NO3 failling to 1 μg at l−1. Phytoplankton density was lowest during upwelling and peaked during onshore winds. The concentration of suspended particulate organic matter varied between 0·7 and 5·5 mg l−1 dry mass, the highest values occurring during heavy wave action. The number of suspended bacteria and ranged from 0·7×105 to 36·2×105 cells ml−1, with peaks lagging 1 day behind those of POM. Biological implications of these measurements are discussed.

Distribution, substratum preference and burrowing behaviour of Lovenia elongata (Gray) (Echinoidea: Spatangoida) in the Gulf of Elat ('Aqaba), Red Sea

Lovenia elongata (Gray), the Indo-West Pacific spatangoid, was studied in the Gulf of Elat ('Aqaba) at the northern end of the Red Sea. The occurrence and local distribution of this macrobenthic burrowing species was determined and related to the particle size distribution of the substrata in which it was found. Its habitat preference is for clean, grassless, sandy bottoms in intertidal areas (beaches and sandbars), and on sublittoral level bottoms of protected bays and lagoons sheltered from heavy wave action. The preferred sediments were found to range from medium to fine sand. The population density, size frequency distribution, growth, reproduction, recruitment, and mortality of a sublittoral population of L. elongata (at Wadi Taba, Sinai) were studied. The mode of life of this burrowing heart urchin was observed in situ and in aquaria. Its burrowing and emergence behaviour are described. Experiments were carried out on the effect of particle size on burrowing behaviour. There are significant differences in the ability of L. elongata to manipulate substrata of varying grain size. The burrowing process was found to be most rapid in natural sand of medium-fine composition, corresponding to the normal sediment of the urchin's habitat. Such sediments were also found to be suitable for successful larval settlement; juveniles did not survive in coarser sand. Differences in population density and spatial distribution between various size groups are attributed to differences in their ability to manipulate the substratum. L. elongata is found to be morphologically suited to sand, particularly medium-fine sand; however, it is restricted by its morphology to shallow burrowing, since it lacks specialized mechanisms for deeper burrowing and funnel building.

Adaptive design of locomotion and foot form in prosobranch gastropods

A comparison of the locomotor types, speed, tenacity, and foot form of nearly 300 species in 52 families of marine prosobranchs has revealed that foot size and shape and even subtle variations of locomotion affect the speed and strength of adhesion to the substratum. Gastropods inhabiting soft substrata move primarily by pedal cilia or by discontinuous locomotion in which shell and foot move alternately. Both types of movement are accompanied by low tenacity. A specialized type of discontinuous locomotion, namely, leaping, surpasses all other methods of movement in speed. Species with ciliary locomotion have a very large foot while those with discontinuous movement have an exceedingly small foot relative to shell size. The majority of prosobranchs inhabit hard substrata, move by continuous pedal muscular gliding, and have moderately high tenacity during movement. Arhythmic pedal locomotion yields lower maximum speeds and tenacities than do rhythmic pedal waves. Foot size and shape relative to shell length in species with arhythmic locomotion vary from very short and broad to long and narrow. Studies of transects at several temperate and tropical marine littoral stations showed that these species are confined to low littoral or sublittoral habitats that are sheltered from heavy wave action. High speed and tenacity are simultaneously attained only by species with rhythmic pedal waves. Speed and tenacity do not represent competing selective pressures on the size and shape of the foot. Speed increases among species as the foot approaches or exceeds shell length and is highest if the foot is also broad; the greatest tenacities are attained by species with a long, broad foot whose dimensions do not exceed that of the shell. The optimal shape for both high tenacity and speed is a broad foot somewhat shorter than the shell; neither speed nor tenacity are much compromised by this form. In general, only species with rhythmic pedal waves whose foot size and shape approximate the optimal form for high tenacity and speed are found in habitats exposed to much wave action. Long rhythmic waves, moving a large proportion of foot area at once, are in theory energetically more economical than small, very rapid waves resulting in the same overall speed, but experiments showed that tenacity is significantly reduced in gastropods which increase speed by enlarging the waves. The optimal wave pattern of a species should be a balance between the demand for speed with the least expenditure of energy, favored by a pattern of many large waves at once, and the demand for tenacity, favored by a pattern of few and small waves. Retrograde ditaxic waves of elongation are the most common pattern encountered among prosobranchs, and are associated with a large range of foot sizes and shapes. Such waves are at least one third as long as the foot, while direct waves and other waves of compression are frequently much smaller. The range of foot forms of species with waves of compression is restricted, tending to be optimal for high tenacity or to be long and narrow. Waves of compression appear to be a specialization with the potential for maintaining high tenacity even at high speeds since the waves can be very small, and for giving superior speed since they can travel very rapidly.

Benthic studies in the south east of Liverpool Bay

The distribution of benthic animals was studied in shallow water in the south east corner of Liverpool Bay into which a new coastal sewage outfall will discharge. Periodic samples were collected from 35 grab stations and at eight of these a beam trawl and epibenthic sledge were also used. A numerical method of analysis proved the most appropriate in order to classify the fauna into various habitats. The group-average sorting procedure was used to produce a dendrogram after a percentage similarity matrix and cluster diagram was made up for each survey. Rarefaction curves complemented the results but in general diversity indices suffered due to the low numbers of animals present over the area. Two primary habitats were defined, one being typical of muddy sand and the other of clean sand. The former was characterized by an alternation of species which was brought about by the re-working feeding mechanisms of deposit feeders reducing the stability of the sediment and thus leading to washing out during storms. Replacement of successive dominant species was by larvae present in the plankton at the time. The sand habitat supported a sparse fauna which varied in both its species composition and density with the amount of silt present in the sediment. This varied with the weather, accretion of fines taking place during calm periods and winnowing out occuring during heavy wave action. Many of the species found during the surveys were common to both habitats but the ratio of their numbers was the distinguishing factor. In the transition zone between the two habitats were several intermediate groups of stations in which there were several co-dominants and the sediment contained an increasing amount of fines. There were also three aberrant groups. In the epifaunal community major changes also occurred with time. The dramatic increase in the number of shrimps during 1971 was thought to have been related to the changes in the silt content of the sand banks whilst the fluctuations in the small crustacean populations may have been due to the shoaling habit of some species.

Spatial and Seasonal Distributions of Marine Algal Communities at Amchitka Island, Alaska

Studies on the benthic marine algae at Amchitka Island since July 1967 entailed considerable systematic effort since there have been no major phycological studies of the Bering Sea area since Kjellman (1889). A series of papers on various groups of benthic marine algae at Amchitka has been initiated (Wynne, 1970). Weinmann's (1969) study of the zonation of littoral algae provided a starting point for our ecological studies. More recently we investigated timedependent changes in the littoral biospace since a knowledge of these changes (or lack of them) will allow us to predict and differentiate to some extent natural and possibly man-induced changes. The major environmental factors affecting the distributions of littoral vegetation are terrestrial climate, sea temperatures, and wave action. The climate is maritime. Air temperature varies little, and sky cover and relative humidity are high throughout the year. These conditions are favorable for the development of littoral vegetation. Sea-surface temperatures range from 1 to 2 C in winter to 8 to 10 C in summer. Wave action can be consistently heavy, particularly in the winter. Dredging data indicate that waves of well over 100 m in wavelength are frequent.l Figure 1 shows the heavy wave action encountered at the Island. This condition would lead one to expect an extensive vertical range of littoral algae and an configuration of vegetation. This was not the case. In general, even on outer exposed points, the vertical range of macroscopic algae was less than 2 m above mean lower low water. This small vertical range is explained by the presence of extensive benches in the littoral. One of these benches is shown in O'Clair and Chew's Figure 1 (this symposium). Providing an unusually uniform habitat, these benches also act as buffers against the long-period waves approaching Amchitka. The outer edges of the benches generally rise abruptly in depth from 2 to 10 m. At these outer edges, most of the energy dissipates as the waves break (Fig. 2), and water piles on the benches, then flows landward and laterally, and eventually seaward via channels between the benches. The process is shown schematically in Figure 3. The buffering action at the benches must be the underlying cause of the absence of an configuration of littoral vegetation. Our first objective was to characterize the distribution of marine algae over This paper is based on work performed under AEC Contract AT(26-1)-171 for Battelle Memorial Institute, Columbus Laboratories. Contribution No. 330, College of Fisheries, University of Washington.

Experiments on some effects of certain environmental factors on Gracilaria verrucosa (Hudson) Papenfuss

At an early stage of an autecological study of Gracilaria verrucosa (Hudson) Papenfuss it seemed possible that the best growth of the plants occurred in places, such as the Achill Sound and the Church Island Channel in the Menai Straits, where, in the absence of heavy wave action, strong currents flowed over the plants. Experiments were carried out, in the sea, to investigate the effect of currents on the growth rate; the apparatus used and described here would probably be suitable for the investigation of current effects on other sedentary marine organisms.These experiments also led to some observations on the effects of illumination on the pigmentation and growth rate of the plants which were supplemented by laboratory experiments. High intensity illumination in the laboratory also demonstrated positive phototropism in the plant.