Subtidal Communities Research Articles

Influence of physical gradients on the structure of a northern rocky subtidal community

We have examined the effect of the estuarine and depth gradients on community structure (diversity, richness, density, biomass) and biomass of trophic guilds in the rocky bottom subtidal community ...

Human Impact Assessment on the Subtidal Macrobenthic Community Structure in the Mondego Estuary (Western Portugal)

The Mondego Estuary consists of two arms, north and south. The north arm is deeper, while the south arm is largely silted up, which causes the freshwater to flow primarily by the north arm. Human impact is significant, due to harbour activities, salt-works, aquaculture farms, and nutrient and chemical discharge from agricultural areas. From December 1989 to September 1990 a reference study on the subtidal macrofauna was carried out, in order to characterize the community structure in relation to physicochemical environmental factors. The subtidal macrofauna in the Mondego Estuary appears to be clearly impoverished compared to a reference Portuguese estuary, the Mira, which has been much less exposed to human impacts. The south arm macrobenthic community consists mainly of infaunal species and appears to be more stable and structured, presenting a higher macrofauna abundance. The north arm community is mainly characterized by sparse mobile epibenthic species populations, exhibiting a lower biodiversity and an impoverished macrofuna, compared to the south arm. The subtidal communities appear to be physically controlled, with emphasis on the type of sediment, salinity and currents. Biologically, due to their distinct physicochemical characteristics, the two arms of the estuary constitute different subsystems. Dredging takes place twice a year along the north arm, and time intervals between dredging operations appear to be inadequate to allow macrofauna recovery. Although structurally more stable, the south arm community, due to the feeble water circulation, may be more exposed to environmental changes. Benthic monitoring of the Mondego Estuary biological communities is clearly necessary in order to assess temporal trends and to establish if the apparent environmental changes are reversible.

DAMAGE ASSESSMENT IN COASTAL HABITATS: LESSONS LEARNED FROM EXXON VALDEZ

ABSTRACT This paper examines alternative designs for the monitoring and assessment of damages of environmental impacts such as oil spills. The optimal design requires sampling at pairs of impacted (oiled) and control (unoiled) sites both before and after the event. However, this design proved impractical in evaluating impacts of the Exxon Valdez oil spill on nearshore subtidal communities, and may be impractical for future monitoring. An alternative design is discussed in which sampling is conducted at pairs of control and impact sites only after the impact.

Do toxic algal blooms affect subtidal soft-bottom communities?

An extensive bloom of the toxic flagellate Chrysochromuha polylepjs occurred over much of the Skagerrak and Kattegat area during May-June 1988 with severe effects on biota in shallow marine habitats. Results reported here are from annual grab-sampling of subtidal soft-bottom communities off the southwest coast of Norway during the period 1986 to 1991 at depths between 69 and 184 m, and describe changes In fauna associated with the toxic algal bloom. In the 3 years prior to the bloom the fauna at each station was persistent and only modest year-to-year changes in the number of species, abundance and species composition were observed. Samples collected 1 yr after the bloom event revealed a significant reduction in both number of species and individuals. The most obvious changes were found at sites situated in accumulation areas characterised by fine sediments indicating a possible connection between the quantity of sinking toxic material and the degree of faunal perturbation. A DCA ordination demonstrated a clear switch in faunal composition in the study area subsequent to the bloom, and a tendency of the fauna to return to the pre-bloom communities 2 yr after the bloom event. Principal species affected were moderately abundant oligochaetes, polychaetes, crustaceans, molluscs and sipunculids. Allocation of each species into the categories 'gaining' and 'losing' according to annual changes in abundance gave a good description of the community disturbance probably induced by the toxic alga. Other possible causes for the observed faunal perturbation may be changes in water temperature, food input, predation interference and changes in sediment composition, but none of these were found to give reliable, alternative explanations for the substantial changes observed in the soft-bottom communities subsequent to the toxic bloom.

Kelp beds as habitat for American lobster Homarus americanus

The American lobster Homarus americanus and kelp Laminaria longicruris and L. saccharina are prominent and often intimately associated members of the subtidal community in the western North Atlantic Ocean. However, no one has identified the nature of this relationship or specifically investigated whether kelp beds are a superior habitat for lobsters. We conducted field studies in 1990 and 1991 at a coastal site centrally located along the Gulf of Maine. USA, to determine how lobsters use kelp beds as habitat. Identically sized and spaced plots of live and artificial (plastic) kelp were established and monitored for lobster population densities. Adjacent featureless sediment plots of identical size served as controls. Lobster population density and biomass were significantly higher in both real and artificial kelp treatments than in non-kelp control plots (p < 0 0001). The change in lobster density was apparent the day following placement of the expenment, so a secondary trophic effect such as attracting prey into treatments is unlikely to have occurred. Thus, kelp beds can affect local lobster population dens~ties by providing shelter for lobsters, thereby concentrating individuals and increasing the local carrying capacity of potential lobster habitats The effect of kelp beds on the local carrying capacity of lobster habitats was further explored by test~ng how lobsters respond to differing patch sizes. A graded size series of circular patches of artificial kelp was established, in which kelp blade density and total area were held constant for each treatment. Treatments were subdivided into four 1 m', two 2 m*, or one 4 m2 patches. Experiments were surveyed for lobster population density and size structure to determine if statistical differences existed among treatments. Lobster density was significantly greater in the smallest patches (p c 0.001). Moreover, lobsters typically occupied the edges of kelp beds, and their abundance within kelp patches corresponded to the patch's perimeter-to-area relationship. This suggests that 'edge effects' influence the local carrying capacity for lobsters by influencing the lobsters' choice of kelp beds as habitat.

Point quadrat versus video transect estimates of the cover of benthic red algae

Point quadrat (PQ) and video transect (VT) techniques were compared to determine the feasibility of using the latter to sample subtidal red algal assemblages. More taxa were found by PQs than by VTs, both overall and on a per quadrat basis. VTs tended to overestimate the percent cover of rock and articulated coralline algae. VTs were incapable of resolving many of the less common red algae. Video sampling required less time in the field than point quadrats but laboratory analysis was very time consunling. Lack of resolution and extensive laboratory analysis make this technique insufficient for sampling benthic red algal populations. However, VTs may be useful in documenting large-scale differences between regions, such as the percent cover of understory kelps or the density of large mobile invertebrates in central California kelp forests. Video sampling has been successfully used in the intertidal but traditional, random point quadrats s t d appear to be the most efficient and accurate way to sample benthlc communities in kelp forests. The advent of quantitative techniques to sample subtidal benthic communities has aided the scientific description of pattern and helped transform marine ecology from a wholly observational to a more experimental science. Techniques to measure percent cover were first developed by terrestrial botanists (e.g. Greig-Smith 1983) and have been successfully adapted to subtidal communities by marine ecologists (e.g. Foster 1975, Cowen et al. 1982, Kennelly 1987). Photography has been used quantitatively in intertidal, subtidal and deep-sea habitats (Bohnsack 1979, Budd 1982, Witman & Sebens 1985, Jaap 1986, Rosman & Boland 1986, Thouzeau & Hily 1986, Meulstee et al. 1988, Edmunds & Witman 1991). Due to recent advances in video resolution and ease of use, it has begun to replace traditional photography as both a qualitative (Potts et al. 1987, Semple & Sharp 1988) and quantita' Present address: Graduate Program in Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island 02912, USA tive (Jaap et al. 1990, Maney et al. 1990, Berklemans 1992) sampling tool. The strength of video is its ability to rapidly acquire data and permit analysis in the laboratory at a later date. Video quadrats and transects are now being used routinely to measure the density and percent cover of subtidal marine organisms (e.g. Edmunds & Witman 1991). As part of a study on the effect of light on understory algae in central California (USA) kelp forests, we have been using point quadrats to estimate algal cover. These involve scoring a small number of random points (usually 10 to 20) along a bar randomly placed on the substrate. This technique is excellent for relatively abundant organisms but, theoretically, undersamples rare species because of the few points used. Whorff & Griffing (1992) recently suggested that point quadrats undersample the substrata while Foster et al. (1991) concluded that point quadrats provide better estimates of the total number of taxa and their total and individual percent cover. An alternative to point quadrat methods for rare species are search techniques (e.g. Nybakken 1978, McClanahan 1990, McClanahan & Muthiga 1992). These have been designed for extremely rare species (i.e. approximately 0.1 ind. m-2; McClanahan & Muthiga 1992) but understory algae in many benthic habitats range from 1 to 30% cover (Foster 1975). In this paper, we present results of a direct comparison of point quadrat (PQ) and video transect (VT) sampling for benthic algal assemblages. Video was used over traditional photo-quadrats because video film is significantly cheaper than still film and more samples can be taken between film changes. To test for and alleviate the potential PQ bias of using too few points, we sampled the video image using 5 times as many random points as the PQ. This should have increased the likelihood of detecting rare species. The number of 0 Inter-Research 1993 204 Mar. Ecol. Prog. Ser. 101: 203-208. 1993

Pit-Digging by the Crab Cancer pagurus: A Test for Long-Term, Large-Scale Effects on Infaunal Community Structure

1. Most studies which investigate the effects of predator disturbance on marine subtidal community structure focus on a scale appropriate to the individual disturbance event. Dramatic events are often observed at this local scale and it is tempting to infer that such disturbances are important at the landscape scale. For the more detailed small-scale studies, such conclusions are usually based on estimates of the rates of disturbance and recovery of individual patches. We argue that such calculations provide only weak tests of importance at the landscape scale and that manipulative field experiments offer much stronger tests

Competitive interactions between related clades: evolutionary implications of overgrowth interactions between encrustin cyclostome and cheilostome bryozoans

Overgrowth interactions between encrusting cyclostome (eight species) and cheilostome (over 50 species) colonies were examined in subtidal communities in the northern Adriatic Sea off Rovinj, Croatia, in May–June 1988 and September–October 1990. Cheilostome colonies occupied ca. 80% of the available substrate space, whereas the cyclostome colonies occupied <5%. Out of 210 recorded interactions, cheilostomes overlapped cyclostomes in 164, while cyclostomes overlapped cheilostomes in only 16; 30 encounters resulted in mutual overlap or at least temporary growth termination along the line of contact. Most of the recorded interactions were for the cyclostomes Diplosolen obelia (Johnston) and Plagioecia patina (Lamarck). Both species elevated their colony margins on contact with a cheilostome in some instances, but D. obelia only overgrew competing cheilostomes in 12/120 encounters, and P. patina never prevailed. I propose that the cheilostomes have a key evolutionary innovation, lacking in cyclostomes, consisting of more rapid ontogenetic development of zooids along colony margins so that encrusting cheilostomes have substantially higher colony margins. This results in an outflow of filtered water along the edge of encrusting cheilostome colonies, while cyclostomes take water in at colony margins. These contrasting feeding currents apparently give a competitive advantage to the cheilostomes where colony margins approach and make contact with those of cyclostomes.

Sedimentology and paleoecology of Pliocene shallow marine conglomerates, Salton Trough region, California

The basal portion of the Pliocene Imperial Formation, Salton Trough region, California, includes conglomerates which were deposited in a rift-related setting at the head of the Gulf of California. Conglomerates and associated pebbly sandstones contain a fully marine fauna and form a transgressive sequence which unconformably overlies a basement of Miocene volcanics. Sequences deposited adjacent to rock cliffs include stratified conglomerate of beach origin and clast-supported boulder conglomerate which formed as talus. These types of sediment were also transported downslope as sediment gravity flows and accumulated in topographically irregular, nearshore areas as a variety of boulder to pebble conglomerates. The conglomerates contain two types of displaced faunal assemblages which reflect different types of rocky shore to shallow subtidal communities. An assemblage dominated by small oysters and barnacles represents a low diversity community which lived on and among volcanic clasts in muddy, protected coves. Another assemblage represents a diverse community of corals, bryozoans, gastropods, bivalves, annelids, barnacles, decapods and echinoids which lived on and among boulders along coastlines. Both assemblages have analogues in the modern Gulf of California. Local depletion of volcanic clasts resulted in deposition of nearshore sandstone which prograded over conglomerate deposits. This sandstone is composed of volcanic rock fragments and abundant skeletal fragments which represent taxa from several coastal communities.

Neotectonics and Taphonomy: Pleistocene Molluscan Shell Accumulations in the Northern Gulf of California

Late Middle to early Late Pleistocene shallow marine deposits west of Bahia Adair, to the northern Gulf od California, have been exposed by tectonic uplift along the coast. Represented sedimentary environments include shallow subtital, tidal falt, tidal estuary, tidal channel and shoreface. The deposits contain a diverse molluscan fauna, often concentrated in shell beds. Based on taphonomic data, stratigraphic geometry and faunal composition, the shell beds are identified as beach berm accumualtions, tidal channel lags, subtidal community beds, or unconformity beds. While the first three types of shell beds form on many continental margins, the unconformity beds reflect the active tectonic setting of the northern Gulf of California

Environmental effects of a herring mass mortality in northern Norway. Impact on and recovery of rockyshore and soft-bottom biotas

Abstract In October 1984 a school of herring died in the small shallow bay of Alterosen, Lofoten, northern Norway. During the decay of the fish, subtidal sediments turned black from sulphides while fats and oils floated ashore covering seaweeds and rocks. In the subtidal the soft-bottom macrofauna was killed. Recovery took more than three years, following typical successional patterns. The polychaetes Capitella capitata and Scolelepis fuliginosa had high abundances during 1985 and 1986. Intertidal damage was limited. In the rocky shore the key algal species, Ascophyllum nodosum, tolerated the pollution, but encrusting algae and sponges, and epiphytes were locally affected. Intertidal sediment fauna was only affected during autumn 1984. The different impact and recovery of the intertidal and subtidal communities was found to be in marked contrast to other cases of long-term organic pollution. The results are discussed in view of various effects of stress and disturbance on communities.

Feeding ecology of benthic mobile predators: experimental analyses of their influence in rocky subtidal communities of the Gulf of Maine

The feeding ecology of mobile predators and their impact on benthic organisms inhabiting the rocky shallow subtidal zone off the coast of Maine, USA, was investigated. To determine diets and degree of prey selection we analyzed gut contents of lobsters ( Homarus americanus Milne-Edwards), two species of crabs ( Cancer irroratus Say and C. borealis Stimpson) and 13 fish species of which pollock ( Pollachius virens (Linnaeus)), cunners ( Tautogolabrus adspersus (Walbaum)), and sculpins (several species) were the most conspicuous species of this assemblage. Dietary data were compared with those on benthic prey availability. In the laboratory, we conducted experiments to test whether there is prey selection in relation to their energetic value. In the field, we conducted two experiments: (1) to test whether fish predators move into the intertidal zone during high tide for feeding purposes; and (2) to test the impact of increased predator abundance in these communities using artificial reefs made of concrete blocks. Large fish and crustacean predators foraged both benthic (mainly invertebrates) and pelagic (mostly fishes) prey. Juveniles or small individuals of crustaceans, mollusks and fish were the most heavily preyed, crabs ( Cancer spp.), mussels ( Mytilus edulis Linnaeus), and herring ( Clupea harengus Linnaeus) being the most common items. Juvenile sea urchins ( Stronglocentrotus droebachiensis (Muller)) were important prey of cunners. In the laboratory, lobsters, cunners and sculpins exhibited strong preference for small crabs and mussels but not for echinoderms. These results were consistent with those obtained with a prey selection index, and were closely related to the energy value of prey, suggesting that these predators behave as energy maximizers. 13 pollock and four cunners were caught in the mid-intertidal zone. These specimens contained a variety of undigested benthic intertidal organisms indicating that they utilize intertidal zones as feeding grounds. These results suggest that fishes may play an important role on the structure and organization of intertidal sessile communities. A significant number of predators colonized the artificial reefs, indicating that the availability of living space is an important factor affecting the distribution and abundance of large mobile predators. These predators reduced the abundance of juvenile or small benthic species, particularly of green sea urchins, in the area surrounding the reefs, suggesting that size-selective predation is presumably the mechanism by which predators influence the structure and organization of benthic communities in these environments.

Disturbance and organization of macroalgal assemblages in the Northwest Atlantic

Large seaweeds are often structurally dominant in subtidal and intertidal rocky shore benthic communities of the N.W. Atlantic. The mechanisms by which these algal assemblages are maintained are surprisingly different in the two habitats. In the subtidal community, kelps are dominant space competitors in the absence of strong grazing interactions. In contrast, the large perennial seaweeds of intertidal zones (fucoids and Chondrus crispus) are competitively inferior to both sessile filter feeders and ephemeral, pioneer algal species. Intertidal seaweed beds are maintained by carnivory of whelks, which reduces filter feeder populations, and by herbivorous periwinkles which reduce ephemeral algal populations. Through most of the intertidal zone, disturbance, both biological and physical, dictates which species shall compete and equilibrium conditions obtain subsequently. The roles of subtidal consumers are quite different. Sea urchins are the major algal herbivores and these voracious animals maintain an equilibrium state in which large tracts of subtidal coralline pavement are kept free of kelp forests. Urchins do not seem to play a successional facilitative role for kelps in the way that periwinkles do for fucoids in the intertidal. Control of herbivore populations is thus a key to the maintenance of subtidal foliose algal beds. It is clear that parasitic amoebas can decimate sea urchin populations so that kelp forest dominance is assured. However, the importance of carnivory in limiting urchins in the subtidal community is unclear in the absence of appropriate manipulation experiments. It is possible that carnivorous decapods and fin fish control sea urchin populations and hence foliose algal abundance, but this must remain speculative. The seaweed-dominated state of the subtidal system is an alternative equilibrium condition to the urchin/coralline alga configuration. The structure of the kelp beds is relatively uniform in responding to frequent small-scale, infrequent large-scale, or no, disturbance.

Conditions of the peracarid populations of subtidal communities in Northern Brittany ten years after the Amoco Cadiz oil spill

Peracarid populations were greatly reduced in 1978 by oil from the Amoco Cadiz. Ten years after the spill, a benthic survey was conducted in the soft-bottom infralittoral communities of the bays of Morlaix and Lannion and the Aber Wrac'h channel to study the state of recovery of peracarid populations. Living in isolated populations in fine sand and muddy sand communities with low potential for immigration, the recolonization and the reconstitution of these perturbed populations was expected to be slow. The amphipod populations from the subtidal channel of Aber Wrac'h, which were initially the most affected by the oil spill were in the least advanced state of recovery. Some species present in abundance before the oil spill were not rediscovered. Nevertheless, ten years after the oil spill, most of the populations had completely recovered.

Long-term changes in populations of subtidal bivalves (Abra alba and A. prismatica) from the Bay of Morlaix (Western English Channel)

Two populations of Abra alba (Wood) and one of A. prismatica (Montagu) (Mollusca: Bivalvia) were studied over a 10 yr period (1977–1987) in two muddy fine-sand subtidal communities of the Bay of Morlaix, France. The survey provided an example of long-term changes in the three Abra spp. populations, which displayed synchronized changes, with a regular annual cycle and increasing densities during 1979–1980 related to the higher concentration of organic matter resulting from the “Amoco Cadiz” oil spill in March 1978. A. alba rapidly adapts its demographic strategy to eutrophic conditions by increasing its reproductive potential, growth, and abundance. During times of eutrophication, A. alba has three spawning periods and three recruitments per year as opposed to two spawning periods and two recruitments per year during oligotrophic conditions. Growth of the juveniles of this species is insignificant until spring for individuals recruited in the autumn, whereas individuals which settle during spring or summer display immediate rapid growth. A. prismatica has a low capacity to adapt to eutrophic conditions. It has one annual period of sexual maturation at the end of the summer, with spawning in September–October and settlement beginning in mid-November. Growth of the juveniles after settlement is also insignificant until April. These results enable comparison of the demographic strategies of these two sympatric species.

Aspects of Histology in Macoma Carlottensis (Bivalvia: Tellinidae) and In Situ Histopathology Related to Mine-Tailings Discharge

Macoma carlottensis Whiteaves, 1880 is abundant (up to approximately 350 nr2) in subtidal infaunal communities on the continental shelf of western United States (Lie &amp; Kelly, 1970; Word, 1979) and in British Columbia fjords (Ellis, 1969). Of thirteen different species of Macoma identified from the coastal waters of southern British Columbia, M. carlottensis is the most tolerant to variations in water depth, sediment particle size, current exposure, and salinity (Dunnill &amp; Ellis, 1969). Observations of the species maintained in sediment trays in our laboratory suggest thatM. carlottensis is an infaunal, primarily sub-surface deposit feeder which generally lies at a depth of 1-4 cm with its left valve downwards. Filter feeding was observed in some instances as indicated by extension of the siphons from 0-5-3 cm above the substrate.

The subtidal gully fish community of the eastern cape and the role of this habitat as a nursery area

The fish fauna of rocky subtidal gullies between Chelsea Point and Skoenmakerskop (South Africa) was sampled between October 1985 and September 1986 using rotenone to investigate the species composition and the importance of this habitat as a nursery area for marine linefish species, A total of 59 species belonging to 28 families were collected. The majority of these were cryptiC fishes such as Clinidae and Cheilodactylidae. Sparldae and Serranidae were represented by juveniles of inshore species. The findings are discussed in relation to previous studies of intertidal pools in the eastern and western Cape, and it is concluded that shallow rocky inter- and infratidal areas are important nursery areas for numerous inshore fishes, including some species caught by recreational line fishermen. The results from this study suggest that previous intertidal studies have over-emphasized the importance of rock pools as nursery areas.

Larval Predation by Barnacles: Effects on Patch Colonization in a Shallow Subtidal Community

Laboratory studies and gut—content analysis suggest that barnacles should be important predators on invertebrate larvae. To determine if predation on larvae limits settlement, we recorded recruitment in artificially produced patches of clear substratum surrounded by monocultures of either living or dead barnacles. Living barnacles inhibited recruitment of colonial ascidians and bryozoans, but had no detectable effect on recruitment of barnacles, serpulid polychaetes, or solitary ascidians. Patch size did not influence the recruitment of any species. The presence of dead barnacle shells influenced spatial pattern of settlement in ascidians and serpulids. Comparison of blank control plates with barnacle—covered plates indicates that these spatial patterns are better explained by larval behavior than by small—scale hydrodynamic effects. Ascidians, which are dominant competitors in this community, settle primarily on shells of dead barnacles rather than on clear primary substratum. Although predation on larvae by barnacles appears to inhibit recruitment of some species, its effects are variable and easily confounded with other determinants of settlement distribution.

Recurrent and Persistent Brown Tide Blooms Perturb Coastal Marine Ecosystem

Throughout the summers of 1985 and 1986 a small (2–3 μm diameter), previously underscribed chrysophyte bloomed monospecifically (>109 cells 1−1) in Long Island embayments. The bloom colored the water dark brown, decimated eelgrass beds through decreased light penetration and caused starvation (tissue weight loss) and recruitment failure of commercially important bay scallop populations. These perturbations portend longterm changes in subtidal communities Similar and concurrent blooms in bays of Rhode Island and New Jersey suggest a meteorological component of the environmental conditions promoting bloom formation. Culture experiments with isolates of the microalga suggest the presence of stimulatory growth factors in the bloom seawater. *** DIRECT SUPPORT *** A01BY040 00002

Long-term trends in algal-dominated rocky subtidal communities on the Swedish west coast — a transitional system?

Rocky subtidal communities on the Swedish west coast were monitored over approximately 16 years (1970–1985) by means of stereophotographic recordings, at intervals, of fixed sites.