Middle Shore Research Articles

Variation in autotrophic and heterotrophic sponge abundance in a shallow water seagrass system

AbstractSponges are well known to feed heterotrophically through suspension feeding, but their relationships with photosynthetic symbionts mean they also have the potential to utilise or release photosynthetically derived carbon. Here, we determined the nutritional mode of sponges that occurred seagrass meadows at two sites in the Wakatobi National Park (Hoga‐1 and 2), Indonesia from the near‐reef flat (d = 1.9 m), middle‐shore (d = 1.2 m) and high‐shore (d = 0.5 m) zones to assess the role they might play in nutrient fluxes. We measured photosynthetic pigment concentrations and in situ net primary production to dark respiration (P:R) ratios of eight sponge species representing 75%–100% of the total sponge assemblage biomass at the two sites. We found that six of eight sponge species were autotrophic, based on their instantaneous P:R ratios and estimated daily oxygen production/consumption. The proportion of autotrophic sponge biomass in the high and middle shore of the seagrass meadows ranged between 40% and 70%, and accounted for 98% and 81% of the biomass in the near‐flat zones. Our findings that many seagrass sponges are autotrophic challenges the general view that sponges are consumers of organic carbon from the water column.

Contrasting drivers of sponge and seagrass assemblage composition in an Indo-Pacific seagrass meadow

AbstractSponges are major components of benthic marine ecosystems across the world. However, despite seagrasses being one of the key ecosystems in tropical environments, their associated sponge assemblages have been poorly studied. In this study we investigated seagrass and sponge assemblage composition in an Indo-Pacific seagrass meadow located surrounding Hoga Island in the Wakatobi National Park, South-east Sulawesi, Indonesia. We examined a continuous seagrass meadow, spatially separated into two sites, categorized into three zones based on the distance to the shore: high-shore, middle shore and near-reef-flat zones. We investigated the species richness, composition and abundance of both seagrass and sponge species across zones and sites. We found that the sponge assemblage composition was significantly different between sites and zones, while the seagrass assemblage was not. This suggests that the seagrass and sponge assemblages have different ecological drivers. Based on our observations, we propose that sponge adaptation to the prevailing environmental conditions in different seagrass zones may be reflected by sponge morphology. Furthermore, seagrass ecosystem management strategies should consider the different drivers of seagrass and sponge distribution patterns as they might be differentially affected by specific anthropogenic stressors.

Wave exposure, shore level, and season of clearing modulate early algal abundance and succession in an intertidal zone

SUMMARYZonation patterns and the structure of intertidal communities are controlled by tidal conditions. Algal diversity, abundance and succession were experimentally tested under different degrees of wave exposure, shore level, and season of clearing at the intertidal zone, Phuket, Thailand. Dead coral patches colonized by algae (20 cm × 20 cm) were cleared at upper, middle, and lower shore levels on sheltered and semi‐exposed shores during both the dry and rainy seasons. Of 17 algal species, including eight Rhodophyta, eight Chlorophyta, and one Phaeophyceae that were recruited on the cleared plots, three species were dominant: Ulva paradoxa, Padina in the Vaughaniella stage, and Polysiphonia sphaerocarpa. Algal diversity on the semi‐exposed shore was higher than on the sheltered shore. In the successional process, U. paradoxa extensively recruited and persisted longer on plots cleared at the middle shore level on the semi‐exposed shores than at the other. It showed a greater abundance in the plots cleared in the rainy season than those cleared in the dry season. Ulva paradoxa persisted for around 5 to 6 months after clearing and was then replaced by the two later species, Padina in the Vaughaniella stage and P. sphaerocarpa. Ulva paradoxa settled more easily and persisted longer at the cleared plots than other algal species because of its opportunistic characteristics and a special physiological adaptation to long periods of emersion. From this study, degree of wave exposure, shore level, and season of clearing likely play important roles in algal recruitment, abundance, and succession patterns.

Calcareous meiofauna associated with the calcareous algaCorallina officinalison bedrock and boulder-field shores of Ceredigion, Wales, UK

AbstractThe intertidal coastline of Ceredigion, Wales, comprises a patchwork of unstable sand and cobble beaches, and stable bedrock areas and boulder-fields. The last two shoreline types support rock-pools with growths of the red algaCorallina officinalis, the thalli of which are a popular substrate for calcareous epiphytes. Replicate samples ofC. officinalis(four per site) were taken from (a) three bedrock sites (Ceinewydd, Aberystwyth Victoria Rocks and Castle Rocks) and (b) three boulder-fields (Llanon, Aberaeron lower shore (Aberaeron LS), Llanina) on the lower shore. The middle shore boulder field at Aberaeron (Aberaeron MS) was also sampled. These replicates were examined for calcareous meiofauna (63–2000 μm) not previously examined as a community: spirorbids, foraminifera, gastropods, bryozoans, ostracods and ophiuroids. These were assigned to sessile and vagile modes of life. The sessile association overwhelmingly dominated bedrock coastlines and the Aberaeron MS, while the vagile association was at its most abundant on theCorallinafrom lower shore, stable boulder-fields. Gastropods were almost entirely limited toCorallinaon boulder-fields. We hypothesize that the boulders induce low-energy turbulence among breaking waves, allowing the less firmly attached vagile meiofauna to dominate onC. officinalisin rock-pools in lower shore boulder-fields. The small attachment area of sessile organisms allows them to settle bedrock sites in greater densities than do vagile organisms at boulder-field sites, which are presumed to require larger foraging areas.

A concise review of the brown macroalga Ascophyllum nodosum (Linnaeus) Le Jolis

Ascophyllum nodosum is a large and common brown alga. The fronds are olive-brown but can appear yellowish when stressed. It is a common, intertidal species around the periphery of the North Atlantic Ocean. It is particularly common on the north-western coast of Europe (from Svalbard to Portugal), including east Greenland, Iceland and the NE coast of N America (from New York to Newfoundland). This intertidal fucoid has long fronds with large egg-shaped airbladders. The fronds can reach 2 m (extremes of 5–7 m) in length. Depending on nutrient availability, the fronds are yellow, and at low tide, they can form extensive beds appearing to be monospecific to the casual observer. This seaweed is long lived and can be a dominant, climax community species of the middle shore. Ascophyllum nodosum is very effective at accumulating nutrients and minerals from the surrounding seawater. Due to the presence of many bioactive components, its harvested biomass is a valuable resource for human enterprise. This species is exploited for use in products such as food, fertiliser, soil conditioners, biostimulants (for phyco-elicitors), animal feed, skin and hair care products, cleaners, de-greasers, equestrian products and nutritional supplements. It is also a popular ingredient in cosmetology and thalassotherapy.

Wave height vs. elevation effect on macroalgal dominated shores: an intercommunity study

Sheltered intertidal environments are characterized by highly diversified macroalgal communities which vary over short spatial scales depending on various environmental pressures. In this study, relations between wave heights and vertical elevation were analyzed in order to identify the respective roles of these factors on the structure of macroalgal communities. We measured both wave heights and elevation on six macroalgal communities of three Northeast Atlantic rocky shores. The effects of these structuring factors on seaweed communities were seasonally evaluated in early summer and late autumn using pressure recorders and digital terrain model data. Results reveal that waves were higher during late autumn than in early summer. At shore-scale, elevation was the main factor determining both the structure and the species composition of the macroalgal communities (13.7% of total variance explained), while hydrodynamics played a lower, although significant role (< 1% of the total variance). Based on the cover of canopy-forming species, intertidal macroalgal communities can be grouped into three clusters, corresponding to three height levels on the shore (higher, middle, and lower shore). Canopy composition and structure were mainly explained by the effect of elevation (45%) in the higher level of the shore and by both elevation (42%) and wave height (2.5%) in the middle shore. On the contrary, low shore communities were affected by a more balanced effect of elevation (explaining 14% of the total variance and wave height associated with site effect explaining 15% of such effect). This study gave new insights of how hydrodynamics and elevation condition the extension of macroalgal communities.

Benthic foraminifera and the polychaete Sabellaria alveolata (Linnaeus, 1767) on the intertidal rocky shore at Ceinewydd, Ceredigion, Wales, UK

Abstract Interactions between foraminifera and macrofauna have been little studied, especially on rocky coasts. This paper examines (a) the foraminiferal tests captured from suspension by the polychaete Sabellaria alveolata in an intertidal area on a rocky shore, and (b) the foraminifera living on those S. alveolata colonies. The rocky coast at Ceinewydd (Ballantine Exposure Scale 6) supports patchy colonies of S. alveolata ~1 cm thick. We collected ~10 cm2 areas to form three Sample Sets of four replicates each at low tide. Two Sample Sets came from a 1 m2 lower shore rock-pool, Sample Set 1 being taken above water and Set 2 below water. Sample Set 3 was collected from a middle shore cobble. Four replicates were taken for each Set. The replicates were disaggregated, soaked in a rose Bengal solution and dried. The residues (106–2000 μm, 7–9 g aliquots) were picked clean of foraminifera. The total (live +dead) assemblage comprised Ammonia batava (53.6%), with lesser Asterigerinata mamilla (11.1%), Bulimina elongata (7.5%) and Elphidium selseyense (5.6%). There were no statistically significant (p

Genetic and morpho-physiological differentiation in a limpet population across an intertidal gradient

Marine organism adaptive capacity to different environmental conditions is a research priority to understand what conditions are important in structuring the spatial distribution of natural populations. In this context, this study evaluates whether potential differences in Siphonaria lessonii morphology (size and shell shape) and physiology (water loss regulation) at different shore heights are linked to genetically distinguishable individuals. To achieve this goal, we compared size-frequency distribution, morphometric, and genetic data (ISSR-PCR technique) of S. lessonii from the upper, middle and lower shore. We complemented these field samplings with laboratory experiments on water loss and mortality. Genetic analysis showed different genetic composition for individuals from the upper and lower shore. This pattern was accompanied by morpho-physiological variations: the upper shore had fewer small limpets, lower shell shape dispersion (with a morphotype characterized by a higher shell), and less water loss and mortality related to air exposure than the lower shore. The results reported herein support the idea that the extreme and unpredictable conditions of the upper shore may impose strong selection pressure on its inhabitants, leading to considerable morpho-physiological differentiation consistent with different genetic composition. This probably plays an important role in structuring the spatial distribution of natural S. lessonii populations with a possible effect on size-structure distribution.

Genomic divergence between Spanish Littorina saxatilis ecotypes unravels limited admixture and extensive parallelism associated with population history.

The rough periwinkle, Littorina saxatilis, is a model system for studying parallel ecological speciation in microparapatry. Phenotypically parallel wave‐adapted and crab‐adapted ecotypes that hybridize within the middle shore are replicated along the northwestern coast of Spain and have likely arisen from two separate glacial refugia. We tested whether greater geographic separation corresponding to reduced opportunity for contemporary or historical gene flow between parallel ecotypes resulted in less parallel genomic divergence. We sequenced double‐digested restriction‐associated DNA (ddRAD) libraries from individual snails from upper, mid, and low intertidal levels of three separate sites colonized from two separate refugia. Outlier analysis of 4256 SNP markers identified 34.4% sharing of divergent loci between two geographically close sites; however, these sites each shared only 9.9%–15.1% of their divergent loci with a third more‐distant site. STRUCTURE analysis revealed that genotypes from only three of 166 phenotypically intermediate mid‐shore individuals appeared to result from recent hybridization, suggesting that hybrids cannot be reliably identified using shell traits. Hierarchical AMOVA indicated that the primary source of genomic differentiation was geographic separation, but also revealed greater similarity of the same ecotype across the two geographically close sites than previously estimated with dominant markers. These results from a model system for ecological speciation suggest that genomic parallelism is affected by the opportunity for historical or contemporary gene flow between populations.

Linking life-history traits, spatial distribution and abundance of two species of lugworms to bait collection: A case study for sustainable management plan

Arenicola spp. are marine benthic polychaetes dug for bait by anglers. Without regulation, this activity can lead to the decrease of lugworms' population meanwhile affecting the physical characteristics of the beach and the biodiversity. Here, we identified through morphology and genetics two species of lugworms, Arenicola marina and A. defodiens, within a Marine Protected Area of the Eastern English Channel (France). For each species, abundance and spatial distribution were assessed using a stratified random sampling and interpolation at four studied sites, as well as some life-history traits. These data were compared to lugworms' collection data to estimate its sustainability and to provide potential management measures. At one site, A. marina was present in large numbers on the higher and middle shore, whereas A. defodiens occupied the lower shore. At the other sites, both species co-occurred on the lower shore, and A. marina individuals were less numerous and lacking recruits. Spawning periods for A. marina occurred in early autumn and in late autumn for A. defodiens. The size at first maturity of A. marina was at 3.8 cm of trunk length (between 1.5 and 2.5 years old). One site (Au) appeared in need for management when linking abundance data with bait collection, where harvest of both species represented ∼14% of the total amount of lugworms and was above the carrying capacity of the beach for A. marina. The retail value associated to lugworm harvesting within the MPA was estimated at the same level as the shrimp retail value. Our results highlight the need for some fishery regulations.

Characterising the microbiome of Corallina officinalis, a dominant calcified intertidal red alga.

The living prokaryotic microbiome of the calcified geniculate (articulated) red alga, Corallina officinalis from the intertidal seashore is characterised for the first time based on the V6 hypervariable region of 16S rRNA. Results revealed an extraordinary diversity of bacteria associated with the microbiome. Thirty-five prokaryotic phyla were recovered, of which Proteobacteria, Cyanobacteria, Bacteroidetes, Actinobacteria, Planctomycetes, Acidobacteria, Verrucomicrobia, Firmicutes and Chloroflexi made up the core microbiome. Unclassified sequences made up 25% of sequences, suggesting insufficient sampling of the world's oceans/macroalgae. The greatest diversity in the microbiome was on the upper shore, followed by the lower shore then the middle shore, although the microbiome community composition did not vary between shore levels. The C. officinalis core microbiome was broadly similar in composition to those reported in the literature for crustose coralline algae (CCAs) and free-living rhodoliths. Differences in relative abundance of the phyla between the different types of calcified macroalgal species may relate to the intertidal versus subtidal habit of the taxa and functionality of the microbiome components. The results indicate that much work is needed to identify prokaryotic taxa, and to determine the nature of the relationship of the bacteria with the calcified host spatially, temporally and functionally.

Microphytobenthos interannual variations in a north-European estuary (Loire estuary, France) detected by visible-infrared multispectral remote sensing

Estuarine intertidal sediments are colonized by photosynthetic microorganisms grouped under the generic term of microphytobenthos (MPB). These microbial assemblages form transient biofilms at the sediment surface and have important ecosystem functions. MPB biofilms are well known to exhibit high microscale patchiness whereas meso- and macroscale spatio-temporal structures are little known. In this work, satellite remote sensing was used to map MPB assemblages at such scales. MPB interannual distribution was investigated in the poly- and mesohaline domain of the north-European estuary (Loire estuary), using a multispectral SPOT image time series (1991–2009). The Normalized Difference Vegetation Index (NDVI) was calculated from two SPOT channels, XS2 and XS3, (red and near-infrared wavelengths, respectively). MPB biofilms were identified by NDVI values between 0 and 0.3. At the scale of the whole intertidal area, the results showed that MPB biofilms in the Loire estuary exhibited perennial structures in both the polyhaline and mesohaline sectors, occupying nearly 90% of the mudflat surfaces. MPB biofilm density was closely associated with intertidal position, with thicker biofilms developing mostly in the upper and middle shore, and formed kilometric longitudinal structures parallel to the shoreline. Mean NDVI values showed a strong positive correlation with mean seasonal air temperature (τ = 0.714, p < 0.05 in the polyhaline domain and τ = 0.810, p < 0.05 in the mesohaline domain), with a strong correlation in the upper intertidal mudflat (between +3 and 4 m isobaths). Negative wind effect was mainly detected in the upper intertidal areas, particularly between the +3 and 4 m isobaths (τ = −0.810, p < 0.05 in the polyhaline domain and τ = −0.910 in the mesohaline).

Grain Size Analysis of a Growing Sand Bar at Sonadia Island, Bangladesh

Sonadia is one of the most important islands of Bangladesh. South-eastern shore of the island, which is a growing sand bar, has to encounter continuous and rapid morphological changes. So study of the sediment characteristics of this Sand Bar is very important. In the present study, grain size analysis of this Sand Bar has been done on the basis of laboratory analysis. Sediment samples were collected from 9 (Nine) stations and texture analysis of sediment was completed following a standard procedure of sieve analysis of sand samples. The average value of median (MD), mean (M), standard deviation (s), skewness (SK) and kurtosis (K) of sediment of the Sand Bar of Sonadia Island was 1.93, 1.87, 0.44, 0.11 and 1.88 respectively. Highest median, mean, standard deviation, skewness and kurtosis of Sand Bar of Sonadia Island was 2.98 (Station 2, Lower Shore), 2.68 (Station 1, Lower Shore), 0.83 (Station 1, Middle Shore and Station 2, Lower Shore), 0.65 (Station 2, Lower Shore) and 3.59 (Station 1, Lower Shore) respectively. Lowest median, mean, standard deviation, skewness and kurtosis of Sand Bar of Sonadia Island was 1.42 (Station 9, Middle Shore), 1.45 (Station 9, Middle Shore), 0.30 (Station 5, Lower Shore and Station 8, Lower Shore), 0.04 (Station 5, Upper Shore) and 0.94 (Station 6, Upper Shore) respectively.

The effects of beach nourishment on benthic invertebrates in eastern Australia: Impacts and variable recovery

Beach erosion is likely to accelerate, driven by predicted consequences of climate change and coastal development. Erosion is increasingly combated by beach nourishment, adding sand to eroding shores. Because a range of engineering techniques exists to nourish beaches, and because these techniques differ in their environmental effects, assessments of ecological impacts need to be tailored and specific. Here we report on impacts and recovery of benthic invertebrates impacted by beach nourishment operations undertaken at Palm Beach (SE Queensland, Australia). Assessments are made based on a beyond-BACI design, where samples were taken once before nourishment and twice afterwards at the impact and two control sites. Because almost all of the sand was deposited on the upper beach and later moved with bulldozers down-shore, we specifically examined whether the effects of nourishment varied at different heights of the beach—a little-studied question which has management implications. Impacts on the fauna were massive on the upper and middle levels of the beach: samples collected two days after the conclusion of nourishment were entirely devoid of all invertebrate life (‘azoic’), whereas weaker effects of nourishment were detectable on the lower shore. Recovery after five months also varied between shore levels. The sediment of the upper level near the dunes remained azoic, the fauna of the middle shore had recovered partially, and the lower level had recovered in most respects. These findings indicate that the height and position of sand placement are important. For example, rather than depositing fill sand on the intertidal beach, it could be placed in the shallow subtidal zone, followed by slow up-shore accretion driven by hydrodynamic forces. Alternatively, techniques that spread the fill sand in thin layers (to minimize mortality by burial) and leave unfilled intertidal refuge islands (to provide colonists) may minimize the ecological impacts of beach nourishment.

Comprehensive assessment of heavy metal contamination in sediment of the Pearl River Estuary and adjacent shelf

Total metal concentrations (Cr, Ni, Cu, Zn, and Pb), acid volatile sulfide and simultaneously extracted metals (AVS–SEM), and heavy metal fractionation were used to assess the heavy metals contamination status and ecological risk in the sediments of the Pearl River Estuary (PRE) and adjacent shelf. Elevated concentrations at estuarine sites and lower concentrations at adjacent shelf sites are observed, especially for Cu and Zn. Within the PRE, the concentration of heavy metals in the western shore was mostly higher than that in the middle shore. The metals from anthropogenic sources mainly occur in the labile fraction and may be taken up by organisms as the environmental parameters change. A combination of total metal concentrations, metal contamination index and sequential extraction analysis is necessary to get the comprehensive information on the baseline, anthropogenic discharge and bioavailability of heavy metals.

Seasonal Dynamics of Phytoplankton and Environmental Factors around the Chagwi-do off the West Coast of Jeju Island, Korea

The dynamics of phytoplankton abundance with seasonal variation in physicochemical conditions were investigated monthly at 10 stations around the Chagwi-do off the west coast of Jeju Island, Korea, including inshore, middle shore, and offshore in the marine ranching area from September 2004 to November 2005. Water temperature varied from 12.1 to 28.9°C (average 18.8°C), and salinity from 28.9 to 34.9 psu (average 33.7 psu). The chlorophyll a concentration was 0.02-2.05 µg L1 (average 0.70 µg L1), and the maximum concentration occurred in the bottom layer in April. A total of 294 phytoplankton species belonging to 10 families was identified: 182 Bacillariophyceae, 52 Dinophyceae, 9 Chlorophyceae, 12 Cryptophyceae, 6 Chrysophyceae, 4 Dictyophyceae, 13 Euglenophyceae, 6 Prymnesiophyceae, 5 Prasinophyceae, and 5 Raphidophyceae. The standing crop was 2.21-48.69x104 cells L1 (average 9.23x 104 cells L1), and the maximum occurred in the bottom layer in April. Diatoms were most abundant throughout the year, followed by dinoflagellates and phytoflagellates. A phytoplankton bloom occurred twice: once in spring, peaking in April, and once in autumn, peaking in November. The spring bloom was represented by fourChaetoceros species andSkeletonema costatum; each contributed 10–20% of the total phytoplankton abundance. The autumn bloom comprised dinoflagellates, diatoms, and phytoflagellates, of which dinoflagellates were predominant.Gymnodinium conicum, Prorocentrum micans, andP. triestinum each contributed over 10% of the total phytoplankton abundance.

Doses of darkness control latitudinal differences in breeding date in the barnacle Semibalanus balanoides

This paper reports the first study of breeding in the boreo-arctic barnacle Semibalanus balanoides in which latitudinal variation in timing of egg mass hardening has been examined simultaneously over the geographical scale involved, thereby excluding temporal confounding of the data. The timing of autumn egg mass hardening on the middle shore was established in 2002 and 2003 at ten stations ranging latitudinally from Trondheim (63°24′N) to Plymouth (50°18′N). To assess variation at local scale (&lt;10 km), breeding was studied on three shores at each of two Irish locations (Cork and Galway). At Oban (Scotland) and Cork, the effect of shore height on timing of breeding was investigated. A strong influence of latitude and day length on timing of breeding was found in both 2002 and 2003. In both years, barnacles bred much earlier (when day length was longer) at high rather than low latitudes. No significant effect of environmental temperature or insolation on timing of breeding was detected. Shores no more than 10 km apart showed minimal difference in middle shore breeding date (&lt;4 days). However, upper shore barnacles bred significantly earlier (by 7–13 days) than middle shore animals. The data indicate that breeding is controlled by period of daily darkness, with high shore animals encountering longer effective ‘nights’ because of the opercular closure response to emersion (which will reduce light penetration to tissues). Predictions concerning the effects of global changes in climate and cloud cover on breeding and population distribution are made. It is suggested that increased cloud cover in the northern hemisphere is likely to induce earlier breeding, and possibly shift the present southern limit of Semibalanus southwards.

COMPARISONS OF THE BIOLOGY OF THE INTERTIDAL SUBANTARCTIC LIMPETS NACELLA CONCINNA AND KERGUELENELLA LATERALIS

Two limpet species occur intertidally on subantarctic South Georgia, the patellid Nacella concinna and the siphonanid Kerguelenella lateralis. N. concinna is confined to the lower shore close to LWS; K. lateralis occurs in middle shore pools, so their distributions do not overlap. N. concinna has a much narrower thermal niche (-12.9°C to +15.6°C) than K. lateralis (-17.8°C to +31.8°C). Environmental data are presented to show that the upper lethal temperature of N. concinna is low enough to prevent the limpet living higher on the shore. Both limpet species are slow-moving, but K. lateralis shows increasing speed with rising temperature, peaking at 15-20°C. In contrast, N. concinna moves actively down to — 1.9°C (when sea water freezes), but there is a steady decrease in speed of locomotion above +2°C. Locomotion ceases at 14°C in N. concinna (c.f. 30°C in K. lateralis). Both species exhibit very low tenacities, but in N. concinna tenacity decreases with increasing shell length. In K. lateralis there is no effect of temperature on tenacity. Both species show a positive allometric relationship between foot area and shell length. N. concinna feeds upon microbial films and microepiflora, but K. lateralis eats colonial diatoms and Enteromorpha bulbosa. Observations on shell middens of the kelp gull Larus dominicanus showed that the gulls did not eat K. lateralis, though they ate great quantities of the less accessible N. concinna. Gulls ate N. concinna as small as 11 mm shell length (within the size range of K. lateralis). Experiments on gulls demonstrated an unwillingness to eat K. lateralis, probably because the siphonariid extrudes a viscid white mucus when the foot is touched.

SEASONAL AND SPATIAL VARIATION IN EPIPELIC DIATOM ASSEMBLAGES IN THE SEVERN ESTUARY

The species composition of epipelic algal assemblages on intertidal mudflats was investigated at three sites in the Severn estuary, England, between March 1990 and February 1991. Diatoms were the dominant algal group, comprising greater than 95% of living cells in 100 out of 106 lens tissue samples. Euglenoid algae were abundant at one site during the summer months. Although over 60 diatom taxa were identified, only 15–20 species occurred regularly, with 12 species being dominant in the samples. Upper and middle shore diatom assemblages were dominated by Navicula pargemina during the spring and autumn, and by Nitzschia epithemioides during the early summer months. High relative abundances of Rhaphoneis minutissima were recorded at lower shore stations throughout the year and on upper and middle shores during the winter months. These lower shore assemblages contained high proportions of epipsammic species (Coscinodiscus sp., Cymatosira belgica). Principal component scores (obtained from assemblage composit...

The temperature tolerances ofTetraclita squamosa(Crustacea: Cirripedia) andSeptifer virgatus(Bivalvia: Mytilidae) on a sub‐tropical rocky shore in Hong Kong

Mid‐tidal levels on Hong Kong's exposed shores are dominated by sessile barnacles (Tetraclita squamosa) and mussels (Septifer virgatus).Such shores experience dramatic elevations in air and rock temperatures in mid summer that have been causally linked to mortalities of these species.Tetraclita squamosaandS. virgatushad lower body temperatures relative to their rock substratum. Mussel inhabitants of high‐ and low‐zoned pools were in temperature equilibrium with contained water. On the rocks, body temperatures ofT. squamosawere higher than those ofS. virgatus.Middle shore individuals had higher body temperatures than lower shore ones. Individuals in the sun had higher body temperatures than those in the shade. Factors possibly affecting body temperature are orientation to the incident sunlight, height on the shore and the contact area between the individual and the substratum.Mean heat coma temperature (HCT) values for submergedSeptifer virgatuswere 45.0d̀C and 44.5d̀C for individuals of between 10‐20mm and 30‐50mm shell length, respectively. Mean median lethal temperature (LT50) values for submergedS. virgatuswere the same for both size classes (47.0d̀C). A longer exposure in air resulted in a lower LT50value; a shorter exposure in air resulted in a higher LT50value.Water content ofSeptifer virgatuswas between 89.21–90.26%, i.e. including the water from the mantle cavity. In air temperatures of between 30–32d̀C and after long periods of exposure,Septifer virgatusshowed significant water loss, i.e. of between 13.87‐15.52% body water content. In air temperatures of between 33–36d̀C and after relatively short exposures,Septifer virgatusshowed small water losses, i.e. between 1.92‐5.74% body water content.On Hong Kong's exposed rocky shores, sessile components of the middle shore environment are thermally stressed in summer when low tides occur in the afternoon in combination with periods of cloudless sunny days and when rock temperatures may climb to ≅ 50d̀C through solar insolation. Lower‐zoned individuals and high‐zoned pool residents are buffered from the adverse effects of such environmental extremes. Mortality occurred mainly in the sun‐exposed shore populations of bothTetraclita squamosaandSeptifer virgatusand is particularly evident when the above combination of climatic and tidal factors first occurs in July.