Interactions between two introduced species: Zostera japonica (dwarf eelgrass) facilitates itself and reduces condition of Ruditapes philippinarum (Manila clam) on intertidal flats

Dwarf eelgrass (duckgrass; Zostera japonica) and Manila clams (Ruditapes philippinarum) are two introduced species that co-occur on intertidal flats of the northeast Pacific. Through factorial manipulation of clam (0, 62.5, 125 clams m⁻²) and eelgrass density (present, removed by hand, harrowed), we examined intra- and interspecific effects on performance, as well as modification of the physical environment. The presence of eelgrass reduced water flow by up to 40% and was also observed to retain water at low tide, which may ameliorate desiccation and explain why eelgrass grew faster in the presence of conspecifics (positive feedback). Although shell growth of small (20–50 mm) clams was not consistently affected by either treatment in this 2-month experiment, clam condition improved when eelgrass was removed. Reciprocally, clams at aquaculture densities had no effect on eelgrass growth, clam growth and condition, or porewater nutrients. Overall, only Z. japonica demonstrated strong population-level interactions. Interspecific results support an emerging paradigm that invasive marine ecosystem engineers often negatively affect infauna. Positive feedbacks for Z. japonica may characterize its intraspecific effects particularly at the stressful intertidal elevation of this study (+1 m above mean lower low water).     

Characterization of disjunct populations of Zostera marina (eelgrass) from California: genetic differences resolved by restriction-fragment length polymorphisms

Comparative restriction-fragment analysis was used to analyze the nuclear ribosomal DNA, and alcohol dehydrogenase-1 loci of Zostera marina L., for variation within and among populations. Eelgrass is a perennial marine flowering plant that is widespread and ecologically significant throughout the temperate northern hemisphere. A chemical method was developed to obtain restriction-quality DNA without CsCl fractionation from experimentally relevant quantities of seagrass tissues (0.5 to 1.0 g). The yield was 25 g g^-1 fresh weight. The three morphologically distinct forms of Z. marina from disjunct populations examined in this study were found to be genetically distinct; morphologically similar populations were indistinguishable genetically. Genetic distinction also correlated with habitat depth, as subtidal and intertidal populations were clearly divergent. Homologous probes for the 17S and 28S ribosomal DNA genes were used to map 24 restriction sites on the rDNA repeat of Z. marina, which was determined to be about 14 kb in length. At least 1 length mutation and 5 restriction-site changes were identified that distinguished Z. marina populations from San Diego and Monterey Bay (Del Monte Beach) from Z. marina populations from Elkhorn Slough and Tomales Bay. Estimated sequence variation (100×p) between eelgrass populations ranged from 0.00 to 0.69. Individual plants were observed to contain as many as four different rDNA-repeat length variants. The mean number of rDNA-repeat length variants per individual in Z. marina was about two. Intrapopulation variation in rDNA-repeat type was observed in only one individual from the Tomales Bay population.     

Shell ecophenotype in the blue mussel (<Emphasis Type="Italic">Mytilus edulis</Emphasis>) determines the spatial pattern in foraging behaviour of an oystercatcher (<Emphasis Type="Italic">Haematopus ostralegus</Emphasis>) population

When feeding on blue mussels (Mytilus edulis), oystercatchers (Haematopus ostralegus) either stab into the mollusc’s gaping valves or hammer through its dorsal or ventral shell. Whilst the selectivity of hammering and stabbing oystercatchers for specific prey morphologies has been well studied, the way in which the effects of environment on M. edulis morphology can in turn affect feeding methods of H. ostralegus is very poorly understood. Based on morphological analyses on randomly selected shells from three intertidal zones, this study failed to detect differences in morphology or distribution of dorsally and ventrally hammered shells but confirms the finding of previous authors that hammering oystercatchers select thinner mussels than stabbing birds. Additionally, we show that this difference in optimal prey morphology can lead to spatial patterns in oystercatcher feeding behaviour. Whilst at the low intertidal and higher mid intertidal zones, characterised by comparatively thick shells, most empty shells had apparently been stabbed, hammering was the dominant feeding behaviour at the lower mid intertidal zone, where shells were thinner. Preference of hammering birds for smaller mussels was not ubiquitous. Sagittal shell shape was predominantly influenced by allometric growth effects and had only minor effect on prey selection. All oystercatchers preferred less inflated mussels, with the degree of shell inflation gradually increasing with higher intertidal elevation. Our results illustrate the importance of small-scale patterns in prey ecophenotypes in determining the distribution and feeding dynamics of wading birds.     

Ecology of two littoral species of caprellid amphipods (Crustacea) from Washington,USA

Ecology of 2 littoral species of caprellid amphipods is compared. Populations of Caprella laeviuscula, a periphyton scraper/filter-feeder, are most dense on eelgrass Zostera marina L. In the absence of C. laeviuscula, periphyton biomass increases 411% in protected Z. marina beds. The light absorption spectra of periphyton and Z. marina are similar, and C. laeviuscula, by its periphyton removal, may allow Z. marina to grow in areas where it would otherwise be excluded. C. laeviuscula is aggressively dominant over sympatric caprellids, and seasonal predators of C. laeviuscula are absent during winter, the period when periphyton would be most limiting to Z. marina. Populations of Deutella californica, a predator, are most dense on the hydroid Obelia dichotoma (L.). In the O. dichotoma epibiotic community, D. californica is the primary predator, but removal did not change the composition of the community structure. In the absence of other macropredators, the structure of the O. dichotoma epibiotic community depends more on the seasonality of O. dichotoma occurrence than on organismal interactions.     

Estimates of outcrossing rates and of inbreeding depression in a population of the marine angiosperm Zostera marina

Despite the great diversity of pollination and fertilization mechanisms observed in marine plants, little is known about the causes or maintenance of this variation. In this study, I estimated outcrossing rates and levels of inbreeding depression in Zostera marina L. (eelgrass), providing the first empirical test of hypotheses about the evolution of breeding systems in plants with submerged flowers. This study also addressed temporal separation of female and male flowering (dichogamy) in eelgrass as a mechanism promoting an outcrossing mating system, and whether the mating system in eelgrass is related to the degree of dichogamy in the field. Outcrossing rates (0<t<1) estimated from two polymorphic allozyme loci indicate that the Z. marina population in intertidal and subtidal habitats in False Bay, Washington, USA, was highly outcrossing in both 1991 (intertidal t=0.905, subtidal t=1.0) and 1992 (intertidal t=0.775, subtidal t=0.611). The outcrossing rates were positively associated with the degree of dichogamy in 1992; intertidal plants exhibited a greater temporal separation of female and male flowering and a higher outcrossing rate than did subtidal plants. Inbreeding depression at seed set was estimated from hand pollinations (self- and outcross) on 20 reproductive individuals from the False Bay population. Averaged across all maternal parents, a greater proportion of outcrossed flowers set seed than selfed flowers; i.e., inbreeding depression was detected. Plants exhibited genetic variation for inbreeding depression, detected as a significant pollination treatment × maternal family interaction in a log-likelihood analysis. By the end of the seed-maturation period (7 mo after intial seed set) some families showed outbreeding depression, i.e., greater fitness in progeny derived from selfing than in progeny from outcrossing. The inbreeding depression in the False Bay population may be an important selective factor contributing to the maintenance of dichogamy and an outcrossing mating system, as proposed for aquatic plants.     

The contribution of seagrass beds (<Emphasis Type="Italic">Zostera noltii</Emphasis>) to the function of tidal flats as a juvenile habitat for dominant,mobile epibenthos in the Wadden Sea

Since the substantial loss of subtidal eelgrass (Zostera marina L.) in the 1930s, seagrass beds in the Wadden Sea are limited to the intertidal zone and dominated by Z. noltii Hornem. This study deals with the effect of vegetated tidal flats on quantities of mobile epifauna and proves empirically the function of seagrass canopies as a refuge for marine animals remaining in the intertidal zone at ebb tide. Drop-trap samples were taken in the Sylt-Rømø Bight, a shallow tidal basin in the northern Wadden Sea, on vegetated and unvegetated tidal flats during July and August 2002, and during the entire growth period of Z. noltii from May to September in 2003. The species composition in Z. noltii and bare sand flats showed minor differences since only two isopod species (Idotea baltica and I. chelipes) occurred on Z. noltii flats exclusively. Juvenile shore crabs (Carcinus maenas L.), brown shrimps (Crangon crangon L.) and common gobies (Pomatoschistus microps Krøyer) were also found abundantly on bare sand flats. However, the results showed significantly higher abundances and production of these dominant species on vegetated tidal flats. Additionally, the analyses of faunal size classes indicated higher percentages of small individuals in the seagrass bed during the entire sampling period. Despite drastic diurnal fluctuations of dissolved oxygen at low tide, faunal density in the residual water layer remaining in seagrass canopies at ebb tide was found to be consistently higher than that found in artificially created tide-pool units. Although species composition of mobile epifauna did not basically differ between vegetated and unvegetated tidal flats, Z. noltii beds are considered to contribute quantitatively to the function of tidal flats, as an extended juvenile habitat for some of the most important species of the Wadden Sea food web.Communicated by O. Kinne, Oldendorf/Luhe     

Effect of nutrient enrichment on growth of the eelgrass Zostera marina in the Chesapeake Bay,Virginia, USA

The addition of two commerical fertilizers, one 5% NH₄NO₃, 10% P₂O₅, 10% K₂O, and the other 10% NH₄NO₃, 10% P₂O₅, 10% K₂O, ahd a dramatic effect on the growth of Zostera marina in the Chesapeake Bay. There was a significant increase in the length, biomass and total number of turions of fertilized plots compared with controls during a 2 to 3 month period. Data from this short-term field experiment suggest that Z. marina beds in the Chesapeake Bay are nutrient-limited, that the grwoth form of Z. marina may be related to the sediment nutrient supply, and that Z. marina may competitively exclude Ruppia maritima by light-shading.     

Ecological energetics of the seaweed zone in a marine bay on the Atlantic coast of Canada. I. Zonation and biomass of seaweeds

An intertidal and underwater survey of the zonation of seaweed in St. Margaret's Bay, NovaScotia, Canada showed 8 major zones as one proceeded away from shore: (1) Fucus and Ascophyllum; (2) Chorda with filamentous browns; (3) Chondrus crispus; (4) Zostera marina; (5) Laminaria digitata with L. longicrusis; (6) Laminaria longicruris; (7) L. longicruris with Agarum cribrosum; (8) Agarum cribrosum with Ptilota serrata. Zostera occurred at the same level as O. crispus but replaced it in sheltered water. Ascophyllum was more abundant in sheltered water. L. digitata was confined to the more exposed, steeper shore. The average horizontal extent of the seaweed zone was 369 m, and the greatest depth of significant amounts of seaweed 20 to 30 m. Laminaria longicruris contributed 36% of the total biomass, and Laminaria spp. and Agarum together constituted 83%, while intertidal seaweeds contributed less than 10% of the biomass. The estimated average total biomass per m of shore line was 1,481 kg fresh weight, 326 kg dry weight, 98 kg carbon, or 980x10³ kcal. When averaged over the whole area of the bay, the corresponding figures were 1.38 kg/m² fresh weight, 0.30 kg/m² dry weight, 91 g/m² carbon or 912 kcal/m². Dry matter of Laminaria was 15 to 27% of fresh weight in blades, 10 to 12% in stipes. The dry matter content of blades was least in spring and highest in autumn, but carbon content and calorific value of dry matter showed little difference with species or season.Contribution to the International Biological Programme CCIBP 108.Bedford Institute Contribution BI 249.     

Significance of salinity and silicon levels for growth of a formerly estuarine eelgrass (Zostera marina) population (Lake Grevelingen, The Netherlands)

Since the early 1980s, the eelgrass, Zostera marina L., population in the saline Lake Gevelingen, The Netherlands, is rapidly declining. An earlier study, in which long-term data on eelgrass coverage in this former estuary were correlated with several environmental variables, showed only one significant correlation: coverage was positively related to water column silicon levels. In addition, a negative correlation with salinity was observed, but this was not significant. In the present study, the effect of silicon and the effect of salinity on the development of Z. marina were investigated experimentally. Enhancement of dissolved silicon concentrations in the water did not stimulate Z. marina above-ground production or an increase in final above- and below-ground biomass. The highly significant correlation between eelgrass coverage and water column silicon levels, thus, remains to be explained. The results of the growth experiments did, however, demonstrate a clear effect of salinity on Z. marina growth. Plants cultured at 22 psu showed a higher production of shoots and leaves, resulting in more above-ground biomass, than plants grown at 32 psu. In addition, below-ground biomass was also higher at 22 psu. Measurements of chlorophyll a fluorescence, performed with a PAM-fluorometer, indicated a reduction of photosynthesis in the high-salinity treatments. Thus, low salinity stimulates development of Z. marina from Lake Grevelingen. Eelgrass from such a historically estuarine area may be more sensitive to high salinities than other, more marine populations. Recovery of the autochthonous eelgrass population is expected to be favoured when the estuarine conditions of the seagrass area are re-established, or when restoration programmes are carried out with allochthonous ecotypes that are less sensitive to high salinities. Received: 23 June 1998 / Accepted: 19 November 1998     

Growth and reproduction of the generalist opisthobranch Haloa japonica: effect of algal seasonality on growth rate

The life history of Haloa japonica (Pilsbry), especially the seasonality of growth and reproduction, was investigated from 1992 to 1995 on a flat rocky shore exposed at low tide at Kattoshi, Hakodate Bay, Japan. To estimate the effect of environmental seasonality on growth rate, we conducted two sets of field cage experiments in which algal food was controlled. H. japonica was shown to have an annual life cycle. Spawning occurred from late April to mid-July. Recruitment occurred in June. Simultaneously, large adults disappeared from the habitat. A rapid increase in wet weight of H. japonica was observed in early spring, and coincided with the bloom of the membranous green alga, Monostroma angicava. Under field cage experiments, individuals feeding on both M. angicava and Polysiphonia japonica grew faster than those feeding on only P. japonica. Growth rates of H. japonica feeding on only P. japonica, however, did not vary from February to April. These results suggest that seasonal variation in the algal community limits the growth of H. japonica in early spring at Kattoshi. This factor is relatively more important as a limiting factor on the growth of H. japonica than the physical environment in this season.     

Relationship between mouthpart structures and in situ feeding habits of species of the family Pontellidae (Copepoda: Calanoida)

Pontellid copepods were collected from the surface waters of a tidal front region in the Bungo Channel (the Inland Sea of Japan) in June 1986 to examine the relationship between the morphology of cephalic appendages and gut contents. In particular, two dominant species,Labidocera japonica Mori andPontellopsis yamadae Mori, were compared in detail. Large setae on the second maxillae ofL. japonica possessed two rows of setules at right angles to each seta along its inner margin except a terminal part which was serrated, whereas the inner margin of those setae ofP. yamadae was entirely serrated. Judging from the structure of the mouthparts, especially the second maxillae, the former species seems to employ both suspension and raptorial feeding modes, in contrast to the latter, which may use only the raptorial mode of feeding. InP. yamadae, the first maxilla and the maxilliped are also modified for carnivory. Gut content analysis supported the morphological evidence for feeding differences, and revealed thatP. yamadae is a carnivore preying mainly on copepodids whileL. japonica feeds omnivorously on copepod nauplii and phytoplankton particles. Since the mouthpart structures of congeners are quite similar to each other, the feeding behavior and habits might also be similar. Within the family Pontellidae, the generaAnomalocera, Calanopia, Epilabidocera, andPontella have mouthpart structures similar to those ofLabidocera, whereas the genusPontellina resemblesPontellopsis. Morphological similarities would suggest that the first group of genera employs both suspension and raptorial feeding modes, and thatPontellina is a carnivore likePontellopsis.     

Relationship between specific dynamic action and food quality in the solitary ascidian<Emphasis Type="Italic"> Ciona intestinalis</Emphasis>

This study aims to determine the influence of the quality of food on specific dynamic action (SDA) in the ascidian Ciona intestinalis and thereby if respiratory measurements of SDA can be used to monitor the quality of a potential food source. Weight specific rates of filtration, ingestion and respiration of C. intestinalis were examined using two different experimental methods. Two food sources were selected, the flagellate Rhodomonas sp. representing a high-quality food source and the eelgrass Zostera marina representing a potentially low-quality food source. In spite of insignificant differences in weight specific rates of filtration and ingestion of Rhodomonas sp. and Z. marina, significant differences in weight specific respiration rates and weight specific ingestion rates of C. intestinalis at different particle concentrations of the two food sources were found. Ingestion of Rhodomonas sp. resulted in a SDA coefficient of 20%, whereas the respiratory increase after ingestion of Z. marina was insignificant. From the experimental results it is concluded that SDA of C. intestinalis is strongly dependent on the quality of the food ingested.Communicated by L. Hagerman, Helsingør     

Impact of epibenthic predation on estuarine intertidal harpacticoid copepod populations

The extent to which energy is transferred directly from benthic meiofauna to epibenthic predators was investigated on an intertidal sand-flat in the Exe estuary, southwest England, during 1981–1982 and compared with data obtained from an intertidal mud-flat in the Lyhner estuary, also in south-west England, between 1978 and 1981. Two species of flatfish (Pleuronectes platessa L. and Platichthys flesus L.), two species of goby [Pomatoschistus microps (Krøyer) and P. minutus (Pallas)], brown shrimp (Crangon crangon L.) and shore crabs (Carcinus maenas L.) are the most common epibenthic predators feeding on the benthic invertebrates in these locatites. Harpacticoid copepods are the only component of the meiofauna to form a significant part of the diet of early juvenile stages of these predators, particularly the invertebrates. Harpacticoids are a more important source of food for predators feeding over the sand-flat than for those feeding on the mud-flat because in the sand-flat alternative prey of suitable size, such as small annelids, are absent. Moreover, the impact of predation on the mud-flat is spread over the whole harpacticoid species spectrum whereas on the sandflat it is confined almost entirely to a single species, Asellopsis intermedia (T. Scott). Flatfish, gobies and shrimp consume daily an estimated 0.01 to 0.1% of the standing stock of A. intermedia and account for between 12 and 22% of the observed reduction in the population of this species between July and October. Therefore, only a very small proportion of total meiofauna biomass is transferred directly to higher trophic levels.     

Reintroduction of eelgrass (Zostera marina) in the Dutch Wadden Sea; review of research and suggestions for management measures

Eelgrass (Zostera marina L.) in the Dutch Wadden Sea historically covered an area varying from 65–150 km² in the eulittoral as well as the sublittoral zones. At present, this area comprises less than 1 km² eulittoral eelgrass stands, with an associated decrease in habitat diversity. The causes for this decline are presumably connected with the ‘wasting disease’ and the closure of the former Zuiderzee in the early 1930s resulting in increased tidal range and increased currents. After a slight recovery of the eelgrass populations on the intertidal flats a definite decline started in the early 1970s, possibly connected to increased turbidity. The present water quality and turbidity do not negatively influence eelgrass growth up to a depth of at least 0.6m below Mean Sea Level. Based on mesocosm experiments and field experiments it is concluded that re-establishment of eelgrass should be possible in sheltered bays and on unexposed tidal flats. The most suitable depths for a reintroduction are those between 0 and 20–40 cm below mean sea level.     

Reintroduction of eelgrass ( Zostera marina ) in the Dutch Wadden Sea; Review of research and suggestions for management measures

Eelgras (Zostera marina L.) in the Dutch Wadden Sea historically covered an area varying from 65–150 km² in the eulittoral as well as the sublittoral zones. At present, this area comprises less than 1 km² eulittoral eelgrass stands, with an associated decrease in habitat diversity. The causes for this decline are presumably connected with the ‘wasting disease’ and the closure of the former Zuiderzee in the early 1930s resulting in increased tidal range and increased currents. After a slight recovery of the eelgrass populations on the intertidal flats a definite decline started in the early 1970s, possibly connected to increased turbidity. The present water quality and turbidity do not negatively influence eelgrass growth up to a depth of at least 0.6m below Mean Sea Level. Based on mesocosm experiments and field experiments it is concluded that re-establishment of eelgrass should be possible in sheltered bays and on unexposed tidal falts. The most suitable depths for a reintroduction are those between 0 and 20–40 cm below mean sea level.     

Genetic differentiation in populations of two Palaemon prawn species at the Atlantic east coast: does gene flow prevent local adaptation?

Two Palaemon prawn species, P. adspersus Rathke and P. squilla (L.), occur along the European Atlantic coast. In areas heavily affected by tides (Netherlands and France), one species, P. squilla, inhabits the intertidal zone, and P. adspersus is found subtidally in the Zostera marina meadows. In areas with small tidal ranges (Sweden) both species occur in the Z. marina zone, and here the body size of P. squilla is reduced, something that in turn reduces reproductive output in this species. Is adaptive improvement in non-tidal P. squilla populations prevented by too extensive gene flow from tidal areas? To wanswer this, genetic differentiation was studied by horizontal starch gel electrophoresis. Each of the two species was sampled at five sites along the European Atlantic coast. Twenty-two enzymes representing 25 loci were scored. Significant genetic heterogeneity between sites was found in both species. Thus the extensive gene flow hypothesis was not supported by the results, and non-tidal populations ought to have the possibility to adapt to local conditions.     

Sterol production and phytosterol bioconversion in two species of heterotrophic protists, Oxyrrhis marina and Gyrodinium dominans

The kinetics and efficiency of sterol production and bioconversion of phytosterols in two heterotrophic protists Oxyrrhis marina and Gyrodinium dominans were examined by feeding them two different algal species (Rhodomonas salina and Dunaliella tertiolecta) differing in sterol profiles. R. salina contains predominantly brassicasterol (≅99%) and <2% cholesterol. The major sterols in D. tertiolecta are ergosterol (45–49%), 7-dehydroporiferasterol (29–31%) and fungisterol (21–26%). O. marina fed R. salina metabolized dietary brassicasterol to produce 22-dehydrocholesterol and cholesterol. O. marina fed D. tertiolecta metabolized dietary sterols to produce cholesterol, 22-dehydrocholesterol, brassicasterol and stigmasterol. G. dominans fed either R. salina or D. tertiolecta metabolized dietary sterols to make cholesterol, brassicasterol and a series of unknown sterols. When protists were fed R. salina, which contains cholesterol, the levels of cholesterol were increased to a magnitude of nearly 5- to 30-fold at the phytoplankton-heterotrophic protist interface, equivalent to a production of 172.5 ± 16.2 and 987.7 ± 377.7 ng cholesterol per mg R. salina carbon consumed by O. marina and G. dominans, respectively. When protists were fed D. tertiolecta, which contains no cholesterol, a net production of cholesterol by the protists ranged from 123.2 ± 30.6 to 871.8 ± 130.8 ng per mg algal C consumed. Cholesterol is not only the dominant sterol, but a critical precursor for many physiologically functional biochemicals in higher animal. As intermediates, these heterotrophic protists increase the amount of cholesterol at the phytoplankton–zooplankton interface available to higher trophic levels relative to zooplankton feeding on algae directly.     

Nutrient enrichment of seagrass beds in a rhode island coastal lagoon

Seagrass and algal beds showed a variety of reponses when the water column was treated with low level additions of ammonium, nitrate and phosphate. The nutrients were added separately to 3 uniform seagrass beds of a temperature coastal lagoon during 1979 and 1980. (1) Ammonium caused the production of dense mats of free-floating green algae Enteromorpha plumosa and Ulva lactuca. It also stimulated growth in both the leaf and root-rhizome fractions of Zostera marina. This growth response in Z. marina was greater in the area where current reached 12 cm · s^-1 than in the area with little or no current. The concentration of nitrogen in the tissue did not change. In contrast, where current was lacking, Z. marina growth increase with ammonium was small, but the concentration of nitrogen in the tissue doubled over that in control plots. The growth of Ruppia maritima was inversely related to the growth of green algae in the same plots. The red alga Gracilaria tikvahiae did not grow better in ammonium, but its tissue reddened. (2) Nitrate additions enhanced the growth of the green seaweeds Enteromorpha spp. and U. lactuca, but not Z. marina or R. maritima. G. tikvahiae, when fertilized in isolation from other plants, showed a marginal response to this nutrient, and the tissue always reddened. (3) Phosphate enhanced growth in Z. marina and R. maritima exposed to moderate current. G. tikvahiae growing alone showed a small growth response to phosphate. The phosphate made no difference in the growth of the green seaweeds. (4) None of the nutrient supplements noticeably altered the species composition of either epiphytic or planktonic algae associated with the beds, although we did detect small increases in their numbers. The rapid and dense growth of green algae in nitrogen-enriched water probably limited growth of adjacent seagrasses and red algae. Because these seaweeds did not use the phosphate, it became available to other plant components. The overall floral response to nutrient addition in seagrass communities depends, therefore, upon the particular nutrient supplied, the ability of alternate species in the area to compete for that nutrient and the velocity of current in the specific area.     

Annual cycle of biomass of a threatened population of the intertidal seagrass Zostera japonica in Hong Kong

The phenology and primary productivity of a population of Zostera japonica (Aschers. & Graebn.) threatened by the construction of Hong Kong's new international airport were studied over a 12-month period. The need to conserve the population, and the small leaf size of Z. japonica rendered traditional destructive or marking techniques inapplicable for percentage cover and biomass estimation. A nondestructive method based on image analysis techniques was therefore devised for repeated estimation of percentage cover, biomass and leaf area index. This technique, which involved random quadrat sampling, photographic recording and image analysis, was able to provide data on the three parameters with acceptable precision and was cost-effective in the field. Z. japonica demonstrated a strongly seasonal cycle of vegetative growth, with different patterns for leaf density (peak in March) and overall bed area (peak in June). Total (above- and below-ground) net primary productivity was estimated at between 344 and 688 g AFDW m⁻² yr⁻¹. Percentage cover of Z. japonica was negatively correlated with total suspended solids (TSS) in the water column while total bed area was negatively correlated with water salinity. Increased sedimentation associated with the new airport project was identified as one important factor affecting the growth of the seagrass, as TSS reached the high level of ≈1 g DW l⁻¹ during the first half of the study period. Sediment traps set in the beds also recorded potential sedimentation rates at between 2.89 and 14.5 mg cm⁻² d⁻¹. This high turbidity resulted in a sharp decrease in the density of Clithon spp., the dominant grazers of epiphytic algae on Z. japonica. Effects of sedimentation and shading on growth of Z. japonica were investigated by field manipulative experiments. Experimental increase of sedimentation rate and shade both resulted in larger decreases in percentage cover and above-ground AFDW compared with the control. Received: 3 March 1997 / Accepted: 14 March 1997     

Stable isotope analysis of two species of anguilliform leptocephali (<Emphasis Type="Italic">Anguilla japonica</Emphasis> and <Emphasis Type="Italic">Ariosoma major</Emphasis>) relative to their feeding depth in the North Equatorial Current region

Anguilla japonica leptocephali are transported from their offshore spawning area to their recruitment areas in East Asia, but their depth distributions, food sources and feeding are still poorly known. This study analyzed carbon and nitrogen stable isotope ratios of leptocephali of A. japonica, Ariosoma major and Ariosoma spp., and of particulate organic matter (POM), their likely food source, at five different depths in 2004–2009. We used mixing models to show that A. japonica appeared to be feeding at depths between 5 and 50 m, but sometimes deeper. A. major appeared to have a tendency of mostly feeding at depths of 50 m or shallower. Although the A. japonica and Ariosoma spp. collected in the same area during the leptocephalus stage appeared to have different feeding ecologies possibly related to different types of POM, their different depth distributions, sizes and transport histories may also help explain these differences.