Contrasting effects of habitat structure on the recruitment and mortality of an epibiotic macroalga

Field surveys over 2 years in contiguous beds of the seagrassesZostera capricorni andPosidonia australis showed that the green algaCodium duthieae was consistently more abundant inZ. capricorni than inP. australis. In 1 year, mature plants were also more abundant at the boundary between the seagrass beds than in either bed. Field experiments and programmes of sampling were used to investigate three potential explanations for the unusual distribution of this alga: (1) that the availability of substrata suitable for attachment of the alga differed between the two seagrass beds; (2) that mortality of matureC. duthieae differed between the seagrass beds; and/or (3) that the intensity of recruitment was different in the two seagrass beds.C. duthieae plants were exclusively epibionts of the bivalveAnadara trapezia. Detailed sampling showed that the abundance ofA. trapezia was similar in both seagrass beds and that the distribution of bivalves suitable as substrata forC. duthieae plants was not obviously related to proximity to the boundary between the beds. Two experiments investigated the survival ofC. duthieae plants in each bed. In the first, matureC. duthieae plants transplanted into theP. australis bed suffered similar rates of mortality to plants which were disturbed and moved within theZ. capricorni bed or which were left undisturbed in theZ. capricorni bed. Fewer of the host bivalves were recovered from theZ. capricorni bed, however, indicating that the mechanism of mortality differed between the beds, hosts being more frequently dislodged in theZ. capricorni bed. Removal of the leaves of the seagrasses had consistently greater effects on near-bottom current velocities in theZ. capricorni bed than in theP. australis bed and significantly increased mortality ofC. duthieae in theZ. capricorni bed. Survival of plants was greater in plots of artificial leaves ofP. australis placed in theZ. capricorni bed than in plots of the naturalZ. capricorni leaves or plots where the natural leaves were removed. Most mortality in theZ. capricorni bed was due to dislodgement of the alga and its bivalve substratum. Corresponding manipulations of leaves in theP. australis bed had consistently smaller effects on survivorship of both the alga and its host. Patterns in the recruitment of the alga most clearly reflected the distribution of adults.C. duthieae recruits were 5 times more abundant in theZ. capricorni bed and at the boundary between the two beds than in theP. australis bed. The results demonstrate how habitat structure, provided by the canopy of leaves of the two species of seagrass, can have contrasting effects on the recruitment and mortality of a macroalga. In the case ofC. duthieae, it appears that the differential pattern of recruitment is the primary determinant of the distribution of adult plants.     

A molecular investigation of the genus Ecklonia (Phaeophyceae,Laminariales) with special focus on the Southern Hemisphere

Brown algae of the order Laminariales, commonly referred to as kelps, are the largest and most productive primary producers in the coastal inshore environment. The genus Ecklonia (Lessoniaceae, Phaeophyceae) consists of seven species with four species in the Northern Hemisphere and three in the Southern Hemisphere. It was recently transferred to the family Lessoniaceae based on phylogenetic analyses of nuclear and chloroplastic markers, though the type of the genus was not included and its relationship with allied genera Eckloniopsis and Eisenia remained unresolved. The present study is the first to produce a phylogeny focussed on the genus Ecklonia. It included sequences from nuclear, mitochondrial, and chloroplastic DNA, for most of the distribution range of the three current Southern Hemisphere species (Ecklonia radiata, Ecklonia maxima, and a sample of a putative Ecklonia brevipes specimen), sequences for East Asiatic species (Ecklonia cava, Ecklonia kurome, and Ecklonia stolonifera), as well as the closely related genera Eckloniopsis and Eisenia. Results confirmed E. radiata and E. maxima as two distinct species in South Africa, E. radiata as a single species throughout the Southern Hemisphere (in South Africa, Australia, and New Zealand) and East Asiatic species as a distinct lineage from the Southern Hemisphere clade. Results further pointed out a close sister relationship between Eckloniopsis radicosa and two Eisenia species (including the type species: Eisenia arborea) to the genus Ecklonia suggesting that the genera Eckloniopsis and Eisenia are superfluous.     

Patch formation by herbivorous fish in a temperate Australian kelp forest

Summary The odacid fish Odax cyanomelas feeds on the kelp Ecklonia radiata, an important component of subtidal reef habitats on the central coast of New South Wales, Australia. Herbivory by Odax has a major impact on the structure and dynamics of discrete patches within larger stands of kelp at Cape Banks. This three-year study showed that each year, between August and October, approximately the same patches of kelp were denuded by preferential feeding on the meristem and primary laminae. This coincided with a variable pulse of Ecklonia recruits to the cleared patches, thereby generating patches of a single age-class of plants. Neighbouring areas of Ecklonia forest, non cleared by Odax, consisted of larger, perennial plants, which exhibited more gradual changes in abundance. The seasonal impact of Odax appeared to be due to a change in the behaviour of female Odax during their spawning period. Observations suggested that females aggregate at traditional sites prior to spawning with territorial males in adjacent areas of kelp forest. An alternative hypothesis, that Odax preferentially attacked stands of one-year old Ecklonia plants, was rejected by a field experiment; the establishment of experimental stands of one-year old plants did not lead to increased damage due to Odax or any change in the use of space by the fish. The generality of this effect of fish herbivory is unknown, but this and other Odax species are widely distributed throughout temperate Australia, where Ecklonia is the dominant laminarian alga. The effects of pulsed herbivory by Odax is contrasted to the more continuous grazing by sea urchins in the same system. The latter herbivore has been shown to maintain areas free of Ecklonia, the long-term effects of herbivory by Odax remain unclear.     

Settlement of seaweeds on coastal structures

Coastal structures are constructed principally to protect the coast line. However, these structures also can act as artificial substrata for seaweeds. In particular, armor blocks, such as tetrapods, prove to be good algal substrata. Our field observations on the vegetation and standing crop of seaweed communities on armor blocks led us to the following conclusions: 1) Ecklonia cavacommunities grew on tetrapods that had been placed to coincide with the maturation period of E. cava. The communities have been maintained for more than four years. 2) The standing crop of seaweeds on an offshore breakwater composed of tetrapods placed seven years before was almost the same as that found under natural conditions. 3) Variations in roughened surfaces (pebbles or scores and grooves 3 or 46 mm wide) did not affect the growth of Sargassum spp. However, a greater number of Ecklonia stolonifera plants were observed attached to roughened surfaces compared to smooth surfaces.     

Recent changes in the seaweed community of Western Prince Edward Island: implications for the seaweed industry

Prior to 1980, seaweed beds of western Prince Edward Island, Canada, were dominated by Chondrus crispus Stackhouse, associated with 37 other species of common algae. Furcellaria lumbricalis (Huds.) Lamour. occurred in 4.6% of the samples from the Pleasant View bed but never reached measurable levels of cover. By 1991 F. lumbricalis was reported in 67% of the samples from this bed. Furcellaria lumbricalis biomass was negatively correlated with C. crispus biomass and positively correlated with Phyllophora Grev. spp. biomass. F. lumbricalis biomass and cover peaked in subtidal areas below 4 m at 359 ± 299 SD fresh g 0.25 m^–2. Harvesters avoid concentrations of F. lumbricalis, resulting in a 5% content of this species in the commercial harvest. Systematic harvesting located concentrations of F. lumbricalis in seven of 10 commercial beds. Proposed causes for change in species dominance from C. crispus to F. lumbricalis range from general environmental change, natural succession to excessive human perturbation.     

Extracellular secretion of phenolic substances from living brown algae

The chemical structures of ultraviolet (UV)- absorbing substances secreted from the healthy living brown algae, Eisenia bicyclis and Ecklonia kurome, were demonstrated. The living activity of algal cells was critically examined using a confocal laser-scanning microscope after incorporation of fluorescein diacetate (FDA) into the cells. Using thin-layer chromatography (TLC), reversed-phase three-dimensional high-performance liquid chromatography (RP-3D-HPLC) and gas chromatography-mass spectrometry (GC-MS), it was found that the UV-absorbing substances (λ_max 265–270 nm) secreted from the living brown algae mainly consisted of the three monomeric bromophenols, 2,4-dibromophenol, 2,4,6-tribromophenol and dibromo-iodophenol, but not phloroglucinol or phlorotannins. The other minor compounds detected in the secretions were as follows: benzothiazole, fatty acids (14:1, 16:0 and 18:0 acids), franesol, 3-hydroxy-2,4,4-trimethylpentyl 2-methylpropanoate and squalene. Exudation of phloroglucinol and phlorotannins was ascertained to take place only after the cell death of these algae. These results indicate that, whilst the algae are alive, polymeric phlorotannins are strictly kept within the algal body, and only monomeric bromophenols are secreted into the seawater medium.     

Relationships between bed age,bed size,and genetic structure in Chesapeake Bay (Virginia,USA) eelgrass (<Emphasis Type="Italic">Zostera marina</Emphasis> L.)

Genetic structure and diversity can reveal the demographic and selective forces to which populations have been exposed, elucidate genetic connections among populations, and inform conservation strategies. Beds of the clonal marine angiosperm Zostera marinaL. (eelgrass) in Chesapeake Bay (Virginia, USA) display significant morphological and genetic variation; abundance has fluctuated widely in recent decades, and eelgrass conservation is a major concern, raising questions about how genetic diversity is distributed and structured within this metapopulation. This study examined the influence of bed age (<65years versus<6years) and size (>100ha versus<10ha) on morphological and genetic (allozyme) structure and diversity within Chesapeake Bay eelgrass beds. Although both morphology and genetic diversity varied significantly among individual beds (F _ST=0.198), neither varied consistently with bed age or size. The Chesapeake eelgrass beds studied were significantly inbred (mean F _IS=0.680 over all beds), with inbreeding in old, small beds significantly lower than in other bed types. Genetic and geographic distances within and among beds were uncorrelated, providing no clear evidence of isolation by distance at the scale of 10's of km. These results suggest that local environmental conditions have a greater influence on plant morphology than do bed age or size. They support the hypotheses that eelgrass beds are established by multiple founder genotypes but experience little gene flow thereafter, and that beds are maintained with little loss of genetic diversity for up to 65 years. Since phenotypic and genotypic variation is partitioned among beds of multiple ages and sizes, eelgrass conservation efforts should maximize preservation of diversity by minimizing losses of all beds.     

Community dynamics of intertidal soft-bottom mussel beds over two decades

Macrozoobenthos communities in the North Sea showed pronounced changes over the past decade in relation to an increasing number of invasive species and climate change. We analysed data sets spanning 22 years on abundance, biomass and species composition of intertidal soft bottom mussel beds near the island of Sylt (German Bight) in the Northern Wadden Sea, based on surveys from 1983/1984, 1990, 1993 and from 1999 to 2005. Mussel bed area and blue mussel biomass decreased, and a change in the dominance structure in the associated community comparing 1984 to mid-1990s with the period from 1999 to 2005 was observed. Coverage of the mussel beds with the algae Fucus vesiculosus decreased since the end of the 1990s. Within the study period biomass and densities of the associated community increased significantly. Dominance structure changed mainly because of increasing abundances of associated epibenthic taxa. Apart from the Pacific oyster Crassostrea gigas all other alien species were already present in the area during the study period. Community changes already started before Pacific oysters became abundant. An attempt is made to evaluate effects on the observed changes of decreasing mussel biomass, ageing of mussel beds, decreasing fucoid coverage and increasing abundances of invader. All four factors are assumed to contribute to changing community structure of intertidal mussel beds.     

Documentation of sites of intertidal blue mussel (Mytilus edulis L.) beds of the Lower Saxonian Wadden Sea, southern North Sea (as of 2003) and the role of their structure for spatfall settlement

Field surveys (dating back to 1950) and aerial photograph series (dating back to 1966) were evaluated to determine sites of intertidal blue mussel (Mytilus edulis) beds at the Wadden Sea coast of Lower Saxony. Maps were prepared indicating sites of blue mussel beds during the last decades. A table gives additional information on the presence (or absence) of blue mussel beds at each site at the time of large-scale surveys. Altogether 187 sites of M. edulis beds were recorded in the investigation area. In spring 1996, there were still only 19 sites where mussel beds still occurred, although at 51 sites residual mussel-bed structures were present, e.g. shell bases of former beds or protruding patches (which had been occupied by M. edulis before the beds vanished) and open spaces. At that time, the majority of the sites contained neither mussel beds nor mussel-bed structures. The analysis of recent data confirmed that mussel larvae have preferred to settle in sites of present mussel beds and sites with bases of former mussel beds. There was no preferential selection of one of these categories (settled beds vs. shell bases). On the other hand, the presence of mussel beds or mussel bed structures is not obligatory for settlement, because sites without those structures were also re-settled by the spatfall in 1996, even though on a smaller scale.     

A field test of porous carbonated blocks used as artificial reef in seaweed beds of <Emphasis Type="Italic">Ecklonia cava</Emphasis>

Marine forests are the main primary producer in coastal waters, supplying food to fish and shellfish as well as providing their spawning and growing sites. It is important to conserve marine forest in order to protect coastal marine environments. A “Marine Block” comprised of steelmaking slag particles combined by CaCO₃ has several merits: (1) solidification of CO₂, (2) stability and safety, (3) porous materials covered with CaCO₃, (4) harmony with the seabed environment, (5) recycling of iron by-products, and (6) the ability to be mass produced in coastal ironworks. The objectives of this study were to observe the algal succession, the growth of Ecklonia cava and other attached organisms on Marine Blocks and concrete blocks compared to natural seaweed beds. In November 2001, five 1 m³ Marine Blocks and five 1 m³ concrete blocks were installed close to E. cava beds around the coastal frontage of Jogashima at the mouth of Tokyo Bay. Scuba-diving observations were continued until April 2006. In the field, a large number of seaweed and animal species were observed on the Marine Blocks, and it was found that the speed of succession, the plant length and the wet weight of E. cava were greater on Marine Blocks than on concrete blocks. The main advantages of Marine Blocks are considered to be the recycling of steelmaking slag, the absorption and solidification of atmospheric CO₂, and the conservation of coastal environments.     

<Emphasis Type="Italic">Eisenia bicyclis</Emphasis> bed coverage off Oshika Peninsula,Japan, in relation to sporophyte survival and <Emphasis Type="Italic">Strongylocentrotus nudus</Emphasis> abundance

It has recently been reported that the Eisenia bicyclis beds on the coast of Tomarihama in the Oshika Peninsula in Miyagi Prefecture (northern part of Japan) have been progressively receding from the offshore side of the beds every year. In this study, to determine the factors causing the change in the distribution of E. bicyclis beds as well as the extent of the change, an ecological survey of E. bicyclis and grazing animals was conducted. A 150 m-long fixed line was set up in the rocky coastal area on the Oshika Peninsula in Miyagi Prefecture, and the number of E. bicyclis individuals inhabiting a 1-m width was then measured. In addition, the numbers of individuals of sea urchins (Strongylocentrotus nudus) and abalone (Haliotis discus hannai) were measured. Although the E. bicyclis bed extended to the 64-m point on the fixed line at the time of the survey in August 1999, by July 2001, its lower limit was at the 53-m point; thus, a retreat of the outer edge of the bed of 11 m was observed in 2 years. The density of adult E. bicyclis sporophytes in the bed was reduced from 1.08 to 1.80 individuals (ind.) m⁻², respectively. Juvenile sporophytes were also observed, but most of those that had settled near the lower limit of the bed failed to survive to become adult sporophytes. There were few sea urchins, the main grazing animal on the site, inside the E. bicyclis bed (3.5 ind. m⁻² in October 2001) while there were many near the bed lower limit and on the deep-site side (12.9 ind. m⁻² in October 2001). These results indicate that the high grazing pressure of sea urchins reduced the survival of juvenile sporophytes near the bed lower limit, thus impeding the recruitment of juvenile sporophytes to replace adult sporophytes that died due to senescence, storm action and/or grazing.     

Effects of stock loading and planting distance on the growth and production ofGracilaria chilensis in rope culture

Through experimental manipulation of stock loading and planting distances in rope culture, this study evaluates the hypothesis that optimum density for growth and production ofGracilaria chilensis vary within a bed. Small packages ofGracilaria disposed at short distances exhibit higher growth and production rates in the landward ends of ropes than larger packages of seaweed disposed at longer distances. In these experiments, the landward end of ropes faces estuarine conditions, with reduced salinities and increased sediment loads during low tides. The interacting effects of stocks loading and position in the rope can affect growth and production rates so much as to nullify the 3.5-fold summer increments in production that occur in these estuarine beds of southern Chile. It is concluded that through manipulation of planting distances and stock loading, the production of this species could be increased up to 35% in some areas.     

Comparison between community structures of fishes in <Emphasis Type="Italic">Enhalus acoroides</Emphasis>- and <Emphasis Type="Italic">Thalassia hemprichii</Emphasis>-dominated seagrass beds on fringing coral reefs in the Ryukyu Islands,Japan

To clarify differences in community structures and habitat utilization patterns of fishes in Enhalus acoroides- and Thalassia hemprichii-dominated seagrass beds on fringing coral reefs, visual censuses were conducted at Iriomote and Ishigaki islands, southern Japan. The numbers of fish species and individuals were significantly higher in the E. acoroides bed than in the T. hemprichii bed, although the 15 most dominant fishes in each seagrass bed were similar. Cluster and ordination analyses based on the number of individuals of each fish species also demonstrated that fish community structures were similar in the two seagrass beds. Species and individual numbers of coral reef fishes which utilized the seagrass beds numbered less than about 15% of whole coral reef fish numbers, although they comprised about half of the seagrass bed fishes. Of the 15 most dominant species, 5 occurred only in the two seagrass beds, including seagrass feeders. Ten other species were reef species, their habitat utilization patterns not differing greatly between the two seagrass beds. Some reef species, such as Lethrinus atkinsoni and L. obsoletus, showed ontogenetic habitat shifts with growth, from the seagrass beds to the coral areas. These results indicate that community structures and habitat utilization patterns of fishes were similar between E. acoroides- and T. hemprichii-dominated seagrass beds, whereas many coral reef fishes hardly utilized the seagrass beds.     

The microdistribution of mayflies (Ephemeroptera) in myriophyllum beds in Pennington Creek,Johnston County,Oklahoma

I studied the population densities and the microdistributions of the four most abundant mayfly (Ephemeroptera) nymphs in Myriophyllum heterophyllum beds in Pennington Creek, Johnston County, Oklahoma, from May to December, 1977. The section of the stream studied has relatively constant flow rates and temperatures throughout the year due to continuous inflow from groundwater. This uniform physical environment allows the plant beds to grow throughout the year and provide a permanent habitat. The four mayflies are Tricorythodes fictus (Tricorythidae), Caenis delicata (Caenidae), Baetis favistriga and B. quilleri (Baetidae). All four species are of similar size and feed by scraping periphyton from the surface of the Myriophyllum. T. fictus and the two species of Baetis have overlapping microdistributions in the leafy, upper one-half of the plant bed. T. fictus and C. delicata have overlapping microdistributions in the lower portion of the plant bed and roots. Each species has similar population densities where their distributions overlap. There is no significant selection for position with respect to current flow in the Myriophyllum for any of the species. Food and space are always abundant, especially in the top one-half of the Myriophyllum. There are no major changes in these trends over the year.     

Phycological research in the development of the Chinese seaweed industry

The term seaweed industry is employed in a broad sense and includes production both of commercial seaweeds such as Laminaria and Porphyra by phycoculture and of processed seaweed products, such as algin, agar and carrageenan.Before the founding of the People's Republic, China had a very insignificant seaweed industry, producing small quantities of the purple laver Porphyra and the glueweed Gloiopeltis by the primitive rock-cleaning method and the kelps Laminaria and Undaria by the primitive stone-throwing method, both aiming at enhancing the growth of the wild seaweeds. Also, a small quantity of agar was manufactured by the traditional Japanese method of gelling, freezing, thawing and drying the product. The small production was not sufficient to meet the demand of the Chinese people who for ages have appreciated seaweeds and their products for food. Therefore, large quantities of seaweeds and seaweed products had to be imported from various countries, for instance, Eucheuma and Gracilaria from Indonesia and other southeastern Asian countries, Laminaria and agar from Japan, even Porphyra from the USA. Annual Laminaria import from Japan generally amounted to over 10 000 tons and in some years approached 20 000–30 000 tons. Some quantities of the glueweed Gloiopeltis and the vermifuge weed Digenea simplex were exported, mainly to Japan.Since the founding of the People's Republic of China in October, 1949, China has exerted efforts to build up a self-supporting seaweed industry. Now after a lapse of 30-some years, a sizable seaweed industry has been developed. China is now able to produce by phycoculture more than one million tons of fresh seaweeds, including Laminaria, Undaria, Porphyra, Eucheuma, Gracilaria etc. and several thousand tons of seaweed extracts, including algin, agar, carrageenan, mannitol and iodine. At present, China still imports some quantities of seaweeds and seaweed products from various countries but is able to produce sufficient quantities to meet the people's need and even to export some quantities of the seaweeds Laminaria, Undaria and Porphyra and the seaweed products algin and mannitol.At the Tenth International Seaweed Symposium, I presented a paper on the Marine Phycoculture of China, in which I emphasized on the methods of cultivation (Tseng 1981b). Therefore I would like to take this opportunity to supplement the last lecture by presenting a paper on the role of phycological research in the development of China's seaweed industry.     

Spatio-temporal distribution patterns of the invasive macroalga Sargassum muticum within a Danish Sargassum-bed

Sargassum muticum was first observed in Scandinavia in Limfjorden (Denmark) in 1984, where it is now the most abundant and conspicuous macroalga. Despite the ecological importance of Sargassum, few studies have described seasonal patterns within Scandinavian Sargassum beds. We quantified the dynamics of macroalgae among years and seasons along a depth transect through a typical Sargassum bed in Limfjorden. The annual investigations (summer transects 1989–1999) showed a gradual increase in the dominance of Sargassum, especially at the 2–4-m depth interval. Significant seasonal dynamics in macroalgal abundance and assemblage structure were observed in this depth interval; the mean cover of Sargassum varied from ca. 5% (autumn and winter) to 25% (mid-summer). In comparison, encrusting algae had high and relatively stable covers throughout the year (ca. 20%). Other perennial macroalgae had low mean covers (<2%) characterized by a few patches of higher abundances. Except from a spring bloom, filamentous algae had low covers throughout the year. Within this relatively uniform bed, Sargassum abundance was positively related to boulders >10 cm in diameter and species richness was negatively correlated to depth and stones <10 cm in diameter, and non-correlated to other algal form-groups or grazer densities. Thus, in Limfjorden, the distribution of Sargassum is determined by large- (>6 m) and small-scale (<1 m) depth differences where low light limits Sargassum at depth, physical disturbance and sediment stress limits Sargasum in shallow waters, and the presence of stable boulder substratum facilitate Sargassum. Competition for space from other macroalgae and herbivory are probably of minor importance.     

Integrated seaweed cultivation on an abalone farm in South Africa

Land-based abalone aquaculture in South Africa, based on the local species Haliotis midae, started in the early 1990s and has grown rapidly in the last decade, with 13 commercial farms now producing over 850 t per annum. Over 6,000 t per annum of kelp Ecklonia maxima are now harvested for this purpose, and some kelp beds are reaching maximum sustainable limits. Research into seaweed aquaculture as feed (Ulva and some Gracilaria) for abalone started in the late 1990s on the southeast coast (where there are no kelp beds) using abalone waste water. A growing body of evidence suggests that a mixed diet of kelp plus other seaweeds can give growth rates at least as good as compound feed, and can improve abalone quality and reduce parasite loads. A pilot scale Ulva lactuca and abalone integrated recirculation unit using 25% recirculation was designed and built on the south west coast of South Africa using one 12,000-L abalone tank containing 13,200 15 ± 2.5 g abalone, connected to two 3,000-L seaweed tanks containing an initial starting biomass of 10 kg of seaweed, replicated 3 times. In an 18-month period, there were no significant differences in abalone health or growth rates, sediment build up and composition, mobile macro fauna densities and species between the recirculation or the flow-through units. Transfer of oxygen generated by the seaweeds to the abalone tanks was poor, resulting in the recirculated abalone tanks having lower (33%) dissolved oxygen concentrations than a comparable flow-through abalone unit. Seaweed nutrient content and specific growth rates in the units were comparable to seaweeds cultivated in fertilized effluent (SGR = 3.2 ± 3.4%.day⁻¹; Yield = 0.2 ± 0.19 kg.m².day⁻¹). Indications were that at this low recirculation ratio the seaweeds in the units were nutrient limited and that there were no negative effects to the abalone being cultivated in such a recirculation unit at this recirculation ratio.     

The role of the blue mussel Mytilus edulis in the cycling of nutrients in the Oosterschelde estuary (The Netherlands)

The fluxes of particulate and dissolved material between bivalve beds and the water column in the Oosterschelde estuary have been measured in situ with a Benthic Ecosystem Tunnel. On mussel beds uptake of POC, PON and POP was observed. POC and PON fluxes showed a significant positive correlation, and the average C:N ratio of the fluxes was 9.4. There was a high release of phosphate, nitrate, ammonium and silicate from the mussel bed into the water column. The effluxes of dissolved inorganic nitrogen and phosphate showed a significant correlation, with an average N:P ratio of 16.5. A comparison of the in situ measurements with individual nutrient excretion rates showed that excretion by the mussels contributed 31–85% to the total phosphate flux from the mussel bed. Ammonium excretion by the mussels accounted for 17–94% of the ammonium flux from the mussel bed. The mussels did not excrete silicate or nitrate. Mineralization of biodeposition on the mussel bed was probably the main source of the regenerated nutrients.From the in situ observations net budgets of N, P and Si for the mussel bed were calculated. A comparison between the uptake of particulate organic N and the release of dissolved inorganic N (ammonium + nitrate) showed that little N is retained by the mussel bed, and suggested that denitrification is a minor process in the mussel bed sediment. On average, only 2/3 of the particulate organic P, taken up by the mussel bed, was recycled as phosphate. A net Si uptake was observed during phytoplankton blooms, and a net release dominated during autumn. It is concluded that mussel beds increase the mineralization rate of phytoplankton and affect nutrient ratios in the water column. A comparison of N regeneration by mussels in the central part of the Oosterschelde estuary with model estimates of total N remineralization showed that mussels play a major role in the recycling of nitrogen.     

Reedbed structure and habitat preference of reed passerines during the post-breeding period

Post-breeding habitats of nine passerine species were studied in the permanently flooded reed beds of ponds near the village Pusté úlany (SW Slovakia) in 2002. Structural features of reed beds and the abundance of all birds studied were sampled at two sites and eight study plots. The reed bed structure and abundance of four passerines differed considerably between the individual study plots. The variations in vegetation structure were also caused by winter reed burning in the SE part of study area. Stems in burnt reed beds were smaller and thinner than unburnt. Abundance of the reed warbler and to a lesser extent sedge warbler was higher in tall reed beds, while the bearded tit preferred thick reed stems. Position of reed beds along the shore — open water gradient was an important factor in terms of abundance at the study plots in the case of the reed bunting. This species showed a strong preference for the reed bed edge with open water. Horizontal distribution of other species seems to be random, however, the birds with the exception of Savi’s warbler preferred the upper stratum of reed bed in vertical distribution. Our field data corroborate with those found in experimental studies concerning with ecomorphological characteristics of birds. Some inconsistencies, however, could also be caused by other factors (e.g. foraging preferences). Comparison of breeding and post-breeding habitat requirements of the studied bird species confirmed a more or less random distribution of the majority of species in the post-breeding period. Some variation was found in the cases of only the reed warbler and bearded tit.     

Current state of seaweed resources in Spain

Commercial seaweeds in Spain are harvested on the north and northwest coasts. They are mainly agarophytes and carrageenophytes (Gelidium spp. and some Irish moss-like species, respectively), although some Phaeophyceae species (Fucus spp. and Laminaria spp.) are also exploited for alginates. No industrial seaweed cultivation is carried out in Spain at present. Spain's total commercial seaweed harvest can be estimated at 6,528 ± 2,076 t dry wt year^–1. Gelidium spp. are by far the most harvested, attaining 5,135 ± 1,761 t dry wt y^–1. To date, Spain's commercial seaweed harvest is low compared with that of other countries, but data on field standing crops and productivities of commercial taxa suggest that harvesting could be increased greatly.