Coralline algal facies and their palaeoenvironments in the Late Eocene of Northern Italy (Calcare di Nago, Trento)

Summary The Calcare di Nago is a carbonate unit of Middle-Late Eocene (Bartonian and Priabonian) age which is well exposed at the north-eastern end of Lake Garda, on the western margin of the Lessini Shelf (Southern Alps). This unit is highly fossiliferous as far as the coralline red algae and large foraminifera are concerned. Corals, bryozoans, echinoderms, and molluscs are also present. The present study deals with the relationships among the coralline taxa, the coralline growth-forms, and their facies development in the Priabonian part of the type section of the Calcare di Nago. The taxonomic investigation led to the identification of 15 coralline red algal species belonging to 7 non-geniculate and 2 geniculate genera. One species of Peyssonneliacean (red alga) and one of Halimedacean (green alga) were also recognized. The quantitative and qualitative analyses based on coralline red algae and large foraminifera enabled five facies to be distinguished: Algal crust-branch rudstone, Algal/Discocyclina packstone, Coralalgal boundstone, Rhodolith mound wacke/packstone, and Rhodolith pavement. According to the coralline assemblages, coralline growth-forms, and large foraminiferal associations, the five facies reflect solid and soft substrate types. Some of these facies are dominated byin situ rhodoliths, others by reworked algal debris. In the architecture of an interpreted prograding carbonate ramp, shallow water facies are dominated by members of the subfamily Mastophoroideae, while deeper water facies are dominated by those of the subfamily Melobesioideae and family Sporolithaceae. There is a significant increase both in size and in constructional voids of the rhodoliths with depth. A concomitant decrease in algal species diversity with depth has been also recognized. LargeDiscocyclina assemblages are localized across the inner and mid ramp boundary.Pellatispira andBiplanispira are present only in the uppermost mid-ramp.Nummulites, Assilina, andSpiroclypeus are dominant together with small orthophragminids both in the mid- and uppermost outer ramp facies.     

Direct observations on the sublittoral marine algae of Argyll,Scotland

Summary The distribution with depth of sublittoral marine algae has been investigated at 11 sites on the west coast of Scotland. More than half of the species found were confined to the sublittoral zone. The greatest variety of species was found in shallow water at sites sheltered from excessive turbulence. With increasing depth the number of species found steadily decreased. There was no evidence of a specifically distinct algal flora confined to deeper water. No algae were found deeper than 36 m below ELWS at any site, but the lower limit of algal growth was reduced to 9 m in a turbid water loch and to 3 m at a site where the herbivorous echinoderm Ophiocomina nigra was abundant. Fairly distinct algal communities were found on different substrates. The major communities recognised underwater were algae attached to stable substrata, algae attached to unstable substrata and epiphytic on other algae, especially on the stipes of Laminaria hyperborea. On stable rock in areas subject to water movement L. hyperborea forest was the dominant vegetation whereas on unstable substrata and in sheltered localities L. saccharina was dominant.     

Deep-water rhodolith productivity and growth in the southwestern Atlantic

Rhodolith beds are an important source of marine calcium carbonate worldwide. Dense aggregations are found in deep water in the tropical southwestern Atlantic. In order to understand the distribution of coralline algae that build rhodoliths, algae responses were measured in light levels over their depth range. Qualitative samples were obtained by dredging at 90–100 m depth 80 km offshore of Cabo Frio Island, southeastern Brazil. Histological sections indicate that Mesophyllum engelhartii (Foslie) Adey was the most frequent coralline algae in 22 sampling stations. Its lumpy and thin thallus is characterized by raised multiporate sporangial conceptacles, with cells of similar size and shape around the pore canal. Accretion growth rates were below detectable levels for two rhodolith species. Photosynthetic peak was reached at 0.5–1.5 % of the maximum surface irradiance (10–30 μmol photons m^?2 s^?1) showing an extremely narrow P–I curve of net primary production. Readings of primary production at irradiance close to saturation and lower light levels (5 μmol photons m^?2 s^?1) showed no significant difference, suggesting that rhodoliths are acclimated to restricted light ranges in deep water.     

Reef habitats and associated sessile-benthic and fish assemblages across a euphotic–mesophotic depth gradient in Isla Desecheo, Puerto Rico

Quantitative surveys of sessile benthos and fish populations associated with reef habitats across a 15–50 m depth gradient were performed by direct diver observations using rebreathers at Isla Desecheo, Puerto Rico. Statistically significant differences between depths were found for total live coral, total coral species, total benthic algae, total sponges and abiotic cover. Live coral cover was higher at the mid-shelf (20 m) and shelf-edge (25 m) stations, whereas benthic algae and sponges were the dominant sessile-benthic assemblage at mesophotic stations below 25 m. Marked shifts in the community structure of corals and benthic algae were observed across the depth gradient. A total of 119 diurnal, non-cryptic fish species were observed across the depth gradient, including 80 species distributed among 7,841 individuals counted within belt-transects. Fish species richness was positively correlated with live coral cover. However, the relationship between total fish abundance and live coral was weak. Abundance of several numerically dominant fish species varied independently from live coral cover and appeared to be more influenced by depth and/or habitat type. Statistically significant differences in the rank order of abundance of fish species at euphotic vs mesophotic stations were detected. A small assemblage of reef fishes that included the cherubfish, Centropyge argi, sunshine chromis, Chromis insolata, greenblotch parrotfish, Sparisoma atomarium, yellowcheek wrasse, Halichoeres cyanocephalus, sargassum triggerfish, Xanthichthys ringens, and the longsnout butterflyfish, Chaetodon aculeatus was most abundant or only present from stations deeper than 30 m, and thus appear to be indicator species of mesophotic habitats.     

Patterns of distribution and abundance of large brown algae and invertebrate herbivores in subtidal regions of northern New Zealand

The distribution and abundance of subtidal laminarian and fucoid algae, echinoids, and four species of herbivorous gastropods were assessed. Line transects and stratified random quadrats were used, encompassing different depths and seven localities ranging over 1000km in northern New Zealand. This sampling was carried out to form a quantitative basis on which to pose hypotheses for later experimental work. Several patterns were apparent, both within and between localities. Fucoid algae were characteristic of depths between 0–6 m, while laminarian algae had peaks of abundance at 3–6 m and from 10–17 m. Intermediate depths were characterized by areas bare of large brown algae, with high densities of echinoids and gastropods. This pattern of a bimodal depth distribution of algae and abundant grazers at intermediate depth was a consistent feature of the northern coastal sites and the semi-exposed offshore islands. Size—frequency distributions for algae at these sites showed that the largest fucoids generally occurred in very shallow water, while the dominant laminarian alga tended to have larger individuals at depths of 10–14 m. In all cases, the biomass distribution with depth of the algal species mirrored their numerical abundance.

Differences between localities were associated both with environmental factors and the unique algal floras of some offshore islands. Echinoids and gastropods were rare at the two most exposed localities, which were also at the extremes of the geographic range. At these sites, depths between 6–10 m were occupied by large brown algae. Deep-water fucoid algae occurred at two offshore localities but the predominant species at one of them was peculiar to that island group.

Because of the general consistency between localities, and the few exceptions to these patterns, hypotheses are considered which could be tested on a local scale to account for echinoid and algal abundances and depth distributions. Our results indicate that the chief questions concerning the structuring of subtidal algal communities are demographic in nature and that an assessment of the status of individual organisms, rather than biomass or productivity, is the appropriate approach to resolve these questions.     

空间层次及人工基质对着生藻类建群特性的影响

为全面了解着生藻类在建群中群落变化的生态学特性,揭示着生藻类的建群规律,在以丝状藻类为优势藻的生态塘中,采用花岗岩和瓷砖为附着材料,设置水体底部和中部为附着位点,进行频次为10d的采样分析。结果表明,生态塘中共检出8门73属117种着生藻类,其中以硅藻、蓝藻、绿藻为优势类群。同时不同人工基质和不同空间层次条件下着生藻类的建群特征较一致,早期以单细胞硅藻如舟形藻(Navicula sp.)、脆杆藻(Fragilaria sp.)、曲壳藻(Achnanthes sp.)等为优势,后期以丝状藻类如鞘丝藻(Lyngbya sp.)、颤藻(Oscillatoria sp.)、伪鱼腥藻(Pseudanabaena sp.)等为优势;研究结果发现不同人工基质(花岗岩和瓷砖)对着生藻类的种类组成、细胞密度、生物量和藻类多样性无显著影响,花岗岩和瓷砖上附着的着生藻类具有较高的相似性;但不同的空间层次对着生藻类建群特征影响明显,水体底部具有更多的硅藻种类数,中部具有更多的绿藻,随着建群时间的发展,蓝藻比例不断增加;就生物量而言,底部的着生藻类叶绿素a显著高于水体中部,但两者的细胞密度无显著性差异;随着建群过程的发展,水体底部的着生藻类生物量达峰值所需的时间比中部更长。通过相关性分析,生态塘中着生藻类的生长主要受总磷的影响。     

Colonization of subtidal macroalgae in a fucalean-dominated algal assemblage,southwestern Australia

The influence of fucalean canopy species and dominant understory macroalgae on algal colonization was investigated to evaluate whether layering contributes to patterns in algal diversity. Patterns in recruitment were compared among total-clearing, understory-removal, canopy-removal, and undisturbed plots (plot area = 0.25 m²), using a randomized block design in depths <10 m and 10–20 m at Woody Island, Western Australia. To evaluate if propagules were available in the water column above the canopy layer, settlement plates (plate area = 0.04 m²) were deployed in depths <10 m, 10–20 m, and >20 m. A total of 198 macroalgal species was recorded. Biomass of the understory species Osmundaria prolifera Lamouroux and Botryocladia sonderi Silva was similar between canopy-removal and undisturbed plots. Diversity of macroalgae was similar in the presence and absence of a canopy layer. Taxa found in the canopy showed different patterns in recruitment. Cystoseiraceae recruited predominantly in total-clearings in both depth strata. Sargassaceae recruited most abundantly in depths <10 m. Density of canopy taxa on settlement plates was similar with depth (20–30 juveniles per plate), and juveniles were mainly Cystoseiraceae. In contrast to kelp beds or forests, patterns in algal colonization appeared to be maintained by environmental factors or processes other than the direct effects of layering in the subtidal fucoid-dominated assemblages at Woody Island. Handling editor: K. Martens     

新疆古尔班通古特沙漠南部沙垄不同部位藻类的垂直分布特征

 通过2005和2006年夏季两次对新疆古尔班通古特沙漠南部荒漠土壤中藻类植物进行采样、培养、鉴定和生物量测定,分析了该区沙漠沙垄不同部位藻类物种组成、优势种和生物量的垂直变化。结果表明：1）在不同地貌部位的不同层次中,藻类的种类组成均有所差异。2）藻类优势物种主要集中在0～2 cm层次中,2 cm以下的土层中藻类细胞很少,绝大部分土层以具鞘微鞘藻(Microcoleus vaginatus)为第一优势种,有些部位的土层中第一优势种是伪双点颤藻(Oscillatoria pseudogeminata),如垄间低地和迎风坡的1～2 cm中。此外,大型集胞藻（Synechocystis crassa）、舟形藻 (Navicula sp.)、卵圆双眉藻(Amphora ovalis)数量也较多。3）除垄顶外,藻类生物量在其它3个部位不同层次的差异呈极显著水平（p<0.01）,生物量由表及里锐减;在同一土层,藻类生物量基本表现为垄间低地最高,迎风坡与背风坡居中, 垄顶最低,不同地貌部位对藻类生物量影响呈显著或极显著水平。     

POPULATION PARAMETERS AND PHOTOSYNTHETIC CAPABILITIES OF CORALLINE RHODOLITHS: BIOGEOGRAPHIC IMPLICATIONS

Rhodoliths are bed forming, unattached coralline algae that incorporate large quantities of carbonate into their thalli in oceans worldwide. Lithophyllum margaritae and Neogoniolithon tricotomun are common Rhodoliths from the Gulf of California, however, little is known about their biogeographic distribution, and their ecological and physiological characteristics. As a consequence, the objective of this study was to define the temporal population dynamics, growth rates and photosynthetic capabilities of L. margaritae and N. tricotomun from the Gulf of California. The Gulf of California is characterized by annual temperature fluctuations that can exceed 20° C. Rhodolith beds exceeding one km in length were observed from the intertidal zone to a depth of approximately 20 m at the southern Gulf. Percent cover estimates for spherical unattached individuals of L. margaritae and N. tricotomun ranged from 10 to 100%. Growth rates were determined by staining the rhodoliths with Alizarine red and evaluating the linear increment of calcium carbonate after an incubation period in the field. Annual growth rates averaged 1.8 mm year⁻¹ in L. margaritae and 3.4 mm year⁻¹ in N. tricotomun , however, growth rates varied seasonally. The photosynthetic response of both species as a function of temperature was evaluated in the laboratory from 10° to 30° C. Maximum net photosynthetic rates peaked at approximately 25 to 30° C in both species, suggesting that maximum growth rates occur during the warmest months of the year. These results lead to a better understanding of the biogeographic ranges of subtidal coralline algae.     

Structure of Periphytic Community on Artificial Substrata: Influence of Depth, Slide Orientation and Colonization Time in Karstic Lake Visovačko, Croatia

This study determined the potential for periphyton growth on artificial surfaces using plexiglas slides exposed for different lengths of time, orientations and depths in the lower, lentic area of River Krka during the summer of 1995. During the research period, 88 taxa were identified. The most abundant group was Bacillariophyceae with 57 taxa. Species Cyclotella comta (Ehr.) Kutz., Achnanthidium affine (Grunow) Czarn., Cocconeis placentula Ehr. and Eolimna minima (Grunow) Lange-Bertalot were present in all samples. The results show significant differences in abundance, number and composition of algae between growth surfaces and depths. The number of species present shows statistically significant differences with respect to all three variables (days of colonization, depth and orientation), while the total number of cells per square centimeter varies with respect to the days of colonization and the depth only. The highest growth potential was observed after 60 days of exposition on the upper side of the horizontally placed surface at a depth of 1 m. During the same exposition time, the lowest growth potential was noted on the vertical slide at a depth of 18 m. Physical and chemical parameters and their relations to algal periphyton assemblages were also monitored.     

DISTRIBUTION AND RECRUITMENT OF SUBTIDAL GENICULATE CORALLINE ALAGE1

Geniculate coralline algae are common members of kelp forest communities. The structure provided by their stiff branches greatly influences the abundance and species composition of benthic animals and can affect associated algae by inhibiting recruitment, but the branches are themselves substrate for a large number of other taxa. However, other than qualitative observations, little is known about the within-site distribution, recruitment, and growth of these algae. We examined the distribution of the dominant corallines at a subtidal site in central California. Abundances of Calliarthron tuberculosum (Post. & Rupr.) Dawson, Bossiella californica ssp. schmittii (Manza) Johans., Calliarthron cheilosporioides Manza, Corallina vancouveriensis Yendo, and unidentifiable juveniles were determined at depths of 10, 15, and 20 m and on horizontal rock, vertical rock, and cobble. Calliarthron tuberculosum was most abundant (≤39% cover) at all depths, growing primarily on horizontal surfaces. Vertical surfaces and cobbles were dominated by B. californica ssp. schmittii (40 and 15% cover, respectively). These two most abundant species had the highest cover at 15 m. Calliarthron cheilosporioides and C. vancouveriensis were relatively rare (<1% cover) and generally grew on horizontal rocks and at shallower depths. Unidentified juveniles were also rare and occurred mainly on horizontal rocks and cobbles at 20 m. The settlement and growth rates of coralline crusts and the initiation and growth rates of young erect fronds from these crusts were determined in clearings made in the spring and fall at the three depths. Crust densities and diameters were highest at 10 m and in spring clearings. Settlement and growth tended to decrease with increasing depth. Trends were similar in fall clearings, but initial settlement was lower. Initiation and growth of fronds decreased with depth and were also higher in fall clearings. These variations in depth and substrata distribution, as well as settlement and growth, suggest there is considerable variation in the population biology between species in this group of subtidal plants.     

In situ study of relative electron transport rates in the marine macroalga <Emphasis Type="Italic">Fucus vesiculosus</Emphasis> in the Baltic Sea at different depths and times of year

The brown alga Fucus vesiculous is one of the few marine species in the Baltic Sea. Fucus vesiculosus shows high morphological and physiological variability as a response to its environmental conditions. The salinity in the Baltic Sea is 4–5 psu, compared to 35 psu in the Atlantic. Photosynthesis of algae is usually measured after collection and transportation to constant culture conditions. However, in this study, relative photosynthetic electron transport rates, calculated from chlorophyll a fluorescence parameters were compared in algae collected from 1 and 4 m depths by SCUBA divers. Measurements of light response curves from the same individuals of F. vesiculosus at different depths and times of the year have, to our knowledge, not been made previously. Measurements were performed on four different occasions during the spring of 2005 (25 February, 3 and 29 April, and 26 May) in the Baltic Sea, using rapid light curves generated with a Diving PAM. In addition, samples were collected for photoinhibition studies in the laboratory. The light response curves obtained in situ at 1 and 4 m depths for F. vesiculosus showed lower values of light saturation with depth. When algae from 1 and 4 m depths were exposed to high irradiances of photosynthetically active radiation (1,400 μmol photons m⁻² s⁻¹), algae from 1 m depth showed a higher degree of photoinhibition in comparison to algae from 4 m depth.     

The distribution of seagrasses in Dominica, Lesser Antilles

Seagrass beds are the largest organism-built marine habitat in Dominica, yet have only been surveyed since 2007. Standardized examinations along a depth gradient between 0 and 24 m, focusing on magnoliophyte species composition and benthic cover of shoots at 17 seagrass bed sites, were carried out between September 10 and December 7, 2008. The Cymodoceaceae Syringodium filiforme (Kuetzing 1860) and Halodule wrightii (Ascherson 1868), as well as the Hydrocharitaceae Halophila decipiens (Ostenfeld 1902), H. stipulacea (Fosskal & Ascherson 1867) and Thalassia testudinum (Banks ex K?nig 1805) displayed distinct regional and horizontal distribution patterns. Syringodium filiforme is the island's dominant seagrass along the western and northern coasts, occurring at depths between 2 and 18 m and with a mean benthic cover ranging from 0.9-10% along the West coast. Along the North coast it grew between 0.2 and 1 m depth with a mean maximum benthic cover of 48.9%. Halodule wrightii grew along the North and West coasts, in depths between 1 and 14m in areas of recent and chronic disturbances. Its delicate morphology and sparse benthic cover (< 0.1%) did not constitute seagrass beds. Halophila decipiens grew along the deep, shallow and lateral margins of west coast S. filiforme beds and monospecifically in depths between 3 and 24m. Halophila stipulacea, an invasive species, was widespread along 45km of the West coast and was found in depths between 5 and 24m. Both Halophila species formed extensive beds at depths beyond the survey limit of 24m thus playing a potentially important role in the resettlement of shallow areas after storms. H. decipiens and H. stipulacea are currently the second and third most common seagrasses on the island respectively, despite their absence along the North coast. T. testudinum was confined to North coast's sheltered reef flats at depths Im or less with mean a benthic cover ranging from 2 to 76%. It grew monospecifically in the most turbulent and in the calmest locations, yet intermixed with S. filiforme in areas of moderate turbulence. Strong surge along the West coast (October 15-16, 2008), associated with Hurricane Omar, caused uprooting and burial of seagrass beds in varying degrees, in particular along the shallow margins between 2 and 10m depth. This event also demonstrated the dynamic nature of Dominica's shallow seagrass bed margins and the resistance level of individual beds to storm disturbances.     

Patterns in shallow subtidal marine assemblages along the coast of New South Wales

Shallow subtidal areas of rocky reefs in central and southern New South Wales may best be described as a mosaic of habitats, the distributions of which are seemingly related to depth, wave exposure and a number of biological processes, particularly herbivory. The Fringe habitat is generally found only in the most shallow waters. Forests of the laminarian alga Ecklonia radiata are often found at intermediate depths. In deeper, or more sheltered water, sponges, ascidians and red algae are more abundant and the abundances of sea urchins and other invertebrate grazers decline. Overlying this broad-brush pattern are patches of crustose coralline algae (the Barrens habitat), the distributions of which are not clearly related to depth. Invertebrate herbivores, and sea urchins in particular, are abundant in the Barrens habitat. The Barrens habitat was most represented at the more southern locations. At the two most northern locations, reefs were shorter in length and dominated by ascidians (Pyura species).     

Rhodolith beds are major CaCO3 bio-factories in the tropical South West Atlantic

Rhodoliths are nodules of non-geniculate coralline algae that occur in shallow waters (<150 m depth) subjected to episodic disturbance. Rhodolith beds stand with kelp beds, seagrass meadows, and coralline algal reefs as one of the world's four largest macrophyte-dominated benthic communities. Geographic distribution of rhodolith beds is discontinuous, with large concentrations off Japan, Australia and the Gulf of California, as well as in the Mediterranean, North Atlantic, eastern Caribbean and Brazil. Although there are major gaps in terms of seabed habitat mapping, the largest rhodolith beds are purported to occur off Brazil, where these communities are recorded across a wide latitudinal range (2°N-27°S). To quantify their extent, we carried out an inter-reefal seabed habitat survey on the Abrolhos Shelf (16°50'-19°45'S) off eastern Brazil, and confirmed the most expansive and contiguous rhodolith bed in the world, covering about 20,900 km(2). Distribution, extent, composition and structure of this bed were assessed with side scan sonar, remotely operated vehicles, and SCUBA. The mean rate of CaCO(3) production was estimated from in situ growth assays at 1.07 kg m(-2) yr(-1), with a total production rate of 0.025 Gt yr(-1), comparable to those of the world's largest biogenic CaCO(3) deposits. These gigantic rhodolith beds, of areal extent equivalent to the Great Barrier Reef, Australia, are a critical, yet poorly understood component of the tropical South Atlantic Ocean. Based on the relatively high vulnerability of coralline algae to ocean acidification, these beds are likely to experience a profound restructuring in the coming decades.     

Epibiotic macroalgae on the scallop <Emphasis Type="Italic">Mizuhopecten yessoensis</Emphasis> in Peter the Great Bay,Sea of Japan

The composition and structure of the epibiotic flora of the Japanese scallop Mizuhopecten yessoensis were studied on the basis of data from long-term (1979 to 2007) observations on the scallop beds in Peter the Great Bay (Sea of Japan). In all, 52 species of macroalgae belonging to three phyla were found on the scallop shells; 3 species were new records for the benthic flora of the area studied. Red algae constituted the bulk of the species richness of algal epibionts; brown algae were represented by the lowest number of species. Species of Chlorophyta predominated in terms of biomass; species of Rhodophyta were found in lower numbers. The main form of the thallus of epibiotic algae was bushy or filamentous. The ratio of common to rare species was 30 : 70. As compared to the benthic flora, the epibiotic flora on the scallop shells was characterized by a greater number of warm-water species.     

Rhodolith formation induced by reef erosion in the Red Sea,Egypt

Along the northwestern margin of Safaga Island (Northern Bay of Safaga, Red Sea, Egypt) a small fringing reef (several hundred meters long, up to 2 m high) and small patch reefs are developed due to the local current regime which is favorable for coral growth. Corals and reef rock are encrusted by coralline algae, predominantly by branchedLithophyllum kotschyanum. Owing to destructional processes dominated by sea urchin activities, fragmentation of (1) corals, (2) reef rock, and (3) coralline algae takes place resulting in the formation of almost mono-specific, branchedLithophyllum kotschyanum rhodoliths. Rhodolith formation takes place in various reef environments: (1) in depressions on the reef flat where ellipsoidal rhodoliths develop, with interlocking and fusing branches leading to a coralline algal framework; (2) in discharge channels where smaller elongated rhodoliths occur; (3) in leeward positions between reef flat and seagrass meadows, where a dense belt of spheroidal to ellipsoidal rhodoliths is formed; scattered rhodoliths occur in adjacent seagrass beds. The formation and preservation of rhodoliths requires a complex interplay of destruction, growth, transportation, movement, and stabilization.     

Recent increase of filamentous algae in shallow Swedish bays: Effects on the community structure of epibenthic fauna and fish

A summary is presented of estimates of distribution and growth of filamentous algae and its effect on the structure and functioning of epibenthic fauna and fish communities in shallow bays on the Swedish west coast. As a consequence of coastal eutrophication vegetation cover has gradually increased during the last decade, and during 1990's most bays in the Skagerrak-Kattegat area were variously covered with filamentous algae during spring and summer (May–July). In some areas filamentous algae (mainlyCladophora andEnteromorpha) completely covered the bottom. In field studies it was demonstrated that increased cover and dominance of filamentous algae result in structural changes of the epibenthic fauna community. Field studies showed that species richness and biomass of epibenthic fauna increased in a sandy bay with a moderate increase (30 to 50%) of filamentous algae cover. At higher cover (90%), biomass of epibenthic fauna was reduced, however, to the same level as for the sandy habitat, although the dominant epibenthic species were different. Heavy growth of epiphytic filamentous algae on eelgrass resulted in reduced biomass and a shift in the species composition of the epibenthic fauna community. Fish assemblage structure was also related to changes in vegetation. In eelgrass beds, fish species numbers were reduced with increasing cover of epiphytic filamentous algae, and at rocky bottoms with kelp algae (dominated byFucus), fish biomass decreased with increasing cover of attached filamentous algae. Further, foraging efficiency of juvenile cod and settling success of plaice were reduced as a response to increasing dominance of filamentous algae.     

Littoral flow rates within and around submersed macrophyte communities

• 1 The magnitude and range of water flow rates were measured within and adjacent to plant beds at different depths and among different dominant submersed plant species in the littoral zones of two lakes with contrasting morphometry.
• 2 There was very little variability in within-bed flow rates, either for locations within or among beds. However, when significant differences occurred in within-bed flow rates, the higher rates occurred predominately near the bottom of the Scirpus subterminalis bed where the plant surface area to water volume ratio was lowest.
• 3 Factors such as bed depth and dominant species had little effect on within-bed flow rate variance. Flows external to the plant beds were dissipated within 10–15 cm of the outer plant-bed boundary even under severe external flow-rate conditions (flow rate ～ 30cms^?1).
• 4 The mean within-bed flow rate was 0.07cms^?1 and individual experiment means ranged from 0.03 to 0.46cms^?1. These flow rates resulted in estimates of laminar flow boundary layer thickness, 1 mm from the leading edge of the leaf, ranging from 9.1 to 2.3mm. These estimates are much larger than submersed macrophyte leaf thicknesses themselves (<1 mm).