Strength, drag, and dislodgment of two competing intertidal algae from two wave exposures and four seasons

Intertidal macroalgae often experience greater risk of dislodgment with increasing size because of underscaling of breaking force of their stipes relative to drag on their thalli. This ratio (breaking force/drag) indicates safety from breakage at a given flow speed, with values greater than one indicating safety from breakage and values lower than one indicating danger of breakage. We examined this force ratio for the largest thalli of two species of co-dominant, red algae, Chondrus crispus Stackhouse and Mastocarpus stellatus Stack. In With. (Guiry), in four seasons at two wave exposures. During fall and winter, the largest thalli in both populations were dislodged resulting in a decrease in mass of the largest thalli found. This decrease was greater for Chondrus than for Mastocarpus, but their mass-specific force ratios (at 0.55 m s⁻¹) were equal indicating similar size-specific risk of dislodgment. The equality of force ratios was underlain by two similarities: (1) breaking force was independent of mass and not different between species; (2) mass-specific drag was not different between species. These similarities were underlain by dissimilar causes: (i) similarity in breaking force (the product of cross-sectional area and material strength) occurred because greater material strength of Mastocarpus compensated for greater mass-specific cross-sectional area of Chondrus; (ii) similarity in mass-specific drag (a function of planform area and the coefficient of drag) occurred because greater drag coefficients for Mastocarpus compensated for greater mass-specific planform areas of Chondrus. The similarity in force ratio, if it held at season- and site-relevant flow speeds, would suggest that during seasons of minimal growth and high wave exposure, the mass of the largest thalli of both species should be the same. Chondrus, however, had a greater mass at both sites in all seasons. Chondrus experienced greater decreases in mass probably because it grew larger and larger thalli are less safe. Extrapolation of a site-relevant force ratio for Chondrus in the fall revealed (1) that the site-relevant force ratio did not differ between exposures even though the mass-specific force ratio was greater at the protected site, and (2) a paradoxical result that all Chondrus thalli studied ought to have dislodged, but had not. This paradox may be resolved by consideration of the protection conferred by canopies of Chondrus: a canopy may effectively raise its site-relevant force ratio. Perhaps differences in protection conferred by different canopies explain why larger Chondrus persist with Mastocarpus even given a similarity in mass-specific force ratio.     

Comparative studies on the allelopathic effects of two different strains of Ulva pertusa on Heterosigma akashiwo and Alexandrium tamarense

Allelopathic effects of fresh tissue and dry powder of a nonsexual and a sexual strain of the macroalga Ulva pertusa on the growth of the microalgae Heterosigma akashiwo and Alexandium tamarense were evaluated using long-term coexistence culture systems in which several concentrations of macroalga fresh tissue and dry powder were used. The effects of macroalga culture medium filtrate on the two HAB algae were also investigated. Moreover, isolation co-culture systems were built to confirm the existence of allelochemicals and preclude the growth inhibition by direct contact. Short-term algicidal effect assays of macroalgae on H. akashiwo were carried out to measure the rate of algal cell lysis. The results of the coexistence assays showed that the growth of H. akashiwo and A. tamarense was strongly inhibited by fresh tissue and by dry powder of both strains of U. pertusa. The allelochemicals were lethal to H. akashiwo at relatively higher concentrations. The macroalga culture medium filtrate exhibited no apparent growth inhibitory effect on the two HAB algae under initial or semicontinuous filtrate addition, which suggested that continuous release of small quantities of rapidly degradable allelochemicals from the fresh tissue of both strains of U. pertusa was essential to effectively inhibit the growth of H. akashiwo and A. tamarense.     

Quantitative and qualitative analyses of dissolved organic matter released from Ecklonia cava Kjellman, in Oura Bay, Shimoda, Izu Peninsula, Japan

Release of dissolved organic matter (DOM) was studied on macroalga, Ecklonia cava Kjellman, by in situ field bag experiments, which were designed to keep the algal body under natural field condition, in Oura Bay, Shimoda, Izu Peninsula, Japan, from August 2003 to May 2005. The experiments were conducted 6 times in different seasons. The concentrations of dissolved organic carbon (DOC) in the experimental bag generally increased during the first 1-2 days, showing the release of DOC from algae. The daily release rates of DOC varied between 0.12 and 5.8 mgC per g (dry wt) of the algal blades per day. The seasonal variability of the DOC release rate was similar to those of the growth and the photosynthetic rates reported by the previous studies on E. cava in the same location. The fractions of the DOC release of the net primary production accounted for 18-62%. These contributions are comparable to other macroalgae in the previous studies (27-43%) using other macroalgae. Analysis of organic composition of DOM using gas chromatography (neutral aldose composition) and spectrometry (UV-visible and fluorescence) indicates that E. cava usually release extracellular products containing mucilaginous polysaccharides containing fucose as a main constituent and colored dissolved organic matter (CDOM). The release of these organic compounds is likely performed as the ordinary metabolism of brown algae. An exceptional release of a considerable amount of protein and carbohydrates mainly composed of mannose was found in June 2004, probably reflecting leakage of the intracellular material by an autolysis of the algal cells. The high DOM release rate in the present study suggests that macroalgae would be one of the important DOM producers in coastal regions.     

Direct Measurements of Drag Forces in C. elegans Crawling Locomotion

With a simple and versatile microcantilever-based force measurement technique, we have probed the drag forces involved in Caenorhabditis elegans locomotion. As a worm crawls on an agar surface, we found that substrate viscoelasticity introduces nonlinearities in the force-velocity relationships, yielding nonconstant drag coefficients that are not captured by original resistive force theory. A major contributing factor to these nonlinearities is the formation of a shallow groove on the agar surface. We measured both the adhesion forces that cause the worm’s body to settle into the agar and the resulting dynamics of groove formation. Furthermore, we quantified the locomotive forces produced by C. elegans undulatory motions on a wet viscoelastic agar surface. We show that an extension of resistive force theory is able to use the dynamics of a nematode’s body shape along with the measured drag coefficients to predict the forces generated by a crawling nematode.     

Interactions of Patella and macroalgae with settling Semibalanus balanoides (L.)

Biological interactions affecting densities of settling and newly-settled Semibalanus balanoides (L.) have been investigated by manipulative field experiments on the Isle of Man.The effects of sweeping by fucoid clumps of different species and Patella browsing have been compared on moderately-exposed shores. Patella allowed barnacle settlement by preventing growth of competing green algae, but reduced post-settlement densities. Small clumps of Fucus spiralis L., F. vesiculosus L, and F. serratus L. all reduced settlement considerably more so than limpets. F. serratus had the greatest sweeping effect.Interactions between macroalgae and Semibalanus balanoides have been investigated at all levels on sheltered shores and low down on more exposed shores. In the Fucus spiralis and F. vesiculosus zones, post-settlement numbers were higher than in adjacent areas where the canopy was removed. Barnacles did not settle readily in the Ascophyllum zone in either experimental or control areas. Settlement occurred in the upper part of the Fucus serratus zone in experimental areas where the canopy was removed but not in control areas. No settlement occurred in either treatment or control areas lower in the F. serratus zone. At all levels on the shore fucoid canopies seemed to reduce cyprid settlement, but the effect was greatest amongst F. serratus where there was total prevention. High on the shore the effect of enhanced post-settlement survival under the canopy outweighs reduction of cyprid settlement thus there are greater numbers in the controls. Competition with red algal turfs was shown to set the lower limit of the barnacle zone on a vertical pier face.     

Field measurement of the dynamics of the bull kelp Durvillaeaantarctica (Chamisso) Heriot

Seaweed habitats and morphological development are strongly affected by wave forces. Novel measurements were made of the force dynamics of the large intertidal macroalga Durvillaeaantarctica under the influence of wave action. Synchronized video, a pressure sensor and a resistance wave gauge provided data describing the wave field. The response of seaweeds to waves was gauged using instrumentation mounted directly on the seaweed, including accelerometers and displacement and force transducers. These field measurements were used to estimate forces and bending moments acting at the holdfast, where failure is most likely to occur. For waves of the order of 0.5 m high, we measured maximum forces on the stipe of around 300 N and blade accelerations that exceeded 30 m s⁻². During large wave events, inferred bending moments at the base of the stipe reached average values of around 140 N m. There was a decoupling between the blade response and the force experienced at the stipe base. Furthermore, changes in water depth throughout the tidal cycle had a systematic effect on blade accelerations and moments at the holdfast.     

The effects of habitat complexity and hydraulic conditions on the establishment of benthic stream macroalgae

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Algal assemblages associated with Stegastes sp. territories on Indo-Pacific coral reefs: Characterization of diversity and controls on growth

Herbivorous fish occupy an important niche in coral reef ecosystems. Specifically, damselfish of the genus Stegastes have been shown to have a significant impact on coral community structure and algal assemblages. This study investigated the algal communities associated with Stegastes territories of the Indo-Pacific (Fiji, Solomon Islands, and Tonga), while concurrently examining the effects of nutrient enrichment and herbivore exclusion (alone and in unison) on these communities. Results evidenced differences in species composition, percent cover, and algal growth rate between Stegastes territories and non-Stegastes sites and between control sites and treatment sites. Stegastes territories consistently displayed a greater abundance of turf algae than non-Stegastes sites; the two main genera of turf algae observed at all sites were Polysiphonia and Ceramium. Although non-Stegastes sites in Fiji, the Solomon Islands, and Tonga showed a greater percent coverage of macroalgae, they contained fewer algal species compared to Stegastes territories. In Fiji, red macroalgae decreased in the herbivore exclusion treatments, while brown macroalgae increased significantly in the herbivore exclusion and nutrient treatments. The combined effect of the herbivore exclusion and nutrient treatment at this location yielded a significantly increased turf algae growth rate compared to control sites. Growth rates of turf algae in the Solomon Islands and Tonga increased significantly in caged treatments, suggesting that damselfish of the genus Stegastes can play an important role in maintaining cropped algal beds. In summation, the results demonstrated that Stegastes sustains distinct algal assemblages which may be disrupted by reduced grazing and/or eutrophication.     

Shell form,behaviour, and tolerance to water movement in the limpet Patina pellucida (L.) (Gastropoda: Prosobranchia)

The diotocardian gastropod Patina pellucida (L.) is a macro-herbivore common upon the fronds of laminarian algae. Considerations of shell shape, adhesive tenacity, and current tolerance, together with calculations of expected hydrodynamic drag, indicate that this limpet is effectively streamlined. Smaller individuals are relatively more resistant to dislodgement than are larger ones. Experimental animals of 6.0–13.5 mm shell length resisted constant flows of up to 0.9-1.3 m s^?1. An orientation, in which the longitudinal axis of a limpet is aligned parallel to the direction of the prevailing flow, is induced by currents faster than ≈ 0.5 m s^?1, and is an adaptation to conditions of directional water streaming. P. pellucida appears to show a seasonal change in orientatory preference, individuals tending to point distally with respect to the algal lamina in spring and proximally in autumn: this change may be associated with the annual growth cycle of the host plant.     

Getting a grip on the intertidal: flow microhabitat and substratum type determine the dislodgement of the crab Pachygrapsus crassipes (Randall) on rocky shores and in estuaries

Hydrodynamic forces can affect survival as well as limit the movement of motile benthic animals. An animal's danger of dislodgement depends on the hydrodynamic forces it experiences in its microhabitat relative to the force required to dislodge it (tenacity) from the substratum. We measured water flow and substratum characteristics in two different habitats of the shore crab Pachygrapsus crassipes: a wave-swept rocky shore and an intertidal mudflat. The maximum water velocities and accelerations in the microhabitats of the crabs at the wave-swept site were three times and two times greater, respectively, than at the mudflat site. In the laboratory, we measured the tenacity of crabs of various sizes on different substrata, and also measured their drag, lift and added-mass coefficients. Using these data, we calculated the flow conditions under which crabs would be overturned or sheared off the substratum in their two habitats. The net horizontal force (drag plus acceleration reaction) required to dislodge a crab on a rugose rock substratum was an order of magnitude greater than on smooth rock and two orders of magnitude greater than on mud. Our calculations indicate that, under non-storm conditions, crabs will not be dislodged from the substratum in either the mudflat or the wave-swept habitat when grasping the substratum with maximum tenacity. Moving crabs have lower tenacity and our calculations predict that hydrodynamic forces will restrict the mobility of large crabs more than that of small ones on smooth, but not on rugose rock.     

Turbinaria ornata (Phaeophyceae) varies size and strength to maintain environmental safety factor across flow regimes

Water motion in coastal areas can produce hydrodynamic forces that damage or dislodge benthic macroalgae if the tissues of macroalgae are not sufficiently strong. Some macroalgae vary their morphology and strength in response to ambient water motion, but little is known of how morphology and strength of macroalgae change relative to one another across flow regimes. Here, we use Turbinaria ornata, an ecologically important macroalga, to study how both the morphology and strength of macroalgae vary with ambient water motion. Typically, T. ornata exhibits weakening of its stipe when sexually mature, leading to breakage from the substratum and dispersal, which is beneficial for reproduction. Across three flow regimes, adult T. ornata increased its size but decreased its strength as water motion decreased. However, the strength of T. ornata relative to the maximum hydrodynamic forces it is expected to encounter (the environmental safety factor) did not differ between flow regimes. Our results showed that T. ornata can conform to its local flow habitat by varying both size and strength, similar to other macroalgae. Varying multiple traits between flow regimes suggested that T. ornata is capable of surviving a wide range of flow conditions, which may permit more control over the timing of its weakening, breakage from the substratum, and dispersal, even with future increases in flow velocities (e.g., large waves from storms) that are expected to occur frequently with climate change.     

Establishment of the introduced kelp Undaria pinnatifida in Tasmania depends on disturbance to native algal assemblages

Despite recent rapid increases in the occurrence of nonindigenous marine organisms in the marine environment, few studies have critically examined the invasion process for a marine species. Here we use manipulative experiments to examine processes of invasion for the Asian kelp Undaria pinnatifida (Harvey) Suringar at two sites on the east coast of Tasmania. Disturbance to reduce cover of the native algal canopy was found to be critical in the establishment of U. pinnatifida, while the presence of a stable native algal canopy inhibited invasion. In the first sporophyte growth season following disturbance of the canopy, U. pinnatifida recruited in high densities (up to 19 plants m⁻²) while remaining rare or absent in un-manipulated plots. The timing of disturbance was also important. U. pinnatifida recruited in higher densities in plots where the native canopy was removed immediately prior to the sporophyte growth season (winter 2000), compared with plots where the canopy was removed 6 months earlier during the period of spore release (spring 1999). Removal of the native canopy also resulted in a significant increase in cover of sediment on the substratum. In the second year following canopy removal, U. pinnatifida abundance declined significantly, associated with a substantial recovery of native canopy-forming species. A feature of the recovery of the native algal canopy was a significant shift in species composition. Species dominant prior to canopy removal showed little if any signs of recovery. The recovery was instead dominated by canopy-forming species that were either rare or absent in the study areas prior to manipulation of the canopy.     

Living inside a sponge skeleton: the association of a sponge,a macroalga and a diatom

The relationships between sponges and macroalgae have been poorly investigated, especially in temperate waters. This work describes the consortium between the dictyoceratid sponge Dysidea pallescens and the red alga Acrochaetium spongicola permeating spongin fibres in the Mediterranean Sea; moreover, this is the first report of a diatom, Amphora pediculus, living also inside the spongin skeleton. The annual trend of the total algal biomass did not vary over time in relation to the temperature, irradiance and incorporation of foreign bodies. Our analyses, conducted by light and electron microscopy, suggest that both the macroalga and the diatom invade the skeleton of the sponge from the external environment, and that the benthic diatom, epiphyte of the macroalga, is passively carried inside the fibres through the growth of Acrochaetium spongicola. All the examined samples of D. pallescens showed that the macroalga permeated at least some fibres, while the presence of the diatom was occasional. The superficial layer of the sponge, thin and reticulate, likely allows the passage of the light and ensures the algal survival inside the sponge tissue. The high occurrence of the association with A. spongicola, together with the morphological adaptations of the sponge favouring the algal life, suggest that the relationship is mutualistic. The possible benefits for the involved partners are hypothesized. The taxonomy and ecology of endozoic Acrochaetiales are controversial due to the reduced size of thalli, the absence of peculiar diagnostic characters and unknown reproductive structures. Therefore, detailed studies on the relationships between the algae and their hosts will provide helpful information for the algal identification.     

Metabolic consequences of living in a wave-swept environment: Effects of simulated wave forces on oxygen consumption, heart rate, and activity of the shell adductor muscle of the abalone Haliotis iris

Animals in wave-exposed habitats must constantly contend with the hydrodynamic forces of lift and drag. In this study, we investigated aspects of the metabolic response of Haliotis iris to simulated wave forces varying in magnitude up to 9.6 N applied to the shell at 69° to horizontal, alternately from anterior and posterior directions, with a period of 10s. Shell adductor muscle activity (electromyogram, EMG), heart rate, and oxygen consumption were monitored during force application and during extended recovery. EMG spiking was absent at zero force, but increased markedly with increasing force, in synchrony with the wave cycle. In contrast, heart rate was unaffected by wave forces and varied by only 5% over the whole range of applied forces. During force application, oxygen consumption increased by 10-25% above resting rates and remained elevated throughout a 5-hour recovery period, indicating a switch to anaerobic metabolism. It is concluded that living in a wave-swept environment is metabolically costly for abalone although this may be compensated by improved food availability and more efficient ventilation induced by external flow.     

Drag, drafting, and mechanical interactions in canopies of the red alga Chondrus crispus.

Dense algal canopies, which are common in the lower intertidal and shallow subtidal along rocky coastlines, can alter flow-induced forces in their vicinity. Alteration of flow-induced forces on algal thalli may ameliorate risk of dislodgement and will affect important physiological processes, such as rates of photosynthesis. This study found that the force experienced by a thallus of the red alga Chondrus crispus (Stackhouse) at a given flow speed within a flow tank depended upon (1) the density of the canopy surrounding the thallus, (2) the position of the thallus within the canopy, and (3) the length of the stipe of the thallus relative to the height of the canopy. At all flow speeds, a solitary thallus experienced higher forces than a thallus with neighbors. A greater than 65% reduction in force occurred when the thallus drafted in the region of slower velocities that occurs in the wake region of even a single upstream neighbor, similar to the way racing bicyclists draft one behind the other. Mechanical interactions between thalli were important to forces experienced within canopies. A thallus on the upstream edge of a canopy experienced 6% less force than it did when solitary, because the canopy physically supported it. A thallus in the middle of a canopy experienced up to 83% less force than a solitary thallus, and forces decreased with increasing canopy density. Thus, a bushy morphology that increases drag on a solitary thallus may function to decrease forces experienced by that thallus when it is surrounded by a canopy, because that morphology increases physical support provided by neighbors.     

Intermittent aeration affects the bioremediation potential of two red algae cultured in finfish effluent

The high cost of aeration needed to tumble culture macroalgae is a limiting factor for integration with land-based finfish culture. Toward reducing this electricity cost, we compared intermittent aeration (16 h on:8 h off) with continuous aeration (24 h on) on the productivity of two strains of Chondrus crispus (Basin Head and Charlesville) and Palmaria palmata from Atlantic Canada between May and June 2011. Algal fronds were cultured under a 16:8-h light/dark photoperiod in 50-L tanks supplied with finfish effluent (49 μmol L^?1 of ammonium and 11 μmol L^?1 of phosphate) at a mean water flow rate of 0.4 L min^?1. Nitrogen (N) influx was 1.8 gN m^?2 day^?1, and phosphorus (P) influx was 0.9 gP m^?2 day^?1, with uptake rates ranging from 0.02 to 2.4 gN m^?2 day^?1 and ?0.2 to 0.4 gP m^?2 day^?1. On average, the macroalgae culture system (algae and biofilms) removed 1.0 gN m^?2 day^?1 (51.9 %). The growth of macroalgae (pooled across treatment and strain) ranged from 0.5 to 1.6 % day^?1, which accounted for a yield of 2.2 to 5.4 g DW m^?2 day^?1. Switching off aeration at night improved the growth rate of Basin Head Chondrus by 146 % and had no effect on growth rate or nitrogen and carbon removal by P. palmata and Charlesville Chondrus. Growth and yield of Basin Head Chondrus under intermittent aeration were over two times greater than both Charlesville Chondrus treatments.     

Seasonal changes in situ grazing of the mesoherbivores Idotea baltica and Gammarus oceanicus on the brown algae Fucus vesiculosus and Pylaiella littoralis in the central Gulf of Finland, Baltic Sea

The in situ grazing experiments were performed in the shallow water rocky habitat of the northern Baltic Sea during ice-free season 2002. In the experiments the effects of algal species and choice on the grazing of the mesoherbivores Idotea baltica (Pallas) and Gammarus oceanicus Segerstråle were tested. Salinity, temperature, concentration of nutrients in water and macroalgae and net production of macroalgae were considered as random effects in the analysis. The invertebrate feeding rate was mainly a function of the net photosynthetic activity of Pylaiella littoralis (L.) Kjellman and Fucus vesiculosus L. Feeding rate increased significantly with decreasing algal photosynthetic activity. When the two algal species were incubated together invertebrates fed primarily on P. littoralis. Low selectivity towards P. littoralis coincided with its high photosynthetic activity. The presence of F. vesiculosus did not modify the invertebrate feeding on P. littoralis. The results indicated that (1) the grazing on F. vesiculosus depended on the availability of P. littoralis, (2) the photosynthetic activity of algae explained the best the variation in grazing rate and (3) the grazers are not likely to control the early outbreak of filamentous algae in the northern Baltic Sea by avoiding young and photosynthetically active algae. The likely mechanism behind the relationship is that the increased photosynthetic activity of macroalgae coincides with their higher resistance to herbivory.     

The effects of grazing on the distribution and composition of low-shore algal communities on the central coast of Portugal and on the southern coast of Britain

A red algal turf is often found just below the barnacle/limpet zone of many European shores, especially on steep shores of moderate exposure. The hypothesis that grazing by limpets determines the upper limit of distribution of this red algal turf was tested on moderately exposed shores in Portugal and Britain. We also aimed to assess whether the grazing effect is modified at various spatial scales. Grazers were excluded by fences, with half-fenced and unfenced controls. Exclusion plots were rapidly colonised by green ephemeral algae in the months immediately after the beginning of the experiment (summer); these algae were later replaced by perennial algae. The percentage cover of turf-forming macroalgae showed a significant increase at both locations. The upper limit of distribution extended more than 50 cm on most of the shores studied. In contrast, control and half-fenced plots remained devoid of algae. After 2 years, ungrazed plots were mainly colonised by a red algal turf (e.g. Caulacanthus ustulatus, Gelidium spp., Laurencia pinnatifida) in Portugal, while canopy cover (Fucus serratus and Himanthalia elongata) dominated in Britain in marked contrast to the grazed plots. Physical factors acting at both local and geographical scales may explain these differences. However, although physical factors probably have an important influence on the identity, size and abundance of sublittoral fringe macroalgae, grazers play a major role in directly setting their upper limits. The effect of grazing by limpets was not consistent for all of the morphological algal groups and spatial scales considered in the present study. The effect of grazing on the cover of turf algae varied between Portugal and Britain (location scale), while effects on ephemeral and canopy algal cover varied at the shore scale within location.     

CONSUMPTION OF ULVA LACTUCA (CHLOROPHYTA) BY THE OMNIVOROUS MUD SNAIL ILYANASSA OBSOLETA (SAY)

Marine invertebrate grazing on temperate macroalgae may exert a significant “top-down” control on macroalgal biomass. We conducted two laboratory experiments to test (1) if consumption by the omnivorous mud snail Ilyanassa obsoleta (Say) on the macroalga Ulva lactuca Linnaeus was a function of food quality (nitrogen content) and (2) if grazing on benthic macroalgae occurred at significant rates in the presence of alternative food sources in the sediment (detritus, larvae, benthic microalgae). Grazing rates were higher for N-enriched macroalgae; however, all snails lost weight when grazing on macroalgae alone, indicating that U. lactuca was a poor food source. The presence of sediment from two sites, a sandy lagoon and an adjacent organic-rich muddy tidal creek, did not affect consumption of macroalgae in microcosm experiments, and the grazing snails were capable of significantly reducing macroalgal biomass associated with both sediment types. Grazing rates by this omnivore were as high as 10.83 mg wet weight·individuals ^{− 1}·d ^{− 1} and were similar to those recorded for herbivorous species. In situ loss rates calculated from average grazing rates per individual and snail abundances (up to 3.5 g dry weight·m ^{− 2}·d ^{− 1}) also were comparable with those calculated for herbivorous species. This level of grazing could remove up to 88% of new macroalgal growth at the lagoon site where the N supply was relatively low but had a much smaller effect (18% of new growth) at the high-nutrient creek site. Snails facilitated macroalgal growth at both sites by increasing tissue N content by 40%–80%. Consumption and digestion of macroalgae aided in the recycling of nutrients temporarily bound in the algae and resulted in enrichment of surficial sediments. Increased N sequestration in the sediments also was associated with an interruption of snail burrowing behavior due to persistent anoxia in sediments rich in decaying algal material. Our data suggest that in shallow lagoons where mud snails and benthic macroalgae coexist, grazing may influence N retention in macroalgal biomass.     

Macroinvertebrate response to different species of macroalgal mats and the role of disturbance history

Over the last 20 years, the Mondego estuary, Portugal has experienced excessive growth of macroalgae especially in the inner parts of the system, with several algal species implicated. In this study, we compare the effects of morphologically different species, the red alga Gracilaria verrucosa and the green macroalga Enteromorpha intestinalis on macrobenthic assemblages, by a field experiment whereby the biomass of algae was manipulated and the resultant changes in macrofauna abundance evaluated. The experiments were carried out in different areas (a relatively undisturbed sea grass bed and an upstream eutrophic area) experiencing different degrees of overall enrichment.Measurements of sediment redox potential revealed a rapid anoxia with a significant increase in algal biomass after 4 weeks. The effects of macroalgae were different at the two sites, being more marked in the eutrophic area. In addition, the effects of Gracilaria and Enteromorpha were significantly different, with Enteromorpha having a greater detrimental effect for most of the macrofauna, in particular Cyathura carinata, Scrobicularia plana, Cerastoderma edule and Alkmaria romijni. However, three of the most abundant invertebrates (Hydrobia ulvae, Hediste diversicolor and Capitella capitata) showed significant increases in abundance in weed affected compared to weed-free plots. Gracilaria had less of an impact on macrobenthic assemblages leading to a more enriched community. Between-site differences in overall impact were related to their previous disturbance history.