The effects of sea urchin grazing and drift algal blooms on a subtropical seagrass bed community

Subtropical seagrass beds can be subject to relatively high levels of direct herbivory and large blooms of drift algae, both of which can have important effects on the floral and faunal components of the community. Caging experiments were used to investigate these factors in a Thalassia testudinum bed in Biscayne Bay, Florida. Abundance of sea urchins, Lytechinus variegatus, and drift algae was manipulated within the cages. Naturally occurring levels of urchin grazing do not appear to affect the T. testudinum population. With experimentally increased urchin densities in the winter, seagrass shoot density and aboveground biomass decreased significantly. Similar effects were not detected in the summer, indicating that the impact of grazing on T. testudinum is lessened during this time of year. Shoot density was more vulnerable to grazing than aboveground biomass. This may be a result of grazing-induced increases in seagrass productivity, in which the remaining shoots produce more or longer leaves. In the winter, drift algal blooms form large mats that cover the seagrass canopy. Under the normal grazing regime these algal blooms do not have significant negative effects on the seagrass. With increased grazing pressure, however, there is a synergistic effect of grazing and drift algae on seagrass shoot density. At intermediate urchin density (10 per m(-2)), cages without algae did not undergo significant decreases in shoot density, while those with algae did. At the high density of urchins, the number of seagrass shoots in cages both with and without algae decreased, but the effect was more pronounced for cages with algae. Invertebrate abundance at the field site was low relative to other seagrass beds. There were no discernible effects, either positive or negative, of urchin and algae manipulations on the sampled invertebrate community.     

Benthic macroalgae as a dispersal mechanism for fauna: influence of a marine tumbleweed

Effective dispersal is problematic for benthic organisms without planktonic larvae; rafting and vertical migrations are mechanisms that can potentially be employed by such fauna, but these strategies entail considerable predation risk as well as other disadvantages. Unattached, but non-floating, “drift” algae harbor large numbers of fauna and may serve as an alternative dispersal mechanism in some systems. This paper reports field manipulations in Florida Bay, Florida, USA designed to determine (1) if such algae can disperse benthic animals, and (2) if dispersal efficiency varies as a function of two common substrata types: seagrass and bare sediment. A live immersion stain was used to mark faunal associates of Laurencia spp. algal clumps in situ. The fidelity of molluscs, decapods, ophiuroids, and fishes to stationary algal clumps was then compared with the fidelity of these animals to clumps that were forced to tumble over a given distance with a blower apparatus; these experiments were performed over both sand and seagrass substrata. Measurements of frequency, spatial extent, and rate of algal drift were made to aid in assessing the potential importance of benthic algae as a dispersal mechanism.

Algal clumps often rolled in a manner similar to that of terrestrial tumbleweeds; mark-recapture work showed that algal clumps can move up to 0.5 km/day and that algal drift is a frequent phenomenon. The algal masses were effective transporters of benthic fauna, including mobile shrimps and fishes; dispersal was more efficient over sand than over seagrass. Dispersal of fauna via this mobile habitat should entail lower risk than other adult dispersal stratagems such as vertical migration or rafting; this mechanism would be most advantageous for brooding species or those with limited planktonic phases. Differential fidelity to clumps tumbling across seagrass versus sand suggests that the algae could facilitate exchange of fauna between isolated seagrass patches.     

Toward a landscape approach in seagrass beds: using macroalgal accumulation to address questions of scale

An experimental investigation of drift macroalgal accumulation in seagrass beds was conducted to determine if the relationship between passively dispersed plant structure and the spatial arrangement of rooted macrophytes differed when examined across two spatial scales. Experiments were performed from December 1992 to April 1993 at four different sites in Tampa Bay, Florida, utilizing artificial seagrass units (ASUs) of uniform shoot length and density but with different areal dimensions [1 m² (S) versus 4 m² (L)]. Drift macroalgae were also collected from 1 m×1 m plots of natural seagrass at each of the experimental sites from November 1990 to May 1992 to determine the relationship between macroalgal abundance and structural characteristics of natural seagrass. Disproportionately higher amounts of macroalgae were captured in L compared to S plots suggesting that macroalgal accumulation does not scale up directly with the areal dimensions of ASU patches. Higher amounts of algae recovered in L plots is in accordance with patterns expected if algae accumulate in zones of attenuated water flow. Neither seagrass shoot density nor blade length could adequately describe the patterns of algal accumulation. These combined results suggest that explanations for trapping/retention of passively dispersed particles should extend beyond traditional measures of vegetation complexity.     

Microhabitat use by a post-settlement stage estuarine fish: evidence from relative abundance and predation among habitats

Seagrass beds provide food and shelter for many fish species. However, the manner in which fishes use seagrass bed habitats often varies with life stage. Juvenile fishes can be especially dependent on seagrass beds because seagrass and associated habitats (drift macroalgae) may provide an effective tradeoff between shelter from predation and availability of prey. This study addressed aspects of habitat use by post-settlement pinfish, Lagodon rhomboides (Linneaus), an abundant and trophically important species in seagrass beds in the western North Atlantic and Gulf of Mexico. Abundance of post-settlement fish in seagrass beds was positively related to volume of drift macroalgae, but not to percent cover of seagrass, indicating a possible shelter advantage of the spatially complex algae. Tethering experiments indicated higher rates of predation in seagrass without drift macroalgae than in seagrass with drift macroalgae. Aquarium experiments showed lower predation with higher habitat complexity, but differences were only significant for the most extreme cases (unvegetated bottom, highest macrophyte cover). Levels of dissolved oxygen did not differ between vegetated and unvegetated habitats, indicating no physiological advantage for any habitat. Seagrass beds with drift macroalgae provide the most advantageous tradeoff between foraging and protection from predation for post-settlement L. rhomboides. The complex three-dimensional shelter of drift macroalgae provides an effective shelter that is embedded in the foraging habitat provided by seagrass. Drift macroalgae in seagrass beds is a beneficial habitat for post-settlement L. rhomboides by reducing the risk of predation, and by providing post-settlement habitat within the mosaic (seagrass beds) of adult habitat, thus reducing risks associated with ontogenetic habitat shifts.     

Drifting algal mats as an alternative habitat for benthic invertebrates: Species specific responses to a transient resource

Patchy occurrences of benthic drift algae (i.e. loose lying macroalgal mats) may increase habitat complexity on normally bare soft bottoms, but at the same time, extensive amounts of drifting algal mats are known to stress the benthic fauna. This paper presents results of the first detailed study of the fauna associated with drift algal mats in the northern Baltic Sea. In order to assess the importance of drifting algae as an alternative habitat for benthic fauna, benthic drift algal mats were sampled on shallow (2-9 m) sandy soft bottoms in the outer archipelago of the ?land Islands (Finland). Species composition, abundance and biomass of the macrofauna associated with algal mats were recorded. The results show that drifting algae at times can harbour very high abundances of invertebrates (up to 1116 individuals/g algal dryweight), surpassing invertebrate densities recorded in seagrass communities. The algal fauna varied between sites and over time, and factors such as ambient benthic fauna, exposure to wind-wave disturbance, depth, and algal coverage and condition influenced the invertebrate community composition of the algal mats. Abundance increased while individual biomass of the animals decreased over time (summer season; July-October). A series of laboratory experiments were conducted in order to test the ability of a few important benthic species to move up into, and survive in a drifting algal mat. Macoma balthica, Hydrobia spp., Nereis diversicolor and Bathyporeia pilosa were used in the experiments, and significant differences in their survival and mobility within drifting algae were recorded. This study shows that benthic species differ significantly in their ability to utilise the algal mats, with mainly opportunistic and mobile taxa such as Hydrobia spp., Chironomidae and Ostracoda benefiting from the algae, whereas infaunal species such as M. balthica and B. pilosa are negatively affected. The occurrence of eutrophication induced drifting macroalgal mats has increased significantly during the last decade in the northern Baltic Sea. Hence, the importance of drifting algae as a stress factor and as an alternative habitat for benthic fauna increases.     

Distribution and seasonal biomass of drift macroalgae in the Indian River Lagoon (Florida, USA) estimated with acoustic seafloor classification (QTCView, Echoplus)

Three areas of the Indian River Lagoon, Florida (USA) were surveyed to show seasonal changes in the distribution and biomass of macroalgae and seagrass. Acoustic seafloor discrimination based on first and second echo returns of a 50 kHz and 200 kHz signal, and two different survey systems (QTCView and ECHOplus) were used. System verification in both the field and a controlled environment showed it was possible to distinguish acoustically between seagrass, sparse algae, and dense algae. Accuracy of distinction of three classes (algae, seagrass, bare substratum) was around 60%. Maps were produced by regridding the survey area to a regular grid and using a nearest-neighbor interpolation to provide filled polygons. Biomass was calculated by counting pixels assigned to substratum classes with known wet-weight biomass values (sparse algae 250 g m^− 2, dense algae 2000 g m^− 2, seagrass 100 g m^− 2) that were measured in the field. In three study areas (Melbourne, Sebastian Inlet, and Cocoa Beach), a dependence of algal biomass on depth and season was observed. Seagrass most frequently occurred in water less than 1 m deep, and in November, seagrass beds tended to be covered by dense algae that also extended up- and downstream of shoals in the Lagoon. In March, the pattern was similar, with the exception that some areas of previously dense algae had started thinning into sparse algae. Macrophyte biomass was lowest in May in the Melbourne and Cocoa Beach study areas, with the opposite situation in the Sebastian Inlet study area. In May, seagrass areas were largely devoid of dense algae and most algae accumulations were sparse. In August, dense algae covered large areas of the deep Lagoon floor while shoals were largely free of algae or had only sparse cover. We suggest this summer pattern to reflect moribund algae being washed from the shallows to deeper channels and from there being removed from the lagoonal ecosystem either through tidal passages into the open ocean or by degradation and breakdown in situ. The differences between the study areas indicate high spatial and temporal variability in biomass and distribution of macrophyte biomass in the Indian River Lagoon.     

Differentiation of benthic marine primary producers using stable isotopes and fatty acids: Implications to food web studies

A two-dimensional biomarker approach, using stable isotopes (δ¹³C, δ¹⁵N) and fatty acids, was used to evaluate differences both amongst and within benthic primary producer types (seagrass, fleshy red algae, calcareous red algae, brown algae, and seagrass periphyton) that are typical of the nearshore, temperate Australian region. The primary source of variance (as examined by permutational ANOVA) for all biomarkers examined was amongst primary producer types, as opposed to amongst species within type. δ¹³C showed a clear separation (Monte Carlo p < 0.05) between seagrass (range of means = −10.1 to −14.0‰) and macroalgae (−14.6 to −25.2‰), but could not differentiate amongst the algal types examined. Similarly, distinct δ¹⁵N signatures (p < 0.05) were found only for seagrass (range of means = 3.6-4.1‰) versus calcareous red algae (4.6-5.5‰), with all other types overlapping in their mean δ¹⁵N values. In contrast, multivariate analysis of fatty acid data (using Canonical Analysis of Principal coordinates; CAP) distinguished not only between seagrass and macroalgae, but also between red and brown algae (and to a limited extent between the calcareous and fleshy red algal types). The principal unsaturated fatty acids in the samples were C₂₀ polyunsaturates (found primarily in the macroalgae and periphyton), and C₁₈ mono- and polyunsaturates, with high proportions of 18:2n-6 and 18:3n-3 typical of the seagrasses. The C₁₈ monounsaturate 18:1n-7 was one of the most diagnostic compounds for the red algae examined, being present in very low amounts in seagrass and virtually absent in the brown algae. Conversely, brown algae were high in 18:4n-3, with 20:4n-3 particularly diagnostic of the kelp Ecklonia radiata. In contrast to stable isotopes, fatty acids helped distinguish different algal groups, thereby providing support that a two-dimensional approach using stable isotopes and fatty acids is likely to provide the most useful tool to distinguish primary producers in food web structure.     

Seasonal abundance and distribution of drift algae and seagrasses in the mid-Indian river lagoon,Florida

The distribution of seagrasses in a 15-ha area in the mid-Indian River lagoon on Florida's central east coast was mapped. Halodule wrightii Aschers. dominated in shallow (< 0.4 m) and Syringodium filiforme Kutz. in deeper water (> 0.5 m). Thalassia testudinum Banks ex König occurred as scattered patches. Areal coverage of monospecific stands of the three major seagrasses was: Syringodium 35%, Halodule 14%, Thalassia 6% and bare sand 21%. Mixed species stands, mostly Syringodium with Hallodule, covered 25% of the total study area. Above-ground seagrass biomass was maximum in summer (June–July) and minimum in late winter (February–March). Summer maxima ranged from 60 g dry wt. m^?2 for Syringodium to ～ 300 g dry wt. m^?2 for Thalassia, with Halodule intermediate at 160 g dry wt. m^?2.Because distribution of unattached benthic macroalgae (“drift algae”), primarily Gracilaria spp., was highly aggregated, aggregations were first mapped, followed by stratified quadrat sampling in order to estimate total drift algal abundance. In April 1982, high-density patches covering a few hectares averaged 409 g dry wt. m^?2. At maximum abundance, averaged over the entire 15-ha mapped area, drift algal biomass was 164 g dry wt. m^?2; mean above-ground seagrass biomass was only 49 g dry wt. m^?2. Other large expanses of the lagoon had similar accumulations of drift algae; densities of some accumulations exceeded 15 000 g dry wt. m^?2. Year-to-year variability of seagrass and drift algal abundance was high and may be related to variations in light levels.Drift algae harbor high densities of animals and at times may be quantitatively more important locally than seagrasses in terms of habitat, nutrient dynamics and primary production.     

Effects of an epiphytic alga on abundances of fish and decapods associated with the seagrass Zostera capricorni

During a field experiment to test effects of changes to leaf height and density of Zostera capricorni on associated fish and decapods, more than one-third of the substratum at some sites was covered by the epiphytic brown alga Giffordia mitchelliae. This paper reports apparent effects of the alga on the 13 most abundant species of fish and decapods, and interactions between effects of the alga and those due to height and density of the seagrass leaves. Abundances of three species were significantly lower, and those of two species were significantly greater where the alga occurred. There were interactions between effects of the alga and those of leaf height and/or density for another four species. In all four cases, when the alga had an effect, it was to decrease abundance. The effects of Giffordia were generally opposite to those reported for drift algae in seagrass habitats elsewhere. The lower abundances of seven species where algal cover was great are best explained by an existing model for seagrass habitats that predicts reductions in abundance of individuals when the ratio of plant material to substratum is very high.     

Seagrass meadows mixed with calcareous algae have higher plant productivity and sedimentary blue carbon storage

Seagrass meadows capture and store large amounts of carbon in the sediment beneath, thereby serving as efficient sinks of atmospheric CO₂. Carbon sequestration levels may however differ greatly among meadows depending on, among other factors, the plant community composition. Tropical seagrass meadows are often intermixed with macroalgae, many of which are calcareous, which may compete with seagrass for nutrients, light, and space. While the photosynthetic CO₂ uptake by both seagrasses and calcareous algae may increase the overall calcification in the system (by increasing the calcium carbonate saturation state, Ω), the calcification process of calcareous algae may lead to a release of CO₂, thereby affecting both productivity and calcification, and eventually also the meadows’ carbon storage. This study estimated how plant productivity, CaCO₃ production, and sediment carbon levels were affected by plant community composition (seagrass and calcareous algae) in a tropical seagrass‐dominated embayment (Zanzibar, Tanzania). Overall, the patterns of variability in productivity differed between the plant types, with net areal biomass productivity being highest in meadows containing both seagrass and calcareous algae. Low and moderate densities of calcareous algae enhanced seagrass biomass growth, while the presence of seagrass reduced the productivity of calcareous algae but increased their CaCO₃ content. Sedimentary carbon levels were highest when seagrasses were mixed with low or moderate cover of calcareous algae. The findings show that plant community composition can be an important driver for ecosystem productivity and blue carbon sequestration.     

Detection of inconspicuous epiphytic algae supporting food webs in seagrass meadows

Summary Detritus from common seagrasses and other marine angiosperms may often be a less important basis for estuarine food webs than previously believed. In NW Gulf of Mexico seagrass meadows, epiphytic algae have high productivities, palatability, and a more important trophic role than common large plants have. Interdisciplinary field experiments show (1) intensive night-time ingestion of epiphytes by various invertebrate detritivores, (2) very high productivity of epiphytic algae on seagrasses, and (3) assimilation of epiphytes rather than seagrasses, as measured by ¹³C comparisons. These combined data show that many naturally concentrated and potentially competing invertebrates in Gulf of Mexico seagrass meadows feed largely on the algal overgrowth on seagrass blades, even when such algae appear to be sparse. Primary productivity of these epiphytic algae can equal that of the seagrasses, per blade or per unit biomass. Animal ¹³C values tracked epiphytic values rather than seagrass values when comparisons were made over six sites. These measurements reinforce the view that epiphytic algae can be the primary basis of the food web in seagrass meadows.Contribution No. 608 of The University of Texas Marine Science Institute     

Varying foraging strategies of Labridae in seagrass habitats: Herbivory in temperate seagrass meadows?

The diets of five species of Labridae in south-western Australia were examined to determine whether: (1) grazing of seagrass and epiphytic algae is a prominent feature of the food web within the deeper seagrass meadows of this temperate region; (2) levels of grazing differ among different seagrass systems; and diets differ among these closely-related species. Fish were collected seasonally from three seagrass habitats mainly comprising either Posidonia sinuosa, Posidonia coriacea or Amphibolis griffithii between the summer of 1996/97 and spring of 1997. Consumption of considerable amounts of algae and seagrass by Odax acroptilus and seagrass by Haletta semifasciata indicates that macrophyte grazing by fish is a component of the trophic dynamics of south-western Australian seagrass meadows. O. acroptilus and H. semifasciata were both omnivorous, feeding on a range of epifauna, infauna and flora, whereas Siphonognathus radiatus, Neoodax balteatus and Notolabrus parilus were carnivorous, feeding predominantly on motile epifauna, such as molluscs and crustaceans. The level of macrophyte grazing is likely to be underestimated in temperate offshore meadows of P. sinuosa and A. griffithii where omnivorous labrids, monacanthids and terapontids are abundant. Stable isotope data for O. acroptilus from the study region suggest that animal prey is more important to tissue maintenance than macrophyte material. Macrophytes may be grazed to acquire attached animal prey or for fulfilling energy requirements. Based on the distribution of prey, it appears that species in A. griffithii meadows forage within and below the seagrass canopy, whilst species in P. sinuosa meadows are likely to forage towards the basal area of this seagrass.     

Heavy Metals in Seagrasses and Algae of Pos'et Bay, Sea of Japan

The contents of heavy metals in Sargassum algae and seagrasses from Pos'et Bay in the Sea of Japan were studied. The concentrations of Fe, Mn, Cu, Zn, and Cr in the algae and seagrass leaves were correlated with each other. The concentrations of heavy metals in brown algae and seagrasses from Pos'et Bay were largely close to background levels. Increased contents of some metals found in macrophytes in the area of the port of Pos'et were due to local environmental pollution; around Furugel'm Island, to periodical upwelling and drift currents from the mouth of the Tumannaya River; and, at Cape Deger, to the cyclonic current.     

Composition,export, and import of drift vegetation on a tropical,plant-dominated,fringing-reef platform (Caribbean Panama)

For 15 months, the composition and abundance of drift vegetation were determined from a plantdominated fringing reef at Galeta Point, Caribbean Panama. Five nets located downstream of the reef platform continuously sampled 1.0–1.3 ha of reef flat which included 137–202 m of fore reef. Time series and multiple correlation analysis were done to evaluate the dependence of drift biomass on selected physical and biological factors. Export and import rates and turnover times were derived and compared between the dominant species. Floating leaves, branches, and seeds of higher plants were the major components of imported drift with 52% of the dry weight mass, followed by algae and seagrass each with 19%, the water hyacinth Eichhornia with 2%, and floating tar with 8%. Exported biomass from the reef platform was higher in the dry-season (late November–March) than in the wet-season (April-early November). Within the 1.0–1.3 ha sampling area, export estimates ranged from 37–294 kg mo^-1 for the seagrass Thalassia, 3–171 kg mo^-1 for the alga Laurencia, and 3–74 kg mo^-1 for the alga Acanthophora. Multiple correlation models indicated that meteorological and hydrographic conditions explained between 31 to 65% of the variance in the drift biomass and that the best predictors of exported biomass were tidal elevation and wind speed (3 week lag). Export rates increased with high tides and strong winds and decreased with elevated water temperatures. Autocorrelations of drift biomass were generally highest at 2 week intervals, suggesting that the quantity of drift removed from the platform was, in part, related to spring and neap tide cycles. Export rates were also affected by the morphology of the vegetation, development of uprights, and location on the reef platform. Import rates of terrestrial-plant debris, the hyacinth Eichhornia, the seagrass Syringodium, and the brown alga Sargassum did not exhibit pronounced seasonal patterns in abundance and averaged 60.2, 1.9, 1.1, and 2.7 g d^-1m^-1, respectively. Wind speed was negatively correlated with Sargassum abundance, suggesting that strong winds depleted it from nearshore waters. Floating tar averaged about 10 g d^-1m^-1, the highest reported in the Caribbean. The plant-dominated fringing reef at Galeta Point is shown to be a major source, as well as a recipient, of drift vegetation.     

Seagrass recovery in the Northern Wadden Sea?

Aerial surveys on seagrass (Zostera spp.) indicate a three to fourfold increase in bed area from 1994 to 2006 with up to 100 km² or 11% of intertidal flats in the Northfrisian Wadden Sea (coastal eastern North Sea), observed at seasonal maximum in August when flying during low tide exposure 300 to 500 m above ground. When viewed from the air, difficulties in distinguishing between seagrass and green algae and a lack of contrast on dark-coloured mudflats are sources of error in areal estimates. Particularly the positioning of beds remote from shores was imprecise. However, the consistency in method over time gives confidence to the inferred positive trend which is opposite to the global pattern. Both, the spatial pattern and a recent decrease in storminess suggest that sediment stability is the key factor for seagrass dynamics in this tidal area. On exposed sand flats, high sediment mobility may be limiting and along the sheltered mainland shore land claim activities with high accretion rates may cause a scarcity of seagrass. The potential area of seagrass beds may be twice as large as the realized maximum in 2006 but eventually the rising sea level will reverse the observed seagrass expansion.     

Distribution of seagrasses along the Indian coast

Seagrass environments, from the main coast of India, Lakshadweep and Andaman Islands, were surveyed for seagrass and marine algal composition. Extensive seagrass meadows and the maximum number of species (seven genera and 12 species) occurred along the Tamil Nadu coast. Seagrasses were observed from intertidal to subtidal regions down to 8 m depth. Thalassia hemprichii (Ehrenberg) Aschers. and Cymodocea serrulata (R. Brown) Aschers. & Magnus were the dominant seagrasses in the subtidal zones. Halophila beccarii Aschers. was restricted to the intertidal mudflats in association with mangroves. The rich growth of seagrasses along the Tamil Nadu coast and Lakshadweep can be attributed mainly to high salinity, clarity of the water and sandy substratum. One hundred species of marine algae were recorded from the seagrass environments of India.     

Structure of major seagrass beds from three coral reef atolls of Lakshadweep, Arabian Sea, India

Detritus-based marine ecosystems such as mangrove and seagrass are of immense ecological importance. Major seagrass meadows from three coral atolls of the Lakshadweep group (Arabian Sea) were studied for their floral components. Seagrass beds were heterogeneous, comprising mainly of Thalassia hemprichii and Cymodocea rotundata, in Agatti and Kavaratti and it was observed to be monospecific (T. hemprichii) in the Kalpeni lagoon. Maximum (0.34 km²) and minimum (0.005 km²) extent of seagrass beds were observed in Kavaratti and Agatti lagoons, respectively. Seagrass weight (dry) of 43.97, 30.88 and 0.74 t were estimated from Kavaratti, Kalpeni, and Agatti, respectively. Maximum biomass occurred from 0–2 m depth, mainly contributed by the aboveground shoots, and was found to be negatively correlated with depth (r=0.71, p<0.05). Sediments were devoid of seed reserves indicating seagrass growth mainly by vegetative propagation. Epiphytes, on an average, contributed 7.5% of the seagrass biomass and were dominated by algae such as Melobesia spp., Microcoleus lyngbyaceus and Ceramium spp. Epiphytic biomass, too, decreased with increasing depth. Associated marine algae were represented by 66 species, dominated by rhodophytes.     

Grazing effects of a gammaridean Amphipoda,Ampithoe sp., on the seagrass,Syringodium isoetifolium,and epiphytes in a tropical seagrass bed of Fiji

In the pure stand of tropical seagrass,Syringodium isoetifolium, in a small oceanic island, Fiji, grazing effects of the seagrass-associated gammarid,Ampithoe sp., on seagrass and epiphytes were assessed in October 1989, November 1991, November 1992. Density of the gammarid was estimated with two methods, mesh bag method and tuft method. During the three years surveyed the density of the gammarid increased remarkably from 1989 to 1991, with heavy epiphytism. Gut contents of the gammarid were examined. Grazing rates on seagrass leaf with and without epiphytic blue-green algae were measured in a bottle experiment. Litter bag experiments were conducted using different mesh sizes each containing seagrass only and seagrass and gammarids. The seagrass leaf biomass in the litter bag reduced abruptly in both bags. After one week, 78–86% of seagrass biomass disappeared from the bags. Enhancement of decomposition of seagrass leaf by the gammarid grazing was observed. Oxygen consumption and ammonium excretion rates were measured simultaneously in bottle experiments. Carbon budget in the seagrass bed was estimated as follows: 0.9 gC m⁻² day⁻¹ in seagrass growth, gammarid grazing was about a half of it and further assimilated a half of it, about 0.1 gC m⁻² day⁻¹, and more than half of it become CO₂ by respiration. Grazing effects on epiphyte and seagrass growth and production were discussed through the carbon budget and indirect interactions between seagrass, epiphytes and associated gammarids to explain the temporal change of seagrass and epiphyte dynamics.     

Effects of naturally acidified seawater on seagrass calcareous epibionts

Surface ocean pH is likely to decrease by up to 0.4 units by 2100 due to the uptake of anthropogenic CO2 from the atmosphere. Short-term experiments have revealed that this degree of seawater acidification can alter calcification rates in certain planktonic and benthic organisms, although the effects recorded may be shock responses and the long-term ecological effects are unknown. Here, we show the response of calcareous seagrass epibionts to elevated CO2 partial pressure in aquaria and at a volcanic vent area where seagrass habitat has been exposed to high CO2 levels for decades. Coralline algae were the dominant contributors to calcium carbonate mass on seagrass blades at normal pH but were absent from the system at mean pH 7.7 and were dissolved in aquaria enriched with CO2. In the field, bryozoans were the only calcifiers present on seagrass blades at mean pH 7.7 where the total mass of epiphytic calcium carbonate was 90 per cent lower than that at pH 8.2. These findings suggest that ocean acidification may have dramatic effects on the diversity of seagrass habitats and lead to a shift in the biogeochemical cycling of both carbon and carbonate in coastal ecosystems dominated by seagrass beds.     

Plant-herbivore interactions in seagrass meadows

During the past two decades we have gained much insight into the factors that regulate the productivity of seagrass dominated ecosystems, especially those at low latitudes. Here, we review and reassess the importance of plant-herbivore interactions in seagrass meadows, focusing on recent studies that have examined: 1) grazing on live seagrass leaves; 2) consumption of epiphytic algae growing on seagrass leaves; and 3) consumption of planktonic algae from the waters surrounding seagrass meadows. The major conclusion is that, in contrast to what has been reported in much of the literature on food webs in seagrass meadows, a diverse grazing pathway continues to represent an important conduit for the transfer of energy from the primary producers to higher order consumers. This remains true, although in many areas consumption of seagrasses is reduced in an historical context, owing to the overharvesting of many large species of herbivorous waterfowl, turtles and mammals.We also summarize our view of the important gaps in understanding the broadly defined topic of herbivory in seagrass-dominated ecosystems. We suggest that future studies should focus on: understanding the foraging strategies of seagrass herbivores; quantifying the impact of herbivory on seagrass demography, including effects on sexual reproduction, the fate of flowers, and the production of fruits and seeds; and documenting the commonness of compensatory responses to grazing. In addition, the role of chemical defenses in seagrass species remains inadequately investigated. Studies of the roles of nutritional content (as measured by C/N/P ratios) and chemical defenses are also fertile grounds for future studies of epiphytes and their grazers, as are additional experiments to quantify the relative roles of top-down and bottom-up factors as they determine algal growth and abundance. There is also a need to expand the geographical scope of studies of epiphyte-grazer interactions from cold temperate to sub-tropical and tropical waters. Suspension feeders also need to be studied more broadly, with additional experiments required to quantify their effects on water clarity and their ability to fertilize pore waters, and whether benefits from these activities balances the costs of shading and competition for space that can result from both epifaunal and infaunal suspension feeders.