Epiphyte Biomass Research Articles

A protocol to estimate epiphyte biomass in a forest management unit: case of Cameroon

ABSTRACTVascular epiphyte biomass is poorly studied in Congo Basin forest. The aim of this study was to develop a protocol to estimate epiphyte biomass. Epiphytes were collected on 77 phorophytes of diameter 57–169.5 cm, oven-dried for biomass. Allometric equations for epiphyte biomass were developed with diameter and phorophyte height as predictor variables using regression analysis. The mean epiphyte biomass was 24.48 kg/phorophyte with a standard deviation of ±49.99. Cylicodiscus gabunensis and Triplochiton scleroxylon hosted the greatest epiphyte biomass with a mean vascular epiphyte biomass of 71.65 and 62.3 kg/phorophyte, respectively. Kruskal–Wallis test (p = 0.005) showed that there was a significant difference between epiphyte biomass with respect to phorophyte species. The relationship found between epiphyte biomass and dendrometric variables of the phorophytes were intermediate (Adj.R2 < 0.630) meaning epiphyte biomass increased with increasing tree diameter. However, models with diameter input variables had greater prediction than input variables of total height. Based on the Akaike Information Criterion (AIC), the allometry lnY = −21.37 + 5.21 x ln(D) −0.35 x ln(H) with AIC (183) was the best equation for the prediction of epiphyte biomass. These findings shall contribute to REDD+ (Reducing Emissions from Deforestation and Forest Degradation) strategy for climate change mitigation in Congo Basin.

Species interactions can shift the response of a maerl bed community to ocean acidification and warming

Abstract. Predicted ocean acidification and warming are likely to have major implications for marine organisms, especially marine calcifiers. However, little information is available on the response of marine benthic communities as a whole to predicted changes. Here, we experimentally examined the combined effects of temperature and partial pressure of carbon dioxide (pCO2) increases on the response of maerl bed assemblages, composed of living and dead thalli of the free-living coralline alga Lithothamnion corallioides, epiphytic fleshy algae, and grazer species. Two 3-month experiments were performed in the winter and summer seasons in mesocosms with four different combinations of pCO2 (ambient and high pCO2) and temperature (ambient and +3 °C). The response of maerl assemblages was assessed using metabolic measurements at the species and assemblage scales. This study suggests that seasonal variability represents an important driver influencing the magnitude and the direction of species and community response to climate change. Gross primary production and respiration of assemblages was enhanced by high pCO2 conditions in the summer. This positive effect was attributed to the increase in epiphyte biomass, which benefited from higher CO2 concentrations for growth and primary production. Conversely, high pCO2 drastically decreased the calcification rates in assemblages. This response can be attributed to the decline in calcification rates of living L. corallioides due to acidification and increased dissolution of dead L. corallioides. Future changes in pCO2 and temperature are likely to promote the development of non-calcifying algae to the detriment of the engineer species L. corallioides. The development of fleshy algae may be modulated by the ability of grazers to regulate epiphyte growth. However, our results suggest that predicted changes will negatively affect the metabolism of grazers and potentially their ability to control epiphyte abundance. We show here that the effects of pCO2 and temperature on maerl bed communities were weakened when these factors were combined. This underlines the importance of examining multi-factorial approaches and community-level processes, which integrate species interactions, to better understand the impact of global change on marine ecosystems.

Unexpected resilience of a seagrass system exposed to global stressors.

Despite a growing interest in identifying tipping points in response to environmental change, our understanding of the ecological mechanisms underlying nonlinear ecosystem dynamics is limited. Ecosystems governed by strong species interactions can provide important insight into how nonlinear relationships between organisms and their environment propagate through ecosystems, and the potential for environmentally mediated species interactions to drive or protect against sudden ecosystem shifts. Here, we experimentally determine the functional relationships (i.e., the shapes of the relationships between predictor and response variables) of a seagrass assemblage with well-defined species interactions to ocean acidification (enrichment of CO2 ) in isolation and in combination with nutrient loading. We demonstrate that the effect of ocean acidification on grazer biomass (Phyllaplysia taylori and Idotea resecata) was quadratic, with the peak of grazer biomass at mid-pH levels. Algal grazing was negatively affected by nutrients, potentially due to low grazer affinity for macroalgae (Ulva intestinalis), as recruitment of both macroalgae and diatoms were favored in elevated nutrient conditions. This led to an exponential increase in macroalgal and epiphyte biomass with ocean acidification, regardless of nutrient concentration. When left unchecked, algae can cause declines in seagrass productivity and persistence through shading and competition. Despite quadratic and exponential functional relationships to stressors that could cause a nonlinear decrease in seagrass biomass, productivity of our model seagrass-the eelgrass (Zostera marina)- remained highly resilient to increasing acidification. These results suggest that important species interactions governing ecosystem dynamics may shift with environmental change, and ecosystem state may be decoupled from ecological responses at lower levels of organization.

Habitat fragmentation has some impacts on aspects of ecosystem functioning in a sub-tropical seagrass bed

Habitat fragmentation impacts ecosystem functioning in many ways, including reducing the availability of suitable habitat for animals and altering resource dynamics. Fragmentation in seagrass ecosystems caused by propeller scarring is a major source of habitat loss, but little is known about how scars impact ecosystem functioning. Propeller scars were simulated in seagrass beds of Abaco, Bahamas, to explore potential impacts. To determine if plant-herbivore interactions were altered by fragmentation, amphipod grazers were excluded from half the experimental plots, and epiphyte biomass and community composition were compared between grazer control and exclusion plots. We found a shift from light limitation to phosphorus limitation at seagrass patch edges. Fragmentation did not impact top-down control on epiphyte biomass or community composition, despite reduced amphipod density in fragmented habitats. Seagrass and amphipod responses to propeller scarring suggest that severely scarred seagrass beds could be subject to changes in internal nutrient stores and amphipod distribution.

Experimental assessment ofPosidonia oceanica-associated gastropods grazing on an early-successional biofilm community: nutrient availability and species-specific effects

Although grazing is considered an essential process controlling epiphyte biomass on seagrass leaves, there is still a lack of fundamental knowledge about the species-specific consumption rates of the most common grazers in Mediterranean meadows. This study experimentally assessed the effect of Posidonia oceanica-associated gastropod grazing on early successional biofilm and the species-specific relationship between biofilm consumption rates and biofilm biomass. Two biofilms on artificial substrata, both developed in situ (in a P. oceanica meadow), one under ambient conditions and the other under nutrient-enriched conditions, were offered in aquaria assays to nine species of grazers found in P. oceanica meadows. Biofilm consumption rates and their association with biofilm biomass were assessed. It was found that: (i) there was a positive association between biofilm consumption and biofilm biomass up to 20 mg Chl a·m−2 for Bittium reticulatum, Gibbula ardens, Jujubinus exasperatus and Tricolia pullus; (ii) Alvania montagui, B. reticulatum and Jujubinus striatus showed the highest consumption rates and are thus expected to be amongst the leading consumers in early-successional epiphytic communities; (iii) there was not an increase of consumption rate when a substratum colonized under nutrient-enriched conditions was offered to any of the nine studied species. This study provides species-specific consumption rates knowledge that is useful for the assessment of the strength of grazer–epiphyte interactions and trophic fluxes in P. oceanica meadows.

The absorption and evaporation of water vapor by epiphytes in an old‐growth Douglas‐fir forest during the seasonal summer dry season: Implications for the canopy energy budget

AbstractOur goal was to determine how epiphytic lichens and bryophytes affect canopy latent heat fluxes in an old‐growth Douglas‐fir forest when the canopy was dry. The epiphyte water content (WCe expressed as a percent of dry weight) of representative epiphytic foliose lichens, fruticose lichens, and bryophytes was measured in the laboratory after 1 to 12 hr of exposure at five different values of vapor pressure deficit (VPD). After 12 hr of exposure, WCe increased fivefold to sixfold as VPD decreased from 1849 to 132 Pa. In addition, we measured WCe in the field using strain gauges. These field measurements were used to calibrate the models described below. Two models were created to estimate the potential latent heat flux from epiphytes at the canopy scale (LEe). The first model combined measured total biomass of epiphytes with a model that estimated the laboratory determined VPD‐dependent changes in WCe of the lichens/bryophytes (VPD method). The second model estimated LEe by scaling the change in WCe of epiphyte‐laden branches that were continuously monitored in situ in the canopy by a strain gauge (SG method). Both methods showed a strong diurnal trend in LEe when VPD was less than 645 Pa. Prior to sunrise, the epiphytes absorbed water, corresponding to a latent heat flux of 5 to 15 W/m2 per unit ground area, whereas after sunrise, the epiphytes lost water at a rate of −10 to −20 W/m2. For short periods, epiphytes may contribute a significant portion of the latent heat flux from Douglas‐fir forests.

Abundance, performance, and feeding preference of herbivorous amphipods associated with a host alga-epiphyte system

Mesograzers can benefit their host macrophytes by selectively grazing on epiphytic algae, such as well documented for seagrass meadows and other systems where hosts seem to be less palatable than epiphytes. However, where both host and epiphytes are algae and are likely to be palatable, the predictions about the feeding preferences of mesograzers and, thus, their potential ability in controlling the epiphyte biomass are unclear. Also, the performance consequences of feeding on host algae versus epiphytes, what can explain the feeding preferences of mesograzers, have been rarely tested. The food value of the host alga Sargassum filipendula and two of its common epiphytic algae, Hypnea musciformis and Dictyota cervicornis, for mesograzers was contrasted by investigating the abundance, performance, feeding preference, and feeding rates of the herbivorous amphipods Cymadusa filosa and Sunamphitoe pelagica. Sampling was carried out seasonally between January and November 2012 in a subtropical shore from the Brazilian south-eastern coast (23°32′S, 45°10′W). The abundance of both herbivores was positively correlated with S. filipendula biomass. However, only C. filosa had a positive relationship with H. musciformis biomass during the season of lower S. filipendula abundance. Juveniles of S. pelagica were able to survive only when consuming S. filipendula. In contrast, C. filosa survived at similar rates on epiphytes and S. filipendula, but long-term feeding on epiphytes resulted in lower growth and reproduction. Both herbivores preferred to consume S. filipendula in choice assays, but only C. filosa was able to feed on epiphytes at similar rates as it did on S. filipendula in no-choice assays. These mesograzer species may differ from each other in the mechanisms determining their association with S. filipendula. For C. filosa, epiphytes should be an alternative source of food, mainly in periods of lower host availability. Thus, the role of epiphytes on mesograzer-Sargassum interaction seems to vary temporally and among herbivores. Overall, these results contrast with the general patterns about mesograzer preferences towards epiphytes reported for other systems. Therefore, the feeding preferences of mesograzers in host-epiphyte systems and, thus, their potential impact may depend on the identity of hosts and epiphytes.

Response of seagrass (Thalassia testudinum) metrics to short-term nutrient enrichment and grazing manipulations

The relative influence of top-down and bottom-up factors in seagrass communities has received considerable attention; however, individual seagrass attributes do not always respond in consistent fashion across studies. Because these metrics have been used as indicators of ecosystem stress, it is necessary to examine whether seagrass response to nutrient enrichment and grazing is persistent. To do so, we examined seagrass and epiphyte response to field manipulations of nutrients and grazing in Big Lagoon, FL, USA where previous studies were conducted with similar methodologies. Our manipulations were successful in elevating water column and porewater nutrient levels. Nutrient additions caused significant increases in tissue nitrogen contents (p=0.01), but they did not affect seagrass or epiphyte biomass, shoot density, or percent cover. Aboveground biomass was significantly reduced in both treatments involving grazing simulations when compared to the fertilizer addition only treatment (p=0.001). Fish counts revealed greater numbers of pinfish in fertilized plots compared to all other treatments (p=0.000), suggesting the fish were attracted to plots in which fertilization resulted in elevated tissue nitrogen contents. Overall, effects of fertilization and grazing on various seagrass attributes showed both similarities and differences in comparison to other studies from the same and different locations, raising the question about how predictable and informative the responses of individual seagrass characteristics for discerning the relative, long-term influence of these factors on seagrass populations.

Environmental factors structuring benthic primary producers at different spatial scales in the St. Lawrence River (Canada)

The influence of environmental factors controlling the biomass of submerged aquatic macrophytes, cyanobacterial mats, and epiphyton was examined at three nested spatial scales within the St. Lawrence River: (1) along a 250-km-long upstream–downstream river stretch, (2) among three fluvial lakes located within that river stretch and (3) within each fluvial lake at sites located upstream, at the mouth, and downstream of the St. Lawrence River tributaries. Over its 250-km-long course, large increases of water colour (fivefold), suspended matter (tenfold), dissolved organic carbon (DOC) (twofold) and dissolved N and P concentrations (2.5-fold) were observed in the St. Lawrence River, showing the cumulative effects of human activities on water quality. In contrast, biomass of submerged vascular macrophytes dropped tenfold along the sampled reach whereas biomass of epiphytes and cyanobacterial mats rose significantly. Biomass of the three benthic primary producers (PP) was explained (59 %) by the combined effects of conductivity, TP and spatial structure. Macrophyte biomass was related to changes in conductivity (+), biomass of epiphyton responded to DIN:TDP ratio (+) and light extinction coefficient (+) and cyanobacterial mats coincided with differences in DOC (+) and NH4+ (−). Within-lake structure was the most important spatial component for all benthic PP, suggesting that local effects, such as enrichment by the inflow of tributaries, rather than upstream–downstream gradients, determined the biomass of benthic PP. Our study shows that the sum of local tributary inflows exerts major overall pressures on benthic PP in the St. Lawrence River and that conversely, small-scale management of individual watersheds, can markedly improve local ecological condition of the river ecosystems.

Large variations in composition and spatial distribution of epiphyte biomass on large trees in a tropical montane forest of northern Thailand

Large amounts of epiphytic matter (EM), which comprises living epiphytes and canopy humus, play important roles in tropical montane forests. However, little is known about the composition and spatial distribution of EM in tropical Asia because the tall and complex tree structures have hampered direct and precise measurements of EM. Using the three-dimensional mapping method, we examined the composition and distribution of EM on three large trees and estimated the EM mass in a tropical montane forest in northern Thailand. EM mass per unit cover area (mean dry weight ± SE) varied widely among their physiognomies from 80.0 ± 12.7 g m−2 in sparse bryophytes, to 5882 ± 1451 g m−2 in orchid mats composed mainly of vascular epiphytes and canopy humus. With the increase of tree size, the EM mass on individual trees increased from 73.4 to 480.9 kg, and the proportions of vascular epiphytes and canopy humus to the total EM mass increased, whereas that of bryophytes decreased. The proportions of bryophytes and canopy humus varied among positions within the tree crown, and the distribution pattern of EM within the tree crown was different between the trees. Although the number of sample trees was small, our study provides an evidence that large trees contribute to the accumulation of a large mass of EM attributed to developments of vascular epiphytes and canopy humus.

Quantifying the influence of small omnivorous fishes on seagrass epiphyte load.

The influence of two cryptic, seagrass-inhabiting omnivorous fishes, the fan-bellied leatherjacket Monacanthus chinensis and the variable sabretoothed blenny Petroscirtes variabilis, on seagrass epiphyte biomass are described. Overall, M. chinensis significantly reduced epiphyte biomass by 35·1% after 18 h in experimental aquaria, whilst P. variabilis showed a non-significant 15·7% reduction. It is concluded that some cryptic omnivorous species play an important role in epiphyte removal in seagrass beds.

Even moderate nutrient enrichment negatively adds up to global climate change effects on a habitat-forming seaweed system

Coastal marine ecosystems have been under high anthropogenic pressure and it can be assumed that prevalent local perturbation interacts with rising global stressors under proceeding climate change. Understanding their effective pathways and cumulative effects is of high relevance not only with regard to future risk assessment, but also for current ecosystem management. In benthic mesocosms, we factorially tested the effects of one global (combined elevated seawater temperature and CO2 concentration) and one local (nutrient enrichment) stressor on a common coastal Baltic seaweed system (Fucus vesiculosus). Both treatments in combination had additive negative impacts on the seaweed—epiphyte—mesograzer system by altering its regulatory mechanisms. That is, warming decreased the biomass of two mesograzer species (weakened top-down control), whereas moderate nutrient enrichment increased epiphyte biomass (intensified bottom-up control), which ultimately resulted in a significant biomass reduction of the foundation seaweed. Our results suggest that climate change impacts might be underestimated if local pressures are disregarded. Furthermore, they give implication for local ecological management as the mitigation of local perturbation may limit climate change impacts on marine ecosystems.

Modelling Primary Producers in Some Shallow Spanish Lakes

Increased nutrient loading is regarded as one of the major causes of a shift from macrophyte to phytoplankton dominance in shallow lakes. This work investigates the responses of phytoplankton and epiphyton biomass, macrophytes and algal species assemblages along a trophic gradient in some shallow Mediterranean lakes. A unimodal response of epiphyton biomass to increasing phosphorus levels was observed, while phytoplankton developed exponentially and submerged aquatic plant coverage decreased. At TP levels above 100 μg L−1 (~30 μg-P L−1 by element) phytoplankton began to dominate over the other primary producers and led the system to a turbid state, mainly dominated by cyanobacteria. This could be regarded as a critical threshold to avoid algal turbid states, especially for shallow Mediterranean lakes. Macrophyte typology was also important in modulating this response. On average, the unimodal maximum of epiphyton biomass was about 20 μg g−1 of macrophyte dry weight and hydrophytes had higher epiphytic biomass than helophytes. The coverage of submerged aquatic vegetation was not related to the epiphytic and phytoplanktonic biomass, but to the distribution of their species. Further studies on epiphyton-plant ecology are needed, especially for specific aquatic macrophyte conservation and lake management.

Variability in patterns of macro-epiphytic leaf community of Posidonia oceanica in the Islands of Kuriate: Western coast of Tunisia

Objective: To test the response of the epiphyte community structure and biomass of the Posidonia oceanica (P. oceanica) leaves to natural disturbance. Methods: Sampling of P. oceanica was carried in winter and summer on three sites in Kuriate Islands (western coast of Tunisia) subject to different environments disturbances. Shoots of P. oceanica were preserved in seawater-formalin (5%) solution for macro-epiphytes species identification in the laboratory. The samples were examined for leaf surface per shoot and the coverage (expressed as a percentage of leaf surface) of each morphological group, then carefully scraped with a razor blade. Epiphytes and scraped leaves were oven-dried at 60 °C for 48 h. Biomass was expressed as g dry weight/shoot. Results: The biomass and the percentage cover of macro-epiphytic leaves showed seasonal variation. The highest values of epiphytic leaves were detected in summer whereas the lowest values were registered during winter. ANOVA showed that Kuriate Islands functioned as a single ecosystem in terms assemblage of macro-epiphytic leaves since no significant variation was detected for biomass and percentage cover at the scale site. Our study showed that natural disturbance had no effect on the assemblage distribution and the biomass of macro-epiphyte on the leaves of P. oceanica between the scales of site, whereas variability at the smallest scale was detected. ANOVA showed that exposure to wind and current had no effect on the biomass of macro-epiphytes leaves. Conclusions: Biomass and assemblages of macro-epiphytic leaves of P. oceanica were high in summer and homogenous between all sites investigated. Natural disturbances such as exposure to wind have no effect on the distribution and the biomass of epiphytes on the shallow meadow.

Influence of aquatic plant architecture on epiphyte biomass on a tropical river floodplain

Tropical floodplains are highly productive because of seasonal replenishment of water and nutrients, which substantially boost primary productivity. This study examined how the architecture of aquatic macrophytes affect the light and water quality and consequently the attachment and biomass of epiphytes on a floodplain in northern Australia. Results show that macrophyte structural complexity is not only important for water column light penetration but also for the development of epiphytes on macrophytes. Emergent grasses with simple vertical structure and high plant densities, limit light penetration and consequently the development and biomass of epiphytic algae. In contrast, submerged macrophytes growing just below the water surface, allow greater light penetration. The complex architecture of submerged macrophytes also provides a large surface area for the development of a dense covering of epiphytic algae. Other plant structural forms (e.g., plants with floating leaves) have a simple structure, variable light penetration and low epiphytic algae biomass. The emergent grass Pseudoraphis spinescens (R.Br.) Vickery also had low light penetration but the horizontal alignment of stems across the water surface allow greater exposure to sunlight of the stems and the consequent development of epiphytic algae. We conclude that (1) the complex structure of submerged plants effectively creates a “false bottom” in deeper waters so that they function similarly to the floodplain's littoral zone, and (2) that their extremely large surface area for attachment allows greater production of epiphytic algae than would occur on the sediment surface.

Spatial scale variability in shoot density and epiphytic leaves ofPosidonia oceanicaon Kerkennah Island (Tunisia) in relation to current tide effects

AbstractA study was undertaken of the patterns of spatial variability, epiphytic biomass and distribution of epiphytic fauna and flora ofPosidonia oceanica. Samples were taken at four stations located approximately 4 km apart, exposed to different current conditions. Stations A and B, situated near the Oued Mimoun tidal channel with its relatively strong bi‐directional flows, were affected by high current tide. The other two stations, North Oued Mimoun (L1) and South Oued Mimoun (L2), were located further from the channel, in low current tide conditions. Sampling was conducted in the Attaya area of Kerkennah Island (Tunisia) in August 2009 at depths between 2 and 3 m, with the results indicating differences among the stations. Shoot density decreased when exposed to high levels of hydrodynamic activity generated by current tides whereas the epiphytic biomass ofP. oceanicaleaves decreased at sheltered stations located far from the channel. Epiphytic algae such as Heterokontophyta, Rhodophyta and Chlorophyta, and epiphytic fauna represented by Bryozoa, Hydrozoa, Annelida, Porifera and Tunicata, dominated the epiphytic assemblages and were abundant at the station most exposed to high current tide hydrodynamics. Cyanobacteria, however, were dominant in stations exposed to low current tide.

Non‐consumptive effects of avian predators on fish behavior and cascading indirect interactions in seagrasses

Top–down impacts of avian predators are often overlooked in marine environments despite evidence from other systems that birds significantly impact animal distribution and behavior; instead, birds are typically recognized for the impacts of their nutrient rich guano. This is especially true in shallow seagrass meadows where restoration methods utilize bird perches or stakes to attract birds as a passive fertilizer delivery system that promotes the regrowth of damaged seagrasses. However, this method also increases the local density of avian piscivores that may have multiple unexplored non‐consumptive effects on fish behavior and indirect impacts to seagrass communities. We utilized laboratory and field experiments to investigate whether visual cues of avian predators impacted the behavior of the dominant demersal fish in seagrass habitats, the pinfish Lagodon rhomboides, and promoted cascading interactions on seagrass‐associated fauna and epiphytes. In laboratory mesocosms, pinfish displayed species specific responses to models of avian predators, with herons inducing the greatest avoidance behaviors. Avoidance patterns were confirmed in field seagrass meadows where heron models significantly reduced the number of fish caught in traps. In a long term field experiment, we investigated whether avian predators caused indirect non‐consumptive effects on seagrass communities by monitoring fish abundances, invertebrate epiphyte grazers, and the seagrass epiphytes in response to heron models, bird exclusions, and bird stakes. On average, more fish were recovered under bird exclusions and fewer fish under heron models. However, we found no evidence of cascading effects on invertebrate grazers or epiphytes. Bird stake treatments only displayed a simple nutrient effect where higher bird abundances resulted in higher epiphyte biomass. Our results indicate that although birds and their visual cues can affect fish and epiphyte abundance through non‐consumptive effects and nutrient enrichment, these impacts do not propagate beyond one trophic level, most likely because of dampening by omnivory and larger scale processes.

Herbivory in a subtropical seagrass ecosystem: separating the functional role of different grazers

Seagrass meadows provide many important ecosystem services, but they are threatened by human activities and are in decline globally. In particular, eutrophication arising from human activities promotes algal growth, which negatively affects seagrass. Herbivores consume algae and can, therefore, reduce eutrophication effects, but they may also consume seagrass. Little is known, however, about grazer-epiphyte-seagrass interactions in subtropical seagrass in the Indo-Pacific. We used a 5 wk exclusion experiment to quantify the influence of different grazers in seagrass (dominated by Zostera muelleri) in Moreton Bay, eastern Australia. Our results show that herbivory does indeed affect seagrass-epiphyte dynamics in this region and that different grazers can exert different effects in seagrass ecosystems. In particular, exclusion of small mesograzers (i.e. amphipods and juvenile shrimp) caused epiphyte biomass to increase by up to 233%. Exclusion of medium mesograzers (i.e. small fish and prawns) resulted in increases of up to 10% in seagrass cover, 53% in shoot height and 29% in shoot density. Large mesograzers (i.e. adult fish) and macrograzers (i.e. turtles and dugong) did not appear to play a role in the study system. These results demonstrate that mesograzers can be important in controlling epiphytic algae in subtropical Indo-Pacific seagrass, and show that different mesograzers can affect seagrass-epiphyte dynamics in different ways. It is critical that the functional effects of different herbivores be considered when implementing programs for seagrass conservation and restoration.

Inshore capture-based tuna aquaculture impact on Posidonia oceanica meadows in the eastern part of the Adriatic Sea

Mapping and monitoring of the seagrass Posidonia oceanica in the eastern (Croatian) part of the Adriatic Sea since 2004 indicates a significant decline in meadow density in an area impacted by inshore capture-based tuna aquaculture. The density and overall condition of P. oceanica meadows impacted by tuna farms near Fulija Islet was compared to two reference sites (Iž Island and Mrtovnjak Islet). The factors with the most significant influence on P. oceanica meadows were found to be the input of organic matter originating from the cages, as well as high epiphyte biomass caused by nutrient enrichment. Significant differences in nutrient concentrations were found between the sites impacted by tuna farms (Fulija Islet) and the control stations. Shoot density of the P. oceanica meadows decreased at the stations in close vicinity to the tuna farm, which suggests that the tuna farm activity strongly affected the surrounding meadows.

Bathymetric variation of epiphytic assemblages on Posidonia oceanica (L.) Delile leaves in relation to anthropogenic disturbance in the southeastern Mediterranean.

A survey of the epiphytic leaves of Posidonia oceanica was conducted along a depth transect at both the control station Attaya in the Kerkennah Islands and the disturbed Mahres station on the Sfax coast (Tunisia). Samples were collected by scuba divers at depths of 5, 10, 15, and 20m in July 2008. We evaluated whether the pattern of spatial variability of the macroepiphyte assemblages of leaves of Posidonia oceanica differed in relation to anthropogenic interference. The results indicate that the decrease in shoot density and leaf length according to depth was low at Mahres. The biomass of epiphytic leaves and the percentage cover of epiphytic assemblages decreased with depth for both stations and heavily at Mahres, this decline being related to anthropogenic disturbance. This study shows that the highest values of epifauna and epiflora were detected at the disturbed station Mahres. Macroalgae assemblages decreased with depth at both stations and were dominated by Rhodophyta, whereas the percentage cover of the epifauna leaf that decreases according to depth was dominated by Hydrozoa and Bryozoa. Changes in epiphyte assemblages, epiphytic biomass, percentage cover, and species richness in proportion to Heterokontophyta, Rhodophyta, Cyanobacteria, Hydrozoa, Porifera, and Tunicata between the two stations constitute promising tools for detecting environmental disturbance.