Seagrass Epiphytes Research Articles

Microplastics in seagrass ecosystems: A review of fate and impacts

Microplastics have been detected in seagrass ecosystems, raising concerns about their potential impacts on the ecological functions of seagrasses. Seagrass meadows are biodiversity hotspots as they provide habitats to diverse fish and invertebrates. They also play a crucial role in nutrient cycling, capturing carbon, and buffering coastal erosion. This review aims to present the fate of microplastics in seagrass ecosystems and their impacts on the ecosystems. A total of 66 scientific articles have been reviewed. The review highlights that seagrass meadows intercept microplastics, though the relevant results are currently inconclusive. Some microplastics attach to the epiphytes on seagrass blades or the seagrass blades while some accumulate in the seagrass sediment, causing enrichment of microplastics in seagrass meadows. Nonetheless, a few studies did not observe such intercepting effects. Microplastic enrichment, where observed, could be due to near-bed turbulent kinetic energy that entraps denser sinking microplastics. Microplastics can directly affect seagrasses by blocking light and nutrient transfer, affecting their shoot or leaf turnover, degenerating root and causing oxidative stress. However, a study on Zostera marina L. found that short-term exposure to microplastics did not significantly impact bicarbonate utilization and photosynthetic efficiency. Microplastic additives, particularly bisphenol A reduced chlorophyll and caused peroxide accumulation in Cymodocea nodosa. The presence of biodegradable plastics in the sediment might alter the distribution and interaction of seagrass species. Seagrasses could be affected indirectly through the potential impacts of microplastics on seagrass epiphytes but more studies are needed to confirm this. Desorption of pollutants sorbed on microplastics could negatively affect seagrass meadows. Further research could focus on the impacts of microplastic accumulation on the seagrass ecosystem and the processes therein, including nutrient cycling. Disintegration-oriented techniques and alternatives to conventional plastics are two strategies to mitigate microplastic prevalence in the environment.

Nitrogen fixation associated with epiphytes on the seagrass Zostera marina in a temperate lagoon with moderate to high nitrogen loads

AbstractAs part of a long-term study on the effects of nitrogen (N) loading in a shallow temperate lagoon, we measured rates of N2 fixation associated with seagrass (Zostera marina) epiphytes during the summer from 2005 to 2019, at two sites along a gradient from where high N groundwater enters the system (denoted SH) to a more well-flushed outer harbor (OH). The data presented here are the first such long-term N2 fixation estimates for any seagrass system and one of the very few reported for the phyllosphere in a temperate system. Mean daily N2 fixation was estimated from light and dark measurements using the acetylene reduction assay intercalibrated using both incorporation of 15N2 into biomass and a novel application of the N2:Ar method. Surprisingly, despite a large inorganic N input from a N-contaminated groundwater plume, epiphytic N2 fixation rates were moderately to very high for a seagrass system (OH site 14-year mean of 0.94 mmol N m−2 d−1), with the highest rates (2.6 mmol N m−2 d−1) measured at the more N-loaded eutrophic site (SH) where dissolved inorganic N was higher and soluble reactive phosphorus was lower than in the better-flushed OH. Over 95% of the total N2 fixation measured was in the light, suggesting the importance of cyanobacteria in the epiphyte assemblages. We observed large inter-annual variation both within and across the two study sites (range from 0.1 to 2.6 mmol N fixed m−2 d−1), which we suggest is in part related to climatic variation. We estimate that input from phyllosphere N2 fixation over the study period contributes on average an additional 20% to the total daily N load per area within the seagrass meadow.

Fingerprinting macrophyte Blue Carbon by pyrolysis-GC-compound specific isotope analysis (Py-CSIA).

There is a need for tools to determine the origin of organic matter (OM) in Blue Carbon Ecosystems (BCE) and marine sediments to (1) facilitate the implementation of Blue Carbon strategies into carbon accounting and crediting schemes and (2) decipher changes in ecosystem condition over decadal to millennial time scales and thus to understand and predict the stability of BCE in a changing world. Pyrolysis-GC-compound specific isotope analysis (Py-CSIA) is applied for the first time in marine environments and BCE research. We studied Australian mangrove, tidal marsh and seagrass sediments, in addition to potential sources of OM (Avicennia, Posidonia, Zostera, Sarcocornia, Ecklonia and Ulva species and seagrass epiphytes), to identify precursors of different biomacromolecule constituents (lignin, polysaccharides and aliphatic structures). Firstly, the link between bulk δ13C and δ13C reconstructed from compound-specific δ13C showed that the pyrolysis approach allows for the isotopic screening of a representative portion of the OM. Secondly, for all samples, the C isotope fingerprint of the carbohydrate products (plant polysaccharides) was the heaviest (13C enriched), followed by lignin and aliphatic products. The differences in δ13C among macromolecules and the overlap in δ13C among putative sources reflect the limitations of bulk δ13C analyses for deciphering OM provenance. Thirdly, phanerogams specimen had the heaviest carbohydrate and lignin, confirming that seagrass-derived lignocellulose can be traced based on δ13C. Consistent differences for individual compounds were identified between seagrasses and between Avicennia and Sarcocornia using Py-CSIA. Fourth, ecosystem shifts (colonization of seagrass habitats by mangrove) on millenary time scales, hypothesized in previous studies on the basis of bulk δ13C and Py-GC-MS, were confirmed by Py-CSIA. We conclude that Py-CSIA is useful in Blue Carbon research to decipher OM sources in marine sediments, identify ecosystem transitions in palaeoenvironmental records, and to understand the role of different OM compounds in Blue Carbon storage.

Pigment-Based Chemotaxonomy of Seagrass Epiphyte Communities; Variables to Consider and uses in Ecosystem Assessment and Monitoring

Pigment-Based Chemotaxonomy of Seagrass Epiphyte Communities; Variables to Consider and uses in Ecosystem Assessment and Monitoring

Phytoplankton Response to Changes in Water Quality and a Flood Event in Hervey Bay, Queensland

Dynamic, integrated and complex processes dominate nearshore marine ecosystems. Industrial, agricultural and population growth delivers pollution and excess nutrient and sediment loads to nearshore marine zones. Climate change also poses serious threats to the delicate balance that exists between and within marine ecosystem processes. Methods to monitor, characterise and model nearshore marine systems have been developed to predict responses to changes in the physical, chemical and biological parameters that constitute elements of a model. Such models are useful to management authorities as they provide a tool to mitigate adverse impacts to marine ecosystems. Phytoplankton community structure provides a sensitive early warning for change to marine ecosystems. It is also a vital component of models derived to determine light attenuation and requirements for healthy seagrass and coral habitats. The multifactorial physicochemical drivers of change to marine microalgae abundance and community structure were investigated in Hervey Bay following two floods, a cyclone and destruction of 1000 km2 of seagrass. A water quality gradient from point sources to offshore sites was identified. Correlations between chlorophyll a (Chl a) and both soluble reactive phosphate (SRP) and total suspended solids was revealed. Diatoms dominated the phytoplankton community structure with seasonal and site differences detected amongst 150 phytoplankton species. Chl a, Secchi depth, SRP and pH maximised the rank correlation with phytoplankton assemblage structure (ρ = 0.63) to best ‘explain’ the link between water quality and phytoplankton (BIOENV). Indicator species include Cylindrotheca closterium for the nearshore degraded sites and Rhizosolenia sp. and Guinardia sp. (G. flaccida and G. striatula) for the offshore more pristine habitats. Following the flood in February 1995, Cylindrotheca closterium, Pseudonitzschia sp. and Skeletonema costatum bloomed at Pulgul Creek (a treated sewage effluent site), and Thalassionema frauenfeldii, Cerataulina bicornis and dinoflagellates bloomed at the mouth of the Mary River (agricultural runoff). Discharge-driven flood impact and trigger values were analysed to assist management authorities in their decision making around potential seafood biotoxin problems associated with harmful algal blooms (HAB’s). Results highlight the importance of species determination rather than Chl a assessment when evaluating the health of an ecosystem. Spring blooms of Trichodesium sp. indicate an additional nutrient source during high traffic of commercial whale watching vessels. Flood impacts highlight the need to schedule dredging, construction and other land management and development activities outside of flood periods. Commercial fishing and swimming should be avoided following a flood due to the health risks associated with toxic phytoplankton species (HAB’s) and for the health and safety of swimmers, consumers and other aquatic species. Future research to replicate this study in Hervey Bay and in other estuaries holds much value to further explore the phytoplankton species identified as bio-indicators of ecosystem health in the present study. Research yet to be published, addresses the links between water quality and seagrass epiphytes, light attenuation and the habitat requirements of seagrass in Hervey Bay.

Diversity and composition of algal epiphytes on the Mediterranean seagrass Cymodocea nodosa: a scale-based study

Abstract Cymodocea nodosa, a typical marine angiosperm species in the Mediterranean Sea, hosts a range of epiphytic algae. Epiphyte abundance varies at different spatial scales, yet epiphyte diversity and community composition are poorly understood. This study explores the epiphytes on C. nodosa from two reference meadows (Thasos, Vrasidas) and one anthropogenically stressed meadow (Nea Karvali) in the northern Aegean Sea (Kavala Gulf, Greece). A nested destructive sampling design at three spatial scales (metres, hundreds of metres, kilometres) and stereoscopic/microscopic observations were used. Light microscopy revealed a total of 19 taxa of macroalgae populating the leaves of C. nodosa. The most commonly encountered taxa with highest cover (%) were Hydrolithon cruciatum and Feldmannia mitchelliae. DNA sequencing (18S rDNA) confirms the presence of a number of dinoflagellate and red algal epiphytes, and this represents the first application of DNA metabarcoding to study the diversity of seagrass epiphytes. Epiphytic communities studied at species/taxon and functional (Ecological Status Groups) levels separated the reference low-stressed meadows from the degraded one, with the functional approach having higher success. The ecological evaluation index classified the studied meadows into different Ecological Status Classes according to anthropogenic stress.

Pathways to Spatial Subsidies by Kelp in Seagrass Meadows

Spatial subsidies often occur from highly productive donor ecosystems to recipient systems with low productivity, but little is known about subsidies between highly productive ecosystems. Here, we investigated whether the kelp Ecklonia radiata, which is frequently dislodged from reefs, provides spatial subsidies in adjacent seagrass meadow where in situ productivity is already high. We clarified the pathways of the allochthonous kelp as a spatial subsidy by adding 15N-enriched kelp to Posidonia sinuosa seagrass meadows, tracing the uptake of kelp-derived nitrogen by seagrass epiphytes and epifauna. We observed a 32.5–52.2% loss of kelp biomass (wet weight), representing almost 13,000 μg of 15N from the labelled kelp. This corresponded with an increase in δ15N and 15N biomass of detritivores and epiphytes, showing either a direct or indirect (via epiphytes) assimilation of kelp-derived nutrients. The densities and biomass of the gastropod Strigosella lepidus and shrimps were higher in seagrass plots with input of E. radiata, but not for amphipods or the biomass of epiphytes. We estimate that the detritivores and epiphytes in the seagrass plots assimilated about 3% of the 15N released from the labelled kelp, and based on a data from previous study, the seagrass itself could have assimilated up to a further 6% of the released kelp 15N. Our findings show that in a highly productive seagrass ecosystem, where autochthonous food sources are available, the input of allochthonous resources such as kelp can supplement those resources and appears to play an important a role as a vector for a spatial subsidy.

Pelagic Sargassum as an emerging vector of high rate carbonate sediment import to tropical Atlantic coastlines

Abstract Since 2011, pelagic Sargassum has inundated Caribbean, West African, and northern Brazilian shorelines in increasing volumes. These events are linked to the emergence of a major new Sargassum bloom region in the Atlantic Ocean, and annual high-volume Sargassum beachings are seemingly becoming an established norm. Resultant socio-economic and ecological implications are widespread and potentially serious, but an important question that has so far received no attention is whether these Sargassum inundations might represent a new source of carbonate sediment in affected coastal areas. This sediment derives from calcareous epiphyte communities that colonise Sargassum (e.g., bryozoans, serpulid worms, and red algae), and if volumetrically significant, may help to counteract aspects of Sargassum beachings thought to reduce sediment supply and decrease coastal stability. Here we determine the carbonate contents of Sargassum from coastal waters of the Mexican Caribbean. Integrating these with volumetric data on beached Sargassum, we then estimate total epiphytic carbonate import during 2018 at 11 sites along a 60 km section of the Quintana Roo coast, Mexico. Based on measured mean carbonate content of Sargassum (2.09% wet weight; 95% confidence interval [CI]: 1.83–2.32), and estimates of annual beached Sargassum (7.0 × 103 kg drained weight·m−1 of shoreline; 95% CI: 6.9–7.2), our findings indicate that Sargassum beachings in the Mexican Caribbean contributed an average of 179 kg CaCO3·m−1 of shoreline (95% CI: 173–185) in 2018: close to our upper estimate of seagrass epiphyte contributions (210 kg·m−1). Although quantitative data on Sargassum beachings from other locations are sparse, numerous media reports suggest the scale of these events is comparable for many exposed tropical Caribbean and Atlantic shorelines. This represents the first documentation of pelagic Sargassum as a major vector of coastal sediment import, the significance of which has likely only arisen since the onset of large-scale inundations in 2011.

Ocean acidification alters the responses of invertebrates to wound-activated infochemicals produced by epiphytes of the seagrass Posidonia oceanica

Ocean acidification (OA) influences the production of volatile organic compounds (VOCs) by seagrass leaves and their associated epiphytes. We hypothesize that the perception of “odour” produced by seagrass leaf epiphytes will change with seawater acidification, affecting the behaviour of seagrass-associated invertebrates. To test this hypothesis, we collected epiphytes from leaves of Posidonia oceanica growing at two pH conditions (7.7 and 8.1) and identified the most abundant genera of diatoms. We tested the VOCs produced at pH 8.1 by the epiphytic communities in toto, as well as those produced by selected diatoms, on various invertebrates. A complex set of species-specific and concentration-dependent chemotactic reactions was recorded, according to the pH of seawater. In particular, VOCs produced by individual diatoms triggered contrasting reactions in invertebrates, depending on the pH. The perception of epiphyte VOCs is likely to vary due to alteration of species ability to perceive and/or interpret chemical cues as infochemicals or due to changes in the structure of VOCs themselves. Thus, OA alters the fine-tuned chemical cross-talks between seagrass epiphytes and associated invertebrates, with potential consequences for the structure of communities and food webs of seagrass ecosystems.

Census Estimates of Algal and Epiphytic Carbonate Production Highlight Tropical Seagrass Meadows as Sediment Production Hotspots

Tropical shelf, platform and reef-lagoon systems are dominated by calcium carbonate (CaCO ) sediments. However, data on habitat-specific CaCO sediment production rates by different sediment producing taxa are sparse, limiting understanding of where and in what form CaCO sediment is produced, and how overall sediment budgets are influenced by habitat type and scale. Using novel census methodologies, based primarily on measures of plant biovolumes and carbonate content, we assessed habitat-scale production by two ubiquitous biogenic CaCO sediment producers, calcareous green algae and seagrass epiphytes, across southern Eleuthera Bank, Bahamas (area -140 km ). Data from species-specific plant disaggregation experiments and from X-ray diffraction (XRD) analysis of calcified plants also allowed us to resolve questions about the size fractions and mineralogies of the carbonates produced. Production rates varied significantly among habitats (range: 1.8-237.3 g CaCO m yr ), collectively totaling -0.98 M kg annually across the study area. Outputs comprise similar amounts of aragonite and high Mg-calcite, with -54% of the CaCO produced being contributed as mud-grade (<63 μm) sediment. Our analysis also reveals that habitat type and extent - especially of medium and high density seagrass beds - strongly influence the amounts and types of carbonate sediment generated. Dense seagrass beds were identified as the dominant per unit area production sites, contributing -17% of total CaCO despite covering only 0.5% of the study area. These findings have direct relevance for quantifying present-day sediment budgets and for predicting changes in sediment generation at the system scale in responses to modified habitat extent and productivity.

An evaluation of factors controlling the abundance of epiphytes on Zostera marina along an estuarine gradient in Yaquina Bay, Oregon, USA

Epiphytes on seagrass (Zostera marina) growing in the lower intertidal were examined along an estuarine gradient within Yaquina Bay, Oregon over a period of 4 years. The Yaquina Estuary receives high levels of nutrients from the watershed during the wet season and from the ocean during the dry season. Mean epiphyte biomass per unit seagrass leaf surface area (epiphyte load) peaked during the summer, and thus epiphyte load was higher during dry season than wet season in both marine and riverine dominated regions. Epiphyte load was greater in marine than in riverine dominated areas in both wet and dry seasons, although only dry season differences were significant. There was no evidence that grazers controlled epiphyte load differences. Annual DIN concentration was inversely related to epiphyte load, principally because of elevated wet season dissolved inorganic nitrogen from river inputs. While there was a positive annual relation of epiphyte load to PO4 concentration, it is not clear that phosphorus becomes a limiting nutrient for epiphyte growth. Water column light attenuation tends to increase linearly with distance from the estuary mouth, while both epiphyte load and Z. marina biomass tend to decrease. Both seagrass and seagrass epiphytes may be increasingly light limited in the upper estuary, and thus, epiphyte loads may have proportionally more impact on seagrass occurrence in this estuarine region.

Altered epiphyte community and sea urchin diet in Posidonia oceanica meadows in the vicinity of volcanic CO2 vents

Ocean acidification (OA) predicted for 2100 is expected to shift seagrass epiphyte communities towards the dominance of more tolerant non-calcifying taxa. However, little is known about the indirect effects of such changes on food provision to key seagrass consumers. We found that epiphyte communities of the seagrass Posidonia oceanica in two naturally acidified sites (i.e. north and south sides of a volcanic CO2 vent) and in a control site away from the vent at the Ischia Island (NW Mediterranean Sea) significantly differed in composition and abundance. Such differences involved a higher abundance of non-calcareous crustose brown algae and a decline of calcifying polychaetes in both acidified sites. A lower epiphytic abundance of crustose coralline algae occurred only in the south side of the vents, thus suggesting that OA may alter epiphyte assemblages in different ways due to interaction with local factors such as differential fish herbivory or hydrodynamics. The OA effects on food items (seagrass, epiphytes, and algae) indirectly propagated into food provision to the sea urchin Paracentrotus lividus, as reflected by a reduced P. oceanica exploitation (i.e. less seagrass and calcareous epiphytes in the diet) in favour of non-calcareous green algae in both vent sites. In contrast, we detected no difference close and outside the vents neither in the composition of sea urchin diet nor in the total abundance of calcareous versus non-calcareous taxa. More research, under realistic scenarios of predicted pH reduction (i.e. ≤ 0.32 units of pH by 2100), is still necessary to better understand cascading effects of this altered urchin exploitation of food resources under acidified conditions on ecosystem diversity and function.

Evaluating the Role of Seagrass in Cenozoic CO2 Variations

Marine seagrass angiosperms play an important role in carbon sequestration, removing carbon dioxide from the atmosphere and binding it as organic matter. Carbon is stored in the plants themselves, but also in the sediments both in inorganic and organic forms. The inorganic component is represented by carbonates produced by calcareous organisms living as epiphytes on seagrass leaves and rhizomes. In this paper, we find that the rate of seagrass epiphyte production (leaves and rhizomes), averages 400 g m-2 yr-1, as result of seagrass sampling at seven localities along the Mediterranean coasts, and related laboratory analysis. Seagrasses have appeared in the Late Cretaceous, becoming a place of remarkable carbonate production and C sequestration during the whole Cenozoic era. Here, we explore the potential contribution of seagrass as C sink on the atmospheric CO2 decrease by measuring changes in seagrass extent, which is directly associated with variations in the global coastal length associated with plate tectonics. We claim that global seagrass distribution significantly affected the atmospheric composition, particularly at the Eocene-Oligocene boundary, when the CO2 concentration fell to 400 ppm, i.e. the approximate value of current atmospheric CO2.

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.

Mesopredator foraging success in eelgrass (Zostera marina L.): Relative effects of epiphytes, shoot density, and prey abundance

In seagrass habitats, structural complexity influences predator foraging success and prey survival by moderating encounter rates and behaviors of predators and prey. Studies of the effects of structural complexity on predator–prey interactions typically vary seagrass shoot density, biomass, or architecture. Filamentous epiphytic algae commonly grow on seagrass blades and add structure to seagrasses, but the effect of epiphytes on the outcome of predator–prey interactions has rarely been explored. This study determined the relative effects of simulated seagrass epiphytes, eelgrass (Zostera marina) shoot density, and prey density on the foraging behaviors of a common predator, the juvenile giant kelpfish Heterostichus rostratus (Girard, 1854) and on escape responses of their prey, grass shrimp Hippolyte californiensis (Holmes, 1895). Juvenile giant kelpfish hunted for grass shrimp in a laboratory experiment in which two levels of grass shrimp density were crossed with three levels of structural complexity (low density artificial eelgrass, low density artificial eelgrass with artificial epiphytes, and high density artificial eelgrass). Increased structural complexity had variable effects on juvenile kelpfish activity levels, but enhanced their success at catching prey after detection, resulting in no net effect of structural complexity on the total number of prey captured. Grass shrimp made fewer escape attempts in high vs. low structural complexity, which may have enhanced kelpfish foraging success because kelpfish rarely pursued fleeing prey. Prey density did not influence kelpfish behaviors or the total amount of prey they consumed. These results suggest that it is important to consider how different forms of habitat complexity affect the mechanisms by which predators hunt for prey in structured marine habitats.

Winter Nutrient Pulse and Seagrass Epiphyte Bloom: Evidence of Anthropogenic Enrichment or Natural Fluctuations in the Lower Florida Keys?

Nutrient enrichment continues to disrupt marine ecosystem function worldwide. Assessing eutrophication in seagrass ecosystems such as the Great White Heron National Wildlife Refuge (GWHNWR), Florida Keys is critical for protecting the diverse community that depends on the intertidal and subtidal seagrass beds. We quantified water column nutrients, chlorophyll a, tissue nutrients in macroalgae and Thalassia testudinum, and epiphyte percent cover on seagrasses on tidal flats seasonally over 1 year at three sites: Howe Key, Water Keys, and Upper Harbor Key. Water column nutrients (total nitrogen, total phosphorus, nitrate–nitrite, ammonium, dissolved inorganic nitrogen, dissolved organic nitrogen, soluble reactive phosphorus, dissolved organic phosphorus) increased from fall to winter at Water Keys. These increased nutrients coincided with a bloom of the epiphyte Spyridia filamentosa on seagrasses that exceeded 40 % cover at Water Keys and Upper Harbor Key in two consecutive seasons. Seagrasses at all sites had chronic percent cover of small epiphytes exceeding 60 %. Additionally, low δ13C in T. testudinum tissues at Upper Harbor Key compared to the other sites suggested variations in carbon sources across the study area. Spatial patterns in macrophyte nitrogen-to-phosphorus (N:P) ratios point to sources of nutrients from the inhabited islands of the Keys and from the Gulf of Mexico. Water column total nitrogen and total phosphorus concentrations exceeded the numeric nutrient criteria for the study area suggesting that the area should be monitored closely.

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.

Ocean acidification outweighs nutrient effects in structuring seagrass epiphyte communities

Summary Developing a framework for assessing interactions between multiple anthropogenic stressors remains an important goal in environmental research. In coastal ecosystems, the relative effects of aspects of global climate change (e.g. CO2 concentrations) and localized stressors (e.g. eutrophication), in combination, have received limited attention. Using a long‐term (11 month) field experiment, we examine how epiphyte assemblages in a tropical seagrass meadow respond to factorial manipulations of dissolved carbon dioxide (CO2(aq)) and nutrient enrichment. In situ CO2(aq) manipulations were conducted using clear, open‐top chambers, which replicated carbonate parameter forecasts for the year 2100. Nutrient enrichment consisted of monthly additions of slow‐release fertilizer, nitrogen (N) and phosphorus (P), to the sediments at rates equivalent to theoretical maximum rates of anthropogenic loading within the region (1.54 g N m−2 d−1 and 0.24 g P m−2 d−1). Epiphyte community structure was assessed on a seasonal basis and revealed declines in the abundance of coralline algae, along with increases in filamentous algae under elevated CO2(aq). Surprisingly, nutrient enrichment had no effect on epiphyte community structure or overall epiphyte loading. Interactions between CO2(aq) and nutrient enrichment were not detected. Furthermore, CO2(aq)‐mediated responses in the epiphyte community displayed strong seasonality, suggesting that climate change studies in variable environments should be conducted over extended time‐scales. Synthesis. The observed responses indicate that for certain locations, global stressors such as ocean acidification may take precedence over local eutrophication in altering the community structure of seagrass epiphyte assemblages. Given that nutrient‐driven algal overgrowth is commonly cited as a widespread cause of seagrass decline, our findings highlight that alternate climate change forces may exert proximate control over epiphyte community structure.

Uptake of dissolved inorganic nitrogen by shallow seagrass communities exposed to wave-driven unsteady flow

In natural seagrass systems, regular oscillatory motion-like that caused by surface wind-waves-enhances uptake of dissolved inorganic nitrogen (DIN) relative to current-driven, unidirectional flows. A mobile field flume was deployed to measure the uptake of DIN by intact shallow seagrass communities exposed to unidirectional, oscillatory, and combined flows. The flume volume was spiked with N-15-labeled DIN to measure nutrient uptake rate constants for the entire system (S) and to determine height-specific nutrient uptake rates of seagrass epiphytes (rho). In oscillatory and combined flows, S depended positively on the water speed, but also inversely on a modified Keulegan-Carpenter number (KC). This ratio compares the wave-orbital excursion distance to canopy element spacing. Experiments characterized by relatively low KC values were associated with significantly higher uptake efficiencies than experiments characterized by high KC values. Uptake efficiencies for high KC conditions were similar to those measured in comparable unidirectional flows. Measured canopy flow attenuation was also found to increase with KC, a result in line with expectations from a parameterized conceptual model. Damping variability alone could not explain the observed oscillatory flow uptake enhancement however, a result that seemingly highlights the role of both element flexural response and unsteady boundary layer mechanics in the exchange process. In all flow conditions, DIN uptake rates of epiphytes harvested from the bottom of the blades (rho(bot)) were lower than those for epiphytes harvested from the upper portion of the blades (rho(top)). While uptake by the top epiphytes exhibited flow dependency, uptake by bottom epiphytes did not.

Evaluation of the concurrent use of multiple descriptors to detect anthropogenic impacts in marine coastal systems

This study is aimed at evaluating the effectiveness of multiple environmental descriptors to detect anthropogenic impacts in marine coastal system. To achieve this objective, data obtained from an area subjected to different sources of pollution were compared with those of reference areas through multivariate and univariate analyses. The following environmental components were used: metal concentration in sediments, structure of soft-bottom macrozoobenthos assemblages, biometrical and lepidochronological variables of the seagrass Posidonia oceanica, epiphytes and vagile fauna of P. oceanica meadows. Differences between disturbed and reference conditions were clearly detected by chemical analyses and soft-bottom assemblages, Posidonia oceanica descriptors showed variable sensitivity and vagile fauna associated to seagrass meadows did not highlight significant differences between conditions. Results showed the effectiveness of the use of multiple descriptors and both univariate and multivariate approaches to evaluate the ecological quality of coastal marine areas when more sources of pollution are concurrently present.